Stress-induced orthorhombic O phase in TiAl alloys

The orthorhombic O phase precipitation within the D0(19)-alpha(2) phase has attracted increasing attention recently in high Nb containing TiAl (high Nb-TiAl) alloys since the precipitation temperature is close to the expected service temperature of the alloys. In this study, in-situ synchrotron high energy X-ray diffraction (HEXRD) reveals that the O phase precipitates at 550 degrees C while it dissolves into the alpha(2) phase at 750 degrees C during heat treatments. However, under external stress the O phase unexpectedly precipitates from alpha(2) phase at 800 degrees C and even 900 degrees C. The O phase formation proceeds further in the presence of a critical stress promoted by internal stress accumulation in the alpha(2) phase, whereas the reverse O ->alpha(2) phase transformation takes place when the internal stresses are relaxed. Additionally, it has been revealed that the O phase preferentially precipitates from specifically oriented alpha(2) grains with one of their <112<overline>0> directions aligned perpendicular and their < 0001 > directions rotated by an angle of 120 degrees out of the external load axis. This alpha(2) phase orientation facilitates the alpha(2)-> O crystal transition during uniaxial compression. Transmission electron microscopy (TEM) study shows that stress-induced alpha(2)-> O transformation is governed by small atomic shifts in the alpha(2) lattice. In addition, the selective growth of certain O variants via shuffling along an [112<overline>0](alpha 2) direction is found to accommodate the external strain component in this direction.