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

Iron-based ternary Fe-V-Si and binary Fe-Si alloy specimens were nitrided in a NH3/H2 gas mixture at 580 °C (853 K) with a nitriding potential of rN = 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 Si3N4. Moreover, the precipitation rate of Si3N4 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 Si3N4-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 Si3N4 precipitates and are obstacles to the movement of dislocations necessarily introduced in the ferrite matrix by growing Si3N4 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, Si3N4 precipitation.