Deformation twinning and feature size mediated strain hardening behavior in a medium-entropy alloy at the mesoscopic scale

MP35N medium-entropy alloy with low stacking fault energy (SFE) is widely used in the biomedical field due to its excellent biocompatibility and mechanical property matching. With the miniaturization of products in the biomedical field, the deformation behavior of MP35N alloy has changed, which is different from the traditional plastic deformation at the macroscopic scale. In the present paper, tensile experiments of MP35N alloy at the deformation of 293 K and 93 K at the mesoscopic scale were carried out to analyze its mechanical properties and deformation mechanism. The results show that the reduction of the deformation temperature leads to a synergistic increase in the strength and plasticity of MP35N alloy. The feature size (ratio of sample thickness to grain size, t/d) and deformation twins are decisive in the mesoscopic deformation process. At the same deformation temperature, the reduction of the feature size makes the deformation twin occupy a dominant position in the competition with dislocations. At the deformation temperature of 93 K, the strain corresponding to the first inflection point decreases from 0.18 to 0.12 as t/d decreases from 8.21 to 1.52. However, the alloy with t/d of 8.21 preferentially produces the second inflection point. The decrease in deformation temperature drives the inflection point down to a larger strain for the same feature size. The reduction of deformation temperature gives dislocations an advantage in competition with deformation twins in the initial deformation stage. The decrease in deformation temperature promotes the emergence of multi-level deformation twin structures in the later deformation stage, causing the second inflection point to move toward larger strains.