Microstructure Evolution during Isothermal Aging for Wrought Ni-Based Superalloy Udimet 520

Abstract

The evolution of microstructure during the isothermal aging at 1173 K was investigated for the wrought Ni-based superalloy Udimet 520 solution-treated at 1393 K for 4 h followed by various cooling rates. The age-hardening behavior was observed during the isothermal aging for the water-quenched (WQ) and air-cooled (AC) specimens after the solution treatment, while it could not be detected for the furnace-cooled (FC) specimen. No primary γ′ particles were observed in any continuously cooled samples. For the WQ and AC specimens, the size of the secondary γ′ precipitates increased during the isothermal aging along the Ostwald ripening and their morphology evolved from a spherical shape to an intermediate shape between spherical and cuboidal ones. On the contrary, the secondary γ′ particles exhibited an octodendritic shape for the as FC specimen, and the octodendritic character of the secondary γ′ particles was emphasized during the isothermal aging resulting in the splitting of the secondary γ′ particles. It was found that the splitting of γ′ particles occurs during the isothermal aging for the Alloy 80A with a lower volume fraction of γ′ phase around 20％.

Fig. 8 FE-SEM images of Udimet 520 solution-treated at 1393 K/4 h/FC followed by the aging treatment at 1173 K/1 h (a), 10 h (b), 100 h (c) and 1000 h (d).