Quasi-Seeding Mechanism in Lamellae Alignment of TiAl Alloys During Directional Solidification
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ASED alloys are considered to be potential candidates for the new-generation aero-engine applications due to their low density, excellent high-temperature mechanical properties, and good oxidation resistance.[1–4] Among various microstructures of TiAlbased alloys, fully lamellar microstructure exhibits an excellent balance between the tensile property and ductility when the lamellar microstructure is parallel to the load axis.[5,6] However, the worst samples in which the lamellar microstructures are normal to the growth direction were usually produced by simple directional
solidification. Thus, a seeding mechanism has been developed and then used to align the lamellar microstructure of TiAl alloys. And it is suggested that the seed material must meet the following four requirements[7,8]: 1. 2.
3.
4. YUJUN DU is with the State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China and also with the Western Superconducting Technologies Co., Ltd., Xi’an 710018, China. JUN SHEN, YILONG XIONG, QIUDONG LI, and HENGZHI FU are with the State Key Laboratory of Solidification Processing, Northwestern Polytechnical University. Contact e-mail: [email protected] Manuscript submitted October 12, 2018. Article published online June 12, 2019 4166—VOLUME 50A, SEPTEMBER 2019
The a phase is the primary solidification phase. Upon heating above the a fi a2 + c eutectoid temperature, the lamellar microstructure is stable and the a2 phase is simply disordered to the a phase. Upon heating, the a phase is thermodynamically stable and the volume fraction of the a phase increases by thickening a lamellae instead of by nucleating a new a lamellae so that the high-temperature a phase has the same orientation as that of the a2 lamellae in the original lamellar microstructure. Upon cooling, the process is reversed and the original orientation of the lamellar microstructure is restored.
Based on these requirements, two aspects of the seed are assured: one is the preparation of the seed ingot with appropriate lamellae orientation (requirement 1), and the other is the availability, or more precisely, the thermodynamic stability of the lamellar microstructure
METALLURGICAL AND MATERIALS TRANSACTIONS A
of the seed ingot (requirements 2 to 4). When the requirement 1 is satisfied, the seed ingot, in which the lamellar microstructure is uniform and parallel to the longitudinal direction of the ingot, can be obtained due to the a solidification. When the seed material meets the requirements 2 to 4 further, the lamellae orientation can remain unchanged after the heat treatment. According to these requirements, it is easy to identify if one alloy can be used to prepare a qualified seed for lamellae alignment. However, for most TiAl alloys, it is difficult to satisfy these four requirements simultaneously despite the fact that they can achieve either the requirement 1 or the requirements 2 to 4. To prepare a qualified seed ingot, extensive studies have been performed so far, but only several TiAl alloys (Ti-43A
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