Whisker formation and the mechanism of superplastic deformation

The morphology and formation process of the whiskers which developed during the superplastic deformation of 7475 Al were studied as a Function of temperature, strain rate and externally applied electric field. The whiskers were as fine as 0.1 mu m and as long as 100 mu m. Their density 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 revealed that the whiskers formed from flow lines which developed from grain boundary sliding. This strongly suggests that the mechanism of grain boundary sliding could also be viscous how of a liquid-like, grain boundary material.