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

In this study, the effect of heat treatment on characteristics of the gamma'-Ni-3(Al, Ti) phase, as the main strengthening precipitates of the nickel-based superalloy Rene (R) 80 was evaluated in terms of morphology and distribution of the gamma' phase, gamma (matrix)-gamma'(strengthening precipitates) lattice mismatch, and hardness of the alloy. For this purpose, at first, the solution heat treatment was performed at 1204 degrees C on the alloy. Then, heat treatment cycles were carried out at three temperatures, namely 871 degrees C, 1095 degrees C, and 1130 degrees 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 gamma-gamma' 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 gamma' phase with holding time at 871 degrees C, while the values were seen to remain constant and even decrease at 1095 degrees C and 1130 degrees C, respectively, due to simultaneous dissolution of the gamma' phase during extended periods of time. At all of the three temperatures, the gamma-gamma' lattice mismatch increased with time and showed larger negative values, while an increase in temperature from 871 degrees C to 1130 degrees C led to lower gamma-gamma' lattice mismatch. The largest lattice mismatch (- 0.653%) was observed at 871 degrees C after a heat treatment time of 1000 s. Hardness of the Rene (R) 80 alloy at 871 degrees C increased with time, while different behaviors were observed at 1095 degrees C and 1130 degrees 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 gamma and gamma' phases, which is affected by the characteristics of the gamma' phase and its lattice mismatch with the gamma phase.