Abstract
Mutual transformation between α2 and ωo phases has been an interesting topic in recent years. In this study, martensitic α2 was obtained by air-cooling from 1250 °C in Ti-37.5Al-12.5Nb (at%) alloy while four ωo variants formed in the βo phase matrix during the cooling process. Nonetheless, only one ωo variant was observed at the periphery of the α2 plates in the βo phase and the orientation relationship between these two phases was [0001] α2//[\({1\overline{2}10}\)] ωo; (\({11\overline{2}0}\)) α2//(0002) ωo. Thin γ plates precipitated within the α2 phase and were thought to be related to the appearance of ωo phase. The redistribution of the compositions during the phase transformations was studied by energy dispersive X-ray spectroscopy analysis. The corresponding mechanisms of the phase transformations mentioned above are discussed.
Highlights
High Nb-containing TiAl (Nb-TiAl) alloys have been considered as potential materials for high-temperature applications due to their low density, high strength, good oxidation resistance, and creep properties [1,2,3]
Thin foils used for transmission electron microscopy (TEM) observation were prepared by twin-jet electro-polishing in a solution of
The microstructure is composed of α2 plates and βo matrix which can be identified by TEM as in Figures 1b and 2b
Summary
High Nb-containing TiAl (Nb-TiAl) alloys have been considered as potential materials for high-temperature applications due to their low density, high strength, good oxidation resistance, and creep properties [1,2,3]. Stark et al showed that the amount of ωo phase increased with the content of Nb in high Nb-TiAl alloy [4,5]. Numerous studies have reported the ωo phase transformations in high Nb-TiAl alloys, indicating that the ωo phase is stable at 700–900 ◦ C [10,11]. These studies have mainly focused on the transition process between the βo and ωo phases [6,7,8,9,10,11,12,13] or the α2 to βo phase [14,15,16].
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