Preparation And Mechanical Properties Of AZ91 Alloy Composite Reinforced With Nano-Carbon Materials

Carbon nanotubes (CNTs) and graphene (GN) are the excellent reinforcements of magnesium matrix composite due to their fantastic mechanical properties, such as high strength and high elastic modulus. How to obtain the well-dispersed CNTs and GN with good interface bonds in the matrix is the keypoint to prepare the Mg-based composite with excellent performance. In this work, AZ91 alloy composites reinforced with nano-carbon materials (CNTs, MgO-coated CNTs (MgO@CNTs), graphene-nanosheets (GNPs) and reduced-graphene oxide (RGO)) have been fabricated by using powder metallurgy, followed by hot extrusion and T4 treatment. The effects of content of nano-carbon materials on the microstructure and mechanical properties of the as-synthesized composites have been studied. The main contents can be summarized below:(1) A layer of MgO nanoparticles can be coated on the surface of CNTs through a simple co-precipitation, and two types of MgO nanoparticles were found on the surface of CNT:single crystal and polycrystalline MgO nanoparticles. The carboxylic acid groups and some defects, such as notch, destroyed out-walls, etc. have been successfully implanted on the surface of CNT after purification, which is beneficial for the nucleation and growth of MgO nanoparticles on the CNT surface. MgO@CNTs exhibits the excellent stability due to the strong interfacial bonds have been formed between CNTs and MgO.(2) The four nano-carbon materials (CNTs, MgO@CNTs, GNPs and GO) can be distributed in the matrix of Mg alloy by a method combined the liquid stirring technology with powder metallurgy process. After comparison of the distribution of the four carbon materials in the matrix of Mg alloy, it is found that GO exhibits the best dispersibility, second is the GNPs, again is the MgO@CNTs, and the worst is the CNT particles.(3) The grain size of the Mg-based composites can be refined by the four carbon materials. Moreover, the average grain size of AZ91 alloy with the four carbon materials decreased firstly, and then increased with increasing the amount of carbon materials. There is no obvious gap among the four carbon materials on the effect of the grain size refinement. However, the grain size refinement of GO is slightly higher than the other three carbon materials.(4) CNTs with coating of MgO is beneficial for the enhancement in the mechanical properties of the composites. Because the interfacial bonds between CNTs and the matrix of Mg alloy and the distribution of CNTs can be improved remarkably through the coating of MgO.(5) The mechanical properties of AZ91 alloy with GNPs can be improved significantly. Because GNPs with a two-dimensional plane structure have large contact area with the matrix of Mg alloy, which is beneficial to form a strong interfacial bonds between GNPs and the matrix of Mg alloy.(6) AZ91 alloy with filling of GO will results in the improvement in mechanical properties. Because GO can be uniformly distributed in the matrix of Mg alloy, which is beneficial to result in a good distribution of RGO by the in situ reaction between GO and Mg element. Meanwhile, the strength of the RGO/a-Mg interface can be improved significantly by the interface reaction product, MgO.(7) After characterizing the interfacial structures of the as-synthesized four composites, we found that CNTs are mainly incoherency with the matrix of a-Mg in the AZ91-CNTs composite. However, in AZ91-MgO@CNTs composite, there are two types of bonds were formed at the CNTs/MgO interface:nanoscale-contact bonds and diffused interfacial bongs. In addition, the orientation relationships of ((?))MgO//((?)α-Mg and [011]MgO//[(?)]α-Mg with the semi-coherent interface characterization were observed between MgO and the matrix of a-Mg. The interatomic spacing misfit at the MgO/a-Mg interface is 6.5%, which indicates a strong interface bonds formed between MgO and the matrix of a-Mg. In AZ91-GNPs composite, GNPs have been formed a strong interface bond with the matrix of a-Mg because of the large contact area between GNPs and the matrix of a-Mg. In AZ91-RG0 composite, a strong interface bonds were formed between RGO and MgO due to the bridging by chemical bonds. Moreover, the orientation relationships of ((?))MgO//((?))a-Mg and [011]MgO//[(?)]a-Mg have been confirmed by the HRTEM observation. The interatomic spacing misfit at the MgO/a-Mg interface is about 8.6%, which indicates that the semi-coherent interface have been formed between MgO and the matrix of α-Mg. The interface bonding strength between RGO and the matrix of α-Mg have been improved significantly by MgO nanoparticles formed through the interfacial reaction between GO and Mg element.(8) The four carbon materials have the similar effect on the YS and elongation of the composites:with increasing the amount of the carbon materials the YS and elongation increasing firstly, and reached a maximum, then decreased. In conclusion, GO exhibits the excellent superiority in the improvement of tensile strength (UTS and YS), ductility and microhardness of AZ91 alloy, as compare to MgO@CNTs, CNTs and GNPs reinforements.