ENVIRONMENTAL-EFFECTS ON THE ROOM-TEMPERATURE DUCTILITY OF POLYSYNTHETICALLY TWINNED (PST) CRYSTALS OF TIAL

Environmental effects on the room temperature ductility of polysynthetically twinned (PST) crystals of TiAl with two different lamellar orientations have been investigated through tensile tests conducted in four different atmospheres at strain rates in the range of 2.0 x 10(-4) to 1.0 x 10(-1) s-1. The tensile ductility of TiAl PST crystals is sensitive to test environment. It is higher when tested in vacuum or in dry air than in air or in hydrogen gas. Such environmental loss in ductility decreases with increasing strain rate. These results suggest that the environmental embrittlement of TiAl PST crystals can be interpreted in terms of hydrogen embrittlement. The fracture mode of TiAl PST crystals whose lamellar boundaries are inclined to the tensile axis also depends on test environment and strain rate. On the basis of the results obtained, the mechanism of the environmental embrittlement of TiAl PST crystals is discussed, with particular emphasis on the interaction of the lamellar boundaries with hydrogen atoms.