Thermomechanical treatments for a dual phase cast high entropy alloy

Thermomechanical treatments modify the cast microstructure of a dual phase hypoeutectic high entropy alloy. The as-cast microstructure consists of an FCC matrix and a BCC eutectic phase forming a semi-continuous network. The as-cast material is subjected to various thermomechanical treatments: I) hot compression; II) hot compression with strain rate jumps; III) multi-stage hot compression with intermediate stages of isothermal soaking; IV) hot compression followed by annealing. The material recovers rapidly after the strain rate jump and no dependence of the yield stress on the previous deformation step is observed. Much of the stress relaxation occurs within seconds during the hold for the multi-stage tests. A steady-state stress is reached for the first holding cycle, while progressively slower softening is observed for later holding cycles. The eutectic BCC phase fragments during hot deformation, while discontinuous dynamic recrystallisation associated with the formation of Σ3 occurs in the FCC phase. Precipitates are formed at high temperatures and pin the high-angle grain boundary motion. The rotation of the eutectic BCC phase and the precipitates during plastic deformation also promotes local lattice rotation of the FCC phase. As a result, a fine and intricate substructure is formed in the interdendritic spaces. Complete recrystallisation is not achieved during either hot deformation or annealing, suggesting that practical grain refinement of the investigated alloy would only be achieved at larger strains or a combination of larger deformations and intermediate annealing treatments.