Strength and ductility of CrFeCoNiMo alloy with hierarchical microstructures

Abstract

Single-phase multi-principal-element alloys (MPEAs) with face-centered cubic (FCC) structure generally exhibit low yield strength but superb ductility and strain hardening capability. In this work, we demonstrate that enhancing yield strength while retaining good ductility of single phase FCC MPEAs can be achieved by developing hierarchical microstructures. A non-equiatomic Cr20Fe6Co34Ni34Mo6 alloy with single-phase FCC structure was fabricated and immediately cold-rolled with ∼70% thickness reduction after a liquid nitrogen bath. The cold-rolled sample was then annealed at various temperatures in a range of 675–1150 °C for various periods. As annealing temperature exceeds 800 °C, annealed samples exhibit coarse-grained microstructure and low yield strength, while high strain hardening rate and good ductility. As annealing temperature is lower than 800 °C, annealed samples develop hierarchical microstructures that comprise high density of annealing nano-twins in recrystallized fine grains (grain size ∼ 1 μm) and stable dislocation walls in non-fully recrystallized fine grains. The addition of Mo in the system is found to be very effective in retarding the recrystallization and grain growth, promoting formation of recrystallized fine grains. Mechanical tensile testing reveals that such kind of hierarchical microstructure enhances yield strength of single phase FCC MPEAs (1.1 GPa) while retains good ductility (uniform elongation of ∼29%) and high ultimate tensile strength (1.3 GPa). Grain boundaries, twin boundaries and dislocation walls act as strong barriers for dislocation motion, enhancing yield strength and strain hardening capacity. Dislocation walls also act as sources for dislocations and deformation twins at large deformation stages, retaining a good ductility. Engineering such hierarchical microstructures should thus be an efficient strategy in enhancing mechanical properties of FCC MPEAs with low or medium stacking fault energies.