ENHANCED FE MOMENT IN NITROGEN MARTENSITE AND FE16N2 (INVITED)

Nitrogen martensite was prepared by treating fine Fe powder with NH3/H -2 gas mixtures at temperatures around 665 degrees C. Upon quenching t o a temperature T-q, the gamma phase which had formed at the elevated temperature undergoes a martensitic transformation to form nitrogen ma rtinsite (alpha' Fe-N alloy), a tetragonal material. Heat treating thi s material for 1-2 h at 140+/-10 degrees C produced the alpha'' phase Fe16N2. The alpha' phase occurred along with gamma Fe-N. From x-ray li ne intensities, the amount of alpha' phase was ascertained. The alpha' phase exhibits a room-temperature moment of 250+/-10 emu/g. Fe16N2 is formed along with alpha Fe and also there is retained N-austenite. Us ing XRD and conventional magnetic measurement procedures, one obtained 280+/-10 emu/g for the saturation moment of Fe16N2. The experimental Fe moment, 2.88 mu(B), is in excellent agreement with the most recent band-structure calculations, 2.85 mu(B). The ternary systems (Fe,M)(16 )N-2 were studied with M=Mn or Ni. gamma Fe-Mn nitride readily forms, but it does not undergo the gamma-->alpha' transformation. The alpha' phase and possibly the alpha'' phase form in the Fe-Ni nitride.