Influence of thermal exposure on the microstructures and mechanical properties of a superalloy

The effects of long-term aging on microstructures and their influence on tensile and stress-rupture behavior of a corrosion resistant nickel-base superalloy are investigated. Samples are aged isothermally at 1073, 1123, or 1173 K for different times of up to 10,000 h and mechanical tests are performed on samples in both standard heat treatment (SHT) and aged conditions. Scanning electron microscopy (SEM) studies reveal that the coarsening kinetics of γ′ follows a linear law at different temperatures with the calculated activation energy, i.e., 255 kJ/mol, for γ′ growth according to Lifshitz-Slyozof-Wagner (LSW) theory. After long-term aging for more than 1 khours, σ phase appears in the alloy. The kinetics of σ formation can be described by the Johnson-Avrime-Mehl equation. Tensile experiments at room temperature and 1173 K and endurance experiments at 1173 K/274 MPa are performed to test the effect of σ phases on these properties and no remarkable harmful effect is found. γ′ coarsening can be used to explain the reduction of yield stress, which is tested by the Labusch-Schwarz hardening theory. Although the presence of the σ phase clearly does affect the fracture process, the σ phase does not embrittle the alloy.