Dual effects of pre-strain on continuous and discontinuous precipitation of L1₂-strengthened high-entropy alloys

The effects of pre-strain on the continuous precipitation (CP) and discontinuous precipitation (DP) as well as mechanical properties of L1₂-strengthened HEAs were thoroughly studied. It is found that the pre-strain has a dual effect on the L1₂ precipitation behavior depending on the pre-strain level. At low pre-strains, the plastic deformation increases the dislocation density without significantly affecting the grain structure, which accelerates the CP reaction by promoting the CP nucleation and growth, leading to the CP-dominant microstructure. At high pre-strains, the server plastic deformation induces the formation of a high density of deformation bands and subgrains, the boundaries of which provide preferred nucleation sites for the DP reaction. In addition, the high stored energy induced by the cold deformation enhances the grain boundary migration, which promotes the DP growth and concurrent recrystallization. Mechanical tests further reveal that the pre-strain substantially improves the strength of the alloys without significantly deteriorating the ductility. The contributions of dislocations, grain boundaries, and precipitates to the strengthening of the pre-strained and aged alloys were quantitatively evaluated.