Enhanced Fe moment in nitrogen martensite and Fe16N2 (invited)

Abstract

Nitrogen martensite was prepared by treating fine Fe powder with NH3/H2 gas mixtures at temperatures around 665 °C. Upon quenching to a temperature Tq, the γ phase which had formed at the elevated temperature undergoes a martensitic transformation to form nitrogen martinsite (α′ Fe-N alloy), a tetragonal material. Heat treating this material for 1–2 h at 140±10 °C produced the α″ phase Fe16N2. The α′ phase occurred along with γ Fe-N. From x-ray line intensities, the amount of α′ phase was ascertained. The α′ phase exhibits a room-temperature moment of 250±10 emu/g. Fe16N2 is formed along with α Fe and also there is retained N-austenite. Using XRD and conventional magnetic measurement procedures, one obtained 280±10 emu/g for the saturation moment of Fe16N2. The experimental Fe moment, 2.88 μB, is in excellent agreement with the most recent band-structure calculations, 2.85 μB. The ternary systems (Fe,M)16N2 were studied with M=Mn or Ni. γ Fe-Mn nitride readily forms, but it does not undergo the γ→α′ transformation. The α′ phase and possibly the α″ phase form in the Fe-Ni nitride.