Microstructure Evolution and Mechanical Behavior of 625 Alloy During Long-term Thermal Aging Process at 750 °C

In order to clarify the performance of 625 alloy during 10 000 h long term thermal aging at 750 degrees C, the microstructure evolution and mechanical behavior were investigated by SEM, TEM as well as tensile and hardness tests. Results show that a phase transition from the metastable gamma '' phase to the steady-state delta phase occurs in 625 alloy at 750 degrees C. At the initial stage gamma '' phase rapidly precipitates and grows in matrix, and then decreases and disappears with the extension of thermal aging. While the transformed delta phase gradually nucleates and grows in size, and turns to be the main strengthening phase in 625 alloy in the long run. With the prolonging of the thermal aging time, tensile strength, yield strength and hardness of the alloy are continuously improved, but the elongation constantly reduces. After 3000 h thermal aging treatment, the mechanical properties tend to keep stable. There is no significant difference between the two types of delta phase in the reinforcement of 625 alloy.