Effects of gas nitriding pressure on the formation of nanocrystalline AlN in plasma nitrided Fe-9Al-28Mn-1.8C alloy

The effects of gas nitriding pressure on the formation of nanocrystalline nitrided layer and its effect on the performance of the Fe-9Al-28Mn-1.8C alloy (in wt.%) were investigated. Plasma nitriding was conducted at 450 °C for 12 h under nitriding pressures ranging from 133 to 798 Pa. The results evidently demonstrated that, due to the unique as-quenched microstructure of the present alloy, the effect of nitriding and aging could be achieved simultaneously with one-step plasma nitriding scheme. Both the thickness of nitrided layer and the nitrogen concentration at the outmost surface were found to increase with increasing gas nitriding pressure in the range of 133-798 Pa. Detailed microstructural analyses indicated that the nitrided layer is composed predominantly of nanocrystalline face-centered-cubic (FCC) B1-AlN and FCC γ′-Fe4N with minor amount of expanded austenite phase, which accounts for the excellent surface microhardness and corrosion resistance in 3.5% NaCl solution. The increase in gas nitriding pressure led to a thicker nitrided layer and smaller AlN particles, which in turn, resulted in higher surface microhardness and better corrosion resistance.