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

Finite-temperature ductility-brittleness and electronic structures of Al_(3)Sc,Al_(2)Sc and AlSc are studied comparatively by first-principles calculations and ab initio molecular dynamics.Results show that Al_(3)Sc and Al_(2)Sc 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 Al_(3)Sc,Al_(2)Sc and AISc 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 AISc 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.