Theoretical study on the influence of grain size on the strength, toughness and plastic deformation mechanism of pre-cracked polycrystalline high entropy alloys

Abstract

High entropy alloys (HEAs) is a kind of structural material with excellent mechanical properties. The material exhibits excellent damage tolerance at low temperature, which is closely related to its potential crack tip plastic mechanism and overall fracture mode. In the research, the plastic deformation and fracture mechanical properties of polycrystalline CrMnFeCoNi HEAs with pre-crack under type I tensile load were investigated by molecular dynamics (MD) simulation. The effects of grain size and crack length on the strength, toughness and microstructural deformation of the alloy were analyzed. It is found that with the increase of grain size, the strength of the material increases and the toughness decreases. With the increase of crack length, the strength and toughness decrease, and the influence of grain size on the strength is weakened. The plastic mechanism of crack initiation in nanocrystalline HEAs is ductile cracking under the joint action of dislocations, amorphization and grain boundary plasticity, which provides high damage tolerance. As the grain size increases, additional cavities appear near the crack tip or at the grain boundary, resulting in brittle fracture mode along the grain boundary or at the end of the cavity, which greatly affects the plastic properties of the material. The initial crack length has a significant effect on the transformation of the fracture mode. With the increase of the initial crack size, the critical grain size of the brittle fracture mode increases, and its proportion in the whole fracture mode decreases. This study provides a theoretical reference for the design and application of nanocrystalline HEAs.