Factors controlling heteroepitaxial phase formation at intermetallic-Al3Sc/liquid interfaces

Abstract

The mechanism of face-centered-cubic (FCC)-Al formation at an L12-Al3Sc/liquid-Al interface was investigated on the basis of interfacial structure and misfit strains, by using ab initio molecular dynamics (AIMD). These simulations were performed using Born–Oppenheimer dynamics, where pressure and temperature was controlled using a Parrinello–Rahman barostat and Langevin thermostat, respectively. Through this approach, we compared the relative stability of (001)Al3Sc/liquid-Al and (111)Al3Sc/liquid-Al interfaces and examined their effect on the heterogeneous nucleation of FCC-Al. Enhanced interfacial bonding along ⟨001⟩Al3Sc stabilized the (001)Al3Sc/liquid-Al, and formed in-liquid ordered layer resembling (002)FCC. Subsequently, the (001)Al3Sc/liquid-Al interface was subjected to stepwise cooling from 1450 to 950 K. The (002)-ordered layer was found to promote layer-by-layer epitaxial growth of FCC-coordinated regions to ∼25% fraction. During cooling, the resulting misfit strains—at (001)Al3Sc/(002)-ordered layer and (001)Al3Sc/(002)FCC−Al interfaces—ranged from −7.4 to 0.5% within 1450–950 K. The magnitude of such misfit strains reduced significantly between 1250 and 950 K, and this trend coincided with a sharp increase in FCC coordination. Thus, AIMD simulations revealed heteroepitaxial formation of FCC-Al on the (001) faces of intermetallic Al3Sc, and that this mechanism is closely associated with a reduction in misfit strains. Our findings motivate the search for new elements that will stabilize potent L12-like structures and produce grain-refinement in Al-based alloys.