Determination of Phase Equilibria by Experiment of Calonlation in F-Al-X Ternary System

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Phase equilibria among the beta (bcc or B2), alpha (hcp) and gamma (L1_0) phases in Ti-Al-M ternary systems at elevated temperatures have been studied, where the M is beta stabilizing element for pure titanium. The beta (B2) +alpha+gamma three-phase coexisting region exists at temperatures above 1473 K,and it moves towrds a direction of high aluminum and high M concentrations with increasing temperature. The change in the phase equilibria by the addition M is associated with the alpha<tautomer>beta allotropic transformation temperature of pure titanium and can be thermodynamically interpreted in terms of the lattice stability ratios LAMBDA of M to Ti. The LAMBDA for beta against alpha in ternary Ti-Al-M systems, for example, is defined as follows : LAMBDA^<beta*alpha>_i=DELTAﾟG^<beta*alpha>_/DELTAﾟG^<beta*alpha>_. The DELTAﾟG^<beta*alpha>_ refers to the difference in Gibbs free energy between bcc (beta) structure and hcp (alpha) structure for pure M, and DELTAﾟG^<beta*alpha>_ that for … More pure titanium. The values of LAMBDA^<beta*alpha>_i for each element is nearly equal to 10 and at least one oeder of magnitude larger than those of LAMBDA^<alpha*gamma>_i. The reason the LAMBDA^<beta*alpha>_i has a large positive value for each element is that the value of DELTAﾟG^<beta*alpha>_ is fairly smaller than that of DELTAﾟG^<beta*alpha>_, since the temperature to be interest is only 300 K away from the beta*alpha allotropic transformation temperature T^<beta/alpha>_ (1155 K) of pure Ti. When the temperature raises and the difference in temperature from the T^<beta/alpha>_ becomes large, the value of DELTAﾟG^<beta*alpha>_ almost remains unchanged whereas that of DELTAﾟG^<beta*alpha>_ increases, leading to a smaller value of LAMBDA^<beta*alpha>_i. This is responsible for the three-phase coexisting region moving toward the direction of high aluminum and high M concentration as temperature increases.At the temperature below the alpha<tautomer>alpha_2 congruent temperature T^b_ in the binary system (T^b_ : 1452K), alpha_2-Ti_3aL phase appears as a stable phase as well, together with the other three phases. We identified that the beta+alpha+gamma three-phase coexisting region at higher temperatures changes to beta+alpha_2+gamma three-phase coexisting region at lower temperatures through a transition peritectoid reaction (beta+alpha<tautomer>alpha_2+gamma) when the thrid element M stablizes the alpha_2 phase against alpha phase. Less