Supercooled Metallic Glasses Research Articles

Nucleation kinetics in a supercooled metallic glass former

We use molecular dynamics simulations to shed light on the mechanism underlying crystal nucleation in a supercooled metallic glass former characterized by a concurring crystal and amorphous local order based on icosahedral symmetry. At a crossover temperature, well below the melting point, we find that the supercooled phase exhibits glassy dynamics which includes a breakdown of the Stokes-Einstein relation and the emergence of spatially heterogeneous dynamics. In addition, we show that the origin of these phenomena can be related to a structural heterogeneity caused by the increase of icosahedral symmetry upon cooling. We also reveal that crystal nucleation occurs close to the glass transition and takes place in regions of high icosahedral symmetry, which can be interpreted by a strong reduction of the crystal-liquid interfacial energy. This scenario provides a framework for testing the estimation of the nucleation time according to the classical nucleation theory. More specifically, our findings allow to quantify how the heterogeneous character of the supercooled phase affects the predictions of this theory.

Dynamic slowing-down and crystal nucleation in a supercooled metallic glass former induced by local icosahedral order

A long-standing question in condensed matter and material science is understanding how local structural order determines the fate of a liquid on cooling, i.e., whether it crystallizes or experiences dynamic slowing down leading to glass formation. Here we address this issue by focusing on a metallic glass former with concurring crystal and amorphous local order based on icosahedral symmetry. Using atomistic simulations, we study its deep supercooled regime in which we clearly identify spontaneous crystal nucleation close to the glass transition. We find that regions of slow dynamics characterized by a high degree of icosahedral orientational order play a central role in the nucleation process. Our study brings a new perspective into the microscopic mechanism of crystallization from the supercooled phase, which could be of significance for the in silico design of materials with selected glass-forming ability.

Ultrasonic welding of metallic alloy glass to copper

Metallic glasses (MGs) with amorphous structures and tunable compositions have been considered excellent structural and functional materials, especially in the frontier fields of microelectromechanical systems (MEMS), biomedical engineering, energy conversion, etc. Making these materials into desired structures for different applications has always been one of the research focuses in the past decades. Due to high strength and poor plasticity at ambient temperature, MGs are difficult to be formed and manufactured by conventional approaches. Thermoplastic forming, utilizing the superplasticity property of MGs in supercooled liquid region, has been developed to fabricate micro-/nanofeatures. However, the thermal effect inevitably increases the risks of crystallization and oxidation of MGs during forming, leading to material performance degradation and limiting the process feasibility to specific composition systems of MGs. In view of this, a novel strategy proposed recently is to promote plastic deformation and forming of MGs by high frequency ultrasonic vibration at room temperature, which promises to avoid the shortcomings in thermal forming and extend the forming processes of MGs. In this review article, we would like to first discuss the forming mechanisms of supercooled MGs liquid, the joining of MGs and a variety of fabricated structures based on thermoplastic forming. After that, a concept of ultrasonic-assisted forming of MGs is introduced focusing on plastic behavior under high frequency vibration and the latest process advances including shear punching and manufacturing. Finally, the existing problems and the development prospects will be discussed to facilitate the further researches on MGs forming.