Study on VPS-cladding of NiAl intermetallics on theTiAl-based intermetallics

Abstract An innovative technology for VPS-cladding of NiAl intermetallics on thesurface of TiAl-based intermetallics is described in this paper. The oxidation behavior ofNiAl-clad TiAl-based intermetallics was studied under isothermal conditions at 1000°C. Themicrostructural stability of NiAl coatings, prepared by means of the VPS process, is consideredin this article. Micro-analysis showed that a ternary inter-diffusion layer was found in theinterface between the NiAl coating and the TiAl-based substrate. The growth rate of the ternaryinterface layer and EPMA analysis reflect that the growth kinetics were diffusion controlled.Weight gain tests indicated that the NiAl cladding greatly improved isothermal oxidationresistance in air at 1000°C; however, the oxidation kinetics did not obey the parabolic rate law.The oxidation protection is attributed to the formation of the three diffusion barriers: firstly, asacrificial barrier of NiO scale; secondly, a dense inert barrier of a-Al 2 O 3 ,lastly, a barrier of the ordered structural matrix of the P-NiAl coating. The mobility of oxygenwould be low at the relatively low homologous temperature with respect to the high meltingpoint of the NiAl matrix, T ⧸ ( T m ) NiAl = 1273⧸1911=0.67 in thiscase.