Deformation Twinning

Because of the low symmetry hexagonal close packed atomic structure of Mg there are a limited number of deformation mechanisms to accommodate deformation along the crystallographic c-axis. Only one dislocation slip based mechanism can accommodate deformation along the c-axis. Therefore, deformation twinning plays a dominant role in the accommodation of deformation along the c-axis.  To date there have been a number of studies to examine twinning but there are still a number of critical details that need to be addressed to properly inform modeling efforts.  Shown below are the first in-situ images of twin propagation of single crystal magnesium, images captured by Vignesh Kannan at Johns Hopkins University. From these images insight is gained towards the kinetics of twinning and nucleation sites during strain accommodation. Corresponding microstructural analysis elucidates the formation of secondary twins (twins within twins) at small scales that not previously been observed.


Strain Localization Evolution and Failure

High speed imaging is combined with high strain rate loading to probe the evolution of strain localization leading up to failure in Mg alloys. From this study pathways for deformation are identified which lead to undesired shear band formation and crack formation. Show below is a comparison of the strain localization that occurs at a machined defect for three different loading orientations. The left two strain maps demonstrate the ability of two loading orientations to accommodate deformation in regions away from the specimen through hole.  The strain map on the right shows strong localization near the machined defect which leads to the formation of undesirable cracking.