This thesis investigates the formation of carbides and Al-Mn-Fe intermetallic compounds (IMCs) in liquid Mg-9Al-0.7Zn-0.2Mn (AZ91, wt.%) held in uncoated steel/cast iron crucibles at various temperatures. The focus is on (i) how liquid AZ91 reacts with Fe-C crucibles to produce carbides in AZ91, (ii) how Al-Mn-Fe IMCs are influenced by different Fe content picked up from steel crucibles, and (iii) how Al-Mn-Fe IMCs settle in liquid AZ91.
The formation of carbides has been investigated during reactions between solid Fe-xC alloys (x = 0-3.6 wt.%) and liquid Mg-9Al-0.7Zn-0.2Mn-0.001Fe (wt.%, AZ91) at 700-800 °C. Two ternary carbides have been identified in the reaction zone: T2-Al2MgC2 and κ-AlFe3C. The T2-Al2MgC2 carbide was found in the bulk as settled particles at the bottom of crucible, and within the Al-Mn-Fe reaction layer as thin plates; while the κ-AlFe3C formed as a thin layer between the B2-layer and the steels as a diffusion barrier for Al. α-Mg has been found nucleated on the settled T2-Al2MgC2 carbides.
Similar experiments were performed to investigate the influence of Fe content in the melt on the formation of Al-Mn-Fe IMCs using Fe-0.2C crucibles and holding temperatures in the range 640-850 °C. Four types of Al-Mn-Fe IMCs have been identified in the IMC particles: B2-Al(Fe,Mn) as the core, Al8Mn5 as a shell around the B2-core, Al5Fe2 as a thin layer on the B2-core, and Al11Mn4 as plates growing on the surface of particles. The B2-core gradually became exposed to the α-Mg at higher Fe content. Reproducible crystallographic orientation relationships were found between all four Al-Mn-Fe IMCs.
The settling behaviour of IMC particles and sludge formation were directly observed by synchrotron radiography and quantitatively compared with Stokes’ law. During solidification, interactions between the settling Al8Mn5 particles with entrained oxide films were also observed and discussed.