Fracture behavior of twin induced ultra-fine grained ZK61 magnesium alloy under high strain rate compression

In this study ultra-fine grained single pass extruded ZK61 magnesium alloy sheet is processed at temperature 350°C with area reduction ratio 30. The complex dynamic mechanical behavior is studied experimentally with Split Hopkinson pressure bar over wide ranges of strain rates (1000-4000s^-1) along the normal direction. Electron backscattered diffraction, scanning electron microscopy and optical microscopy analysis are employed to reveal the changes in texture and fracture analysis. Positive strain rate sensitivity is observed up to strain rate 3000 s^-1 that is signature of basal slip and extension twinning although the grain size was very small. This finding is contradiction to experimental and theoretical predictions suggesting the elimination of twinning induced deformation in magnesium alloys in very fine small grains <3 μm. Flow stress and a fraction of extension twinning are apparently decreased as the applied strain rate increased from 3000 to 4000 s^-1; that is attributed to adiabatic rise in temperature, early reorientation of crystal. Extensive grain reorientation causes the nucleation of extension twinning that changes fiber texture to strong double peak basal texture under high strain rate compression. Nucleation of principle crack was attributed to twin-dislocation interaction that provides dynamic recrystallized grains followed by the adiabatic shear band at the boundary of coarse grains. Contrary, secondary cracks are nucleated and propagated due to high local stresses at the triple junction of grain boundaries. Besides, scanning electron microscopy revealed ductile fracture under high strain rate compression.