Long-term strength and creep resistance of a polycrystalline Re-containing nickel-based superalloy

The work was devoted to the study of the long-term strength and creep resistance of the recently developed heat resistant nickel-based superalloy SDZhS-15 intended for manufacturing of discs for gas turbine engines. The as-cast alloy was subjected to homogenization heat treatment, hot forging at subsolvus temperatures with intermediate annealing, solution treatment at various temperatures and aging or only to aging. A predominantly fine-grained structure was obtained in the workpieces after forging. It was revealed that solution treatment at T > Ts − 50, where Ts is the solvus temperature of the γ'-phase, led to a decrease in the volume fraction of the primary γ'-phase and an increase in the volume fraction of the γ' precipitates together with a significant growth of γ grains (up to d > 50 μm). Solution treatment at Т = Ts − 50 allowed maintaining a relatively fine-grained structure (dγ =10 – 20 μm) and ensured the precipitation of the secondary γ'-phase with a size of about 0.1 μm upon cooling in air. Three microstructure conditions were obtained, for which long-term strength and creep resistance tests were performed in the range of temperatures 650 – 850°C and stresses 400 –1200 MPa. The highest values of the long-term strength were achieved for a relatively fine-grained condition obtained after solution treatment at Ts − 50 and aging. To evaluate the service life of the superalloy, the methodology based on the calculation of the Larson-Miller parameter was used. It was shown that the long-term strength (creep resistance) of the SDZhS-15 alloy in the optimal condition was higher as compared to the industrial disc nickel-based superalloys EP741NP and Udimet 720. Microstructure examination of the creep tested samples did not result in significant microstructure changes and especially in the formation of topologically close-packed phases. After creep tests, microcracks were observed along the grain / interphase boundaries, which apparently resulted from the development of boundary diffusion and sliding.

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1. State Assignment of the Institute for Metals Superplasticity Problems of the Russian Academy of Sciences - AAAA-A17-117041310215-4