Stress-induced orthorhombic O phase in TiAl alloys

Abstract

The orthorhombic O phase precipitation within the D0₁₉-α₂ 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 °C while it dissolves into the α₂ phase at 750 °C during heat treatments. However, under external stress the O phase unexpectedly precipitates from α₂ phase at 800 °C and even 900 °C. The O phase formation proceeds further in the presence of a critical stress promoted by internal stress accumulation in the α₂ phase, whereas the reverse O→α₂ phase transformation takes place when the internal stresses are relaxed. Additionally, it has been revealed that the O phase preferentially precipitates from specifically oriented α₂ grains with one of their <11$\overline{2}$0> directions aligned perpendicular and their 〈0001〉 directions rotated by an angle of 120° out of the external load axis. This α₂ phase orientation facilitates the α₂→O crystal transition during uniaxial compression. Transmission electron microscopy (TEM) study shows that stress-induced α₂→O transformation is governed by small atomic shifts in the α₂ lattice. In addition, the selective growth of certain O variants via shuffling along an [11$\overline{2}$0]_α2 direction is found to accommodate the external strain component in this direction.