A study of phase transformations in the Ti-Al-V system.

Thermodynamic assessment of the Ti-Al-V system has been carried out using the Thermo-Calc. The database resulting from the present work gives calculated phase equilibria which are in good agreement with experimentally determined phase diagrams. The beta→B2 ordering process has been modelled as a second order transformation and an ordering model for the ternary B2 phase was proposed, which makes the Gibbs energy of the B2 phase converge to that of the disordered beta phase at the beta→B2 ordering temperature. The calculation of phase diagrams and beta→B2 ordering temperatures can thus be made from the same thermodynamic database. The predicted B2/beta boundary for Al-V rich alloys is in good agreement with experiments, while there is a discrepancy between the predicted and the experimentally determined beta/B2 boundaries for Ti-V rich alloys, probably due to the use of composition-independent w(2)/w(1) ratios in the present B2 model. It has been predicted that alpha2+gamma cannot be in equilibrium with disordered beta. Phase equilibria in arc-melted ingots of Ti-Al-V alloys have been studied using TEM, SEM and EDX analysis. Thermodynamic modelling has also proved to be a powerful approach to predict the as-solidified microstructures of these alloy ingots, even though they did not solidify under equilibrium conditions. Experiments have been performed to study metastable phase transformations in melt spun Ti-Al-V alloy ribbons. The results show that both thermodynamics and kinetics play an important role for phase selection in the alloy ribbons. Transient nucleation effects appear to be dominant in ≤50mum thick alloy ribbons. Large cooling rates tend to: (a) favour the primary phase selection in the order of: gamma>alpha>beta, (b) suppress solid state transformations, (c) lower the effective alpha →alpha2 ordering temperature, (d) to suppress completely the alpha →alpha2 ordering process, and (e) lead to massive transformations such as alpha→gammam and alpha →alpha2. An ordered o phase has been observed in the metastable B2 phase in Ti-Al-V alloys. Diffuse o formation has been observed for the first time in the disordered metastable P phase in V-50A1 and V-50Al+(≤30at%Ti) alloys. It has been shown that the o stability is related to both the unit cell volume (Vbeta) and the number of free electrons (e) per atom of the metastable beta/B2 phase. Small values of (e/Vbeta) lead to the formation of an w phase closer to the 3-D crystalline o structure