Effect Of Ba Content On Microstructure And Mechanical Properties Of Al-7Si Alloy

Coarse flake-like eutectic Si particles would reduce mechanical properties of Al-Si alloy,especially ductility,which limits its wider application in automative industry.Modification of eutectic silicon by addition of certain elementsr could improve the mechanical properties of Al-Si alloys.In order to satisfy the requirements for the cast alloys suitable for automotive body structure,dutile Al-Si alloy have been developed,such as Aural-2,Silafont-36.These alloys are modified by Sr to improve plasticity,but heat treatment is necessary to spherize rod-like Si particles to improve ductility.In this paper,the effect of Ba content on the microstructure,mechanical properties and fracture behaviorof Al-7Si alloys was investigated through optical microscopy(OM),scanning electron microscopy(SEM),X ray diffraction(XRD),differential scanning calorimetry(DSC),transmission electron microscopy(TEM)analysis method,and the mechanism of Si particle modification was also proposed.The results are as follows:(1)Addition of Ba modified the morphology of eutectic silicon from coarse plate-like into rod-like or granular.It is found that with the addiction of 0.15 wt.% Ba,eutectic silicon was translated into fine fibrous and spherical.When the content of Ba is further increased,the eutectic silicon is transformed to rod-like again.The addition of Ba can cause formation of Ba containing phase and voids.With the increase of Ba content,the size of Ba containing phase increases and the porosity arises.(2)With the increase of Ba content,the tensile strength and elongation of Al-7Si alloy firstly increase and then decrease.When the Ba content is 0.15 wt.%,the tensile strength and elongation of the alloy reach the peak value,which are 184.4MPa and 15.1%,respectively.(3)The transition of fracture mode as a function of Ba content can be generally described as follows: brittle fracture(0wt.%Ba)?ductile fracture(0.15 wt.%Ba)?brittle fracture(1.00 wt.%Ba).(4)When the Ba content is 1.00 wt.%,the nucleation temperature of eutectic silicon was decreased by 2.74°C,which indicates that nucleation was depressed by the addition of Ba.It was observed that Ba aggregated near the twin boundary in Si particle and makes the twin branch,the mechanism of Ba modification is proposed as the impurity induced twinning mechanism.