Research On The Preparation And Property Of Insitu Al₂O₃ Particle Reinforced ZL109 Matrix Composites

The α-Al₂O₃p/ZL109 composites were successfully synthesized from Al-SiO2 system by melt in situ reaction methods.Optical microscope（OM）,X ray diffraction（XRD）and scanning electron microscope（SEM）equipped with energy dispersive spectrometer（EDS）were used to analyze the phases,distribution of particles and morphology of composites.In this paper,research is being directed to the synthesis mechanism of composite containing in-situ Al₂O₃ from Al-SiO₂ system in the melt.The influence of Al₂O₃ particle volume fraction,particle size and heat treatment on the microstructure and tensile properties of the composite were researched,and the fracture modes and strengthening mechanism were also investigated.The formation of Al₂O₃ phase from Al and SiO2 was thermodynamically feasible,and the reaction begun to take place at 720℃.The results revealed that the reaction of Al and SiO₂ was mainly dependent on the migration and exchange of Al and Si elements,which was a diffusion-reaction process.The particles were identified to be of shape and size similar to that of the SiO2 particles.Two reaction temperature points occurred at 720℃ and 1050℃ in Al-SiO2 system.The γ-Al₂O₃ was first produced during the reaction of Al and SiO₂ at 720℃ and the phase transformation of γ-Al₂O₃ to α-Al₂O₃ occurred at higher temperatures（1050℃）.It resulted in a volume contraction in the process of transformation.The α-Al₂O₃p/ZL109 composites were fabricated from SiO2-ZL101 A system by combining melt in situ reaction and semi-solid shearing methods with the aid of ultrasonication.The homogeneous distribution of α-Al₂O₃ particles were acquired under the condition of 900r/min shearing speed for 3min at semi-solid state.A further step of ultrasonic treatment to the melt followed at 670℃ for 5min,the porosity was basically eliminated and α-Al₂O₃ reinforced particles distributed uniformly in the matrix.In Al₂O₃p/ZL109 composites,α-Al₂O₃ particles were mainly found to disperse along the grain boundaries in the matrix.The average size of α-Al dendrite arm gave a remarkable decrease from 79.88μm for matrix alloy to 34.45μm for 10 vol.%α-Al₂O₃p（30μm）/ZL109 composites.The size of the dendrite arm in the composites with 5vol.% α-Al₂O₃ p gave a decrease from 43.65μm to 31.65μm and reduced by27.5% as α-Al₂O₃ size decreased from 30μm to 0.5μm,indicating that α-Al₂O₃ p played significant role in grain refinement.The higher the volume fraction and the smaller the size of α-Al₂O₃ p,the better the grain refinement.After heat treatment,the eutectic Si phase with approximately 3⁵μm in size existed in particle,which was mainly distributed at the grain boundaries in the form of spherical or bulk.The tensile strength of the α-Al₂O₃p/ZL109 composites increased as α-Al₂O₃ volume fraction increases from 0vol.% to 5vol.%,and then decreased as α-Al₂O₃ particle volume fraction increases from 5vol.% to 10 vol.%,while the elongation decreased continuously.As the size of α-Al₂O₃（5vol.%）increased from 0.5μm to30μm,the strength of α-Al₂O₃p/ZL109 composites increased first and then decreased as well.The in situ composite with 5vol.% α-Al₂O₃ particle（3μm）presented the greatest mechanical properties with tensile strength and elongation.After heat treatment,the tensile strength increased by 10.9% and elongation decreased by 20.5%,the transition of brittle-to-ductile fracture in composites occurred.The hardness of composites enchanced as α-Al₂O₃p（30μm）volume fraction increase from 0vol.% to 10 vol.%,the hardness of the as-cast composites with 10vol%α-Al₂O₃ p was 107.4HV and increased by 27.93% in comparison with that of the matrix,and they were much effective for increasing the hardness after heat treatment,which reached to 118.6HV.The hardness of the as-cast and heat-treated composites with 5vol.% α-Al₂O₃ p enchanced with the size of particles increased first and then decreased.The hardness of α-Al₂O₃p（3μm）/ZL109 composite was 117.2HV after heat treatment,which was higher than other composites.