Effect of crack size and its position on cyclic deformation behavior of refractory high-entropy alloy HfNbTiZr

The influence of crack position and its size on short-range ordering (SRO) and high-temperature cyclic deformation behavior of refractory high-entropy alloy (RHEA) HfNbTiZr using bicrystal models has been investigated by combined Monte-Carlo (MC) and molecular dynamics (MD) simulations. It was revealed that the SRO process is affected not only by the crack size, namely the crack opening value, but also by its position. For the first time, using atomistic MC/MD modeling to get the steady-state atomic structure subjected to diffusion process, it was demonstrated that during material relaxation, cracks with small crack opening value or small crack tip radius located in the GB of RHEA HfNbTiZr can promote the formation of stable hexagonal close-packed (HCP) martensite phase. The martensite phase is enriched in Hf and Ti atoms which is required to satisfy equilibrium condition from both side of the moving interface. The stable HCP structure, which is absent when the cracks are larger or located within the grain, improves the material performance under subsequent high-temperature cyclic deformation in Mode I perpendicular to the GB plane. An improved cyclic deformation performance of alloy with smaller cracks compared to material with larger cracks results from the disappearance of the original GBs when a stable martensite phase grows from the crack tips along the boundaries. Since, as a result, the high local stresses initiated by the GBs disappear at the crack tip, this process prevents further crack propagation. The obtained results provide new insights into the role of microstructural features, namely the influence of cracks and their distribution, on the deformation behavior of RHEA.