Penetration of foreign elements connected with internal precipitation: a computer-based description and first experimental verification

Abstract

Metallic alloys often show a change in structure in high temperature applications caused by a penetration of a foreign element from the gas phase and a simultaneous precipitation of compounds of this element with components of the alloy. The kinetics of these processes can be described by a simple time law in cases where the compounds formed are very stable, and the solubility product of the reacting components in the matrix is very small in this case. However, for cases where less stable compounds are formed and the corresponding solubility product is relatively high, a general solution has not been given so far. In this paper a computer-based method is presented for a quantitative description of the kinetics in the latter case. The method is based on the solution of differential equations using the finite difference technique. The basic relations are derived from considerations of the processes of diffusion, precipitation of several compounds of different stabilities and their transformations. The method is applied to the carburization of Ni-25wt.%Cr in a CH 4-H 2 gas mixture at temperatures from 850 to 950°C. In this example a quantitative description of the experimental results is achieved. Possibilities for further applications of this method are also discussed.