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Flow Characteristics of Highly Constrained Metal Wires
Abstract
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.
... In fact, the high Cr cast iron is nearly a natural brittle matrix composite material. From the toughening theory of the brittle matrix composites [1][2][3], it should be possible that the toughness of these materials could be further improved by introducing continuous ductile fibers or wires into them. Recently, there has been a significant effort to explore the potential use of the ductile phase reinforcement in the toughening of relatively brittle matrix materials. ...
... The interface between the matrix and the reinforcements plays an important role in the mechanical performance of MMCs. Many studies [1,7,8,9] indicated that a high work of rupture of ductile reinforcements was encouraged by a partial debonding at the interface. Analytical and numerical models [9,10,11,12] showed that partial decohesion (a result of debonding at the interface and/or multiple fracture of the matrix) was beneficial to a high work rupture, and therefore a high toughness. ...
... There have been significant efforts to explore the potential use of ductile phase reinforcement in the toughening of relatively brittle metal matrix. The properties of the interface between the matrix and reinforcement are the most important factors in the manufacture of metal matrix composites [1,9,13]. One of the most important factors in the fabrication of MMCs is the compatibility (wettability and reactivity) between the matrix and reinforcement. ...
... The matrix fracture nucleates at brittle ceramic particles i.e., ZrO 2 , TiO 2 , Y 2 O 3 , and SrO. A strong interfacial bond between the ceramic particle and aluminium matrix is advantageous in terms of toughness [25,26]. The fracture toughness of Al 2 O 3 P/steel composites was investigated by Schlenther et al [27] and it was noticed that the cracking sources were influenced by increasing the brittle ceramic materials. ...
... Reddy [24] in his study concluded that yield strength and fracture strength increased with the increase in volume fraction of SiC p particles, whereas ductility of the SiC p -Al6061 composites reduced. Similarly, Iqbal et al [25] in their study demonstrated that hybridization effect reduced the resistance to crack initiation. Along with this the nano sized ceramic particles prohibited the crack propagation in the matrix which ultimately increased the fracture toughness of the prepared composites. ...
During the service life of components, they encounter several cyclic loadings that consequently generate stress which encourages crack interaction ultimately deteriorating materials’ performance. The present study aims to investigate the fracture behaviour of aluminium 6061 reinforced with 0-15 wt.% ZrO2 particles (at step of 5 wt.%) fabricated using stir casting technique. Compositional analysis through X-ray diffraction was performed on the prepared composite samples. The fracture toughness of prepared composites is investigated through bending test by using single edge notch bend (SENB) specimens. In addition, the computation of fracture toughness was also performed by finite element analysis (FEA) approach, and the numerical results obtained from the FEA were compared with the experimental value. Furthermore, fractography and microstructural tests were carried out in order to investigate the influence of reinforcement weight percentage on the failure behaviour of the composites that had been prepared. The results show that with inclusion of ZrO2 (15 wt.%) in aluminum 6061 matrix the maximum fracture toughness of 500.83 MPa.mm0.5 was observed. The study performed through FEA highlights the stress phenomena and result are in good agreement with the experimental results.
... Similarly, different models, such as the Ravichandran model and the Godes-Gurland model [22,23], have been developed to predict the fracture toughness. For example, the strain energy release rate of WC-Co hardmetal is the sum of fracture resistance of the brittle WC phase and the binder phase by assuming crack propagating in a straight type in the Ravichandran model [24][25][26]. However, these models also suit only effectively to prismatic-shaped WC-Co hardmetals. ...
... toughness K IC of the WC-Co hardmetal can be deduced from eq. (5), and is shown as eq. (7) [24][25][26]: ...
The hardness–toughness trade-off relationship is an essential challenge when creating high-performance WC–Co hardmetals. In this work, by combining plasma milling and low-pressure sintering, a dual-scale plate-like WC–8Co hardmetal is fabricated. The hardmetal simultaneously exhibits ultra-high strength (1701 kgf/mm²) and fracture toughness (21.77 MPa*m1/2), with bimodal grain size distributions of plate-like WC with average diameter of 1.1 μm and 1.4 μm, respectively. This breaks the normal trade-off relationship between the strength and the fracture toughness, which is because of the enhancement of crack propagation resistance owing to the synergetic effect of the plate-like WC and the bimodal grain structure. Existing models for hardness and fracture toughness prediction are invalid for WC–Co hardmetals with oriented plate-like WC grains. Modified models, based on the Lee-Gurland hardness model and the Ravichandran fracture toughness model, are developed by inducing factors R, which is the proportion of the WC(0001) plane in the indented plane, and can be determined by XRD analysis, to represent morphology features of WC. The modified models are universal and can reliably predict the hardness and fracture toughness of WC–Co hardmetals with both equiaxial and plate-like WC grains.
... More insight on the failure of constrained elastic-plastic layers was provided by the work of Ashby et al. (1989). Ashby et al. (1989) observed that the failure strength of a constrained metal wire exceeded the uniaxial yield strength of an unconstrained wire up to a factor of six. ...
... More insight on the failure of constrained elastic-plastic layers was provided by the work of Ashby et al. (1989). Ashby et al. (1989) observed that the failure strength of a constrained metal wire exceeded the uniaxial yield strength of an unconstrained wire up to a factor of six. Failure of the constrained metal wires was found to be triggered by unstable cavitation instead of failure by necking and cup-and-cone fracture as observed in unconstrained wires. ...
This thesis contributes to the understanding of the deformation and fracture of methyl methacrylate (MMA)-based polymers in the context of void growth. The first part of the thesis focuses on the prediction of void growth during solid state nanofoaming of polymethyl methacrylate (PMMA). These predictions may contribute to the development of polymeric foams with a thermal conductivity close to that of air. The second part of the thesis explores the fracture behaviour of structural adhesive (e.g. MMA)-based joints. An adhesive layer within such a joint is prone to defects such as (micro)voids and (micro)cracks. The ability to accurately predict the failure strength of adhesive joints as a function of pore or crack size is essential in order to design reliable structures based on adhesive bonding technology. A one dimensional void growth model is developed to simulate cavity expansion during solid-state nanofoaming of PMMA by CO$_2$ in the first part of the thesis. To that end, tensile tests on two PMMA grades of markedly different molecular weight are conducted close to the glass transition temperature and over two decades of strain rate. The void growth model makes use of fitted constitutive laws for each PMMA grade and the effect of dissolved CO$_2$ is accounted for by a shift in the glass transition temperature of the PMMA. Solid-state nanofoaming experiments are performed on the two PMMA grades to validate the void growth model. The morphology of the foams (and the limit in attainable porosity) is found to be sensitive to the molecular weight. The measured porosity versus foaming time curves are in good agreement with those predicted by the model, for porosities below the maximum observed porosity. The observed limit of achievable porosity is interpreted in terms of cell wall tearing; it is deduced that the failure criterion is sensitive to cell wall thickness. The tensile strength of a centre-cracked elastic layer, sandwiched between two elastic substrates, and subjected to remote tensile stress, is predicted in the second part of the thesis. An analytical theory is developed by making use of a cohesive zone at the crack tip to predict the strength of the joint as a function of the relative magnitude of crack length, layer thickness, plastic zone size, specimen width, and elastic modulus mismatch ratio. Joint design maps are constructed, revealing competing regimes of fracture. The analytical theory is verified by finite element calculations, and validated by means of two experimental case studies.
... The advantage of such a microstructure is that the coarse α-Mo grains contribute to the toughness by crack trapping, while the fine α-Mo grains are favorable to the strength of the alloy through finegrain strengthening. Moreover, the coarse α-Mo grains improve ductility and retain high extent of deformations attributed to the stronger capability for dislocation storage [27]. However, local aggregation of adjacent coarse α-Mo grains and occasional clusters of Mo 3 Si/Mo 5 SiB 2 intermetallics are observed in that bimodal Mo-Si-B alloy, which are thought to cause high stress concentrations, resulting in a reduced toughening effect. ...
Mo-Si-B alloys consisting of α-Mo, Mo3Si, and Mo5SiB2 phases have high melting point above 2000 °C and have potential as ultra-high temperature structural materials. However, the room-temperature fracture toughness falls short of meeting the practical application, and enhancing the toughness through grain coarsening inevitably leads to a trade-off with the strength of the Mo–Si–B alloy. Bimodal structure design is expected to improve both the toughness and strength simultaneously. In the present work, Mo–12Si–8.5B alloy with a unique bimodal microstructural design was successfully prepared via multistep mechanical alloying, followed by hot pressing. The bimodal structure comprised fine (~ 0.65 μm) α-Mo grains and partial coarse (~ 1.25 μm) α-Mo grains, which exhibited a uniform distribution and formed a continuous matrix embedded with fine Mo3Si and Mo5SiB2 intermetallics. Such microstructure enhanced the fracture toughness and compressive strength simultaneously, exhibiting 14.3 MPa m1/2 and 3130 MPa, respectively. The coarse α-Mo grains involved increasing dislocation motion and storage capacity, playing an important toughening role as the crack trapper. The uniform distribution of the bimodal α-Mo grains combined with the continuous matrix promoted uniformity of the strain distribution and alleviated the plastic constraint on α-Mo imposed by intermetallics, which then maximized the extent of crack trap toughening of α-Mo. The existence of fine α-Mo grains strengthened the alloy.
... Occasionally, debonding occurs such that the underlying quartz is observed. This failure mode and level of tensile strength is suggestive of cavitation whereby voids can grow without limit from vanishingly small initial defects [39,40]. ...
... Second, the fracture toughness can be affected by some physical properties. According to the Ashby model [33], the toughness increment ∆K C can be expressed as Equation (10): ...
//Nbss and α-Nb5Si3 phases were detected. Meanwhile, Nb2C was observed, and the crystal forms of Nb5Si3 changed in the C-doped composites. Furthermore, micron-sized and nano-sized Nb2C particles were found in the Nbss layer. The orientation relationship of Nb2C phase and the surrounding Nbss was [001]Nbss//[010]Nb2C, (200) Nbss//(101) Nb2C. Additionally, with the addition of C, the compressive strength of the composites, at 1400 °C, and the fracture toughness increased from 310 MPa and 11.9 MPa·m1/2 to 330 MPa and 14.2 MPa·m1/2, respectively; the addition of C mainly resulted in solid solution strengthening.
... For soft films, the critical bifurcation stretch occurs at the limit of short wavelengths, i.e. κ = KH f → ∞. A typical bifurcation mode in Fig. 5(c) shows that the maximum displacement occurs at the center cord of the film and decreases along the film thickness direction, indicating the failure might nucleate from defects in the bulk of the film in the form of cavitation, reminiscent to the classical work of Ashby et al. (1989). In contrast, for the case with a stiffer film, the damage is expected to first occur at the film-matrix interface. ...
The necking instability is a precursor to tensile failure and rupture of materials. A quasistatically loaded free-standing uniaxial specimen typically exhibits necking at a single location, corresponding to a long wavelength bifurcation mode. If confined to a substrate or embedded in a matrix, the same filament can exhibit periodic necking thus creating segments of finite length. While periodic instabilities have been extensively studied in ductile metal filaments and thin sheets, less is known about necking in hyperelastic materials. There is a renewed interest in the role of necking in novel materials to advance fabrication processes and to explain fragmentation phenomena observed in 3D printed active biological matter. In both cases materials are not well described by existing frameworks that employ J2 plasticity, and existing studies ignore the role of misfit stretches that may emerge in these systems through chemical or biological contraction. To address these limitations, we first experimentally demonstrate the role of the matrix on the necking and fragmentation of a compliant embedded filament. Using a strain softening generalized hyperelastic model, our analytical bifurcation analysis explains the experimental observations and agrees with numerical predictions. The analysis reveals 3 distinct bifurcation modes: a long wavelength necking, recovering the Considere criterion; a periodic necking observed in our experiments; and a short wavelength mode characterized by localization along the center cord of the filament and independent of the film-to-matrix stiffness ratio. We find that the softening coefficient and the filament misfit stretch can significantly influence the stability threshold and observed wavelength, respectively. Our results can guide the design and fabrication of composite materials and explain the fragmentation processes observed in active biological materials.
... Most importantly, it is not obvious that an asymptotic pressure even exists and if the lengthscale independent property of bulk cavitation translates to interfacial cavitation. Finally, a major driver of earlier studies on cavitation has been their observation (30,42,43). However, interfacial failure, is rarely considered from the viewpoint of cavitation, and has instead been interpreted as an interfacial fracture and delamination process (44)(45)(46)(47)(48)(49). ...
Cavitation has long been recognized as a crucial predictor, or precursor, to the ultimate failure of various materials, ranging from ductile metals to soft and biological materials. Traditionally, cavitation in solids is defined as an unstable expansion of a void or a defect within a material. The critical applied load needed to trigger this instability – the critical pressure – is a lengthscale independent material property and has been predicted by numerous theoretical studies for a breadth of constitutive models. While these studies usually assume that cavitation initiates from defects in the bulk of an otherwise homogeneous medium, an alternative and potentially more ubiquitous scenario can occur if the defects are found at interfaces between two distinct media within the body. Such interfaces are becoming increasingly common in modern materials with the use of multi-material composites and layer-by-layer additive manufacturing methods. However, a criterion to determine the threshold for interfacial failure, in analogy to the bulk cavitation limit, has yet to be reported. In this work we fill this gap. Our theoretical model captures a lengthscale independent limit for interfacial cavitation, and is shown to agree with our observations at two distinct lengthscales, via two different experimental systems. To further understand the competition between the two cavitation modes (bulk versus interface) we expand our investigation beyond the elastic response to understand the ensuing unstable propagation of delamination at the interface. A phase diagram summarizes these results, showing regimes in which interfacial failure becomes the dominant mechanism.
... The effect of large carbides in steel has been thoroughly studied in the end of last century and the behavior approaches closely that of a ferrite-cementite composite, albeit some deviations mostly due to deformation constrains are observed (see Figure 7, for instance). Ashby et al. [42] have investigated the effects of constrains caused by the interfacial strength in a brittle matrix (glass)-ductile reinforcement (lead). They demonstrated that toughening by the lead reinforcement only happened when the lead-glass interface was weakened, so that the lead wire could debond and then fully deform plastically, reaching necking. ...
... Hence, the strain to fracture of the alloy decreases with increase in the silicide phase fraction, morphology and distribution. In few cases the constraint on the Nb ss phase can be relaxed through interface debonding [82,83]. Thus, the weak interface is beneficial in relaxing the constraint and can promote the plastic dissipation in the ductile phase. ...
This paper reflects on the potentiality and development of the Nb-Si alloys for the hot section components of the futuristic turbine engine. The Nb-Si alloys consist of ductile Nbss phase as the matrix in which the brittle and oxidation resistant silicide phases (Nb3Si/Nb5Si3) arrange as dispersion or vice-versa. Thereby, the fracture toughness, high temperature strength and creep resistance along with the oxidation resistance can be balanced. However, further improvement in the room temperature fracture toughness, the resistance to intermediate pest damage and the high temperature oxidation is necessary to use them as turbine airfoils. Therefore, the concept of various processing techniques and developing into multicomponent systems has been in practice to obtain better combination of both room and high temperature properties. The present paper highlights the importance of the alloying addition on the phase formation, microstructure, mechanical and oxidation properties and the consolidated literature results are presented. Further, the fracture behavior and the need for the development of new coatings to these alloys are discussed. Finally, the present paper highlights the potentiality of Nb-Si alloys as the hot section material and the future directions of research and development are discussed.
... Most importantly, it is not obvious that an asymptotic pressure even exists and if the lengthscale independent property of bulk cavitation translates to interfacial cavitation. Finally, a major driver of earlier studies on cavitation has been their observation 30,40,41 . However, interfacial failure, even when it occurs locally, has not been previously considered from the viewpoint of cavitation, and has instead been interpreted as an interfacial fracture and delamination process [42][43][44][45][46][47] . ...
Cavitation has long been recognized as a crucial predictor, or precursor, to the ultimate failure of various materials, ranging from ductile metals to soft and biological materials. Traditionally, cavitation in solids is defined as an unstable expansion of a void or a defect within a material. The critical applied load needed to trigger this instability - the critical pressure - is a lengthscale independent material property and has been predicted by numerous theoretical studies for a breadth of constitutive models. While these studies usually assume that cavitation initiates from defects in the bulk of an otherwise homogeneous medium, an alternative and potentially more ubiquitous scenario can occur if the defects are found at interfaces between two distinct media within the body. Such interfaces are becoming increasingly common in modern materials with the use of multi-material composites and layer-by-layer additive manufacturing methods. However, a criterion to determine the threshold for interfacial failure, in analogy to the bulk cavitation limit, has yet to be reported. In this work we fill this gap. Our theoretical model captures a lengthscale independent limit for interfacial cavitation, and is shown to agree with our observations at two distinct lengthscales, via two different experimental systems. To further understand the competition between the two cavitation modes (bulk versus interface) we expand our investigation beyond the elastic response to understand the ensuing unstable propagation of delamination at the interface. A phase diagram summarizes these results, showing regimes in which interfacial failure becomes the dominant mechanism.
... Perhaps the most notable seminal study is that of Gent and Lindley [22], which reported unusual rupture process in rubbers. That unstable rupture is now commonly referred to as cavitation [23,24,25], and is linked to the initiation of damage and fracture [26,27,28,29]. However, the mechanical instability induced by application of external loads beyond a critical threshold is an extremely fast and uncontrollable process; attempts to experimentally study these internal ruptures are thus challenging [30,31]. ...
The reciprocal theorems of Maxwell and Betti are foundational in mechanics but have so far been restricted to infinitesimal deformations in elastic bodies. In this manuscript, we present a reciprocal theorem that relates solutions of a specific class of large deformation boundary value problems for incompressible bodies; these solutions are shown to identically satisfy the Maxwell-Betti theorem. The theorem has several potential applications such as development of alternative convenient experimental setups for the study of material failure through bulk and interfacial cavitation, and leveraging easier numerical implementation of equivalent auxiliary boundary value problems. The following salient features of the theorem are noted: (i) it applies to dynamics in addition to statics, (ii) it allows for large deformations, (iii) generic body shapes with several potential holes, and (iv) any general type of boundary conditions.
... The Ashby et al.'s model [44] could be used to estimate the toughening effects of Nb-Ti-Si-based alloys. The factors contributing to the increment of toughness ( K C ) can be expressed by the following equation: ...
The synergistic effect of Mo and Zr additions on microstructure evolution, room-temperature fracture toughness and microhardness of Nb-22Ti-15Si-xMo-yZr (x=4,8, y=3,6) alloys manufactured by laser directed energy deposited (L-DED) have been investigated. The major phases in as-deposited 4Mo-3Zr alloy are Nb solid solution (Nbss), Nb3Si and γ-Nb5Si3. The Nb3Si phases disappear with increasing Mo content to 8 at.% or increasing Zr content to 6%. γ-Nb5Si3 precipitates formed in the Nbss of as-deposited xMo-yZr alloys due to the cyclic re-heating, and the γ-Nb5Si3 precipitates and Nbss matrix exhibits orientation relationship (OR) of [001]Nbss//[1¯112]γ and (110)Nbss//(011¯0)γ in as-deposited 4Mo-6Zr alloy. The microstructure of xMo-yZr alloys became relative homogeneity and the Nbss matrix showed the better continuity after the heat treatment (1400°C/30h). The Nb3Si phase has transformed into Nbss and α-Nb5Si3 with the OR have been determined as {100}Nbss//{100}α and{111}Nbss//{111}α in the heat-treated 4Mo-3Zr alloy. When Mo content increase to 8at.%, a part of γ-Nb5Si3 phase also transformed into the α-Nb5Si3 phase with the OR have been determined as {112¯0}γ//{110}α and {101¯0}γ//{111}α after heat treatment. Among the as-deposited alloys, the 4Mo-6Zr alloy has the highest fracture toughness KQ with the lowest hardness. The average KQ of 4Mo-6Zr and 8Mo-6Zr alloys increased by 28% and 30% after the heat treatment, and reached 13.61 and 11.43MPa•m1/2 respectively. The hardness of Nb-22Ti-15Si-xMo-yZr alloys is significantly decreased after the heat treatment.
... The solution of this problem in various formulations is widely used for constructing the constitutive relations for porous solids, powders (Green, 1972;Gurson, 1977;Perrin and Leblond, 2000), as well as for analyzing the compacting processes (Carroll and Holt, 1972;Carroll et al., 1986). In addition, the expansion of a spherical cavity is a typical fracture mechanism for a number of materials (Ashby et al., 1989;Faye et al., 2017;Shang and Wu, 2018). There are known applications of solutions for spherical cavity expansion to assess the stability of underground structures (Mo et al., 2020), as well as to indentation and penetration modeling (Bishop et al., 1945;Johnson, 1985;Warren and Forrestal, 1998;Macek and Duffey, 2000;Durban and Masri, 2008;Masri, 2019), including polymeric materials indentation under the finite strain (Tvergaard and Needleman, 2011). ...
The paper presents an analytical solution to the coupled problem of spherically symmetric elastic-plastic deformation accompanied by heating of the material due to the plastic dissipation, which in turn changes the mechanical properties of the material. The dependence of both elastic and plastic mechanical properties of the material on temperature can be arbitrary. Both elastic and plastic deformations are assumed to be finite. The thermal expansion of material is neglected. Heating is assumed to be adiabatic. Instead of a spatial coordinate, temperature is considered as an independent monotonic variable. This made it possible to reduce the problem to solving the first order ODE. The obtained solution is valid for any incompressible hyperelastic solid with arbitrary pressure-independent non-singular yield condition, perfectly-plastic or isotropic strain-hardening/softening. Example of solution for the linear thermal-softening and strain hardening material with tension-compression asymmetry in yielding is given.
... The perfect bonding imposes lateral constraints on the inclusions, prohibiting the full advantage of the particle's ductility. As pointed out in Ashby et al. [47], the forcedisplacement curve for a bonded (constrained) particle is quite different than that for an unconstrained material, as measured in an ordinary tensile test. The degree of constraint is an important factor affecting the amount of energy absorbed in stretching, thus the fracture toughness. ...
：ZrO2 (3Y-TZP) matrix composites with 30 vol % Zr metallic particles were obtained by spark plasma sintering (SPS) using a colloidal processing method. The microstructure and mechanical properties of this novel ceramic–metal composite have been studied. The fracture toughness of composites is slightly higher than the values corresponding to monolithic zirconia. Scanning electron microscope (SEM) observations of the crack path show that the major contributions to toughening are the resulting crack blunting and branching that occurs at crack tips in the metallic particles before the onset of crack propagation. Plastic deformation of the metallic particles is strongly influenced by the constraint induced by the different phase arrangements. This system can be considered as a particulate composite with a periodic residual stress field, in which the metal phase is under strong compression due to the residual thermal stresses as a consequence of the coefficient of thermal expansion mismatch. Therefore, the plastic deformation of the metallic particles in this composite is likely to be reduced to a large extent.
... The WC-Co cemented carbides are typical cermet materials whose hardness and wear resistance are attributed to the hard phase of WC and the toughness mainly stems from the Co binder phase (Ortner et al., 2014). In addition to the high hardness and wear resistance, good toughness is also strongly demanded for the cermets that are widely applied as mining tools, molds and impact drills in various industry fields (Ashby et al., 1989;Evans & McMeeking, 1986;Exner, 1979;Riesch et al., 2013;. It is generally considered that the plasticity and toughness of the cermets are determined by the binder metal due to its intrinsic mechanical features (García et al., 2018). ...
Using the typical WC–Co cemented carbide as an example, the interactions of dislocations within the ceramic matrix and the binder metal, as well as the possible cooperation and competition between the matrix and binder during deformation of the nanocrystalline cermets, were studied by molecular dynamics simulations. It was found that at the same level of strain, the dislocations in Co have more complex configurations in the cermet with higher Co content. With loading, the ratio between mobile and sessile dislocations in Co becomes stable earlier in the high-Co cermet. The strain threshold for the nucleation of dislocations in WC increases with Co content. At the later stage of deformation, the growth rate of WC dislocation density increases more rapidly in the cermet with lower Co content, which exhibits an opposite tendency compared with Co dislocation density. The relative contribution of Co and WC to the plasticity of the cermet varies in the deformation process. With a low Co content, the density of WC dislocations becomes higher than that of Co dislocations at larger strains, indicating that WC may contribute more than Co to the plasticity of the nanocrystalline cermet at the final deformation stage. The findings in the present work will be applicable to a large variety of ceramic–metal composite materials.
... The improved fracture toughness for the bimodal MSBZ alloys in comparison with the UFG-MSBZ alloy is significantly associated with the establishment of coarse α-Mo grains that tend to exhibit higher ductility and toughness. In fact, the coarse α-Mo grain has stronger ability to store dislocations and also dislocations can glide more easily in the CG grain than in the UFG grain due to the larger size and fewer obstacles of the grain boundaries [27], which support evidence for a higher strain hardening in the coarse α-Mo grain, thereby showing higher in-situ ductility. And the characteristic of high dislocation activities in the coarse α-Mo grain can be easily observed in the deformed sample, as typically shown in Fig. 8. ...
Designing a bimodal α-Mo grain structure in Mo-12Si-8.5B-ZrB2 alloy was achieved via mechanical alloying followed by hot pressing. This bimodal structure consisted of a major of nano-scaled α-Mo grains and partial micron-scaled α-Mo grains. The effective cooperation of these fine and coarse α-Mo in the Mo-12Si-8.5B-ZrB2 alloy was very beneficial for improving its toughness as well as strength. The high toughness was mainly attributed to the in-situ toughening and crack trapping effects originating from the coarse α-Mo. While the high strength was owing much to the grain boundary strengthening of the fine α-Mo. When the volume fraction ratio of the fine and coarse α-Mo grains was approximately 6:1, the fracture toughness value reached 13.1 MPa·m1/2 in the bimodal alloy, showing a 18% increase compared to an ultrafine-grained alloy. Simultaneously, a high compressive strength with 2998 MPa was also maintained.
... Thus, for a thin metal layer bonding two ceramic blocks, Dalgleish et al. [7] have observed rapid cavity growth under tension normal to the layer. For a ceramic reinforced by Al particles, a single dominant cavity has been observed in some of the particles crossed by a fracture surface [8], and similar observations have been made in model experiments [9]. Also in a metal matrix composite reinforced by ceramic fibers, a cavitation instability can occur in the metal between fiber ends, dependent on the material parameters [10]. ...
Full three dimensional cell models containing a small cavity are used to study the effect of plastic anisotropy on cavitation instabilities. Predictions for the Barlat-91 model (Int. J. Plast. 7, 693-712, 1991), with a non-quadratic anisotropic yield function, are compared with previous results for the classical anisotropic Hill-48 quadratic yield function (Proc. Royal Soc. Lond. A193, 281-297, 1948). The critical stress, at which the stored elastic energy will drive the cavity growth, is strongly affected by the anisotropy as compared to isotropic plasticity, but does not show much difference between the two models of anisotropy. While a cavity tends to remain nearly spherical during a cavitation instability in isotropic plasticity, the cavity shapes in an anisotropic material develop towards near-spheroidal elongated shapes, which differ for different values of the coefficients defining the anisotropy. The shapes found for the Barlat-91 model, with a non-quadratic anisotropic yield function, differ noticeably from the shapes found for the quadratic Hill-48 yield function. Computations are included for a high value of the exponent in the Barlat-91 model, where this model represents a Tresca-like yield surface with rounded corners.
... 4 The toughness increment is obtained mainly from the crack bridging effect with the stretched ductile phase on the propagated crack surface. 5,6 Powder metallurgy (PM) is a conventional technique for fabrication of ceramic-metal composites. 7 This method generally consists of three basic steps: mechanical mixing of ceramic and metal powders, die compaction, and sintering. ...
TiO2–Co composite powders with various Co contents were prepared by the alloying-recomposition-oxidation-sintering process. For comparison, conventionally mixed TiO2–Co composites with the same compositions were sintered at 1000, 1100, 1200, 1300, and 1400℃. Structural characterizations were performed using X-ray diffraction, field-emission scanning electron microscopy, energy-dispersive spectroscopy, and transmission electron microscopy. All of the sintered samples were more densified. A melted matrix was observed at a temperature higher than 1300℃. The flexural strength and the fracture toughness of the TCA sample were higher than those of the conventionally mixed TiO2–Co sample at the same sintering temperature, while the Vickers hardness exhibited the opposite relationship. The flexural strength and the fracture toughness of the TCA sample increased until a Co content of 14 vol%, followed by decrease at 18 vol%, while those of the conventionally mixed TiO2–Co sample increased in the entire Co content range. The highest flexural strength and fracture toughness were observed for T14CA sintered at 1400℃ (161.3 MPa and 6.39 MPa m−1/2, respectively). Consequently, the desirable Co content in the TiO2–Co composite prepared by the alloying–recomposition–oxidation–sintering process was 14 vol%.
A novel method of preparing continuous NiTif‐reinforced Ti/Al‐laminated composites is proposed via ultrasonic consolidation (UC) and vacuum hot pressing. The microstructure of the composites is characterized by X‐ray diffraction, scanning electron microscopy, and electron backscattering diffraction. The tensile properties of the composites are measured by a universal testing machine. Results indicate that the composite exhibits multilayer structure consisting of residual NiTi fiber, intermetallic compounds (IMCs) layer, reaction layer, Ti layer, and Al layer. The intermetallic layer is composed of Al3Ti and Al3Ni, and some Ni‐rich precipitates are dispersed in the matrix. And with the introduction of UC assisted, grain refinement occurs at the IMC/Al interface, texture variation emerges at the NiTi fiber/IMCs interface, and a weakening of texture strength is observed at both interfaces. Compared to NiTif‐reinforced Ti/Al‐laminated composites without UC assisted, both the tensile strength (from 872.4 to 826.5 MPa) and failure strain (from 17.5% to 20.4%) of NiTif‐reinforced Ti/Al‐laminated composites with UC assisted increase due to the contribution of UC assisted. The reinforcing and fracture mechanisms of the UC assisted are also discussed.
Herein, the development of directional solidification for a novel high‐temperature Mo–20Si–52.8Ti (at%) ternary alloy using a modified Bridgeman type apparatus is presented. The resulting alloy exhibits a microstructure consisting of a body‐centered cubic solid solution (BCC ss ) and a hexagonal silicide (Ti,Mo) 5 Si 3 with approximate volume fractions of 50% for each phase. The phases exhibit a crystallographic orientation relationship with and . Different solidification velocities are imposed, which reveal an inverse relationship to the lamellar spacing according to a Jackson–Hunt type scaling. Mechanical characterization using Vickers indentation demonstrates that the BCC ss accommodates plasticity through dislocation motion, while the silicide phase exhibits high hardness and brittleness, serving as a crack initiation site. Crack propagation is arrested and deflected at the interface to the BCC ss . Fracture toughness measurements via indentation yield a fracture toughness of 3.7 MPa√m for the silicide, somewhat higher than previously reported values for Nb‐, Mo‐, and Cr‐based silicides at room temperature. The directionally solidified specimens show an enhanced fracture toughness attributed to a greater BCC ss length scale; thus, combining the ductile and hard phases results in a ductile‐phase toughened intermetallic composite. The findings open up new possibilities for the design of advanced intermetallic composites with improved toughness performance.
Microstructural characteristics and mechanical properties of as-cast Nb-16Si-22Ti-2Al-2Cr-xV (x = 2, 5, 10, 15 and 20 at.%) and Nb-16Si-22Ti-2Al-2Cr-ySn (y = 1, 3, 5 and 7 at.%) alloys are systematically investigated in this study. The individual addition of V or Sn obviously promotes the eutectoid decomposition of metastable phase (Nb3Si). All alloys consist of the primary Nb5Si3 and the eutectic Nbss/Nb5Si3 structures. For the V-added alloys, the low content of added V (2 at.%) cannot change the constituent phases (Nbss and α-Nb5Si3) of Nb-16Si-22Ti2Al-2Cr alloy. However, the other V-added alloys consist of the primary β-Nb5Si3 and the eutectic structures of Nbss/β-Nb5Si3. Similarly, addition of 1 at.% Sn also cannot affect the constituent phases of Nb-16Si-22Ti-2Al-2Cr alloy, while the β-Nb5Si3 and Nb3Sn are formed with the increase in Sn content. Moreover, the addition of V or Sn promotes the formation of chaotic eutectics and suppresses the appearance of reticulate and cellular eutectics. The fracture toughness of the alloys is continuously elevated by the V addition, while it gets weakened when the content of added Sn exceeds 1 at.%. The high-temperature compressive strength of the V-added alloys increases when the V content is below 10 at.%, and then it decreases with increasing V content. The Sn-added alloys, except 3Sn alloy, exhibit higher compressive strength than Nb-16Si-22Ti-2Al-2Cr alloy, due to the increase in volume fractions of brittle phases (β-Nb5Si3 and Nb3Sn). Moreover, the crystallographic orientation relationships among the constituent phases are also explored before and after high-temperature compression.
In this study, the effect of Ge addition on microstructural evolution and mechanical properties of hypereutectic Nb-16Si-22Ti-2Al-2Cr (at.%) alloy was explored in detail. The microstructural evolution of these alloys containing Ge was characterized by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM). The mechanical properties including room-temperature fracture toughness and high-temperature compression strength were measured. Only Nbss and β-Nb5Si3 were observed in all Ge-added alloys, which indicated that Ge intensively promoted the formation of β-Nb5Si3 and restrained the formation of Nb3Si. Moreover, the volume fraction of β-Nb5Si3 increased with Ge addition. Ge could improve the fracture toughness of Nb-16Si-22Ti-2Al-2Cr alloy. 1Ge (Nb-16Si-22Ti-2Al-2Cr-1Ge) alloy exhibited the max KQ of 18.47 MPa•m1/2. All alloys with added Ge, except 9Ge alloy, showed the higher toughness than that of Nb-16Si22Ti-2Al-2Cr alloy, owing to the presence of chaotic eutectics of Nbss/β-Nb5Si3. In addition, Ge could not only improve the elastic modulus and nanohardness of constituent phases, but also elevate the high-temperature compression strength of the alloys. Moreover, some orientation relationships between Nbss and Nb5Si3 before and after high-temperature deformation were observed by EBSD method.
The necking instability is a precursor to tensile failure and rupture of materials. A quasistatically loaded free-standing uniaxial specimen typically exhibits necking at a single location, thus corresponding to a long wavelength bifurcation mode. If confined to a substrate or embedded in a matrix the same filament can exhibit periodic necking and fragmentation thus creating segments of finite length. While such periodic instabilities have been extensively studied in ductile metal filaments and thin sheets, less is known about necking in hyperelastic materials. Nonetheless, in recent years, there has been a renewed interest in the role of necking in novel materials, for the advancement of fabrication processes and to explain fragmentation phenomena observed in 3D printed active biological matter. In both cases materials are not well described by the existing frameworks that employ J2 deformation theory plasticity, and existing studies do not account for the significant role of misfit stretches that may emerge in these systems through chemical, or biological contraction. To address these limitations, in this paper we begin by experimentally demonstrating the role of the surrounding matrix on the necking and fragmentation of a compliant filament embedded in a tunable rubber matrix. Using a generalized hyperelastic model with strain softening, our analytical bifurcation analysis explains the experimental observations and is shown to agree with numerical predictions. The analysis reveals three distinct bifurcation modes: the long wavelength necking, thus recovering the Considère criterion; the periodic necking observed in our experiments; and a short wavelength mode that is characterized by localization along the center cord of the filament and is independent of the film-to-matrix stiffness ratio. We find that the softening coefficient and the filament misfit stretch can significantly influence the stability threshold and observed wavelength, respectively. Our results can guide the design and fabrication of novel composite materials and can explain the fragmentation processes observed in active biological materials.
This article presents and overview of the fatigue of thermostructural alloys focusing on structural steels, titanium and its alloys, nickel and its alloys and thermal barrier coatings. Factors contributing to the fatigue behavior of these thermostructural alloys such as stress ratio, operating environment and microstructural features have been discussed. Differences in fatigue crack growth rate trends at varying stress ratios (low and high) are attributed to crack closure with 1% offset collapsing “closure corrected” low stress ratios with a “closure-free” high stress ratio. The fatigue and fracture micromechanisms at near threshold, Paris regime and high stress intensity factor are discussed for the thermostructural alloys. Fracture maps of the alloys and their characteristic features which are viable for predicting fatigue was discussed in line with current literature. The implications of the results are then discussed for the design of robust and fatigue-resistant thermostructural alloys.
Ultrafine-grained or nanocrystalline Mo-12Si-8.5B alloys possess more excellent strength than those of the coarse-grained ones, but they often exhibit limited fracture toughness at room temperature. It’s very important to simultaneously enhance the strength and toughness of Mo-12Si-8.5B alloy for their practical applications. In the present study, nanometer Mo-ZrO2(Y2O3) composite powder was successfully prepared by a combination of sol-gel and one-step high-temperature reduction in hydrogen. A series of Mo-12Si-8.5B-ZrO2 (Y2O3) alloys composed of different proportions of coarse and fine Mo grains were fabricated via spark plasma sintering using nano- and micro-Mo powders as raw materials. With an increase of the mass ratio of the nanometer Mo to the micrometer Mo in the alloy, the hardness, flexural strength and fracture toughness first increase and then drop. As the mass ratio of the nanometer Mo powder to the micrometer Mo powder is 3:7, the bimodal structural alloy possesses the excellent comprehensive mechanical properties, including the hardness of 9.75 GPa, the flexural strength of 630 MPa, and the fracture toughness of 12.82 MPa m1/2. Grain refinement of the Mo, grain boundary strengthening in the boundary between the nanometer/micrometer Mo grains and the second-phase strengthening of the nano-ZrO2(Y2O3) particles attribute to the enhancement of the hardness and flexural strength. The improved fracture toughness mainly results from the crack arrest effect of the coarse grain Mo.
This article presents an overview of the fatigue and fracture mechanisms and toughening of some group of intermetallics. Fracture toughness and fatigue crack growth of α2 and γ-based titanium aluminides as well as niobium aluminides and molybdenum disilicides are elucidated. Following a brief introduction for these group of intermetallics, the toughening mechanisms of fracture for these intermetallics under monotonic and cyclic loading was explored. Materials and mechanistic models are also presented for the prediction of deformation twinning and transformation deformation toughening ratios to understand the fracture toughening contribution of the reinforcements. Finally, the implications for the design of next generation mechanical components and structures are discussed.
Four NbSi based ultrahigh temperature alloys with compositions of Nb-16Si-20Ti-1ZrC-xZrB2 (x = 0, 0.5, 1, 3) (at.%) were prepared by vacuum non-consumable arc melting. The microstructure evolution, fracture toughness, and fracture behavior of four alloys were investigated. Results showed that four alloys consisted of Nbss, γ-(Nb,X)5Si3, and (Nb,X)3Si phases. The low content of ZrB2 facilitates the disintegration of (Nb,X)3Si, while the high content of ZrB2 stabilizes (Nb,X)3Si. The maze-like Nbss/γ-(Nb,X)5Si3 eutectic structure forms in Nb-16Si-20Ti-1ZrC-0.5ZrB2 alloy. With the increase of ZrB2 content, the room-temperature fracture toughness of Nb-Si-Ti-ZrC-based alloy increases and decreases. When the addition amount of ZrB2 is 0.5 at.%, the room-temperature fracture toughness is the largest of 18.4 MPa·m1/2, and the fracture mode changes from quasi-cleavage fracture to dimple fracture. Compared with the base alloy, it is increased by 28%. It is the highest value of as-cast arc melting that has been reported so far. The maze-like eutectic of the high content of Nbss (54.9 ± 5.9%) phase promotes the formation of microcracks and inhibits the propagation of cracks in Nb-16Si-20Ti-1ZrC-0.5ZrB2 alloy.
Brittle fracture often compromises the durability of glassy polymers. This can be mitigated by reinforcing the matrix with a filler, activating a range of toughening mechanisms. Therefore, it is desirable to better understand the mechanical response of polymer composites, but a direct visualization of the mechanical fate of second-phase inclusions upon material fracture was previously unavailable. Here, rubbery poly(hexyl acrylate) particles, cross-linked with an optical force probe (OFP), are dispersed within an epoxy matrix, the material is fractured, and particle–crack interactions are visualized with confocal laser scanning microscopy. The dual-fluorescence character of the OFPs allows the differentiation between particles that remain intact and those that are stressed beyond bond scission upon interaction with a propagating crack. The localized activation of OFPs reveals stress gradients within the particles and crack direction pathways, hence providing a new layer of information over fracture events in polymer composites.
Nb-Si ultra-high temperature alloys have been a hot topic in aviation materials, but low fracture toughness limits the application. Ultrasound treatment has a significant advantage on the grain refinement and properties improvement, successfully induced into hypoeutectic Nb-Si-Ti ternary alloys to improve primary morphology and fracture toughness. In this work, Nb-16Si-22Ti alloy was treated by ultrasound of 0 s, 80 s, 120 s, 160 s and 200 s. Meanwhile, the coupled physical fields were simulated in Comsol Multiphysics to reveal the mechanism of ultrasonic action. Results show that γ-Nb5Si3 appears with ultrasound induced, and the solidification path is close to the equilibrium transition. Primary Nbss phases were coarsened and spheroidized after ultrasound treatment, and fracture toughness improved by 69.16%. Simulation results show that the strong acoustic active zone is at the height of 15 mm in the ingot, with maximum sound pressure of 36.3 MPa. The high sound pressure can break dendrites. Maximum fluid velocity reaches 0.03 m/s in the longitudinal section and 0.02 m/s in the transversal section with several circulations. Acoustic streaming and cavitation effects break dendrites and spheroidize Nbss phase, benefit to elements distribution uniformly, equilibrium transition and fracture toughness improvement.
The microstructural evolution and mechanical properties of Nb–10Hf–15W–(0/1/3/5)Si (at.-%) alloys were investigated. The as-cast 0Si and 1Si alloys had a single Nb solid solution (NbSS) phase, while the 3Si and 5Si alloys consisted of NbSS matrix and the eutectic of NbSS plus Nb3Si. After heat treatment, the microstructure contained primary NbSS and NbSS + α-Nb5Si3 eutectoid in 3Si and 5Si alloys. Fracture toughness of as-cast and heat-treated samples at room temperature monolithically decreased with increasing Si content. However, the α-Nb5Si3 phase improved compressive strength of the NbSS alloy at 1250 and 1350°C, with no degradation in the ductility or formability of the NbSS/Nb5Si3 microstructure. Finally, the failure mechanisms of the NbSS/α-Nb5Si3 microstructure at room and high temperatures were discussed.
Volume 19 is a resource for basic concepts, alloy property data, and the testing and analysis methods used to characterize fatigue and fracture behavior of structural materials. Contents include fatigue mechanisms, crack growth and testing; fatigue strength prediction and analysis; fracture mechanics, damage tolerance, and life assessment; environmental effects; and fatigue and fracture resistance of ferrous, nonferrous, and nonmetallic structural materials. Statistical aspects of fatigue data, the planning and evaluation of fatigue tests, and the characterization of fatigue mechanisms and crack growth are also covered. Practical applications and examples of fracture control in weldments, process piping, aircraft systems, and high-temperature crack growth and thermos-mechanical fatigue are also included. For information on the print version of Volume 19, ISBN 978-0-87170-385-9, follow this link.
Single NbSS phase alloy (Nb-37Ti-13Cr-2Al-1Si) and two-phase NbSS/Nb5Si3 alloy (hypoeutectic composition of Nb-16Si-24Ti-2Al-2Cr) powders prepared by the plasma rotating electrode atomization technique (PREA) were spark plasma sintered (SPS), and the microstructure, fracture toughness, deformation and fracture mode of the two SPS and arc melting (AM) alloys were characterized. With a coarsened grain size, the AM and SPS single NbSS phase alloy had a small difference in fracture toughness and it failed in a cleavage mode. As for the SPS two-phase NbSS/Nb5Si3 sample containing an ultra-fine ductile/stiffening NbSS/Nb5Si3 microstructure, approximately 1–2 μm in phase size, a fracture toughness of as high as 16.1 MPa·m1/2 was obtained, owing to operation of the active dislocation of a/2 〈111〉 and dimple failure of the fine NbSS phase. However, the AM two-phase NbSS/Nb5Si3 microstructure had a coarsened NbSS phase and a low fracture toughness of 9.6 MPa·m1/2, resulting from the movement of a 〈100〉 dislocation in the {001} cleavage planes and a cleavage failure mode of the coarsened NbSS phase.
A novel Mo–28Ti–14Si–6C–6B (atom %) alloy was designed for ultrahigh-temperature applications. Arc-melting and appropriate heat-treatment at 1600°C results in the targeted four-phase microstructure comprising Moss, Mo5SiB2 (T2), Ti5Si3 and TiC. Previously developed three-phase alloys, comprising Moss–T2–Ti5Si3 and Moss–T2–TiC, are outperformed by this novel alloy as an adequate balance of oxidation resistance, creep strength and fracture toughness is attained: The oxidation behavior is characterized by substantial oxide scale formation at 1200°C being accompanied by a mass loss of around -50 mg/cm² after 100 h of cyclic oxidation. The oxide scale is found to be composed of a top TiO2 scale and an underlying duplex SiO2/TiO2 scale. Minimum creep rate of the alloy was in the order of 10–7 s–1 at 1200°C and 300 MPa and its room-temperature fracture toughness was (12.8 ± 1.2) MPa(m)1/2.
Mechanisms of ductile failure in brittle matrix composites with metal reinforcements are investigated using a mechanistic approach to incorporate material anisotropy. Rapid void expansion with increasing remote strain and at nearly constant remote strain (cavitation) is modeled. Fracture energy, total energy absorbed, and cavitation limit is obtained as a function of material anisotropy. It is observed that for initial void volume fraction less than or equal to 10-4 cavitation phenomenon is seen. The critical stress at cavitation is found to increase with decreasing void size until it reaches the cavitation limit. Further, it is observed that the ductile fracture of metal reinforcements in the brittle matrix composite is sensitive to the material anisotropy.
The main tasks of studying molybdenum silicides for use at elevated temperature include development of materials with a melting point Tme ≥ 2000°C, possibly with a low brittle-ductile transition temperature and resistance to high temperature oxidation. The possibility is considered of increasing corrosion resistance of a model alloy consisting of molybdenum solid solution and Mo3Si. The effect of Sc or Nd alloying additives, as well as preliminary high-temperature annealing. i.e., pre-oxidation in order to create a protective oxide layer on the alloy surface of a model alloy, is evaluated. Non-isothermal oxidation of alloys is performed using combined thermogravimetric and differential thermal analysis methods. The phase composition of the scale formed is determined by an X-ray diffraction method. As a result of these studies it is established that preliminary oxidation firing of Mo–Mo3Si–REE (about 3 at.%) alloys at 1100°C for 5–10 min provides conditions for forming a layer of SiO2 and REE molybdates Sc2(MoO4)3 and Nd2Si2O7 at the alloy surface. Oxidation in a dynamic regime for treated alloys in the range 500–850°C proceeds much more slowly than for the initial samples. This can be explained by the difficult access of oxygen to the reaction zone through a protective layer of non-volatile oxides. In addition, preliminary oxidation of Mo–Mo3Si alloyed with Sc or Nd during subsequent oxidation leads to an increase in the transition temperature for the process from the stage of oxide formation to intense MoO3 sublimation.
Titanium-aluminum alloy metal matrix composites (MMC) and Ti-Al intermetallic matrix composites (IMC), reinforced with continuous SCS6 SiC fibers are leading candidates for high temperature aerospace applications such as the National Aerospace Plane (NASP). The nature of deformation at fiber / matrix interfaces is characterized in this ongoing research. One major concern is the mismatch in coefficient of thermal expansion (CTE) between the Ti-based matrix and the SiC fiber. This can lead to thermal stresses upon cooling down from the temperature incurred during hot isostatic pressing (HIP), which are sufficient to cause yielding in the matrix, and/or lead to fatigue from the thermal cycling that will be incurred during application, A second concern is the load transfer, from fiber to matrix, that is required if/when fiber fracture occurs. In both cases the stresses in the matrix are most severe at the interlace.
The friction coefficient of thermal sprayed ceramic coatings is relatively high, and the traditional introduction of lubricants will result in the decline of mechanical properties of coatings, which severely limits their applications. Herein, YSZ/Ag self-lubricating coatings were prepared on 316 L stainless steel substrates via thermal spraying technology, vacuum impregnation and in-situ synthesis for the first time. The morphologies, mechanical properties and vacuum tribological performance of composite coatings were studied in detail by comparing with YSZ coatings. Results show that silver particles distribute in the pores, cracks of YSZ coatings, and the filling of defects by silver improves the micro-hardness, fracture toughness and cohesive strength of the coatings. In addition, owing to the formation of lubricating film during the sliding process in vacuum, the friction coefficient of the coating is reduced by about 2 times and the wear rate is reduced by about 600 times after the introduction of silver. At the same time, the addition of the silver lubricant makes the wear of the coupling ball significantly reduced. Since the lubricating film can be continuously replenished and repaired, the YSZ/Ag self-lubricating coating has a relatively long service life (>100,000 sliding cycles).
The effects of (0–60%) vol% (70 vol% ZrB2 + 30 vol% SiC) additions on microstructure and properties of NbMo substrate fabricated by hot-pressing were studied at room temperature. Types of formed phase were decided by the amount of ZrB2 and SiC additives. The effective eutectic phase was observed in 15 vol% (70 vol% ZrB2 + 30 vol% SiC)-NbMo, which was attributed to the addition of SiC. 30 vol% (70 vol% ZrB2 + 30 vol% SiC)-NbMo had the highest relative density of 98.69%. Compared with x ZrB2-NbMo composites, the addition of SiC could further improve the hardness of NbMoss in x (70% ZrB2 + 30% SiC)-NbMo when the value of x was same, and NbMoss in 60 vol% (70 vol% ZrB2 + 30 vol% SiC)-NbMo had the highest hardness of 6.82 GPa. Only the 15 vol% (70 vol% ZrB2 + 30 vol% SiC) addition could improve the compressive strength of NbMo matrix. The reasons for the low strength of 30, 45, 60 vol% (70% ZrB2 + 30% SiC)-NbMo were the lack of ductile phase and the large amount of hard phase production.
Nearly three decades ago, the field of mechanics was cautioned of the obscure nature of cavitation processes in soft materials [Gent. Cavitation in rubber: a cautionary tale. Rubber Chemistry and Technology, 1990, 63, 3]. Since then, the debate on the mechanisms that drive this failure process is ongoing. Using a high precision volume controlled cavity expansion procedure, this paper reveals the intimate relationship between cavitation and fracture. Combining a Griffith inspired formulation for crack propagation, and a Gent inspired formulation for cavity expansion, we show that despite the apparent complexity of the fracture patterns, the pressure-volume response follows a predictable path. In contrast to available studies, both the model and our experiments are able to track the entire process including the unstable branch, by controlling the volume of the cavity. Moreover, this minimal theoretical framework is able to explain the ambiguity in previous experiments by revealing the presence of metastable states that can lead to first order transitions at onset of fracture. The agreement between the simple theory and all of the experimental results conducted in PDMS samples with shear moduli in the range of 25-246 [kPa], confirms that cavitation and fracture work together in driving the expansion process. Through this study we also determine the fracture energy of PDMS and show its significant dependence on strain stiffening.
The transverse rupture strength of hot-pressed and annealed composites of magnesium oxide and dispersed metallic phases (nickel, iron, cobalt) increases with increasing volume fraction of metal and annealing temperature. The strengthening effect of the metal is attributed to an inhibition of grain growth while flaw healing occurs during the annealing of the composites.
Some effects of intact particles in the crack wake on the fracture toughness of ceramics have been analyzed. The result have been applied to interpretation of the toughening of ceramics by strong well-bonded metal particles and by whiskers. Trends in toughness with microstructure have been predicted for various reinforced ceramic systems.
Fracture mechanics of a metal-matrix composite containing ductile-metallic fibers is described. Experimental verification of the proposed descriptions has been established with an aluminum-base composite containing uni-directional stainless-steel fibers. Theoretical description of the plastic energy dissipation shows that crack propagation across fibers is very difficult because of the high energy density represented by the fiber. The fiber contribution is an increasing function of volume fraction which results in the critical stress-intensity factor increasing with fiber content. On the other hand, crack propagation between fibers is very easy because the inter-fiber spacing limits the plastic energy dissipation in the matrix. The critical stress intensity for crack propagation between fibers is a decreasing function of volume fraction.
Measurements of absorbed energy in a miniature Charpy test have been made on brittle-fibre ductile-matrix composites (tungsten wires in copper, copper 10 w/o tin and phosphor bronze). The variation of work-to-fracture with fibre volume-fraction has been found and for ductile matrices supports the theory of G.A. Cooper and A. Kelly given in 1967. The effect of fibre diameter on work-to-fracture has been determined over a large range of diameters (10 μm to 5 mm) and this also shows general agreement with the above theory. It is predicted theoretically that a nonhomogeneous arrangement of fibres will lead to a higher work-to-fracture and experimental evidence is provided to support this prediction.
The role of elastic, thermoelastic, and interfacial properties in the toughening of a brittle matrix by metallic second-phase particles was studied. Two composites were studied: glass+partly oxidized Ni particles (thermal expansion coefficient of the glasses lower than, equal to, and higher than that of Ni) and glass+partly oxidized Al particles (thermal expansion and elastic moduli equal). Weak interfacial bonding between the nickel and its oxide and developed stress concentrations are the major toughness limitations found in the glass/Ni composites. When the thermal expansion coefficient and elastic modulus of the second phase are sufficiently greater than that of the glass matrix, a propagating crack will bypass the particles. When the thermal and elastic stresses are minimized and satisfactory bonding is achieved (glass/Al composites), a 60x toughness increase was realized.
Critical stress intensity factor, and related parameters have been measured in three-point bending for 18 different combinations of different volume fractions of cobalt (5 to 37%) and grain size of tungsten carbide (0.7, 1.1 and 2.2 m). In particular, a study was made of the correlations between the strength and mechanical and microstructural parameters, such as L
Co,C
WC, L
Co/D
WC, L
Co
2
/D
WC,H
V and wear resistance. A hypothesis for the mechanism of fracture has been proposed following an analysis of these results and a study of the mode of fracture.
The transverse rupture strength of hot-pressed and annealed composites of magnesium oxide and dispersed metallic phases (nickel, iron, cobalt) increases with increasing volume fraction of metal and annealing temperature. The strengthening effect of the metal is attributed to an inhibition of grain growth while flaw healing occurs during the annealing of the composites.The strength of magnesium oxide hot-pressed with nickel fibres is not affected by the volume fraction of fibre or the annealing temperature, and is comparable to the strength of porous magnesia. However, the work of fracture, though insensitive to heat-treatment, increases by at least two orders of magnitude for a moderate volume fraction of randomly oriented fibres. Mechanisms of energy absorption during the fracture of composites containing weakly bonded, non-aligned fibres are discussed. They include the work done in plastically deforming the fibre as it is withdrawn from its socket. It is concluded that this mechanism may be of importance in composites containing very weakly bonded ductile fibres.
Experiments have been designed to study how a material reinforced with aligned fibres fails at the root of a notch. Ductile and brittle tungsten wires and silica fibres have been introduced into a copper matrix made either by casting or by electrodeposition. A completely notch-insensitive composite can be produced provided splitting is tolerated parallel to the fibres. In these experiments splitting appears to occur in shear. If splitting does not occur then it is shown experimentally for thin sheets that fracture is governed by the established rules of fracture mechanics. The contribution of the matrix to the work of fracture is assessed. An important result is that the work of fracture varies linearly with the fibre diameter and in a copper matrix at room temperature is less than 107 ergs. cm−2 for a fibre diameter of 20 μ.