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.

Liquid dynamics and glass formation of Gd_(55)Co_(20)Al_(25)metallic glass with minor Si addition

Liquid dynamics plays an essential role in glass formation.Here we observed a distinct change of liquid dynamics in Gd_(55)Co_(20)Al_(25)metallic glass induced by microalloying Si element.In the equilibrium melt,minor Si(0.5 at.%)addition leads to a more fragile liquid behavior and a smaller strength of liquid-liquid transition with the transition strength(F)decreasing from 0.76 to 0.35.However,in the supercooled liquid,Si-doped liquid exhibits a remarkable enhanced fragile-to-strong transition(FST),and the value of FST factor f increases sharply from 1.63 to 3.84,resulting in a stronger liquid behavior and more sluggish crystallization kinetics for Gd_(55)Co_(20)Al_(24.5)Si_(0.5)metallic glass.Moreover,minor Si addition promotes the formation of a crystal-like structure with a size of 1-2 nm.The interactions between the crystal-like structures and other local favored clusters frustrate the further growth of crystal-like phases,thus stabilizes the amorphous structure.As a result,the glass-forming ability(GFA)was largely improved.The critical diameter of Gd_(55)Co_(20)Al_(25)metallic glass increased from 2 to 7 mm with 0.5 at.%Si addition without deterioration of the magnetocaloric effect.This study provides valuable insight for understanding the distinct effect of microalloying on GFA of metallic glasses from the aspect of the evolution of the liquid.