Influence of carbon nanoparticle inoculation on the structure of ML5 casting magnesium alloy

Abstract

The object of this study is to investigate the structure of cast magnesium alloy (Mg-Al-Zn) inoculated with nano particles of different allotropic forms of carbon. The aim of this study is to improve the processability and quality of critical load bearing magnesium alloy part castings. The tasks of the study are: attainment of efficient control of the alloy structural features by introduction of incremental additions of carbon nano particles; determination of the relationship between the quantity of the added inoculating carbon agent and α - solid solution grain size; determination of the influence of allotropic forms of carbon on the morphologic characteristics of the intergranular borders and the eutectoid (a+Mg17Al12); and comparative analysis of the phase composition of the standard ML5 alloy and the experimental alloy ML5 inoculated with carbon black, nanographite, and single-wall carbon nanotubes. The phase composition was determined by X-ray diffraction analysis using copper radiation. Micro X-ray spectral analysis was performed. The following findings were obtained. Using an optical microscope α – solid solution grain size was measured in the standard and inoculated alloy. In the samples of the alloy variants inoculated with 0.1 % wt. of carbon black, nanographite, and single-wall carbon nanotubes, the grain size was reduced by appox. 50 % compared with the standard alloy. In the samples of the alloy inoculated with 0.1 % wt. of nanographite and single wall carbon nanotubes, eutectoid (a+Mg17Al12) precipitates along the grain boundaries were thinner than those in the standard alloy samples and those inoculated with carbon black. X-ray diffraction analysis revealed no new phase formation. The X-ray diffraction patterns of the alloy samples with nanographite and nanotubes display weak peaks of free carbon, which may indicate the presence of free carbon in small quantities in the structure. The scientific and practical originality of the obtained results consists of the following: a technology of inoculation with incremental additions of allotropic forms of nano carbon, namely carbon black, nanographite, and single-wall carbon nanotubes, has been tested on a standard cast magnesium alloy (Mg-Al-Zn system) in an environment similar to the industrial production of critical magnesium cast components. It has been shown that the introduction of nanocarbon in a quantity of 0.1 % wt. can have a beneficial effect on the structural characteristics of ML5 cast magnesium alloy with no change in its phase composition. Nanographite and single wall carbon nanotubes with a regular structure contribute to the formation of thinner intergranular boundaries than amorphous carbon black.