Dislocation-Precipitate Interactions in Magnesium-Neodymium Alloys

Abstract

The dislocation-precipitate interaction in Mg-Nd alloy has been characterized by in situ straining in transmission electron microscopy (TEM). Postmortem study on deformed samples has been conduct on various technique including weak-beam dark field and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Bulk properties data has been collected from the tensile test. In addition to experimental results, supplementary computational molecular dynamic (MD) simulation and first principle calculation provide insight and validation for found mechanisms. The unit dynamic process between extended β1 precipitate and basal dislocation is characterized via viewing dislocation-precipitate interaction at basal plane. Dislocations are observed to glide along precipitate broad facet. A dislocation-theory based analysis suggests that the shape, spacing and orientation (with respect to the glide plane) of β1 precipitates may favor glide of pinned dislocations along interfaces as opposed to the classical mechanism of bowing and looping around the precipitate. The analysis also suggests the dislocation relaxation is the root cause for the interface gliding mechanism. Further in situ straining and postmortem TEM dislocation analysis investigation revealed that basal {a} type dislocations interacted differently with β1 (Mg3Nd) and β’’’ (Mg3-7Nd) precipitates. For β’’’ precipitates, dislocations directly shear the precipitates. For interactions with β1 precipitates, dislocations were observed to by-pass the precipitates and then cross-slip to prismatic planes. The cross-slip associated with β1 precipitates occurred at either the departure or front end of the lenticular shaped phase. Dislocations with screw characteristic are able to overcome β1 precipitate by double cross-slip. The cross-slip mechanism was explored by MD simulations, and found to be similar to the Friedel-Escaig mechanism in pure Mg. The role of cross-slip in a β1-precipitate-dominant microstructure in enhancing ductility is discussed. The cross-slip mechanism introduces dislocation kink receiving strong lattice resistance along propagating direction, which is suspected to prevent following dislocation motion and finally leads to dislocation pile-up. The dislocation pile-up phenomenon was then observed in in situ straining experiment. Analysis of the images from the in situ experiments as well as postmortem TEM of bulk deformed samples reveals that, in the presence of a dislocation pile-up of sufficient intensity, slip is transmitted through the β1 precipitates. This impediment to dislocation motion from planar precipitate facets is analyzed using dislocation theory and density functional theory calculations. It is shown that the β1 precipitate “walls” result in a strong but not impenetrable barrier to slip transmission. The imposed dislocations pile-up effect on β1 precipitates is systematically characterized from tensile and compressive sample. In the tensile samples, contrary to the one single plane shearing as expected from classic precipitate cutting mechanism, several shearing events occur at a series slip plane parallel to each other, resulting in a region of deformation layer. The precipitates are significantly deformed within the layer but still remain connected. MD simulations validate the experiment result and show that deformation gradually spread across the layer, indicating a new mechanism to accommodate imposed displacement from shearing events. In the compressive samples, precipitates are more vulnerable to fragment into small pieces, which is suspected to associate with the buckling effect from c axis compressive stress