Optimization of mechanical properties of a Ni–Mo–Cr alloy by structural modifications induced by changes in heat treatment

The influence of modifications of heat treatment applied to a commercial Ni–25 wt.%Mo–8 wt.%Cr age-hardenable alloy (HAYNES® 242™) on microstructure and mechanical properties were investigated. The standard heat treatment (aging at 650°C for 72 h), producing coherent long-range ordered domains with the Ni2(Mo, Cr) stoichiometry, was modified by short duration thermal exposures at temperatures at between 760 and 980°C (preceding the alloyʹs aging). The intermediate heat treatment (IHT) affected the size of the long-range order domains and produced precipitations of a μ phase on the grain boundaries. The size of the ordered domains depended on the temperature of the IHT. The largest domains occurred after the IHT at 760°C. With the increase in the IHT temperature, the domain size decreased. The yield strength of the alloy increased with increasing domain size. The amount of grain boundary particles, and not the size of the ordered domains, controlled the ductility of the material subjected to the IHT: The more grain boundary precipitates there were, the lower tensile ductility. Optimization of the tensile properties (strength and ductility) of the examined alloy occurred when the conventional aging was preceded by the IHT at 760°C.