Nucleation of coupled body-centered-cubic and closed-packed structures in liquid Ni-Cr alloys

In the last four decades, the structure of pre-critical solid nuclei within a liquid phase of single component face-centered-cubic (FCC) materials has been extensively investigated. These studies demonstrated that formation of equilibrium-FCC is mediated by metastable-body-centered-cubic (BCC) structured pre-critical nucleus. However, it is unknown if such nucleation mechanism is applicable to multi component structural alloys that forms FCC and BCC structures as equilibrium phases. We have investigated this matter using Ni-35at.%Cr and Ni-50at.%Cr binary alloys via molecular-dynamics simulations. Our results indicated that the pre-critical and critical nucleus comprised of coupled Cr-rich BCC and Ni-rich closed-packed (FCC and hexagonal-closed-packed) structures. Formation of such multi-structured pre-critical nucleus was facilitated by prior phase-separation of alloy-melt into Cr-rich and Ni-rich liquid pockets. BCC and closed-packed regions inside the nucleus formed with non-equilibrium compositions that evolved over time. Thus, a non-conventional nucleation mechanism in concentrated Ni-Cr alloys allows them to solidify into FCC-BCC microstructures. (c) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.