Microstructural stability of long term aging treated Ti-22Al-26Nb-1Zr orthorhombic titanium aluminide
(State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China)
Abstract: The microstructure development of lamellar structure of an orthorhombic Ti2AlNb-based Ti-22Al-26Nb-1Zr alloy, including B2 decomposition and spheroidization of O phase, was investigated. The results show that the lamellar structure is fabricated by heating the samples in the single B2 phase field and cooling slowly in the furnace. Aging treatments are conducted in the (O+B2) phases field by air cooling. After aging at 700 °C for a short time within 100 h, there is no significant change of microstructures, whereas the coarsening of lamellae is observed in the long-term aged microstructure. Ti-22Al-26Nb-1Zr alloy exhibits microstructural instability including the severe dissolution of B2 lamella, discontinuous precipitation and spheroidization of O phase during the long term aging process at 700 °C up to 800 h. In addition, a pronounced formation of branch-shaped O phase lamella is observed for the alloy aged over 100 h.
Key words: orthorhombic titanium aluminide; microstructural evolution; aging treatment; spheroidization