Melt processing and characterisation of lightweight metal composites reinforced by nanocarbon

Abstract

Increasing the specific strength of lightweight structural metals such as magnesium, which has two-thirds the density of aluminium, has the potential to significantly widen their application and improve the system energy efficiency and performance across transport and defence industries. Reinforcing magnesium matrices with nano-sized particles to form metal matrix nanocomposites (MMNCs) has shown promise as a way of achieving an increase in specific properties, albeit without the traditionally associated decrease in ductility. This study manufactured and characterised magnesium-based MMNCs to investigate the impact of nanoparticle reinforcements on the mechanical properties of the composites, their microstructure and the theoretical models used for the potential strengthening mechanisms. Multi-walled carbon nanotubes (MWCNTs) can be considered as the ideal reinforcing nanoparticle due to their exceptional mechanical properties, geometry and low density, which can take full advantage of the proposed strengthening mechanisms that occur when nanoparticles are added to a metal matrix. MWCNTs are however notoriously difficult to be homogenously dispersed in a metal matrix and are poorly wetted by molten metal; therefore, nickel coated MWCNTs (NiCNTs) and silicon carbide nanoparticles are also investigated as a potential solution for wettability in this study. Magnesium alloy AZ91D was used as the metal matrix material and the MMNCs were made using metal melt-stirring combined with a nanoparticle pre-dispersion technique. Melt processing and casting techniques are favoured in component manufacturing processes due their scalability and cost-effectiveness. The melt stirring processing parameters such as casting temperature, stirring time and stirring speed were thoroughly examined and modified to successfully produce MMNC samples. The compression mechanical properties of the AZ91D-MWCNT composites showed no change for a range of MWCNT concentrations for the melt stirring processing parameters tested. Theoretical studies of nanoparticle dispersion and wettability combined with electron microscopy of the cross sections of the samples showed that the pre-dispersion and melt stirring processes were insufficient to homogenously disperse the MWCNTs in the metal matrix, therefore NiCNTs were utilised. Whilst no significant differences in compression mechanical properties were seen for the AZ91D-NiCNT samples, a 13% increase in hardness was achieved. An 11% and 20% increase in hardness was achieved for samples of AZ91D composites reinforced with equal concentration of silicon carbide nanoparticles and whiskers, respectively. The poor wettability of MWCNTs by AZ91D melt, apparent from SEM imaging of the composite fracture surfaces, suggested that in order to take advantage of the superior mechanical properties of MWCNTs, a coherent interface between the MWCNT and the Mg matrix is necessary. The nickel coating and silicon carbide nanoparticles, known to wet with molten magnesium, may have provided a coherent interface that resulted in the measured increase in hardness.