Nanostructured iron-nitrogen martensite synthesized from Fe79Mo10B10Cu1 magnetic nanocomposite alloy

Abstract

A Fe79Mo10B10Cu1 nanocomposite alloy was prepared by rapid solidification processing and subsequent isothermal annealing of ribbons with 6 nm, α-Fe grains embedded in an amorphous matrix. The nanocrystalline alloy was then nitrided in a gas mixture of ammonia and hydrogen at 650 and 700 °C for varied time between 300 and 7200 s followed by quenching. A majority (~55 vol%) of nitrogen austenite γ-(Fe,N) with a grain size of ~20 nm was produced when nitriding conditions were at a temperature of 700 °C in 12% NH3–88% H2 for 3600 s. The nitrided alloy has a magnetization of 50 ± 20 A·m2/kg at an applied field of 1.19 MA/m. Retained α-Fe, the only ferromagnetic phase in the sample, is responsible for the magnetization. And paramagnetic phases of γ-(Fe,N) and η-Fe3Mo3N account for ~80% of the material by volume. After the high-temperature nitridation, the alloy was soaked in liquid nitrogen and then tempered at 117 °C. Nanostructured iron-nitrogen martensite (α′) was transformed from the nitrogen austenite phase in the nitrided Fe79Mo10B10Cu1 alloy, and was characterized to have an amount of ~20 vol% and a grain size of ~12 nm. This iron-nitrogen martensite nanocomposite has a magnetization of 65 ± 8 A m2/kg at an applied field of 1.19 MA/m and coercivity of 14 ± 6 kA/m.