The Internal-Nitriding Behavior of Co-Fe-Al Alloys

Abstract

Co-10Fe, Co-20Fe, and Co-40Fe alloys containing3 at.% Al were internally nitrided inNH₃/H₂ mixtures over the range700-1000°C. The kinetics of thickening of theinternal-reaction zone followed the parabolic rate law, suggesting thatsolid-state diffusion was rate controlling. Nitrogenpermeabilities were obtained for each alloy. AlN was theonly nitride to form for all materials and at alltemperatures. At high temperature, the nitride precipitatesformed hexagonal plates near the surface, theprecipitates becoming more blocky near the reactionfront. Precipitate size increased with increasing depthin the alloy and increasing temperature, becauseof competition between nucleation and growth processes.Increasing iron content increased the reacion kineticsdue to increased nitrogen solubility with increasing iron content.