Electronic origin and interfacial stability of the newly identified Al5Sc2Si phase in Sc micro-alloyed Al–Mg–Si–Cu alloys: A first-principles and experimental study

A previously unreported Al 5 Sc 2 Si phase was identified in a Sc-micro-alloyed Al–Mg–Si–Cu alloy, and its atomic-scale characteristics were elucidated through a synergistic integration of first-principles calculations and high-resolution transmission electron microscopy (HRTEM). Comprehensive investigations encompassing structural optimization, thermodynamic and dynamical stability, electronic structure, and interfacial configuration were performed, complemented by first-principles tensile simulations to assess its intrinsic mechanical response. The Al 5 Sc 2 Si phase crystallizes in a tetragonal lattice, and its negative formation energy, positive elastic constants, and phonon spectra free of imaginary frequencies collectively confirm thermodynamic, mechanical, and dynamical stability. Analyses of the electronic density of states and band structure reveal metallic bonding with pronounced Al–Sc–Si orbital hybridization, which underpins the structural robustness of the phase. Experimental diffraction and STEM–EDS mapping validate the predicted orientation relationship: (004) Al5Sc2Si // (002) Al , demonstrating coherent lattice matching with the Al matrix. Moreover, the moderate elastic modulus and Poisson’s ratio obtained from DFT simulations indicate a balanced combination of strength and ductility, favorable for reducing anisotropy and improving interfacial compatibility. Overall, this study establishes a combined computational-experimental framework for understanding and designing Al–Sc–Si-based alloys, providing thermodynamic and mechanical insights valuable for future phase-diagram modeling and alloy development. These findings highlight the fundamental physics governing coherent interfacial stabilization in light-alloy systems.