A model austenitic Fe-Ni-Cr alloy was subjected at 1000 deg C to either nitridation at P_(N_2) = 0.9 atm, carburisation in a gas of unit carbon activity or to a series of exposures where the alloy was simultaneously carburised and nitrided or nitrided and subsequently carburised. Nitridation produced only lamellae of Cr_2N, whereas carburisation produced two carbide zones, an inner layer of M_(23)C_6 and an outer layer of M_7C_3. Carbonitridation produced a structure indistinguishable from that formed during attack by carbon only, suggesting that nitrogen plays little role in precipitation reactions under these circumstances. However, carburisation following nitridation led to the formation of zones of lamellar carbides, with the same crystal structures as those formed during carburisation, but with lamellar morphologies similar to those of the pre-formed nitrides. No nitrides were observed following this sequential attack. It is speculated that the carbides nucleate on the preexisting nitrides and displace the nitrogen into the alloy matrix. The highly oriented nitrides/carbides also act as rapid paths for oxidant diffusion into the metal and this increases the volume fraction of carbide that is formed, compared with carburisation.
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