Effects of Impact Deformation on Material Microstructure

Abstract

The effects of varying ambient temperature and strain rate on the microstructure of metals deformed to large plastic strains are investigated. At room temperature, dynamically-deformed samples (DDS) become softer, in contrast to quasi-statically deformed samples, which show hardnesses close to the saturation value. The microstructure of DDS is characterized by deformation twins and equiaxed micron-sized grains; softening is caused by conventional recrystallisation. At liquid nitrogen temperature, variation of deformation rate leads to changes in the density, character and arrangement of dislocations, as well as the size and configuration of dislocation cells/(sub)grains. A threshold strain rate of 103s-1 is identified for the formation of localized deformation bands, and amorphization is a deformation mechanism in resultant microstructures with grains smaller than ~13nm; this offers an explanation for the `inverse Hall-Petch effect?. A model that illustrates the initiation and propagation of amorphous phases during deformation is proposed. Based on experimental results, suggestions on techniques for grain refinement through severe deformation are proposed.