Study On The Fabrication And Properties Of AZ91 Matrix Composites Reinforced By In-situ Mg₂Si Particles

In this paper, the fabrication of Mg₂Si/AZ91D composites from Mg-SiO₂ system has been studied. The influence of alkaline elements Ca, Sr, rare earth element Y, high-energy ultrasonic and heat treatment on the morphology and the size of Mg₂Si particles is studied in details. Microstructure and phase contents of Mg₂Si/AZ91 composites were investigated by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscope (TEM), energy dispersive spectrometer (EDS) and X-ray diffraction (XRD).The results indicate that the addition of Ca, Sr, and Y can significantly refine the morphology of Mg₂Si, and make the size of Mg₂Si much smaller. By adding 0.9% Ca, 0.1% Sr, and 0.5%RE(80%Y), all Mg₂Si grains change to multilateral ones. Meanwhile, the size of Mg₂Si grain is between 0.8 and 5μm, the tensile strength and elongation of Mg₂Si/AZ91composite are 214.7 MPa and 2.18%, respectively.The influence of ultrasonic time and ultrasonic power on the microstructure and properties of Mg₂Si/AZ91 composites have also been studied. It is shown that the best modification result has been obtained as the ultrasonic treatment time and power are 7min and 1.2kW, respectively. The tensile strength of Mg₂Si/AZ91 composite is 215.6MPa, which increases 18.3% as compared to that without ultrasonic field. The tensile fracture analysis indicates that the composite experiences cleavage fracture and intergranular fracture during room temperature tensile.While adding alkaline earth and rare earth, and applying ultrasonic field at the same time, the size of Mg₂Si turns smaller. The edges of block Mg₂Si particles are found to be passivated, and Mg₂Si particles are found to disperse more evenly. The tensile strength and elongation are 231.3 MPa and 2.65%, respectively.After solution treatment+aging treatment, the morphology of the granular Mg₂Si has not changed, and the size of Mg₂Si has not coarsened, which proves that Mg₂Si phase is a thermal stable phase under this heat treatment condition. After heat treatment, the tensile strength and elongation of Mg₂Si/AZ91 by adding alkaline earth and rare earth under the ultrasonic field reaches 242.3MPa and 2.86%, respectively.