The internal-nitriding behavior of Co-Fe-Al alloys

Co-10Fe Co-20Fe, and Co-40Fe alloys containing 3 at. % Al were internally nitrided in NH3/H-2 mixtures over the range 700-1000 degrees C. The kinetics of thickening of the internal-reaction zone followed the parabolic rate law, suggesting that solid-state diffusion was rate controlling. Nitrogen permeabilities were obtained for each alloy. AlN was the only nitride to form for all materials and at all temperatures. At high temperature, the nitride precipitates formed hexagonal plates near the surface, the precipitates becoming more blocky near the reaction front. Precipitate size increased with increasing depth in the alloy and increasing temperature, because of competition between nucleation and growth processes. Increasing iron content increased the reacion kinetics due to increased nitrogen solubility with increasing iron content.