Sub-nanometer open volume regions in a bulk metallic glass investigated by positron annihilation

Flores, Katharine M.; Sherer, E.; Bharathula, Ashwini; Chen, H.; Jean, Y. C.

doi:10.1016/j.actamat.2007.01.040

Cited by 96 publications

(58 citation statements)

References 41 publications

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“…The peening-induced rejuvenation of the annealed glass is a stronger effect when the peening is conducted at 77 K than at room temperature, just as expected from the discussion of damage and repair rates in §2.1. In a Zr-based BMG cold-rolled to thickness reductions of up to 50%, ΔH rel first increases and then decreases, matching well with changes in free volume indicated by positron-annihilation lifetime spectroscopy (PALS) [38].…”

Section: Stored Energy From Plastic Deformationmentioning

confidence: 65%

Stored energy in metallic glasses due to strains within the elastic limit

Greer

Sun

2016

Philosophical Magazine

108

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Room temperature loading of metallic glasses, at stresses below the macroscopic yield stress, raises their enthalpy and causes creep. Thermal cycling of metallic glasses between room temperature and 77 K also raises their enthalpy. In both cases, the enthalpy increases are comparable to those induced by heavy plastic deformation, but, as we show, the origins must be quite different. For plastic deformation, the enthalpy increase is a fraction (<10%) of the work done (WD) (and, in this sense, the behaviour is similar to that of conventional polycrystalline metals and alloys). In contrast, the room temperature creep and the thermal cycling involve small strains well within the elastic limit; in these cases, the enthalpy increase in the glass exceeds the WD, by as much as three orders of magnitude. We argue that the increased enthalpy can arise only from an endothermic disordering process drawing heat from the surroundings. We examine the mechanisms of this process. The increased enthalpy ('stored energy') is a measure of rejuvenation and appears as an exothermic heat of relaxation on heating the glass. The profile of this heat release (the 'relaxation spectrum') is analysed for several metallic glasses subjected to various treatments. Thus, the effects of the small-strain processing (creep and thermal cycling) can be better understood, and we can explore the potential for improving properties, in particular the plasticity, of metallic glasses. Metallic glasses can exhibit a wide range of enthalpy at a given temperature, and small-strain processing may assist in accessing this for practical purposes.

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Section: Stored Energy From Plastic Deformationmentioning

confidence: 65%

Stored energy in metallic glasses due to strains within the elastic limit

Greer

Sun

2016

Philosophical Magazine

108

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“…18 and 32 among others. However, the excess free volume generated during plastic deformation is not stable, 33 and it coalesces to form nanovoids once the externally applied stress is removed, as experimentally verified by Li et al 34 and Flores et al 35 Nanoindentation of ball-indented plastic regions showed that these nanovoids act as sites for the initiation of shear bands. 18 Because the shot-peening process involves repeated indentations that induce successive plastic strains in the surface layers, it is likely that the effective free volume in those layers increases considerably.…”

Section: A Pressure Sensitivitymentioning

confidence: 88%

Free-volume dependent pressure sensitivity of Zr-based bulk metallic glass

et al. 2009

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Instrumented indentation experiments on a Zr-based bulk metallic glass (BMG) in as-cast, shot-peened and structurally relaxed conditions were conducted to examine the dependence of plastic deformation on its structural state. Results show significant differences in hardness, H, with structural relaxation increasing it and shot peening markedly reducing it, and slightly changed morphology of shear bands around the indents. This is in contrast to uniaxial compressive yield strength, s y , which remains invariant with the change in the structural state of the alloys investigated. The plastic constraint factor, C = H/s y , of the relaxed BMG increases compared with that of the as-cast glass, indicating enhanced pressure sensitivity upon annealing. In contrast, C of the shot-peened layer was found to be similar to that observed in crystalline metals, indicating that severe plastic deformation could eliminate pressure sensitivity. Microscopic origins for this result, in terms of shear transformation zones and free volume, are discussed.

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“…The free volume model provides a simple and practical model to describe the plasticity of amorphous alloys. The applicability of the model has been well verified and it can qualitatively explain many mechanics properties of amorphous alloys [74][75][76] . In 1979, Argon 77 used "shear transformation" to explain the plastic deformation of metallic glasses and the theory meets well with the experimental observations.…”

Section: Free Volume Modelmentioning

confidence: 98%

The Development of Structure Model in Metallic Glasses

et al. 2017

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Metallic glasses (amorphous alloys) have grown from a singular observation to an expansive class of alloys with a broad range of scientific interests. Their unique properties require a robust understanding on the structures at the atomic level while alloys in this class have a similar outlook on the microstructure. In this review, we went through the history of the majority studies on the structure models of metallic glasses, and summarized their historical contributions to the understanding of the structure metallic glasses and the relationship between their structures and properties.

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Sub-nanometer open volume regions in a bulk metallic glass investigated by positron annihilation

Cited by 96 publications

References 41 publications

Stored energy in metallic glasses due to strains within the elastic limit

Stored energy in metallic glasses due to strains within the elastic limit

Free-volume dependent pressure sensitivity of Zr-based bulk metallic glass

The Development of Structure Model in Metallic Glasses

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