Phase transformation mechanism and microstructure of a Y-doped TiAl gas-atomized powders

In this study, the phase transformation mechanism of a yttrium-containing R-solidified TiAl alloy (Ti-43Al-9 V0.3Y at.%), prepared by gas atomization, was systematically investigated. X-ray diffraction, electron backscatter diffraction, scanning electron microscopy, and transmission electron microscopy were utilized to comprehensively analyze the morphology and microstructure of powders with varying sizes as well as the form and distribution of yttrium and its influence on the phase transformation of the powders. The results show that the solidification phase structure of the powders exhibits significant variations: the ultra-fine powder consists of alpha' martensite and remaining R phase, while the medium-sized powder is solely composed of R0 phase. The largesized dendritic powder comprises R0, alpha' martensite and alpha 2 phase. With an increase in powder size, there is a corresponding increase in the content of alpha 2 phase, whereas the content of martensite initially rises and subsequently declines. Additionally, yttrium is present in the form of multiscale Y-rich precipitates (YAl2 and Y2O3) within the matrix, and the segregation degree gradually increases with increasing powder size. The primary factors contributing to the disparity in solidification structure include cooling rate and segregation defects. A faster cooling rate and a higher supercooling degree will inhibit the R -> alpha transition, while the Y-rich precipitated phase forms a pre-existing strain zone around it, providing an effective site for martensitic nucleation. In summary, these findings offer novel insights into the mechanism of phase transformation in yttrium-containing R-solidified TiAl alloy, thereby contributing to further advancements in the theory of rapid solidification for TiAl alloys.