THE EFFECT OF ORIENTATION AND LAMELLAR STRUCTURE ON THE PLASTIC BEHAVIOR OF TIAL CRYSTALS

Crystals of binary TiAl(48.1-51.6 at.% Al) containing a single set of lamellae were grown using the floating zone method, and the spacings between the lamellae were changed by altering the crystal growth rate and the alloy composition. The plastic behavior of TiAl was found to depend strongly on the lamellar spacing and the angle between the loading axis and the lamellar planes. When the stress is applied parallel or perpendicular to the lamellar planes, shear deformation proceeds on {111} planes which cross the lamellae, the specimens are very strong and they exhibit low ductility. However, when the stress is applied at intermediate angles, slip occurs on {111}planes parallel to the lamellae at a low applied stress and good ductility is observed. The mode of deformation in the alpha-2 phase, i.e. {1121BAR}[1126BAR] or {1010BAR}[1210BAR] slip, strongly affects the plastic behavior of lamellar TiAl compounds. A fine and uniform distribution of lamellae is necessary to obtain high strength and good ductility.