Alloying mechanism of beta stabilizers in a TiAl alloy

The effects of beta stabilizers such as Fe, Cr, V, and Nb on the microstructures and phase constituents of Ti52Al48-XM (X = 0, 1.0, 2.0, 4.0, or 6.0 at. pct and M = Fe, Cr, V, and Nb) alloys were studied. The dependence of the tensile properties and creep resistance of TiAl on the alloying elements, especially the formation of B2 phase, was investigated. Fe is the strongest B2 stabilizer, Cr is second, V is an intermediate stabilizer, and Nb is the weakest stabilizer. The composition partitioning of Fe, Cr, V, and Nb in the gamma phase is affected by the formation of B2 phase. The peaks of the tensile strengths and creep rupture life of Ti52Al48-XM generally occur at the maximum solid solution of these elements in the gamma phase, which is just before the formation of B2 phase. Ti52Al48-0.5Fe shows an attractive elongation of 2.5 pct at room temperature, and the Ti52Al48-1V, Ti52Al48-Cr, and Ti52Al48-2Nb alloys have about 1.1 to 1.3 pet elongation at room temperature. The increase of tensile strengths and creep resistance with increasing Fe, Cr, V, and Nb contents is chiefly attributed to the solid-solution strengthening of these elements in the gamma phase. The appearance of B2 phase deteriorates the creep resistance, room-temperature strengths, and ductility. With respect to the maximum solid-solution strengthening, an empirical equation of the Cr equivalent [Cr] is suggested as follows: [Cr] = Cr + Mn + 3/5V + 3/8Nb + 3/2 (W + Mo) + 3Fe = 1.5 to 3.0. The solid-solution strengthening mechanism of Fe, Cr, V, and Nb at room temperature arises from the increase of the Ti 3s and Al 2s binding energies in Ti-Ti and Al-Al bonds, and the retention of the strength and creep resistance at elevated temperatures in Ti52Al48-XM is mainly attributed to the increase of the Ti 3s and Al 2s binding energies in Ti-Al bonds in gamma phase. The decrease of the Ti 3p and Al 2p binding energies in Ti-Ti, Ti-Al, and Al-Al bonds benefits the ductility of TiAl.