Study On Microstructures Evolution And Creep Behavior Of Mg-9Al-1Si-1SiC Composite By Two-step ECAP Formation

Semisolid stirring assisted ultrasonic vibration was used to fabricate the composites of 1wt.%SiC nanoparticles reinforced Mg-9Al-1Si?indicated as Mg-9Al-1Si-1SiC?.The obtained composites were subjected to equal channel angular pressing?ECAP?under constant and decreasing temperature conditions,respectively.Optical microscopy?OM?,scanning electron microscopy?SEM?equipped with energy dispersive spectrometer?EDS?,X-ray diffractometer?XRD?and transmission electron microscopy?TEM?were applied to analysis the evolution of microstructures of Mg-9Al-1Si-1SiC composite during ECAP process.Tensile test at room temperature and creep at high temperature were conducted to study mechanical properties at room temperature and creep properties at high temperature of Mg-9Al-1Si-1SiC composite under different ECAP process.The internal relationship between microstructure and mechanical properties was reveled.The results are shown as follows:?1?The microstructures of as-cast Mg-9Al-1Si-1SiC composite were composed of primary?-Mg matrix,secondary?-phase(Mg₁₇Al₁₂)and Chinese script-shaped phases?Mg₂Si?.The Mg₁₇Al₁₂2 phase was basically dissolved in the matrix after solid solution treatment;?2?The grain size of ECAPed Mg-9Al-1Si-1SiC composite was significantly reduced compared with that of as-cast composite.The volume fraction of dynamic recrystallization increased gradually.The grain size of Mg-9Al-1Si-1SiC composite decreased initially and increased afterwards with the decreasing of ECAP passes.The average grain size of the composite was minimum?19.50?m?after two passes;Simultaneously,basic texture?0002?intensity of Mg-9Al-1Si-1SiC composite during the ECAP process;The coarse Mg₂Si phase and the residual Mg₁₇Al₁₂2 phase were gradually fragmented and the distribution of them were more uniform.At the same time,the amount of fine precipitations(Mg₁₇Al₁₂)increased gradually;?3?The ultimate tensile strength?UTS?and elongation?EL?of Mg-9Al-1Si-1SiC composite increased with the increasing of ECAP passes.The UTS and EL of Mg-9Al-1Si-1SiC composite after ECAP four passes were maximum and reached 284 MPa,6.11%,respectively.However,the yield strength increased initially and decreased afterwards.The yield strength?YS?of Mg-9Al-1Si-1SiC composite achieved the maximum?185 MPa?after ECAP two passes,which resulted from the minimum grain size of Mg-9Al-1Si-1SiC composite after ECAP two passes leading to grain refinement strength;?4?The grain size of ECAPed Mg-9Al-1Si-1SiC composite significantly reduced under decreasing temperature conditions compared with that under constant.The average grain size of Mg-9Al-1Si-1SiC composite subjected to ECAP?400-2P+320-2P?was smallest?6.05?m?.Mg₂Si was fragmented and distributed most uniformly and the volume fraction of precipitations(Mg₁₇Al₁₂)was largest?38.9%?.The UTS and YS of Mg-9Al-1Si-1SiC composite subjected to ECAP?400-2P+320-2P?achieved 312MPa,206 MPa,respectively.However,the EL of Mg-9Al-1Si-1SiC composite processed by ECAP?400-1P+360-2P+340-1P?was largest and reached 7.21%,which was related the volume fraction of precipitations;?5?The difference of the strength of Mg-9Al-1Si-1SiC composite processed by different ways of ECAP deformation was mainly affected by fine grain reinforcement and Orowan strengthening but the contribution of two strengthening effects to increased strength was different.In addtition,the difference of elongation of Mg-9Al-1Si-1SiC composite processed by different ways of ECAP deformation was mainly affected by texture and volume fraction of precipitations;?6?The creep test was conducted at 448 K/80 MPa.The steady creep rate of Mg-9Al-1Si-1SiC composite processed by ECAP?400-2P+320-2P?was largest so that the creep resistance of it was worst.The results indicated that the creep properties of Mg-9Al-1Si-1SiC composite decreased with the decreasing of the average of grains.