Nickel–aluminide coating of TiAl by a two-stage process

Nickel–aluminide protective coatings on the surface of TiAl alloys were produced by pack aluminization of preliminarily nickel-coated samples. The deposition of the nickel layer from a solution was carried out initially by an electroless, and subsequently by an electrolytic process, providing good adhesion of Ni to the TiAl alloy surface. Two bed regimes were used: a high-activity one (with pure aluminum as the coating source) and a low-activity one (with Fe2Al5 as the source). The effect of the bed composition and of the thickness of the preliminary Ni layer was examined. The layers were examined by optical microscopy, SEM with EDS and X-ray analysis. The coatings obtained with the high-activity bed contained Al-rich phases of low symmetry, making for high brittleness and poor adhesion of the surface layers; those obtained with the low-activity bed, with NiAl as the outermost layer, are oxidation resistant at high temperatures and have a ductile structure with good adhesive properties.