Temperature dependence of yield stress, tensile elongation and deformation structures in polysynthetically twinned crystals of Ti-Al

Abstract

The plastic deformation behaviour of polysynthetically twinned (PST) Ti-Al with three different orientations has been studied in tension and compression as a function of temperature in a range from −196 to 1100°C. With increase in temperature, the yield stress decreases rather rapidly at low temperatures and then decreases gradually at intermediate temperatures for all orientations studied although the temperature dependence at low temperatures is less significant for an orientation where shear deformation occurs parallel to the lamellar boundaries. When the loading axis is perpendicular to the lamellar boundaries, the yield stress again rapidly decreases with increasing temperature at high temperatures. This is also the case for PST Ti-Al whose lamellar boundaries are parallel to the loading axis, although a small and broad anomalous yield stress peak is observed at 800°C for this orientation. When the lamellar boundaries are inclined at an intermediate angle to the loading axis, the yield stress gradually decreases with increasing temperature without showing any anomalous peak or rapid decrease at high temperatures. For the parallel and intermediate orientations, the tensile elongation is constant up to 200°C and begins to increase with increasing temperature at 400°C and continues to increase until 600°C. Above 800°C, tensile deformation for these two orientations is accompanied by the occurrence of necking at the part of specimens where recrystallization starts. For the perpendicular orientation, the tensile elongation is essentially zero at all temperatures studied and fracture occurs always parallel to the lamellar boundaries, indicating the weakness of such boundaries under tensile stress even at high temperatures.