Journal of Magnesium and Alloys (Sep 2020)
Microstructure and tensile properties of magnesium nanocomposites fabricated using magnesium chips and carbon black
Abstract
In this study, carbon black (0, 0.01, 0.03 and 0.08 wt%) and AZ31 (Mg–3Al–1Zn) magnesium chips were used to fabricate carbon black-reinforced magnesium matrix composites with extrusion or a combination of extrusion and high-ratio differential speed rolling. After hot pressing at 693 K and extrusion at 623 K with an extrusion ratio of 22, the magnesium chips coated with carbon black were soundly bonded into a bulk composite material. The grain sizes of the extruded materials were similar with a size of 48.2–51.5 µm despite the difference in the amount of carbon black. The yield strength and ultimate tensile strength increased from 177 to 191 MPa and from 240 to 265 MPa, respectively, as a result of the addition of 0.01% carbon black; however, a further increase in the strength was marginal with additional carbon black. The same trend was observed in the strain hardening behavior. The tensile elongation increased by to the addition of 0.01% carbon black (from 15.8% to 17.4%) due to the increased work hardening effect, but decreased with additional carbon black due to its agglomeration and poor dispersion at higher concentration. After high-ratio differential speed rolling (HRDSR) on the extruded materials and subsequent annealing, the AZ31 and AZ31 composites had a similar fine grain size of 16.3–17.9 µm. The annealed HRDSR composites showed the best mechanical properties at a higher content of carbon black (0.03%) compared to that (0.01%) for the extruded composites. This resulted from the enhanced dispersion effect of the carbon black due to the high shear flow induced during the HRDSR process. The extruded composites exhibited the three distinct hardening stages (stage II, stage III and stage IV), while the annealed HRDSR composites mainly displayed the stage III hardening. The addition of carbon black increased the strain hardening rate at all the strain hardening stages in both of the extruded and annealed HRDSR materials. At the initial hardening stage, the strain hardening rates of the extruded composites were higher than those of the annealed HRDSR composites, but this became reversed at the later stage of hardening. Possible explanations for this observation were discussed. The strength analysis suggests that dislocation-carbon black interaction by Orowan strengthening and dislocation generation due to a difference in thermal expansion between matrix and carbon black are the major strengthening mechanisms.
