Abstract
                Long-term isothermal exposure tests were conducted on cast nickel base superalloy grade IN-738 alloy to evaluate the variation of gamma prime particle coarsening behavior after different rejuvenated heat treatment conditions as attempts to restore the useful microstructure. Different microstructures due to various heat treatments were obtained for long-term tests. These different microstructural characteristics should also provide the different behaviors in long-term phase stability of gamma prime phase. The long-term exposure tests (up to 2,500 hours) were conducted at high temperatures (1000?C) to assess gamma prime particle coarsening behavior. Metallographic work had been performed on isothermally aged samples to characterize gamma prime coarsening as a function of heating time. At all investigated heating times, the gamma prime particles in all tested specimens were coarser than the initial re-heat-treated ones. The gamma prime particles appeared to coarsen more into round shape or agglomerated between them comparing to those of each re-heat treatment. The degree of coarsening, as evidenced by gamma prime particle size, increased with increasing heating time. At the given heating temperature, the gamma prime particle size increased, and the area fraction of the secondary gamma prime particle slightly decreased with increasing heating time. The re-heat-treated microstructure obtained after heat treatment at 1175?C/ 2 hrs. (AC), 1055?C/ 1 hr. (AC) and 845?C/ 24 hrs. (AC) provided the most precipitated phase stability in term of minimum coarsening rate after long-term exposure at 1000?C.