The electronic mechanism behind the brittle fracture of trialuminide alloys was investigated using the full-potential linearized augmented plane-wave total-energy method within the local density functional approach. The bulk phase stability, elastic constants, antiphase boundary energy, superlattice intrinsic stacking fault energy, and cleavage energy on various crystallographic planes were determined. In general, trialuminide alloys had a large elastic modulus, small Poisson ratio and small shear modulus to bulk modulus ratio. An extremely high antiphase boundary energy (670mJ/m2) on the (111) plane was found for Al3Sc; indicating that the separation between ½<110> partials of a <111>(111) super-dislocation was small. Since the total super-dislocation had to be nucleated essentially at the same time, a high critical stress factor for dislocation emission at the crack tip was suggested. The high antiphase boundary energy on the (111) plane was attributed to the directional bonding: Sc(d-electron)-Al(p-electron). Moderately high values of superlattice intrinsic stacking fault energy (265mJ/m2) on the (111) plane and an antiphase boundary energy (450mJ/m2) on the (100) plane were found for Al3Sc. The brittle fracture of trialuminide alloys was attributed to higher stacking-fault energies and to a lower cleavage strength compared to those of a ductile alloy (such as Ni3Al). While the (110) surface had the highest surface energy, the cleavage strength (19GPa) of Al3Sc was independent of the crystallographic planes. The directional Sc-Al bond became even stronger on the (110) surface, which was suggested to explain the preferred (110)-type cleavage observed in experiments.

Electronic, Elastic, and Fracture Properties of Trialuminide Alloys. Al3Sc and Al3Ti. C.L.Fu: Journal of Materials Research, 1990, 5[5], 971-9