On the Roles of Oxidation and Vaporization in Surface Micro-structural Instability during Solution Heat Treatment of Ni-base Superalloys

Micro-structural instability at the surface that develops during solution heat treatment of a typical third generation Ni-base superalloy, CMSX10N has been reported. It is shown that elemental Ni vaporizes from the surface during solutioning leading to de-stabilization of γ phase. With increasing extent of vaporization, a phase mixture of β, γ′, and the refractory (W and Re-rich) precipitates occur within the surface layers resulting in the complete breakdown of the cuboidal γ/γ′ phase morphology that is usually observed. It is demonstrated that the conditions at the surface have a marked effect on the vaporization kinetics and subsequent evolution of surface phases—the presence of a continuous dense oxide such as Al₂O₃ or the presence of sacrificial Ni-foils interspersed in the furnace significantly suppresses elemental vaporization from the sample surface.