High toughness magnesium alloy through severe plastic deformation and short annealing

• Main Author: Fong, Kai Soon
• Other Authors: Tan Ming Jen
• Format: Theses and Dissertations
• Language: English
• Published: 2018
• Subjects: DRNTU::Engineering::Materials
• Online Access: http://hdl.handle.net/10356/74211
• Institution: Nanyang Technological University

In this work, a thermomechanical treatment using severe plastic deformation (SPD) via a modified constrained groove pressing (CGP) technique and short duration post-annealing was introduced to improve the mechanical properties of AZ31 magnesium alloy plate. This method is effective for producing fine-grained microstructure in thin magnesium plates and also enhances the mechanical strength without sacrificing the fatigue and corrosion properties. In this process, the magnesium plate is subjected to repeated deformation at elevated temperature using a specified pressing sequence designed to accelerate the grain refinement process. This technique has unique advantages over other principal SPD techniques such as Equal Channel Angular Pressing (ECAP) and High-Pressure Torsion (HPT). Firstly, it circumvents the difficulties faced in ECAP and HPT such as buckling and size limitation, respectively. Secondly, it is possible to implement the pressing procedures into rolling process which will benefit continuous processing of long magnesium strip. Thirdly, CGP was demonstrated as an effective grain refinement process for improving the microstructure of squeeze-casted AZ31 alloy. This opens up to the possibility of producing fine-grained magnesium plate with improved properties by using lower cost magnesium preform prepared by casting. Investigation of the deformation and temperature sequences in CGP process revealed interesting dependency of these conditions on strain, texture development and microstructure. Finite element (FE) simulation showed that the total strain imparted in the material was affected by the constraints imposed on the material by the die just before shear deformation and also by the compressive and stretching of the material during die filling. Experimental studies showed that decreasing processing temperature limit static recrystallization, recovery and grain growth and resulted in the development of micro and sub-micron grains. As such, effective choice of deformation and temperature sequences could positively influence the total strain, texture development and grain refinement efficiency. Electron backscatter diffraction (EBSD) analysis of the microstructure evolution during modified CGP revealed that dislocation boundaries of initially low angle grain boundaries (LAGB) were developed in the grain interiors and at the vicinity of the grain boundaries. This evidence suggests that mechanisms of both rotational dynamic recrystallization (rDRX) and continuous dynamic recrystallization (cDRX) were responsible for the efficient grain refinement and was unique to this process. The microstructure stability in the strain-induced fine and ultrafine grain boundaries prepared by modified CGP was investigated by annealing and interpreted by grain growth kinetic equations. From this study, it was revealed that despite the lower microstructure stability, as indicated by the low activation energy, the average grain sizes can be maintained considerably smaller than the parent material at shorter annealing. The hot tensile behaviour and deformation mechanism of the modified CGP processed alloy with short annealing was investigated at elevated temperatures and under various strain rates. From the room temperature tensile tests, the yield strength and elongation to failure were significantly improved by 34% and 11%, respectively. The results of the strain rate sensitivity and activation energy suggested that climb-controlled dislocation creep is the dominant deformation process associated with lattice diffusion. Enhanced ductility was measured at 523 K and 1×10-3 s−1 where a maximum elongation to failure of 100±2.5% was obtained, and dynamic recovery was observed as the main restoration process. From the biodegradation studies, it was established that the average corrosion rate was marginally higher after modified CGP. This reduction in corrosion resistance can be attributed to the non-uniformity of the microstructure; some grains contain high dislocation and some grains were almost without dislocation. This introduced strain energy difference and stress differences which worsen the corrosion properties despite having a higher area of grain boundaries. It was further established from the study of the fatigue crack growth rate that the CGP process does not significantly alter the fatigue performance. The above studies suggest that CGP enhances the yield strength of magnesium alloy without significantly affecting the fatigue and corrosion performance and that combining CGP treatment with coating technologies are viable approaches to improve the performances of magnesium material for biomedical implants. New insights into the microstructure evolution and mechanical properties of severe plastically deformed AZ31 magnesium alloy through modified CGP have been gained through this study. From an application point of view, this study has demonstrated very clearly the potential of the CGP process for preparing fine-grained AZ31 plate with enhanced mechanical strength and improved ductility by adopting the appropriate heat treatment strategy. The process can be made continuous through adaptation of its design into rolling process. It is reasonably believed that this thermomechanical processing route can be established for a wide range of commercial application in the foreseeable future, particularly for improving the mechanical properties of biodegradable magnesium alloy.