Mechanical Relaxation-to-Rejuvenation Transition in a Zr-based Bulk Metallic Glass

Cooling under Applied Stress Rejuvenates Amorphous Alloys and Enhances Their Ductility

The effect of tensile stress applied during cooling of binary glasses on the potential energy states and mechanical properties is investigated using molecular dynamics simulations. We study the three-dimensional binary mixture that was first annealed near the glass transition temperature and then rapidly cooled under tension into the glass phase. It is found that at larger values of applied stress, the liquid glass former freezes under higher strain and its potential energy is enhanced. For a fixed cooling rate, the maximum tensile stress that can be applied during cooling is reduced upon increasing initial temperature above the glass transition point. We also show that the amorphous structure of rejuvenated glasses is characterized by an increase in the number of contacts between smaller type atoms. Furthermore, the results of tensile tests demonstrate that the elastic modulus and the peak value of the stress overshoot are reduced in glasses prepared at larger applied stresses and higher initial temperatures, thus indicating enhanced ductility. These findings might be useful for the development of processing and fabrication methods to improve plasticity of bulk metallic glasses.

Rejuvenation of Zr-Based Bulk Metallic Glasses by Ultrasonic Vibration-Assisted Elastic Deformation

The rejuvenation of Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti₅ bulk metallic glasses (BMGs) by ultrasonic vibration-assisted elastic deformation (UVEF) was studied herein. The UVEF-treated samples demonstrate an obvious rejuvenation and have a higher relaxation enthalpy and a smaller range of supercooled liquid regions than the as-cast samples. The fracture of the rejuvenated amorphous alloy is mainly ductile fracture, and shear deformation occurs in the deformation region. It is also found that as the amplitude increases, the free volume of the rejuvenated amorphous alloy increases, the yield strength and the elastic modulus decrease, and the formability increases. The free-volume content is used to characterize the degree of rejuvenation, and a mathematical model of the relationship between the ultrasonic amplitude and free volume is established. In addition, it is found that the ultrasonic vibration stress induces the additional free volume in the Zr-based bulk metallic glasses and improves the plasticizing behavior. The temperature rise caused by the ultrasonic thermal effect does not induce additional free volume.

Ultrafast extreme rejuvenation of metallic glasses by shock compression

Structural rejuvenation of glasses not only provides fundamental insights into their complicated dynamics but also extends their practical applications. However, it is formidably challenging to rejuvenate a glass on very short time scales. Here, we present the first experimental evidence that a specially designed shock compression technique can rapidly rejuvenate metallic glasses to extremely high-enthalpy states within a very short time scale of about 365 ± 8 ns. By controlling the shock stress amplitude, the shock-induced rejuvenation is successfully frozen at different degrees. The underlying structural disordering is quantitatively characterized by the anomalous boson heat capacity peak of glasses. A Deborah number, defined as a competition of time scales between the net structural disordering and the applied loading, is introduced to explain the observed ultrafast rejuvenation phenomena of metallic glasses.

Glass transition memorized by the enthalpy-entropy compensation in the shear thinning of supercooled metallic liquids

To unravel the true nature of glass transition, broader insights into glass forming have been gained by examining the stress-driven glassy systems, where strong shear thinning, i.e. a reduced viscosity under increasing shear rate, is encountered. It is argued that arbitrarily small stress-driven shear rates would 'melt' the glass and erase any memory of its thermal history. In this work, we report a glass transition memorized by the enthalpy-entropy compensation in strongly shear-thinned supercooled metallic liquids, which coincides with the thermal glass transition in both the transition temperature and the activation Gibbs free energy. Our findings provide distinctive insights into both glass forming and shear thinning, and enrich current knowledge on the ubiquitous enthalpy-entropy compensation empirical law in condensed matter physics.