Nanocomposites and an extremely hard nanocrystalline intermetallic of Al-Fe alloys prepared by mechanical alloying

Abstract

Mechanical alloying of Al_100-xFe_x, for x = 2.5, 5, 10, 15 and 20¹, alloys was carried out for 20 h followed by cold consolidation. Mechanical alloying resulted in the formation of supersaturated solid solution of Fe in Al for alloys containing 2.5-10% Fe, while Al-20% Fe alloy formed nanocrystalline metastable Al₅Fe₂. During mechanical alloying, intermetallic phase formed only when the crystallite size dropped below a critical value (20 nm). Final crystal size of the nanocrystalline alloys formed after 20 h of mechanical alloying was related to the Fe content in the alloy. Hall-Petch slope was observed to decrease below a crystallite size of about 15 nm indicating possible transition of hardening to softening of nanocrystalline Al-Fe alloys. Nanocomposite of α-Al and nanocrystalline intermetallics viz. Al₆Fe, Al₅Fe₂, and Al₁₃Fe₄ in the alloy was formed after annealing at temperature above 673 K, which was identified as the peak ageing temperature for the nanocrystalline Al-Fe alloys. The highest ever reported microhardness (12.4 GPa) for any Al-based alloy was measured for nanocrystalline Al-20% Fe alloy after annealing at peak ageing temperature (673 K) for 2 h.