SPONTANEOUS MAGNETIZATION OF ALPHA''-FE16N2 PRECIPITATES IN BULK FE-N

Precipitates of alpha''-Fe16N2 amounting to a fraction of 10 to 44 wt% in alpha-Fe (25 to 90 wt%) in the presence of residual gamma-Fe (0 to 30 wt%) are obtained by nitriding pure iron with NH3-H2 gas at temper atures in the austenite phase field, martensitic transformation to alp ha' by quenching into water and liquid nitrogen, followed by the trans formation alpha' --> alpha + alpha'' upon tempering at 130 to 150-degr ees-C. During that transformation, in situ measurements show a signifi cant though small increase of the magnetization of the material. The r oom temperature values of the specific magnetization and spontaneous p olarization of alpha''-Fe16N2, sigma(s) = (250 +/- 20) Am2/kg and J(s) = (2.3 +/- 0.2) T, are extracted from the mean specific magnetization of the multiphase material. The alpha'' precipitates have typical dim ensions of about 1 to 5 nm.