Chiang Mai Journal of Science

Long-term isothermal exposure tests were conducted on IN-738 alloy to evaluate the variation of gamma prime particle coarsening behavior after different rejuvenated heat treatment conditions. The long-term exposure tests (up to 2,500 hours) were conducted at two temperatures (900^oC and 1,000^oC) to assess gamma prime particle coarsening behavior. Metallographic work was performed on  isothermally aged samples to characterize gamma prime coarsening as a function of aging time and temperature. At all aging temperatures and/ or times investigated, the gamma prime particles in all tested specimens were more coarsening than the initial reheat treated ones. The gamma prime particles appeared to be more coarsening into round shape or agglomerated between them comparing to the initial ones of  each reheat treatment. The degree of coarsening, as evidenced by gamma prime particle size, increased with increasing heating time and temperature. In most cases, the amount of secondary or very fine gamma prime precipitates decreased with increasing heating temperature and finally disappeared. It was found that the lower solutioning temperature provided the higher rate of γ' coarsening at both elevated temperatures. In contrast, the higher solutioning temperature kept slow rate of γ' coarsening resulting in lower average γ' particle size.