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.