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The Role of Metal Plasticity and Interfacial Strength in the Cracking of Metal-Ceramic Laminates
Abstract
Two models based on elastic-plastic fracture mechanics and fiber bridging are developed to study the role of plastic yielding in metals and the interfacial strength of metal/ceramic laminates. There are two types of damage observed in metal/ceramic laminates: multiple cracking and macroscopic crack propagation. The former occurs around the macroscopic crack tip and thus distributes the damage and enhances the composite's toughness. The present models establish that there exists a critical metal/ceramic layer thickness ratio above which multiple cracking dominates and that this ratio decreases (hence increasing the possibility of multiple cracking) as the ratios of metal yield stress over ceramic strength, metal modulus over ceramic modulus, and metal/ceramic interfacial strength over ceramic strength increase. Good agreement between the present models and experimental results is observed for both damage modes, i.e. multiple cracking vs macroscopic crack propagation, and for critical stress intensity factors. The elastic-plastic fracture mechanics and fiber-bridging models predict that multiple cracking is ensured if the metal layer thickness is 2.5 times larger than the ceramic layer thickness, regardless of the metal/ceramic properties.
... Akin to tunnel cracks, a critical layer thickness ratio, below which the composite will fail by single macroscopic cracking, is demonstrated experimentally and theoretically [155]. Using a metal/ceramic laminated composite as an example, the multiple cracking mode is favored if the thickness of metallic layers is at least 2.5 times greater than that of ceramic layers, regardless of the constituent properties [400]. In addition, the morphology of multiple cracking should be particularly highlighted, with features of two completely broken brittle layers bridged by one intact ductile layer in between them [346]. ...
... The dimensionless coefficient β reflects the geometrical constraint of heterogeneous laminated structure, which is roughly estimated by either elastic fracture mechanics, β = 1/(1 − 2ν) = 2.94 if taking v = 0.33 for typical metals, or Prandtl's solution, β = (2 + π)/1.732 = 2.97 [400]. At a finer microstrucural scale, the β level can be increased up to 5 [155]. ...
... Consequently, the stress intensity of ductile components is progressively enhanced, and may send an "reversed" force back to the α 2 -Ti 3 Al layer particularly when the stress level of ductile components becomes larger than that of α 2 -Ti 3 Al layers [345,381]. This inference is theoretically supported by the presence of high bridging stress [155,400] and experimentally confirmed by initial extension and subsequent shrink of strain concentration areas as observed by high-resolution DIC strain mapping [137]. Therefore, the direction of local stress transfer might be inversed (Fig. 18d), which enables the sufficient leverage of the ability to impede severe strain localization nearby the crack tip and disperse plastic deformation in wider dimensions. ...
In this paper, we systematically proposed the strategy of tailoring strain delocalization to evade long-standing strength-ductility trade-off dilemma. The scientific contribution is to define and, for the first time, to expand the category of strain localization into the whole deformation process, including elastic lattice distortion, plasticity-relevant statistical behaviors (dislocation, twinning, shear/slip bands, necking, etc.), and crack-dependent damage accumulation. The viewpoint we proposed is that the achieving of strength-ductility synergy depends on the delocalizing of aforementioned localized strains. Using hierarchical materials as an example, the design of heterogeneous structure significantly influences the strain delocalization behaviors in terms of internal stress/strain (elastic stage), local strain evolution (plastic stage), and cracking (fracture stage). Relationships among the heterogeneous microstructure, microscopic stress/strain evolution, macroscopic mechanical properties are established. In particular, we assess their influences on strain delocalization from the perspective of slip transfer, plastic stability, damage micromechanics, and crack propagation. A methodological framework is then suggested to understand the materials behaviors in the future using the rapidly developed physics-based multi-dimensional computational models and advanced in situ strain characterization techniques. Innovations towards excellent strength-ductility synergy and expanding applications are increasingly advocated, through promoting strain delocalization and indentifying the current challenges and future opportunities.
... There are two different damage modes observed during the fracture of metal/ceramic laminates ( Fig. 1): one is the formation of a zone of multiple cracks near the tip of a macroscopic crack, and the other is the continuous propagation of a macroscopic crack in the ceramic layers while the metal layers remain intact. As the criterion for the transition from single to multiple cracking is a key parameter for the design of metal/ceramic laminates, this subject has been investigated by many authors [6][7][8][9][10][11]. Fracture maps have been constructed to enable the designer to determine the fracture mode for a given laminate with certain given geometrical and mechanical properties of the ceramic and the metal layers. ...
... Factors affecting the transition from single to multiple cracking have been studied by many authors [6][7][8][9][10][11], and different fracture maps has been constructed to enable predicting the fracture mode for a given laminate with certain given geometrical and mechanical properties of the ceramic and the metal layers (Fig. 3). The main geometrical parameter, which affects the transition from single to multiple cracking mode is the laminate thickness ratio, / m c t t (t m and t c are the metal and ceramic layers thickness respectively) while the mechanical properties include the ceramic fracture strength and the constrained yield stress of the metal layer. ...
... The plastic hardening of metals helps to activate the multiple cracking mode, (as it increase the bridging traction), while the plastic yielding does the opposite. Further work of Huang and Zhang [11] introduced a new parameter affecting the cracking mode, which is the ceramic fracture stress,σ c . A fracture map was established, from which was concluded that as long as the metal layer thickness is 2.5 times larger than the ceramic layer thickness, (t m /t c >2.5), multiple cracking always occurs for a well-bonded laminate, regardless of the metal/ceramic properties. ...
... thus the toughness of the composite decreased drastically. On the other hand, stress easily concentrates at the brittle phase [16]. In this case, TiAl 3 is easy to become a crack source and deteriorates the toughness of the composites. ...
... The fracture modes of ceramic−metal laminated composites include single crack propagation and multiple crack propagation. Compared with single crack propagation, multiple crack propagation can significantly improve the toughness of the ceramic−metal laminated compo- sites [16][17][18][19][20]. When the lamellae, particularly the metal layers, fracture in the mode of multiple cracking, multiple cracks are able to absorb fracture energy much more efficiently than a single crack. ...
... When cracks occurred in C1, the stress at the crack tip reached σ max , while that at the un-fractured C2 was σ 0 . If σ max > σ 0 , C2 would fracture in the multiple-cracking mode [16], as shown in Fig. 8 (a). However, if σ max < σ 0 , C2 would fracture in the singlecracking mode [16], as shown in Fig. 8(b). ...
We prepared Al/TiC composites with different ceramic volume fractions (15, 25 and 35 vol.%) using ice-templating and pressure infiltration. The thickness of the lamellar layer and the porosity in the ceramic layer of the TiC scaffolds were controlled by varying the slurry concentration. The Al/15 vol.%TiC composite had a thick metal layer and a low-density ceramic layer, which effectively dissipated the stress at the crack tip and fractured in a multiple-crack-propagation mode, giving bending strength of 355 ± 3 MPa and fracture toughness of 81 ± 2 MPa·m1/2. However, the Al/25 vol.%TiC and Al/35 vol.%TiC composites had much higher bending strength (417−500 MPa) but lower fracture toughness (46−33 MPa·m1/2) as compared to the Al/15 vol.%TiC composite, and they fractured in a single-crack-propagation mode. In addition, an increase in the brittle TiAl3 phase with increasing ceramic volume at the fracture surface greatly deteriorated the toughness of the Al/TiC composites. Finally, the relationship between cracking mode and structure features in the laminated composites was discussed to account for the toughening mechanism.
... The thickness of soft metal layers has also an influence on stress intensity factors for macroscopic crack propagation: with increasing metal layer thickness, the stress intensity factor and the critical stress intensity for crack reinitiation increase as well [80,81]. ...
... In metal-ceramic multi-layer coatings, cracks are initiated at interfaces and developed in hard and brittle ceramic layers and block at next interface [80,83,84]. Huang and Zhang [80], based on microscopic observations, showed that two different damage modes in metal-ceramic multi-layer laminates occur (Figure 16.8). ...
... In metal-ceramic multi-layer coatings, cracks are initiated at interfaces and developed in hard and brittle ceramic layers and block at next interface [80,83,84]. Huang and Zhang [80], based on microscopic observations, showed that two different damage modes in metal-ceramic multi-layer laminates occur (Figure 16.8). According to Figure 16.8, in one mode a zone of multiple cracks is formed near the tip of a macroscopic crack, while in the second mode the continuous propagation of one of the cracks occurs in the ceramic layers with ductile metal layers remaining untouched. ...
The aim of this chapter is to present and discuss the correlations between coating deposition and properties of physical vapor deposition (PVD) coatings, and the degradation mechanisms of hard coatings caused by dynamic loading. PVD coatings are very attractive for the aerospace, chemical, and oil and gas industries due to their anticorrosion properties, oxidation resistance, high hardness, and low coefficient of friction. Due to the fact that many devices during their service are exposed to shock loads, the knowledge of degradation mechanisms of PVD coatings under dynamic load is particularly important.
PVD coatings have been successfully employed as wear-resistant coatings. They improve fatigue strength and cavitation erosion resistance. However, hard coatings easily fracture in a brittle mode. Mechanical behavior of coatings depends on which degradation mechanism dominates during deformation. Degradation mechanism is related on the coating structure (morphology, phase composition, number of layers in a coating, thickness of each layer, thickness of a whole coating), mechanical properties (Young's modulus, hardness, adhesion) as well as on properties of substrate, and a rate and frequency of impacts.
Moreover, in order to get a coating with special properties, composite or multi-layer coatings are produced. A tricomponent TiAlN or Ti(C,N) coating, combining the advantages of high hardness of TiC, high ductility of TiN, and high adhesion strength, possesses much better mechanical properties than single-phase TiC or TiN. Similarly, multi-layer coatings have shown better properties in many aspects than monolayer coatings, for example, cavitation erosion resistance of TiN/Ti multi-layer coating was better than single-phase monolayer TiN.
... Mechanical properties of metal-ceramic multilayers are important because these systems are prevalent in electronic devices [1], biomedical implants [2] and advanced coatings [3,4], where mutually exclusive features such as hardness and toughness are contributed by the ceramic and metal layers, respectively. As a result, fracture mechanics of metal-ceramic multilayers has been extensively studied in the literature [5][6][7][8]. In these studies, the metal and ceramic layers are assumed to exhibit isotropic and bulk (where continuum assumption holds good) properties. ...
... Upon further loading, the ceramic layers in front of the metal layer also relax some stress by developing microcracks ahead of the propagating crack tip -a process known as crack renucleation. Depending upon the extent of plasticity in the metal layers, renucleation in the ceramic layers can take place as one dominant crack in the same plane of the macroscale crack or as multiple cracks in different planes [5,6]. Another strengthening mechanism is the deflection and/or termination of the cracks at the interfaces. ...
... In other words, crack renucleation always takes place in the ceramic layers [30]. Mechanistic models are available in the literature that take in account the extent of plasticity [5,6] to predict whether the crack renucleation is going to be single or multiple crack mode. For the ratio of metal and ceramic layer thickness, Young's modulus and fracture stress for our multilayer specimens, the models predict a multiple mode crack renucleation in the ceramic layer, yet we observed single-mode renucleation in the metal layer. ...
Fracture of Ti/TiN multilayer specimens was studied in situ inside a transmission electron microscope. Fracture toughness for cracks propagating perpendicular to the multilayers is found to be thickness dependent, varying from 1.45 to 2.45 MPa-m0.5 as the specimen thickness increased from 150 to 300 nm. Single-mode crack renucleation was observed in the metals layers, which is anomalous to the continuum-based elastic–plastic multilayer fracture model predictions. This is explained by the ultrafine columnar grain structure in the metal layers.
... Finally, consider crack growth in a direction transverse to the layer orientations, but in which crack extension must occur by the sequential renucleation of cracks in successive ceramic layers across intervening metal layers (orientation 3; Figures 7(c) and 12) (Shaw et al., 1993Shaw et al., , 1996 Pateras et al., 1995a,b; Shaw, 1996 Shaw, , 1998 Cao and Evans, 1991; Huang and Zhang, 1995; Figure 12 Optical micrograph of the same region of a polished cross-section of a copper/aluminum oxide multilayer subject to orientation 3 crack growth, (a) Before crack renucleation, (b) after crack renucleation (after Shaw et al., 1993)., 1993a,b). This mode of crack growth, hereafter designated perpendicular crack growth, must be analyzed in a different manner than crack growth subject to orientation 2 loading, where the crack front has unrestricted access to the ceramic phase. ...
... The previous discussion has focused on the fracture behavior of multilayered composites without any pre-existing stress concentration. If flaws or notches exist that span the width of several brittle layers, it should be noted that different behavior may result upon initial loading than if the layers originally are relatively free of flaws (Shaw et al., 1996; Huang and Zhang, 1995). Specifically, the stress concentration associated with the presence of the notch may trigger Class I behavior in composites where Class II behavior would result in the absence of the notch (Zok and Horn, 1990). ...
Multi-layered metal/ceramic composites are being developed for the past few decades and are extremely beneficial in the various engineering applications. The extensive application history and the processes adopted for manufacturing them are explained. The fracture characteristics and failure mechanisms of multi-layered metal/ceramic composites under different loading conditions are elucidated. Depending on these features, the parameters to be analyzed during designing are described and their reliability is addressed. Finally, the self-healing ability, enhancing the mechanical performance, such as the flexural properties and reliability, is elaborated along with the limiting factors.
... The crack pattern undergoes a transition that is mainly influenced by the ratio of restrained metal yield stress (σ m ) to ceramic strength (σ c ) and the ratio of the thickness of the alloy layer (t m ) to the thickness of the ceramic layer (t c ) [42]. Specifically, multiplecracking extension occurs when t m /t c exceeds the critical value (t m /t c ) crit , and the critical value (t m /t c ) crit increases with the increase in the alloy layer/ceramic layer elastic modulus ratio (E m /E c ) [43,44]. As mentioned above, as the sintering temperature increases, only a small amount of alloy liquid fills the ceramic layer into the denser lamellar structure, and a large number of ceramic particles are interconnected with each other, gradually increasing the strength (σ c ) of the ceramic layer and also gradually increasing the modulus of elasticity (E c ), thus making the critical value (t m /t c ) crit gradually increase. ...
To address the issue of inadequate strength and plasticity in magnesium matrix composites, SiC preforms were prepared using the freeze-casting process. The effects of sintering temperature on the microstructure, mechanical properties, and fracture behavior of SiCp/AZ91 magnesium matrix composites were studied by controlling the density of SiC preforms through low-temperature sintering. The results indicate that as the sintering temperature decreases, the reaction products in the SiC layer decrease, resulting in lower SiC preform density and increased content of AZ91 alloy filling in the layer. The increased alloy content in the ceramic layer not only inhibits crack initiation but also hinders crack propagation, thereby endowing the SiCp/AZ91 laminated material with excellent compressive strength and compressive strain. At the sintering temperature of 900 °C, the SiCp/AZ91 laminated material exhibits impressive compressive strength and strain values of 623 MPa and 8.77%, respectively, which demonstrates an excellent combination of strength and toughness.
... Moreover, when the crack propagates through the interfaces between Sn and Ni or Ni and NdFeB, it will experience a change in crystallographic structure, microstructure and mechanical properties. As a result, the crack tip will be blunted by plastic deformation, and the crack path will be deflected near the interfaces [26][27][28][29]. In this case of Ni/Sn coated NdFeB magnet, the blunting of crack tip can be induced by the plastic deformation of Ni coating when the crack goes through Sn to Ni or NdFeB to Ni. ...
Metallic coating by electroplating is commonly attractive for improving the corrosion resistance of sintered NdFeB magnets. However, its tailoring of mechanical characteristics for sintered NdFeB magnets has been seldom concerned. Herein, the impact toughnesses of sintered NdFeB magnets with various metallic coatings (Ni or Ni/Sn) were comparatively investigated. The results indicate that the impact toughnesses of sintered NdFeB magnets are both improved by Ni coating and Ni/Sn bilayer coating. And Ni/Sn bilayer coating exhibits more enhancement of the impact toughness, increased by 41.6 % compared with the original magnet. Moreover, the microstructural observations of the metallic coatings and the fracture were conducted, and the enhanced mechanism of impact toughness for the magnet is analyzed. These findings may provide a reference for toughening the brittle materials. © The Korean Magnetics Society. All rights reserved. and 2018 Journal of Magnetics.
... Toughening by a multi-layered coating architecture is attributed to the effects of interfaces on dissipating crack energy and deflecting cracks [92,93,94,95,96]. Several toughening mechanisms, as demonstrated in Figure 5, have been suggested: crack splitting and deflecting at weak interface between layers [97], a favorable stress distribution due to the layered structure [98], ductile layer ligament bridging and crack tip blunting at a strong interface [99]. ...
Aircraft engine components, such as compressor blades, vanes, and impeller blisks/wheels, when operating in a sandy environment, can experience severe erosion damage due to ingestion of sand particles. As erosion progresses, the substantial amount of material removal not only leads to further aerodynamic losses but results in the structural weakening of blades as well. Replacement of the parts, whose erosion damage limits have been reached, significantly increases the maintenance and downtime costs. Applying erosion resistant coatings on airfoil surface has been proven as an effective approach to extending the serviceable life of gas turbine engine components. This chapter provides a comprehensive review of the factors affecting erosion damage to engine components, the basic requirements that erosion resistant coatings need to satisfy, the various coating systems that have been developed with excellent erosion resistance, as well as the erosion durability testing techniques applied to evaluate and qualify erosion-resistant coatings. Several technical challenges related to the development of erosion resistant coatings are also briefly discussed.
... Damage mechanisms in metal/ceramic composites with lamellar microstructures have not been studied in depth yet. Previous studies of cracking patterns in metal/ceramic composites under tensile loading were performed on composites fabricated by diffusion bonding and focused mainly on multiple cracking in ceramic layers ahead of a macroscopic through crack [15][16][17][18][19]. Initiation and accumulation of damage within the ceramic lamellae, mainly in the form of transverse cracking (Fig. 1), has been observed under compressive loading. ...
Metal/ceramic composites with lamellar microstructures are a novel class of metal-matrix composites produced by infiltration of freeze-cast or ice-templated ceramic preforms with molten aluminium alloy. The cost-effectiveness of production and relatively high ceramic content make such composites attractive to a number of potential applications in the automotive, aerospace and biomedical engineering. A hierarchical lamellar microstructure exhibited by these composites, with randomly orientated domains in which all ceramic and metallic lamellae are parallel to each other, is the result of the ice crystal formation during freeze-casting or ice-templating of preforms from water–ceramic suspensions. In this paper, a single-domain sample of metal/ceramic composite with lamellar microstructure is modelled theoretically using a combination of analytical and computational means. Stress field in the sample containing multiple transverse cracks in the ceramic layer is determined using a modified 2-D shear lag approach and a finite element method. Using finite element modelling, the shear layer thickness is determined and used as input in the analytical model. Degradation of stiffness properties of the sample due to multiple transverse cracking is predicted using the equivalent constraint model.
... Ze względu na wytrzymałość mechaniczną i estetykę uzupełnień metalowo-ceramicznych połączenia materiałów odgrywają ważną rolę w technikach dentystycznych dotyczących korony i mostu protetycznego, od czasu, kiedy tego rodzaju protezy zębowe są wykonywane [1]. Najsłabszym ogniwem tych konstrukcji wydaje się być połączenie tych różnych materiałów [2]. Na etapie łączenia elementów konstrukcyjnych protez stałych z materia- łem służącym do wykonania estetycznego licowania imitującego zęby własne pacjenta, stan powierzchni oraz jej przygotowanie mają decydujące znaczenie dla trwałości pracy. ...
... In ceramic/metal laminates, there is a critical thickness ratio (t m /t c ) crit ≈2.5 of the metal layer thickness (t m ) to the ceramic layer thickness (t c ). If t m /t c is higher than the critical thickness ratio, fracture pattern switches from single to multiple [15,27] . In the present study, laminates with t m /t c ≈0.33 are expected to fail through the single, dominant cracking mode. ...
Al2O3/Al-steel mesh-Al laminated composites with "sandwich" Al-steel mesh-Al layer as interlayer, which was consisted of two Al foils and one steel mesh, were prepared by vacuum hot pressing at 580 degrees C and 1.5 MPa. The results indicate that the interface of Al2O3/Al is bonded tightly and no reaction is observed. Intermetallic compound (IMC) is detected in Al/steel interface which improves the connection conditions between the steel and Al. The laminated composites have much higher fracture toughness, work-of-fracture than that of the Al2O3 monolith, as well as having a close strength to Al2O3. Crack blunting and arresting, crack bridging, interface debonding, and ductile deformation of "sandwich" Al-steel mesh-Al layer are the main reasons for improving the toughness and work-of-fracture of laminated composites. Low-velocity impact results suggest that the Al2O3/Al-steel mesh-Al laminated composites have good impact resistance.
... Furthermore, in metal-reinforced ceramic laminates, it has been shown that K o scales directly with increasing reinforcement layer thickness. [33,44,46,47] This is due to the larger far-field applied stresses necessary to drive the local fracture event because the brittle phase is farther from the crack tip. ...
A brittle intermetallic, Nb3Al, reinforced with a ductile metal, Nb, has been used to investigate the resistance curve and cyclic fatigue behavior of a relatively coarse laminated composite. With this system, the toughness of Nb3Al was found to increase from ∼1 MPa√m to well over 20 MPa√m after several millimeters of stable crack growth; this was attributed to extensive crack bridging and plastic deformation within the Nb layers in the crack wake. Cyclic fatigue-crack growth resistance was also improved in the laminate microstructures compared to pure Nb3Al and Nb-particulate reinforced Nb3Al composites with crack arrester orientations in the laminate providing better fatigue resistance than either the matrix or pure Nb.
... These chipped sites were formed from localized partially removal of coating materials through crack formation, propagation and inter-connection as a result of the continuous impingement from sand particles. As the multilayered architecture enhances coating toughness due to the roles of interfaces in dissipating crack energy and deflecting cracks [44,45], the erosion performance of multilayered coatings was reported to be affected by the number and thickness of the individual layers [46]. It has been reported that when the interface area per volume increases, toughening is enhanced first and then achieves the optimum effect at a specific value of interface area/ volume [47] (or the number of layers and individual layer thickness). ...
Applying hard coatings on airfoil surfaces is proven to be an effective approach to mitigating erosion damage to engine components. Nanolayered or multilayered coatings, because of their capability of tailoring hardness and toughness through modifications in the chemistry and architecture of layer constituents, have been explored as potential candidates for this specific application. In this study, nanolayered CrAlTiN (CrN/AlTiN) coatings with different modulation periods, along with multilayered CrAlTiN-AlTiN coatings having different number of layers and different thickness of individual layers, were fabricated, characterized and evaluated. All the coatings significantly outperformed the CrN baseline in erosion resistance, and their performance was strongly affected by the bilayer period of the nanolayered coatings or the layer architectural characteristics of multilayered coatings.
... The underlying fracture mechanisms in metal-ceramic laminates, often characterized by multiple cracking and macroscopic crack propagation, have been extensively studied at the bulk scale [12][13][14][15][16][17]. However, these mechanisms and the associated theories may not be directly applicable to the nanometer to micrometer thick multi-layers. ...
Thin film specimens of titanium - titanium nitride multilayer erosion resistant coating were prepared using liftout technique in Focused Ion Beam - Scanning Electron Microscope (SEM). The fracture toughness of the thin film specimen was measured in situ using a cantilever bending experiment in SEM to be 11.33 MPa/m0.5 , twice as much as conventional TiN coatings. Ti–TiN multi-layer coatings are part of a new class of advanced erosion resistant coatings and this paper discusses an experimental technique to measure the fracture toughness of these coatings.
... In the applications these materials would be exposed not only to severe mechanical stresses, but also to aggressive gasses resulting in oxidation and corrosion. Many researchers have reported the mechanical properties of the ductile/brittle microlaminates, such as deformation and fracture process [4][5][6][7][8]. However, a detailed investigation into the high-temperature oxidation behavior of these materials has not been publicly reported. ...
Two NiCoCrAl/ZrO2–Y2O3 microlaminates (A and B) were fabricated by electron beam physical vapor deposition, which were different in layer number and metal-layer thickness. The layer number was 20 and 26, respectively. And the metal-layer thickness was 35 and 14μm, respectively. The microstructure and isothermal oxidation behavior were investigated. During the exposure in air at 1000°C for 100h, the t to m phase transformation occurred in the ceramic layers, and oxide scales formed at the surfaces of not only the outer metal-layers but also the internal metal-layers for the microlaminates. The oxidation rate of microlaminate B was greater than that of microlaminate A. Their overall mass gains were significantly dependent on the number and thickness of the metal-layers. The oxidation products were also influenced by metal-layer thickness. Oxide scales of the 35μm thick metal-layer microlaminate (A) consisted mainly of α-Al2O3 and θ-Al2O3, while the oxidation products of the 14μm thick metal-layer microlaminate (B) were the mixture of α-Al2O3, θ-Al2O3 and Cr2O3. It was also found that the growth of the oxide scale adjacent to the top YSZ layer was controlled by the oxygen diffusion, and that the growth of the oxide scale adjacent to the internal YSZ layer was controlled by the metal ionic diffusion.
... A number of diverse brittle intermetallics-ductile metal laminates have been produced, including Ti-Al 3 Ti [22], Nb-Cr 2 Nb [18], Nb-Nb 3 Al [2], TiAl-TiNb [23], FeAl-Tic [24], Al-Al 2 O 3 [25][26][27], Al 2 O 3 -Cu [6], Al-NiAl [28], Mo-NiAl [29]. Among these laminate composites, the Ti6Al4V-Al 3 Ti laminate, fabricated from Ti6Al4V and Al foils by reaction synthesis in open air, has a great technological advantage and attracts special attention [30][31][32]. ...
The crack propagation and damage evolution in metal (Ti6Al4V)-intermetallic (Al3Ti) laminate composites were investigated. The composites (volume fractions of Ti6Al4V: 14%, 20% and 35%) were tested under different loading directions (perpendicular and parallel directions to laminate plane), to different strains (1%, 2%, 3%) and at different strain rates (0.0001 and 800–2000s−1). Crack densities and distributions were measured. The crack density increases with increasing strain, but decreases (at a constant strain) with increasing volume fraction of Ti6Al4V. Differences in crack propagation and damage evolution in MIL composites under quasi-static (10−4s−1) and dynamic (800–2000s−1) deformation were observed. The fracture stress does not exhibit significant strain-rate sensitivity; this is indicative of the dominance of microcracking processes in determining strength. Generally, the crack density after dynamic deformation is higher than that after quasi-static deformation. This is attributed to the decreased time for crack interaction in high-strain rate deformation. The effect of crack density, as quantified by a damage parameter, on elastic modulus and stress–strain relation were calculated and compared with experimental results.
In this work, we present a comparative analysis of the structure and properties of cathodic-arc multilayer TiZrN/NbN and TiSiN/NbN nanolaminates. It has been found that both nanolaminates are reasonable for tribological applications inasmuch demonstrate improved mechanical properties like super hardness of 36–44 GPa and Young’s modulus of 396–435 GPa. The multilayer structure of both nanolaminates reduces the interfacial shear stress of the coating; however, the TiSiN/NbN nanolaminate possesses higher fracture toughness and bonding strength. The complete cohesive-adhesive failure of the TiSiN/NbN coating from the substrate occurs at a load of 47 N. Phase segregation, microstructure evolution, and grain refinement strengthening are the alternative backgrounds for enhanced mechanical and adhesive properties of TiSiN/NbN nanolaminate, while for TiZrN/NbN nanolaminate it is the solid-solution hardening. In summary, the presence of thin amorphous-crystalline TiSiN layers enhances the strength of nanolaminates better than the solid-solution TiZrN crystalline layers.
In this work high strength and tough metal‐ceramic laminated composites are fabricated by spark plasma sintering (SPS) of Ti3Al(Si)C2 MAX‐phase filled preceramic papers (TAC) and ductile Nb foils. The sintering is carried out at 1250 °C and 50 MPa for 5–20 min. Various stacking techniques are used to obtain Nb/TAC laminated composites with different architectures. SPS results in the formation of reaction layer (RL) with a complex composition, which changes the thickness insignificantly with increasing sintering time. The possible formation mechanism of RL is discussed. The bending strength of Nb/TAC composites is decreased from 410 to 350 MPa when lowering the thickness of ceramic layer. The maximum fracture toughness of 10.2 MPa·m1/2 is achieved for the composite with similar individual layers thickness. The toughening is explained by complex fracture mechanisms associated with deflection and branching of cracks at interfaces, delamination, plastic deformation of Nb layers, multiple cracking and crack deflection in ceramic TAC layers.
Many natural materials demonstrate ideal design inspirations for the development of lightweight composite materials with excellent damage tolerance. One notable example is the layered architecture of nacre, which possesses toughness an order of magnitude higher than its constituent parts. Man-made nacre-like ceramic/polymer composites obtained through direct infiltration of polymer in ceramic scaffolds have been shown to produce improved mechanical properties over other composite architectures. Replacing the polymer phase with metal could provide higher damage tolerance but the infiltration of metal into complex ceramic scaffolds is difficult due to the surface tension of molten metal. To address this, bioinspired nacre-like micro-layered (µL) alumina scaffolds with different ceramic fractions from 18 to 85% were infiltrated with aluminium alloy 5083 via pressureless and squeeze casting infiltrations techniques. The scaffolds were created using a bi-directional freeze-casting and one-step densification method. As a result, the µL alumina/aluminium composites displayed significant extrinsic toughening mechanisms with both high strength and toughness. The mechanical performance was highly dependent on the interface, microstructure, and composition. The nacre-like composites with 18% alumina and AlN interface displayed a maximum resistance‐curve toughness up to around 70 MPa.m½ (35 MPa.m½ at the ASTM limit) and a flexural strength around 600 MPa.
Unidirectional freeze casting followed by sintering and infiltration treatments has been proved to be a promising method to produce lamellar composites with good strength and toughness. However, it remains a great challenge to tailor the microstructure, particularly the lamellar orientation at the centimeter scale and the architectural features over multiple scales. Herein, we prepared Al/(Al2O3–TiC) composites with a long-range ordered lamellar architecture via bidirectional freeze-casting and melt-infiltration techniques. The incorporation of a certain amount of TiC (∼3 μm in diameter) into the Al2O3 slurry increased the solidification velocity and decreased the ceramic layer thickness. Furthermore, it greatly facilitated the infiltration of liquid Al into the interlamellar channels and in-layer cavities, thus improving the mechanical properties of the composites. The maximum flexural strength (474 ± 8 MPa) and crack-growth toughness (42.7 ± 2.7 MPa m1/2) appeared in the composite containing ∼27 vol% ceramics with Al2O3:TiC = 5:5. Moreover, the composites exhibited anisotropic mechanical properties. The flexural strength loaded in the direction parallel to the ceramic layers was higher than that perpendicular to them, while the toughness exhibited an opposite trend. The main toughening mechanisms included crack blunting, crack deflection, plastic deformation of the metal layers and multiple cracking. This work offers a cost-effective and scalable method for the fabrication of laminated composites with exceptional damage tolerance.
The bio-inspired 2024Al/B4C composites with a laminate-reticular hierarchical architecture were constructed by squeeze casting of 2024Al into loose freeze-cast ceramic scaffolds. This pressurized infiltration process provided a clean and well-bonded interface without physical gaps. By regulating the initial suspension concentration (20, 25, 30 and 35 vol%), the effects of different ceramic content on the microstructure, damage-tolerance behavior and toughening mechanisms of the composites parallel and perpendicular to the ice-growth direction were investigated. The strength and toughness in the longitudinal direction were greater than that in the transverse direction. The 2024Al/20 vol% B4C composite in the longitudinal direction yielded the highest flexural strength of 658 MPa, crack-initiation toughness (KIc) of 18.4 MPa·m1/2 and crack-growth toughness (KJc) of 27.5 MPa·m1/2. The unique damage-tolerant properties were attributed to multiple toughening mechanisms, including crack deflection, branching and blunting, ductile-ligament bridging and multiple-crack propagation, as evidenced by the stable crack growth and rising R-curve behavior during fracture. The significantly decreased damage tolerance in the transverse direction was mainly due to inadequate toughening tools. On the other hand, both the flexural strength and fracture toughness reduced remarkably as the ceramic content increased. The 2024Al/35 vol% B4C composite fractured in a single-crack mode and the crack growth path was almost straight, showing a relatively low flexural strength (502 MPa) and crack-initiation toughness (9.1 MPa·m1/2). The toughening mechanism was discussed in terms of the relationship between structural characteristics and cracking mode.
Herein, we have deposited Cr/CrN/Cr/CrAlN multilayer coatings with various modulation ratios on TC11 alloy substrate using cathodic arc system. The influence of various modulation ratios on microstructure and Al2O3 sand erosion behavior of coatings is systematically studied. Results reveal that the coatings are about 200 nm per cycle and total thickness is 8 μm. Five groups of coatings exhibit high hardness (>3000 HV0.025). The coating with modulation ratio of 12 adhesion can reaches 55 N. The residual stress increases with the decrease of the modulation ratio, but the increase is generally low (less than −2 GPa). In addition, according to sand erosion test, it is found that sand erosion resistance of multilayer coating is significantly around 5 times higher than TC11 alloy matrix. The erosion morphology shows that a large number of irregular cracks and layered spalling appear on the surface of the coating, indicating that the cracks are constantly initiated under the continuous impact of the sand and gravel，and finally gather together and then spalling. Moreover, dynamic response and stress field of the coating under the impact of single sand (Al2O3) are studied by numerical simulations. It is determined that coating cracking is caused by high tensile stress under CrAlN layer. In addition, according to crack propagation morphology and influence of different interfaces between multilayered structures on crack tips, propagation/termination mechanism of cracks is analyzed in detail. Cracks are easy to initiate in hard CrAlN layer and consume a lot of energy after propagating into soft Cr layer, thereby ending at next soft and hard interfaces. These results provide experimental and theoretical support for the study of high tenacity and anti-erosion coating.
To address the issue that B4C ceramics are difficult to be wetted by aluminum metals in the composites, TiB2 was introduced via an in-situ reaction between TiH2 and B4C to regulate their wettability and interfacial bonding. By pressure infiltration of the molten alloy into the freeze-cast porous ceramic skeleton, the 2024Al/B4C–TiB2 composites with a laminate-reticular hierarchical structure were produced. Compared with 2024Al/B4C composite, adding initial TiH2 improved the flexural strength and valid fracture toughness from (484±27) to (665±30) MPa and (19.3±1.5) to (32.7±1.8) MPa·m1/2, respectively. This exceptional damage resistance ability was derived from multiple extrinsic toughening mechanisms including uncracked-ligament bridging, crack branching, crack propagation and crack blunting, and more importantly, the fracture model transition from single to multiple crack propagation. This strategy opens a pathway for improving the wettability and interfacial bonding of Al/B4C composites, and thus produces nacre-inspired materials with optimized damage tolerance.
This paper concentrates on the research progress of titanium alloys and their diffusion bonding fatigue characteristics, and summarizes the laws of fatigue crack initiation and growth of titanium alloys with/without welding. The chemical composition, classification, and common welding method of titanium alloys are stated, with emphasis on the features and advantages of diffusion bonding. The phenomena of slip band formation and dislocation movement under cyclic loading are described, and the mechanism of fatigue crack initiation is clarified. The selection of microstructures is a common method to optimize mechanical properties of titanium alloys. Previous studies suggested that the laminated structure is an important mode to realize the low fatigue crack growth rate of titanium alloys. Improper parameters of the welding process can cause joint defects, and further heat treatment can reduce joint defects while improving the fatigue life and strength. Finally, the multilayer and heterogeneous laminates of titanium alloys produced by diffusion bonding are briefly described to realize the possibility of high damage tolerance.
Freeze casting is a promising approach for assembling lamellar metal-ceramic composites with an exceptional combination of strength and toughness. Although these mechanical properties can be optimized by regulating the microstructure of porous ceramic structures, the effect of lamellar thickness is rarely mentioned, especially for B4C/Al composites. Herein, by controlling the freezing temperature, we used freeze casting to create nacre-like B4C scaffolds with identical ceramic content yet different lamellar thicknesses and then infiltrated them with 2024Al alloy. The effects of lamellar thickness on the damage-tolerance behavior and toughening mechanisms are discussed. The refinement of lamellae decreases the probability of observing catastrophic flaws in ceramic layers, increasing strength from 534 ± 14 to 578 ± 15 MPa and increasing crack-initiation toughness (KIc) from 9.2 ± 0.6 to 11.4 ± 0.2 MPa m1/2. These composites exhibit higher damage tolerance resulting from several toughening mechanisms, such as plastic deformation, crack deflection and blunting, and the uncracked-ligament bridging of ductile metal layers, which is reflected in the stable crack propagation during fracture and rising R-curve behavior. Importantly, coarsening of the structure of composites allows the fracture behaviors transform from single to multiple crack propagation, thus absorbing much more fracture energy, with the valid crack-growth toughness (KJc) enhancing markedly from 17.1 ± 2.4 to 29.8 ± 2.3 MPa m1/2.
The nacre-inspired Al-Si/TiB2 composites were successfully prepared by freeze casting and pressure infiltration. The microstructures and mechanical properties of nacre-inspired Al-Si/TiB2 composites were studied by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and mechanical testing. The results show that the high performance of Al-Si/TiB2 composites can be attributed to the clean interfaces between TiB2 and Al and several toughening mechanisms, such as crack blunting, crack branching, crack deflection, plastic deformation of Al layer, and bridging of the uncracked fracture process zone. Specifically, the compressive strength, three-point bending strength and KIC of composites corresponding to LS were 640-710 MPa, 629 MPa, and 16.4 MPa m1/2, respectively. The fracture behaviors of the Al-Si/TiB2 composites have been discussed in detail in this work. It was found that single cracks were accompanied by the propagation of multiple micro-cracks in the layered composites. The precipitation of Si particles at the TiB2/α-Al interface and the Al phases infiltrated in the TiB2 layers play a great role in the formation of single crack fractures and multiple micro-cracks fractures, respectively, in the nacre-inspired Al-Si/TiB2 composites.
Freeze casting is a versatile approach for the design of lamellar metal−ceramic composites with unique combination of strength and toughness. However, previous studies mainly focused on ceramic factors such as content and lamellae structure, seldom concerning the effects of metal property and interfacial structures, which, in practice, are key factors in determining the mechanical performances of the composites. In this work, we prepared three kinds of Al/TiC lamellar-interpenetrated composites with different matrix compositions (pure Al, 6061Al (Al−0.4Cu−1.0Mg−0.6Si) and ZL107 (Al–7Si–5Cu)) via freeze casting and pressure infiltration, aiming at clarifying the roles of matrix property and interfacial reaction on the mechanical properties and fracture mechanisms of the composites. The flexural strengths of pure Al/TiC, 6061Al/TiC and ZL107/TiC composites reached 355 ± 10, 415 ± 15 and 459 ± 18 MPa, while the toughness values (characterized by crack-growth toughness) were 81.0 ± 2.0, 57.6 ± 1.2 and 43.4 ± 1.5 MPa m1/2, respectively. The exceptional damage tolerance of these lamellar composites was attributed to multiple toughening mechanisms such as crack deflection, uncracked-ligament bridging of ductile layers and plastic deformation of the metal matrix. However, the presence of Si in the 6061Al and ZL107 alloys weakened the stability of TiC and promoted interfacial reaction, leading to the formation of a certain number of (Al1-m, Sim)3Ti and Al4C3, which greatly weakened the toughness of the composites. Due to the combined effects of alloy plasticity, lamellar-interpenetrated structure and interfacial reaction, the fracture of the materials changed from a multiple cracking mode in the Al/TiC composite to a single crack propagating mode in the 6061Al/TiC and ZL107/TiC composites.
We proposed a new strategy of combining dynamic freeze casting with pressure infiltration and successfully prepared an antler-bone-inspired gradient metal-ceramic composite. As exemplified by Al-7Si-5Cu/Al 2 O 3 , the resultant composite exhibits the unique mechanical properties of being externally hard, strong, and wear-resistant but internally soft and tough owing to its compositional and structural gradients and delicate lamellar microstructures. This study offers a feasible, flexible, and scalable approach to designing and preparing functionally gradient materials with biomimetic structural features and exceptional properties.
Freeze casting has been widely used in the preparation of nacre-inspired composites because of its prominent advantages in controlling the fine structure of ceramic scaffolds. However, the ceramic-phase content in composites prepared by this method is much lower than that in nacre due to the limitations of the dispersion and viscosity of the ceramic slurry. In this work, we combined the ideas of freeze casting and in-situ reaction to prepare nacre-inspired lamellar composites with high hard-phase contents. We first prepared the TiO 2 -−SiO 2 porous scaffold with an initial solid loading of 20 vol% via unidirectional freeze casting and then infiltrated it with a 6061Al alloy. During the reaction process, the TiO 2 -SiO 2 layers were progressively transformed into Al 2 O 3 layers, and the Al layers were transformed into Al 3 Ti. Composites with different hard-phase volume fractions (approx. 29%, 38%, 52% and 65%) and different interfacial bonding strengths could be obtained by altering the infiltration temperature. Increases in the hard-phase content and the interfacial bonding strength substantially enhanced the bending and compressive strengths of the composites, with maximum values of 350 MPa and 854 MPa, respectively, achieved in the composites with approx. 65 vol% hard phases. However, the highest fracture toughness of 42.9 MPa m 1/2 and maximum fracture work of 3.6 kJ/m ² appeared when the hard-phase content in the composite was approx. 38 vol%, sharing an appropriate interfacial bonding strength. A robust interface favors stress transfer but impedes crack deflection, while a weak interface fails to bear loads, even though it is conducive to crack deflection. A moderately-strong interface together with a considerable amount of layered soft phase contributes to the generation of multiple-cracking, and thereby greatly toughens the metal−ceramic composite.
Nacre-inspired laminated composites have been proven to possess a unique combination of strength and toughness. In this study, we fabricated nacre-mimetic Cu/TiC composites via unidirectional freezing of aqueous TiC slurries containing different amounts of NiO additives, followed by ice sublimation, carbothermal reduction of NiO to Ni during sintering and then gas-pressure infiltration of the Cu melt. The introduction of Ni greatly facilitated the densification of ceramic lamellae and enhanced the interfacial bonding between Cu and TiC. The resultant composites displayed outstanding damage tolerance and anisotropic electrical conductivities. Specifically, for an ∼31 vol% TiC–Cu composite containing 24 wt% Ni in the ceramic lamellae (based on the TiC content), a fracture toughness (K Jc ) of 72.5 ± 1.0 MPa·m 1/2 , work of fracture of 53.4 ± 3.5 kJ/m ² , bending strength of 725 ± 11 MPa and longitudinal electrical conductivity of 22.7 MS/m (∼60% of the Cu matrix) were achieved, which were approx. 81%, 536%, 122% and 97% higher than those of the Ni-free composite, respectively. Noticeable toughening was demonstrated to be a consequence of multiple cracking, plastic deformation and uncracked-ligament bridging of the metal layers, as well as crack deflection and blunting. On the other hand, significant strengthening resulted from tailoring the microstructures in the ceramic layers and at the Cu/TiC interface as a result of Ni doping. We believe that the facile strategy adopted herein provides an effective way to solve the problems of wetting and bonding related to metal infiltration and can be readily extended to the preparation of other nacre-inspired metal−ceramic composites.
ZrB2/MO laminated composites, in which ZrB2 containing 10 vol% nano-SiC whiskers and 10 vol% SiC particles were taken as matrix layer and Mo as interfacial layer were prepared by roll-compaction and hot-pressing sintering at 1950 degrees C for 1 h under 20 MPa pressure in flow argon. The results show that the ZrB2/MO laminated composites have a high fracture toughness of 9.3 +/- 0.21 MPa-m(1/2), and the bending strength can be increased to 400 36 MPa by the addition of Si and B into the Mo layers, which would avoid cracking from layer interface and improve the brittlement of Mo at room temperature. The main mechanisms of enhancing toughness are attributed to crack bluntness, crack deflection, and crack parallel propagation along interfacial layers. MoB, ZrB and Mo5SiB2 were formed as the reaction among ZrB2, SiC and Mo, which would limit the increase of the strength and toughness.
The bending load-deflection curves and fracture characteristics of a range of laminated Ti-(TiBw/Ti) composites were tested under five modes. The results show that ductile-brittle transition with decreasing Ti layer thicknesses and increasing volume fractions of TiBw, which is attributed to the constrained plastic deformation mechanism accompanying with high stress traixiality and tensile stress. The laminated composites with weak interfaces display superior fracture toughness under notched crack arrested orientation, which is related to the delamination cracks and multiple tunnel cracks. Moreover, Single tunnel crack propagation and periodic multiple tunnel cracks were observed in the laminated composites under mode IV, which depend on the thickness and ultimate strength ratio of Ti layer and TiBw/Ti composite layer. There are many interfacial delamination cracks and multiple tunneling cracks presented under mode V, playing an effective role in toughening the laminated composites. In addition, with decreasing the Ti layer thickness, laminated composites reveal obvious size effect characterized by more tunnel cracks.
Novel Ceramic-Fiber-Reinforced-Metal-Intermetallic-Laminate (CFR-MIL) composites, Ti-Al3Ti-Al2O3-Al, were synthesized by reactive foil sintering in air. Microstructure controlled material architectures were achieved with continuous Al2O3 fibers oriented in 0° and 90° layers to form fully dense composites in which the volume fractions of all four component phases can be tailored. Bend fracture specimens were cut from the laminate plates in divider orientation, and bend tests were performed to study the fracture behavior of CFR-MIL composites under three-point and four-point bending loading conditions. The microstructures and fractured surfaces of the CFR-MIL composites were examined using optical microscopy and scanning electron microscopy to establish a correlation between the fracture toughness, fracture surface morphology and microstructures of CFR-MIL composites. The fracture and toughening mechanisms of the CFR-MIL composites are also addressed. The present experimental results indicate that the fracture toughness of CFR-MIL composites determined by three- and four-point bend loading configurations are quite similar, and increased significantly compared to MIL composites without ceramic fiber reinforcement. The interface cracking behavior is related to the volume fraction of the brittle Al3Ti phase and residual ductile Al, but the fracture toughness values appear to be insensitive to the ratio of these two phases. The toughness appears to be dominated by the ductility/strength of the Ti layers and the strength and crack bridging effect of the ceramic fibers.
How to defeat the conflict of strength vs toughness and achieve unprecedented levels of damage tolerance within structural materials is a great challenge for designing microstructure-sensitive materials. The nanostructured metallic multilayers (NMMs) are widely used as essential components of high performance microelectronics and interconnect structures owing to their smart, tunable internal features and their outstanding mechanical properties. The deformation and fracture of NMMs during their service processes has been identified as an important factor influencing their reliability. The present authors had systematically investigated the size and interface effects on the mechanical properties, such as hardness/strength, tensile ductility, fracture toughness, deformation and fracture mechanisms of Cu/X (X=Cr, Nb, Zr) nanolayered films/micropillars, in addition to their microstructure evolUtion. In this paper, based on these experimental results achieved by the present authors, as well as the Progresses at home and abroad made in the deformation and fracture behavior of NMMs, the correlation of microstructure-size constraint-mechanical performance in NMMs (and nanolayered micropillars) is reviewed, and the universities in their deformation and fracture modes and the related mechanisms are revealed. Finally, a brief prospect on the studies of NMMs in future in the light of manipulation of the internal features, origin and dynamics of dislocations and the high performance of NMMs at extreme is discussed.
Fracture prediction in subsurface reservoirs is critical for exploration through exploitation of hydrocarbons. Methods of predicting fractures commonly neglect to include the stratigraphic architecture as part of the prediction or characterization process. This omission is a critical mistake. We have documented a complex heterogeneous fracture development within the eolian Tensleep Sandstone in Wyoming, which arguably is one of the least complex reservoir facies. Fractures develop at four scales of observation: lamina-bound, facies-bound sequence-bound, and throughgoing fractures that span the formation. We documented a detailed facies and fracture-intensity model using LIDAR-scanned outcrops located at the Alcova anticline in central Wyoming. Through this characterization, we reveal the existence of a striking variability in fracture intensity caused by original depositional architecture, overall structural deformation, and diagenetic alteration of the host rock.
A brief overview is given of some recent work concerning characterisation of the mechanical response of interfaces in MMCs to externally-applied loads. Approaches based on either critical stress levels or strain energy release rates are described. It has been established that cavity formation in particulate aluminium-based MMCs is governed by the attainment of a critical hydrostatic stress at the interface. For titanium alloys reinforced with (carbon-coated) SiC monofilaments, on the other hand, it is speculated that initial debonding may occur as soon as a significant normal tensile stress is set up at the interface, but that subsequent opening up of a cavity then depends on the hydrostatic stress in the vicinity. The effects of these features on deformation and failure of the composite are briefly explored. Finally, the application of interfacial fracture mechanics to substrate/coating systems, and hence to layered planar composites, is briefly examined This is potentially a more rigorous approach to interfacial debonding than one based on analysis of stresses alone, but prediction of crack initiation can be problematic and application to particulate and fibrous systems is hampered by the level of geometrical complexity in the analysis.
In this paper, coupled (thermal and structure) finite element analysis has been employed to analyze the influence of interlayer material and geometry in MgO.ZrO2-GG coatings subjected to thermal loading. Coatings with NiAl, NiCrAlY, NiCoCrAlY interlayers and with different combinations of these interlayer materials were modeled. All models had a coating to substrate thickness ratio of 1:10 and with different interlayer thickness of 0.1, 0.2 and 0.3 mm. Nominal and shear stresses at the critical interface regions (film/interlayer/substrate) were obtained and compared. The results showed that the interlayer thickness and material combinations have a significant influence on the level of the developed thermal stresses. It was also concluded that the finite element technique can be used to optimize the design and the processing of ceramic coatings.
The present paper develops a numerical technique named FSMS for simulating the crack growth of multilayered composites. Numerical simulations for the crack growth of multilayered ceramic/metal composites are carried out. The effects of some factors such as thickness ratio, initial crack length, material properties and dimensions of the structure on the crack growth are investigated. Numerical results show good agreement with experiments. FSMS is also a simple numerical method to solve crack problems of complex composite structures.
In this study, crack-front debonding behavior in layered materials is analyzed by the boundary element method (BEM). Interaction between a single main crack and interface cracks is demonstrated based on the calculated stress intensity factors of the debonding crack and main crack. Evidence is given indicating that the growth of the debonding at the main crack front is stable because of interfacial friction and subsequent debonding at the bridging position is thus accompanied by friction.
Experiments are described in which lead sheets, sandwiched between two pairs of steel plates separated by a pre-specified gap midway along the specimen, have been loaded in tension. The nominal stress at the onset of global yielding was found experimentally to be about twice the unconstrained yield stress of the metal, over a range of values for the initial gap. The nominal stress was then found to fall in an approximately linear fashion with increasing displacement, as the load-bearing section necked down. This behaviour is quantitatively consistent with experimental data for the fracture energy of metal-ceramic laminates loaded in bending, which failed by the propagation of a single dominant crack, assuming that rupture of the bridging ligaments made the dominant contribution. An attempt is made to predict the nominal stress at the onset of global yielding, using an analytical treatment of the initial small scale yielding around the notch and a slip line field solution for fully developed global plasticity. This gave an overestimate of the stress at the onset of global yielding, relative to the experimental data, by a factor of between two and three. This is attributed to the use of a simplified notch geometry in the slip line field solution.
The technique this study focuses on is the use of explosive consolidation to create distinct interfacial systems from metallic and metallic-cermet powder mixtures. One of the advantages of this technique is that undesired intermetallic formation between the constituents (which can happen with other techniques such as hot-pressing) is usually avoided due to the relatively low temperatures involved. In addition, interfacial strengths actually higher than the surrounding bulk material are achieved. Finally, since extremely high dislocation densities are created by the passage of the shock wave, the strength contribution from dislocation pile-ups is maximized.
Fatigue crack growth in fiber-reinforced metal-matrix composites is modeled based on a crack tip shielding analysis. The fiber/matrix interface is assumed to be weak, allowing interfacial debonding and sliding to occur readily during matrix cracking. The presence of intact fibers in the wake of the matrix crack shields the crack tip from the applied stresses and reduces the stress intensity factors and the matrix crack growth rate. Two regimes of fatigue cracking have been simulated. The first is the case where the applied load is low, so that all the fibers between the original notch tip and the current crack tip remain intact. The crack growth rate decreases markedly with crack extension, and approaches a “steady-state”. The second regime occurs if the fibers fail when the stress on them reaches a unique fiber strength. The fiber breakage reduces the shielding contribution, resulting in a significant acceleration in the crack growth rate. It is suggested that a criterion based on the onset of fiber failure may be used for a conservative lifetime prediction. The results of the calculations have been summarized in calibrated functions which represent the crack tip stress intensity factor and the applied load for fiber failure.
Models for the debonding of a fiber embedded in a brittle matrix are proposed and analyzed. Attention is restricted to systems having a residual compressive stress acting across the fiber/matrix interface. Debonding, as well as pullout after the fiber breaks, is accompanied by frictional sliding. Fiber—matrix interaction is modeled by a cylindrical cell with two sets of boundary conditions: one modeling an isolated fiber—matrix unit and the other a matrix containing an array of unidirectional fibers. The elastic properties of the fiber are taken to be transversely isotropic about the fiber axis, while the matrix is assumed to be isotropic. The debonding process is treated within the framework of fracture mechanics as a mode 2 crack. Two idealizations of friction are considered: a constant friction stress independent of normal compression across the interface, and Coulomb friction. Approximate closed form solutions to the model are presented. These are assessed using results from an accurate numerical analysis.
For an infinite, remotely stressed elastic-plastic solid containing an isolated void a state may be reached, in which the void grows without bound, even though the remote stresses and strains are kept fixed. Such cavitation instabilities are determined here for power hardening elastic-plastic solids subject to axisymmetric stress states. The relatively simple analysis for a spherical void under spherically symmetric conditions is first briefly reviewed. Subsequently, the effect of an axisymmetric stress state is studied for the case of a cylindrical void, where the problem is also governed by ordinary differential equations. For a spherical void under axisymmetric stressing cavitation instabilities are determined by a numerical procedure, which couples a finite element solution for an inner region with a perturbation solution for an outer region. It is found that the critical stress levels are significantly increased by deformation hardening.
There have been observations showing that relatively thick metal layers in metal-ceramic laminates lead to the formation of multiple periodic cracks within a zone near a pre-crack, distributing damage and significantly enhancing the composite's toughness. Several models including linear elastic fracture mechanics and shear-lag analysis are developed in the present work in order to study the competition between multiple cracking and single-crack extension as damage modes. It is established that there is a critical thickness ratio for metal-ceramic layers above which multiple cracking dominates. Moreover, this critical thickness ratio is inversely proportional to the corresponding moduli ratio such that the competition between damage modes is governed by the metal-ceramic layer stiffness ratio. Plastic hardening of metals helps to activate the multiple-cracking damage mode, while plastic yielding does the opposite.
The role of fiber debonding and sliding on the toughness of intemetallic composites reinforced with ductile fibers is examined. The toughness is shown to be a function of the matrix/fiber interface properties, residual stresses and the volume fraction, size and flow behavior of the fibers. Mechanical testing and in situ microstructural observations were carried out on a Ti-25at.%Ta-50at.%Al intermetallic matrix reinforced with W-3Re fibers. The fibers were coated with a thin oxide layer in order to induce debonding and prevent interdiffusion between the fiber and the matrix. The ductility, high strength and debond characteristics of coated tungsten-rhenium fibers promote a large increase in toughness. However, the mismatch in thermal expansion coefficients is the source of large residual tensile stresses in the matrix that induces spontaneous matrix cracking. Matrix cracking and composite toughness are examined as a function of the interfacial properties, residual stresses and properties of the fiber.
The problem of crack progression in a laminate consisting of alternate brittle and ductile layers has been addressed. A finite element analysis has been used to calculate stresses in the vicinity of a crack and the results rationalized on the basis of low and high stress bounds associated, respectively, with small-scale yielding and with a shear lag at the interface. Preliminary experiments conducted on Al2O3/Al laminates have been used to assess the crack extension criterion, upon comparison with the stress analysis. Implications for the strength and toughness of laminates are briefly presented.
A cavitation instability occurs when an isolated void in an infinite, remotely stressed elastic-plastic solid grows without bound under no change of remote stress or strain. The cavitation instability can be thought of as a process in which elastic energy stored in the remote field drives the plastic expansion of the void. The paper begins with a brief review of cavitation under spherically symmetric stress states and then goes on to consider the problem for cavitation states under general axisymmetric stressing. It is found that the criterion for cavitation under multiaxial axisymmetric stressing depends on the attainment of a critical value of the mean stress, to a reasonably good approximation. A set of recent experiments is discussed in which cavitation instabilities appear to have occurred. The last section of the paper reviews available theoretical results for the dilatation rates of isolated voids. The most commonly used formulae underestimate the dilatation rate under stress states with moderate to high triaxiality.
A high-toughness gamma-TiAl with a lath microstructure has been characterized. The presence of a laminated structure that consists of alpha(2) plates between gamma laths has been shown to govern the toughness. The lath microstructure promotes dissipation through a combination of process zone and crack bridging mechanisms. Mechanical twinning in the gamma-phase contributes to the toughness through the formation of a twin process zone, while regions consisting of intact lath colonies act as bridging ligaments in the crack wake. The measured steady-state toughness of about 25 MPa sq rt m is consistent with preliminary estimates of dissipation, which indicate that similar contributions arise from deformation within the bridging ligaments and from twinning in the process zone. 14 refs.
The collinear periodical array of microcracks ahead of a semi-infinite crack (macrocrack) is considered. A close form solution in terms of complex stress potentials is given, assuming that a remote, macroscale, stress intensity factor is given. The exact solution of the interaction of a macrocrack with a single microcrack is given. Results demonstrate that for relatively close location (with respect to crack length) of microcracks to the macrocrack tip, the microcrack spacing becomes important. For microcrack spacing (period) greater than 10 crack lengths the interaction can be taken as for a single microcrack, and, for distance greater than two microcrack lengths, the local stress intensity factor can be taken as equal to that remotely applied (for cases with crack spacing greater than two crack lengths). In other cases the macro-microcrack interaction is significant.
Brittle solids can be toughened by incorporating ductile inclusions into them. The inclusions bridge the crack and are stretched as the crack opens, absorbing energy which contributes to the toughness. To calculate the contribution to the toughness it is necessary to know the force-displacement curve for an inclusion, constrained (as it is) by the stiff, brittle matrix. Measured force-displacement curves for highly constrained metal wires are described and related to the unconstrained properties of the wire. The constraint was achieved by bonding the wire into a thick-walled glass capillary, which was then cracked in a plane normal to the axis of the wire and tested in tension. Constraint factors as high as 6 were found, but a lesser constraint gives a larger contribution to the toughness. The diameter of the wires (or of the inclusions) plays an important role. Simple, approximate, models for the failure of the wires are developed. The results allow the contribution of ductile particles to the toughness of a brittle matrix composite to be calculated.
Investigations of cracking in multilayered ceramic/metal composites are presented. Two aspects are considered: crack renucleation across intact single metal layers and subsequent crack extension. Crack renucleation criteria are determined and compared with predictions. High-resolution strain-mapping techniques are employed to determine the surface strain fields surrounding cracks. Good agreement is found between these experimental measurements and the predictions of a small-scale yielding model. Subsequent crack progression occurs either by the extension of a dominant, nearly planar crack or by the formation of a zone of periodically spaced cracks. Both patterns are analyzed. The dominant cracking behavior is found to depend on the volume fraction and yield strength of the metal.
Alumina has been liquid state bonded with pure Al and Al-4 wt% Mg and the mechanical behavior of the bonds has been characterized. It has been shown that failure never occurs by interface cracking. Instead, when the bond layer is thick, the bond strength is limited either by plastic flow or by ductile fracture in the metal. Conversely, when the bond layer is thin, failure occurs in the ceramic, limited by the strength of the ceramic. The Al alloy bond has also been shown capable of arresting cracks by plastic blunting.RésuméOn a lié de l'alumine avec de l'aluminium pur et de l'aluminium à 4% en poids de magnésium, à l'état liquide, et l'on a caractérisé le comportement mécanique des liaisons. La rupture n'a jamais lieu par rupture interfaciale. Par contre, quand la couche de liaison est épaisse, la résistance de la liaison est limitée soit par l'écoulement plastique, soit par la rupture ductile dans le métal. Réciproquement, quand la couche de liaison est mince, la rupture a lieu dans la céramique, et elle est limitée par la résistance de la céramique. La liaison en aluminium allié est également capable d'arrêter les fissures par émoussage plastique.ZusammenfassungAluminiumoxid wurde mit reinem Al und mit Al-4 Gew.-%Mg im schmelzflüssigen Zustand kontaktiert; das mechanische Verhalten dieser Verbindungen wurde untersucht. Der Bruch tritt niemals an der Grenzfläche auf. Stattdessen ist die Festigkeit der Verbindung begrenzt entweder durch plastisches Flieβen oder duktilen Bruch des Metalls, wenn die Verbindungsschicht dick ist. Ist sie dagegen dünn, dann tritt Bruch in der Keramik auf, entsprechend der Festigkeit der Keramik. Auch bei der Al-Legierung wurde gezeigt, daβ sie Risse durch plastisches Abstumpfen behindern kann.
Matrix fracture in brittle-matrix fiber composites is analyzed for composites that exhibit multiple matrix cracking prior to fiber failure and have purely frictional bonding between the fibers and matrix. The stress for matrix cracking is evaluated using a stress intensity approach, in which the influence of the fibers that bridge the matrix crack is represented by closure tractions at the crack surfaces. Long and short cracks are distinguished. Long cracks approach a steady-state configuration, for which the stress intensity analysis and a previous energy balance analysis are shown to predict identical dependence of matrix cracking stress on material properties. A numerical solution and an approximate analytical solution are obtained for smaller cracks and used to estimate the range of crack sizes over which the steady-state solution applies.
A fiber-reinforced ceramic subject to tensile stress in the fiber direction can undergo extensive matrix cracking normal to the fibers, while the fibers remain intact. In this paper, the critical conditions for the onset of widespread matrix cracking are studied analytically on the basis of fracture mechanics theory. Two distinct situations concerning the fiber-matrix interface are contemplated : (i) unbonded fibers initially held in the matrix by thermal or other strain mismatches, but susceptible to frictional slip, and (ii) fibers that initially are weakly bonded to the matrix, but may be debonded by the stresses near the tip of an advancing matrix crack. The results generalize those of the Aveston-Cooper-Kelly theory for case (i). Optimal thermal strain mismatches for maximum cracking strength are studied, and theoretical results are compared with experimental data for a SiC fiber, lithium-alumina-silicate glass matrix composite.
A model is developed for fatigue growth of matrix cracks in metals reinforced with aligned continuous elastic fibers. The mechanics of elastic cracks bridged by frictionally constrained fibers is used to develop the model, which provides estimates of the tip value of the stress intensity factor amplitude, ΔKTIP. It is found that when the applied load amplitude is held fixed during fatigue crack growth, ΔKTIP, and thus the rate of growth approach an asymptotic value independent of crack length. The residual strength after fatigue crack growth is also discussed. In some cases, the residual strength is unaffected by prior fatigue growth. But, in another regime, the matrix crack length allows fibers to begin breaking before the matrix crack grows. The strength is then inversely proportional to the square root of fatigue crack length.