Investigation of γ' precipitates growth kinetics in CMSX-۴ based on the Johnson-Mehl-Avrami model

CMSX-۴ is a second-generation single-crystal nickel-based superalloy. Through aging heat treatments, the characteristics of γ’ precipitates, including their size, volume fraction, and morphology, can be controlled in this alloy. Therefore, the main goal of this research is to investigate the effect of aging temperatures (۸۰۰-۱۱۴۰ ℃) and times (۱-۲۴ h) on the kinetics of γ’ precipitate formation and growth. As the aging temperature increases, the morphology changes from an irregular shape to a cubic one. The most regular cubic morphology is observed at ۹۰۰ ℃. Due to the competition between diffusion and matrix supersaturation, the volume fraction of precipitates initially increases and then decreases with increasing temperature and time. Kinetic studies of γ’ precipitate growth and formation using the Johnson –Mehl -Avrami method show that, the rate of transformation and growth of γ’ precipitates increases with increasing aging temperature. The average value for the Avrami exponent (n) is approximately ۴ in the range of ۸۳۵-۹۰۰ ℃, which indicates regular and cubic growth of γ’ precipitates within this temperature range. Based on this model, a TTP diagram was plotted for CMSX-۴. The nose of TTP curve, indicating the optimal conditions for γ’ phase formation, which is obtained at approximately ۸۵۰ °C.

کلیدواژه ها:

CMSX-۴ superalloy ، Aging ، γ’ Growth Kinetics ، JMAK Model