Characterization of Bifilms and Oxide Inclusions in Investment Cast IN100

Abstract

Oxide bifilms are a proposed casting inclusion reported to have been observed in vacuum investment cast polycrystalline Ni-base superalloys. Ongoing research seeks to determine if current superalloy casting practices can result in the formation of oxide bifilms, and subsequently if it is possible to observe and characterize this phenomenon. The effect of casting atmosphere, turbulence, filtering, hot isostatic pressing (HIP), and heat treatment have been investigated to identify the critical parameters that can result in bifilm formation in Ni-based superalloy IN100. Tensile and fatigue testing is used to identify the effects of each casting and processing parameter on bifilm formation and the resultant effects on mechanical properties. Characterization of mechanical test specimens seeks to identify the role of bifilms in the fracture mechanics of the conditions utilized, as well as the prevalence of bifilm formation and the chemical composition(s) of oxide bifilms. The characterization methods used were scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS), and Auger electron spectroscopy (AES). Across the range of process parameters investigated, bifilms were not indicated by mechanical testing or identified by the characterization methods employed. A nano-layer of sulfur has been identified on the surface of dendritic casting porosity and characterized by in-situ fractographic Auger analysis.