Study On Microstructure And Creep Behavior Of SiC Nanoparticles Reinforced Mg-9Al-lSi Composite Deformed By ECAP

High-temperature stable Mg₂Si phase reinforced Mg-Al-Si composite is the first creep resistant magnesium matrix composite for automobile power system.However,network Mg17Al12 and coarse Chinese-script shape Mg₂Si phases in as-cast Mg-Al-Si composite can significantly reduce mechanical properties due to the role of matrix fragmentation,which limit its wide application.Therefore,improving the size and morphology of the second phase has an important significance for performance improvement of Mg-Al-Si composite.1wt.%SiC nanoparticles reinforced Mg-9Al-1Si?indicated as Mg-9Al-1Si-1SiC?composite fabricated by semisolid stirring assisted ultrasonic vibration was subjected to ECAP in this paper.Optical microscopy?OM?,scanning electron microscopy?SEM?,X-ray diffractometer?XRD?,energy dispersive spectrometer?EDS?and transmission electron microscopy?TEM?were applied to analyze the effect of SiC nanoparticles on the microstructure of Mg-9Al-1Si composite and the microstructure evolution of solution-treated Mg-9Al-1Si-1SiC composite after different ECAP passes.The mechanical properties at room temperature and creep properties at high temperature of the composites were systematically studied.The results showed that the addition of lwt.%SiC nanoparticles could refine matrix grain,Mg17Al12 and Mg₂Si phases of as-cast Mg-9Al-1Si composite.But the morphology of Mg17Al12 phase was still network structure and the MgaSi phase still exhibited Chinese-script type.In the matrix,the distribution of SiC nanoparticles was relatively uniform,but SiC nanoparticles clusters located at the Mg17Al12 and Mg₂Si phases.The as-cast Mg-9Al-1Si and Mg-9Al-1Si-1SiC composites were subjected to ECAP for 4 passes directly,the matrix grain was refined and the Mg17Al12 and Mg₂Si phases were broken significantly.Compared with the as-extruded Mg-9Al-1Si composite,the grain size was finer and the smaller Mg17Al12 and Mg₂Si particles were more uniform in as-extruded Mg-9Al-1Si-1SiC composite.In addition,the distribution of SiC nanoparticles was improved after ECAP.So that the as-extruded Mg-9Al-1Si-1SiC composite exhibited better mechanical properties.However,a small amount of massive Mg17Al12 phase was still existed in directly extruded Mg-9Al-1Si-1SiC composite,which would become the source of microcracks initiation due to stress concentration during the tensile test at room temperature.This would hinder the further improvement of mechanical properties.The Mg-9Al-1Si-1SiC composite was subjected to solution treatment to remove the Mg17Al12 phase before extrusion,and then solution-treated Mg-9Al-1Si-1SiC composite was subjected to ECAP for different passes.The results showed that precipitated Mg17Al12 phase exhibited smaller particles after different ECAP passes and the amount of precipitated Mg17Al12 particles was the largest after ECAP for 2 passes.The Mg₂Si phase was fragmentation gradually and the distribution was homogeneous with the increase of ECAP passes.And after ECAP for 2 passes,the average size of matrix grain was the smallest.The ultimate tensile strength and elongation of the composites improved gradually with the increase of ECAP passes,and which reached up to 296MPa and 8.8%respectively after 4 passes.But the yield strength reached the maximum after 2 passes,which was decline after 4 passes.Under the creep conditions of 473K/70MPa,the creep properties of as-cast Mg-9Al-1Si-1SiC composite were superior to that of as-cast Mg-9Al-1Si composite.However,the creep properties of as-extruded Mg-9Al-1Si-1SiC composite were significantly lower than that of as-cast composites,and the higher steady creep rate and the inferior creep properties were gained with reducing grain size.The creep stress exponent values were in the range of 5.51-6.89 and the creep activation energy values were in the range of 86?111kJ/mol of as-cast Mg-9Al-1Si-1SiC composite at temperatures ranging from 448K to 498K and under stresses ranging from 70MPa to 90MPa,and thus the diffusion-controlled dislocation-climbing and the second-phase particles enhancement were suggested to be the dominant creep mechanisms.