Common mechanism for controlling polymorph selection during crystallization in supercooled metallic liquids

Despite the fundamental and technological importance, the physical scenario of polymorph selections during crystallization in metals remains poorly understood. Here, through the extensive molecular dynamics simulation, we study the crystallization pathway of face-centered cubic (fcc) metals aluminum and copper. We reveal two different crystallization pathways at ambient pressure: involving the metastable body-centered cubic (bcc) polymorph (Cu) or not involving the bcc polymorph (Al). Interestingly, by varying pressure on AI, we successfully control polymorph selection and observe crystallites containing the metastable bcc polymorph. These findings are in line with the phonon dispersion analyses, which indicate that the bcc structure of Cu at ambient pressure and of Al at high pressure remain stable whereas that of Al at low pressure is thermally unstable. More importantly, we demonstrate that it is the profile of the free energy surface related to the phase stability, rather than the strong cohesive interactions of metals, encodes the crystallization pathway in metals. Our findings not only shed important light on a common thermodynamic mechanism of polymorph selections, but also pave a new way for better controlling the crystallization pathway in industrial and metallurgical applications by regulating the stability of the intermediate metastable phases. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.