Electronic, elastic, and fracture properties of trialuminide alloys: Al 3 Sc and Al 3 Ti
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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 Ll 2 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 V^llO) 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(/?-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.
I. INTRODUCTION
Transition metal trialuminide base intermetallic alloys have received increasing attention recently due to their low densities, oxidation resistance, and high melting points. These attractive characteristics make trialuminide alloys (e.g., Al3Ti) potential candidates for high temperature structural materials. However, the lack of ductility has limited the application of these alloys.1'2 Most trialuminide alloys are stabilized in the DO22 structure. The low ductility is often thought to be associated with the lack of equivalent slip systems to satisfy the von Mises criterion for slip deformation in polycrystals with the DO22 structure. Consequently, there is considerable effort attempting to increase the symmetry of trialuminide alloys to the related Ll 2 structure as a way to provide enough equivalent slips to ensure strain compatibility. However, the resultant Ll 2 structurebase alloys still remain brittle under the influence of a tensile stress and fail by transgranular cleavage.3"7 This paper represents our theoretical effort to understand the electronic mechanism behind brittle fracture of the Ll 2 structure-base trialuminide alloys from firstprinciples calcu
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