Development of Microstructural Damage in Ni-Based Alloys During Creep

Ni-based model alloys with a base composition of Ni-20 mass pct Cr-3 mass pct Mo that were precipitation strengthened by the γ′ phase were studied in regards to their failure mechanisms as part of the fundamental research for achieving a creep rupture strength of 100 MPa at 1023 K (750 °C) and 10⁵ hours. The microstructure, which was interrupted by transient creep, as well as the minimum creep rate and accelerated creep at 1123 K (850 °C) and 80 MPa was observed. The microstructure around the grain boundaries was altered remarkably with strain-induced grain boundary migration, while the γ′ particle size increased linearly inside the grains with increasing temperature and time. Furthermore, the volume fraction of the γ′ phase and the amount of precipitation on the grain boundary were associated with the size of the precipitate-free zone (PFZ), which is a major factor in creep damage. The appropriate precipitations inside the grains and at the grain boundaries were very effective for suppressing PFZ. Consequently, the creep properties can be improved by controlling PFZ in the proximity of grain boundaries for a superior balance of creep strength and ductility.