α(α2)/γ Phase Equilibria In Ti-Al-∑X(X=B, C, Fe, Si, Cr, Nb) Multi-Component Systems

It has been shown that additions of alloying elements such as B, C, Fe, Si, Cr-Nb, Cr-Si and Nb-Si to γ-TiAl-based alloys is an effective method to improve the properties of the alloys. However, the α(α₂)/γphase equilibria in the Ti-Al-X(X=B, C, Fe, Si) ternary systems have not been studied systematically, and the α(α₂)/γphase equilibria in the Ti-Al-Cr-Si, Ti-Al-Cr-Fe, Ti-Al-Nb-Si and Ti-Al-Nb-Fe quarternary systems are never reported. In the present work, the α(α₂)/γphase equilibria in Ti-Al-B, Ti-Al-C, Ti-Al-Fe, Ti-Al-Si, Ti-Al-Cr-Fe, Ti-Al-Cr-Si, Ti-Al-Nb-Fe and Ti-Al-Nb-Si systems have been determined using alloy-equilibrium-diffusion treatment, diffusion couple and electron probe microanalysis (EPMA). Recent work in our group has indicated that the grand potential method (GPM) is effective to calculate phase equilibrium in binary systems. In the present work, the applicability of GPM in ternary systems has been studied systematically and the α/βand the α/γphase equilibria in the Ti-Al-Nb ternary system has been calculated by GPM. The α(α₂)/γphase equilibria at 1000¹250℃in Ti-Al-X(X=B, C, Fe, Si) ternary systems were determined by alloy-equilibrium-diffusion treatment and EPMA. According to the obtained data, the partial vertical sections of Ti-Al-X(X=B, C, Fe, Si) ternary systems have been plotted at 0.1at.%B, 0.2at.%C, 0.3at.%Si and 1.0at.%Fe, respectively. The results are as following: (1) The addition of B to the Ti-Al binary system results in a shift of both the α(α₂)/(α(α₂)+γ) and the (α(α₂)+γ)/γboundaries to the Al-rich side and 0.1at.%B addition decreases Tαby 18℃. The addition of C leads to a shift of the (α(α₂)+γ)/γboundary to the Al-rich side but the effect of 0.2at.%C addition on Tαis very slight. The addition of 0.3at.%Si increases Tαby 80¹10℃and 1.0at.%Fe addition by 90¹10℃. However, 1.0at.% Fe addition leads to a limited shift of the (α(α₂)+γ)/γboundary to the Ti-rich side, whereas the effect of 0.3at.%Si addition on the (α(α₂)+γ)/γboundary is quite slight. (2) The solubilities of B, C and Si in the α(α₂) phase are larger than those in the γphase, whereas the solubility of Fe in the α(α2) phase is smaller than that in the γphase. With increasing temperature, the partitioning ratio KFeα/γincreases gradually. (3) A third phase Ti5Si3 appears in Ti-46Al-1.0Si alloy and obviously restrain coarsening of primary lamellae of the alloy at elevated temperature. A new method of diffusion couple was designed to determine the α/γphase equilibrium in quarternary systems. By using this method, the α/γphase equilibria at 1150¹250 ℃in the Ti-Al-Cr-Fe, Ti-Al-Cr-Si, Ti-Al-Nb-Fe and Ti-Al-Nb-Si quarternary systems have been determined. Based on the measured α/γphase equilibrium data, the partial isothermal squares for the Ti-Al-Cr-Fe, Ti-Al-Cr-Si, Ti-Al-Nb-Fe and Ti-Al-Nb-Si quarternary systems have been obtained. The results show that the addition of Cr to the Ti-Al-Fe ternary system increases the partitioning ratio KFeα/γat given temperature, whereas the addition of Cr to the Ti-Al-Si ternary system decreases the partitioning ratio KSiα/γ. Both the additions of Si and Fe to the Ti-Al-Cr ternary system increase the partitioning ratio KCrα/γat given temperature. In this work, the principle, equations, approaches and programs for calculating the miscibility gap and the dissimilar phase equilibrium in ternary system by GPM are given systematically. Based on the theoretical analysis, a series of phase diagrams with miscibility gap and dissimilar phase equilibrium in ternary systems at a given temperature have been calculated out with various thermodynamic parameters. The results show that the calculation of phase equilibrium in ternary system by GPM has characteristics of clear geometrical relationship, convenient approach and wide application. Comparing with the usual optimization method for calculating the minimum value of the Gibbs free energe, GPM in ternary system has no convergent problem and has a high calculation efficiency. Based on the GPM in ternary system above, a new method to calculate the phase transition temperature in ternary system was proposed and is also applied to calculate the α→(α+γ) transition temperature Tαin Ti-Al-Nb system. The calculated results agree well with the experimental data. According to experimental data of phase equilibria in ternary system, the methods (the T0 line method and the chemical potential equality method) to determine the binary interaction parameters in metastable phases were proposed. By these methods, the interaction parameters IαA lNb, IγA lNb and IγN bTi are obtained with the subregular solution model based on the experimental data of α/βand α/γphase equilibria in the Ti-Al-Nb ternary system. The interaction parameters, IβT iAl, IγT iAl, IαTi2Al, IαN bTi, IβN bTi and IAβlNb were calculated using the experimental data in the Ti-Al,Al-Nb and Nb-Ti binary systems. Using the obtained thermodynamic parameters above, the phase equilibria α/β, α/γ, α2/γin the Ti-Al binary system, L/βin the Al-Nb binary system, α/βin the Nb-Ti binary system, α/β( at 1150℃and 1400℃) and α/γ(at 1150℃, 1200℃, 1250℃, 1300℃and 1400℃) in the Ti-Al-Nb ternary system were calculated. The results are consistent well with the experimental phase diagram.