The Al2O3（p）/Al nano-composites were fabricated from Al-K2ZrF6-Na2B4O7 system by sonochemistry in situ reaction. The fabrication mechanisms, including high intensity ultrasonic influence on microstructures and reinforcement particles-aluminum matrix interface, were investigated by X-ray diffraction （XRD）, scanning electron microscopy （SEM） and transmission electron microscopy （TEM）. The XRD results show that the component of the as-prepared composites is Al2O3 reinforcement. The SEM analysis results indicate that Al2O3 particles are uniformly distributed in the aluminum matrix. The TEM results show that the morphologies of Al2O3 particles present in nearly sphere-shape, the sizes are in the range of 20-100 nm, and the interfaces are net and no interfacial outgrowth is observed. Analysis with secondary development Image-J software shows that Al2O3 recoveries are firstly improved and then decreased with increasing ultrasonic power. When the power is 0.4 kW, the distribution is the best, and a maximum number of particles are obtained. The reaction mechanisms were investigated.
CHEN Deng-binZHAO Yu-taoZHU Hai-yanZHENG MengCHEN Gang
（ZrB2＋Al2O3＋Al3Zr）/A356 composites were synthesized by melt direct reaction from A356-（K2ZrF6＋KBF4＋Na2B4O7） system.The phase compositions and the microstructures of the as-prepared composites were investigated by XRD,SEM and TEM.The results show that the reinforcements are composed of ZrB2 and Al2O3 ceramic phase particles and Al3Zr intermetallic particles.The ZrB2 particulates are easy to join together to form some particle clusters and distribute along the α（Al） grain boundary.The morphologies of the ZrB2 particulates are in hexagon-shape with the size of about 50 nm.The TEM investigation results of Al3Zr indicate that Al3Zr grows in the form of facet with the length-diameter ratio of about 20.The morphologies of Al2O3 particles are in rectangular-shape and ellipsoidal-shape,with the size of about 0.1 μm.In addition,the interfaces of the matrix and particles are net and no interfacial outgrowth is observed.
In-situ TiB2/7055Al nanocomposites are fabricated by in situ melt chemical reaction from 7055Al-K2TiF6-KBF4 system under high intensity ultrasonic field,and the mechanism and kinetic model of in-situ melt chemical reaction are investigated.X-ray diffraction （XRD） and scanning electron microscope （SEM） analyses indicate that the sizes of in-situ TiB2 nanoparticles are in the range of 80-120 nm.The results of ice-water quenched samples show that the whole process contains four stages,and the overall in-situ reaction time is 10 minutes.The in situ synthesis process is controlled mainly by chemical reaction in earlier stage （former 3 minutes）,and by the particulate diffusing in later stage.The mechanism of key reaction between Al3Ti and AlB2 under high intensity ultrasonic in the 7055Al-K2TiF6-KBF4 system is the reaction-diffusion-crack-rediffusion.Furthermore,the reactive kinetic models in 7055Al-K2TiF6-KBF4 system are established.
In situ （Mg2Si＋MgO）/Mg composites fabricated from AZ91-A12（SiO3）3 under high-energy ultrasonic field were investigated by XRD, DSC and SEM. The results indicate that the size, morphology and distribution of the in situ Mg2Si particles are greatly optimized with the assistance of the high-energy ultrasonic field. The amounts of the in situ Mg2Si particles are increased, the sizes are refined, the distributions become uniform, and the morphologies are changed to smooth olive-shape or spherical shape. The amounts of brittle fl-Mgl7All2 phases are decreased and the morphologies are granulated. The values of the tensile strength ab and HB hardness are increased. These are due to the cavitation effects and acoustic streaming effects induced by the high-energy ultrasonic field.
The influence of solution treatment on the microstructure and properties of Mg2Si/AZ91D composites fabricated from Mg-SiO2 system via in-situ processing method was investigated.The results show that coarse Chinese script shape Mg2Si phases can be formed by adding SiO2 into AZ91D magnesium alloy with Si content up to 1.5% of the alloy melt.During solution treatment,the morphology and distribution of the coarse Chinese script shape Mg2Si phases are modified.Meanwhile,the β-Mg17Al12 phase is dissolved into the magnesium matrix.With increasing holding time,the coarse Mg2Si phases tend to dissolve,break and spheroidize.After solution treatment at 420 ℃ for 16 h,Mg2Si phases become the finest and relatively well-distributed phase.The tensile strength and elongation are increased by 14.9% and 38.9%,respectively.It is believed that the Mg2Si phases continuously dissolve and break,and finally the spheroidized Mg2Si particles are obtained due to the interface tension of Mg2Si/Mg interface.
In-situ ZrB2/AZ91D magnesium matrix composite was successfully synthesized with AI/K2ZrF6＋NH4BF4 by means of Direct Melt Reaction. The fabricated ZrB2/AZ91D magnesium matrix composite through direct melt mixing method was investigated. Results from X-ray diffraction （XRD） and energy dispersive X-ray spectroscopy （EDS） confirmed the existence of ZrB2 particles in the AZ91D alloy, and most ZrB2 particles were in the size range of just several microns, some even to 100 nm. The cast specimens were studied through corrosion testing and heat treatment. The average grain size of AZ91D decreased markedly from about 250 pm to 50 IJm. In addition, the shape and size of the ,β-MglTAI12 phase as well as the morphologies of primary a-Mg in the magnesium matrix composite were greatly changed. The network structure of the β-MglTAI12 phase was broken into small blocks and the size of a-Mg decreased significantly.
