The In-situ Reaction Syntheis Of Sic Particles In Al-Si Melts

The high scope video microscope (HSVM), X-ray diffractometer (XRD), field emission scanning microscopy (FESEM) and electron probe micro-analyzer (EPMA) were used to systematically study the structural evolution of carbonaceous particles such as TiCx and Al4C3, and the methods for in-situ synthesis of SiC in Al-Si melts were proposed. Meanwhile, the thermodynamic and critical conditions of the in-situ synthesis of SiC particles in Al-Si melts were discussed. Based on above works, the in-situ reaction mechanism of the synthesis of SiC particles in Al melts was built up, which could be confirmed by experiments. Moreover, the in-situ synthesized SiC particles reinforced Al-Si based composites initially were fabricated, the mechanical properties of which such as tensile strength and wear resistance, etc. were measured.The major results of the present study are as follows:(1) The in-situ synthesis of SiC particles via the structural evolution of TiCx in Al-Si meltsIt is found that the Si atoms could promote the structural evolution process of TiCx, the final products of which would depend on two parameters, that is melt temperature and Si contents in Al melts. When the reaction temperature is lower than890℃, TiCx would transform into Al4C3phase; When the melt temperature is higher than890℃, the evolution products of TiCx depends mainly on the Si content in Al melts:When the Si content increases from7wt.%to30wt.%, TiCx would keep stable. Besides, a little Ti3SiC2is formed. When the Si content is higher than30wt.%, all TiCx particles could transform into SiC.In the view of crystallography, it is the crystal defects such as carbon vacancy etc. that results in the structural evolution and in-situ reaction of TiCx in Al-Si melts. Through calculation based on the free Gibbs energy, it is known that higher temperature and Si contents in the melt could promote the structural-evolution process of TiCx in Al-Si melts. It is proposed that, compared with C atoms, directional atoms such as Si and Al atoms which possess larger atomic radius, could result in lattice distortion of TiCx, which would accelerate the structural-evolution process of TiCx. When the degree of structural-evolution reaches the threshold value, the directional atoms such as Si, Al would react with C atoms and the reconstruction of these atoms would occur, forming series of transiting phases. Finally SiC products would be fabricated in Al-Si melts.(2) The structural-evolution of Al4C3in Al-Si melts and the in-situ reaction mechanism of synthesized SiCIt is found that the Si atoms in Al-Si melts could drive the structural evolution process of Al4C3, the final products of which would depend on two parameters such as holding time and Si contents in Al melts. When the Si content is low (20wt.%), Al4C3phase could not evolve during short holding time (5min.). However, with the increase of holding time (30min.), Al4C3would transform into Al4SiC4and SiC phases; When the Si content is high (more than25wt.%), Al4C3would transform into SiC during short holding time (5min.). With the increase of holding time, the SiC particles would keep unchange.In the view of the non-stabilization doping principle, the in-situ reaction mechanism of synthesis of SiC in Al melts can be described as this:The directional atoms (such as Si, Al atoms) could spread to Al4C3phase, which would stimulate the evolution, forming series of transiting phases. The whole process undergoes "atom-dope and in-situ reaction" more than once, and finally in-situ synthesizes SiC phases.(3) The investigation of the tensile strength and wear resistance of the in-situ synthesized SiC particles reinforced Al-Si based compositesBased on the in-situ reaction mechanism, a novel method to in-situ synthesize SiC particles in Al melts is proposed. Utilizing this method, the in-situ synthesized SiC particles reinforced Al-Si alloy can be fabricated. In order to eliminate the weak effect of TiAlxSiy phases, boron element could be added into melts forming in-situ TiB2, and (SiC+TiB2) hybrid-particles reinforced Al-Si composites can be fabricated. Using MM200model wear-resistant testing machine, HRS-150model Rockwell hardness tester, etc., the properties such as wear-resistance and Rockwell hardness have been compared between the untreated and reinforced Al-Si alloys.Using particles reinforcement to fabricate high-strength and wear-resistant piston materials is one of hot topics in the engineering industry. In our work,(SiC+TiB2) hybrid particles reinforced Al-Si alloys were fabricated using the method of in-situ synthesizing SiC particles and boron element optimization in Al-Si melts. It is found that the existence of SiC and TiB2particles can significantly improve the high-temperature tensile strength and wear resistance of piston alloys.