Oxynitride dendritic strengthening in pre-melted aluminium flakes through controlled in situ nitridation

In situ nitridation of pre-compacted aluminium flakes is shown to be a key to lightweight composites for many engineering applications such as more efficient and cost-effective construction vehicles. This new composite has the mechanical properties of high-strength streels while being ∼3.4 times lighter. This composite was produced by: (i) compaction of micrometric Al flakes into porous blanks, (ii) partial melting through rapid heating to 870 K in N₂ and (iii) annealing of the pre-melted blanks at 750 K in N₂, where different Al flake deoxidisers were used to control a sufficiently low level of H₂O/O₂ activating in situ nitridation. The nitrided composites contained stellar dendrites composed of Al, AlN and γ-Al₂O₃, in which the ceramic fraction increases over the nitridation stage. The traces of H₂O in the N₂ atmosphere were found to be beneficial for strengthening, due to oxidation and suppression of the composite dendrites from overgrowing and segregation.