Healing Process of Casting Pores in a Ni-based Superalloy by Hot Isostatic Pressing

Hot isostatic pressing （HIP） with a pressure of 180 MPa at a temperature of 1170 ℃ was introduced to an investment cast Ni-based superalloy （Mgl） turbocharger blade to explore the healing process of casting pores generated during investment casting. Optical micrograph and scanning electron naicroscopy （SEM） observations indicate that eutectic pores are the main cast defects in the as-cast blade before HIP. These pores normally locate at the solidification front of γ/γ＇ eutectic with a size of a few micrometers to a few tens of micrometers. After HIP for 4 h, most of the pores were closed. Based on phase characteristics, these pores were healed by the formation of γ matrix with finer and irregular-shaped γ＇ precipitates. Healing interface can be easily distinguished by SEM. Line scan by using energy dispersive X-ray spectroscopy （EDS） reveals a much higher Ti and Al concentration in the healing interface. It is proposed that solute diffusion toward the casting pores during HIP results in the formation of γ, and the much higher concentration of γ＇-forming elements Al and Ti near the healing interface contributes to the precipitation of γ＇ in the healed region in the succeeding cooling process after HIP.

Suppression of grain boundary migration at cryogenic temperature in an extremely fine nanograined Ni-Mo alloy

Microindentation creep tests on an electrodeposited extremely fine(4.9 nm) nanograined(ng) Ni-14.2 at.% Mo(Ni-14.2 Mo) at both room temperature(RT) and liquid nitrogen temperature(LNT) demonstrated that lowering temperature retarded softening in the ng Ni-Mo alloy. The obtained strain rate sensitivity at LNT was one order of magnitude lower than that at RT. Microstructural characterization revealed that mechanically-driven grain boundary(GB) migration was greatly suppressed by lowering temperature,which might be ascribed to the presence of solute Mo atoms that significantly retarded coupled GB motion at LNT. Deformation was instead carried by shear bands.