Coherency strain and precipitation kinetics: crystalline and amorphous nitride formation in ternary Fe–Ti/Cr/V–Si alloys

Specimens of iron-based binary Fe-Si alloy and ternary Fe-Me-Si alloys (with Me=Ti, Cr and V) were nitrided at 580 degrees C in a NH3/H-2-gas mixture applying a nitriding potential of 0.1atm(-1/2) until nitrogen saturation in the specimens was attained. In contrast with recent observations in other Fe-Me-1-Me-2 alloys, no mixed (Me-1, Me-2) nitrides developed in Fe-Me-Si alloys upon nitriding: first, all Me precipitates as MeN; and thereafter, all Si precipitates as Si3N4. The MeN precipitates as crystalline, finely dispersed, nanosized platelets, obeying a Baker-Nutting orientation relationship (OR) with respect to the ferrite matrix. The Si3N4 precipitates as cubically, amorphous particles; the incoherent (part of the) MeN/-Fe interface acts as heterogeneous nucleation site for Si3N4. The Si3N4-precipitation rate was found to be strongly dependent on the degree of coherency of the first precipitating MeN. The different, even opposite, kinetic effects observed for the various Fe-Me-Si alloys could be ascribed to the different time dependences of the coherentincoherent transitions of the MeN particles in the different Fe-Me-Si alloys.