Cross-scale precipitation and thermomechanical strengthening of a powder-metallurgy CoCrNi–5Mo medium-entropy alloy

To overcome the low room-temperature strength of CoCrNi medium-entropy alloy (MEA) for structural applications, Mo was introduced via a powder metallurgy route, followed by cold rolling and subsequent annealing. Mo addition promotes the formation of σ (Cr-Mo) and μ (Co-Mo) intermetallic phases, with the σ phase exhibiting a cross-scale distribution at both submicron and micron levels. After thermomechanical processing, the precipitates become more uniformly dispersed, and the matrix grain size refines from 43.69 μm to 5.34 μm. Compared with the base alloy, yield strength increases from 405 ± 21 MPa to 755 ± 17 MPa and ultimate tensile strength from 823 ± 13 MPa to 957 ± 15 MPa, while maintaining 17 ± 2% total elongation. The strength improvement is due to precipitation, grain boundary, solid solution, and hetero-deformation-induced (HDI) strengthening.