Fabrication, microstructure and compressive behavior of ZC63 Mg-microballoon foam composites

ZC63 Mg alloy-fly ash microballoon foam composites were fabricated by melt stir technique with the use of fluxes. The influence of various processing parameters such as melt temperature, type and dimensions of the stirrer, pouring temperature and mould type on distribution of the fly ash microballoons in the Mg matrix alloys was presented. Density measurements of the composites revealed that the addition of fly ash microballoons significantly reduced the density of the composites. As the volume fraction of microballoons increased, the density of the composites decreased. Composites in the as-cast condition exhibited smaller dendrite arm spacing/cell size compared to unreinforced ZC63 Mg alloy. The average dendrite arm spacing/cell size of the composites was about 10 Pm compared to 100 mu m of the unreinforced magnesium alloy. The microstructure of the composites demonstrated even distribution of the microballoon in the Mg alloy matrix and there was no sign of fly ash cluster or residual porosity. Also, it was observed that there were some fly ash microballoons filled with Mg matrix alloy. SEM micrographs showed those fly ash microballoons, eutectic phase and other intermetallic compounds segregated at the magnesium cell boundaries. Examination of the composite interface indicated that the eutectic and other intermetallic compounds phases were able to heterogeneously nucleate on the fly ash microballoons. SEM, XRDA and EDXA of the composites showed clear evidence of reaction product at fly ash/matrix interface. On the basis of XRDA and EDXA, composition, structure and thermodynamic analysis, the main interfacial phase between the fly ash and ZC63 Mg alloy was identified as MgO. The compressive properties of the foam composites were characterized. The foam composites produced in the present work demonstrated typical behavior of an elastic-plastic foam material in compression. They had an initial linear elastic region, followed by a long region of deformation at a relatively flat stress level. The composites containing microballoons up to 20 vol% significantly revealed higher energy absorption in comparison to the solid ZC63 matrix alloy or the composite containing 25 vol% microballoons. (C) 2006 Elsevier Ltd. All rights reserved.