WHISKER FORMATION AND THE MECHANISM OF SUPERPLASTIC DEFORMATION

The morphology and formation process of the whiskers which developed d uring the superplastic deformation of 7475 Al were studied as a Functi on of temperature, strain rate and externally applied electric held. T he whiskers were as fine as 0.1 mu m and as long as 100 mu m. Their de nsity and length increased with test temperature and with decrease in strain rate, and with an applied electric field. The formation of the whiskers could not be explained by a dislocation or vacancy diffusion mechanism. Rather, their morphology and the high strain rates involved suggested that the mechanism is more likely viscous flow of a liquid- like substance originally at the grain boundaries. SEM observations re vealed that the whiskers formed from flow lines which developed from g rain boundary sliding. This strongly suggests that the mechanism of gr ain boundary sliding could also be viscous how of a liquid-like, grain boundary material.