Abstract

    A reasonable experimental literature has now been built up on Ti-Al-X ternary phase diagrams. However, although these basic systems give insight to phase equilibria in certain commonly used γ-TiAl based alloys it is difficult to interpret phase relationships in multicomponent alloys using just this information.Thermodynamic calculations via the CALPHAD route1 offer a means by which phase equilibria in multi-component alloys can be predicted and the present paper presents typical results which can now be achieved using this methodology. As well as basic relationships between the γ-TiAl, α2-Ti3Al, β and α phases, the ordering of the β phase to B2 and the effect of O will be analysedand discussed.

Introduction

    The computer calculation of phase equilibria in multi-component alloys via the CALPHAD route1 is becoming increasingly popular and is now being used in practical applications for predicting phase equilibria in a number of the more commonly used metallic and intermetallic alloys. These range from Ni-based superalloys2 to Al-alloys3. Further, they can be used to predict phase formation under conditions well away from equilibrium. This paper will present results that can now be obtained in γ-TiAl based intermetallic alloys.

    The CALPHAD method first requires that sound mathematical models exist for describing the thermodynamic properties of the various phases that can appear in an alloy. The coefficients used by the models are then held in databases that are accessed by software packages such as Thermo-Calc4 which then perform a series of calculations, usually via Gibbs energy minimisation, to provide the user with detailed information on phase equilibria.These calculations can be augmented with kinetic modelling to provide answers for phase formation under conditions that can deviate substantially from equilibrium1,5,6,7.

    Attempts at modelling of γ-TiAl based alloys have mainly concentrated on the binary phase diagram8,9,10 or ternary subsystems11,12.However, although these basic systems give insight to phase equilibria in certain commonly used γ-TiAl based alloys it is difficult to interpret phase relationships in multi-component alloys using just this information.

    The present paper provides a brief introduction to the CALPHAD method and then presents a series of calculations for phase equilibria in a variety of γ-TiAl based alloys.

Background to the Calculation Method

    The roots of the CALPHAD approach lie in the mathematical description of the thermodynamic properties of the phases of interest. Details of modelling procedures can be found in the reviews of Ansara13 and Saunders and Miodownik1. All types of models require input of coefficients that uniquely describe the properties of the various phases and these coefficients are held in databases that are either in the open literature or proprie-tary.

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