High-temperature fracture and fatigue-crack growth behavior of an XD gamma-based titanium aluminide intermetallic alloy

A study has been made of the effect of temperature (between 25 °C and 800 °C) on fracture toughness and fatigue-crack propagation behavior in an XD-processed, γ-based titanium aluminide intermetallic alloy, reinforced with a fine dispersion of ∼1 vol pct TiB2 particles. It was found that, whereas crack-initiation toughness increased with increasing temperature, the crack-growth toughness on the resistance curve was highest just below the ductile-to-brittle transition temperature (DBTT) at 600 °C; indeed, above the DBTT, at 800 °C, no rising resistance curve was seen. Such behavior is attributed to the ease of microcrack nucleation above and below the DBTT, which, in turn, governs the extent of uncracked ligament bridging in the crack wake as the primary toughening mechanism. The corresponding fatigue-crack growth behavior was also found to vary inconsistently with temperature. The fastest crack growth rates (and lowest fatigue thresholds) were seen at 600 °C, while the slowest crack growth rates (and highest thresholds) were seen at 800 °C; the behavior at 25 °C was intermediate. Previous explanations for this “anomalous temperature effect” in γ-TiAl alloys have focused on the existence of some unspecified environmental embrittlement at intermediate temperatures or on the development of excessive crack closure at 800 °C; no evidence supporting these explanations could be found. The effect is now explained in terms of the mutual competition of two processes, namely, the intrinsic microstructural damage/crack-advance mechanism, which promotes crack growth, and the propensity for crack-tip blunting, which impedes crack growth, both of which are markedly enhanced by increasing temperature.