Structure, thermal stability and magnetic properties of mechanically alloyed (Fe-Al)-30vol%B powders

An elemental powder mixture of Fe-50 at.%Al with addition of 30 vol% of B was subjected to mechanical alloying (MA). Phase transformations occurring in the material throughout MA were investigated by Mossbauer spectroscopy. Examination of thermal behaviour of the MA product by differential thermal analysis revealed four exothermic peaks. Structural and phase transformations induced in the MA powders by heating in a calorimeter up to 530, 630, 730 and 1000 °C were investigated by X-ray diffraction and Mossbauer spectroscopy. The powders containing 30% of B in the as-milled state and after heating up to 1000 °C were examined by scanning electron microscopy. Magnetic properties of the MA product before and after heating up to 1000 °C were investigated. It was found that MA of the (Fe-Al)-30 vol%B powders resulted in the formation of a composite structure with boron particles embedded in the predominantly amorphous Fe-Al-B matrix. Additional Mossbauer measurements performed on MA products with a smaller boron content of 5, 10 and 20 vol% evidenced the crucial role of boron addition in promoting the formation of Fe-Al-B amorphous phase by MA process. The structure of the (Fe-Al)-30 vol%B powders after their controlled heating comprised of fine B particles distributed in the two-phase matrix of nanocrystalline (FeAl or AlFe2B2-type) and amorphous phases or in the nanocrystalline matrix (AlFe2B2 + Al13Fe4), depending on the temperature up to which the material was heated. To the best of our knowledge, the nanocrystalline AlFe2B2 phase was obtained for the first time by mechanical alloying followed by heat treatment.