Anomalous Oxidation of Intermetallics

Abstract

AbstractMoSi2, (γ-NiAl and TiAl with Cr additions are of interest for high temperature applications in oxidizing environments, where an oxide layer such as SiO2 or Al2O3 should form to protect the base material. At elevated temperatures (600-1700°C), a protective SiO2 layer forms on MoSi2, while near 500°C pesting and/or accelerated oxidation could disintegrate the material to powder as Mo and Si oxidize to form a complex, thick, non-protective oxide layer. Use of γ -TiAl is limited by poor oxidation resistance, whereby layered mixed oxides of TiO2 and Al2O3 form. With the addition of Cr from 4 to 34 at%, results are varied: protective Al2O3 formation, mixed oxide formation as with TiAl or more rapid oxidation than TiAl. NiAl is currently used as a diffusion coating on Ni-based superalloys and is being considered for use as a structural material itself because of its excellent oxidation resistance, i.e. forming α-alumina above 1000°C. Recent work indicates that pure NiAl oxidized under low oxygen partial pressures in a contained atmosphere develops nonprotective oxide scales similar to accelerated oxidation of MoSi2. This study explores the parameters defining protective behavior of these intermetallics and attempts to describe and explain anomalies at low temperatures and pressures.