The interpretation of grain boundary offsets during creep
About: This article is published in Scripta Metallurgica.The article was published on 1976-10-01. It has received 11 citations till now. The article focuses on the topics: Grain boundary diffusion coefficient & Grain boundary strengthening.
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TL;DR: Sintering occurs when packed particles are heated to a temperature where there is sufficient atomic motion to grow bonds between the particles as mentioned in this paper, and the conditions that induce sintering depend on the material, its melting temperature, particle size, and a host of processing variables.
Abstract: Sintering occurs when packed particles are heated to a temperature where there is sufficient atomic motion to grow bonds between the particles The conditions that induce sintering depend on the material, its melting temperature, particle size, and a host of processing variables It is common for sintering to produce a dimensional change, typically shrinkage, where the powder compact densifies, leading to significant strengthening Microstructure coarsening is inherent to sintering, most evident as grain growth, but it is common for pore growth to occur as density increases During coarsening, the grain structure converges to a self-similar character seen in both the grain shape distribution and grain size distribution Coarsening behavior during sintering conforms to classic grain growth kinetics, modified to reflect the evolving microstructure These modifications involve the grain boundary coverage due to pores, liquid films, or second phases and the altered grain boundary mobility due to these phases
226 citations
TL;DR: In this paper, the processes that occur at grain boundaries during high temperature creep at low strain rates have been studied by scanning electron microscopy of a fiducial grid and by transmission electron microscope of pure aluminium.
Abstract: The processes that occur at grain boundaries during high temperature creep at low strain rates have been studied by scanning electron microscopy of a fiducial grid and by transmission electron microscopy mainly of pure aluminium. These and other observations are interpreted in terms of a dislocation model for grain boundary behaviour during high temperature creep.
120 citations
TL;DR: In this paper, an enhanced diffusion through an activated layer at the grain boundaries has been modeled for the case of tungsten sintered with transition element additions, where both constant heating rates and isothermal sintering are considered.
Abstract: Activated solid-state sintering relies on the addition of low concentrations of grain boundary segre-gating species to increase diffusion rates. In this article, enhanced diffusion through an activated layer at the grain boundaries has been modeled for the case of tungsten sintered with transition element additions. Both constant heating rates and isothermal sintering are considered. As in classical treatments, sintering is divided into three stages, but modifications are proposed based on recent observations and theories regarding packing coordination, pore morphology, pore location, grain growth, and pore-grain boundary separation. The intermediate and final stages of sintering are al-lowed to overlap based on the amount of closed porosity to account for both pore closure early in the process and the gradual increase in packing coordination with densification. Mean curvature theory is used to estimate pore curvature during the intermediate stage of sintering. In the final stage, pores are modeled on both the corners of a tetrakaidecahedron and on its square facets. The pore location has only a small effect on densification, while the grain boundary mobility is more of a factor. The model allows pore-grain boundary separation to match experimentally measured grain sizes. The model predictions are compared to dilatometer curves of pure tungsten and tungsten sintered with additions of Co, Fe, Ni, and Pd. For the Co- and Fe-activated samples, the model is modified to account for an increase in diffusional activation energy due to dissolution of the activator in tungsten.
60 citations
TL;DR: In this article, the authors show that at these low rates of deformation, the transport of matter occurs by the migration of vacancies rather than by the glide of dislocations.
Abstract: The creep rate in a land-based power station must be less than 10−11 s−1. At these low rates of deformation the transport of matter occurs by the migration of vacancies rather than by the glide of dislocations. A quantitative understanding of these diffusional processes is, therefore, important. First type of diffusional creep (Nabarro-Herring (N-H)): the sources and sinks of vacancies are grain boundaries. The vacancies may diffuse through the bulk of the grain or along the grain boundaries (Coble (C)). Second type (Harper-Dorn (H-D)): the vacancies diffuse from edge dislocations with their Burgers vectors parallel to the major tensile axis to those with Burgers vectors perpendicular to this axis. The coherence of the polycrystalline aggregate is maintained by sliding along the grain boundaries. The three mechanisms of vacancy migration, grain boundary sliding, and dislocation glide may all interact. The theories of N-H and C creep in pure metals are established and confirmed, but H-D creep and grain boundary sliding are less well understood.
55 citations
TL;DR: In this paper, a detailed and quantitative investigation of the stress-state dependence of superplastic cavitation in fine-grained aluminum alloys has been carried out to develop clear evidentiary support to build future models.
Abstract: A detailed and quantitative investigation of the stress-state dependence of superplastic cavitation in fine-grained aluminum alloys has been carried out to develop clear evidentiary support to build future models. Several stress states, such as uniaxial tension, plane-strain tension, plane-strain compression, shear, and equibiaxial tension have been examined. Tests were carried out to large strain in an interrupted manner under a constant effective strain rate (
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$$\dot \varepsilon $$
e
) in the range of 10−4 to 10−2 s−1. Measurements of volume fraction, population density, and size distribution of cavities, made by image analysis via optical microscopy, show continuous emergence of new cavities as well as growth of cavities during superplastic straining. The total cavity volume fraction (V) increases exponentially with strain. The cavity growth rate, represented by η (equal to d ln V/de
e
), as well as the cavity population evolution rate with strain (dN
c
/de
e
, where N
c
is the cavity number/unit area) are found to increase with normalized mean hydrostatic tensile stress (σ
m
/ σ
e
). An empirical equation for the biaxial forming limit in terms of the principal surface strains (e
1 and e
2) has been defined for a fixed cavity volume, as given by e
1=a V
b
− α e
2, where a and b are constants determined from e
1 values for plane strain (e
2=0). The value of b is found to be 0.2 to 0.3, and α is 0.4 to 1.0.
21 citations
References
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TL;DR: In this paper, a finite-element computational method was used to model the creep of polycrystalline solid by attributing a Newtonian-viscosity to the grain boundaries and a power-law viscosity to the interior.
238 citations
TL;DR: This article measured grain boundary sliding on polycrystalline specimens of Magnox AL80, a magnesium-0.78 wt% aluminium alloy, at successive strains during creep at 200° C under a stress of 2800 psi.
Abstract: Measurements of grain boundary sliding have been made on polycrystalline specimens of Magnox AL80, a magnesium-0.78 wt% aluminium alloy, at successive strains during creep at 200° C under a stress of 2800 psi. Three independent methods were used to determine the strain due to sliding (egb) at the surface and two to determineegbin the interior of the specimens. The one direct method of measuringegbin the interior used oxide markers introduced by extruding a composite billet. The values ofegbobtained from the offsets in these interior markers were found to agree with those given by the three sets of measurements made on the surface, but not with those from the indirect method for the interior which relies on the measurement of grain strain via grain shape changes.
27 citations
TL;DR: In this paper, the creep resistance of a two-dimensional system of hexagonal grains is analyzed for both Nabarro-Herring and grain-boundary sliding rate-controlling mechanisms.
Abstract: The creep resistance of a two-dimensional system of hexagonal grains is analysed for both Nabarro-Herring and grain-boundary sliding rate-controlling mechanisms Both mechanisms are shown to operate sequentially When one is much faster than the other the shear and normal stresses on the boundaries are shown to differ from those in an elastic body The creep rate is related to the boundary stresses and is shown to be independent of orientation of the uniaxial applied stress to the hexagonal array The grains are allowed to roll over neighbouring grains and it is shown that this can increase the creep rate The conditions under which grains can exchange neighbours are discussed, and also the effect of the resulting irregular shapes on the creep rate
25 citations
TL;DR: In this paper, the relative displacement of grains across a common grain boundary during diffusional creep deformation is determined by using marker line displacement measurements across the grain boundary, where the only measurements necessary are of marker line displacements across a grain boundary.
Abstract: A procedure is presented which allows the determination of the relative displacement of grains across a common grain boundary during diffusional creep deformation. This relative displacement comprises the strain produced by accretion of material at the grain boundary and by grain-boundary sliding. The only measurements necessary are of marker line displacements across the grain boundary.
9 citations
3 citations