Microstructural engineering of medium entropy NiCo(CrAl) alloy for enhanced room and high-temperature mechanical properties

Abstract

This work demonstrates the development of a strong and ductile medium entropy alloy by employing conventional alloying and thermomechanical processing to induce partial recrystallization (PR) and precipitation strengthening in the microstructure. First, the cold working of homogenized alloy resulted in a highly deformed microstructure. On annealing at 700 °C, B2 ordered precipitates heterogeneously nucleate on the highly misoriented sites. These B2 promotes particle stimulated nucleation (PSN) of new recrystallized strain-free grains. The migration of recrystallized grain boundaries leads to discontinuous precipitation of L12 ordered regions in highly dense lamellae structures. We also observe near-spherical L12 precipitates in the non-recrystallized grains. Atomic-scale compositional analysis reveals a significant amount of Ni confined to the Phase Boundary (PB) regions between B2 and L12 precipitates, indicating Ni as a rate-controlling element for coarsening the microstructure. On 20 h of annealing, the alloy comprises a composite microstructure of recrystallized and non-recrystallized grains, B2 particles at the grain boundaries (GBs), and coherent L12 precipitates inside the grains. The microstructure results in a 0.2% yield stress (YS) value of ∼1060 MPa with ∼25% elongation at ambient temperature and retains up to ∼910 MPa at 670 °C. Examination of deformed microstructure reveals excessive twinning, formation of stacking faults, shearing of L12 precipitates, and accumulation of dislocations at around the B2 precipitates and GBs attributed to high strain hardening of the alloy.