Microstructural evolution of a titanium- and cobalt-modified nickel-based repair alloy during exposure to high temperatures

- The microstructure and tensile property for different aging time was analyzed.
- η and σ phase were observed and the reasons of precipitated behavior were analyzed.
- γ′ phase, grain boundary carbides, harmful phase influence strength of the alloy.
- Coherent γ′ size, grain boundary carbides, harmful phase influence plasticity.

Spray forming (SF) is a near net shape manufacturing process in a wide variety of alloys. A titanium- and cobalt-modified repair alloy has been prepared by the SF process and was presented excellent properties of friction and wear in previous published article. In this paper, the microstructural evolution of the alloy exposed at 750 °C for different isothermal aging time was analyzed. Microstructure observations reveal that equiaxed grains formed in the ingots whether after aging process or not. The size and the volume fraction of γ′ precipitates increase with the ongoing of aging process, which could result in the change of the lattice misfit of γ′/γ and the precipitation of new σ, η phases. Intergranular fracture and transgranular fracture are main fracture mechanisms in aged state. Its tensile properties were attributed to the size and the volume fraction of γ′ precipitates, γ′/γ misfit, grain boundary carbides and the number of harmful phases (σ and η phases) during aging process.