Atomistic Modelling of Thermal-Cycling Rejuvenation in Metallic Glasses

Abstract

Cycling of a metallic glass between ambient and cryogenic temperatures can induce higher-energy states characteristic of glass formation on faster cooling. This rejuvenation, unexpected because it occurs at small macroscopic strains and well below the temperatures of thermally induced structural change, is important, for example, in improving plasticity. Molecular-dynamics simulations elucidate the mechanisms by which thermal cycling can induce relaxation (reaching lower energy) as well as rejuvenation. Thermal cycling, over tens of cycles, drives local atomic rearrangements progressively erasing the initial glass structure. This arises mainly from the heating stage in each thermal cycle, linked to the intrinsic structural heterogeneity in metallic glasses. Although, in particular, the timescales in MD simulations are shorter than in physical experiments, the present simulations reproduce many physically observed effects, suggesting that they may be useful in optimizing thermal cycling for tuning the properties of metallic glasses and glasses in general.

Accessing a broader range of energy states in metallic glasses by variable-amplitude oscillatory shear

“…To remediate the latter issue, a number of processing methods are employed in order to rejuvenate metallic glasses; for example, cold rolling, high pressure torsion, irradiation, elastostatic loading, and surface treatments like shot peening [4]. More recently, it was found that a particularly elegant and minimally invasive approach to enhance potential energy is to thermally cycle glasses between the room and cryogenic temperatures [7][8][9][10][11][12][13][14][15][16][17]. Despite considerable efforts, however, the development of efficient processing methods to access a broader range of energy states in metallic glasses and, at the same time, maintain their structural integrity remains a challenging problem.…”

Role of thermal expansion heterogeneity in the cryogenic rejuvenation of metallic glasses

“…One possible interpretation of the effect, suggested by Hufnagel [6], is that the rejuvenation is due to the creation of internal stresses during the temperature cycling. The origin of such stresses could be twofold: local heterogeneities in the temperature field due to the very fast heating process which were investigated by some of us in a recent molecular dynamics study [7]. Alternatively, an heterogeneous thermal expansion coefficient, similar to the heterogeneity in elastic constants, would also create internal stresses even if the temperature field is homogeneous.…”

Yielding transition in stable glasses periodically deformed at finite temperature

“…Common examples include high-pressure torsion, cold rolling, wire drawing, irradiation, elastostatic loading, as well as surface treatments like shot peening [6]. Here, we highlight a recent discovery that thermal cycling between the room and cryogenic temperatures can lead to rejuvenation of metallic glasses due to heterogeneous thermal expansion [7][8][9][10][11][12][13][14][15][16][17][18]. Remarkably, it was also recently demonstrated experimentally and by means of atomistic simulations that cooling across the glass transition temperature under applied stress rejuvenates amorphous alloys and increases their ductility [19,20].…”