Nitriding of iron-based ternary Fe–V–Si alloy: The precipitation process of separate nitrides

Abstract Iron-based ternary Fe–V–Si and binary Fe–Si alloy specimens were nitrided in a NH 3 /H 2 gas mixture at 580 °C (853 K) with a nitriding potential of r N = 0.1 atm −1/2 until nitrogen saturation in the specimens was achieved. In contrast to previously investigated iron-based ternary Fe–Al–Cr and Fe–Ti–Cr alloys, no so-called “mixed” nitrides developed in the Fe–V–Si alloy. Instead, in the first stage, all vanadium precipitated as crystalline VN and, subsequently, all silicon precipitated as amorphous Si 3 N 4 . Moreover, the precipitation rate of Si 3 N 4 in the nitrided ternary Fe–V–Si alloy was much lower than in the binary Fe–Si alloy nitrided under identical conditions. This much lower Si 3 N 4 -precipitation rate is attributed to the presence of first precipitated VN: coherency strains caused by the (semi-) coherent VN precipitates interact with the strain fields surrounding the Si 3 N 4 precipitates and are obstacles to the movement of dislocations necessarily introduced in the ferrite matrix by growing Si 3 N 4 precipitates. This interpretation is supported by additional experiments, in which the first precipitated VN platelets were coarsened by annealing before subsequent nitriding led to, now much faster, Si 3 N 4 precipitation.