B. Alling, A. Karimi, L. Hultman and I. A. Abrikosov
Applied Physics Letters 92 071903 (2008)
TiAlN is a widely used hard coatings material system. The NaCl-structure solid solution Ti1-xAlxN1-y have superior properties compared to pure TiN. One of the reasons is that the material hardens during operation as the solution decomposes into coherent c-AlN and Ti rich Ti1-xAlxN. However, until now it has been an open question how nitrogen sub-stoichiometry, a phenomena common in transition metal nitrides, affects this decomposition. We show by means of extensive first-principles calculations that a small amount of nitrogen vacancies in Ti1-xAlxN enhances decomposition as Ti clusters around the vacancies. In the Ti-rich, nitrogen poor regime on the other hand nitrogen sticks to Ti while Al tends to form nitrogen free fcc-Al or TiAl alloys or compounds.
A simultaneous consideration of the two compositional degrees of freedom: Ti to Al ratio and the stoichiometry of the N sublattice, gives rise to a complex curvature of the formation energy surface.
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