On the anomalous temperature dependence of fatigue-crack growth in γ-based titanium aluminides

Abstract

Recently, there has been considerable interest in {gamma}-TiAl based titanium aluminides as light-weight structural materials, particularly in the aerospace industry where they are under consideration for gas-turbine engine applications. As they are candidate materials for elevated temperature use, an understanding of their fatigue and fracture properties at temperatures comparable to operating conditions is essential. In this regard, an anomalous temperature dependence of fatigue-crack propagation and the fatigue threshold ({Delta}K{sub TH}) has been reported for {gamma}-based alloys between 25 and 800--850 C in the air environments. Based on observations that the effect is less striking in vacuo compared to air, Larsen et al. attributed such behavior to environmental embrittlement and a lack of ductility at intermediate temperatures (on the assumption that {Delta}K{sub TH} should increase with increasing temperature in the absence of embrittlement). Since this latter assumption is questionable, the authors considered here the role of crack-surface oxidation and its tendency to retard crack-growth rates, particularly at near-threshold levels, via crack-tip blinting and shielding by oxide-induced crack closure. In this study, the role of this mechanism on fatigue-crack growth rates in {gamma}-based titanium aluminides is evaluated as a function of temperature, and an alternative explanation is presented for the anomalous temperaturemore » dependence of the near-threshold fatigue behavior.« less