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Free keywords:
Cobalt alloys; Metastable phases; Nickel alloys; Phase transitions; Superalloys, Antiphase boundaries; Co-based superalloys; Deformation resistance; Elemental segregation; High temperature; Moving dislocation; Stress condition; Superlattice intrinsic stacking faults, Creep
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/6lt;11¯2gt; type displacement in the (1¯11) plane and APB with a/2lt;110gt; type displacement nucleating in the (11¯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/6lt;112gt; 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. © 2021
