Microstructural evolution and grain refinement mechanisms of a Ni-based superalloy during a two-stage annealing treatment

In authors' previous work [Mater. Charact., 141(2018)212-222], it was found that the heterogeneous deformed microstructures can be replaced by the relatively homogeneous recrystallized grains through a single annealing treatment. However, there are still some relatively large recrystallized grains. In this study, two-stage annealing treatment tests for the deformed samples are designed to better refine the grains and improve the homogeneity. Results showed that the two-stage annealing treatment can better refine grains and improve the microstructural homogeneity compared with single annealing treatment. Thus, the more uniform distribution of grains is obtained. Based on the new experimental findings, the mechanisms for refining and homogenizing the deformed coarse grains are discussed. It is found that the delta phase can easily precipitate around the boundaries of re crystallization grains in the first annealing stage when the temperature is 900 degrees C. The precipitated delta phase can effectively inhibit the grain growth. However, with increasing annealing time, the precipitation rate of delta phase gradually decreases due to the decreased Nb element content. So, the pinning force becomes smaller and smaller, and the follow-up formed recrystallization grains grow to a relatively large size. Therefore, the holding time of first stage annealing treatment has a great influence on the final grain size and the content of delta phase. During the second stage annealing treatment, the increasing nucleation rate induced by the higher temperature and the pinning effects of delta phase make the uniform and fine grains. The feasible parameters are as follows: the first stage annealing treatment is 900 degrees C for 9-12 h and the second stage annealing treatment is 980 degrees C for 60 min.