Ascertaining the plastic deformation mechanisms of polycrystalline extruded Zn through in situ SEM/EBSD mechanical tests

Abstract

The plastic deformation micro-mechanisms of extruded pure Zn deformed in tension along the extrusion direction were investigated by means of in situ scanning electron microscopy (SEM) integrated with electron back-scatter diffraction (EBSD). Plastic deformation began with the activation of 〈a〉 basal slip in grains with the highest Schmid factor while, the incompatibility of deformation between neighbour grains was accommodated by grain boundary sliding. The geometrically necessary dislocation density increased sharply from 1.53 × 10¹³ m⁻² to 9.03 × 10¹³ m⁻² when applied strain reached 6.7%, and this increase coincides with the strong initial strain hardening region. The incompatibility of deformation between neighbour grains was accommodated by grain boundary sliding at strains above 3.3%, which somehow limited the strain hardening rate. Evidence of 〈c + a〉 pyramidal II slip was also found through slip trace analysis from the early stages of deformation, i.e. 1.6% strain, but it was always limited to a small fraction of suitably oriented grains. Moreover, transmission electron microscopy (TEM) observations showed that many 〈c + a〉 pyramidal dislocations were dissociated into the basal plane and became sessile. {$10\bar{1}2$}<$10\bar{1}\bar{1}$> compression twins were nucleated at 3.3% strain and the fraction of grains undergoing twinning as well as the area fraction of twins increased proportionally to the applied strain. Twinning was favoured by the fiber texture and the twin variant with the highest Schmid factor was primarily activated in each grain. The contribution of twinning to the total strain was limited (around 11% when the applied strain was 16.7%). The strain hardening rate decreased sharply beyond 6.7% and the hardening contribution of basal slip was balanced by grain boundary sliding and compression twinning. Finally, a high fraction of sub-grain boundaries that trigger recrystallization at larger strains was found at 16.7%. These observations reveal the sequence and interaction of plastic deformation mechanisms in Zn, which may help design novel Zn alloys with improved mechanical properties.