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 inhigh Nb containing TiAl (high Nb-TiAl) alloys since the precipitation temperature is close to the expected servicetemperature of the alloys. In this study, in-situ synchrotron high energy X-ray diffraction (HEXRD) reveals thatthe O phase precipitates at 550 ◦C while it dissolves into the $\alpha_2$ phase at 750 ◦C during heat treatments. However,under external stress the O phase unexpectedly precipitates from $\alpha_2$ phase at 800 ◦C and even 900 ◦C. The Ophase formation proceeds further in the presence of a critical stress promoted by internal stress accumulation inthe $\alpha_2$ 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 $\alpha_2$ grainswith one of their <11$\bar{2}$0> directions aligned perpendicular and their 〈0001〉directions rotated by an angle of 120◦ out of the external load axis. This $\alpha_2$ phase orientation facilitates the $\alpha_2$ → O crystal transition during uniaxialcompression. Transmission electron microscopy (TEM) study shows that stress-induced $\alpha_2$ → O transformation isgoverned by small atomic shifts in the $\alpha_2$ lattice. In addition, the selective growth of certain O variants viashuffling along an [11$\bar{2}$0]$\alpha_2$ direction is found to accommodate the external strain component in this direction.