Experimental Investigation Of Mg₂B₂O₅w/AZ91D Composite Fabricated By Vacuum-Gas Pressure Infiltration Process

In this thesis Vacuum-Gas Pressure Infiltration process was developed for fabrication of discontinuous phase reinforced Metal Matrix Composite and the whiskers perform forming technique by wet forming method without any binder was also developed.The micrographs reveal reasonably uniform distribution and random orientation of the Mg₂B₂O₅ whiskers in the as cast Mg₂B₂O₅w/AZ91D composite fabricated by Vacuum-Gas Pressure Infiltration process casting, and without pores defect in the composite. The SEM micrographs shows that microstructure of the Mg₂B₂O₅w/AZ91D composite is very fine structure. The hardness, elastic modulus, tensile strength and especially the elongation to failure of the composite were increased significantly compared to magnesium matrix alloys. On the other hand the thermal expansion coefficient of the composite was reduced obviously. It indicates that the addition of Mg₂B₂O₅ whisker causes the AZ91D magnesium matrix to obtain strengthening and toughening simultaneously.The interfacial phenomena in the composite are systematically investigated by using TEM, XRD and DSC analysis. The investigation shows that there are not interfacial reaction products at the Mg₂B₂O₅w/AZ91D interface. The composition analysis in surrounding matrix of the whiskers was surface scanned by means of EPMA. The results show that Mg and O elements were enrichment on the surfaces of the whiskers. Based on the experiment results, the bonding situation at the Mg₂B₂O₅w/AZ91D interface is analyzed and discussed in the thesis.The solid solution treatment made the hardness of the Mg₂B₂O₅w/AZ91D magnesium composite decreased, whereas the ageing treatment made the composite displaced the peak hardness of ageing 201HV. The solid solution ageing treatment could make the hardness of the composite rise to 30%. However, as the ageing time increased to 24h, the hardness of the composite is reduced to 183 HV.