MECHANISMS OF PHASE-FORMATION DURING POSTDISCHARGE NITRIDING

The evolution of the microstructure during post-discharge nitriding of pure iron at 773 degrees K in a diluted N-2 atmosphere has been studi ed with an emphasis on the initial stages. Phase formation starts afte r 30 s with the precipitation of gamma'-Fe4N needle-like nitrides on t he surface. Coverage of the surface then quickly proceeds by continuou s needle nucleation and needle branching. Simultaneously with surface coverage, nitride wedges grow into the matrix and eventually form a co ntinuous layer mainly by lateral spreading. The longest nitriding time results II the formation of a compact nitride bilayer. Mechanisms for phase nucleation in this process are discussed and compared to those proposed for ion and conventional gas nitriding. In general, kinetics of phase formation are similar to ion plasma nitriding, while the morp hology and crystallographic precipitation characteristics tend to foll ow the trends observed in conventional gaseous nitriding.