Segregation-assisted phase transformation and anti-phase boundary formation during creep of a γ′-strengthened Co-based superalloy at high temperatures

Abstract In this study, the creep defects and segregation-assisted local phase transformation processes in a γ′-strengthened Co-based superalloy crept at 982 °C/248 MPa-1% and 1000 °C/137 MPa-1% were analyzed. A non-coplanar deformation configuration, i.e. a repetition of superlattice intrinsic stacking faults and antiphase boundaries (…SISF-APB…), was discovered in a Ni-free Co-based alloy, and found to be assisted by the local elemental segregation and favored more at higher stress conditions. A SISF with a/6 1 1 ¯ 2 > type displacement in the ( 1 ¯ 11 ) plane and APB with a/2 type displacement nucleating in the ( 1 1 ¯ 1 ) plane were observed to be connected. The γ former segregation-assisted local γ′→γ phase transformation process near LPD was estimated to evolve in the order: γ′→metastable γ→γ+γ′1. The formation of metastable γ generates a new γ/γ′ interface with misfit strain/stress, which is responsible for the nucleation of dislocations, accompanied by the creep stress, thus generating APBs in the γ′ phases. The separation of γ and γ′1 seems to occur by a directional redistribution of γ and γ′ formers along the near-( 1 ¯ 11 ) plane and toward the SISF side. The W segregation-assisted γ′→χ phase transformation along the planar fault is estimated to decrease the deformation resistance by forming SISFs with single-layer a/6 type displacement, while the Co segregation-assisted γ′→γ phase transformation at the LPD is believed to improve the deformation resistance by trapping the moving dislocation at the local γ/γ′ interface.