Experimental Phase Equilibria and Isopleth Section of 8Nb-TiAl Alloys

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.

Solid-Solid Phase Transformations and Their Kinetics in Ti-Al-Nb Alloys

“…By considering the fact that Nb is suspected to stabilize ωo [62,63], this large difference between the transformation temperatures is surprising. In a recent DTA investigation of alloys with 8 at.% Nb, Xu et al [53] determined the transformation temperature from ωo to (βTi)o for Al contents between 32 and 43 at.% Al to lie around 850 °C. When compared to alloy Ti-39.8Al-9.4Nb (A3) and Ti-36.4Al-9.7Nb (A4) with slightly higher Nb contents, these values are 60 °C below the measured transformation temperatures of 906 °C (A3) and 908 °C (A4).…”

“…By considering the fact that Nb is suspected to stabilize ω o [62,63], this large difference between the transformation temperatures is surprising. In a recent DTA investigation of alloys with 8 at.% Nb, Xu et al [53] determined the transformation temperature from ω o to (βTi) o for Al contents between 32 and 43 at.% Al to lie around 850 • C. When compared to alloy Ti-39.8Al-9.4Nb (A3) and Ti-36.4Al-9.7Nb (A4) with slightly higher Nb contents, these values are 60 • C below the measured transformation temperatures of 906 • C (A3) and 908 • C (A4). With in situ HEXRD, Stark et al [25] determined the transformation temperature in a Ti-45Al-10Nb alloy to be 870 ± 40 • C. For an alloy with the same composition, Chladil et al [52] determined the transformation temperature to be 913 ± 10 • C by conventional XRD and in situ HEXRD.…”