A RATIONALE FOR DEFORMATION TWINNING IN NANOCRYSTALLINE MAGNESIUM AND MAGNESIUM AZ80 ALLOY

Recent reports have shown that the generation of nanotwins in nanostructured grains can further enhance both the strength and ductility of fcc alloys; in such alloys, twinning can be far more prominent in nanoscale grains than for their coarse-grained counterparts where deformation twinning rarely occurs. Mg presents a more perplexing challenge, as the opposite phenomena occurs; twinning is a prevalent deformation mode for coarse-grained materials, but has been observed to occur more infrequently as the grain size decreases, and rarely at the nanoscale, which is reported to be due to the high stress needed to nucleate a partial dislocation from a boundary, and the lower energy needed for dislocation slip to preferentially occur. In this preliminary report, we demonstrate that the unique thermomechanical processing conditions offered by cryomilling result in the nucleation and growth of deformation twins in nanostructured grains. It is hypothesized that the high rate of deformation, combined with the reduced temperature facilitate the high stresses needed for twins to nucleate at a grain boundary. Transmission electron microscopy and the corresponding nano-diffraction patterns show evidence of the same compression twinning systems that occur in coarse-grained materials. These results point to a promising approach for the design of nanocrystalline Mg-alloys with superior strength and ductility for advanced structural applications.