Size-related Mechanical Properties of Pure Magnesium
Magnesium (Mg) is the lightest structural metal in the world. Therefore, Mg alloys hold great promise for weight-saving applications in the automotive and aerospace industries. However, the hexagonal close-packed (HCP) structure of Mg alloys results in limited dislocation plasticity and alternative deformation mechanisms such as deformation twinning that leads to poor formability and mechanical performance of Mg alloys. This inherent low ductility in Mg restricts its broad applicability as a high performance structural material. Further understanding of the fundamental deformation behavior in Mg and its alloys is therefore critical in order to identify potential processing routes that could enable high strength and ductility performance in Mg alloys.
Dimensional refinement is regarded as an efficient way to tune the mechanical properties of materials. To probe the size-related mechanical properties of Mg and also the related fundamental deformation mechanisms, a series of in situ transmission electron microscopy nanocompression (TEM), nanotension and nanobending tests were conducted on single crystal and polycrystalline Mg with different crystallographic orientations. The external dimensions of the samples studied ranged from approximately 100 to 900 nm. The effect of crystal size was studied in regard to both deformation twinning behavior and dislocation behavior in single crystal Mg oriented for deformation twinning and basal slip, respectively. The influence of different grain boundary structures on the mechanical properties of polycrystalline Mg were also investigated using bicrystalline Mg specimens with constrained physical dimensions.
For deformation twinning in Mg, it was found that there is strong crystal size effect. The formation of nanotwins was obtained in small specimens resulting in high strength (GPa level), high ductility and significant strain hardening, characteristics that have not been observed before in bulk materials. The nanotwinned structure is explained to be a result of the confined volume and large surface area in the small samples. The nucleation mechanism for deformation nanotwins was studied by computational simulations, and it was found that intrinsic nucleation of deformation twinning in Mg can be influence by the correlated nucleation of twinning dislocations, resulting in a nanotwinned structure.
A strong size effect on the dislocation behavior in Mg was also discovered through the in situ TEM tests. Through systematic investigation three different size regimes were identified where the strength levels and dislocation plasticity were distinctly different. In the largest samples, three-dimensional dislocation plasticity was found; both the microstructure and the mechanical behavior were similar to those found in bulk. As the sample size decreased, two-dimensional dislocation plasticity became dominant, resulting in limited ductility and localized shear along the basal plane. Finally, in the extremely small samples (≤ 100 nm), multiple slip systems were activated under ultra-high stresses and exceptional ductility was reached. Corresponding high-resolution TEM (HRTEM) observations revealed a significant contribution from non-basal slip systems to the entire plastic deformation in these smallest samples. Presumably, the ultra-high stress decreased the anisotropy of the critical resolved shear stress (CRSS) between different slip systems, resulting in non-basal slip that generated a more homogenous deformation and much better ductility. The in situ TEM experiments were further compared to detailed molecular dynamic simulations. These observations of the reduction of CRSS anisotropy and ultra-high strength plasticity are discussed in light of future processing opportunities for high strength and high ductility structural materials.
Lastly, the influence of different grain boundary structures on the deformation mechanisms and the mechanical properties of Mg at small scales was investigated by performing in situ SEM/TEM compression tests on polycrystalline Mg. Using electron backscatter diffraction (EBSD), it was observed that a low angle grain boundary served neither as an effective source of dislocation nucleation nor an effective barrier to mobile dislocations, resulting in localized shear. By comparison, high angle grain boundaries served as effective sources for dislocations and deformation twins, resulting in more stable and sustained plastic deformation.
Taken together, the observations and analysis in this thesis give novel and powerful insight into the fundamental plasticity mechanisms in pure Mg. The experiments presented here are both rigorous and creative, generating insightful and powerful conclusions into Mg metallurgy with a high potential impact for light-weighting strategies in structural materials.