Evolution of Short-to-Medium Range Orders During the Liquid-Liquid Phase Transition of a Pd-Si Metallic Glass

Liquid-liquid phase transition (LLPT) is a universal phenomenon that occurs in different types of liquids. Understanding its mechanism can help solve the long-standing mystery of liquid and amorphous structures. In metallic liquids, LLPT has been widely reported to be observed in the supercooled liquid region in multicomponent alloy systems. However, few observations of LLPT were reported in binary metallic systems, mainly due to the poor thermal stability of these systems in the supercooled liquid region. Herein, the phase transition of Pd82Si18 metallic glass in its supercooled liquid region was studied by in situ synchrotron X-ray scattering and transmission electron microscopy (TEM). The in situ synchrotron diffraction data revealed the precipitation of fcc crystals after 200 s of annealing at 638 K. Before 200 s, the position of the first broad diffraction peak, Q(1), shifted toward lower momentum transfer (Q) values and the peak broadened, indicating the occurrence of LLPT at the initial stage of annealing. The structural changes occurring during LLPT were analyzed based on the pair distribution function; the changes were characterized by a transition from short- to medium-range orders as per the reduced atomic pair distribution function curve G(r). The intensity of peaks up to the fourth nearest neighbor shell in the G(r) curve exhibited different variations trends during LLPT. The peaks were classified into two groups: those indicating an fcc structure and those indicating a six-membered tricapped trigonal prism (6M-TTP; a typical medium-range order observed recently in Pd-based metallic glasses) structure. The number of peaks associated with the 6M-TTP structure gradually decreased during annealing. In contrast, the number of peaks associated with the fcc structure gradually increased at medium-range scale and decreased at short-range scale before 200 s. An analysis of the G(r) peaks indicated that LLPT is characterized by a transition from the 6M-TTP-type atomic cluster to a new type of cluster. This new type of cluster shows an atomic correlation similar to that observed in the fcc structure in the medium-range scale; however, its short-range order deteriorates. Further, high-angle annular dark field scanning TEM images revealed nanoscale structural heterogeneities during LLPT. The SAED and HRTEM results confirmed that the sample annealed for a short duration (i.e., before 200 s) with a nanoscale heterogeneous structure is amorphous, thus demonstrating the coexistence of two liquid phases. Notably, one of the two liquid phases is prone to crystallization under ion milling, thereby forming a crystal-amorphous network structure. The crystals formed due to ion milling exhibit the fcc structure and have the same crystal orientation. This research provides new evidence to unravel the LLPT mechanism in supercooled metallic liquids. Further, it presents a new model for explaining the complex structure and phase transition in metallic liquids.