Ageing Behavior Of SiCp/AZ91D Magnesium Matrix Composites

Ageing behavior of the SiC particles reinforced AZ91D magnesium matrixcomposites was studied. As-cast and extruded matrix alloys and composites wereageing treated, effects of the aging treatment on the microstructure and mechanicalproperties of the composites and the matrix alloy were studied. The influence of theSiC particles on precipitation behavior of the matrix alloy was discussed, the effectsof the size,number, geometry and distribution of the precipitation on the mechanicalproperties of the copmposites were also studied. The optimized heat-treatmentparameters were obtained based on the research above.T6treatment was employed on both the as-cast matrix alloy and composites at175℃, and the peak ageing time was determined by the results of the tensile tests.Results showed that ultimate tensile strength(UTS) of the composites reached to thepeak value at28h,while it took38h for the matrix alloy. As-cast composites reachedto peak ageing in advance of the matrix alloy.TEM results showed that high densitydislocation zones formed around the SiC particles because of the addition of theSiC particles. These high density dislocations not only can contribute tohomogeneous nucleation of the precipitation, but also can promote the growth ofthe precipitation as diffusive paths of the solute elements. As a result of these twoeffects, ageing process of the composites can be evidently acceleratedthermodynamically and kinetically.Precipitation zone at grain boundaries wraped the SiC particles and distributedin the form of network as a result of the necklace distribution of the SiC particlesalong the grain boundaries. From TEM and SEM observation, it can be seen thatsecondary phase precipitated firstly at the intersections of the particles and the highstress zones formed at the sharp corners of the particles. Precipitation growed intothe grain in lamellar form, and it tended to spherized along with the ageing time.These characteristics of the microstructure evolution corresponded to theclassical discontinuous precipitation. In the mid-to late ageing process, thereexisted continuous nano-scale precipitation in the grains far away from SiCparticles. In the late ageing process, precipitation in grains tended to grow into rodshape whose length can reach to micron scale. Results of the mechanical propertiesof the peak-aged composites showed that YS and UTS of the composites reached to158MPa and231MPa respectively, which were increased by40%and41%compared with those of the T4-treated composites.Hot extrusion can refine the grains greatly and modify particle distribution.However, some second phases precipitated unevenly during hot extrusion. If the as- extruded material was subjected to T5treatment, the homogeneously precipitatedsecondary phases growed up fast which went against the improvement of themechanical properties.So T4treatment with following aging treatment at175℃was chose. Precipitation zone of the aging-treated extruded composites was in theform of the strip, which was due to the strip distribution along the extrusiondirection of the SiC particles. Observation of the Mg17Al12precipitation inextruded SiCp/AZ91D composites showed that secondary phase precipitatedfirstly at the intersections of the particles and the high stress zones formed at thesharp corners of the particles and the chain-shape distribution zone of the smallerparticles crushed by extrusion.Along with the aging time, lamellar precipitationaround the particle gradually growed up, but it would not become larger with thetime going on, upon when precipitation inside the grains began to generate and ittended to grow into rod shape from particle.As-extruded alloys were T4treated. Grain size increased obviously and yieldstrength decreased. As-extruded composites was also T4treated,but grain size ofthe composites increased slightly,yield strength decreased a little. Secondary phaseof the as-extruded materials after T6treated precipitated firstly at the interfacebetween the matrix and the SiC particle and grain boundaries in the matrix.Continuous nano-scale precipitation was observed in the mid-to late ageingprocess,and along with the ageing time,the precipitation had a transition fromgranular shape to rod shape.Discontinuously precipitated secondary phase at thegrain boundaries was in lamellar form and had various orientations.After ageing at175℃for44h, the extruded alloy has the best tensilestrength with TYS of224MPa and UTS of360MPa. However, the extrudedcomposites can reach the best tensile strength with YS of320MPa and UTS of428MPa, and it takes only32h to reach the peak strength. Composites reached topeak ageing12h earlier than matrix alloy which was mainly because of theintroduced high energy interface by the addition of the SiC particles and the highdensity dislocations around the SiC particles.The increments of the UTS and YS ofthe composites and the matrix alloy were close, which demonstrated that agingefficiency of these two was equivalent The elongation of both the composites andthe alloy decreased dramatically which was due to the brittle secondary phaseformed at the grain boundaries.Ageing treatment had little effect on elasticmodulus.High temperature tensile test results showed that ageing treatment canmake contribute to the strength below200℃.For temperature above250℃,ageingwas harmful to the mechanical properties.