Microstructure and nanohardness distribution in a polycrystalline Zn deformed by high strain rate impact

Polycrystalline Zn with an average grain size of about 300 μm was deformed by direct impact Hopkinson pressure bar at a velocity of 29 m/s. An inhomogeneous grain structure was found consisting of a center region having large average grain size of 20 μm surrounded by a fine-grained rim with an average grain size of 6 μm. Transmission electron microscopy investigations showed a significant dislocation density in the large-grained area while in the fine-grained rim the dislocation density was negligible. Most probably, the higher strain yielded recrystallization in the outer ring while in the center only recovery occurred. The hardening effect of dislocations overwhelms the smaller grain size strengthening in the center part resulting in higher nanohardness in this region than in the outer ring.