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Orientation-dependent dynamic shearing properties and underlying failure mechanisms of laser powder bed fusion of CoCrFeNi high-entropy alloy
Abstract
In order to study the influence of building orientation on the dynamic shearing properties and failure mechanism of laser powder bed fusion (LPBF) of CoCrFeNi high-entropy alloy (HEA) under impacting load, a series of high speed impact tests were systematically conducted on a split Hopkinson pressure bar (SHPB) device at different strain rates. For this purpose, flat hat-shaped samples with a preset shear region were designed. The samples HS0 and HS90 respectively possessed a shear direction parallel and perpendicular to the building orientation. The respective columnar grain orientations of HS0 and HS90 influence the synergistic interaction between dislocation slip and deformation twinning, which is reflected in the differences in mechanical properties. It was observed that HS0 had higher shear yield strength whereas HS90 exhibited better plasticity. Moreover, the two samples show different adiabatic shear sensitivities as adiabatic shear bands (ASBs) were observed when the shear strain reached ~9.5 and 11.2, respectively. Typical dynamic responses such as adiabatic temperature rise and grain refinement took place in the shear regions. Further, the formation of ultrafine equiaxed grains may be explained using the classical rotational dynamic recrystallization (RDRX) mechanism, which can furnish a sound theoretical basis to understand the deformation behaviors and mechanisms of LPBF CoCrFeNi HEA under impact loads.
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CoCrFeMnNi high entropy alloy (HEA) parts were fabricated by laser powder bed fusion (LPBF), and their dynamic compressive properties at different temperatures as well as the resulting microstructures were analyzed. The HEAs showed an unprecedented strength-ductility combination, especially at a cryogenic temperature of 77 K and a high strain rate of 3000 s−1. Under this testing condition, the yield strength (YS) of the HEAs amounted to 665 MPa. Regardless of the testing temperature, the deformation mechanism of all investigated HEAs was dominated by a synergistic effect consisting of deformation twinning and dislocation pile-up around twins. The fraction of twin boundaries and dislocation density within the deformed microstructure of the HEA correlated with the test temperature. At 77 K, the formation of nanotwins together with dislocation slip prevailed and contributed to pronounced twin-twin and twin-dislocation interactions which effectively restricted the dislocation movement and, hence, contributed to a higher YS as well as strain hardening rate in comparison to that of the HEAs at room temperature of 298 K. The LPBF-fabricated HEAs showed unpronounced thermal softening even at a high testing temperature of 1073 K. Continuous dynamic recrystallization was restricted in the HEA because of its inherent sluggish dislocation kinetics and low stacking fault energy.
Spatter is an inherent, unpreventable, and undesired phenomenon in laser powder bed fusion (L-PBF) additive manufacturing. Spatter behavior has an intrinsic correlation with the forming quality in L-PBF because it leads to metallurgical defects and the degradation of mechanical properties. This impact becomes more severe in the fabrication of large-sized parts during the multi-laser L-PBF process. Therefore, investigations of spatter generation and countermeasures have become more urgent. Although much research has provided insights into the melt pool, microstructure, and mechanical property, reviews of spatter in L-PBF are still limited. This work reviews the literature on the in situ detection, generation, effects, and countermeasures of spatter in L-PBF. It is expected to pave the way towards a novel generation of highly efficient and intelligent L-PBF systems.
Deformation-induced hcp nano-lamellae with various widths and interspacings were observed in the CoCrNi medium-entropy alloy (MEA) under high strain rate and cryogenic temperature in the present study. Higher hardness was found in the cryogenic-deformed samples compared to the room temperature-deformed samples without hcp phase. Then, size effects of embedded hcp nano-lamellae on the tensile behaviors in the fcc CoCrNi MEA were investigated by molecular dynamics simulations. The overall strengthening was found to have two components: phase strengthening and extra interface strengthening, and the interface strengthening was observed to be always stronger than the phase strengthening. Both overall strengthening and interface strengthening were found to increase with increasing width and decreasing interspacing of embedded hcp nano-lamellae. The samples with small spaced hcp nano-lamellae are even stronger than the pure hard hcp phase due to the extra interface strengthening. The samples with larger width of embedded hcp nano-lamellae can provide stronger resistance for dislocation slip and transmission. Nanotwins were observed to be formed in the embedded hcp nano-lamellae. Higher density of phase boundaries and newly formed twin boundaries can provide more barriers for dislocation glide in the other slip systems, resulting in higher strength for samples with smaller interspacing.
For millennia, humans have exploited the natural property of metals to get stronger or harden when mechanically deformed. Ultimately rooted in the motion of dislocations, mechanisms of metal hardening have remained in the cross-hairs of physical metallurgists for over a century. Here, we performed atomistic simulations at the limits of supercomputing that are sufficiently large to be statistically representative of macroscopic crystal plasticity yet fully resolved to examine the origins of metal hardening at its most fundamental level of atomic motion. We demonstrate that the notorious staged (inflection) hardening of metals is a direct consequence of crystal rotation under uniaxial straining. At odds with widely divergent and contradictory views in the literature, we observe that basic mechanisms of dislocation behaviour are the same across all stages of metal hardening.
We report deformation bands ahead of the crack tip in a CrCoNi alloy under impact loading, and analyze the effects of deformation twinning on strain localization in the form of shear banding. Due to low stacking fault energy, upon high-strain-rate deformation this medium-entropy alloy forms twins with thickness and spacing of only a few nanometers. These nano-twins are embedded dynamically not only ahead of any developing shear band to retard its advance, but also in its interior to work harden. This dual effect helps to suppress runaway shear banding instability that instigates failure and contributes to the energy absorption as recorded in instrumented Charpy tests.
Hot tearing mechanisms of a high-entropy alloy CoCrFeNi additively manufactured by selective laser melting have been investigated. Intergranular hot cracks are present regardless of various parameters used, suggesting poor laser-based printability for the alloy. Elemental segregation does not exist at the grain boundary that favours the hot cracking. We find that severe residual stress induced by the large grain size is the root cause for the inter-granular cracking. The classic Rappaz-Drezet-Gremaud model is used to predict the characteristic depression pressure limit beyond which hot tearing will occur for the selective laser melting of metals and alloys.
One of the most important issues related to adiabatic shear failure is the correlation among temperature elevation, adiabatic shear band (ASB) formation and the loss of load capacity of the material. Our experimental results show direct evidence that ASB forms several microseconds after stress collapse and temperature rise reaches its maximum about 30 μs after ASB formation. This observation indicates that temperature rise cannot be the cause of ASB. Rather, it might be the result of adiabatic shear localization. As such, the traditional well-accepted thermal-softening mechanism of ASB needs to be reconsidered.
Owing to the reduced defects, low cost, and high efficiency, the additive manufacturing (AM) technique has attracted increasingly attention and has been applied in high-entropy alloys (HEAs) in recent years. It was found that AM-processed HEAs possess an optimized microstructure and improved mechanical properties. However, no report has been proposed to review the application of the AM method in preparing bulk HEAs. Hence, it is necessary to introduce AM-processed HEAs in terms of applications, microstructures, mechanical properties, and challenges to provide readers with fundamental understanding. Specifically, we reviewed (1) the application of AM methods in the fabrication of HEAs and (2) the post-heat treatment effect on the microstructural evolution and mechanical properties. Compared with the casting counterparts, AM-HEAs were found to have a superior yield strength and ductility as a consequence of the fine microstructure formed during the rapid solidification in the fabrication process. The post-treatment, such as high isostatic pressing (HIP), can further enhance their properties by removing the existing fabrication defects and residual stress in the AM-HEAs. Furthermore, the mechanical properties can be tuned by either reducing the pre-heating temperature to hinder the phase partitioning or modifying the composition of the HEA to stabilize the solid-solution phase or ductile intermetallic phase in AM materials. Moreover, the processing parameters, fabrication orientation, and scanning method can be optimized to further improve the mechanical performance of the as-built-HEAs.
The mechanical behavior of the single phase (fcc) CrMnFeCoNi high-entropy alloy (HEA) is examined in the dynamic regime. A series of experiments by dynamic-loading hat-shaped specimens using stopper
rings to control the displacement are performed, and the alloy resists adiabatic shear-band formation up to a very large imposed shear strain of ~7. It is proposed that the combination of the excellent strain hardening ability and moderate thermal-softening effect retard shear localization. Recrystallized ultrafine-grained grains (diameters of 100-300 nm) with twins are revealed inside the shear band. Their
formation is explained by the rotational dynamic recrystallization mechanism. The stability of the structure at high strain rates strongly suggests a high ballistic resistance for this class of alloys.
Single-phase CrCoNi medium-entropy alloys (MEA) are emerging recently as an interesting class of metallic materials, but the dynamic response of this MEA at high strain rates remains unknown. Here we have produced this MEA with various heterogeneous microstructures, using cold rolling followed by annealing at various temperatures. The high-strain-rate response of the MEAs was characterized using hat-shaped specimens in Hopkinson-bar experiments. A combination of high dynamic shear yield strength and large uniform dynamic shear strain was observed, exceeding all other metals and alloys reported so far. Even better dynamic shear properties was revealed when the experiments were conducted at cryogenic temperature. The strong strain hardening under dynamic shear loading can be attributed to the dynamic grain refinement and deformation twinning that accompany the homogeneous shear deformation. When compared to room temperature, the efficiency of grain refinement was found to be enhanced at cryogenic temperature, with a higher density of multiple twins, stacking faults, Lomer-Cottrell locks, and hcp phase via phase transformation inside the grains, which could be responsible for the better dynamic shear properties under cryogenic environment.
A new high-entropy alloy, AlCoCr1.5Fe1.5NiTi0.5, with dual-phase body-centered cubic structure, exhibits excellent compression strength and large plasticity upon quasi-static and dynamic loadings. Positive strain-rate sensitivity on the yielding strength is available, and the work-hardening behavior weakens with increasing the strain rates due to the adiabatic heating effect upon dynamic loadings. Considering the adiabatic temperature rise converted by plastic deformation work at high strain rates, the flow behavior is characterized by employing the modified Johnson-Cook model.
The mechanical behavior of a single phase (fcc) Al 0.3 CoCrFeNi high-entropy alloy (HEA) was studied in the low and high strain-rate regimes. The combination of multiple strengthening mechanisms such as solid solution hardening, forest dislocation hardening, as well as mechanical twinning leads to a high work hardening rate, which is significantly larger than that for Al and is retained in the dynamic regime. The resistance to shear localization was studied by dynamically-loading hat-shaped specimens to induce forced shear localization. However, no adiabatic shear band could be observed. It is therefore proposed that the excellent strain hardening ability gives rise to remarkable resistance to shear localization, which makes this material an excellent candidate for penetration protection applications such as armors.
Constitutive response Adiabatic shear band formation and failure mechanism a b s t r a c t Dynamic deformation and shear localization of ultrafine-grained (~120 nm) pure titanium are examined. The strain hardening can be considered as having two regimes: below and above a strain ~0.04; at this point there is a drastic decrease in the slope. The strain-rate sensitivity of ultrafine-grained titanium is found to be approximately the same as its coarse grained counterpart. Based on experimentally determined parameters, the Zerilli-Armstrong equation is modified to describe the mechanical response of the ultrafine-grained titanium over the strain rate range 10 À5 to 10 3 s À1. Adiabatic shear banding is examined in a forced shear configuration where large strain is imposed in a narrow region. The microstructure inside the adiabatic shear band consists of a mixture of elongated grains and equiaxed nanograins (~40 nm) that are significantly smaller than the initial grains (~120 nm). The formation of equiaxed nanograins is modeled through a mechanism of rotational dynamic recrystallization. This further reduction in grain size from the one generated by ECAP is interpreted in terms of the Zener-Hollomon parameter for quasistatic and dynamic deformation. The adiabatic shear band eventually fractures by a combination of brittle and ductile failure.
Adiabatic shear banding (ASB) is a unique dynamic failure mechanism that results in an unpredicted catastrophic failure due to a concentrated shear deformation mode. It is universally considered as a material or structural instability and as such, ASB is hardly controllable or predictable to some extent. ASB is modeled on the premise of stability analyses. The leading paradigm is that a competition between strain (rate) hardening and thermal softening determines the onset of the failure. It was recently shown that microstructural softening transformations, such as dynamic recrystallization, are responsible for adiabatic shear failure. These are dictated by the stored energy of cold work, so that energy considerations can be used to macroscopically model the failure mechanism. The initial mechanisms that lead to final failure are still unknown, as well as the ASB formation mechanism(s). Most of all - is ASB an abrupt instability or rather a gradual transition as would be dictated by microstructural evolutions? This paper reports thorough microstructural characterizations that clearly show the gradual character of the phenomenon, best described as a nucleation and growth failure mechanism, and not as an abrupt instability as previously thought. These observations are coupled to a simple numerical model that illustrates them.
The equiatomic CoCrFeMnNi high entropy alloy, which crystallizes in the face-centered cubic (FCC) crystal structure, was prepared by the spark plasma sintering technique. Dynamic compressive tests of the CoCrFeMnNi high entropy alloy were deformed at varying strain rates ranging from 1 × 103 to 3 × 103 s−1 using a split-Hopkinson pressure bar (SHPB) system. The dynamic yield strength of the CoCrFeMnNi high entropy alloy increases with increasing strain rate. The Zerilli-Armstrong (Z-A) plastic model was applied to model the dynamic flow behavior of the CoCrFeMnNi high entropy alloy, and the constitutive relationship was obtained. Serration behavior during plastic deformation was observed in the stress-strain curves. The mechanism for serration behavior of the alloy deformed at high strain rate is proposed.
Selective Laser Melting (SLM) is a particular rapid prototyping, 3D printing, or Additive Manufacturing (AM) technique designed to use high power-density laser to melt and fuse metallic powders. A component is built by selectively melting and fusing powders within and between layers. The SLM technique is also commonly known as direct selective laser sintering, LaserCusing, and direct metal laser sintering, and this technique has been proven to produce near net-shape parts up to 99.9% relative density. This enables the process to build near full density functional parts and has viable economic benefits. Recent developments of fibre optics and high-power laser have also enabled SLM to process different metallic materials, such as copper,aluminium, and tungsten. Similarly, this has also opened up research opportunities in SLM of ceramic and composite materials. The review presents the SLM process and some of the common physical phenomena associated with this AM technology. It then focuses on the following areas: (a) applications of SLM materials and (b) mechanical properties of SLM parts achieved in research publications. The review is not meant to put a ceiling on the capabilities of the SLM process but to enable readers to have an overview on the material properties achieved by the SLM process so far. Trends in research of SLM are also elaborated in the last section.
An equiatomic CoCrFeMnNi high-entropy alloy, which crystallizes in the face-centered cubic (fcc) crystal structure, was produced by arc melting and drop casting. The drop-cast ingots were homogenized, cold rolled and recrystallized to obtain single-phase microstructures with three different grain sizes in the range 4-160 mu m. Quasi-static tensile tests at an engineering strain rate of 10(-3) s(-1) were then performed at temperatures between 77 and 1073 K. Yield strength, ultimate tensile strength and elongation to fracture all increased with decreasing temperature. During the initial stages of plasticity (up to similar to 2% strain), deformation occurs by planar dislocation glide on the normal fcc slip system, {1 1 1} < 1 1 0 >, at all the temperatures and grain sizes investigated. Undissociated 1/2 < 1 1 0 > dislocations were observed, as were numerous stacking faults, which imply the dissociation of several of these dislocations into 1/6 < 1 1 2 > Shockley partials. At later stages (similar to 20% strain), nanoscale deformation twins were observed after interrupted tests at 77 K, but not in specimens tested at room temperature, where plasticity occurred exclusively by the aforementioned dislocations which organized into cells. Deformation twinning, by continually introducing new interfaces and decreasing the mean free path of dislocations during tensile testing ("dynamic Hall-Petch"), produces a high degree of work hardening and a significant increase in the ultimate tensile strength. This increased work hardening prevents the early onset of necking instability and is a reason for the enhanced ductility observed at 77 K. A second reason is that twinning can provide an additional deformation mode to accommodate plasticity. However, twinning cannot explain the increase in yield strength with decreasing temperature in our high-entropy alloy since it was not observed in the early stages of plastic deformation. Since strong temperature dependencies of yield strength are also seen in binary fcc solid solution alloys, it may be an inherent solute effect, which needs further study.
Due to their strong strain hardening, face-centered cubic (FCC) high/medium entropy alloys (H/MEAs) have remarkable resistance to shear localization even under forced dynamic shear. In this work we investigated the dynamic mechanical behavior of an FCC Al0.1CoCrFeNi HEA via a series of split Hopkinson pressure bar (SHPB) experiments at 77K and room temperature. This HEA exhibited very strong strain hardening capacity, remarkable strain rate sensitivity and moderate temperature dependence. Such a combination of mechanical properties should be strongly unfavorable for adiabatic shear bands (ASBs). However, its dynamic compressive failure at 77K was found to be caused by crack propagation within the primary ASBs along the directions of maximum shear stress. Detailed examination of the specimens using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) revealed convincing microstructural evidences for massive dynamic recrystallization (DRX) prior to ASB. We suggested that the expansion and coalescence of these DRX softened regions served as the root cause for the occurrence of ASBs and the final catastrophic fracture of specimens. The severe local shear flow, substantial adiabatic temperature rise and extremely fast quenching within the ASB resulted in ultrafine grains therein. Our results and analyses provided a plausible understanding of the formation mechanisms of ASBs in the FCC HEA under impact loading.
This study presents an experimental investigation on elucidating the influence of build orientation on the microstructure and macroscopic response of the selective laser melted (SLM) Ti-6Al-4V alloy under compressive loading at varying strain rates. Quasi-static (at 0.001 s⁻¹) uniaxial compression and dynamic impact tests (up to 4500 s⁻¹) were carried out on specimens with their building directions either being parallel, diagonal or normal to the loading direction. The initial microstructure, dynamic compression stress–strain response, strain rate sensitivity, strain rate hardening effect, adiabatic temperature rise, and fracture behaviours are discussed. The results show that although the build orientation has a negligible influence upon the adiabatic temperature rise, the yield strength and strain-hardening rate of the SLM Ti-6Al-4V alloy are both build orientation and strain-rate dependent. Fractographic examination of the fragmented specimens under quasi-static and impact loading reveals ductile dimples and smooth surfaces, which indicate the coexistence of both ductile and brittle fractures. Although build orientation barely affects the fracture topology of quasi-static compressed specimens, it has a notable impact on facture morphology for dynamically loaded specimens.
The microstructural modification for cellular structures can achieve the improvement of dynamic mechanical properties of a selective laser melted FeCoNiCrMo0.2 high-entropy alloy (SLM-FeCoNiCrMo0.2 HEA) and can expand its promising applications in the field of high-velocity deformation. In this work, FeCoNiCrMo0.2 HEAs with cellular structures in different sizes were produced by selective laser melting (SLM) with different process parameters. The dynamic mechanical properties and microstructure of the SLM-FeCoNiCrMo0.2 HEA were studied. The dynamic mechanical properties of the SLM-FeCoNiCrMo0.2 HEA increased with decrease of average size of cellular structures, and the values of them were sensitive to strain rates. The energy absorption, compressive strength and yield strength of the SLM-FeCoNiCrMo0.2 HEAs reached 315.6 MJ/m³, 2.2 GPa and 775.6 MPa, respectively at a strain rate of 2,420 s⁻¹, under the process parameters of laser power and scanning speed of 330 W and 800 mm/s, respectively, where the corresponding average size of cellular structures in the HEAs was 483.6 nm. The value of strain-hardening rate of the SLM-FeCoNiCrMo0.2 HEA was about 5.1 GPa at a strain level of 0.1, which was much higher than that of the powder-metallurgy FeCoNiCrMo0.2 HEA. The cellular structure was formed inside the molten pool with segregation of Mo on the boundary. Deformation localization appeared in the cellular structures, forming several deformation bands after high strain-rate deformation. The elemental segregation strengthening and dislocation strengthening are considered to be the main strengthening mechanisms in SLM-FeCoNiCrMo0.2 HEA.
High-entropy alloys (HEAs) containing nanotwins exhibit simultaneous high strength and great plasticity. The instantaneous formation mechanism of nanotwins in a shear band in the hot-extruded FeCoNiCrMo0.2 HEA has been investigated. A series of material characterization and dynamic-loading experiments were carried out to study microstructures and dynamic-mechanical behavior of this alloy. The FeCoNiCrMo0.2 HEA is a single face-centered cubic (FCC) crystallographic phase and has coarse twins with sizes of about 20 μm. A shear band with a width about 25 μm appeared in the FeCoNiCrMo0.2 HEA at the nominal strain of 19.60. The shear band consists of elongated sub-grains, nanotwins, and equiaxed grains. Some elongated sub-grains are composed of nanotwins lamellae. The shear band containing nanotwins has a high strength of about 1,500 MPa and good plasticity, and the true strain can reach 3.6. Moreover, nanotwins mainly exist in a lamellar form, and the thickness of these lamellae ranges from 1 nm to 7 nm. Nanotwins in the shear band are of a single FCC phase with Mo-poor contents at twin boundaries, and the twin plane is (111). Twins in the shear band and the matrix have the same orientation but different lattice constants. Severe shear deformation plays a key role in the instantaneous formation of deformation nanotwinning in the FeCoNiCrMo0.2 HEA.
In this study, high speed impacting tests are systematically conducted on a split Hopkinson pressure bar device to investigate the strain rate and temperature dependence of dynamic compressive properties of Ti-6Al-4 V (TC4) fabricated by selective laser melting (SLM), the ranges of strain rate and temperature are 2000–6000/s and 25–650 ℃, respectively. The results reveal that the yield strength and ultimate compressive strength of the SLM-TC4 alloy increase with the increasing strain rate and the decreasing temperature, showing obvious strain rate and temperature sensitivities. The high speed impacting load intensifies the texture of the SLM-TC4 alloy significantly. Adiabatic shear band (ASB) is more likely to evolve at higher temperatures and strain rates, submicron equiaxed grains formed in the ASB and the surrounding area are mainly ascribed to the combination of dynamic recrystallization, deformation-induced twinning and transverse α-lath splitting. Within the ASB, grains with {0001} pole orientation are rotated by approximately 45° with respect to the shear direction, indicating that the recrystallized grains are able to reorient themselves to accommodate to the shear deformation. The findings in this work provide a theoretical basis to understand the deformation behavior and mechanism of SLM-TC4 alloy under impacting loads, thus is helpful to widen the application of SLM technique and products.
In this study, we have investigated the effects of heat treatment and loading direction on anisotropic compressive behaviour of additively manufactured (AM) Ti–6Al–4V ELI (extra low interstitials). AM samples were fabricated by powder bed fusion (PBF) processing, heat-treated between 800 °C and 950 °C, and compressed along two directions (i.e. parallel and perpendicular to the building direction) with a strain rate of 10⁻³·s⁻¹ at room temperature. The heat treatment effectively leads to increasing ductility of as-built by decomposing the α′ phase to α and β phases. The heat-treated samples show a significantly lower yield strength than the as-built sample, which has a higher pre-existing dislocation density. As the annealing temperature increases, the yield strength decreased with a broadening of α lath thickness. Furthermore, perpendicular loaded samples have significantly higher strain hardening rate and ductility than parallel loaded samples. However, most of these anisotropic responses disappeared with indistinct morphologies. Anisotropic properties can be ascribed to the directional features of the microstructure resulting from the additive manufacturing process.
High-entropy alloys (HEAs), recently emerging alloy materials with numerous excellent performances, may have a wide application prospect in impact engineering. However, previous research regarding the mechanical behavior of HEAs has primarily focused on quasi-static testing, whereas the dynamic mechanical behavior of HEAs at high strain rates remains elusive. In this paper, the unusual simultaneous strength-plasticity enhancement and the inhibition of the high strain rate embrittlement of CrMnFeCoNi HEA in impact tension were revealed via split Hopkinson tensile bar (SHTB) with high-speed photography. Quantitative microstructural analysis indicates that the cooperation of twins and dislocations is the crucial mechanism for the synchronous enhancement of strength-plasticity in this alloy under impact tension. A thermo-viscoplastic constitutive model based on dislocations and twins evolution was developed to describe dynamic mechanical behavior. The high plastic hardening under dynamic tension was revealed to be induced by high dislocation forest hardening and strong resistance of twins to dislocation motion. The excellent combination of dynamic strength-plasticity of CrMnFeCoNi HEA makes it becoming a promising candidate for impact engineering applications.
Deformation behavior of a FeCrNi medium entropy alloy (MEA) prepared by powder metallurgy (P/M) method was investigated over a wide range of strain rates. The FeCrNi MEA exhibits high strain-hardening ability, which can be attributed to the multiple deformation mechanisms, including dislocation slip, deformation induced stacking fault and mechanical twinning. The shear localization behavior of the FeCrNi MEA was also analyzed by dynamically loading hat-shaped specimens, and the distinct adiabatic shear band cannot be observed until the shear strain reaches ∼14.5. The microstructures within and outside the shear band exhibit different characteristics: the grains near the shear band are severely elongated and significantly refined by dislocation slip and twinning; inside the shear band, the initial coarse grains completely disappear, and transform into recrystallized ultrafine equiaxed grains by the classical rotational dynamic recrystallization mechanism. Moreover, microvoids preferentially nucleate in the central areas of the shear band where the temperature is very high and the shear stress is highly concentrated. These microvoids will coalesce into microcracks with the increase of strain, which eventually lead to the fracture of the shear band.
Cross-scale coordination
Laser-based additive manufacturing has the potential to revolutionize how components are designed. Gu et al. suggest moving away from a strategy that designs and builds components in a serial manner for a more wholistic method of optimization for metal parts. The authors summarize several key developments in laser powder bed fusion and directed energy deposition and outline a number of issues that still need to be overcome. A more integrated approach will help to reduce the number of steps required for fabrication and expand the types of structures available for end-use components.
Science , abg1487, this issue p. eabg1487
ABSRACT
Investigations into the microstructural characteristics and evolution mechanisms of 90W-Ni-Fe alloy during deformation under high strain rates were conducted using a Split Hopkinson Pressure Bar (SHPB) system. With increasing strain rate and strain the 90W-Ni-Fe alloy deformation behavior changes from homogenous plastic deformation to localized shear deformation resulting in adiabatic shear bands (ASBs). At strain rates below 4779s⁻¹, 90W-Ni-Fe alloy undergoes a homogenous deformation, and in the tungsten and γ- (Ni, Fe) phase the distribution of dislocations, dislocation tangles and the formation of substructures such as dislocation cell structures and subgrains appear randomly. Up to the strain rate of 6146s⁻¹, an ASB with a width of about 50μm is formed in the deformed specimen, and the microstructure in the ASBs undergoes significant dynamic recrystallization, which is more pronounced in the γ-(Ni, Fe) phase than in the tungsten phase. The evolution mechanism of the microstructure was determined through calculating the temperature rise of specimens during the deformation. A rotational dynamic recrystallization mechanism is proposed which provides a reasonable explanation for the development of the fine equiaxed grains in the ASBs. This study provides an important contribution to the design of self-sharpening tungsten alloy kinetic energy penetrators.
Three factors govern adiabatic shear localization: strain hardening (or softening), strain-rate hardening, and thermal softening. It is typically associated with large shear strains (>1), high strain rates (10³-10⁷ s⁻¹), and high temperatures (40-100% of melting point), all of which happen within narrow regions with widths of about 1-200 μm.
It is often an undesirable phenomenon, leading to failure, but there are situations where it is desirable, e. g., the generation of machining chips. Here, we review the development of both theoretical and experimental achievements, from the initiation of shear bands to their propagation with emphasis on three aspects: novel experimental techniques, novel materials, and nano/microstructural effects. The principal characteristics of adiabatic shear bands in metallic materials at the nano- and micro-scale are described. Bands that were formerly identified as transformed actually consist of nanocrystalline/ultrafine grains. These grains result from the breakup of the microstructure by a rotational recrystallization process.
The evolution of the microstructure inside shear bands and their interactions for hcp, bcc, and fcc alloys, high-entropy alloys, nanocrystalline alloys, and metallic glasses are analyzed mechanistically.
The gaps in the field and opportunities for future research are identified. Modern experimental characterization and computational techniques enable a more profound and predictive understanding of adiabatic localization and its avoidance in advanced materials.
The relative density of high-entropy alloys fabricated with selective laser melting (SLM) was enhanced via a process optimization method based on polynomial regression analysis. The resulting alloy exhibited a high relative density (99.71%) and excellent tensile properties (tensile strength = 720 MPa; post-fracture elongation = 31.85%). Optimization has been more commonly based on volumetric energy density (VED), but the unreliability of this method has been evidenced by the large difference in relative densities achieved using the VED-based approach. The high strength of alloy samples was investigated based on structural characterization, and the formation mechanisms of various defects are discussed. There is no micro-segregation or secondary phase according to both theoretical calculations and experimental observation. The strengthening mechanism of SLM FeCoCrNi sample has been reported by quantitative analysis of dislocation strengthening and fine grain strengthening. It was found that the dislocation strengthening contributed more than fine grain strengthening to the increased yield strength in SLM. This deeper insight into the processing and the strengthening mechanisms is expected to contribute to improved mechanical properties of SLM products, thereby increasing the potential for industrial applications of SLM.
CrMnFeCoNi high-entropy alloys (HEA) with various microstructures have been produced using cold rolling followed by critical annealing at various temperatures. Shear deformation behaviors of this HEA with various microstructures at a wide range of strain rates (2 × 10⁻³-5 × 10⁴ s⁻¹) have been characterized using hat-shaped specimens. Strain hardening exponent and strain rate sensitivity have been obtained for various microstructures. No shear localization was observed up to shear strain of 8 for all microstructures under lower strain rates (2 × 10⁻³-1 × 10¹ s⁻¹), while stress drop and shear localization were found to occur at various critical shear strains for various microstructures under dynamic shear loading (5 × 10⁴ s⁻¹). A new formula, considering competition of strain hardening and strain rate hardening against thermal softening, was proposed to estimate the critical shear strains under dynamic shear loading and the predicted results were found to be in a fairly good agreement with the experimental data. Based on micro-hardness testing, the strain hardening due to the microstructure evolution was found be much stronger under dynamic shear loading than that under quasi-static loading at the same interrupted shear strain, which can be attributed to the more efficient grain refinement and the triggered hierarchical deformation nanotwins under dynamic shear deformation.
Dynamic loading of the Fe50Mn30Co10Cr10 high-entropy alloy (HEA) was carried out using a split Hopkinson pressure bar at room temperature. The effects of microstructures and loading strain rates on the adiabatic shear susceptibility of an Fe50Mn30Co10Cr10 HEA were evaluated for the first time by comparing metallographic observation, the stress collapse time, the critical values of strain corresponding to the adiabatic shearing initiation, and the adiabatic shearing formation energy per unit volume. The results show that the adiabatic shear susceptibility increases with the increase of loading strain rate. The smaller the grain size and the more hexagonal close-packed (hcp) phase of the Fe50Mn30Co10Cr10 HEA, the greater the strain hardening effect and the stronger the localized deformation resistance under the same dynamic loading conditions, and the lower the adiabatic shear susceptibility.
To widen the applications of FeCoCrNi high-entropy alloys (HEAs) fabricated via selective laser melting, their mechanical properties must be improved, and annealing plays an important role in this regard. In this study, the microstructure, residual stress, and mechanical properties of the as-printed specimen and specimens annealed at 773–1573 K for 2 h were compared. As the annealing temperature increased, the specimen structure recrystallized from all columnar grains to equiaxial grains containing numerous annealing twins. The dislocation network, which formed during the solidification process under considerable shrinkage strain, decomposed into dislocations. The residual stress, yield strength, and hardness decreased, while the plasticity and impact toughness increased. During the deformation of as-printed and low-temperature-annealed specimens, the dislocation network remained unchanged and provided resistance to the dislocations moving within it, thus strengthening the specimen. The tensile strength remained largely unchanged owing to the reduction in the residual stress during low-temperature annealing, as well as the formation of the twinning network and dislocation wall under large deformation upon high-temperature annealing. Meanwhile, the ductility greatly increased, thus increasing the potential for industrial application of HEAs.
One of the most important issues related to dynamic shear localization is the correlation among the stress collapse, temperature elevation and adiabatic shear band (ASB) formation. In this work, the adiabatic shear failure process of pure titanium was investigated by dynamic shear-compression tests synchronically combined with high-speed photography and infrared temperature measurement. The time sequence of important events such as stress collapse, ASB initiation, temperature rise and crack formation was recorded. The key characteristics of ASB, such as width, critical strain, temperature, propagation speed and cooling rate were systematically studied. The maximum propagation velocity of ASB is found in this work to be about 1900 m/s, about 0.6Cs (Cs is the shear wave speed). The maximum temperature within ASB is in the range of 350–650 °C, while the material close to ASB is also heated. The cooling rate of ASB is on the order of 10⁶ °C/s, indicating that it needs a few hundreds of microseconds for the ASB to cool down to the ambient temperature. One important observation is that the apparent temperature rise occurs after ASB initiation, which indicates that it might not be the causation but the consequences of ASB. Further efforts are called for confirmation of this notion because of its significance.
A sample of 304L stainless steel manufactured by Laser Engineered Net Shaping (LENS) was characterized in 3D using TriBeam tomography. The crystallographic, structural, and chemical properties of the as-deposited microstructure have been studied in detail. 3D characterization reveals complex grain morphologies and large orientation gradients, in excess of 10∘, that are not easily interpreted from 2D cross-sections alone. Misorientations were calculated via a methodology that locates the initial location and orientation of grains that grow during the build process. For larger grains, misorientation increased along the direction of solidification. For grains with complex morphologies, K-means clustering in orientation space is demonstrated as a useful approach for determining the initial growth orientation. The gradients in misorientation directly tracked with gradients in chemistry predicted by a Scheil analysis. The accumulation of misorientation is linked to the solutal and thermal solidification path, offering potential design pathways for novel alloys more suited for additive manufacturing.
To identify the critical issues that affect the evolution of microstructure during additive manufacturing, we investigated the influence of process parameters on the evolution of the dimensional and surface quality, microstructure, internal defects, and mechanical properties in 316L stainless steel (SS) components fabricated using laser engineered net shaping (LENS®), a directed energy deposition (DED) additive manufacturing (AM) technique. The results show that the accumulation of un-melted powder particles on the side walls of deposited sections can be avoided by selecting a laser under-focused condition. Moreover, we report that the variation of melt pool width is more sensitive to laser power than to the depth of the melt pool. The formation of a so-called “hierarchical” microstructure with cellular morphology is attributable to a combination of layer deposition and rapid solidification, which are characteristics of AM. Finally, we discuss microstructure evolution and defect formation, particularly the formation of multiple interfaces and the presence of un-melted powder particles and pores, in light of the dynamic convective fluid flow and rapid solidification that occur in the melt pool. X-ray computed tomography (X-CT) was used to precisely map the spatial distribution of pores in the DED components. The evolution of microstructure during DED is discussed in the context of related thermal phenomena in an effort to provide fundamental insight into the mechanisms that govern defect formation.
The microstructure and texture evolution near the adiabatic shear band (ASB) in TC17 Titanium alloy, during hot compression, were studied by electron back-scattered diffraction (EBSD). The ASB was induced in the bulk cylindrical TC17 Titanium alloy with an initially equiaxed microstructure, by hot compression with a relatively low strain rate. When the equiaxed-microstructure TC17 Titanium alloy was compressed at 600 °C, 700 °C, and 850 °C, two kinds of ASB were observed: D-ASB, which is almost parallel to the diagonal of the longitudinal section, and H-ASB, which is almost parallel to the horizontal of the longitudinal section. Dynamic recrystallization took place in β phase grains at the ASB center, while dynamic recovery dominated at the transition region. The large angle grain boundaries (LAGBs) were mainly concentrated in β phase, while the small angle grain boundaries (SAGBs) were mainly concentrated in α phase distributed along the ASB direction. Meanwhile, both α and β phase grain sizes slightly increased with increasing hot compression temperature and deformation, except for the alloy compressed at 850 °C with 70% deformation due to the phase transformation. At the ASB center, prismatic textures existed in α phase grains except for the basal textures in the alloy deformed at 600 °C. In β phase grains, the {001} planes were always normal to CD, while the {111} planes were normal to CD in the initial microstructure of the TC17 alloy. Furthermore, in β phase grains, the cubic texture {100} 〈001〉 dominated in the TC17 alloy deformed at 600 °C, 700 °C, and 850 °C. With increasing compression temperature, the crystal plane parallel to the ASB direction in α phase grains changed from {0001} to {112¯0} while it was stable in β phase grains, which is not beneficial to the mechanical performance of this alloy.
One of the most important issues related to dynamic shear localization is the correlation among the stress collapse, temperature elevation and adiabatic shear band (ASB) formation. In this work, the adiabatic shear failure process of pure titanium was investigated by dynamic shear-compression tests synchronically combined with high-speed photography and infrared temperature measurement. The time sequence of stress collapse, ASB initiation, temperature rise and crack formation was recorded. The key characteristics of ASB, such as temperature, critical strain, propagation speed and cooling rate were systematically studied. The propagation velocity of ASB is dependent on the impact velocity and the maximum velocity is found in this work to be about 1900 m/s, about 0.6C s (C s is the shear wave speed). The maximum temperature within ASB is in the range of 350–650℃, while the material close to ASB is also heated. One important observation is that the apparent temperature rise occurs after ASB initiation, which indicates it could not be the causation but the consequences of ASB.
An equiatomic CoCrFeNiMn high-entropy alloy was prepared by induction melting and a progressive combination of mechanical alloying and compaction via spark plasma sintering done at temperatures of 800 °C and 1000 °C. The chosen methods of preparation had a significant impact on the microstructure and mechanical properties of the alloy. In comparison, the as-cast alloy had a much coarser microstructure while simultaneously obtaining inferior mechanical properties compared to those of the 8-h mechanically alloyed and spark plasma sintered alloy compacted at 1000 °C, which achieved a hardness of 424 ± 7 HV, and the alloy compacted at 800 °C showed a lower but still highly comparable hardness of 352 ± 12 HV. Both alloys showed good thermal stability, as expressed by almost negligible hardness changes during 100 h of annealing at temperatures of 400 °C and 600 °C. The investigated alloys also showed their superiority during compressive stress-strain tests at ambient and elevated temperatures of 400 °C and 600 °C. At ambient temperature, the highest compressive yield strength of 1534 MPa was observed for the sample compacted at 800 °C. As the temperature of the compressive test increased, the investigated alloys reduced their compressive yield strengths.
Dynamic recrystallization (DRX) is often observed in conjunction with adiabatic shear banding (ASB) in polycrystalline materials. The recrystallized nanograins in the shear band have few dislocations compared to the material outside of the shear band. In this paper, we reformulate the recently-developed Langer-Bouchbinder-Lookman (LBL) continuum theory of polycrystalline plasticity and include the creation of grain boundaries. While the shear-banding instability emerges because thermal heating is faster than heat dissipation, recrystallization is interpreted as an entropic effect arising from the competition between dislocation creation and grain boundary formation. We show that our theory closely matches recent results in sheared ultrafine-grained titanium. The theory thus provides a thermodynamically consistent way to systematically describe the formation of shear bands and recrystallized grains therein.
In this paper, a selective laser melting (SLM) physical model describing the melt pool dynamics and the response of downward-facing surface morphology evolution of overhanging structure under different laser processing conditions was proposed, in which an enormous difference in thermal conductivity and laser absorption capacity between the as-fabricated part and powder material was taken into consideration. The underlying thermal physical mechanism of the dross formation phenomenon during SLM preparing overhanging surface was revealed by numerical simulation analysis and experimental studies. It was found that both high and low laser volume energy density (ω) resulted in an inferior downward-facing surface quality. As an optimal processing parameter (60–80 J/mm³) was settled, the overhanging structure obtained a relatively smooth downward-facing surface due to the sound melt pool dimension and steady melt flow behavior. The experimental studies were compared with the simulated results, showing a good agreement with the predictions obtained in the simulations. It was interesting to find that the variation rules of surface quality and densification level of overhanging structure with different ω were exactly converse. As the ω decreased from 80 J/mm³ to 60 J/mm³, the surface roughness could be reduced from 59 μm to 33 μm while, contrarily, the porosity was elevated from 3.2% to 8.4%. In order to fabricate complicated metal parts with lower risk, four solutions for improving the processability of hard-to-process overhanging structure were provided.
The plastic work-heat conversion coefficient is one key parameter for studying the work-heat conversion under dynamic deformation of materials. To explore this coefficient of 7075-T651 aluminum alloy under dynamic compression, dynamic compression experiments using the Hopkinson bar under four groups of strain rates were conducted, and the temperature signals were measured by constructing a transient infrared temperature measurement system. According to stress versus strain data as well as the corresponding temperature data obtained through the experiments, the influences of the strain and the strain rate on the coefficient of plastic work converted to heat were analyzed. The experimental results show that the coefficient of plastic work converted to heat of 7075-T651 aluminum alloy is not a constant at the range of 0.85–1 and is closely related to the strain and the strain rate. The change of internal structure of material under high strain rate reduces its energy storage capacity, and makes almost all plastic work convert into heat. © 2017 The Chinese Society of Theoretical and Applied Mechanics; Institute of Mechanics, Chinese Academy of Sciences and Springer-Verlag Berlin Heidelberg
To enhance the toughness of metal matrix nanocomposites, we demonstrate a strategy that involves the introduction of spatial arrays of nanoparticles. Specifically, we describe an approach to synthesize a microstructure characterized by arrays of fiber-like nanoparticle-rich (NPR) zones that contain spherical nanoparticles of B4C (sn-B4C) embedded in an ultrafine grained (UFG) aluminum alloy matrix. A combination of cryomilling and hot-extrusion was used to obtain this particular microstructure, and the mechanical behavior and operative strengthening and deformation mechanisms were investigated in detail. When compared to an equivalent unreinforced material, the presence of the array of NPR zones contributed to a 26% increase in tensile strength. Moreover, when compared to a nanocomposite containing a homogeneous distribution of nanoparticles, a 30% increase in toughness was observed. High nanohardness values obtained for the NPR zones and the observation of “pull-out” phenomena on fracture surfaces, suggest that the NPR zones behave as “hard” fiber-like units that can effectively sustain tensile loading and thereby enhance the strengthening efficiency of sn-B4C. Also, the presence of the array of NPR zones surrounded by nanoparticle-free (NPF) zones led to an enhancement in strength with limited loss in ductility. This behavior was rationalized on the basis of a low value of the Schmid factor in regions adjacent to NPR zones, coupled with the ease of dislocation movement in NPF zones. Finally, the ratio of the plastic zone size to the size of the “hard” NPR zones is proposed as an important factor that governs the overall toughness of the nanocomposite.
In the present work, the engineering, high strength ARMOX500T steel was submitted to a Kalthoff and Winkler type impact test in view of evaluating its crack arrest capability under dynamic loading. From an impact velocity of the order of 150 m/s, the crack propagation is seen to be preceded by adiabatic shear banding (ASB) leading to a premature plate failure. The whole chronology of the plate failure mechanisms (weak localization, strong localization in the form of ASB then cracking) was observed thanks to the use of an ultra-high speed camera. Further digital image analysis allows for establishing displacement fields describing the kinematics induced by both adiabatic shear banding and crack propagation, in the perspective of being implemented into an embedded band/crack based model in the context of dynamic plasticity and fracture.
The equiatomic CoCrFeMnNi alloy is now regarded as a model face-centered cubic single-phase high-entropy alloy. Therefore, determination of its intrinsic properties such as the temperature dependencies of elastic moduli and thermal expansion coefficient are important to improve understanding of this new class of material. These temperature dependencies were measured over a large temperature range (200-1270 K) in this study.
Numerous studies have examined the microstructural evolution of adiabatic shear bands through the utilization of the forced shear or “tophat” test specimen. While the geometry of this specimen does not allow for the microstructure to play a dominant role in the location of a shear band, the forced shear specimen has been shown to be particularly useful for characterizing the influence of parameters such as strain rate, temperature, strain, and load on the microstructural evolution within a shear band. Additionally, many studies have also utilized this geometry to advance the understanding of shear band development in a number of materials.In this study we systematically examine the influence of integrated loading states on the dynamic shear localization response of high-purity Fe by varying the geometry of the forced shear specimen. Post-mortem characterization was performed to quantify the width of the localizations and to examine the microstructural and textural evolution of shear deformation in a body centered cubic (BCC) metal. Increased instability in mechanical response is strongly correlated with development of enhanced intergranular misorientations and high angle boundary evolution. Stress state was also critical to the localization process. Single-component, simple shear configurations were found to promote instability over multi-component stress states. In addition, these geometries resulted in traditional BCC deformation shear textures, while multi-component stress states led to less developed textures.