| dc.description.abstract | Magnesium (Mg) and its alloys are promising lightweight and high specific strength structural materials, especially for transportation and aerospace applications. However, pure Mg and many Mg alloys exhibit low room-temperature ductility and formability, which is due to the low-symmetry hexagonal close-packed crystal structure and high plastic anisotropy. Therefore, a comprehensive understanding of the fundamental deformation behavior in Mg and its alloys is critical to further improve the mechanical performance of Mg. In this dissertation, advanced characterization techniques (scanning electron microscopy, transmission electron microscopy, etc.) were used to study the activation and suppression of non-basal slips (non-basal < a > and < c + a >) and extension twinning in deformed pure Mg and Mg alloys.
In the first part, the effect of Schmid factor was examined on the activation and suppression of < c + a > dislocation and extension twinning. Textured Mg-3Al-1Zn (AZ31) alloy was selected as the model material. Regarding < c + a > dislocations, statistical analyses on the normal-direction compressed AZ31 revealed that the activation and suppression of < c+a > dislocations in individual grains are primarily dictated by the overall texture rather than the crystallographic orientation of each grain. (Global texture prevails!) Regarding extension twinning, statistical analyses on the deformed Mg under multiply deformation conditions (normal-direction compression and tension, rolling-direction compression and tension, and 45 degree compressions) revealed extension twinning generally obeys the Schmid law for individual grains rather than the global stress state that favors extension twinning or not. (Schmid factor of individual grain prevails!)
In the second part, the effect of non-rare earth (RE) alloying was investigated on the activation and suppression of < c + a > dislocation and extension twinning. Moreover, the operation of specific deformation mode on the sample ductility was discussed. Non-RE Mg (pure Mg and AZ31) with similar initial microstructures (i.e., grain size and texture) were chosen. To uncover the fundamental mechanisms for the much-improved ductility in AZ31, statistical analyses on the pure Mg and AZ31 after rolling-direction tension at different strain levels were performed. The observations revealed a significant disparity of non-basal dislocation activities between the pure Mg and AZ31. For the pure Mg, < c + a > dislocations were activated since the early stage of plastic deformation. For AZ31, < c + a > dislocations were largely absent at all strain levels, even in the strain-to-failure samples. The promotion of the non-basal < a > dislocation activities and the suppression of < c+a > dislocations in AZ31 are expected to offer more sustainable hardening, which could elucidate the absence of apparent shear banding and much-improved ductility in AZ31 compared to pure Mg.
In the third part, the effect of RE alloying was investigated on the activation and suppression of < c + a > dislocation and extension twinning, and the influence on sample ductility was discussed. The microstructure of a newly developed Mg RE alloy (Mg–2Zn-0.3Ca-0.2Ce-0.1Mn) alloy with an exceptionally high tensile ductility was characterized. Compared with the pure Mg and AZ31 alloy microstructure at the same deformation condition in the second part, our results revealed that the high ductility of the ZXEM2000 could be attributed to four key factors: weaker texture, finer grain size, reduced twinning, and increased cross-slip frequency of < c + a > dislocations.
Taken together, this dissertation explores the roles of Schmid factor and alloying on the operation of non-basal slip and extension twinning in Mg and Mg alloy. Both above deformation mechanisms have a significant impact on the ductility of Mg and Mg alloys. The fundamental mechanisms uncovered in this work are anticipated to help guide the future Mg alloy design, especially in the area of simultaneously achieving high strengthen, high ductility, and improved formability. | en |
