The effect of nitrogen, niobium and temperature of the high temperature oxidation of titanium aluminide alloys

Abstract

This is especially attractive for reducing the engine weight and increasing the thrust/weight ratio. Increasing the operating temperature and thereby the engine efficiency has approached the maximum possible operating temperature for currently used materials. A model set of three intermetallics -- unalloyed [gamma]-TiAl, unalloyed [alpha]-Ti 3Al and a nearly stoichiometric Ti2NbAl (an orthorhombic intermetallic structure) -- was chosen and investigated under a variety of experimental conditions. Gas mixtures of varying oxygen concentrations in both argon and nitrogen allowed the effect of oxygen concentration on oxidation kinetics to be investigated. The Ti 2NbAl-alloy was specifically chosen to examine the effect of Nb in a intermetallic compound (with Nb substituting for one Ti in the Ti 3Al intermetallic structure). This is in contrast to Nb as a simple alloy addition to y-TiAl or c 2-Ti 3Al. Exposures at temperatures of 700°C and 800°C for 20 and 100 hours were studied (compared to temperatures of 900°C or higher used by other investigators), since these temperatures promoted a more protective scale. Additionally, the scale composition is sensitive to temperature and restricts the conventional practice of corrosion study acceleration by raising the experimental temperature. The resulting corrosion scales were analyzed using electron microscopy (scanning and transmission), x-ray diffraction and thermogravimetric analysis. The effect of nitrogen in the gaseous environment was explored. The impact of Nb on the oxidation of the intermetallics was found to be more protective than what might be expected from examining the thermodynamics and kinetics of the Ti-Al-Nb-O-N system. At times an alternating layered scale structure resulted and the effect of N, Nb and the lower temperatures used in this study is summarized. The oxidation rate of the Ti 2NbAl-alloy fell between the rates for y-TiAl and c 2-Ti 3Al, with the scale on y-TiAl growing at the slowest rate of the three compositions. However, the superior mechanical properties of Ti2 NbAl-alloy combined with the acceptable corrosion resistance promote its consideration for replacing those portions of the engine operating at intermediate temperatures that would result in significant weight and cost savings.