Phase Evolution, Stability and Magnetic Behavior of Lightweight Al–Fe Aluminide-Based Nanocomposites Processed by Mechanical Alloying, Cryomilling, and Annealing

Abstract

Attempts are made to synthesize Al₅Fe₂ aluminide-based composites by mechanical alloying (MA) and cryomilling (CM). The XRD and TEM results of the milled samples confirm the formation of a major B2-AlFe phase (0.2887 ± 0.0003 nm; cP2) along with the minor amount of Al₅Fe₂ phase. Nanocrystalline grains of ≈16 nm and an average particle size of 4.0 ± 0.36 μm are evident. A significant refinement in the crystallite size (≈10 nm) and average particle size (1.0 ± 0.03 μm) is achieved after 10 h CM of 60 h MAed powder. CM enhances the phase fraction of the Al₅Fe₂ phase. The DSC thermogram discerns three exothermic heating events due to phase transformation. These can be corroborated by the structural transformation of the B2-AlFe phase to the orthorhombic Al₅Fe₂ phase. The phase obtained as a result of 60 h of MA transforms to orthorhombic Al₅Fe₂ along with a minor amount of pre-existing B2-AlFe structure after annealing at 600 °C. It becomes more stable after annealing at 900 °C. Further, the 60 h milled sample displays soft ferromagnetic properties. The saturation magnetization decreases on CM and annealing due to phase transition from B2-AlFe to Al₅Fe₂ phase. Coercivity is reduced when the MA sample is annealed due to an increase in crystallite size and a reduction in lattice strain.