A variety of vitrification kinetics in bulk metallic glasses

We all know what glass is, don’t we? We just point to the nearest window and it is that transparent sheet. Humans have been producing glass for at least 6,000 years, way before they knew how to smelt iron. Actually, the chemistry is well known: to produce window glass we just heat a mixture of sodium carbonate (soda), calcium oxide (lime) and silicon (IV) oxide (silica or, less technically, sand) and we have soda glass. If we want our glass to withstand temperature changes and be tougher, we add some boron oxide and, instead of a calcium silicate, we get a borosilicate with a fancy name (Pyrex). We can add other elements to get other special characteristics in our glass. It is as if we knew what we are doing. But we are not. The glass transition or vitrification, understood as a good theoretical description of what is going on during glass formation, is one of the most fascinating and still unsolved problems in condensed matter physics.

A diverse class of materials, a variety of viscous liquids and colloidal suspensions, happens to solidify in a nonequilibrium state where there is no long-range order as in crystals, under conditions that are still not fully understood. This transition, which can be reversed, is not a phase transition, as it occurs over a temperature range rather than at a particular temperature. Decreasing temperature is usually the parameter triggering viscous liquids through the transition.

One of the major questions is whether vitrification in viscous liquids— that is, the transformation from a supercooled liquid in metastable equilibrium into a nonequilibrium glass—and devitrification—taking place on heating the glass through its glass transition—are exclusively related to the primary structural relaxation process, the so-called α-relaxation process, which is attributed to cooperative motion of several structural units, or rather other atomic motions play a role.

Contrary to polymeric and molecular systems, bulk metallic glasses (BMGs) are often considered model candidates to investigate the glass transition process as they do not have any reorentational or intramolecular motion, which could influence their vitrification. They are typically multicomponent alloys with large size mismatch and heterogeneous chemical affinity between the constituents. It results that the liquid structure is very densely packed with a pronounced structural and chemical short- and medium-range order.

In these glass-forming alloys, both diffusion and viscous flow start to develop solid-like features upon cooling far above the temperature above which the material is completely liquid (liquidus temperature). Already at the melting point, viscosities of BMG formers are several orders of magnitude larger than those of regular metals and alloys. Because of these observations and others, we could infer that their multicomponent nature should reflect on the heterogeneous dynamic behaviour of BMGs, implying that, apart from the α-relaxation, there should exist multiple different atomic motions acting at a variety of time scales.

Now, a team of researchers shows ¹ that multicomponent bulk metallic glasses can display several vitrification kinetics in standard conditions, i.e., without any geometrical restriction or prolonged annealing.

The scientists studied exhaustively the vitrification kinetics and the atomic mobility of a gold-based BMG former in a broad time and temperature range using fast scanning calorimetry. This technique allows a characterization over a wide range of time scales. Combining fast scanning calorimetry results with those of other techniques, they were able to characterize the α-relaxation time over more than four decades.

As a result, they find that two aspects of glassy dynamics are decoupled: vitrification is delayed with respect to what it would be expected, accounting exclusively for the α-relaxation. This implies that fast atomic mechanisms that do not contribute to the α-relaxation steer vitrification.

As the existence of multiple mechanisms of vitrification is a general pattern in all kinds of glasses, the presence of the decoupling α-relaxation/vitrification kinetics can be anticipated in a wide variety of them. This conclusion has profound implications in our understanding of the fundamental properties of glasses.

Author: César Tomé López is a science writer and the editor of Mapping Ignorance

Disclaimer: Parts of this article may be copied verbatim or almost verbatim from the referenced research paper.