Fracture mechanisms of a Mo alloyed CoCrFeNi high entropy alloy: In-situ SEM investigation

This paper is focused on an experimental investigation on the fracture property and mechanisms of a Mo alloyed high entropy alloy (HEA) CoCrFeNiMo0.2 that demonstrates a promising damage tolerance capacity under room temperature. The employed experimental means is the recently developed in-situ scanning electron microscope (SEM) testing system that allows observation on the deforming material surface in real time and under variable spatial resolutions. A dedicated fracture test on the HEA is performed, and the overall process of crack nucleation and propagation is recorded thoroughly via the in-situ SEM. A remarkable finding of the tested specimen is the elevated steady-state crack-tip-opening angle (CTOA) during the stable crack extension, indicating a satisfactory crack resistance capacity of the studied material. Further analysis on the crack propagation demonstrates that the widespread Cr-rich hard intermetallic particles act as the weak sites for microvoid nucleation, and the high energy ductile fracture mechanism characterized by the microvoid nucleation, growth and coalescence is well identified. Intensified plastic straining along the crack propagation path in a zigzag pattern is remarked, and multiple deformation mechanisms are recognized in the crack tip plastic zone, involving both slip and twinning. The observed microstructural changes are consistent with the macroscopic evaluation on the fracture toughness of the material, and they are accountable for the excellent damage tolerance capacity of the investigated HEA.