Abstract
First-principles evolutionary algorithms are employed to shed light on the phase stability of Al–Nb intermetallics. While the tetragonal Al3Nb and AlNb2 structures are correctly identified as stable, the experimentally reported Laves phase of AlNb3 yields soft phonon modes implying its dynamical instability at 0 K. The soft phonon modes do not disappear even upon elevating the temperature in the simulation up to 1500 K. X-Ray diffraction patterns recorded for our powder-metallurgically produced arc cathodes, however, clearly show that the AlNb3 phase exists. We propose that AlNb3 is dynamically stabilised by ordered antisite defects at the Al sublattice, leading also to a shift of the Nb content from 75 to ∼81 at.%. Unlike the defect-free AlNb3, the antisite-stabilised variant hence falls into the compositional range consistent with our CALPHAD-based phase diagram as well as with the previous reports.
Highlights
Intermetallics containing Al—the most abundant metal in the Earth’s solid crust—and transition metals (TMs) exhibit resistance to oxidation and corrosion, considerable hardness, low density, and high melting points [1,2]. Owing to their low diffusivity in Al, TMs perfectly meet the requirements for long-term strengthening of aluminides at elevated temperatures, which is an important prerequisite for applications in high-performance automobiles, railway cars, airplanes, spacecrafts or light ships
The stability range of δ-AlNb3 turns out to be peculiar in the same way as for the previous CALPHAD
Various Al1− x Nbx structural candidates were generated by the evolutionary algorithm USPEX and subsequenty relaxed by Density Functional Theory (DFT)
Summary
Intermetallics containing Al—the most abundant metal in the Earth’s solid crust—and transition metals (TMs) exhibit resistance to oxidation and corrosion, considerable hardness, low density, and high melting points [1,2]. Owing to their low diffusivity in Al, TMs perfectly meet the requirements for long-term strengthening of aluminides at elevated temperatures, which is an important prerequisite for applications in high-performance automobiles, railway cars, airplanes, spacecrafts or light ships. In terms of thin films, physical vapour deposition (PVD) techniques, such as magnetron sputter deposition or cathodic arc deposition (CAD), allow for synthesis of materials from the vapour phase [4]. In the case of CAD the formation of intermetallic phases on the cathode surface due to the exposure to the plasma affects the plasma properties, such as ion charge states and energies [5,6,7]
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