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

Mechanical alloying of Al100−xFex, 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.