Creep behavior and microstructure evolution of Ni-based alloy IN617 at 1000 °C

Abstract

The microstructure evolution of IN617 during creep under 30 MPa, 23 MPa, and 16 MPa at 1000 °C was characterized after creep ruptured tests and interrupted tests. As the applied stress was decreased, three different microstructure characteristics were found (i.e., dynamic recrystallization, classic creep voids, and precipitates coarsening), which were related to microstructural factors such as dislocation configuration, grain boundary (GB) weakening, and nitrogen diffusion and brittle-phase precipitation. The results indicated that the high stress of 30 MPa promoted the significant multiplication of dislocations that rearranged into high-density and low-energy substructures, leading to recrystallization and ductile rupture. When stress was decreased to 23 MPa, dislocations only formed loose tangles causing intragranular strengthening, and GB weakening was involved because most intergranular precipitates that could have exerted the pinning effect dissolved. Therefore, intergranular rupture occurred under the combined effect of intragranular strengthening and GB weakening. As stress was as low as 16 MPa, precipitates that existed in the as-received condition completely dissolved and abundant nitrogen diffused into the specimen from the furnace atmosphere through creep cracks, promoting the formation of brittle nitrides (π phase and AlN). The brittle π phase forming continuous networks along GBs was vulnerable to internal cracking, and the needle AlN precipitating inside grains caused the AlN/matrix interfacial cracking, which exerted a synergetic effect on brittle fracture.