Grain refinement of non-equiatomic Cr-rich CoCrFeMnNi high-entropy alloys through combination of cold rolling and precipitation of σ phase

Grain refinement of non-equiatomic Cr-rich CoCrFeMnNi high-entropy alloys with an fcc structure was studied focusing on the precipitation of a Cr-rich σ phase along grain boundaries and shear bands. We found for the first time that the σ phase can be precipitated at the grain boundaries and the shear bands in the alloys at a significantly fast precipitation rate. The σ phase effectively suppressed the grain growth of the fcc matrix during recrystallization of the alloy after conventional cold rolling, resulting in a fine-grained microstructure with a mean diameter of approximately 1 µm. The alloys with this fine microstructure containing the σ phase exhibit high strength over a wide temperature range due to grain boundary strengthening and precipitation strengthening caused by the σ phase. Moreover, the alloys exhibit large ductility not only at room and high temperatures, but also at low temperatures. Deformation-induced twinning plays an important role in this ductility.