EXPERIMENTAL CHARACTERIZATION OF DEFORMATION MECHANISMS IN MAGNESIUM AND THE EFFECT OF ALLOYING ON DUCTILITY

Magnesium (Mg) is a promising structural material with low density and high specific strength. The use of wrought Mg products has been stunted by poor formability, which is related to underlying anisotropic deformation mechanisms. The number of independent easy glide basal slip systems does not satisfy the Von Mises criterion, and activation of slip and twining holds the key to creating formable Mg. The current study provides experimental observations of the deformation mechanisms activated in commercial and developmental Mg alloys. dislocations in pure Mg were observed to non-conservatively dissociate onto the basal plane, resulting in a sessile configuration. By contrast, the dislocations in AZ31 do not dissociate, and the dissociation in Mg-3Er is also much smaller than for pure Mg. HREM images revealed that the suppressed dissociation may be related to the spreading of partial dislocation cores on pyramidal planes. The more compact dislocations maintain their mobility and lead to improved ductility. Extension twinning is another significant deformation mechanism that affects the ductility of Mg. Observations made in this study indicate that the strain concentration at the tip of a twin can be released by the formation of dislocations. The high density of dislocations observed in and around twin tips indicate that twin-dislocation and twin-GB interactions play an important role in twin propagation. For Mg-0.1Ca, texture weakening was observed to be a critical mechanism that improves ductility. Ca was observed to segregate at high-angle grain boundaries, which retards GB mobility and activates the nucleation of static recrystallization in deformed grains. The GB energy of the decorated boundaries are less dependent on misorientation, which enables the growth of randomly orientated grains and weakens the texture. In total, investigations of the deformation mechanisms in hot-rolled polycrystalline pure Mg, AZ31, Mg-3Er and Mg-0.1Ca presented in this thesis have elucidated the mechanisms that govern ductility. Control of dislocation cores, deformation twinning, and texture reduction highlight promising paths for the design of ductile Mg alloys

Densification and characterization of rapid carbothermal synthesized boron carbide

Submicrometer boron carbide powders were synthesized using rapid carbothermal reduction (RCR) method. Synthesized boron carbide powders had smaller particle size, lower free carbon, and high density of twins compared to commercial samples. Powders were sintered using spark plasma sintering at different temperatures and dwell times to compare sintering behavior. Synthesized boron carbide powders reached >99% TD at lower temperature and shorter dwell times compared to commercial powders. Improved microhardness observed in the densified RCR samples was likely caused by the combination of higher purity, better stoichiometry control, finer grain size, and a higher density of twin boundaries.Army Research Laboratory (W911NF-12-2-0022); Defense Advanced Research Projects Agency (W31P4Q-13-10001); National Science Foundation (1540027

Twin boundary migration mechanisms in quasi-statically compressed and plate-impacted Mg single crystals

Twinning is a prominent deformation mode that accommodates plasticity in many materials. This study elucidates the role of deformation rate on the atomic-scale mechanisms that govern twin boundary migration. Examination of Mg single crystals deformed under quasi-static compression was compared with crystals deformed via plate impact. Evidence of two mechanisms was uncovered. Atomic-level observations using high-resolution transmission electron microscopy revealed that twin boundaries in the -axis quasi-statically compressed single crystals are relatively smooth. At these modest stresses and rates, the twin boundaries were found to migrate predominantly via shear (i.e., disconnection nucleation and propagation). By contrast, in the plate-impacted crystals, which are subjected to higher stresses and rates, twin boundary migration was facilitated by local atomic shuffling and rearrangement, resulting in rumpled twin boundaries. This rate dependency also leads to marked variations in twin variant, size, and number density in Mg. Analogous effects are anticipated in other hexagonal closed-packed crystals.Open access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]