A Model for Boundary Diffusion Controlled Creep in Polycrystalline Materials

Coble, R. L.

doi:10.1063/1.1702656

Cited by 2,121 publications

(694 citation statements)

References 9 publications

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“…This process, now known as Nabarro-Herring diffusion creep, predicts a creep rate of the form shown in Eq 1 with n = 1, p = 2 and D = D ' , where D ' is the coefficient for lattice self-diffusion. At a later date, Coble [9] noted that vacancies may also diffuse along the grain boundaries and this leads to the process now known as Coble diffusion creep where n = 1, p = 3 and D = D gb , where D gb is the coefficient for grain boundary diffusion.…”

Section: Potential Creep Mechanisms At Low Stresses When N =mentioning

confidence: 99%

Fifty years of Harper–Dorn creep: a viable creep mechanism or a Californian artifact?

2007

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Fifty years ago, in a series of classic creep experiments conducted at the University of California in Berkeley, Harper and Dorn obtained unique experimental data revealing the possibility of a new and heretofore unrecognized flow process occurring in pure aluminum when tested at low stresses and at temperatures very close to the melting temperature. This flow mechanism, subsequently designated Harper-Dorn creep, has been the center of much argument and speculation in the ensuing years. The present paper looks back over the last half-century and charts the various developments in attempts to obtain a more detailed understanding of whether Harper-Dorn creep is (or is not) a viable creep process. Examples are presented for both metals and non-metals. It is concluded that, although it appears Harper-Dorn creep may occur only under restricted conditions associated with high purity materials and low initial dislocation densities, nevertheless there is good evidence supporting the validity of this creep mechanism as a viable and unique flow process.

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Section: Potential Creep Mechanisms At Low Stresses When N =mentioning

confidence: 99%

Fifty years of Harper–Dorn creep: a viable creep mechanism or a Californian artifact?

2007

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“…However, the beneficial effect of nanocrystallinity is weighed by the increased strain rate sensitivity [2][3][4][5][6][7] and increased creep rates compared to films with larger grain size. Nanocrystalline metals allow for large material volumes occupied by grain boundaries that control dislocation nucleation [4,8] and grain boundary mediated creep [9] while affecting the relative contribution of thermal and stress driven inelastic processes [10][11][12]. These often competing mechanisms result in increased rate sensitivity which is not uniform across time scales: diffusion controlled processes are important at slow loading rates and may dominate in materials with large percentage of grain boundaries.…”

Section: Introductionmentioning

confidence: 99%

Strain rate sensitivity of nanocrystalline Au films at room temperature

et al. 2010

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Dimitrios, "Strain rate sensitivity of nanocrystalline Au films at room temperature" (2010) AbstractThe effect of strain rate on the inelastic properties of nanocrystalline Au films was quantified with 0.85 and 1.76 lm free-standing microscale tension specimens tested over eight decades of strain rate, between 6 Â 10 À6 and 20 s À1 . The elastic modulus was independent of the strain rate, 66 ± 4.5 GPa, but the inelastic mechanical response was clearly rate sensitive. The yield strength and the ultimate tensile strength increased with the strain rate in the ranges 575-895 MPa and 675-940 MPa, respectively, with the yield strength reaching the tensile strength at strain rates faster than 10 À1 s À1 . The activation volumes for the two film thicknesses were 4.5 and 8.1 b 3 , at strain rates smaller than 10 À4 s À1 and 12.5 and 14.6 b 3 at strain rates higher than 10 À4 s À1 , while the strain rate sensitivity factor and the ultimate tensile strain increased below 10 À4 s À1 . The latter trends indicated that the strain rate regime 10 À5 -10 À4 s À1 is pivotal in the mechanical response of the particular nanocrystalline Au films. The increased rate sensitivity and the reduced activation volume at slow strain rates were attributed to grain boundary processes that also led to prolonged (5-6 h) and significant primary creep with initial strain rate of the order of 10 À7 s À1 .

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“…[4]). Despite these differences, the model of Argon and Yip, [46] unlike other models discussed earlier, [30,44] predicts a transition from hardening to nanoscale softening at a realistic value for the grain size.…”

Section: Composite Model Involving Amorphous Boundary Layermentioning

confidence: 91%

“…Early theoretical considerations have indicated that under conditions of low homologous temperatures, very small normalized grain sizes, and small stresses, Coble creep [30] may control the creep behavior of fine-grained materials. Coble creep, in which creep strain is produced by the diffusion of vacancies along the grain boundaries, is represented by the following equation: Second, deformation data reported for nc Ni and nc Cu (Table II) are plotted in the form of normalized creep rate c kT=D gb Gb d=b ð Þ 3 as a function of normalized stress (s/G) on a logarithmic scale in Figure 1(a).…”

Section: Coble Creepmentioning

confidence: 99%

Deformation Mechanisms in Nanocrystalline Materials

Mohamed

Yang

2009

Metall Mater Trans A

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As a result of recent investigations on nanocrystalline (nc) materials, extensive experimental data on the deformation behavior of these materials have become available. In this article, an analysis of these data was performed to identify the requirements that a viable deformation mechanism should meet in terms of accounting for the mechanical characteristics and trends that are revealed by the data. The results of the analysis show that a viable deformation mechanism is required to account for the following: (1) an activation volume the value of which is in the range 10 to 40 b 3 ; (2) an activation energy that is close to the activation energy for boundary diffusion but that decreases with increasing applied stress; (3) the magnitudes of deformation rates that cover wide ranges of temperatures, stresses, and grain sizes; (4) inverse Hall-Petch behavior; and (5) limited ductility. The validity of available deformation mechanisms for nc materials is closely examined in the light of these requirements.

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A Model for Boundary Diffusion Controlled Creep in Polycrystalline Materials

Cited by 2,121 publications

References 9 publications

Fifty years of Harper–Dorn creep: a viable creep mechanism or a Californian artifact?

Fifty years of Harper–Dorn creep: a viable creep mechanism or a Californian artifact?

Strain rate sensitivity of nanocrystalline Au films at room temperature

Deformation Mechanisms in Nanocrystalline Materials

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