Abstract
Twinning is an important mode of deformation in magnesium (Mg) and its alloys at high strain rates. Twinning in this material leads to important effects such as mechanical anisotropy, texture evolution, tension-compression asymmetry, and sometimes non-Schmid effects. Extension twins in Mg can accommodate significant plastic deformation as they grow, and thus twinning affects the overall rate of plastic deformation. We use an experimental approach to study the deformation twinning mechanism under dynamic loading. We perform normal plate impact recovery experiments (with microsecond pulse durations) on pure polycrystalline Mg specimens. Estimates of average TB velocity under the known impact stress are obtained by characterization of twin sizes and aspect ratios developed within the target during the loading pulse. The measured average TB velocities in our experiments are of the order of several m s −1 . These velocities are several orders of magnitude higher than those so far measured in Mg under quasi-static loading conditions. Electron back-scattered diffraction (EBSD) is then used to characterize the nature of the twins and the microstructural evolution. Detailed crystallographic analysis of the twins enables us to understand twin nucleation and growth of twin variants under dynamic loading.
Highlights
IntroductionNormal plate impact experiments were performed, with impact in the extrusion direction at impact velocities of 60–70 m/s
The particle displacement at the free surface of the target is measured by the normal displacement interferometer (NDI), and is numerically differentiated to obtain the rear surface particle velocity
Due to the wave propagation associated with the plate impact experiment, the stress, strain and strain rates are inhomogeneous throughout the thickness of the plates and evolve with time at each location
Summary
Normal plate impact experiments were performed, with impact in the extrusion direction at impact velocities of 60–70 m/s. At this velocity, there is elasticplastic wave propagation in the material but no shocks are generated. In the normal plate impact experiment, a cylindrical plate of material (called the flyer) is carried on a projectile that is launched at high velocities, producing an impact onto a stationary plate (called the target) that is supported in a target holder within an evacuated target chamber. The two plates are parallel to each other and perpendicular to the direction of projectile motion, and the impact causes longitudinal wave propagation in both plates
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