Effect of pre-deformation and subsolvus heat treatment on microstructure and mechanical properties of a PM nickel superalloy

Abstract

In the present work, the effect of the super- and subsolvus solid solution heat treatment with intermediate pre-deformation (ε=6 and 12 %) at subsolvus temperature and final aging on the microstructure and mechanical properties of the Russian powder metallurgy (PM) nickel base superalloy EP741NP has been studied. The experiments were aimed at achieving serrated grain boundaries to improve the creep resistance and reduce crack growth rate. The as-HIPed material was forged at subsolvus temperatures followed by cutting the workpieces for subsequent processing. Three processing routes were applied: i) solid solution treatment at supersolvus temperature with slow cooling and at subsolvus temperature followed by air cooling and aging; ii) the same with 6 % pre-deformation before subsolvus solution treatment and iii) the same with 12 % pre-deformation before subsolvus solution treatment. As a result, three microstructural conditions were obtained. With increasing pre-deformation from 0 to 12 %, the degree of grain boundary serration and the dislocation substructure density increased. Tensile and impact tests showed that 6–12 % pre-deformation led to more ductile fracture and slightly higher ductility at 650 °C. The 6 % pre-deformation condition also exhibited higher creep resistance and impact strength, which was attributed to the positive effect of serrated grain boundaries and dislocation substructure.