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

Abstract

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.