An Investigation On The Microstructure And Corrosion Behavior Of Hot Extrusion Mg Alloy AZ91-SiCp Metal Matrix Composite

Magnesium alloy are of great value for applications in machinery manufacturing,transportation, aircraft, aerospace industries and military field. It is an important means to addan appropriate reinforecement into a magnesium alloy matrix to improve the physical andmechanical properties of extrusion magnesium alloy, but it makes corrosion resistancedeteriorative seriously. Poor corrosion resistance restricts the application and development ofmagnesium matrix composites. In order to establish related scientific theory and broaden theapplication range of magnesium matrix composites, the influence of micron-sized SiC particleand its volume fraction on microstructure of SiCp/AZ91metal matrix composites (MMCs)transversal and longitude section were investigated. The corrosion behavior of SiCp/AZ91MMCs with SiC particle of different volume fraction(2%,5%,10%and15%) wereresearched in3.5wt.%NaCl solution saturated by Mg(OH)2at25℃. The roles of SiC particle,SiC volume fraction and β phase during the corrosion of SiCp/AZ91MMCs were discussed.The corrosion model of SiCp/AZ91MMCs was established and the corrosion mechanism ofSiCp/AZ91MMCs was explained. The negative difference effect of SiCp/AZ91MMCs wasinvestigated.The influence of SiC particle and volume fraction on microstructure of SiCp/AZ91MMCs transversal and longitude section were investigated by SEM and TEM. The results ofinvestigation illustrated that:(1) the main phases of SiCp/AZ91MMCs included SiC particle,α-Mg matrix, β phase and Al-Mn phase.(2) The SiC particle and β phase which appeared asclusters of small particles were aligned in the longitude extrusion direction and disperse in thetransversal section.(3) SiC particle promoted dynamic recrystallization nucleation of α phaseand β phase, which promoted α phase grain refinement. And for β phase, on one hand, SiCparticle made grain refinement; On the other hand, SiC particle changed the position where βphase precipitated. β phase precipitated in internal grain of AZ91matrix, the interfacebetween SiC particle and AZ91matrix was more easily precipitated for β phase. The β phaseprecipitated increasingly around SiC particle as increasing volume fraction of micron-sizedSiC paricle.The influence of SiC particle and volume fraction on corrosion behavior of SiCp/AZ91MMCs transversal and longitude section was researched. The results showed that:(1) The corrosion rate of the SiCp/AZ91MMCs was much greater than that of AZ91magnesiumalloy, and the corrosion rate of SiCp/AZ91MMCs increased as increasing volume fraction ofmicron-sized SiC.(2) The microgalvanic corrosion between AZ91matrix and SiC particle inthe composites did not contribute to the overall corrosion rate significantly.(3) The corrosionmorphology of SiCp/AZ91MMCs was mainly the microgalvanic corrosion in interfacebetween SiC particle and AZ91matrix. The microgalvanic corrosion became more serious asincreasing volume fraction of micron-sized SiC, especially in which SiC particle gathered.The corrosion characteristic of AZ91magnesium alloy and SiCp/AZ91MMCs appearedapparently directivity, which is more serious in corrosion along the extrusion direction.(4)There were two main reasons that led to the greater corrosion rate of SiCp/AZ91MMCs. Onone hand, SiC particle promoted grain refinement of β phase, which increased themicrogalvanic corrosion current density between β phase and α phase. The β phase grain wassmaller, and easy to drop, So that the corrosion rate of SiCp/AZ91MMCs was greater. On theother hand, due to the addition of SiC particle, the position where β phase precipitatedperformed certain characteristics, which promoted the microgalvanic corrosion in theinterface between the SiC particle and AZ91matrix, which promoted more β phase aroundSiC particle to corrode and then drop, SiC particles dropped finally. So that the corrosion rateof SiCp/AZ91MMCs transversal and longitude section increased. The different corrosionperformance between transversal and longitude section could be related to the difference ofvolume fraction of SiC particle.SiCp/AZ91MMCs performed the feature of the negative difference effect. SiC particlepromoted the negative difference effect of SiCp/AZ91MMCs. And the negative differenceeffect of SiCp/AZ91MMCs increased as increasing volume fraction of micron-sized SiC.