Abstract

    At high temperatures, and when subjected to mid-range temperatures for long times, superalloys can reach states which approach equilibrium. Knowledge of stable phase structure at fabrication and working temperatures can, therefore, be very important and experimental determination of Ni-based binary and ternary systems has been reasonably extensive. However, when working with such highly alloyed multi-component materials as superalloys information based purely on experimental determination of lower order systems cannot always be directly applied to ‘real’ alloys. The field of computer aided thermodynamic phase diagram calculations holds substantial promise in this respect as it possible to make pre-dictions for the phase behaviour of multi-component alloysbased on models for the binary and ternary phase diagrams.

    This paper will present a review of results which can now be obtained in alloys from the following multi-component system

Ni-Al-Co-Cr-Hf-Mo-Nb-Ta-Ti-W-Zr-B-C

Current work involving the extension of the database to include Fe and Re will also be presented. A further advantage of the CALPHAD route is that it is possible to predict properties other than those associated with equilibrium diagrams and the application to non-equilibrium solidification and the prediction of APB energies in γ' will be presented.

Introduction

    The computer calculation of phase equilibria in multicomponent alloys is becoming increasingly commonplace and it is now possible to make very accurate predictions for phase equilibria in a number of the more commonly used metallic and intermetallic alloys. These range from steels1 to Ti-aluminides2. This paper will present results which can now be obtained in Ni-based superalloys giving a number of examples of where this methodology has been applied.

    The CALPHAD method first requires that sound mathematical models exist for describing the thermodynamic properties of the various phases which can appear in an alloy. The coefficients used by the models are then held in databases which are accessed by software packages such as Thermo-Calc3 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 which

can deviate substantially from equilibrium4,5.

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