Rapidly solidified Al-Fe-Mo-Si alloys for high temperatures applications

Abstract
Iron solubility in aluminium can be extended through the rapid solidification route, allowing the precipitation of a high volume fraction of intermetallic phases. In the ternary Al-Fe-Si system, the observed metastable hardening phase Al₁₃Fe₃Si is sensitive to ripening. Thermal stability can be improved by modification of the surface energy of the precipitate, i.e. by lattice parameter variation, through addition of a transition metal : vanadium in the 8009 alloy and molybdenum in this study. The volume fraction of the silicide phase Al₁₃(Fe,Mo)₃Si can be monitered by the silicon content. With high enough Mo content, an additional hardening is expected from the Al₁₂Mo phase. In addition to a stabilization effect of the silicide phase, molybdenum can also change the kinetic of transformation of the silicide into the coarse, brittle, equilibrium phase Al₁₃Fe₄. The molybdenum content was kept lower than 1 at% because the lower the liquidus temperature, the easier the atomization process. The powders are elaborated by centrifugal atomization and consolidated by hot extrusion. Phase characterization and mechanical properties in the as-extruded condition, or after high temperature exposure are reported in this study. X-rays diffraction shows mainly the presence of the silicide phase. The microstructure is so fine that only the large particles with an irregular morphology can be detected by SEM. Preliminary MET investigations of the alloy Al-2.5Fe-0.5Mo-1.0Si (at%) show that these large particles are composite precipitates. We also noted some small spherical phases probably also the silicide phase. Such differences in precipitate size could be linked to differents solidification regimes. The mechanical properties of the extruded bars are tested at room temperature after 100 hours exposure at different high temperatures.