Analysis of interface between components in AZ91 magnesium alloy foam composite with Ni-P coated fly ash cenospheres

The interface between the components in magnesium matrix foam composites with fly ash cenospheres has been analysed. The composite based on AZ91 commercial magnesium alloy with hollow aluminosilicate cenospheres was fabricated by the negative pressure infiltration technique. Ni-P coated cenospheres heated to 773 K were used in this study. Analyses of the composite microstructure were carried out by light microscopy, XRD, scanning and transmission electron microscopy. The fabricated composite was characterized by uniform distribution of cenospheres free from damage and unfilled by the AZ91 alloy within the matrix. Analyses of the composite matrix microstructure revealed that it was composed of α-Mg, α+γ divorced eutectic, γ discontinuous precipitates and an Al8Mn5 intermetallic phase. Examinations of the composite interfaces disclosed that Ni3P and Ni phases were formed inside the Ni-P coatings during heating of the cenospheres. Additionally, NiO was created in the external layer of the Ni-P coatings due to the oxidation reaction. The presented results unequivocally revealed that the used Ni-P coating, especially the NiO layer, on the cenospheres prevent reactions between the molten AZ91 alloy and fly ashes.