Effect of Processing Route on the Microstructure and Properties of an Fe-al Alloy with Additions of Precious Metal

Iron-aluminium alloys with a nominal composition of Fe-40at.%Al with additions of small amounts of precious metal were melted in a button arc furnace. These alloys were tested for their corrosion and oxidation resistance. Four of the alloys were selected for further study as they provided improved oxidation and corrosion resistance compared to plain Fe-40at.%Al. Due to the problem of the alloys having large grain size on arc melting, it was decided to produce them by non-equilibrium methods in order to decrease the grain size. Thus, the alloys were then produced by a process of mechanical alloying.Commercial purity iron and aluminium powders were milled for several hours in a planetary ball millwith the precious metal powders in order to produce the alloys required. These powders were tested for apparent density using an Arnold density meter and the density was 2.097 g/cm3. SEM analysis was done to determine the morphology of the powders and XRD was carried out to determine the phases. The powders were found to have irregular shapes, and were initially composed of three phases (FeAl, Fe3Al, Fe0.6Al0.4). Annealing helped form the required FeAl phase (Fe0.6Al0.4), although a mixed iron-aluminium carbide phase (Fe3AlC0.5) was also formed. After sintering of the plain FeAl alloy, it was found that the hardness of the material increased, mainly due to the presence of the carbide phase and a significantly smaller grain size in the material.