Magnetic response and hardness enhancement by spontaneous directional coarsening of Fe2AlB2 in quasicrystal-rich matrix

This study investigates the Al₅₅Cu₂₀Fe₁₅B₁₀ alloy system to understand the characteristics of the Fe₂AlB₂ (Cmmm) phase and its interactions with a quasicrystal-rich matrix. The alloy’s composition was chosen for its boron content, which promotes the formation of well-defined Fe₂AlB₂ crystals. We examined how different heat treatments affect the alloy’s microstructure, magnetic properties and hardness.Microstructural and Electron Backscatter Diffraction analyses revealed the stable icosahedral Al₅₉Cu₂₇Fe₁₂B₂ quasicrystalline phase and a metastable Al₇₀Fe₂₀Cu₁₀ approximant that is isostructural with C2/m Fe₄Al₁₃. Additionally, the alloy contained Al-Cu phases known from meteorites, such as AlCu stolperite and Al₂Cu khatyrkite.Heat treatments at 706^∘C and 828^∘C yielded favorable microstructure for ballistic armor, characterized by Fe₂AlB₂ lamellae embedded within the quasicrystalline matrix, reinforced by AlB₂ precipitation on the grain boundaries. Vickers hardness improvements were attributed to Hall-Petch strengthening and grain orientation effects. Notably, annealing at 943^∘C nearly tripled the magnetic entropy change. Quantum Diamond Microscopy confirmed that Fe₂AlB₂ significantly contributed to magnetization. The improvement of magnetic properties was found to be due to preferential orientation of Fe₂AlB₂ grains along the [010] zone axis, as a consequence of directional coarsening induced by annealing. The enhanced magnetocaloric properties observed at 943^∘C are primarily due to intrinsic changes in the Fe₂AlB₂ phase rather than strain relaxation or phase contributions from the quasicrystalline matrix.