Temperature dependent plastic deformation of magnesium using micro-pillar compression

This study investigates the temperature dependence of the deformation processes of basal slip, pyramidal slip and {101 2} tension twinning in magnesium single crystals. In situ SEM compression between 23°C and -94°C was performed on FIB-milled square shaped micro-pillars taken from selected grains of a polycrystalline sample of pure magnesium. For low temperature micro-pillar compression testing, a novel cryo-stage was designed and employed in combination with the existing pico-indentation system. Post-mortem SEM and TEM were then used to analyse the microstructures of the deformed micro-pillars. [112 1] oriented micro-pillars were compressed at 23°C, -28°C and -94°C to activate basal slip. These pillars deformed by a similar deformation mechanism, irrespective of the test temperature. The Critical Resolved Shear Stress (CRSS) for basal slip increased with decreasing test temperature, which is the CRSS increased by approximately 9 MPa on reducing the temperature from 23°C to -94°C. This trend is explained by the increase in Peierls lattice friction for the glide of dislocations on the basal plane accompanied by a rise in activation free energy for the nucleation of dislocations on the basal plane. For the activation of tension twinning, micro-pillars with a [13 20] loading direction were compressed over a similar temperature range. The micro-pillars deformed mainly by the activation of {101 2} tension twins followed by basal slip within the twinned region. The CRSSs for the twin activation / nucleation and for the twin growth showed no change with temperature. The temperature insensitivity of the twin activation / nucleation is explained by the dominant role of stress-concentrators in twin nucleation during micro-pillars compression whereas temperature insensitivity of the twin growth is explained by the availability of the dislocation sources and mobile dislocation segments required for twin growth. For the activation of pyramidal slip, [0001] oriented micro-pillars were compressed over the same temperature range. The activation of pyramidal slip was confirmed at all the test temperatures. The CRSS for pyramidal slip decreased with a reduction in temperature i.e. a drop in testing temperature from room temperature to -90°C resulted in an approximately 39 MPa lower CRSS for pyramidal slip. This anomalous temperature dependence of pyramidal slip is explained by the balance between the thermally activated processes of dissociation of dislocations and cross-slip of dislocations between different pyramidal planes. Overall, the findings of this study provide a useful dataset for understanding the orientation dependent temperature sensitivity of the dominant deformation modes in magnesium at micron length scale under uniaxial compression at room temperature and to a range of cryogenic temperatures.