| Summary: | 博士 === 國立臺灣科技大學 === 機械工程系 === 107 === In this work, heat treatment (homogenization and ageing heat treatment processes) and extrusion plus A route type equal channel angular pressing (ECAP) severe plastic deformation methods were used to improve the microstructural and mechanical properties of as-cast SiCp/AZ61 magnesium metal matrix composites (Mg MMCs) fabricated by stir casting method. Different weight percentages (0%, 2% and 5%) of SiC particles (SiCp) were considered to study the effects of contents of reinforcements at different treatment conditions. Microstructural changes due to heat treatment processes, the number of ECAP passes and SiCp weight percentages were assessed using optical microscope (OM), scanning electron microscope (SEM), microhardness test and X-ray diffraction (XRD) patterns analysis. Enhanced mechanical properties were analyzed based on the Charpy impact and the uniaxial tensile test data. Furthermore, the brittle-ductile properties were testified by using scanning electron microscopy (SEM) features of Charpy impact and tensile test fracture surfaces. The work-hardening behavior of AZ61 magnesium alloy and SiCp/AZ61 Mg MMCs deformed by ECAP plastic deformation were studied by considering strain hardening rate (θ). The details of plastic deformation mechanisms and plastic deformation stages were identified by using a Crussard-Jaoul method based on the Ludwik equation. The response surface methodology in the design of experiments (DOE) wizard and Gurson-Tvergaard-Needleman (GTN) model were employed to estimate the optimum GTN damage parameters and to validate their significant effects respectively on the ductile fracture behavior of ECAP deformed AZ61 magnesium alloy. Hollomon flow stress was applied to identify uniform deformation and non-uniform deformation regions to investigate the void nucleation and coalescence processes separately. From the results obtained, ageing heat treatment process was seen significant on the 12 h aged 2 wt% SiCp/AZ61 Mg MMC which induced lower microhardness values and results in the formations of particle free regions and discontinuous secondary phases. At a higher number of ECAP passes and higher SiCp weight percentage, higher elastic modulus was seen enhanced. The strength, ductility and work-hardening behaviors were varied for both ECAP plastic deformation and SiCp weight percentage variations. The results of ductile fracture behavior of ECAP deformed AZ61 magnesium alloy showed that varying both stress triaxiality and damage variables simultaneously can greatly affect the curve fitting process of experimental, simulation and GTN model curves. The main contribution of this research work is enhancing the mechanical properties of SiCp/AZ61 Mg MMCs by modifying the presence and amount of microstructural constituent phases and by improving their uniform distribution.
|
|---|
