Interfaces Between ${\mathrm{Al}}_2{\mathrm{O}}_3$ and $\beta \mathrm{\text{−}}{\mathrm{Ni}}_{1-x}{\mathrm{Al}}_x$ Alloys: Complex Structures and Ab Initio Thermodynamics

A methodology to study the structural stability of binary alloy/${\mathrm{Al}}_2{\mathrm{O}}_3$ interfaces is developed by expanding the conventional ab initio thermodynamics to include the dependence on alloy composition. Results on $\beta \mathrm{\text{−}}{\mathrm{Ni}}_{1-x}{\mathrm{Al}}_x/{\mathrm{Al}}_2{\mathrm{O}}_3$ interfaces predict the existence of two types of stable interfaces. The stable interface at equilibrium with Al-rich or strictly $1:1$ alloy contains an ${\mathrm{Al}}_2$-terminated ${\mathrm{Al}}_2{\mathrm{O}}_3$ surface and continues with NiAl layers, and the interface at equilibrium with Ni-rich alloy has Al accumulation but continues with a Ni-rich and then NiAl layers. Works of separation for the two interfaces are close to each other.

Figure 1

Top view of interface supercell. Only half of the NiAl overlayer (balls in black) is shown. Large balls in gray represent O layer. Al1 and Al2 indicate the sites for top Al layer that is nearest to or next nearest to the oxygen layer and Al3 for the Al layer just beneath the oxygen layer.

Figure 2

Interfacial energy versus both $\Delta {\mu }_{\mathrm{Al}}$ and oxygen partial pressure $p_{{\mathrm{O}}_2}$. Alloy compositions are indicated by vertical lines. $\Delta {\mu }_{\mathrm{Al}}(x=0.5)$ is calculated from the data of Ref. 12.

Figure 3

Relaxed interfacial structures for ${\mathrm{Al}}_2$ term-1Al (a) and Oterm-8Al-1Ni (b).