Twin boundary-assisted precipitation of sigma phase in a high-entropy alloy

We reveal a novel phenomenon, i.e., twin boundary-assisted precipitation of sigma phase in a non-equiatomic quinary high-entropy alloy (HEA) upon high-temperature annealing. According to systematic electron microscope studies, sigma phase is frequently observed at incoherent twin boundaries where 9R structures are usually formed. The diffusion of solutes and subsequent precipitation of sigma phase are mainly driven by the Gibbs free energy difference between the 9R structure and the adjacent matrix. The findings provide a fundamental understanding for the nature of sigma phase formation in polycrystalline HEAs.