Effect of deformation temperature on fatigue and fracture behavior in TiAl polysynthetically twinned crystals

The temperature and orientation dependence of cyclic deformation, fatigue life, and fracture behavior in TiAl polysynthetically twinned (PST) crystals were investigated, focusing on the change of plastic strain energy and deformation mode in the gamma domains. Stress-controlled fatigue tests were performed at 1 or 10 Hz using the same stress amplitude in tension and compression (R = -1) over a temperature range from -196 degrees C to 700 degrees C. The fatigue strength at phi = 45 deg (phi being the angle between the loading axis and lamellar planes) decreased monotonically with increasing temperature. At phi = 0 deg, the fatigue strength was high up to 500 degrees C, but the fatigue life decreased rapidly above 600 degrees C because of dynamic recovery and interlamellar separation. The plastic strain energy-stress amplitude curves in specimens fatigued with phi = 45 deg increased monotonically with stress amplitude for all temperatures and for higher temperatures with phi = 0 deg. At 25 degrees C and -196 degrees C with phi = 0 deg, three regions in the plastic strain energy-stress amplitude curves were observed. This anomalous change in the plastic strain energy at lower temperatures was due to a transition in primary deformation mode between twinning and slip by ordinary dislocations in some domain orientations.