A Novel Melt Processing for Mg Matrix Composites Reinforced by Multiwalled Carbon Nanotubes

Carbon nanotubes (CNTs) reinforced Mg matrix composites were fabricated by a novel melt processing. The novel processing consisted of two courses: CNTs pre-dispersion and ultrasonic melt processing. Mechanical ball-milling was employed to pre-disperse CNTs on Zinc (Zn) flakes. Serious CNT entanglements were well dispersed to single CNT or tiny clusters on Zn flakes. The ultrasonic melt processing further dispersed CNTs in the Mg melt, especially tiny CNT clusters. Thus, a uniform dispersion of CNTs was achieved in the as-cast composites. Hot extrusion further improved the distribution of CNTs. CNTs increased both the strength and elongation of the matrix alloy. Notably, the elongation of the matrix alloy was enhanced by 40%. Grain refinement and the pulling-out of CNTs resulted in the evident improvement of ductility for the composites.

Introduction

Carbon nanotubes (CNTs) are regarded as the ideal reinforcement for metal matrix composites because of their extremely high elastic modulus and strength as well as good thermal and electrical properties[1], [2], [3], [4], [5]. Thus, there are many reports about carbon nanotubes reinforced Al, Ti and Cu matrix composites. Nowadays, the solid processing (such as modified powder metallurgy and friction stir processing) and chemical processing (for example, molecular-level mixing) have succeeded in homogeneously distributing CNTs in metallic matrix[6], [7], [8], [9]. The melt processing has achieved great success in CNTs reinforced polymer matrix composites^[10]. However, it has not been successfully developed for CNTs reinforced metal matrix composites because Al, Ti and Cu tend to react with CNTs in molten state. Nevertheless, the melt processing is the most promising approach for the fabrication of CNTs nanocomposites on industrial scale. Therefore, it is of great necessity to study the melt processing for CNTs reinforced metal matrix composites.

Normally, melt processing involves mixing CNTs with molten metals, so the melt processing requires good chemical compatibility between CNTs and melts of the matrix. Unlike Al, Ti and Cu, Mg has good chemical compatibility with CNTs. Thus, the melt processing is very suitable for CNTs reinforced magnesium matrix composites (Mg-CNT composites). Although some researchers have attempted to fabricate Mg-CNT composites by stir casting, good dispersion of CNTs was not well realized by simple mechanical stirring. There are two key factors, the formation of serious CNT entanglement and the wetting degree between CNTs and Mg, which significantly influence the dispersion of CNTs in Mg melts. Good dispersion of CNTs can be realized in Mg melts only if both of these two factors are to be taken care of.

For the serious entanglement of CNTs, it is necessary to pre-disperse CNTs before they are added into Mg melts because there are not any effective methods to disperse CNTs directly in metallic melts. Thus, developing a proper CNTs pre-dispersion method for the melt processing can resolve the serious CNT entanglement. Currently, there are two main methods to segregate CNTs; chemical and mechanical. In the chemical method, acid treatment is applied to modify the carbon nanotubes with some functional groups so as to produce electrical repulsive force between the carbon nanotubes[11], [12]. Generally, it can realize a homogeneous dispersion of carbon nanotubes in aqueous solutions. But the process is too complicated and it can only segregate a small number of carbon nanotubes each time. Compared to chemical method, mechanical method is much simpler. Mechanical ball-milling is one of the main methods, the dispersion effect is good and the process can be easily controlled by adjusting the rotational speed, time and ball-to-powder weight ratio[13], [14], [15]. Therefore, mechanical ball-milling can be a good choice for the pre-dispersion of CNTs.

For the wettability between CNTs and Mg, the answer can be found from our previous research. Recently, we have found that high-energy ultrasonic can be used to disperse the clusters of nanoparticles in metal melts through ultrasonic cavitation^[16]. The ultrasonic cavitation could produce transient (in nanoseconds) micro “hot spots” which could have a temperature of about 5000 °C, pressures above 1000 atm and heating and cooling rates above 10¹⁰ K/s^[17]. It cannot only disperse CNTs but also significantly improve the wettability between CNTs and Mg. Thus, this work tried to use the ultrasonic method to overcome the wettability factors.

In this study, a novel melt processing for Mg-CNT composites was explored and the microstructure and mechanical properties of the composites were investigated.