Temperature dependence of yield stress, deformation mode and deformation structure in single crystals of TiAl (Ti−56 at.% Al)

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Abstract
The plastic deformation behaviour of single crystals of TiAl with a composition of Ti-56 at.% Al has been studied in compression as a function of crystal orientation in the temperature range from −196 to 1100°C. The profile of yield stress-temperature curves for all orientations studied can be divided into three temperature regions; the yield stress rapidly decreases with increasing temperature at both low and high temperatures and an anomalous increase in yield stress is observed at intermediate temperatures, exhibiting a peak at 700–1000°C depending on crystal orientation. Both ordinary and [101] superlattice slip exhibit an anomalous increase in critical resolved shear stress (CRSS). However, the extent of the anomaly associated with the former slip is much smaller than that for the latter slip. The CRSS for [101] slip depends on crystal orientation. In the anomalous temperature region, the CRSS exhibits very small strain-rate sensitivity and thermal reversibility24Jun1996. Superlattice dislocations with b = [101] in specimens deformed in the anomalous temperature region mostly align along their screw orientation and adopt a locked configuration. These features are very similar to those observed for the anomalous strengthening of Ni3Al. The CRSS for ordinary slip is considerably larger than that for [101] superlattice slip at temperatures below 600°C. This results in a limited orientation range for the operation of ordinary slip. In the anomalous temperature region, the strain-rate sensitivity of CRSS for ordinary slip is also very small and 1/2 (110] dislocations with many cusps are observed to align along their screw orientation. The dislocation mechanisms of the anomaly for ordinary slip may be different from those for [101] superlattice slip. Above the peak temperature, [110] slip, twinning and [112] superlattice slip are identified to be operative. Slip along [110] occurs not only on {111} but also on (001) and {110), depending on orientation. Twinning and [112] superlattice slip occur only for limited orientations.