Effect of thermomechanical treatment on the microstructure and mechanical properties of a novel heavily alloyed nickel base superalloy

The work has been devoted to study of microstructure and mechanical properties of a novel heavily alloyed heat resistant nickel base superalloy in the cast and heat treated condition and the thermomechanically treated (TMT) condition. The cast condition after heat treatment including homogenization annealing, solid solution treatment and ageing was characterized by a coarse. grain size and uniformly distributed dispersed gamma' precipitates with a size of d(gamma') = 0.1-0.25 mu m. It was established that TMT under optimal conditions led to development of recrystallization processes, which provided formation of refined recrystallized microstructure with a gamma' grain size of d(gamma) = 2 - 60 mu m. The obtained microstructure condition contained mostly dispersed gamma' precipitates with a size of d(gamma)' = 0.1 - 0.3 mu m. The volume fraction of relatively coarse gamma' particles, which were not dissolved during TMT, was about 10 %. The TMT condition was aged in the same manner as the cast condition. Tensile tests and long-term strength tests were performed for the both superalloy conditions. The strength properties in the TMT condition were found by 30 - 50 % higher than those obtained in the cast and heat treated condition. At the same time, the ductility in the TMT condition was found also significantly higher than in the cast and heat treated condition. The following tensile properties were obtained for instance at room temperature: sigma(UTS)/sigma(0.2) = 1802/1355 MPa and delta = 16 % for the TMT and aged condition, and sigma(UTS)/sigma(0.2) = 1255/1132 MPa and delta = 8.5 % for the cast and heat treated condition. The long-term strength at 650 degrees C in the TMT and aged condition was found also to be significantly higher than that in the cast and heat treated condition. Excellent mechanical properties of the superalloy after TMT are explained in terms of the increased amount of the gamma' phase, the effective solid solution strengthening due to heavy alloying with refractory metals and the refined microstructure obtained after TMT.