| Ni-Al coatings have received extensive applications on Ni-based alloys due to their excellent oxidation resistance. But there are few reports on the Ni-Al coatings applied on Ti-Al based intermetallic alloys. Two alloys with classic compositions (i.e., TiAl:Ti-50Al and Ti3Al:Ti-24A1, at.%) and another two alloys with potential applications in engineering (i.e., Ti-46.6Al-1.4Mn-2Mo-0.3C and Ti-24Al-14Nb-3V-0.5Mo, at.%) were selected as target materials. Ni-Al coatings process was developed by electroplating a Ni film followed by Al-pack cementation. The four Ti-Al based alloys oxidized at900℃for120h, while the four Ni-Al coated alloys oxidized at the same temperature for300h. The cross-sectional microstructures were observed by Scanning Electron Microscope (SEM) equipped with Energy Dispersive X-ray Spectroscopy (EDS). Main conclusion has been drawn as following:Two kinds of Ni-Al coatings on the Ti-50Al alloy were prepared by electroplating Ni films with thicknesses of～30and～20μm, respectively, followed by Al-pack cementation at800℃for1h, and the cross-sectional microstructures show a Ni-rich layer and large numbers of cracks, because of the thick Ni film or the short aluminizing time, which are detrimental for the protective ability of the coatings. In contrast, no Ni-rich layer was observed in the cross-sectional microstructures of those three kinds of Ni-Al coatings on the Ti-50Al alloy prepared by electroplating a Ni film with thickness of about10μm followed by Al-pack cementation at800℃for1,3and5h, respectively, but a Al-rich layer lies between outermost N2Al3layer and the substrate, even the content of Al element is higher than that in the Ni2Al3layer and substrate, which is so-called uphill diffusion phenomenon. Among the three Ni-Al coatings, the one prepared by electroplating a Ni film with thickness of about10μm followed by Al-pack cementation for at800℃for3h possesses a much thicker Al-rich layer, which consists of a TiAl3layer and a TiAl2layer. Many results confirmed that a thick Al-rich layer is beneficial for holding the Al content of the outer Ni-Al layers and the oxidation resistance of the coatings is thus improved. Therefore, the optimal processing is obtained. That is a electroplated Ni film with thickness of about10μm followed by Al-pack cementation at800℃for3h.Ni-Al coatings were made by the above optimal processing on the four Ti-Al based alloys. After exposure at900℃for different time, the oxidation kinetics curves of all the four based alloys follow a linear rate law, while the oxidation kinetics curve of the Ti-24Al-14Nb-3V-0.5Mo (at.%) with Ni-Al coatings follows a parabolic rate law for the first150h and then follows a liner rate law for the next150h, but the oxidation kinetics curves of the other three Ti-Al based alloys with Ni-Al coatings follow a parabolic rate law or a nearly parabolic rate law. For the four coated alloys, although they were exposed for a longer time than the four based alloys, their mass gain per unit and thickness of the oxide scale are apparently smaller. As a result, the Ni-Al coatings can effectively improve the oxidation resistance of the Ti-Al based intermetallic alloys.The oxidation resistance of the four Ti-Al based alloys with Ni-Al coatings depends on the microstructures of the coatings, especially for the Ti-24Al-14Nb-3V-0.5Mo (at.%) alloy with Ni-Al coatings, whose mass gain per unit and thickness of the oxide scale are obviously larger than those of others. Unlike the other three samples, there is no protective β-NiAl layer in the cross-sectional microstructure of Ti-24Al-14Nb-3V-0.5Mo (at.%) coated alloy after oxidizing for300h. Furthermore, after oxidizing at900℃for300h, the cross-sectional microstructure of the Ti-50Al coated alloy is composed of outermost β-NiAl layer, inner T2-Al2NiTi layer and innermost TiAl2layer containing T3-Al3NiNi2phase; As for the Ti-46.6Al-1.4Mn-2Mo-0.3C coated alloy, it consists of outer P-NiAl layer and inner T3-Al3NiNi2layer; On the surface of the Ti-24AI coated alloy, it comprises of outermost β-NiAl layer, T2-Al2NiTi and NiAl phases mixed outer layer, inner TiAl2layer with τ2phase, and innermost τ1-Al13Ni2Ti5phase layer. The high content of Al element of the inner layers is beneficial for holding the Al content of the outermost P-NiAl layer and the oxidation resistance of the coatings is improved. Despite the fact that there is a higher Al content in the inner layers of those three samples, less mass gain per unit and smaller thickness of oxide scale are found in the Ti-24A1(at.%) alloy with Ni-Al coatings rather than the other samples. Accordingly, the compositions and contents of alloying elements have a great influence on the cross-sectional microstructures of the coatings, and further optimized technologies are needed for the preparation of Ni-Al coatings on various Ti-Al based alloys to improve oxidation resistance. |
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