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

In this study, the creep defects and segregation-assisted local phase transformation processes in a gamma '-strengthened Co-based superalloy crept at 982 degrees C/248 MPa-1% and 1000 degrees 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)over bar>2> type displacement in the ((1)over bar>11) plane and APB with a/2<110> type displacement nucleating in the (1 (1) over bar1) plane were observed to be connected. The gamma ' former segregation-assisted local gamma '->gamma phase transformation process near LPD was estimated to evolve in the order: gamma '-> metastable gamma ->gamma+gamma '(1). The formation of metastable gamma generates a new gamma/gamma ' interface with misfit strain/stress, which is responsible for the nucleation of dislocations, accompanied by the creep stress, thus generating APBs in the gamma ' phases. The separation of gamma and gamma '(1) seems to occur by a directional redistribution of gamma and gamma ' formers along the near-((1) over bar 11) plane and toward the SISF side. The W segregation-assisted gamma '->chi phase transformation along the planar fault is estimated to decrease the deformation resistance by forming SISFs with single-layer a/6<112> type displacement, while the Co segregation-assisted gamma '->gamma phase transformation at the LPD is believed to improve the deformation resistance by trapping the moving dislocation at the local gamma/gamma ' interface. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.