Recent developments in the use of quantum mechanics to make structural predictions and provide insights at the atomic level are reviewed with respect to intermetallics. In particular, the question is addressed of why some tetragonal D022 trialuminides (e.g. TiAl3) can be stabilised with the cubic L12 structure by alloying additions but others (e.g. NbAl3) cannot, and why, if the cubic phase is stabilised, it remains brittle.MST/1543

Theoretical predictions of structure and related properties of intermetallics

We describe four criteria for the selection of alloying elements capable of producing castable, precipitation-strengthened Al alloys with high-temperature stability and strength: these alloying elements must (i) be capable of forming a suitable strengthening phase, (ii) show low solid solubility in Al, (iii) low diffusivity in Al, and (iv) retain the ability for the alloy to be conventionally solidified. With regard to criterion (i), we consider those systems forming Al3M trialuminide compounds with a cubic L12 crystal structure, which are chemically and structurally analogous to Ni3Al in the Ni-based superalloys. Eight elements, clustered in the same region of the periodic table, fulfill criterion (i): the first Group 3 transition metal (Sc), the three Group 4 transition metals (Ti, Zr, Hf) and the four latest lanthanide elements (Er, Tm, Yb, Lu). Based on a review of the existing literature, these elements are assessed in terms of criteria (ii) and (iii), which satisfy the need for a dispersion...

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Criteria for developing castable, creep-resistant aluminum-based alloys - A review

Publisher Summary This chapter presents the formalism and applications of cluster expansions to the problem of ab initio calculations of thermodynamic properties of crystalline alloys. The success of the method owes much to the parallel developments of statistical and quantum mechanical methods and, of course, to the availability of powerful computational hardware and software. The technique of orthogonal expansions in cluster functions has revolutionized the treatment of configurationally disordered systems. In addition to offering the most rational and general state-of-order description of alloys, cluster methods provide the rigorous and essential link between quantum and statistical mechanical aspects of first-principles thermodynamic calculations. Cluster functions constitute a complete orthonormal set. This basis set produces a rigorous cluster algebra that is used for systems containing an arbitrary degree of configurational order (or disorder). Cluster methods are, in principle, ideally suited for calculating alloy phase equilibria.

Cluster Approach to Order-Disorder Transformations in Alloys

http://nucapt.northwestern.edu/refbase/files/ActaMat2005.pdf

First-principles calculation of structural energetics of Al-TM (TM = Ti, Zr, Hf) intermetallics

The electronic mechanism behind the brittle fracture of trialuminide alloys is investigated using the full-potential linearized augmented plane-wave (FLAPW) total-energy method within the local density functional approach. To this end, the bulk phase stability, the elastic constants, the anti-phase boundary (APB) energy, the superlattice intrinsic stacking fault (SISF) energy, and the cleavage energy on different crystallographic planes have been determined. A small energy difference (=0.10 eV/unit formula) is found between the DO22 and L12 structures of Al3Ti. In general, the trialuminide alloys have large elastic modulus, small Poisson's ratio, and small shear modulus to bulk modulus ratio. An extremely high APB energy (=670 mJ/m2) on the (111) plane is found for Al3Sc, indicating that the separation between ½(110) partials of a (110)(111) superdislocation is small. Since the total superdislocation has to be nucleated essentially at the same time, a high critical stress factor for dislocation emission at the crack tip is suggested. The high APB energy on the (111) plane is attributed to the directional bonding of Sc(d-electron)-Al(p-electron). The same type of directional bonds is also found for Al3Ti. In addition, moderately high values of SISF energy (=265 mJ/m2) on the (111) plane and APB energy (=450 mJ/m2) on the (100) plane are found for Al3Sc. The brittle fracture of trialuminide alloys is attributed to the higher stacking fault energies and a lower cleavage strength compared to those of a ductile alloy (e.g., Ni3Al). While the (110) surface has the highest surface energy, the cleavage strength (=19 GPa) of Al3Sc is found to be essentially independent of the crystallographic planes. The directional Sc—Al bond becomes even stronger on the (110) surface, which may explain the preferred (110) type cleavage observed by experiment.

Electronic, elastic, and fracture properties of trialuminide alloys: Al3Sc and Al3Ti

The structural energy differences between cubic LI2 and tetragonal DO22 crystal structures are calculated for MAl3 compounds, where M is a group III, IV, or V transition metal. The stability of the DO22 structure relative to L12 increases rapidly as the transition-metal d-electron count increases. Typical values of E(DO22) – E(L12) are 0.1–0.15 eV/atom (9600–14500 J/g-atom) for group III,  0.05 eV/atom ( 4800 J/g-atom) for group IV, and ∼ –0.2 eV/atom (∼ –19000 J/g-atom) for group V trialuminides. Similar trends are calculated for the DO23/L12 energy difference. The calculated electronic densities of states (DOS) show that each structure has a minimum in the DOS distribution at a characteristic d-electron count. The preferred crystal structure for a given compound is the one in which the Fermi level lies in the minimum.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S088429140000532X

P. J. Meschter

Papers

Relative stability of LI2, DO22, and DO23 structures in MAl3 compounds