Probing the Formation of Ultrastable Metallic Glass from Structural Heterogeneity

Ultrastable metallic glasses(SMGs)exhibit enhanced stability comparable to those of conventional glasses aged for thousands of years.The ability to understand why certain alloy compositions and processing conditions generate an SMG is an emerging challenge.Herein,amplitude-modulation dynamic atomic force microscopy was utilized for tracking the structure of Zr50Cu50,Zr50Cu44.5Al5.5 and Zr50Cu41.5Al5.5Mo3 thin film metallic glasses(TFMGs)that were produced by direct current magnetron sputtering at room temperature with the rate of deposition being the only variable.The transition in stability from bulk-to SMG-like behavior resides in the change of relaxation mechanism as the deposition rate is decreased.The formation of SMGs is directly linked with the degree of structural heterogeneity,whereby MGs with greater heterogeneity have a higher potential to form SMGs with more significant enhancement in sta-bility.Slower deposition rates,however,are required to yield the more homogenous structure and lower energy state underlying the ultrastability.Ultrastability is closely linked with the geometric shape and distribution of loosely packed phases,whereby SMGs containing more slender loosely packed phases with a more skewed distribution achieve more significant improvements in stability.This work not only pro-vides direct evidence of the structure of SMGs,but also opens new horizons for the design of SMGs.