Nanoindentation behavior of high entropy alloys with transformation-induced plasticity

Abstract

Nanoindentation of three metastable dual-phase high entropy alloys (HEAs) was performed to obtain their inherent elastoplastic deformation responses. Excellent combination of hardness and elastic modulus in as-cast condition confirmed that, their inherently higher strength compared to other HEAs reported in literature, can be attributed to alloy chemistry induced phase stability. Further, hardness of 8.28 GPa combined with modulus of 221.8 GPa was obtained in Fe-Mn-Co-Cr-Si-Cu HEA by annealing the as-cast material, which is the best hardness-modulus combination obtained to date in HEAs from nanoindentation. On the other hand, although Fe-Mn-Co-Cr-Si HEA showed lower hardness and modulus than Fe-Mn-Co-Cr-Si-Al and Fe-Mn-Co-Cr-Si-Cu HEAs, the former alloy exhibited the highest strain rate sensitivity, as determined from tests performed at five different strain rates. The three alloys also had subtle differences in incipient plasticity and elastoplastic behavior, while retaining similar levels of hardness; and nanoindentation response showed microstructural dependence in friction stir processed, annealed and tensile-deformed specimens. Thus, the study highlighted that while higher strength was achieved by designing a class of HEAs with similar composition, any of the individual alloys can be tuned to obtain enhanced properties.

Nanomechanical Characterization of the Deformation Response of Orthotropic Ti–6Al–4V

“…where E and ν are the elastic modulus and Poisson's ratio of the indented material, while E i and ν i are the elastic constants of the indenter. The Oliver and Pharr [40] method is widely used to determine the elastic modulus of indented materials with relatively good results; [32,35,44,45,47,54,55,60,77,78] however, it simplifies the elastic behavior of the material, treating it as entirely isotropic which is not suitable for accurate characterization of orthotropic elastic materials.…”

“…[ 40,41 ] This method requires relatively small samples in comparison with other methods, and allows the mechanical properties of a wide range of materials to be obtained, from thick solid specimens to thin film coatings of metallic or polymer sheets. [ 35,40,42–48 ]…”

“…[40,41] This method requires relatively small samples in comparison with other methods, and allows the mechanical properties of a wide range of materials to be obtained, from thick solid specimens to thin film coatings of metallic or polymer sheets. [35,40,[42][43][44][45][46][47][48] Indentation procedures have for long been used for determining the surface hardness of metallic materials. The acknowledging of an elastic behavior during the return of the indentation procedure led to the mathematical approach and further development of methods to obtain the elastic moduli of the indented material.…”

On the evolving nature of c/a ratio in a hexagonal close-packed epsilon martensite phase in transformative high entropy alloys

“…The amount of ε undergoing elastoplastic deformation at low strain is different in the two alloys. Nanoindentation of as-cast CS-HEA and Al-HEA in our previous work 26 illustrated that Al-HEA shows greater inherent resistance to deformation in the elastoplastic regime than CS-HEA (characterized by larger elastic regions between short displacement bursts in Al-HEA than CS-HEA). Therefore, higher volume of newly formed ε undergoes elastoplastic deformation in Al-HEA but the magnitude of distortion is low.…”

Additive friction stir deposition: a deformation processing route to metal additive manufacturing

“…Severe plastic deformation processed nanocrystalline and ultrafine grained alloys typically lack work hardening and the associated uniform elongation regime [70]. AFSD processing of low stacking fault energy alloys may activate the twinning or transformation-induced plasticity mechanisms to break down the strength-ductility trade-off paradigm [71,72].…”