Tailoring grain growth and solid solution strengthening of single-phase CrCoNi medium-entropy alloys by solute selection

In the present study, we selected solutes to be added to the Cr Co Ni medium-entropy alloy(MEA) based on the mismatch of self-diffusion activation energy(SDQ) between the alloying elements and constituent elements of the matrix, and then investigated their grain growth behavior and mechanical properties.Mo and Al were selected as the solutes for investigation primarily because they have higher and lower SDQ, respectively, than those of the matrix elements; a secondary factor was their higher and lower shear modulus. Their concentrations were fixed at 3 at.% each because previous work had shown these compositions to be single-phase solid solutions with the face-centered cubic structure. Three alloys were produced by arc melting, casting, homogenizing, cold rolling and annealing at various temperatures and times to produce samples with different grain sizes. They were(a) the base alloy Cr Co Ni,(b) the base alloy plus 3 at.% Mo, and(c) the base alloy plus 3 at.% Al. The activation energies for grain growth of the Cr Co Ni, Cr Co Ni-3Mo and CrCo Ni-3Al MEAs were found to be ～251, ～368 and ～219 k J/mol, respectively, consistent with the notion that elements with higher SDQ(in this study Mo) retard grain growth(likely by a solute-drag effect), whereas those with lower values(Al) accelerate grain growth. The roomtemperature tensile properties show that Mo increases the yield strength by ～40 % but Al addition has a smaller strengthening effect consistent with their relative shear moduli. The yield strength as a function of grain size for the three single-phase MEAs follows the classical Hall-Petch relationship with much higher slopes(>600 MPa μm-0.5) than traditional solid solutions. This work shows that the grain growth kinetics and solid solution strengthening of the Cr Co Ni MEA can be tuned by selecting solute elements that have appropriate diffusion and physical properties.