Deciphering the effect of W and Mo co-additions on recrystallization behavior and strengthening mechanism of CrCoNi medium entropy alloy

Abstract

To pursue superior yield strength and maintain high ductility of CrCoNi medium entropy alloy (MEA) at room temperature, 1.0 at% W and 2.0 at% Mo were added to the CrCoNi alloy to ensure a single-phase solid solution strengthening in face-centered cubic structure. The effects of W and Mo additions on microstructure evolution and strengthening mechanism was investigated in detail. The results indicated that discontinuous dynamic recrystallization is the main recrystallization mechanism during annealing. The co-additions of W and Mo with a higher melting point can strengthen the interatomic bonding force and increase the activation energy of grain growth to 409.62 KJ/mol, which leads to sluggish diffusion and retards the recrystallization progress. The high activation energy for grain growth would restrain the grain growth and obtain significantly finer grains of (CrCoNi)97W1Mo2 alloy than that of CrCoNi alloy under similar thermomechanical processing. Smaller grains can impose a stronger grain boundary strengthening and dominate the yield strength enhancement. The strength improvement resulting from grain boundary is 252 MPa for this alloy with the grain size of 1.20 µm. Besides, the co-additions of W and Mo increase the lattice parameter to 0.3610 nm owing to their larger atomic radius and higher shear modulus, then intrinsic lattice friction calculated by the Hall-Petch equation is significantly increased to 289 MPa, which indicated a stronger solid solution strengthening effect (~47 MPa) by severe lattice misfits. Above all, the comprehensive effect of the above aspects of W and Mo additions has developed superior yield strength of 698 MPa and ultimate tensile strength of 1141 MPa. The yield strength of this new alloy enhances by nearly 300 MPa and maintains a high ductility compared with CrCoNi alloy followed under similar thermomechanical processing.