An innovative annealing treatment method and its mechanism to refine deformed mixed grains of initial aged GH4169 superalloy

Abstract

The deformed mixed grains of the initial aged GH4169 superalloy need to be refined by annealing treatment. However, for the isothermal annealing treatment, the deformed mixed grains can only be refined to about 10 µm because the increase of nucleation rate needs to increase the temperature while the decrease of grain growth rate requires to decrease temperature. In the paper, an innovative method including an aging treatment and a subsequent continuous-cooling recrystallization annealing treatment (CC-RAT) is proposed. This method can not only increase the nucleation rate but decrease the grain growth rate. The effects of heat treatment parameters on microstructure are investigated. Besides, the grain refinement mechanism is studied using a developed cellular automaton (CA) model. The results show that the recrystallization takes place slowly during aging, while large number of δ precipitates appear. In subsequent CC-RAT, increasing starting temperature (Ts), decreasing cooling rate (V) or ending temperature (Te) can remarkably promote recrystallization, thereby refining grain microstructure. This attributes to the increase of recrystallization nucleation rate and the effective control of grain growth rate. In the early stage of CC-RAT, δ precipitates hinder the quick growth of recrystallized grains nucleated firstly. Moreover, the increase of Ts accelerates the nucleation rate, and the decrease of V prolongs the time for nucleation. Both of them promote the rapid formation of new recrystallized nuclei, especially at δ phase boundaries. As the annealing temperature decreases, newborn grains grow slowly, and these new grains restrict the growth of grains nucleated first. Therefore, a fine and uniform microstructure with an average grain size of 5.82 µm is obtained. However, when Ts is too high or V is too low, some coarse recrystallized grains appear. Hence, the feasible process is concluded as follows: aging treatment (900 °C×12 h), followed by CC-RAT (Ts = 990–1010 °C, V=1–2 °C/min, Te=950–970 °C).