Dynamically compressive behaviors and plastic mechanisms of a CrCoNi medium entropy alloy at various temperatures

CrCoNi中熵合金在不同温度下的动态压缩行为及塑性变形机制

As an attractive class of metallic materials,single-phase CrCoNi medium-entropy alloy(MEA)has drawn much attention recently regarding their deformation behaviors,but the dynamically mechanical responses of this alloy at high strain rates remain less studied,especially coupled with extremely low temperatures.In this study,the dynamic deformation behaviors of this CrCoNi MEA were systematically investigated at room temperature(RT)of 298 K and liquid nitrogen temperature(LNT)of 77 K using the split Hopkinson pressure bar(SHPB).This alloy exhibited a combination of higher yield strength and stronger hardening rate upon dynamic compressive deformation when the loading conditions become much harsher(higher strain rate or lower temperature).Detailed microstructure analyses indicated that the strong strain hardening ability during dynamic deformation was mainly attributed to the continuous formation of nanoscale deformation twins.Furthermore,as loaded at LNT,multi-directional deformation twins were activated.Meanwhile,due to the interaction between Shockley partial dislocations and twin boundaries,large-sized deformation-induced FCC-HCP phase transformations at a micrometer scale were also observed within the grains,which not only accommodated the plasticity but also played an important role in improving the hardening capability owing to the appearance of newly generated interfaces.

作为一种极具吸引力的金属材料,单相CrCoNi中熵合金(medium-entropy alloy,MEA)的变形行为最近引起了广泛的关注,但该合金在高应变率,尤其是高应变率低温耦合条件下的力学响应目前仍然鲜有报道.本研究使用分离式霍普金森压杆系统地研究了CrCoNi MEA在室温(room temperature,RT)和液氮温度(liquid nitrogen temperature,LNT)下的动态变形行为.随着应变率升高或温度的降低,该合金在动态压缩时表现出更高的屈服强度(yield strength,YS)和应变硬化率.详细的微观结构表征发现,动态变形过程中较强的应变硬化能力主要归因于纳米级变形孪晶的连续形成.此外,当加载温度降低到LNT时,多取向变形孪晶被激活.同时,由于肖克莱不全位错与孪晶界的相互作用,在晶粒内也观察到了大尺寸的变形诱导的FCC-HCP相变,这不仅容纳了塑性,新形成的界面还会提高合金的应变硬化能力.