Plasticity In High-entropy Alloys Research Articles

In situ atomic-scale observation of deformation-induced reversible martensitic transformation in a CrMnFeCoNi high entropy alloy

High entropy alloys (HEAs) have attracted much attention for their excellent mechanical properties stemming from diverse deformation mechanisms. Particularly, face-centered cubic (FCC) to body-centered cubic (BCC) martensitic transformation is crucial for enhancing the strength and plasticity of HEAs, particularly at cryogenic temperatures. However, the fundamental atomic mechanism underlying martensitic transformation remains elusive, and the impact of martensitic transformation on the mechanical properties of HEAs at room temperature is unknown. Here, we report in situ atomic-scale observations of a reversible martensitic transformation from FCC to body-centered tetragonal (BCT) and ultimately back to FCC in nanostructured CrMnFeCoNi HEA at room temperature under deformation. This martensitic transformation is completed by the synergistic action of 90° partial dislocations slip on (111)FCC plane and atom shuffling, involving the periodic arrangement and slip of two 90° half Shockley partial dislocations a/12[1¯1¯2](111) and one 90° Shockley partial dislocation –a/6[1¯1¯2](111) on three successive (111)FCC atomic planes. Additionally, the reversible phase transformation induced by high stress dissipates strain energies and hinders crack propagation, thereby enhancing the fracture toughness of HEAs. Our findings contribute to a deeper comprehension of the martensitic transformation mechanisms in HEAs, offering valuable insights for improving their mechanical properties.

Nanoindentation into a bcc high-entropy HfNbTaTiZr alloy-an atomistic study of the effect of short-range order.

The plastic response of the Senkov HfNbTaTiZr high-entropy alloy is explored by means of simulated nanoindentation tests. Both a random alloy and an alloy with chemical short-range order are investigated and compared to the well understood case of an elementary Ta crystal. Strong differences in the dislocation plasticity between the alloys and the elementary Ta crystal are found. The high-entropy alloys show only little relaxation of the indentation dislocation network after indenter retraction and only negligible dislocation emission into the sample interior. Short-range order-besides making the alloy both stiffer and harder-further increases the size of the plastic zone and the dislocation density there. These features are explained by the slow dislocation migration in these alloys. Also, the short-range-ordered alloy features no twinning plasticity in contrast to the random alloy, while elemental Ta exhibits twinning under high stress but detwins considerably under stress relief. The results are in good qualitative agreement with our current knowledge of plasticity in high-entropy alloys.

Modulating grain boundary-mediated plasticity of high-entropy alloys via chemo-mechanical coupling

Intrinsic chemical disorder and the subsequent interfacial roughening have posed formidable challenges in elucidating the grain boundary (GB)-mediated plasticity in high-entropy alloys (HEAs). Here using a self-propelling atomistic algorithm to probe the complex energy landscape of the CoCrFeMnNi HEAs, in conjunction with location-specific perturbations across GBs exposed to different environments, we investigate atomic-reconfiguration ensembles near GBs and their sensitivities to various chemo-mechanical conditions. Two distinct modes, collective and random, are discovered, and their partitions are dictated by multiple factors, including the activation energy window, external mechanical loading, and local compositions. Remarkably, Fe disproportionately promotes the collective events and facilitates the slip activities near GBs, while Cr atoms suppress the emission of partial dislocations from GBs. These findings imply promising solutions – via synergistic combination of microalloying, heat treatment, and mechanical loading – to selectively trigger desired plasticity modes at needed deformation stage, and hence to achieve an enhanced tunability of HEAs’ mechanical behaviors.

Plastic behavior of a nanoporous high-entropy alloy under compression

Nanoporous High-entropy alloys (HEA) have attracted increasing attention in recent years due to their remarkable mechanical properties and their capability as storage devices. However, chemical complexity involves fluctuations in the atomic environment, hindering the analysis of plasticity. Here, we perform molecular dynamics simulations of an equiatomic nanoporous single crystal FeCrNiCuCo HEA under compression to unveil the role of pores in the plastic behavior, including a random sample and a sample with short-range order. These results are compared to a nanoporous single crystal Average Atom (AA) sample with the same topology to assess the effect of chemical complexity. We find that the overall elastic and plastic regimes are similar in all samples, in contrast to previous reports for tensile tests. However, some differences can be distinguished between HEA samples and AA. Chemical complexity in the HEA favors dislocation nucleation and larger twinning activity, leading to a faster reduction of pores as reflected by the increase in relative density and decrease in surface-to-volume ratio. Following machine learning methods, linear vacancy clusters were found in all samples. These clusters exhibited a perfectly linear shape in AA and a less-defined shape in the HEA samples. Thus, our work provides new insights into the effect of chemical complexity and nanopores on the plasticity of HEAs under compression.

Microstructure and defect effects on strength and hydrogen embrittlement of high-entropy alloy CrMnFeCoNi processed by high-pressure torsion

High-entropy alloys (HEAs) are considered as new hydrogen compatible materials, but enhancing their yield strength without deteriorating their hydrogen embrittlement resistance is challenging. In this study, various kinds of defects are introduced into a CrMnFeCoNi Cantor alloy by plastic straining via the high-pressure torsion method, and the correlations of applied strain, microstructural features, strength, and hydrogen embrittlement are studied. The unstrained coarse-grained alloy shows elongations over 80% under hydrogen, but its yield strength is only 220 MPa. Twinning is a major deformation mechanism at the early stages of straining, resulting in over 1 GPa yield strength and 9% elongation in the presence of hydrogen. With a further increase in strain, dislocation-based defects including Lomer-Cottrell locks and D-Frank partial dislocations with low mobility are formed, enhancing the strength further. At large strains, nanograins with high-angle boundaries are generated, resulting in over 1900 MPa strength with poor hydrogen embrittlement resistance due to large hydrogen diffusion and hydrogen-enhanced decohesion. These results suggest that twins and defects with low mobility such as Lomer-Cottrell locks and D-Frank partial dislocations are effective to achieve a combination of high yield strength and good hydrogen embrittlement resistance by suppressing the hydrogen-enhanced localized plasticity in HEAs.

Dislocation emission and propagation under a nano-indenter in a model high entropy alloy

The nanoindentation response of a FeNiCrCoCu high-entropy alloy is explored by means of atomistic simulations. In order to study the role of compositional complexity, we compare the behavior of the quinary FeNiCrCoCu alloy with that of an average atom potential fitted to the same overall properties of the alloy. In this way, we reveal the influence of compositional randomness of the multicomponent alloys on the onset of plasticity, deformation mechanisms and residual microstructure. Plasticity is dominated by dislocation activity, with twinning appearing as a complementary deformation mechanism in both models. While the HEA alloy substrate yields for a similar indenter penetration, indentation hardness is higher in the HEA alloy. After mitigation of strain rate effects, major differences are observed, influenced by the more difficult movement of dislocations in the compositionally complex HEA alloy. The results are in agreement with previous studies of plasticity in high entropy alloys and highlight the importance of using average atom models over comparison with single element studies for understanding complex multi principal element alloys.

Bimodality of incipient plastic strength in face-centered cubic high-entropy alloys

ABSTRACT Spherical tip nanoindentation experiments on two typical face-centered cubic high-entropy alloys (HEAs), CoCrFeNi and CoCrFeMnNi in as-solutionized and aged conditions were performed using tips of two different radii. Large datasets of the strength at the first pop-in were obtained and their statistical nature were analyzed to gain insights into the micromechanisms responsible for the onset of incipient plasticity in HEAs that are notionally monophasic. In all the cases examined, the probability density distributions were bimodal in nature. The deconvoluted distributions were utilized to estimate the activation volumes of the underlying deformation mechanisms. They show that when the probed material's volume is relatively small, heterogeneous dislocation nucleation aided by monovacancies occurs at lower indentation stresses; this followed by homogeneous dislocation nucleation at high loads, resulting in strengths corresponding to the theoretical strengths. When a substantially larger volume is sampled, by using a larger radius tip, either the preexisting dislocation mediated ones at low stresses or vacancy cluster /grain boundary aided heterogeneous dislocations nucleation at higher stresses become dominant. Increasing the chemical short-range order in the alloy via high temperature aging leads to overall strengthening of the alloy by enhancing stress required for the homogeneous dislocation nucleation. Implications of such plurality of mechanisms with overlapping strength distributions at HEA's disposal in imparting high strength-ductility combinations are discussed.

Nanoindentation into a high-entropy alloy – An atomistic study

The plastic response of a high-entropy alloy is explored by means of simulated nanoindentation tests. We compare nanoindentation into the Cantor alloy – an equi-atomic CoCrFeMnNi fcc solid solution – with that into an elemental Ni fcc single crystal. Using molecular dynamics simulation, strong differences in the plastic behavior are identified. While the total length of the dislocation network in the Cantor alloy is higher than in Ni, the size of the plastic zone is considerably restricted. Dislocations are more localized in the Cantor alloy, both during indentation, but also during retraction of the indenter; emission of prismatic loops is absent in the Cantor alloy. Besides dislocation-mediated plasticity, considerable twinning occurs. No amorphization could be observed. These simulation results are in good qualitative agreement with our current knowledge of plasticity in high entropy alloys.

Atomistic modeling of nanoscale plasticity in high-entropy alloys

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Size effects on plasticity in high-entropy alloys

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Probing deformation mechanisms of a FeCoCrNi high-entropy alloy at 293 and 77 K using in situ neutron diffraction

The deformation responses at 77 and 293 K of a FeCoNiCr high-entropy alloy, produced by a powder metallurgy route, are investigated using in situ neutron diffraction and correlative transmission electron microscopy. The strength and ductility of the alloy are significant improved at cryogenic temperatures. The true ultimate tensile strength and total elongation increased from 980 MPa to 45% at 293 K to 1725 MPa and 55% at 77 K, respectively. The evolutions of lattice strain, stacking fault probability, and dislocation density were determined via quantifying the in situ neutron diffraction measurements. The results demonstrate that the alloy has a much higher tendency to form stacking faults and mechanical twins as the deformation temperature drops, which is due to the decrease of stacking fault energy (estimated to be 32.5 mJ/m2 and 13 mJ/m2 at 293 and 77 K, respectively). The increased volume faction of nano-twins and twin-twin intersections, formed during cryogenic temperature deformation, has been confirmed by transmission electron microscopy analysis. The enhanced strength and ductility at cryogenic temperatures can be attributed to the increased density of dislocations and nano-twins. The findings provide a fundamental understanding of underlying governing mechanistic mechanisms for the twinning induced plasticity in high entropy alloys, paving the way for the development of new alloys with superb resistance to cryogenic environments.

Transformation induced softening and plasticity in high entropy alloys

Nanostructured high-entropy alloys (HEAs) with excellent properties have recently attracted extensive attention from both academia and industry in recent years. However, as a consequence of the resolution limitations of existing experimental techniques, the effects of dynamical microstructure on the strength-and-plasticity properties in nanoscale HEAs is still not well understood. Here we report the kinetics of strain induced phase transformation from face-centered cubic to body-centered cubic (fcc→bcc) in the single-crystal and nanocrystalline HEA investigated by molecular dynamics simulations. The fcc→bcc phase transformation in HEA occurs when the critical stress induced by severe lattice distortion combined with high local strain effect exceeds the stress required for the nucleation of bcc phase in the matrix. In addition, the emergence of this phase transformation is dominated by strain rate and initial orientation, where maintaining high strength in nanocrystalline HEA is achieved without the sacrifice of ductility attributable to the strain-induced fcc→bcc transformation. Interestingly, owing to the structural dissipation and strain relaxation by the volume expansion, the phase transformation agreeing with N-W relationship could convert the stress state from tensile to compressive in single-crystal HEA. Therefore, the evolution of microstructures and fcc→bcc phase fractions is studied to provide a micromechanical understanding of phase transformation induced plasticity responsible for the combination of high strength and high ductility.