The use of flat-ended projectiles for determining dynamic yield stress III. Changes in microstructure caused by deformation under impact at high-striking velocities

The mechanism of deformation of metals at high velocities has been studied by examining the microstructure of cylinders of mild steel, duralumin and standard silver (of composition Ag 92.5%, Cu 7.5%) of similar hardness, and of steel balls after impact on steel plates at velocities from 300 to 3830 ft. /sec. The means by which the stress on impact was relieved depended on the material, and was first the formation of twins or ‘compression bands’, i. e. by block movement of wedges of material within individual grains, or by cracking. When the applied stress could no longer be relieved in this way, plastic deformation occurred. The amount of residual strain in deformed mild steel and duralumin has been examined by observing changes of microstructure after annealing. Hardness surveys were made on longitudinal sections of the cylinders and the results correlated with the microstructures and with observations of the limits of strain due to impact.