Hot Deformation Behaviors And Microcosmic Mechanisms Of Bimodal Size SiCp/AZ91Magnesium Matrix Composite

In the present study, the bimodal size SiCp/AZ91magnesium matrix composite werefabricated successfully by the semisolid stirring casting technology. The influence of volumeratio between submicron and micron SiCp on the microstructure and mechanical properties ofMg matrix was investigated. The hot deformation behavior of (0.2um1.5vol.%+10um8.5vol.%) bimodal size SiCp/AZ91magnesium matrix composite was investigated at thetemperature of270-420℃and strainrate of0.001-1s-1. Based on the dynamic materialmodeling (DDM), theprocessing maps of the studied bimodal size SiCp/AZ91composite areconstructed to optimize the hot working domains. In addition, the microstructure evolution isanalyzed to validate the established processing maps of bimodal size SiCp/AZ91composite.Microstructure observation was carried out by4XC optical microscope (OM) and MIRA3XMU scanning electron microscope (SEM). The specimens for OM were cut parallel tocompression direction.With the increase of volume ratio, the submicron particle dense regions increase and theaverage grain size decreases. Results show that the submicron SiCp is more conducive tograin refinement as compared with micron SiCp. With the increase of volume ratio, thesubmicron particle dense regions increase and the average grain size decreases. The yieldstrength of bimodal size SiCp/AZ91composite is higher than monolithic micronSiCp/AZ91composite. Among the composite with different volume ratio, the S-1.5+10-8.5composite has the best mechanical properties. The strengthening mechanism of bimodal sizeSiCp/AZ91composite were dislocation strengthening mechanism, grain refinement, theOrowan strengthening mechanism and load transfer effect, however, grain refinementstrengthening mechanism and dislocation strengthening mechanism played a major roleamong them. The true stress-true strain curves of AZ91alloy and S-1.5+10-8.5composite wereobtained and their characteristics were analyzed. The results showed that the steady flowstress (σs) and peak stress (σp) value of alloy and composite increases with increasing strainrate and decreasing compression temperature. The apparent activation energy Q and the stressexponent are obtained by hyperbolicsine type constitutive equations, and the dislocation climbis the main deformation mechanism responsible for the present composite and alloy. The Qvalue increase with increasing temperature and stress rate. It is because that the extent ofDRX increases with increasing temperature, which results in the high power dissipation and Qvalue. Besides, the deformation resistance of the composite increase with increasing strainrate, so more energy is need for deformation, which also contributes to the high Q value. TheProcessing maps are developed by the efficiency of power dissipation and the instability ofdeformation. So, processing maps not only help identify the optimum deformation conditionbut also describe the flow instability domains.The correctness and validity of theprocessing maps of the materials were manifested by the observation ofmicrostructures. The processing maps developed at the strain of0.5exhibits two optimumdomains with higher value of power dissipation: one occurs in the temperature range of270–370C and strain rate from0.001to0.01s-1with maximum dissipation efficiency of38%; the other occurs at420C and0.01s-1with peak dissipation efficiency of24%.Microstructures of the S-1.5+10-8.5composite varied with compressive temperature,strain rate and strain value. For AZ91alloy, the matrix is mainly composed of fine dynamicrecrystallised grains and twins microstructure. The interaction of twins/twins andtwins/dislocations enhanced the new DRX grain nucleation. It can be found that theXDRXof the present composite is higher than that of AZ91alloy at all strain. However, theaverage DRXed grain size of DRX in SiCp/AZ91composite smaller than that of themonolithic AZ91alloy. It is because that the bimodal SiC particles have obvious effect onstimulates DRX nucleation, and the submicron SiCp is conductive to inhibiting the growth ofDRX grains due to its pining effect on grain boundaries.