Solid-State Reaction in Al-Fe Binary System Induced by Mechanical Alloying

Abstract

Mechanical alloying (MA) is a solid-state powder processing method which has the ability to synthesize a variety of new alloy phases including supersaturated solid solutions, nanocrystalline structures, amorphous phases and intermetallic compounds. In this investigation, the interaction between aluminum and iron caused by MA of Fe-xAl (where X ranged from 30 to 90%) was studied as a function of milling time and post heat-treating temperatures. The sequences of structural and/or phase transformation and the behavior of mechanically alloyed powders have been assessed using XRD, hardness and magnetometer. It was found that during mechanical milling of elemental powder Al and Fe, five milling stages were categorized, namely, particle flattening, welding predominance, equiaxed formation, random welding orientation and steady state composite particles. All milled powders showed nano-sized powder mixtures after milling for 20hrs. When Fe-30%Al powder was milled for 150hrs, a partially ordered AlFe phase was obtained. However, when these saturated solid solutions were heat treated at 500 °C, the AlFe intermetallic was precipitated fully ordered. When the Al content was increased up to 40% and milled for 50hr, the XRD pattern showed a broad halo spectrum which showed the formation of an amorphous phase. When a Fe-60%Al powder mixture was mechanically milled for 50hr, the Al5Fe2 intermetallic formed that was associated with an amorphous phase, which transformed into the Al3Fe intermetallic by heat treating at 500°C. In the case of Fe-75% and Fe-90%Al milled for 150hrs only Al peaks appeared and were shifted to higher angles, suggesting that Fe atoms diffused into Al, leading to the formation of a solid solution.