Rejuvenation of metallic glasses by non-affine thermal strain

Abstract

When a spatially uniform temperature change is imposed on a solid with more than one phase, or on a polycrystal of a single, non-cubic phase (showing anisotropic expansion-contraction), the resulting thermal strain is inhomogeneous (non-affine). Thermal cycling induces internal stresses, leading to structural and property changes that are usually deleterious. Glasses are the solids that form on cooling a liquid if crystallization is avoided--they might be considered the ultimate, uniform solids, without the microstructural features and defects associated with polycrystals. Here we explore the effects of cryogenic thermal cycling on glasses, specifically metallic glasses. We show that, contrary to the null effect expected from uniformity, thermal cycling induces rejuvenation, reaching less relaxed states of higher energy. We interpret these findings in the context that the dynamics in liquids become heterogeneous on cooling towards the glass transition, and that there may be consequent heterogeneities in the resulting glasses. For example, the vibrational dynamics of glassy silica at long wavelengths are those of an elastic continuum, but at wavelengths less than approximately three nanometres the vibrational dynamics are similar to those of a polycrystal with anisotropic grains. Thermal cycling of metallic glasses is easily applied, and gives improvements in compressive plasticity. The fact that such effects can be achieved is attributed to intrinsic non-uniformity of the glass structure, giving a non-uniform coefficient of thermal expansion. While metallic glasses may be particularly suitable for thermal cycling, the non-affine nature of strains in glasses in general deserves further study, whether they are induced by applied stresses or by temperature change.

On cryothermal cycling as a method for inducing structural changes in metallic glasses

“…In the process of such a treatment, the surface of the samples should experience the strongest influence of any part of the sample because of the highest temperature gradient. In our previous work 11 , we ruled out any long-term surface effects, as they are insignificant in the resulting changes in the heat of relaxation. However, the results of the present work show that these conclusions might have been premature.…”

“…The bulk rejuvenation effect of thermal cycling was proven by differential scanning calorimetry experiments in our previous work 11 . In the present work, we observed a potential relaxation effect of thermal cycling in the metallic glass surface.…”

“…However, the applicability boundaries of the method for metallic glasses are still unknown. In our previous work 11 , we have observed a rejuvenation effect of cryothermal cycling on metallic glasses with different compositions. The effect was more pronounced for melt-spun ribbons than for bulk samples.…”

“…Meanwhile, there are many methods that have been developed to increase the energy level or rejuvenate metallic glasses 8,9 : irradiation, heavy plastic deformation, elastostatic loading, simple heating above T g and subsequent fast cooling 10 , etc. Recently, another method for metallic glass rejuvenation was found 11 . Due to the intrinsic structural heterogeneity of metallic glasses, the spatial distribution of the properties on the nanoscale can be very wide.…”

Properties of Bulk Metallic Glasses

“…Влияние ма-лой концентрации алюминия на упругие и пласти- Когда пространственно-однородное изменение температуры происходит в образце, имеющем в со-ставе более одной фазы, или поликристалличе-ском образце некубической фазы (с сильной ани-зотропией теплового расширения-сжатия), то де-формация неоднородна. Термоциклирование при криогенной температуре вызывает «омоложение» металлических стекол [80] с достижением состо-яния с повышенной энергией, что приводит к ро-сту их пластичности. Это объясняется внутренней неоднородностью структуры стекла [81], что пред-полагает неравномерное значение коэффициента теплового расширения.…”