Showing papers on "Grain boundary strengthening published in 1983"

Grain boundary sliding and stress concentration during creep

Abstract: Importance of grain boundary sliding to creep intergranular fracture is focussed. Previous metallographic and fractographic studies of creep intergranular fracture on metal bicrystals and polycrystals are briefly reviewed in order to show the close relationship between grain boundary sliding and fracture. Deformation ledge and migration irregularity are shown to be potential sites of stress concentration and crack nucleation on sliding grain boundaries without particles. The effect of grain boundary structure on creep intergranular fracture is discussed on the basis of the effect of grain boundary structure on sliding, the contribution of sliding to the overall creep deformation, and a sliding-fracture diagram. Recent observations of the effect of grain boundary structure on creep intergranular fracture on alpha iron-tin alloy polycrystals are shown.

Microstructure, tensile deformation, and fracture in aged ti 10V-2Fe-3Al

Abstract: In the /3-Ti alloy Ti-10V-2Fe-3Al a variety ofα-andω-aged microstructures with different yield stresses was established by combinations of forging and heat treatment. Tensile tests have shown that plastic deformation and fracture are strongly influenced by the morphology, size, and volume fraction of the different types of a-phase (primary a, secondaryα, grain boundaryα), as well as by the-phase. A detailed microscopical study revealed several deformation and fracture modes. It appears that at several sites stress and strain concentrations and subsequent void nucleation can occur and that the quantitative combinations of the differentα-types determine which sites are active. The dominant deformation mode for the (α +gb) solution treated andα-aged conditions was a strain localization in theα-aged matrix leading to voids at the interface between aged matrix and primary a-phase. In case of theβ-solution treated andα-aged microstructures the grain boundaryα leads to a strain localization in the softα-film and to void nucleation at grain boundary triple points at low macroscopic strains. Based on the above mechanisms it is discussed in detail how varying size, volume fraction, and morphology of theα-phase affect the ductility. The embrittling effect ofω-particles can be largely reduced by a grain refinement.

Helium bubble nucleation at grain boundaries

Abstract: Helium bubble nucleation has been studied at grain boundaries in a ternary austenitic steel after helium implantation in the temperature range 450–600°C. Transmission electron microscopy shows that all interfaces except the coherent twin are preferred nucleation sites for bubbles, and that the density of bubbles is greater on interfaces containing resolvable grain boundary dislocations (GBDs), A model has been developed which accounts for the bubble density as a function of GBD spacing. This model implies that there is a positive binding energy between He and GBDs until the dislocation spacing becomes very small. The diffusion coefficient of helium along GBDa is much lower than that arising from interstitial diffusion in the lattice. It appears that helium migrates in grain boundaries and along GBDs by a slow vacancy-type mechanism.

On the structure of twist grain boundaries in ionic oxides

Abstract: Recent calculations on coincidence twist grain boundaries in oxides with the rocksalt structure have shown that the interfaces are only weakly bound. This is in contrast with experiment in which twist grain boundaries have been observed in MgO. The static lattice relaxation calculation presented here predicts a restructured stable [001] twist boundary in NiO. The results are equally applicable to the other oxides with the same structure.

The effect of grain size on deformation twinning in a textured zinc alloy

Abstract: The incidence of deformation twinning in samples of a commercial rolled zinc-0.1% aluminium-0.05 wt % magnesium alloy has been measured at room temperature as a function of strain, grain size and direction of loading relative to the rolling direction. The volume fraction of twinned material has been measured at the surface and in the bulk. It is shown that this volume fraction is a linear function of both strain and grain size in most cases. Bands of heavily twinned grains are found to form inhomogeneously across the gauge length of tensile specimens, by an autocatalytic mechanism. Several examples are given of the interaction of twins and slip bands at grain boundaries which illustrate the formation and the accommodation of twins. The smaller volume fraction of twins found at the surface compared with the bulk reflects a relaxation of Von Mises criterion.

The Effect of Grain Size on Fatigue Growth of Short Cracks

Abstract: The influence of alloy grain size on growth rates of surface cracks 20 to 500 μm in length was studied in Al 7075-T6 specimens prepared in 12 and 130 μn grain sizes. Grain boundaries temporarily interrupt the propagation of cracks shorter than several grain diameters in length. Linear elastic fracture mechanics is inadequate to describe resulting average growth rates which must instead be characterized as a function of cyclic stress amplitude, σa, and alloy grain size as well as stress intensity range, σK. These observations are rationalized using two models, one that relates crack closure stress to alloy grain size, and a second that relates the development of microplasticity in a new grain in the crack path to grain size. In addition, growth rates were found to be faster in fully reversed loading than in tension-tension loading, especially in the large grained material. Evidence is presented to demonstrate that this is a consequence of the fatigue induced development of a compressive residual surface stress during tension-tension loading. These complex effects, and the role of grain size in determining short crack growth, are discussed.

Helium trapping at dislocations, precipitates and grain boundaries

Abstract: Helium bubble formation was observed in austenitic stainless steels by transmission electron microscopy following implantation of 30 to 1000 appm helium at room temperature and annealing at 700 to 800°C. Helium bubble distributions at dislocations and at various grain boundaries and precipitates were studied. It was found that interfacial dislocations play a dominant role in bubble nucleation at grain and interphase boundaries but not at Tic-matrix interfaces. Particularly high trapping of helium was observed at Tic precipitate-matrix interfaces which is attributed to an inhomogeneous ripening mechanism.

Structural width of low-angle grain boundaries in olivine

Abstract: Electron diffraction and imaging techniques have been used to examine the structure and thickness of low-angle grain boundaries in olivine. For edge-on grain boundaries, a periodic array of dislocations in a boundary gives rise to sets of periodic rods of diffracted intensity that are recorded as streaks in the diffraction pattern. The length of these streaks provides a measure of the structural width of a boundary. For olivine, the structural width of a boundary with a given dislocation spacing is three to four times smaller than that in metals where the boundary width is approximately equal to the spacing between dislocations in the boundary.

The plastic deformation of polycrystals I. Aluminium between room temperature and 400°C

Abstract: The tensile flow stressσ∊ has been studied for super-pure and commercial aluminium and for an Al-21/4% Mg alloy in relation to strain, grain size d, subgrain or cell size and dislocation density ρ. At room temperature and 200°C, the flow stress depends on both ρ1/2 and d −1/2. This is interpreted an evidedce of a requirement inσe that slip can be transmitted across a grain boundary. There is extensive dislocation annihilation during deformation at these two temperatures, particularly in the two aluminiums and particularly in fine grains. The behaviour of the commercial aluminium is unusual. At room temperature, the retained ρ is higher in coarse than in fine grains. However, the flow stress remains higher in the fine grains because of the slip transmission requirement. At 200°C, the dislocation density is so much higher in the coarse grains that the flow stress is also higher. At 400°C, most of the dislocations are annihilated and sharp subgrains form with planar network boundaries. It is propose...

The role of Coble creep and interface control in superplastic Sn-Pb alloys

Abstract: The creep behaviour of superplastic Sn-2 wt% Pb and Sn-38.1 wt % Pb is investigated at temperatures between 298 and 403 K and for grain sizes between 2.5 and 260μm. In Sn-2 wt% Pb with grain sizes larger than ∼ 50 μm, diffusion-controlled Coble creep is found and it is experimentally shown that this type of creep is inhibited in smallgrained specimens. Measurements covering low stresses (∼ 0.1 MPa) and strain rates (∼ 10−10 sec−1) rule out any explanation which relies on a threshold stress for plastic deformation. The observations are explained by a model in which, at low stresses or small grain sizes, Coble creep is rate-limited not by diffusion of vacancies but by the rate of emission and absorption at the curved dislocations in the grain boundaries which are the ultimate sources and sinks of vacancies.

Effect of grain size on creep rate in type 316 stainless steel at 873 and 973 K

Abstract: The influence of grain size on creep rate in AISI type 316 stainless steel has been investigated at 873 and 973 K over a wide range of applied stresses. Grain boundaries contribute to strengthening at high stresses (180–260 MN m−2) at 873 K but this strengthening does not correlate with the available models which attempt to incorporate the Hall–Petch strengthening effect into high temperature strain rate equations. The importance of loading strain in determining creep rate has been indicated. The creep rate was generally constant at 973 K but increased at small grain sizes and at lower stresses and this has been attributed to the increased contribution of grain boundary sliding at fine grain sizes. The difference in the grain size effects on creep at the two temperatures is attributed to the difference in the substructures developed during creep.

Non‐Equilibrium State and Recovery of Grain Boundary Structure

Abstract: The non-equilibrium state of high-angle grain boundary structure caused by the presence of grain boundary defects of dislocation nature is analysed. Such a state is characterized by the presence of long-range elastic fields of the boundary and/or by mechanical instability of the grain boundary structure. The creation of a non-equilibrium grain boundary structure and its recovery to an equilibrium one is considered within the framework of the crystallogeometrical approach for the case of interaction between lattice dislocations and grain boundaries. [Russian Text Ignored].

Hall–Petch relationships of iron and steel

Abstract: The Hall–Petch relationships of iron and steel for the yield σy and flow σf stresses are investigated experimentally and theoretically. A simple theory is presented for the coupling between the slopes of the plots σy–d−1/2 and σf–d−1/2, where d is grain diameter. The physical meaning of the so called ‘intragranular friction stresses’ evaluated from the Hall–Petch plots is critically discussed.

Grain boundary sliding model for superplastic deformation

Abstract: Comparison of the data for superplastic Pb–Sn eutectic with the existing theories on diffusion accommoda ted grain boundary sliding shows mixed agreement. In the superplastic region the observed deformation rates are about 102–103 times faster than Coble creep. In the present study, this discrepancy is resolved by a physically realistic three-dimensional grain rearrangement model for superplastic deformation. This model assumes that the ‘macroaccommodation’ during superplastic flow results in grain rearrangement due to grain boundary sliding and ‘microaccommodation’ at triple points is diffusional. The proposed model carries all the structural features of superplastic deformation and the constitutive equation predicts faster strain rates than the existing theories on diffusion accommodated grain boundary sliding. The model gives an activation energy equal to that for grain boundary diffusion. The model predicts the observed grain size dependence of the strain rate and also a threshold stress for s...

Cleavage fracture of low carbon ferritic steels with fine grain size

Abstract: Micromechanisms of transgranular cleavage fracture in ferritic steels have been investigated, particular attention being paid to the dependence of cleavage fracture stress σf on grain size over the range from 3·5 to 34·3 μm. It is found that σf increases less than linearly with d−1/2, where d is the grain diameter, whereas the Hall–Petch relation holds for the yield stress. Based on the experimental results, where σf was measured in the steels and their grain size and carbide size were changed independently, it is suggested that in the grain size range cited, the predominant mechanism was the propagation of a microcrack, which formed in the discrete carbide, into contiguous ferrite grains. The measured values of σf were discussed in a quantitative manner based on a model into which an equilibrium distribution of dislocations produced by the double pile-up mechanism was incorporated.

Observations of plane-matching grain boundaries by electron channelling patterns

Abstract: Observations of {110} plane-matching grain boundaries have been made by electron channelling patterns (ECPs) for the recrystallization structure of an Fe-3% Si alloy. It has been found that the {110} plane-matching grain boundary plays an important role in the grain growth during recrystallization of the alloy. The grains whose surfaces were near {100} preferentially grew by the migration of a {110} plane-matching grain boundary at the expense of the matrix grain whose surface was near {111} in the direction parallel to {110} plane. The anisotropy of the migration of the {110} plane-matching grain boundaries has been found. A step mechanism is suggested to explain the observed anisotropy of grain boundary migration.

Effect of annealing temperature on structure and properties of fine-grained aluminium

Abstract: Ultrafine-grained Al samples of purity grades 2N7 and 4N were prepared by hydrostatic extrusion. The changes in structure and properties in the grain growth regime after annealing were studied in terms of the role of grain boundary structure. Important differences in grain growth kinetics were observed at annealing temperatures higher than 620 K and grain growth is shown to be dependent on the predominant type of grain boundary. This interpretation is supported by the results of mechanical testing and Hall–Petch analysis, leading to the conclusion that the parameters σ0 and k are more strongly dependent on grain boundary structure than on material purity.