The effect of heat treatment on characteristics of the gamma prime phase and hardness of the nickel-based superalloy Rene®80

Abstract

Abstract In this study, the effect of heat treatment on characteristics of the γ'-Ni3(Al, Ti) phase, as the main strengthening precipitates of the nickel-based superalloy Rene®80 was evaluated in terms of morphology and distribution of the γ' phase, γ (matrix)-γ′(strengthening precipitates) lattice mismatch, and hardness of the alloy. For this purpose, at first, the solution heat treatment was performed at 1204 °C on the alloy. Then, heat treatment cycles were carried out at three temperatures, namely 871 °C, 1095 °C, and 1130 °C, for different periods of time. The microstructures were analyzed by optical microscope and scanning electron microscope. Also, X-ray diffraction analysis was conducted to determine variations of the γ-γ′ lattice mismatch, followed by hardness measurement test, which was performed after each stage of heat treatments. The results showed an increase in the amount and size of the γ' phase with holding time at 871 °C, while the values were seen to remain constant and even decrease at 1095 °C and 1130 °C, respectively, due to simultaneous dissolution of the γ' phase during extended periods of time. At all of the three temperatures, the γ-γ′ lattice mismatch increased with time and showed larger negative values, while an increase in temperature from 871 °C to 1130 °C led to lower γ-γ′ lattice mismatch. The largest lattice mismatch (−0.653%) was observed at 871 °C after a heat treatment time of 1000 s. Hardness of the Rene®80 alloy at 871 °C increased with time, while different behaviors were observed at 1095 °C and 1130 °C, where the hardness decreased for extended heat treatment times. The most significant cause of such changes in hardness might be the coherency elastic strain between the γ and γ' phases, which is affected by the characteristics of the γ' phase and its lattice mismatch with the γ phase.