Mechanical behaviour and underlying deformation mechanisms in coarse- and ultrafine-grained Zn over a wide range of strain rates

Abstract

Spark plasma sintering was used to process bulk ultrafine-grained (average grain size of ∼250nm) samples from high purity nanocrystalline Zn powder. The microstructure of the consolidated samples was investigated and the mechanical behaviour was characterised by means of quasistatic and dynamic compression tests at room temperature and compared to that of coarse-grained counterparts. For both materials in the strain rate regime of 10−4–101s−1 a linear relationship between the logarithm of flow stress and strain rate values was observed since room temperature corresponds to a relatively high homologous temperature of 0.43. The strain rate sensitivity was higher for ultrafine-grained Zn (∼0.18) than for coarse-grained Zn (∼0.12) and twinning in the former sample was not observed in the entire strain rate range. For strain rates higher than 103s−1 the plasticity in coarse-grained Zn was controlled by dislocation drag but partial recrystallization was also observed. In the latter regime the mobile dislocation density was about 1011m−2. However, in ultrafine-grained Zn the relatively large dislocation density (∼1014m−2) and the small grain size limit the dislocation velocity yielding the lack of dislocation drag effects up to 104s−1.