PARTICLE DEGRADATION DURING REMELTING OF AL-1-PERCENT MG ALLOY REINFORCED WITH SICP COATED WITH SOL-GEL-PRODUCED TIO2

TiO2 coatings on SiC particles were produced by the sol-gel process to reduce the reinforcement degradation in Al-1 wt % Mg/SiCp composites during remelting. The reaction kinetics of the degradation was followe d using electron probe analysis and the microstructure of the interfac es was investigated by transmission electron microscopy (TEM). The res ults obtained clearly show that TiO2 coatings reduce the degradation o f the particles during remelting of composites at 700 degrees C. The d egradation becomes detectable after 20 min for the uncoated particles and after 100 min for the TiO2-coated particles. TiO2 reacts during th e fabrication of the composites to form MgO. Titanium does not have ti me to leave the interface and remains in the reaction layer during fab rication in the semi-solid state. The MgO/Ti reaction layer appears to be responsible for the protection of the particles. During remelting, titanium leaves the interface and aluminium diffuses into channels be tween the MgO crystals. MgO then transforms into spinel (MgAl2O4) lead ing, after remelting for 150 min, to a non-uniform reaction zone mainl y composed of large spinel crystals. Particle degradation begins when the reaction zone becomes discontinuous and allows molten aluminium to reach the SiC particles.