Effect Of Hot Extrusion On Microstructures And Mechanical Properties Of AZ91 And SiCp/AZ91 Magnesium Matrix Composite

SiCp/AZ91 magnesium matrix composites with average sizes of 10μm and three volume fractions of 5%, 10% and 15% were fabricated by stir-casting technique, and then these composites and AZ91 alloy were extruded under different conditions. The microstructures of the materials were examined by means of optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The room temperatue mechanical properties of AZ91 alloy and the composites before and after extrusion were studied.The results showed that hot extrusion could significantly reduce the grain size of as-cast AZ91 magnesium alloy, and thus enhance the mechanical properties of the alloy at room temperature. Generally, for the extruded AZ91 alloy with different treatment statuses, the ultimate tensile strength and yield strength decreased while elongation to failure increased obviously with increasing test temperature; With the increase of extrusion temperature under the same extrusion ratio, yield strength and ultimate tensile strength of AZ91 alloy decrease, while elongation increases. Under the same temperature, both yield strength and ultimate tensile strength of AZ91 alloy improve when extrusion ratio increases from 5 to 12, but yield strength and ultimate tensile strength decrease when extrusion ratio is raised from 12 to 23.The mechanical properties of SiCp/AZ91 composite are enhanced greatly after hot extrusion. Reason for this is that casting flaws are eliminated, boundary strength between SiC particles and Mg alloy matrix is enhanced, and that uniformity of reinforced particle distribution is improved after the composite is extruded. SiC particles in extruded composite rod distribute along extrusion direction orderly. Under the same extrusion ratio, mechanical properties of the composites increase gradually with the increasing extrusion temperature. But under the same extrusion temperature, the strength of the composite increases initially and then decreases with the increasing extrusion ratio, this variation trend for strength of the composite with extrusion ratio is consistent with that of AZ91 alloy. The strength of the composite increases and its elongation decreases with the increase of volume fraction of SiC particle after undergoing extrusion deformation.The composite fails by a fracture pattern of formation, growth and aggregation of cavities, and that the fracture mechanism of the composite does not change after extrusion.