Gamma relaxation in bulk metallic glasses

Küchemann, Stefan; Maaß, R.

doi:10.1016/j.scriptamat.2017.04.034

Cited by 78 publications

(32 citation statements)

References 36 publications

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“…In contrast, the material around the shear-band core exhibits non-trivial variations in the evolution of g 2,plat , until both samples are reaching a very similar value in the temperature range of 530-544 K. This non-monotonous behavior of g 2,plat near the shear-band must be a superposition of increased thermal motion and relaxation, as also has been inferred from in-situ scattering experiments during heating of a ball-milled Pd-glass [57]. If thus the temperature-dependent g 2,plat value indeed is linked to another underlying faster relaxation processes (may it be γ- [63,64], or β-relaxations [65]), it is concluded that its intensity varies in the material around the shear-band core. The precise structural origin of this different behavior of g 2,plat remains unknown, but it is well established that thermally-induced structural dynamics in binary model glasses [61] and glassy ZrCuAl [59,66] does promote icosahedral cluster formation, and that shear-banding disrupts the icosahedral network [27].…”

Section: A Temperature Dependence Of the G 2platsupporting

confidence: 53%

Shear banding leads to accelerated aging dynamics in a metallic glass

et al. 2018

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Traditionally, strain localization in metallic glasses is related to the thickness of the shear defect, which is confined to the nanometer scale. Using site-specific x-ray photon correlation spectroscopy (XPCS), we reveal significantly accelerated relaxation dynamics around a shear band in a metallic glass at a length scale that is orders of magnitude larger than the defect itself. The relaxation time in the shear-band vicinity is up to ten-times smaller compared to the as-cast matrix, and the relaxation dynamics occurs in a characteristic three-stage aging response that manifests itself in the temperature-dependent shape parameter known from classical stretched exponential relaxation dynamics of disordered materials. We demonstrate that the time-dependent correlation functions describing the aging at different temperatures can be captured and collapsed using simple scaling functions. These insights highlight how an ubiquitous nano-scale strain-localization mechanism in metallic glasses leads to a fundamental change of the relaxation dynamics at the mesoscale.

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Section: A Temperature Dependence Of the G 2platsupporting

confidence: 53%

Shear banding leads to accelerated aging dynamics in a metallic glass

et al. 2018

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“…Such characteristic barrier energies are in very good agreement with empirically derived β‐relaxation energies in other MGs, but much smaller than those reported for α‐relaxation and larger than for γ‐relaxation . The close correspondence of both relaxation times and energies suggests that the development of viscoelastic (structural) heterogeneities at the scale of a few nanometers is directly linked to sub‐ T g relaxation of the MG, which means that spatial mapping of nanoscale structural fluctuations now reveals direct insight into the process of macroscopic energy release during annealing.…”

Section: Experimentally Resolved Structural Length Scales In As‐prepasupporting

confidence: 82%

“…Supporting evidence for energy storage via a cryogenic testing protocol was provided via dynamical mechanical analysis (DMA). Küchemann and Maass studied the mechanical loss response during cooling of various MGs, which is opposite to the traditionally studied relaxation behavior of α‐ and β‐modes during heating . During cooling, a pronounced maximum of the loss modulus was found around 0.2 T g –0.3 T g for Zr 57 Nb 5 Cu 15.4 Ni 12.6 Al 10 (Vit106a), Zr 66.5 Cu 33.5 , and Pd 77.5 Cu 6 Si 16.5 ( Figure ).…”

Section: Tuning Structure Homogeneously Via Thermal and Mechanical Prmentioning

confidence: 96%

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Elastic Fluctuations and Structural Heterogeneities in Metallic Glasses

Liu

Maaß

2018

Adv Funct Materials

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Suppressing crystallization of a metallic melt results in a disordered solid, also known as a metallic glass. One may conclude that a metallic glass is free of defined structural length scales beyond some atomic-scale value that characterizes short-and medium-range order. While it is well known from atomistic modeling that a metallic glass is structurally heterogeneous, heterogeneities at such a small length scale can hardly be resolved in experiments. This review highlights experimental insights into elastic fluctuations and structural heterogeneities that emerge at scales between a few nanometers and tens to hundreds of micrometers. It distinguishes between structural and property fluctuations in as-cast metallic glasses, and heterogeneities introduced by elastic and inhomogeneous plastic deformation. As-cast glasses reveal elastic fluctuations across 3 orders of magnitude, suggesting a hierarchy of length scales that can be tuned by thermomechanical processing. Similarly, nanoscale strain localization into shear bands drives the formation of structural and elastic heterogeneities at both the nano-and the microscale. It is proposed that both types of fluctuations will allow one to quantitatively define structure-property relationships via measurable length scales-an approach that has largely contributed to the engineering success of crystalline metals.

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“…It is associated to creep behavior which implies that the system reacts to an applied force on a certain temperature-dependent timescale which can be described, for instance, by the Kohlrausch-William-Watts function [3]. The timescale depends on the structural state of the sample and, accordingly, it can be decreased via annealing or enhanced via rejuvenation mechanisms [4,5].…”

Section: Introductionmentioning

confidence: 99%

Nanoscratching of metallic glasses – An atomistic study

Avila

Küchemann

Alhafez

et al. 2019

Tribology International

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Tribological properties of materials play an important role in engineering applications. Up to now, a number of experimental studies have identified correlations between tribological parameters and the mechanical response. Using molecular dynamics simulations, we study abrasive wear behavior via nanoscratching of a Cu 64.5 Zr 35.5 metallic glass. The evolution of the normal and transverse forces and hardness values follows the behavior well known for crystalline substrates. In particular, the generation of the frontal pileup weakens the response of the material to the scratching tip and leads to a decrease of the transverse hardness as compared to the normal hardness. However, metallic glasses soften with increasing temperature, particularly above the glass transition temperature thus showing a higher tendency to structurally relax an applied stress. This plastic response is analyzed focusing on local regions of atoms which underwent strong von-Mises strains, since these are the basis of shear-transformation zones and shear bands. The volume occupied by these atoms increases with temperature, but large increases are only observed above the glass transition temperature. We quantify the generation of plasticity by the concept of plastic efficiency, which relates the generation of plastic volume inside the sample with the formation of external damage, viz. the scratch groove. In comparison to nanoindentation, the generation rate of the plastic volume during nanoscratching is significantly temperature dependent making the glass inside more damage-tolerant at lower temperature but more damage-susceptible at elevated temperatures.

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Gamma relaxation in bulk metallic glasses

Cited by 78 publications

References 36 publications

Shear banding leads to accelerated aging dynamics in a metallic glass

Shear banding leads to accelerated aging dynamics in a metallic glass

Elastic Fluctuations and Structural Heterogeneities in Metallic Glasses

Nanoscratching of metallic glasses – An atomistic study

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