Atypical pathways for lamellar and twinning transformations in rapidly solidified TiAl alloy

Abstract

Additively manufactured alloys undergo a rapid solidification process followed by thermal cycles, which promotes the microstructures to return to equilibrium. The microstructural behavior therein is especially complicated and inconceivable at times, thereby greatly restricting the tailorability of microstructures for additive manufacturing. To improve the understanding of microstructure formation in rapidly solidified TiAl alloy, Ti–47Al–2Cr–2Nb powders were aging-treated to reveal the microstructural evolution behavior on multiple scales, and the microstructural transformation mechanisms were comprehensively analyzed from the viewpoints of thermodynamics and kinetics. The results show that the rapidly solidified TiAl alloy almost retains the α2 phase, which then transforms into nanolamellar structures or equiaxed γ microstructures depending on the temperature. The completely different microstructure selections were determined by the competitive growth of lamellar structures and equiaxed γ grains. More importantly, atypical pathways for lamellar and twinning transformations were first discovered in these microstructural transformation processes. A 6H-type long periodic stacking ordered (LPSO) phase was observed to be in the form of straight laths in the retained α2 phase, and 9R- and 18R-type LPSO phases were discovered in the equiaxed γ grains. The 6H-type LPSO phase transformed into nano γ laths, whose formation driving force was deduced to result from the extremely high elastic energy stored in the retained α2 phase. The 9R- and 18R-type LPSO phases transformed into a more stable γtwin phase to reduce the entire energy of the system. This research may potentially enhance the understanding of the microstructure of additively manufactured TiAl alloy and consequently, the tunability of such microstructure as required.