Influence of Forging and Heat Treatment on the Microstructure and Mechanical Properties of a Heavily Alloyed Ingot-Metallurgy Nickel-Based Superalloy

The newly designed ingot-metallurgy nickel-based superalloy SDZhS-15 intended for disc applications at operating temperatures up to 800-850 degrees C was subjected to homogenization annealing and canned forging at subsolvus temperatures, followed by solid solution treatment and ageing. Mostly a fine-grained recrystallized microstructure was obtained in the forgings. It was revealed that post-forging solid solution treatment at T > (T-s-50), where T-s is the gamma ' solvus temperature, led to a significant gamma grain growth, which in turn led to a decrease in strength and ductility of the superalloy. The solution treatment at (T-s-60)-(T-s-50) allowed to save fine gamma grains (d(gamma) = 10-20 mu m) and to provide the formation of secondary gamma ' precipitates with a size of around 0.1 mu m. In the forged and heat-treated conditions, the superalloy demonstrated superior mechanical properties, particularly excellent creep resistance at 650-850 degrees C in the stress range of 400-1200 MPa. Microstructure examination of the creep-tested samples showed that a decrease in the creep resistance at 850 degrees C can be associated with enhanced diffusivity along gamma grain and gamma/gamma ' interphase boundaries leading to formation of cracks along the boundaries. In spite of the heavy alloying, the topologically close-packed phases were not detected in the superalloy, including in the creep tested samples.