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.
Highlights
Introducing transformation induced plasticity (TRIP) and twinning induced plasticity (TWIP) effects in high entropy alloys (HEAs) enabled development of HEAs with tunable compositions and microstructures and superior mechanical properties[1,2,3]
Cu-HEA shows the highest hardness among the three alloys (6.83 GPa), while Al-HEA exhibits highest modulus of 205.7 GPa
They showed the variation of elastic modulus with alloy chemistry when all five compositions are in the same phase, as well as variation with f.c.c. or h.c.p. crystal structure for a particular alloy composition
Summary
Introducing transformation induced plasticity (TRIP) and twinning induced plasticity (TWIP) effects in high entropy alloys (HEAs) enabled development of HEAs with tunable compositions and microstructures and superior mechanical properties[1,2,3]. The advantages of nanoindentation are that the onset of plastic deformation and the regime of elastic-plastic transition can be captured in detail, and mechanisms at the nanoscale can be studied The latter is of utmost importance in nanocrystalline materials and fine structures like the friction stir processed (FSP) TRIP-HEAs that we are currently studying. The premise for studying these three alloys is that we can obtain the nanoindentation properties (hardness and elastic modulus) of TRIP-HEAs as a class of HEAs, yet capture the subtle differences that result from alloy chemistry-induced phase stability To achieve this objective, and to elaborate the inherent incipient plasticity of these alloys, the comparison of nanoindentation properties is focused on as-cast materials. Some specific experiments on thermomechanically processed (FSP and annealed) and tensile deformed specimens are included in this study
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