γ' Precipitate Dissolution during Heat Treatment of Nimonic 115 Superalloy

In precipitation hardenable materials, it is desirable to determine the precipitate dissolution temperature for homogenizing the microstructure by controlling the size and distribution of the precipitates. In this research, differential thermal analysis, dilatometry technique, heat treatments followed by microstructure evaluation were used to determine the γ′ dissolution temperature of Nimonic 115. It is assumed that the variation of enthalpy is governed by the changes in γ′ volume fraction and γ′ concentration with time and temperature, and any contribution of the coarsening of γ′ is neglected. The values obtained for the solvus temperature of γ′ precipitates by the three methods are all in agreement indicating this temperature at approximately 1160°C. As the solvus temperature of γ′ is higher than the temperature of formation of M23C6 carbides, this is considered as the cause of occurrence of serrations at grain boundaries. During the heat treatments, grain boundary serration is observed, and the results indicate that formation of serration depends on the cooling rate, so that while furnace cooling results in boundary serration, no effect is observed by air cooling.