Effect of Heat Treatment on the Microstructural Evolution and Mechanical Properties of NiW Medium/Heavy Alloys

The influence of various heat treatment temperatures on the microstructural evolution and mechanical properties of the cold-rolled Ni42W10Co1Mo alloy was studied. The results indicate that the tcp phases are transformed from s phase to mu phase with the increase in heat treatment temperatures, and then transformed from mu phase to sigma phase finally. The microstructure evolves from deformed dendrites to a uniform equiaxed grains. After heat treatment at 900 degrees C, the precipitation phases are predominantly needle-like and blocky mu phases with a small quantity of granular mu phases. Recrystallization is also evident. After heat treatment at 1200 degrees C, a substantial amount of granular sigma phases is precipitated accompanying the completion of recrystallization. The extensive precipitation of tcp phases consumes a large number of W atoms, leading to a decrease in solid solution strengthening. The room temperature yield strength is decreased from 1564 MPa to 479 MPa after heat treatment at 1200 degrees C, while the elongation is enhanced to 72.4%. The precipitation of larger-sized mu phases is adverse to ductility. The elongation of the alloy is only 7.1% after heat treatment at 900 degrees C.