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Title:: Progress in the Phase Transformation Mechanisms of Ordered ω and Orthorhombic Phases in TiAl Alloys

Author(s):: SONG Lin1; ZHANG Tiebang1; LIN Junpin2; （1. State Key Laboratory of Solidification Progressing, Northwestern Polytechnical University, Xian 710072, China）（2. State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China)

Keywords:: TiAl alloys; ordered ω phase; orthorhombic phase; phase transformation mechanisms; microstructure

Abstract:: Light-weight high-strength TiAl alloys have gained great expectation in the aerospace market as high-temperature structural materials for the weight reduction of advanced aeroengines. Wrought TiAl alloys containing certain amount of β phase are important categories for future development of these alloys. In order to facilitate hot deformation, β phase is stabilized by β stabilizing elements, leading to a number of complex phase transformations at intermediate temperatures, i.e., the application temperatures, such as ωo and orthorhombic phase transformations as well as the decomposition of metastable α2 phase. These phase transformations have crucial effects on the stability of the microstructures, which are essentially related to the long-term service properties of the alloys. On the other hand, the applied stress also directly affects these phase transformations at intermediate temperatures, accelerating the evolution of the microstructures. In this article, the recent researches on the ωo and orthorhombic phases are summarized in perspective of microstructure characterization. Mechanisms of the phase transformations in the βo phase region, the precipitation of ωo and orthorhombic phases from α2 phase as well as the effects of stress on these phase transformations are reviewed. The effects of the above-mentioned phase transformations on the microstructure stability are summarized and the future research directions are prospected.