Modification mechanisms of the primary Al13Fe4 phase in hypereutectic Al–Fe alloys by La element: Experiments and first–principles study

Abstract

The primary Al₁₃Fe₄ phase prefers to grow along the [010] orientation and form flakes, coarse needles, and lath shapes, which significantly impairs the mechanical properties of hypereutectic Al–Fe alloys. In this work, 1 wt% La addition is shown to greatly improve the morphology of the primary Al₁₃Fe₄ phase and have the most efficient refinement effect on Al–5Fe alloy. According to the data, La atoms mainly formed Al₁₁La₃ phase and were not found in the primary Al₁₃Fe₄ phase. The DSC thermal analysis revealed that La addition increased the crystallization temperature range and decreased the crystallization enthalpy of the primary Al₁₃Fe₄ phase. The results of the first-principles calculation indicated that La preferentially occupied Al sites when entering the primary Al₁₃Fe₄ phase, thus leading to an increase in the formation enthalpy of Al₁₃Fe₄. Meanwhile, the surface energy decreased and the stability of the (010) surface was enhanced when La atoms replaced the specific Al atoms sites in the Al₁₃Fe₄ (010) surface. Moreover, La atoms were rather adsorbed on the Al₁₃Fe₄ (010) surface because of the exothermic nature of adsorption and its negative energy. Therefore, La atoms change the formation enthalpy and surface energy of the Al₁₃Fe₄ phase by doping and adsorption, and finally affect the crystal structure, crystallization process and morphology of the primary Al₁₃Fe₄ phase. The experimental results and theoretical calculations confirm that adsorption and constitutional undercooling are more likely to be the main modification mechanisms of the primary Al₁₃Fe₄ phase via La doping.