Abstract. We present an in situ surface x-ray diffraction study of the clean, oxidized and subsequently annealed surfaces of regularly stepped NiAl(6,7,1). Our results show that the UHV stable, clean surface is not faceted and consists of a regular array of (1,1,0) terraces and (0,1,1) steps. The topmost Al and Ni atoms on the terraces exhibit a rippled relaxation while the step atoms are relaxed towards the bulk. Preferential Al oxidation at 540 K and 6×10⁻⁶ mbar O₂ leads to the formation of a 5 Å thin, disordered alumina layer and induces Al vacancies and Ni anti-sites in the Al-depleted interfacial region. The terrace-step structure of the clean surface is maintained, but strong inward relaxations of the interfacial atoms change the strain field around the steps. Massive (1,1,0) faceting with facets up to 50 times larger than the original terraces occurs after high-temperature annealing, during which the surface oxide develops a complex long-range ordering. These results can be understood by the change of interfacial strain, which removes the energy barrier for mass transport. In addition, unlike in the case of low-index (1,1,0) surfaces, we find the step-induced suppression of twin domain formation in the alumina film grown on NiAl(6,7,1). Our results show that the interplay between oxidation and strain can have dramatic effects on the morphology of vicinal surfaces.

GENERAL SCIENTIFIC SUMMARY
Introduction and background. Binary alloys like NiAl are stable structural materials which are used in high-temperature applications. However, a thorough understanding on the atomic scale of their oxidation, which starts at the surface, is missing. Until now, perfect NiAl(1,1,0) surfaces of single crystals are routinely investigated, showing that an ultra-thin twinned alumina film is formed after oxidation. As a first step towards a better understanding of non-ideal surfaces as encountered in polycrystalline materials, we investigate a vicinal NiAl(6,7,1) surface, consisting of many (1,1,0) terraces and (0,1,1) steps. Due to their low coordination number, step atoms are expected to be much more reactive in the oxidation process.

Main results. Our in situ surface x-ray diffraction results show that the clean surface consists of a regular array of (1,1,0) terraces, which is maintained at the amorphous-alumina/alloy interface after oxidation. However, the strain field around the steps, which is responsible for the stability of the surface, changes during oxidation. High temperature annealing of the oxidized sample leads to an ordering of the oxide and massive (1,1,0) faceting. In contrast to perfect (1,1,0) surfaces, twin domain oxide formation is suppressed on the (1,1,0) terraces of NiAl(6,7,1). This can be understood by strain release along surface steps. Our results show that oxide-induced morphology changes can occur on vicinal alloy surfaces and that strain release at surface steps strongly influences the microstructure of surface oxides.

Wider implications. The interplay between strain and selective oxidation can have disastrous consequences for the morphological stability of vicinal alloy surfaces.

Received 10 August 2009
Published 4 November 2009