Abstract

    In recent years thermodynamic modelling via the CALPHADmethod has been extensively applied to industrial alloys of manytypes. Although pertaining to equilibrium conditions, use has shown that valuable information can be gained for a variety of practical applications. A paper presented at the last Seven Springs meeting gave some theoretical background to the CALPHAD method and described the development of the methodology to Ni-based superalloys. The main purpose of that paper was to provide validation of results against an extensive experimental literature which, at the time, concentrated on γ/γ'  equilibria and liquid/solid equilibria. The present paper will present an extension of the validation process to take into account η formation and provide a review and examples of the practical application of the CALPHAD method to industrial alloys. It will expand on some of the topics briefly raised in the previous paper and demonstrate that the CALPHAD route is readily extendable to conditions that depart from equilibrium. It will also be shown that it can be used to provide fundamental input for calculations of physical and mechanical properties.

Introduction

    In a previous paper the construction of a thermodynamic database for calculation of multi-component phase equilibria in Ni-based superalloys was decscribed1. In conjunction with an appropriate software package, such as Thermo-Calc2 (used in the present studies), phase equilibria for multicomponent   Ni-based superalloys can be readily calculated. Numerous comparisons between calculation and experiment were given for γ/γ' phase equilibria and liquid/solid equilibria, and general phase equilibria calculations for some commercial alloys were shown. The paper concen-trated on γ/γ' alloys touched on aspects to do with the extension of the CALPHAD method to non-equilibrium transformation and also the prediction of fundamental properties such as anti-phase domain boundary (APB) and stacking fault (SFE) energies. Since this previous paper, the database has undergone substantial  development, with the inclusion of new models, and its capabilities extended to new types of alloys, in particular to general NiFe-based superalloys and Re-containing single crystal alloys. As part of this process, validation work has been undertaken by com-panies such as Special Metals Corporation. and Rolls-Royce plc. It has now also been used extensively world-wide and examples of its use are becoming well documented3,4,5,6,7,8. The purpose of the present paper is to first describe the extension of capability to NiFe-based superalloys, with emphasis on the formation of η and γ". A review of examples of its application will then be made, a number of which emphasise that the CALPHAD methodology can be extended to areas outside of the field of equilibrium studies. Finally, the recent extension of the database to include  Re will be discussed.

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