Abstract
A needle-like structure of AlN is observed in Fe-31 Mn-9Al-0.87C-xCr (x=0, 3 and 6) alloys at 1000° C in a nitrogen atmosphere. The reaction front of nitriding moves parabolically with time. The nitriding rate is evaluated on the basis of the penetration depth of the nitriding layer at various lengths of time, and is found to be increased with increasing chromium content in the alloy. A quasi-steady state diffusion model is employed to investigate the nitriding kinetics by nitrogen migration through the alloy matrix. The nitriding rate depends on the solubility of nitrogen as well as the diffusivity of nitrogen in the alloy system. It is argued that the solubility of nitrogen predominates the nitriding, and thus the growth of AlN prevails in the austenitic phase instead of ferrite, due to the higher nitrogen solubility in austenite. In addition, the chromium alloying into the Fe-Al-Mn system increases the lattice parameter, which leads to a higher solubility of nitrogen, and hence a higher nitriding rate is obtained. © 1990 Chapman and Hall Ltd.