Methods and mechanisms for uniformly refining deformed mixed and coarse grains inside a solution-treated Ni-based superalloy by two-stage heat treatment

The uniform refinement mechanisms and methods of deformed mixed and coarse grains inside a solution-treatment Ni-based superalloy during two-stage annealing treatment have been investigated. The two-stage heat treatment experiments include an aging annealing treatment (AT) and a subsequent recrystallization annealing treatment (RT). The object of AT is to precipitate some delta phases and consume part of storage energy to inhibit the grain growth during RT, while the RT is to refine mixed and coarse grains by recrystallization. It can be found that the recrystallization grains will quickly grow up to a large size when the AT time is too low or the RT temperature is too high, while the deformed coarse grains cannot be eliminated when the AT time is too long or the RT temperature is too low. In addition, the mixed microstructure composed of some abnormal coarse recrystallization grains (ACRGs) and a large number of fine grains can be observed in the annealed specimen when the AT time is 3 h and RT temperature is 980 degrees C. The phenomenon attributes to the uneven distribution of delta phase resulted from the heterogeneous deformation energy when the AT time is too short. In the regions with a large number of delta phases, the recrystallization nucleation rate is promoted and the growth of grains is limited, which results in fine grains. However, in the regions with few delta phases, the recrystallization grains around grain boundaries can easily grow up, and the new recrystallization nucleus is difficult to form inside grain, which leads to ACRGs. Thus, in order to obtain uniform and fine annealed microstructure, it is a prerequisite to precipitate even-distributed delta phase by choosing a suitable AT time, such as 12 h. Moreover, a relative high RT temperature is also needed to promote the recrystallization nucleation around delta phase. The optimal annealing parameters range for uniformly refining mixed crystal can be summarized as: 900 degrees Cx12 h + 990 degrees Cx(40-60 min) and 900 degrees Cx12 h + 1000 degrees Cx(10-15 min). (C) 2021 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.

Automated summary

The study investigated the uniform refinement mechanisms in a Ni-based superalloy during two-stage annealing treatment. By selecting suitable times for aging annealing treatment and recrystallization annealing treatment, a mixed microstructure with abnormal coarse recrystallization grains and fine grains can be achieved.