Abstract
The 8Nb isopleth section of a Ti-Al-Nb system is experimentally determined based on thermal analysis and thermodynamic calculation methods to obtain the phase transformation and equilibrium relations required for material design and fabrication. The phase transus and relations for the 8Nb-TiAl system show some deviations from the calculated thermodynamic results. The ordered βo phase transforms from the disordered β/α phases at 1200–1400 °C over a large Al concentration range, and this transformation is considered to be an intermediate type between the first- and second-order phase transitions. Moreover, the βo phases are retained at the ambient temperature in the 8Nb-TiAl microstructures. The ωo phase transforms from the highly ordered βo phase, rather than from α2 or βo with a low degree of atom ordering B2 (LOB2) structure, with Al concentration of 32–43 at% at approximately 850 °C. From the experimental detection, the transition of the ωo phase from the βo phase is considered to be a further ordering process.
Highlights
In the past decade, TiAl-based alloys have been considered promising candidates for high-temperature materials in aerospace and automotive applications because of their excellent properties of low density, high specific yield strength and stiffness, and favorable oxidation resistance and creep properties up to high temperatures [1,2,3]
As reported by Appel et al [3] and Erdely et al [7], both the hot-workability and the ductility can be effectively promoted by the β phase [5,8], which provides a sufficient number of independent slip systems to act as a ductile constituent in the final microstructure [9,10]
This work aims to establish the experimental isopleth and phase equilibria of the 8Nb-Ti-Al system, which have the treating conditions closer to the actual production, to obtain the phase transformation and equilibrium relationships required for material design and fabrication
Summary
TiAl-based alloys have been considered promising candidates for high-temperature materials in aerospace and automotive applications because of their excellent properties of low density, high specific yield strength and stiffness, and favorable oxidation resistance and creep properties up to high temperatures [1,2,3]. TiAl alloys containing high amounts of Nb, based on the γ-TiAl and α2 -Ti3 Al intermetallic, exhibit excellent high-temperature strength and oxidation resistance [4] and have attracted significant attention [3,5]. The solid-state transformation pathway and the microstructure of TiAl-based alloys can be manipulated through β/βo phases [3,7,11]. According to reports by Cheng et al [12], Kobayashi et al [13], and Takeyama et al [14], a multitude of solid-state transformations and resulting microstructural morphologies can be achieved by stabilizing the β phase [3]
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