In-situ scattering study of a liquid-liquid phase transition in Fe-B-Nb-Y supercooled liquids and its correlation with glass-forming ability

Abstract

In-situ synchrotron high-energy X-ray diffraction was used to study the kinetics of structure evolution for two Fe-based bulk metallic glasses with different thermophysical behaviors upon heating and isothermal annealing in the supercooled liquid region. It is found that the structure change of (Fe0.72B0.24Nb0.04)97Y3, an average glass former, follows a continuous disordering process before crystallization, while that of (Fe0.72B0.24Nb0.04)95.5Y4.5, a better glass former with an anomalous exothermic peak below the crystallization temperature, is characterized by a reentrant supercooled liquid behavior. A hidden amorphous phase with a configurationally highly-correlated structure is found at a critical temperature of the anomalous exothermic peak for the (Fe0.72B0.24Nb0.04)95.5Y4.5 supercooled liquid, and then it reenters the disordered phase of lower correlation length at a higher temperature. Synchrotron diffraction and the density measurements together illustrate that the liquid-liquid phase transition accompanies with an unusual density change upon isothermal annealing at the anomalous exothermic peak temperature. Our experimental results suggest that a liquid-liquid phase transition which occurred at the medium-range length scale plays an important role in stabilizing the (Fe0.72B0.24Nb0.04)95.5Y4.5 supercooled liquid. Possible mechanisms for the observed differences and the relationship with the glass-forming ability are discussed based on the results of the pair distribution function analysis.