Thickness of shear bands in metallic glasses

Zhang, Y.; Greer, A. Lindsay

doi:10.1063/1.2336598

Cited by 291 publications

(189 citation statements)

References 37 publications

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“…of 1-1.5 nm (36,37). This dimensional coincidence of two closely packed atomic clusters, albeit the significance of which should not be overestimated, implies a potential intrinsic correlation between STZs and the medium-range orders (38). In addition, STZs with larger sizes may be easy to grow up during plastic flow and serve as the embryos for deformation-induced nanocrystallization within shear bands.…”

Section: Resultsmentioning

confidence: 99%

Experimental characterization of shear transformation zones for plastic flow of bulk metallic glasses

Pan

Inoue²,

Sakurai³

et al. 2008

Proc. Natl. Acad. Sci. U.S.A.

542

231

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T he room-temperature plastic deformation of bulk metallic glasses (BMGs) is known to be inhomogeneous both spatially and temporally and achieved by highly localized shear bands (1-3). Inspired by the increasingly intense scientific and technological interests of BMGs and the efforts of improving their limited plasticity (4-7), there is a compelling need to identify the physical processes responsible for the dynamics and rheology of metallic glasses well below their glass transition temperatures. Historically, several rheological theories have been developed to describe the heterogeneous plasticity of glasses. These models are mainly based on two possible atomic-scale mechanisms, i.e., deformationinduced dilatation or free volume (8-10) and local events of cooperative shearing of atomic clusters termed shear transformation zones (STZs) (11-16). Recently, a cooperative shearing model (CSM) of STZs by Johnson and Samwer (17), together with work of Falk and Langer (13,14) and others (18,19), has been shown to provide an effective interpretation of plasticity in metallic glasses well below their glass temperatures. In the Johnson-Samwer model, structure and energetics correlation in glasses has been established by introduction of the concept of potential energy landscapes in combination with STZs, and the mechanical behavior of BMGs is expected to intrinsically depend on the actual volumes of STZs (17,20). Assessment of the sizes of STZs, or the minimum molecular configurations of inelastic rearrangements in BMGs subjected to stresses, is thus of key importance in understanding the plastic deformation of these amorphous solids. More recently, energetic considerations (17, 21) and molecular dynamics (MD) simulations (13, 19) have quantitatively evaluated the number of atoms in a STZ of glassy materials. Despite the aforementioned intense efforts to identify STZ volumes of glasses, an experimental quantitative measurement of the STZ volumes is still missing.Here, we develop an experimental method to characterize the STZs of BMGs based on the Johnson-Sawmer CSM (17). In the Johnson-Sawmer model, a constitutive description of plastic deformation can be given in an equation where inelastic strain rate is a function of dynamical state variables (i.e., the STZ volumes) in addition to stress and strain. A simplified form of this constitutive equation can be written aṡwhereγ is inelastic strain rate,γ 0 is a constant, k is the Boltzmann constant, T is temperature, W * = 4RG 0 γ 2 C (1 − τ/τ C ) 3/2 ζ is the barrier energy at finite stress 0 < τ < τ C , G 0 , and τ C are the shear modulus and the threshold shear resistance of an alloy at 0 K, respectively, the average elastic limit γ C ≈ 0.027 (17), is the volume of STZ, and constants R ≈ 1/4 and ζ ≈ 3 (17). It should be noted that the normal stress dependence of the shear strength (22) is postulated to be insignificant in this analysis. Thus, by direct differentiation of the activation energy W * , we obtain the activation volume in the CSM:From experiments using strain rat...

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Section: Resultsmentioning

confidence: 99%

Experimental characterization of shear transformation zones for plastic flow of bulk metallic glasses

Pan

Inoue²,

Sakurai³

et al. 2008

Proc. Natl. Acad. Sci. U.S.A.

542

231

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“…Such macro-micro inconsistency results from the strong tendency for shear flow localization in these samples during tests. The shear localization in BMGs usually occurs at $10 nm scale [36], which cannot lead to appreciable plastic flow at macroscopic scale. Based on the definition of DTB presented above, we suggest that these samples still failed in a ductile shear mode rather than a brittle tensile mode, even though they show no macroscopic plastic strain in their bending curves.…”

Section: Resultsmentioning

confidence: 99%

Shear transformation zone volume determining ductile–brittle transition of bulk metallic glasses

et al. 2011

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Three-point bending experiments were performed on as-cast and annealed samples of Zr 52.5 Cu 17.9 Ni 14.6 Al 10 Ti 5 (Vit105) bulk metallic glasses over a wide range of temperatures varying from room temperature (293 K) to liquid nitrogen temperature (77 K). The results demonstrated that the free volume decrease due to annealing and/or cryogenic temperature can reduce the propensity for the formation of multiple shear bands and hence deteriorate plastic deformation ability. We clearly observed a sharp ductile-to-brittle transition (DBT), across which microscopic fracture feature transfers from micro-scale vein patterns to nano-scale periodic corrugations. Macroscopically, the corresponding fracture mode changes from ductile shear fracture to brittle tensile fracture. The shear transformation zone volume, taking into account free volume, temperature and strain rate, is proposed to quantitatively characterize the DBT behavior in fracture of metallic glasses.

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“…As dislocation motion in high-strength crystalline materials becomes increasingly difficult (11), the ductility, i.e., the ability of a material to change shape without catastrophic failure, is often reduced dramatically (6, 7). In bulk metallic glasses, plastic deformation is not enabled by dislocations (12-21) but rather by clusters of atoms that undergo cooperative shear displacements [shear transformation zones (STZs)] (16); in the extreme limit of homogeneous-toinhomogeneous flow transition, shear bands of nanoscale width form (17,(19)(20)(21). The formation of such shear bands causes large strain softening and abrupt rupture of the metallic glasses.…”

mentioning

confidence: 99%

Ductile crystalline–amorphous nanolaminates

Wang

Hamza

et al. 2007

Proc. Natl. Acad. Sci. U.S.A.

443

222

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It is known that the room-temperature plastic deformation of bulk metallic glasses is compromised by strain softening and shear localization, resulting in near-zero tensile ductility. The incorporation of metallic glasses into engineering materials, therefore, is often accompanied by complete brittleness or an apparent loss of useful tensile ductility. Here we report the observation of an exceptional tensile ductility in crystalline copper/copper-zirconium glass nanolaminates. These nanocrystalline-amorphous nanolaminates exhibit a high flow stress of 1.09 ؎ 0.02 GPa, a nearly elastic-perfectly plastic behavior without necking, and a tensile elongation to failure of 13.8 ؎ 1.7%, which is six to eight times higher than that typically observed in conventional crystallinecrystalline nanolaminates (<2%) and most other nanocrystalline materials. Transmission electron microscopy and atomistic simulations demonstrate that shear banding instability no longer afflicts the 5-to 10-nm-thick nanolaminate glassy layers during tensile deformation, which also act as high-capacity sinks for dislocations, enabling absorption of free volume and free energy transported by the dislocations; the amorphous-crystal interfaces exhibit unique inelastic shear (slip) transfer characteristics, fundamentally different from those of grain boundaries. Nanoscale metallic glass layers therefore may offer great benefits in engineering the plasticity of crystalline materials and opening new avenues for improving their strength and ductility.metallic glass ͉ size-dependent plasticity ͉ nanocrystalline materials ͉ amorphous-crystalline interface ͉ tensile ductility A traditional strategy to develop ultrahigh-strength crystalline materials is to limit or inhibit the motion of dislocations required for plastic deformation (1-3) so that a higher applied stress is necessary. Examples of such advanced materials include thin films (4), nanocrystalline metals (5-7), and nanolaminates (8-10). As dislocation motion in high-strength crystalline materials becomes increasingly difficult (11), the ductility, i.e., the ability of a material to change shape without catastrophic failure, is often reduced dramatically (6, 7). In bulk metallic glasses, plastic deformation is not enabled by dislocations (12-21) but rather by clusters of atoms that undergo cooperative shear displacements [shear transformation zones (STZs)] (16); in the extreme limit of homogeneous-toinhomogeneous flow transition, shear bands of nanoscale width form (17,(19)(20)(21). The formation of such shear bands causes large strain softening and abrupt rupture of the metallic glasses. By way of contrast, large compressive plastic strains have been obtained in several bulk metallic glasses (12)(13)(14). Nonetheless, they show nearzero macroscopic ductility when subjected to tensile loading. To our knowledge, there is no experimental evidence currently suggesting that macroscopic metallic glass samples can sustain large tensile plasticity. An interesting question arises whether shear banding remains...

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Thickness of shear bands in metallic glasses

Cited by 291 publications

References 37 publications

Experimental characterization of shear transformation zones for plastic flow of bulk metallic glasses

Experimental characterization of shear transformation zones for plastic flow of bulk metallic glasses

Shear transformation zone volume determining ductile–brittle transition of bulk metallic glasses

Ductile crystalline–amorphous nanolaminates

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