Structural effects observed in rods fabricated from magnesium powder and multi-walled carbon nanotubes using different parameters of forward-backward rotating die extrusion (KOBO)

In this study the effects induced by 0.5 vol% of multiwalled carbon nanotubes (MWCNTs) in magnesium based rods, obtained by cold and hot forward-backward rotating die extrusion method (KOBO) were presented. A composite billets were fabricated from powder, sonically mixed prior to hot pressing under vacuum, and then extruded. Prior the extrusion process the billets were either at room temperature or heated up to 150 °C. The macrostructure and microstructure of rods surface, cross and longitudinal sections were characterized by LM, SEM with EDS and XRD methods. Moreover, the tensile strength, hardness and open porosity were determined for Mg/MWCNTs composites and reference rods of pure magnesium processed by KOBO. Presented studies allow to recognize the phenomena occurring in extruded rods, as well the role of initial billets heating and extrusion rate. After hot KOBO in all rod volume the nanotubes formed an elongated skeleton with some submicro or nanosized MgO content. But when the cold KOBO and higher extrusion rate were applied, two types of microstructure were detected in the rod, and the rod consisted of Mg-MWCNTs/MgO core well connected with surrounding free of skeleton α-Mg layer doped with single MWCNTs and nanosized MgO. That phenomena was not earlier reported in literature. Moreover, the Rietveld XRD examinations revealed a spectacular refinement of metal matrix crystallites in dynamic recrystallization, and this effect occurred due to MWCNTs presence. The MWCNTs application caused the rods properties increase. However, greater mechanical properties and lower porosity were obtained by extrusion of the heated up to 150 °C billet.