| The environmental resistance of alloys based on the binary titanium-aluminum system was investigated from the standpoint of both oxidation behavior and the progressive degradation of mechanical properties. Cast gamma TiAl alloys, in particular Ti-48Al-2Cr-2Nb, were found to oxidize rapidly via the formation of titanium-rich oxide scales under most oxidizing conditions. Factors which were found to play major roles include the presence of nitrogen and water vapor in the oxidizing environment, the level of surface deformation of the test specimen, and the exposure temperature. The presence of nitrogen and the exposure temperature interact in what has been termed the "nitrogen effect." The nitrogen effect promotes accelerated oxidation at elevated temperatures by the formation of rapidly oxidizing titanium nitrides. It was found that this effect gradually diminishes in importance at lower temperatures.; In addition to gamma TiAl alloys, an alloy based on beta titanium strengthened by Ti{dollar}sb2{dollar}AlNb ("orthorhombic") was investigated. Such alloys showed a tendency for rapid oxidation, along with exhibiting a transition from parabolic to linear oxidation at 800{dollar}spcirc{dollar}C.; Oxidation was found to affect mechanical properties, notably ductility, in a similar manner for both types of alloys. In general, the embrittlement can be classified as a surface phenomenon, rather than a bulk one. A transformed layer forms at the scale/alloy interface, along with a morphologically untransformed, interstitially hardened layer. In addition, oxygen penetrates down grain boundaries, resulting in the formation of new phases. For gamma TiAl alloys, the hardening of the untransformed material is most important, as the transformed layer is typically quite thin. For orthorhombic alloys, the transformed layer is more significant in physical extent, and plays a correspondingly larger role. It was found that in general, the orthorhombic alloy was more susceptible to embrittlement. |
