Effect Of ECAP On The Microstructure And Mechanical Properties Of AZ91 Alloy And SiCp/AZ91 Composite

Equal channel angular pressing (ECAP) was applied to the as-extruded AZ91 alloy and SiCp/AZ91 composite fabricated by the stir casting, respectively. The optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM) were used to investigate the microstructure of the alloy and the SiCp/AZ91 composite after ECAP. The basal plane texture was measured qualitatively by the X-ray diffractometer. The mechanical properties of the AZ91 and SiCp/AZ91 composite were evaluated by the tensile test at ambient and elevated temperature.ECAP process was effective in reducing the grain size of AZ91 alloy, especially with the die having internal angleΦ=90°. The yield strength and tensile strength decreased with increasing number of the ECAP pass because of the weakening of the basal-plane texture which formed during hot-extrusion. Due to the grain refinement, the maximal elongation of AZ91 processed for 4 passes reached 565% when tested at the temperature of 300°C with the strain rate of 1.67×10^-3s^-1. The strain rate sensitivity is measured as 0.38.Because of the addition of SiC particle to the AZ91 alloy, the ECAP deformation of SiCp/AZ91 at room temperature was not homogenous. The matrix around the particle was severely deformed. No obvious SiCp broken was observed due to the the lower accumulated strain during ECAP. Meanwhile, the texture evolution was accelerated due to eh introduction of SiC particle.Both the ambient yield stress and ultimate tenile stress of SiCp/AZ91 were decreased after ECAP, suggesting that the basal-plane texture weakening in the composite resulted in the decreasing of the strength. The elongation also decreased after ECAP because of stress concentration around the SiC particle. When the tensile temperature and strain rate were set as 400°C and 1.67×10-4s-1 respectively, the elongation of SiCp/AZ91 after 4 ECAP reached 130%. The SEM observation suggested that the dominant deformation mechanism at high temperature was still grain boundary sliding. Moreover, the sliding was prohibited due to the present of SiC particle.