THE SYNTHESIS AND CHARACTERIZATION OF BULK ALPHA''-FE16N2

Controversy about the magnetic moment of alpha ''-Fe16N2 thin films makes it necessary to examine bulk material. In the original 1951 bulk preparation N-austenite (gamma) prepared by nitriding alpha-Fe at 700-750 degrees C was quenched to give N-martensite (alpha') which then gave alpha '' on long tempering at 120 degrees C the final product contained less than 50% alpha ''. Recent repetition in three different laboratories gave various amounts of alpha '', again less than 50%, with considerable variation in its assessed magnetic moment. X-ray diffraction studies of alpha' tempering, and also transmission electron microscopy of alpha '' precipitation from supersaturated N-ferrite (alpha), showed that slow ordering of N atoms to produce alpha '' occurs only after localized regions of alpha' and alpha reach the Fe8N composition by a clustering process. alpha'-Fe8N can be obtained with a tetragonality c/a equal to that of alpha '', but without the N-ordering characteristic of Fe16N2. The range alpha'-Fe8N-->alpha ''-Fe16N2 offers an explanation for the variability in magnetic properties. The existence of the alpha ''-carbonitride Fe-16(C, N)(2) is noted and possible methods, including alloying, for the production of pure bulk alpha '' are suggested.