Effects of Thermo-mechanical Treatment on the Microstructures and Mechanical Properties of a Medium-Entropy Alloy

The effects of hot-rolling (HR), cold-rolling (CR), and HR + CR followed by aging on the microstructure and mechanical properties of the medium-entropy alloy (MEA) Ni43.4Co25.3Cr25.3Al3Ti3 were investigated. The geometrically-necessary dislocation (GND) density increased with increasing cold rolling reduction to 1.8–2.4 × 1014 m−2, which is eight times the 2.9 × 1013 m−2 found in the as-hot-rolled MEA. Higher dislocation density before aging led to a finer, higher volume fraction of L12 nanoparticles as well as finer grains, which resulted from both accelerated nucleation on dislocations and, possibly, accelerated pipe diffusion via the dislocations. The aged CR MEA exhibited an excellent strength-ductility combination (yield strength ~ 1414 MPa, ultimate tensile strength ~ 1605 MPa, elongation ~ 15%) with precipitate strengthening accounting for 26% of the yield strength, which was better than the mechanical properties of the aged HR MEA (yield strength ~ 897 MPa, ultimate tensile strength ~ 1217 MPa, elongation ~ 32.3%). A combination of hot-rolling and cold-rolling also leads to a good strength-ductility combination (yield strength ~ 1501 MPa, ultimate tensile strength ~ 1651 MPa, elongation ~ 20.8%), with grain boundary strengthening and precipitate strengthening accounting for 62% and 25% of the yield strength, respectively. Substantial partitioning and fluctuations of the principle atoms were observed in as-CR sample due to dislocations interacting with the alloying atoms, i.e. several nano-scaled regions were enriched in Ni, Al, and Ti, and depleted in Cr and Co. In addition, Al and Ti have more substantial local fluctuations in composition than Ni, Co, and Co.