High-Temperature Fracture and Fatigue-Crack Growth Behavior of an XD Gamma-Based Titanium Aluminide Intermetallic Alloy

摘要

A study has been made of the effect oftemperature (between 25 deg C and 800 deg C) on fracturetoughness and fatigue-crack propagation behavior in an XD-processed, #gamma#-based titanium aluminide intermetallicalloy, reinforced with a fine dispersion of ~1 vol pct TiB_2particles. It was found that, whereas crack-initiation toughnessincreased with increasing temperature, the crack-growthtoughness on the resistance curve was highest just below theductile-to-brittle transition temperature (DBTT) at 600 deg C;indeed, above the DBTT, at 800 deg C, no rising resistancecurve was seen. Such behavior is attributed to the ease ofmicrocrack nucleation above and below the DBTT, which, inturn, governs the extent of uncracked ligament bridging in thecrack wake as the primary toughening mechanism. The corres-ponding fatigue-crack growth behavior was also found to varyinconsistently with temperature. The fastest crack growth rates(and lowest fatigue thresholds) were seen at 600 deg C, whilethe slowest crack growth rates (and highest thresholds) wereseen at 800 deg C; the behavior at 25 deg C was intermediate.Previous explanations for this "anomalous temperature effect" in#gamma#-TiAl alloys have focused on the existence of someunspecified environmental embrittlement at intermediatetemperatures or on the development of excessive crack closureat 800 deg C; no evidence supporting these explanations couldbe found. The effect is now explained in terms of the mutualcompetition of two processes, namely, the intrinsicmicrostructural damage/crack-advance mechanism, whichpromotes crack growth, and the propensity for crack-tip blunting,which impedes crack growth, both of which are markedlyenhanced by increasing temperature.