Mechanism of formation of Fe-N alloy in the solid-state reaction process between iron and boron nitride

Processes and resulting products of solid-state reaction between Fe and hexagonal boron nitride (h-BN) were studied by ball milling a mixture of Fe and h-BN with an Fe to h-BN volume ratio of 1:12.5 and/or heat-treating under normal or high pressure. The 30 h-milled mixture was annealed for 1 h under an atmospheric pressure at 770 and 1170 K, respectively. A small amount of gamma'-Fe4N was observed in the mixture annealed at 770 K and a little gamma-Fe(N) was obtained at 1170 K. All Fe atoms in the mixture reacted with N atoms in the BN to form a single phase of epsilon-FexN when the mixture was milled for 60 h or annealed for 1 h under 4 GPa at temperatures of 690-800 K. However, no Fe-B phase was observed in the present experiment. Formation and phase transition of Fe-N alloy in the solid-state reaction process as well as effect of pressure on the reaction and resulting products are discussed in the present paper. The mechanism of formation of Fe-N and Fe-B alloys in the reaction processes are also investigated in terms of the Miedema model and thermodynamics and kinetics. (C) 2004 Elsevier B.V. All rights reserved.