Mathematical model of influence of rapid induction heating on nucleation and growth of precipitates

A mathematical model of the influence of rapid induction heating on the nuc leation and growth of secondary phases in diffusion controlled processes ha s been developed. The total stress produced by the electromagnetic field an d acting on the volume V of a specimen was derived and added to other exter nal stresses applied to the system. Then, the total effective stress was in troduced into mathematical equations of diffusion controlled nucleation and growth processes. In addition, the effect of rapid induction heating on th e age hardening treatment of a selected cast nickel base superalloy, IN738L C, was investigated experimentally. For this purpose, two types of rapid ag ing with equal heating rates were applied: one treatment was induction agin g and the other was salt bath aging. Microstructural characteristics of gam ma' precipitates and hardening behaviour were studied by means of scanning electron microscopy (SEM), electron image analysis, transmission electron m icroscopy (TEM), and hardness testing. According to the results obtained, a lthough the rates of heating in induction and salt bath aging were equal, t he rates of nucleation and growth of gamma' precipitates in induction aging were much faster than those obtained in salt bath aging, especially in the first minutes of the aging process. Furthermore, the characteristics of ga mma' precipitates with induction aging were more favorable than those with salt bath and normal aging. It was observed that the growth rate of gamma' in induction aging deviated considerably from the t(1/3) growth law of the standard modified Lifshitz, Slyozov, and Wagner (MLSW) theory. The remarkab le improvement of microstructural characteristics obtained with induction a ging can be attributed to the existence of the external electromagnetic for ce produced by rapid induction heating. (C) 2001 IoM Communications Ltd.