THE EFFECT OF DIFFUSION BARRIER FORMATION ON THE KINETICS OF ALUMINIZING IN INCONEL-718

Aluminizing of nickel alloy 718 was studied in order to reveal the eff ect of combined alloy additions on aluminide growth kinetics, as oppos ed to pure metal substrates. The low activity pack process was used an d treatment was carried out at 1273 K using ammonium fluoride activato r and Ni-50 wt% Al powder as an aluminium source for treatment times o f 1, 2, 4 and 8 h. The aluminide coatings varied between 40 and 110 mu m in thickness. The microstructures consisted of a NiAl phase with a fine grain size containing small secondary-phase particles at the grai n boundaries. The interface between the coating and the substrate was lined by a lamellar layer exhibiting a two-phase structure, which was enriched in chromium and niobium in addition to containing iron and ni ckel. Weight gain measurements indicated parabolic growth up to 2 h, b eyond which the growth rate slowed down. Microstructures and compositi on profiles revealed that the interlayer, which was enriched in elemen ts insoluble in NiAl, posed a barrier to interdiffusion of the reactin g species and slowed down the growth kinetics of the aluminide.