The influence of temperature and stress on the microstructural changes has been studied in Ni-base Superalloy Rene 80. While some specimens were only thermally exposed, others were exposed under high temperature and stress.
An external tensile stress on the specimen has been found to cause the directional coarsening of $\gamma{'}$ precipitates which result in morphological change in $\gamma{'}$ precipitates. In particular, the degree of morphological change in $\gamma{'}$ precipitate was different around the carbides. This indicates that local stress around a carbide varies relative to location. In case of tensile stress exposure, it is suggested that local stress maxima are generated on either side of a carbide which is perpendicular to stress axis while stress minima are generated above and below a carbide.
Rene 80 exhibited bimodal size distribution of $\gamma{'}$ precipitates after standard heat treatment. Consequently, the variation with exposure time of $\gamma{'}$ precipitate size was complex as compared with the systems which exhibit unimodal size distribution. $\gamma{'}$ precipitates have been found to obey the diffusion-controlled coarsening and activation energy for coarsening to 65.1 Kcal/mole.
