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The microstructure and cyclic oxidation behaviour of Hf and Zr co-doped B2 NiAl single-crystal alloys at 1473 K were investigated. Hf and Zr co-precipitated into one phase during alloy manufacturing. The RE-free single-crystal alloy obtained a double-layered alumina film comprised of outer equiaxed grains and inner columnar grains and exhibited lower oxidation rate than the corresponding poly-crystal alloy, while the co-doped single-crystal alloy displayed higher oxidation rate than the RE-free single-crystal alloy and no synergistic effect was discovered. The effect of Hf and Zr co-doping on the growth behaviour of the alumina film was discussed. It is proposed that the existence of grain boundaries is an important precondition for activating the reactive element effect. The absence of grain boundaries significantly weakened the positive effect of Hf and Zr co-doping and distinctly magnified its negative effect on the cyclic oxidation resistance of the alloy in the meantime. Cyclic oxidation behaviour of Hf and Zr co-doped NiAl single-crystals is compared.The undoped alloy obtains a double-layered alumina film during oxidation.The co-doped alloy shows higher alumina film growth rate than the undoped alloy.The absence of alloy grain boundaries suppresses the reactive element effect. Cyclic oxidation behaviour of Hf and Zr co-doped NiAl single-crystals is compared. The undoped alloy obtains a double-layered alumina film during oxidation. The co-doped alloy shows higher alumina film growth rate than the undoped alloy. The absence of alloy grain boundaries suppresses the reactive element effect.

Negative effect of Hf and Zr co-doping on alumina film growth of nickel-aluminium single-crystals during thermal cyclic oxidation