Effect Of Hot Extrusion Deformation On Microstructure And Mechanical Properties Of Bimodal Size SiCp/AZ61 Composites

In the present study,the bimodal size Si Cp/AZ61 magnesium matrix composites were fabricated by ultrasonic assisted semi-solid stirring casting method.The influence of volume ratios between micron and nano Si Cp on the microstructure and mechanical properties of AZ61 matrix alloys was studied.Immediately after,the 6Vol.%Micro-SiCp+1Vol.%Nano-SiCp/AZ61（denote as‘‘M-6+N-1”）composite material was subjected to hot extrusion deformation,the influence of extrusion temperature and extrusion rate on microstructure,texture and mechanical properties of the M-6+N-1 composite was investigated systematically.The microstructure evolution of the composites during hot extrusion was revealed.In addition,the tensile creep tests of extruded AZ61matrix alloy and M-6+N-1 composite at the same temperature and stress were carried out,and the influence of the addition of bimodal size SiCp on the creep properties of the alloy was studied.The creep properties of extruded M-6+N-1composites at different temperatures and stress states were tested and analyzed.The stress exponent n and creep activation energy Q were calculated,and the creep mechanism of the composites was also discussed.The results show that the grain size of AZ61 matrix alloy was obviously refined after the addition of bimodal size SiCp,and the distribution of Mg₁₇Al₁₂was also improved.With the increase of the volume ratio of 5?m SiCp and 60nm SiCp,the aggregation of nano-SiCp particles was improved,and the average grain size of the composites was decreased gradually.Compared with AZ61matrix alloy,the mechanical properties of the as-cast SiCp/AZ61 composites were improved evidently.When the volume ratio of 5?m SiCp and 60nm SiCp is 6:1,the composite has the best mechanical properties（YS=137.2MPa,UTS=194MPa,EL=4.5%）.After hot extrusion,the solution-treated M-6+N-1 composites have almost completely dynamic recrystallization,the grain size was refined obviously and the distribution of Si Cp was improved.As the increase of extrusion temperature and extrusion speed,the average size of the grain and Mg₁₇Al₁₂ precipitates of the composite increased gradually,but the volume fraction of the precipitated phase decreased.Moreover,the extruded M-6+N-1 composite exhibits a strong basal texture,and the texture strength is mainly affected by the precipitated phase of Mg₁₇Al₁₂.In addition,the yield strength（YS）and tensile strength（UTS）of the composites decreased as the extrusion temperature and speed increase,its elongation（EL）increased with the increase of extrusion temperature,increased firstly and then decreased as increase of extrusion speed.The optimal mechanical properties were obtained when M-6+N-1 composite was extruded with the temperature of 280℃at 0.1mm/s,where the YS=295MPa,the UTS=360MPa and the EL=9.5%.The main strengthening mechanism is fine-grained strengthening caused by grain refinement and Orowan strengthening caused by submicron precipitates.The creep resistance of the extruded M-6+N-1 composite was superior to that of extruded AZ61 matrix alloy under the creep condition of 448K/50MPa.This indicates tha t the addition of bimodal size SiCp can effectively improve the creep resistance of the composites.By analyzing the creep behavior of the extruded M-6+N-1 composite under the conditions of（423K?473K）/（50MPa?70MPa）,it is calculated that the stress exponent and creep activation energy were 2.19?6.73 and 75.09?124.94 kJ/mol respectively.The creep mechanism of the composites is mainly the dislocation climbing mechanism controlled by diffusion and the dislocation slip mechanism controlled by grain boundary diffusion.