Equicohesion: Intermediate Temperature Transition of the Grain Size Effect in the Nickel-Base Superalloy PM 3030

The intermediate temperature transition of the grain size effect on the yield strength of PM 3030 is investigated using compression tests from room temperature to 1200 A degrees C. It is found that grain boundary strengthening is strong at low temperature which is consistent with conventional Hall-Petch hardening. However, the grain boundary contribution to strength diminishes exponentially at intermediate temperature and vanishes at the equicohesion point. Above the equicohesion point, finer grain structure leads to material softening primarily due to grain boundary diffusion and deformation processes. Maximum softening occurs at T (soft-max) which is about 70% of the melting point, then decreases logarithmically with further increase in temperature, and vanishes at the melting point. This can well be rationalized by the overwhelming dominance of volume diffusion over grain boundary diffusion at temperatures close to the melting point, which decreases the impact of grain size on material strength. An exponential transition from the Hall-Petch behavior to the diffusion-based behavior provides an overall better fit of test data as compared to a linear transition. This study provides a contribution to the understanding of equicohesion and variation of the grain size effect on material strength and can be particularly crucial for components used at intermediate temperature.