A COUPLED THERMODYNAMIC KINETIC MODEL FOR DIFFUSIONAL PROCESSES IN SUPERALLOYS/

A coupled thermodynamic/kinetic model for diffusional processes in sup eralloys is described. Use is made of the generalised force-flux equat ions, and therefore an accurate knowledge of thermodynamical propertie s of the system is a prerequisite for the calculations. Calculations c an be carried out in either the lattice-fixed (Kirkendall) or the mass -invariant (Matano) frame. In order to model transformations, e.g. inv olving the gamma and gamma' phases, the concept of an interface sub-sy stem is introduced. The numerical accuracy and stability associated wi th the treatment of the moving boundary is shown to be comparable to m ore traditional analyses which incorporate the Murray-Landis transform ation. The results from the simulations are compared with experimental information, which includes (i) concentration profiles, (ii) movement of inert markers and (iii) observations of porosity formation. Partic ular attention is paid to the simulation of Ni-Al-Cr interdiffusion in the f.c.c. phase, for which reliable thermodynamic and kinetic inform ation is available. Similar comparisons have been made for precipitati on reactions involving the gamma' phase, although the available inform ation is very sparse. Observations made on a number of Ni-Ni-3(Al,Ti) couples can be simulated with a reasonable degree of accuracy. Particu lar advantages of the method are (i) the interface between phases is t reated in a novel way, which avoids numerical difficulties arising fro m the estimation of concentration gradients at phase boundaries, (ii) it can be readily extended to multicomponent systems, and (iii) the tr eatment of the Kirkendall drift of vacancies. It is clear that there i s a great need for experimentation aimed at deducing kinetic data, e.g . diffusional mobilities, particularly for the gamma' phase. (C) 1998 Acta Metallurgica Inc. Published by Elsevier Science Lid. All rights r eserved.