Ultra-fine grained structure and high-content precipitates enable ultrastrong yet strain-hardenable medium-entropy alloy

In this work, the mechanical response as a function of ageing time at 700°C for a cold-rolled complex alloy with a chemical composition of Co40Ni33Cr15Al5Ti7 (at%) was explored. The alloy exhibited an ultrafine-scaled recrystallized microstructure combined with high-content coarse L12-type ordered precipitates after a long-term aging for 100 h at 700°C. The resultant alloy demonstrated an extra strain-hardening mechanical response with yield strength of 2.1 GPa and tensile elongation of ∼12% at room temperature. By transmission electron microscopy observations, it was affirmed that the occurrence of profuse stacking faults as well as deformation induced twins during the critical plastic straining stage was mainly responsible for the observed extra strain-hardened behavior. The long-term aging process at low temperature provides a promising strategy for a precipitation-hardening complex alloy with severe deformation to obtain the superb combination of yield stress and ductility.