Study On Preparation And Properties Of Al-Si Alloy Composites Reinforced With Nano-carbon And Al₂O₃

Aluminum-matrix composite materials have been found a wide range of application prospects owing to their low mass densities, high specific strengths and other exceptional properties. Recently, nano-carbon reinforced aluminum matrix composite materials aroused increasing interests among researchers and some achievements were acquired, but its theory needs to be further developed for the question’s complexity on nano-carbon dispersion. Therefore, covalent functionalization and noncovalent functionalization methods were used to modify surfaces of carbon nanotubes(CNTs) and graphene nanoplatelets (GNPs) to improve dispersion property. Based on the principle:SiO2+Al→Si+Al2O3, Al-Si-Al2O3-CNTs, Al-Si-Al2O3-GNPs and Al-Si-Al2O3-CNTs-GNPs composites were prepared by hot pressing(HP) and hot insostatic pressing (HIP) with different CNTs/GNPs compositions and Al2O3. Structure and properties induced by contents of nano-carbon and preparation parameter on composites were systemly studied. Based on mechanical properties and microstructure of nano-composites, strengthening and fracture mechanisms by multiple reinforcements were analyzed to provide some valuable references and significant theories to design and prepare new aluminum-matrix composite materials.CNTs and GNPs were surface-modified by using gallic acid and rutin solution separately. High solution concentration may weaken the modifying effects of CNTs and GNPs because of desorption phenomenons of active functional groups grafted on the surface. On the contrary, low solution concentration may not have effective modifying effects. Results showed that the appropriate concentration of gallic acid solution is 10ug/ml for CNTs surface modification. The quantity of active functional groups grafted on surface of CNTs were the most and dispersibility of CNTs was the best at this concentration. Results showed that the appropriate concentration of Rutin solution is 0.02ug/ml for GNPs surface modification. Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to investigate the dispersion states and effect of CNTs/GNPs. mechanisms can be analyzed as follows:π-π stacking interactions between the large π-conjugated structure of gallic acid, rutin and CNTs, GNPs; chemical reactions between gallic acid, rutin and the amorphous carbon, defects on the surface of CNTs, GNPs.Surface-modified CNTs, GNPs and Al, SiO2 powers were ball milled and freeze-dried to prepare composite powders. Al-Si based composite materials with different mass fraction of CNTs and GNPs were prepared using vacuum hot pressing sintering. Optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), electron spectroscopy (EDS),electron probe micro-analyzer(EPMA), and transmission electron microscopy (TEM) are used to investigate the characteristics of Al-Si-Al2O3-CNTs, Al-Si-Al2O3-GNPs and Al-Si-Al2O3-CNTs-GNPs composites. Based on micro-structure and mechanical properties results, effects of reinforcements on micro-structure and properties of composites were intensively analyzed. Results showed that relatively better contents of CNTs, GNPs, CNTs+ GNPs were 0.5%,1.0% and 0.75%+0.25%, respectively.Co-operative enhancement effects of multi-phase and multi-scale reinforcements were significantly higher than single enhancement effects. Hardness, compressive strength and shear strength of Al-Si-Al2O3-CNTs composites are 55.2HV,334.1MPa and 84.9MPa, respectively, which have correspondingly improved 57.3%,26.0% and 32.4% compared to CNTs single-reinforced composites.Al-Si based composites in which mass fractions of CNTs, GNPs and CNTs+ GNPs are 0.5%,1.0% and 0.75%+0.25% were prepared using hot isostatic pressing sintering (HIP). In-situ reaction of composites proceeded sufficiently and had the best relative density. Fracture modes of composites included:micro-porous aggregate fracture of matrix, debonding fracture of interface,transgranular fracture of reinforcing particles, tearing of matrix and cracking of defects like inter-spaces. Final failure mechanismsof materials were the results of collective effects of these types. Enhancing effects of reinforcements included: grain refinement strengthening, load transfer strengthening, Orowan mechanism strengthening, large interface strengthening of GNPs.