Strain-rate effects on the mechanical behavior of high-entropy alloys: A focused review

Abstract

To address one of the key challenge areas associated with high-entropy alloys (HEAs)— “Scattered Data with Uncertain Materials Pedigree”, as highlighted in the TMS accelerator study in 2021: Defining Pathways for Realizing the Revolutionary Potential of High Entropy Alloys, this review collates HEA mechanical data over strain-rates, $\dot{\epsilon }$$\dot{\epsilon }$, between 10^-5 and 10⁵ s^-1. We focus the aggregated data on coarse-grained HEAs to isolate processing pathway and grain-size effects, identify uncharted regimes, and establish a strong strain-rate–yield strength relationship. We evaluate the deformation mechanisms in HEAs and develop a deformation mechanism map for FCC-HEA—CoCrFeMnNi. With a brief discussion on strengthening mechanisms and evaluation of aggregated data, we develop simple yield-strength prediction models for FCC $[{\sigma }_{y,model}^{\mathit{FCC}-HEA}=(0.0245{\dot{\epsilon }}^{\frac{1}{4}}+0.1171)\ast T_m$$[{\sigma }_{y,model}^{\mathit{FCC}-HEA}=(0.0245{\dot{\epsilon }}^{\frac{1}{4}}+0.1171)\ast T_m$] and BCC [${\sigma }_{y,model}^{\mathit{BCC}-HEA}=(0.0445{\dot{\epsilon }}^{\frac{1}{4}}+0.5075)\ast T_m$${\sigma }_{y,model}^{\mathit{BCC}-HEA}=(0.0445{\dot{\epsilon }}^{\frac{1}{4}}+0.5075)\ast T_m$] HEAs; $T_m$$T_m$—melting point. These models are simple with parameters that can easily be determined from HEA composition and test condition, yet they capture the essential physics related to bond strength and yield strength; moreover, the models can be coupled with other strengthening sources. Finally, the deformation kinetics of HEAs are examined: the activation-volume range in the thermal-activation regime for FCC-HEAs is 10-100b³ (about one-magnitude lower than conventional FCC metals—100-1000b³), while BCC-HEAs are within the activation-volume range for conventional BCC metals. The activation-volume range for both FCC and BCC-HEAs is the same—0-10b³ in the viscous phonon-drag regime, which is not well documented. In general, this review shows that HEA mechanical data are aggregable to establish a strong trend observed in deformed HEAs despite their compositionally-complex nature.