TEMPERATURE-DEPENDENCE OF YIELD STRESS, DEFORMATION MODE AND DEFORMATION STRUCTURE IN SINGLE-CRYSTALS OF TIAL (TI-56 AT-PERCENT AL)

The plastic deformation behaviour of single crystals of TiAl with a co mposition of Ti-56 at.% Al has been studied in compression as a functi on of crystal orientation in the temperature range from -196 to 1100 d egrees C. The profile of yield stress-temperature curves for all orien tations studied can be divided into three temperature regions; the yie ld stress rapidly decreases with increasing temperature at both low an d high temperatures and an anomalous increase in yield stress is obser ved at intermediate temperatures, exhibiting a peak at 700-1000 degree s C depending on crystal orientation. Both ordinary and [101] superlat tice slip exhibit an anomalous increase in critical resolved shear str ess (CRSS). However, the extent of the anomaly associated with the for mer 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 ther mal reversibility. Superlattice dislocations with b = [101] in specime ns deformed in the anomalous temperature region mostly align along the ir 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 degrees C. This res ults 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 wi th many cusps are observed to align along their screw orientation The dislocation mechanisms of the anomaly for ordinary slip may be differe nt 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.