Precipitation Behavior of Ni-Based Superalloy Alloy 625 for A-USC Power Plants

The age-hardening behavior of the wrought Ni-based superalloy Alloy 625 was investigated in the temperature range between 923 and 1173 K for the application to advanced ultra-supercritical (A-USC) power plants. The carbon content of the alloy was controlled as low as possible to minimize the precipitation of carbides during aging. A two-step increase of hardness was detected for the alloy at temperatures between 1000 and 1100 K; the first increase of hardness results from the precipitation of the metastable gamma '' phase, and the second increase corresponds to the precipitation of the orthorhombic delta phase. In contrast, a single-step increase of hardness was detected below 1000 K derived from the precipitation of gamma '' phase and above 1100 K derived from the precipitation of delta phase. The TTP (time-temperature-precipitation) diagram for the alloy was established on the basis of the results of hardness measurements and microstructure observations, where the nose temperatures of gamma '' and delta phases are determined as 1050 and 1123 K, respectively. The gamma '' particle coarsened along the Ostward ripening. The activation energy for the gamma '' coarsening was evaluated as 202 kJ/mol, which is very close to that for the inter-diffusion of Nb in Ni.