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Title: Investigation on the superplastic hot working of Mg-Li alloys for fabrication of complex structures
Authors: Yang, Haopeng
Degree: Ph.D.
Issue Date: 2019
Abstract: Mg-Li alloys, as a kind of superlight material, have the enormous promising potential for wide applications in various industries due to their superior stiffness-to-weight ratio, low density, good mechanical properties, and biodegradability, which are greatly favored by the aerospace and military fields, where reducing the product weight is critical and crucial, and the biomedical field, where biodegradability can be well applied. These fields usually require parts with complex shapes and geometries, which are generally formed with large deformation. However, due to existence of the hexagonal-close-packed (HCP) structure of Mg, which has only a few slip systems, the material is relatively hard to be deformed compared to the cubic-centered materials at room temperature. In order to efficiently fabricate the complex structures by using Mg-Li alloys, the application of superplastic deformation (SPD) of the material can be utilized, by which the material can achieve an extraordinarily large amount of deformation under certain conditions. Researchers have already been attempting to investigate the superplasticity of Mg-Li alloys. However, most of the studies have focused on how to refine the material microstructure to achieve fine-grain superplasticity, while few of them have attempted to attain better superplasticity through optimizing the route of deformation, which could also be a possible way of enhancing the material superplasticity. An ingenious SPD method, the maximum strain rate sensitivity (m) (Maxm) SPD, was originally introduced for enhancing the superplasticity of Ti alloys. Since the m value acts as an indicator for the resistance to necking and a higher m value always corresponds to a better state of superplasticity and further the larger overall elongation, the best superplastic state of the material can be retained if the maximum m value can be maintained throughout the deformation process. For the conventional SPD methods like constant velocity (Constv) SPD and constant strain rate (CSR) SPD, m cannot always be kept at its maximum value, so the potential superplasticity of the material might be compromised. The idea of Maxm SPD was thus proposed to maintain the largest m value throughout the SPD process by in situ measurement of the m value and dynamic control of the deformation strain rate. However, since the Maxm SPD method is still relatively new and the deformation process is highly dependent on the capacity of experimental equipment, this innovative idea has only been successfully applied in the SPD of Ti alloys so far, but its potential applications to the SPD of other materials have not been fully explored and exploited.
Inspired by the enhanced superplasticity of Ti alloys by Maxm SPD, this research is dedicated to exploring the applicability of Maxm SPD to Mg-Li alloys. In this thesis, the alloy of Mg-9Li-1Al (in wt. %, LA91), which has the duplex phases with HCP and BCC structures, was utilized as the experimental material. By adopting both the traditional and Maxm SPD, the differences between these methods were compared and discussed, and the characteristics of Maxm SPD of Mg-Li alloys were identified. At first, single-step Maxm SPD of the LA91 alloy was studied. The as-received material and the samples further refined by equal channel angular extrusion (ECAE) were applied. It was found that the optimal SPD temperature of LA91 alloy is 573 K. The maximum elongation in the experiment was 563.7 %, obtained by Maxm SPD at the temperature of 573 K by using the 8-pass ECAEed samples, and it was found that ECAEed samples are more favored by Maxm SPD than as-extruded samples owing to the more equiaxed microstructure in the ECAEed samples. Nonetheless, since grain refinement procedures like ECAE are quite complicated and time-consuming, an innovative stepped SPD method was then applied to explore the potential superplasticity of the as-received material in the as-extruded state. Due to dynamic recrystallization, the grains of the as-received alloys can be transformed to be more equiaxed in the first deformation step, which is more favored by the SPD process. The largest elongation of 621.1 % was obtained by CSR-Maxm SPD with the pre-elongation of 250 %, which is even larger than the result in the single-step Maxm SPD experiments by using the grain-refined material. The result showed that the stepped SPD method can largely enhance the superplasticity of the material. Moderate annealing was finally applied to the as-received material and the two-step SPD method was adopted by using the annealed samples. The annealing treatment aimed at producing a more equiaxed grain structure by static recrystallization without severe grain coarsening. However, after the SPD experiments, it was found that the annealed samples exhibited worse superplasticity than the as-received materials. It was indicated that during the annealing process, static recrystallization occurred and expended the strain energy obtained from extrusion, which provides the energy source for dynamic recrystallization during SPD. The material superplasticity was therefore compromised, and the application of annealing for the sake of obtaining a more equiaxed grain structure could not be feasible for further enhancing the material superplasticity. This thesis explores the potential superplasticity of Mg-Li alloys by adopting both single-step and two-step SPD processes, and provides an in-depth and epistemological understanding of Maxm superplasticity of the material. It can be found from this research that Maxm SPD can be well utilized for the hot working of Mg-Li alloys to enhance the material superplasticity. However, how to realize the Maxm SPD of Mg-Li alloys in industrial mass production still needs to be further considered and explored in the future research.
Subjects: Hong Kong Polytechnic University -- Dissertations
Magnesium-lithium alloys
Magnesium alloys
Lithium alloys
Deformations (Mechanics)
Pages: xix, 146 pages : color illustrations
Appears in Collections:Thesis

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