The main hypothesis of this research work is that thermomechanically-processed regions of wrought metals and alloys that undergo similar equivalent plastic strains exhibit similar microstructures and associated characteristics, such as mechanical and physical properties (i.e., properties that are related to the dislocation structures). That is, materials that undergo the same equivalent plastic strain should exhibit the same dislocation structures and dislocation densities, and this should then translate to similar microstructures and associated mechanical properties. It was decided that less than 10% difference would be considered acceptable for verifying this hypothesis. In this work, high-pressure torsion (HPT) was considered as the plastic deformation processing technique to produce the wrought samples. To some extent, the proposed hypothesis has been evaluated indirectly (only for hardness distribution) by the HPT research community. However, the proposed work is novel because no one has directly evaluated this hypothesis using the combined microstructure and hardness methodology proposed in this work.The equivalent strains, which for HPT processing are a function of the number of turns, the radial distance from the disk center, and the disk height, chosen for the foci of this work were 24, 82, 123, 247, and 371. The following microstructural characterization techniques were used to evaluate this hypothesis: Optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction spectroscopy (XRD), atom probe tomography (APT), and transmission electron microscopy (TEM). Vickers and Berkovich microhardness testing were chosen as the mechanical property characterization techniques to evaluate the hypothesis. The model material used was Zn-3Mg (wt.%), which readily undergoes plastic deformation at room temperature (RT) without a tendency for cracking at plastic strain levels lower than 30% [1]. There were several reasons that this material was chosen. In equilibrium, this alloy exhibits a two-phase microstructure. This material is also not difficult to prepare metallographically and obtain OM and SEM images as well as Vickers indents, and the grain size range is usually between 1-100 microns [1]. This material is susceptible to HPT plastic deformation at normal pressures (6 GPa) and can withstand a relatively large number of turns without cracking (i.e., >30) [1]. In addition to evaluating this hypothesis, another objective of this dissertation was to compare the microstructure and hardness of powder-processed Zn-3Mg (wt.%) HPT disks with similar disks processed from Zn-3Mg(wt.%) cast alloys as well as hybrids of the same composition. In particular, the different hardness distributions of these multiple-phase materials and their steady-state deformation behavior are discussed. The overall results could neither verify nor nullify the hypothesis. The rate of strain accumulation was different for the different disk locations, and this is believed to have played the major role in the hypothesis not being accurate. The other reasons are also described in detail. However, evaluation of the hypothesis helped further understanding of the microstructural evolution that takes place during HPT processing. In addition, the results of the microstructure and mechanical property evaluation will facilitate the development of the next generation of Zn-Mg implants with improved biodegradable and mechanical properties. Overall, this work has enhanced our understanding of the effect of HPT processing on the microstructure and resulting mechanical properties of Zn-3Mg (wt.%), and this understanding can be transferred to better understand such processing on other alloys and alloy systems.
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