Finite-temperature ductility-brittleness and electronic structures of AlnSc (n = 1, 2 and 3)

Finite-temperature ductility-brittleness and electronic structures of Al3Sc, Al2Sc and AlSc are studied comparatively by first-principles calculations and ab initio molecular dynamics. Results show that Al3Sc and Al2Sc are brittle at both ground state and finite temperatures, while AlSc possesses a significantly superior ductility. At ground state, AlSc is ductile from Pugh’s and Poisson’s criteria, while it is brittle in Pettifor’s model. The ductility of all Al3Sc, Al2Sc and AlSc improves greatly with the elevated temperature. Especially, the Cauchy pressure of AlSc undergoes a transition from negative to positive. At T > 600 K, AlSc is unequivocally classified as ductile from all criteria considered. In all compounds, the Al–Al bond originated from s–p and p–p orbital hybridizations, and the Al–Sc bond dominated by p–d covalent hybridization, are the first and second strongest chemical bonds, respectively. To explain the difference in mechanical properties, the mean bond strength (MBS) is introduced in this work. The weaker Al–Al bond in AlSc, leading to a smaller MBS, could be the origin of the softer elastic stiffness and superior intrinsic ductility. The longer length of the Al–Al bond in AlSc is responsible for its weaker bond strength. Furthermore, the enhanced metallicity of the Al–Al bond in AlSc would also contribute to its exceptional ductility. The longer length of the Al–Al bond in AlSc is responsible for its weaker bond strength. Furthermore, the enhanced metallicity of the Al–Al bond in AlSc would also contribute to its exceptional ductility.