Secondary phases and interfaces in a nitrogen-atmosphere sintered Al alloy: Transmission electron microscopy evidence for the formation of AlN during liquid phase sintering

A comprehensive transmission electron microscopy study has been made of the secondary phases and their interfaces with the matrix in an alloy of Al-2Mg-2Si-0.25Cu sintered in nitrogen. AlN was detected both at the Al-Mg Si interface and inside MgSi grains as strings of nanocrystallites. MgSi did not exist at the sintering temperature; it was the solidified residue of the sintering liquid. The observation confirms the formation of AlN during liquid phase sintering of aluminium alloys in nitrogen. The likely pore filling processes are discussed in the light of the distribution of the AlN nanocrystallites. Two Al-, Si- and O-rich secondary phases were also identified, suggesting that, in addition to Mg, Si may have also played a role in disrupting the AlO film that enveloped each Al powder particle. These findings improve the fundamental basis for understanding the sintering of aluminium alloys in nitrogen and the role of Si.