γ' Phase in a New Fe-Based Superalloy

As a new Fe-based superalloy, the 05Cr14Ni33Ti3Al2MoNb is a nickel-saving valve alloy developed for exhaust valve in internal combustion engine. The scanning electron microscopy, transmission electron microscopy, x-ray diffractometry and quantitative analysis of precipitated phase were adopted to investigate the morphology of γ' phase and the evolution kinetics during the long-term aging at high temperature. The results show that the γ' phase in 05Cr14Ni33Ti3Al2MoNb alloy has two forms, sphere and cell. The formation of cellular is a discontinuous precipitation with the nucleation on grain boundaries. The growing process of the cellular γ' phase is controlled by the diffusion of alloy elements. After long-term aging at high temperature, the fiber of cellular γ' phase near the front edge of new phase is thick, while that far away from the front edge of new phase is thin. The chemical formula of γ’ phase in the alloy is (Ni, Fe, Cr, Mo)₃(Ti, Al, Nb), in which the content of Mo and Cr are trace. After aging 2000 h at 700 °C, the γ' phase is basically stable in both quantity and dimension, and the evolution kinetics of γ' phase conforms to the Ostwald ripening mechanism. This alloy has the highest strength after aging 1000 h at 700 °C when the average particle diameter of phase γ' is 31.84 nm. At this time, the strengthening mechanism of the alloy conforms to the Orowan mechanism.