Microstructure and mechanical properties of a two-phase TiAl alloy containing Mo in single-, DS- and poly-crystalline forms

The effect of adding Mo on the microstructure and room temperature deformation behavior of polysynthetically twinned (PST) crystals, directionally solidified (DS) and polycrystalline forms of two-phase TiAl alloys were systematically investigated in order to get a basic conception for alloying additions on the two-phase TiAl compounds with the lamellar structure. It was found that the Mo addition in TiAl PST crystals increase both the yield stress and tensile elongation to fracture but the increase in yield stress depends on the angle phi at which the lamellar boundaries lie from the loading axis. The large difference in yield stress between specimens deformed parallel (phi = 0 degrees) or perpendicular (phi = 90 degrees) to the lamellar boundaries and those deformed in intermediate orientations could be explained by the difference in Mo content between the TiAl and the Ti3Al phases. It was also found that the Mo-doped specimens with intermediate orientation fail by cracking zigzag across to the lamellar boundaries, which is the same fracture mode as that of binary specimens with intermediate orientations tested in vacuum. This suggests that Mo atoms are thought to play a role to reduce the environmental embrittlement of binary PST crystals, resulting in increasing the tensile ductility. Tensile elongation as large as 6.5% was observed for a polycrystalline specimen with duplex structure, but it varied with strain rate and test environment. In case of the fully lamellar structure, however, tensile elongation lower than 2% was obtained both in polycrystalline and in DS specimens.