Effects of electric current on solid state phase transformations in metals

Abstract

The influence of an electric current on the following solid state transformations in metals are considered: (1) intermetallic compound formation and growth in diffusion couples, (2) precipitation, (3) crystallization of amorphous alloys and (4) recrystallization and grain growth of cold worked metals. The formation and growth of intermetallic compounds were in qualitative accord with electromigration theory. Regarding precipitation, an electric current can either enhance or retard the precipitation rate, depending on the alloy, the current density and its frequency. Important factors appear to be the effect of current on the quenched-in vacancies and the presence of an internal stress. Both a continuous d.c. current and high current density electropulsing enhanced the crystallization rate of amorphous alloys. The effects are greater than can be explained by simple electromigration theory and suggest the cooperative motion of a larger number of atoms. Electropulsing enhanced the recrystallization rate of cold worked metals, but retarded subsequent grain growth. Enhancement of the recrystallization rate resulted mainly from an increase in the pre-exponential factor of the Arrhenius rate equation, which is considered to refer to the nucleation rate. Retardation of subsequent grain growth resulted from a lower residual dislocation density within the newly-formed grains.