Change of Grain Boundary Precipitates and Fracture Behavior of TG700A Nickel-based Alloy during Stress Rupture Process

The stress rupture tests at 770℃ under different stresses (160, 200, 260 MPa) were conducted on TG700A nickel-based alloy. The microstructure and stress rupture behavior of the alloy were studied. The results show that the original microstructure of the tested alloy was composed of equiaxed austenite; fine spherical γ' phase precipitated in the grains uniformly, and granular Cr-rich M₂₃C₆ carbides existed at the grain boundaries. After stress rupture, no new precipitates were found in the microstrucutre, and coarsened γ' phase near grain boundaries and γ' phase depletion zone at grain boundaries appeared. The stress rupture mode of the tested alloy was mainly intergranular fracture. The localized plastic deformation at grain boundaries led to localized stress concentration, promoting the formation of creep voids or cracks. The crack propagated along the interface between γ and γ' phases or the γ' phase depletion zone.