The Nanoscale Density Gradient as a Structural Stabilizer for Glass Formation

The rapid cooling of a metallic liquid(ML)results in short-range order(SRO)among the atomic arrangements and a disordered structure in the resulting metallic glass(MG).These phenomena cause various possible features in the microscopic structure of the MG,presenting a puzzle about the nature of the MGs’microscopic structure beyond SRO.In this study,the nanoscale density gradient(NDG)originating from a sequential arrangement of clusters with different atomic packing densities(APDs),representing the medium-range structural heterogeneity in Zr_(60)Cu_(30)Al_(10)MG,was characterized using electron tomography(ET)combined with image simulations based on structure modeling.The coarse polyhedrons with distinct facets identified in the three-dimensional images coincide with icosahedron-like clusters and represent the spatial positions of clusters with high APDs.Rearrangements of the different clusters according to descending APD order in the glass-forming process are responsible for the NDG that stabilizes both the supercooled ML and the amorphous states and acts as a hidden rule in the transition from ML to MG.

Entropy-driven atomic activation in supercooled liquids and its link to the fragile-to-strong transition

As a common but critical dynamic crossover in glass-forming liquids(GFLs),the discovery of fragile-to-strong(F-S)transition promises a novel route for understanding supercooled liquid and glass transition.The present work,for the first time,successfully realizes the quantitative prediction of the F-S transition in nine metallic glass-forming liquids,by a counter-intuitive approach that focuses on local atomic activation events,rather than relaxation,upon cooling.The dynamic crossover originates from a disorder-to-order transition by self-regulating behavior of atomic position within a cage controlled by finite atomic activation events,due to the appearance of local cooperative motion of nearest neighborhood atoms.Moreover,the dominant role of entropy in this anomaly has been discovered,and the correspondence between the crossover of configuration entropy involved in activation events and the occurrence of F-S transition has been found.Our work implies that the feature of atomic energy fluctuations reflected by atomic activation events has a close linkage to complex dynamic behaviors of disordered systems.