Free Volume Evolution in Metallic Glasses Subjected to Mechanical Deformation

Li, Qikai; Li, Mo

doi:10.2320/matertrans.mj200785

Cited by 58 publications

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“…Their structure can be changed from one state to another state by heating or mechanical deformation. Many studies have addressed the effects of plastic deformation on the free-volume content of metallic glasses [18][19][20], but very little attention has been paid to the dependence of freevolume evolution during deformation on the initial state of the glass. In this study, rolling deformation was performed on as-cast and pre-annealed Zr 65 Al 7.5 Ni 10 Cu 12.5 Ag 5 BMGs, and their free-volume content evolutions were investigated.…”

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Rolling-induced microstructure change in Zr65Al7.5Ni10Cu12.5Ag5 bulk metallic glass

Zhou

Kong

et al. 2011

Chin. Sci. Bull.

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The microstructures and free-volume evolutions of as-cast and pre-annealed Zr 65 Al 7.5 Ni 10 Cu 12.5 Ag 5 bulk metallic glasses during rolling deformation have been investigated. No phase transformation is detected in the as-cast/rolled specimen. However, the structural stability of the glass against plastic deformation is worse after pre-annealing, indicated by nanocrystallization in preannealed/rolled specimens with large deformation degrees. Moreover, with increasing deformation degree, the free-volume content in a pre-annealed/rolled specimen increases at a lower average rate than that in an as-cast/rolled specimen. Bulk metallic glasses (BMGs) have attracted much attention because of their properties such as high fatigue strength, high hardness, good castability, and excellent wear and corrosion resistances [1][2][3][4][5][6][7][8]. However, during compressive or tensile tests at temperatures far below the glass-transition temperature, failure of BMGs usually occurs along a single shear band without much macroscopic plasticity. Their low global plasticity and ductility limit their applications as engineering materials [9], and much attention has been focused on their deformation and fracture mechanisms. Spaepen showed that plastic deformation in a metallic glass is realized by a series of atomic diffusional jumps associated with free volume [10]. The introduction of free volume into shear bands leads to a local lowering of the viscosity, and, as a result, an abrupt break takes places along the shear band. Obviously, the mechanical properties of metallic glasses are closely related to the free-volume content. Four methods can be used to change free-volume content: cooling the melt at different rates during solidification [11], annealing the metallic glass below the glass-transition temperature [12], reheating the metallic glass to above the glasstransition temperature and then cooling down quickly [13], and plastic deformation [14][15][16][17]. The principle behind these methods is that metallic glasses are thermodynamically metastable. Their structure can be changed from one state to another state by heating or mechanical deformation. Many studies have addressed the effects of plastic deformation on the free-volume content of metallic glasses [18][19][20], but very little attention has been paid to the dependence of freevolume evolution during deformation on the initial state of the glass. In this study, rolling deformation was performed on as-cast and pre-annealed Zr 65 Al 7.5 Ni 10 Cu 12.5 Ag 5 BMGs, and their free-volume content evolutions were investigated. Experimental procedureAlloy ingots of nominal composition Zr 65 Al 7.5 Ni 10 Cu 12.5 Ag 5 (atomic per cent) were prepared by arc melting a mixture of pure Zr (99.9%), Al (99.99%), Ni (99.99%), Cu (99.99%), and Ag (99.99%) under a Ti-gettered argon atmosphere. The ingots were inverted and remelted six times to ensure

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Rolling-induced microstructure change in Zr65Al7.5Ni10Cu12.5Ag5 bulk metallic glass

Zhou

Kong

et al. 2011

Chin. Sci. Bull.

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“…[1][2][3][4][5]11 Despite encouraging progresses made so far, our understanding of shear-banding process in terms of quantitative description and modeling of various shear-banding phenomena observed in experiments are still very limited, largely due to the vast span of the length and time scales in the processes. For instance, the shear bands, especially those in the propagation state, are either too large for MD simulation to simulate their branching and interaction processes, 8,9 or too small for the finite element method ͑FEM͒ to capture its atomistic scale deformation details. 12,13 In this paper, we develop a phenomenological theory on mesoscopic scale to bridge this gap and use phase-field approach to simulate shear-band formation and propagation in metallic glasses.…”

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“…The dilatation can be thermally or mechanically activated. As shown in both experiment [1][2][3][4][5] and atomistic simulations, 8,9 at temperature well below the glass transition temperature T g and under the applied stress Ӷ f , where f is the flow stress, the amorphous solid acts like an elastic medium with few sites with large AVD activated. As approaches f , the AVD sites can be activated by applied stress, 7,15 or by local heating 3,16 due to excessive plastic deformation.…”

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“…The initiation of a shear band is related to the local stress concentration and increase of the number of displaced atoms during deformation via atomic volume dilatation ͑AVD͒. 6,7 As shown by extensive molecular-dynamics ͑MD͒ simulation recently, 8,9 the volumetric change is necessary for atomic displacement in the environment that has no obvious plastic-strain-carrier defects such as dislocations as in crystals. Such mechanism was first hypothesized by Erying 10 for viscous flow in liquids and later by Spaepen in deformation of metallic glasses.…”

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“…However, we shall not follow this definition here as we have known from the atomistic simulations that it is not necessary to have such a large hole, or free volume, for the system to execute plastic flow. 8,9,15 For this reason, we simply use the dilatation of atomic volume defined by v f = v i − v 0 , where v i is the atomic volume defined as, for instance, the volume of the Voronoi polyhedron of the ith atom, v 0 is the molar volume in the undeformed ideal random close packing ͑RCP͒ state. The detailed description of the quantities and their rendition on atomic scale can be found in Refs.…”

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Mesoscopic theory of shear banding and crack propagation in metallic glasses

Zheng

2009

Phys. Rev. B

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We propose a phenomenological theory to model shear banding, shear-band propagation and branching on mesoscopic scales in metallic glasses by using Ginzburg-Landau formulism. The disordering caused by mechanical or thermal agitation is represented by atomic volume dilatation and used as an order parameter. This model captures several important features in the deformation process, namely, shear localization or banding, shear-band propagation and branching, and crack propagation and its velocity. We also assessed the relation between the crack propagation velocity and local heating and the connection between the serrated flow and shear band branching. DOI: 10.1103/PhysRevB.80.104201 PACS number͑s͒: 62.25.Mn, 46.50.ϩa, 61.43.Bn, 61.43.Fs One of the most promising developments in metallic materials in recent years is the invention of bulk metallic glasses ͑BMGs͒. 1,2 BMGs are topologically disordered solids without the long-range translational order as seen in crystalline materials. The disordered structure with atoms packed randomly leads to many unique and outstanding properties. Perhaps the most interesting is the phenomenon called shear localization: when a metallic glass subject to external load reaches the flow stress, plastic deformation occurs in narrow bands. The deformation strain inside the band is many times larger than that outside, causing samples to fail locally and quickly. The shear localization is by far identified as the only mechanism that affects the strength, ductility, and thus application of the BMGs which otherwise have many potential applications derived from the superb properties. 1-4 Shear banding consists of three stages: nucleation, growth and propagation, and final failure; and each stage occurs on a different spatial and temporal scale. Shear-band nucleation occurs in less than microsecond and with the critical nucleation size of about 10 to several hundred nanometers, depending on the material and ambient conditions. 1-5 Once the critical state is approached, the band could grow and propagate; the bands grow into steady state, or maturity with typical thickness of tens of nanometers to hundreds of nanometers and length of many times larger than the thickness, depending on the propagation condition. 1-5 A propagating shear band often becomes unstable, resulting in branching into subbands. [1][2][3][4][5] The mechanical properties of metallic glasses are determined by the detailed behavior of the shearband evolution. For example, a shear band starting propagating would affect yielding, or strength, and its propagation and branching would contribute to ductility as more deformation is concentrated inside the bands, absorbing more deformation energy and thus contributing to higher toughness. In addition, as we identify below, shear-band branching may be a major reason for the serrated flow observed during plastic deformation.The evolution exhibited in a localized shear band is a complex interplay of many factors. The initiation of a shear band is related to the local stress concentra...

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On the Atomic Anisotropy of Thermal Expansion in Bulk Metallic Glass

Liss

Yan

et al. 2011

Adv Eng Mater

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Due to their amorphous structure, glasses are thought to be the most isotropic solid materials -in contrast to crystals, which show anisotropic behavior in almost all physical properties, namely mechanical, thermal, chemical, acoustic, and electric. Particularly bulk metallic glasses consist of a mixture of spherical atoms with lack of well-defined bond angles as they can be found in covalent binding materials like silica and germania. [1] Although discovered in 1960, [2] the atomic structure of metallic glasses is poorly understood despite its importance in governing phenomena including glass transition, plastic flow, and crystallization. Here, we demonstrate that a ZrÀCuÀNiÀAl based bulk metallic glass shows anisotropic behavior on the atomic scale upon thermal expansion. Unlike a fully isotropic and homogeneous medium, the relative change of distance between the nearest neighbor and second nearest neighbor atoms in the metallic glass is significantly different, which has tendencies of a crystalline behavior of the sub-nanometer sized building blocks. [3] The implication is, that these building blocks shear upon expansion and, because of their orientational disorder throughout the bulk of the glass, build up local shear stresses which eventually yield to form defects and free volume at the glass transition. [4] Correlations between the shear elastic modulus and the glass transition temperature have been found empirically and are widely discussed throughout the literature to be universal for a large variety of metallic glasses. [5,6] The present work directly links the glass transition with the yielding due to shear stresses at the atomic level.Lack of long range deformation mechanisms, such as slip, dislocation glide, and twinning in crystalline systems, makes metallic glasses brittle. In addition, they are relatively close-packed, although the atoms are not regularly arranged over larger distances. Models employing a free volume between atoms were established and supported by experimental evidence to explain atomic displacement mechanisms COMMUNICATION Glass transition temperature and plastic yield strength are known to be correlated in metallic glasses. We have observed by in situ synchrotron high energy X-ray diffraction anisotropy of the thermal expansion behavior in the nearest neighbor and second nearest neighbor atomic distances in the building blocks of ZrÀCuÀNiÀAl based bulk metallic glass, leading inevitably to shear. Mechanical yielding of the latter on the atomic scale leads to the glass transition and the increase of the free volume. These experimental results uncover the mechanism, how glass transition and yield strength are linked.

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Free Volume Evolution in Metallic Glasses Subjected to Mechanical Deformation

Cited by 58 publications

References 29 publications

Rolling-induced microstructure change in Zr65Al7.5Ni10Cu12.5Ag5 bulk metallic glass

Rolling-induced microstructure change in Zr65Al7.5Ni10Cu12.5Ag5 bulk metallic glass

Mesoscopic theory of shear banding and crack propagation in metallic glasses

On the Atomic Anisotropy of Thermal Expansion in Bulk Metallic Glass

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