Effects of heat treatment on the microstructure and properties of cold-forged CoNiFe medium entropy alloy

The effects of heat treatment on the microstructure and properties of cold-forged two-dimensional (2D) CoNiFe medium entropy alloy (MEA) are determined. Compared to the as-cast specimen with a columnar crystal structure, the cold-forged CoNiFe MEA has a heavily fragmented microstructure with deformation twins. As the annealing temperature is increased, the grain size becomes larger markedly. Annealing at 900 degrees C yields a fully recrystallized microstructure with a large population of annealing twins and the new orientation exhibits a first order twin relationship (60 degrees < 111 > rotation) during recrystallization. Moreover, the CoNiFe HEA annealed at 900 degrees C possesses excellent ductility (epsilon = 50%) and work-hardening ability (sigma(UTS)-sigma(Y) = 246 MPa, sigma(UTS)-sigma(Y) = 0.5), which depend on the annealing twins, dislocations, as well as micro-shear bands in the grains. Analysis of the fracture surface indicates that the main failure mechanism is ductile. Meanwhile, no phase separation occurs as the temperature is raised from 0 degrees C to 1000 degrees C as shown by the expansion rate versus temperature relationship, indicating that the materials have good stability at high temperature. The thermal expansion coefficient (CTE) of the sample annealed at 1100 degrees C for 1 h is 12.1 x 10(-6) K-1 which is less than that of traditional metals (14.4-16 x 10(-6) K-1).