Development of Thermodynamic Description of a Pseudo-Ternary System for Multicomponent Ti64 Alloy

A method is presented to obtain a thermodynamic description of a multicomponent alloy system in terms of a pseudo-ternary formalism. In the present study, this method was applied to multicomponent Ti64 alloys with the “6” and “4” denoting the nominal 6 wt.% of Al and 4 wt.% of V in this class of Ti alloys. However these alloys, in practice, contain many other alloying elements such as O, C N, Fe, and H which exhibit appreciable effect on the thermodynamic properties of Ti64. The essence of this approach is to treat the multicomponent alloys as a pseudo-ternary Ti-Al _x -V _y , in which the apparent composition of Al _x is the weighted summation of that of Al and those of the α-stabilizers in minor quantities such as O, C and N and that of V _y is the weighted summation of that of V and those of the β stabilizing minor elements such as Fe and H. Three Ti64 alloys with slightly different chemistries were treated as pseudo-ternary alloys, and the β transus and β (and α) approach curves were calculated using this pseudo-ternary thermodynamic description. The calculated results were found to compare favorably with those calculated from a multicomponent thermodynamic database for titanium alloys. This study demonstrates that such a simplified thermodynamic description, when available, can be readily coupled with kinetic models for engineering applications. One example is to predict microstructure evolution of a multicomponent alloy when subjected to a specific heat treatment condition.