Showing papers on "Grain boundary strengthening published in 1981"
TL;DR: In this paper, the authors proposed a mechanism in which differences in the diffusion coefficients of the diffusing species along the grain boundary cause a self-sustaining climb of grain boundary dislocations and motion of their associated grain boundary steps.
286 citations
TL;DR: In this article, a thermomechanical process for grain refinement in precipitation hardening aluminum alloys is reported, which includes severe overaging, deformation, and recrystallization steps.
Abstract: A thermomechanical process for grain refinement in precipitation hardening aluminum alloys is reported. The process includes severe overaging, deformation, and recrystallization steps. Microstructural studies by optical and transmission electron microscopy of grain refinement in 7075 aluminum have revealed that precipitates formed during the overaging step create preferential nucleation sites for recrystallizing grains. The relationship between precipitate density following severe overaging and recrystallized grain density has been investigated; the results show that the localized deformation zones associated with particles larger than about 0.75 μ m can act at preferential nucleation sites for recrystallizing grains. The density of particles capable of producing nucleation sites for new grains is approximately ten times greater than the density of recrystallized grains. A close relationship between dislocation cell size after the deformation step and recrystallized grain density has also been established. Both quantities saturate for rolling reductions larger than approximately 85 pct. The grain size produced in 2.5 mm thick sheet by the optimum processing schedule is approximately 10 μm in longitudinal and long transverse directions and 6 μm in the short transverse direction.
254 citations
TL;DR: In this article, the authors investigated the sliding behavior of grain boundaries and found that an off-coincidence boundary changed into an almost exact coincidence boundary by the absorption of lattice dislocations.
Abstract: Dislocation structures of grain boundaries whose sliding behaviour had already been studied, have been observed by transmission electron microscopy of the slid grain boundaries. The ordered (coincidence) and random boundaries exhibited quite different sliding behaviour and dislocation structures. In the case of ordered boundaries, sliding was difficult and remarkable slide-hardening was shown. Transmission electron microscopy evidenced the presence of a number of EGBDs in these grain boundaries. In contrast, sliding of random boundaries easily took place showing slight slide-hardening. EGBDs were hardly observed in random boundaries. A systematic change in grain boundary misorientation during sliding was observed. An off-coincidence boundary changed into an almost exact coincidence boundary by the absorption of lattice dislocations. These results can be qualitatively explained by a dislocation model of sliding in which the absorption of lattice dislocations into the grain boundary is considered t...
210 citations
TL;DR: In this paper, the grain boundary diffusivity was evaluated as a function of temperature and the concentration behind the migrating boundaries was studied as function of the distance from the surface by microprobe measurements.
143 citations
TL;DR: In this paper, it was shown that complete crystallization of small quantities of glass segregated in grain boundaries in a ceramic, e.g. hot-pressed silicon nitride, will be more difficult than crystallizing a bulk glass of the same composition.
113 citations
TL;DR: In this article, the influence of grain size distribution on the stress-strain rate behavior of superplastic metals has been investigated for steady-state as well as transient loading situations.
112 citations
TL;DR: Grain boundary features can be grouped, according to decreasing size, as follows; grain junction voids that can form linked networks of tubules; arrays of isolated voids and inclusions along the grain boundary surface and a narrow grain boundary film of distorted crystal structure.
104 citations
TL;DR: In this paper, a method of correlating the local intergranular fracture stress with the probable composition of the grain boundary on which fracture initiated has been applied to Ni-Cr steels doped with Sb, Sn, and P so that the relative embrittling potencies of these elements can be compared.
Abstract: The previously developed method of correlating the local intergranular fracture stress with the probable composition of the grain boundary on which fracture initiated has been applied to Ni-Cr steels doped with Sb, Sn, and P so that the relative embrittling potencies of these elements can be compared. The results are discussed in terms of a hypothesis offered to rationalize the observed embrittlement effects. Experiments on the Sb-doped steel with two different intergranular Sb distributions support the position that brittle fracture in these notched specimens is controlled by the grain boundary with the maximum Sb concentration in the highly-stressed volume of material ahead of the notch.
94 citations
TL;DR: In this article, a simple model is developed which describes the reduction of the mean free path of conduction electrons in metals near a grain boundary, which leads to a decrease of the self-energy of flux lines in a layer which is considerably thicker than the perturbed zone of the boundary itself.
Abstract: A simple model is developed which describes the reduction of the mean free path of conduction electrons in metals near a grain boundary. This leads to a decrease of the self-energy of flux lines in a layer which is considerably thicker than the perturbed zone of the boundary itself. The model yields pinning forces which agree, within an order of magnitude, with recent measurements on niobium bicrystals, and with observed values of grain boundary pinning in Nb/sub 3/Sn.
93 citations
TL;DR: In this article, the effects of the stress ratio and the grain size on the fatigue crack growth near the threshold in a low carbon steel were analyzed based on the crackclosure measurement and the microscopic observations of cracktip slip deformation and the fracture surface.
84 citations
TL;DR: In this paper, a grain-boundary-dislocation mechanism for diffusion-induced boundary migration is proposed, and point-defect emission from grain dislocations is necessary for the solutio...
Abstract: A grain-boundary-dislocation mechanism for diffusion-induced boundary migration is proposed. It is suggested that point-defect emission from grain-boundary dislocations is necessary for the solutio...
TL;DR: In this paper, it was shown that a low coefficient of expansion, iron-base superalloy, IN903A, suffers severe tensile embrittlement following high temperature air exposure at 1000 °C.
Abstract: It is shown that a low coefficient of expansion, iron-base superalloy, IN903A, suffers severe tensile embrittlement following high temperature air exposure at 1000 °C. This embrittlement involves a transition to intergranular failure at low strains, with no reduction in yield strength, and is manifested in the room temperature to 800 °C range. In parallel with earlier observations on nickel-base superalloys, ductility is regained at 1000 °C. However, in contrast to these earlier results, air exposure enhances rather than hinders grain growth in the near surface regions, and, in addition, suppresses the occurrence of the jerky flow seen in vacuum-exposed material. Oxygen is demonstrated to be the damaging species, and it is show’n that boundaries are embrittled far ahead of any matrix internal oxidation. Small additions of boron are successful in eliminating the embrittlement, as they were in nickel-base alloys. The results of stress rupture tests are then reviewed, and it is concluded that the rapid failures which occur on air testing are a consequence of embrittled grain boundaries failing in tension, rather than the stress accelerated grain boundary oxidation mechanism previously proposed.
TL;DR: In this article, the tensile and creep properties of oxygen free OF- and oxygen saturated OS-polycrystalline copper have been investigated in the temperature range 25 to 500 °C.
Abstract: The tensile and creep properties of oxygen free OF- and oxygen saturated OS-polycrystalline copper have been investigated in the temperature range 25 to 500 °C. Oxygen increases the yield strength by a factor of 2 or 3, by solid solution hardening, and for coarse-grained copper, causes a severe embrittlement, particularly under creep conditions. Both direct and indirect evidence indicate that the embrittlement is caused by the segregation of oxygen to the grain boundaries in copper, thus promoting grain boundary decohesion and intergranular fracture. Auger electron spectroscopy is used to indicate the presence of oxygen at the grain boundaries in OS-Cu. The embrittling effects of oxygen are reversible in the sense that both tensile and creep ductility are restored when oxygen is removed.
TL;DR: The relationship between the strain rate and the stress in diffusional creep has usually been derived with the assumption that all the grains have the same size as discussed by the authors, but in a normal polycrystaltine solid, however, grains of widely different sizes are usually present.
TL;DR: In this article, the authors show that dislocation profiles extending from the grain boundaries, and associated with ledges on the boundary plane, increase in frequency (the number of profiles per unit length of grain boundary plane) with increasing strain.
TL;DR: In this article, the authors measured the contribution of grain elongation to the total specimen elongation by measuring the size variations of each grain (out of ~ 200 grains) along tension axis taking into account the sign of such variations.
TL;DR: In this paper, a transmission electron microscope study of grain boundary precipitation in AlZn and Al-Zn-Mg alloys was conducted with emphasis on the influence of localized boundary structure.
Abstract: A transmission electron microscope study of grain boundary precipitation in Al-Zn and Al-Zn-Mg alloys has been conducted with emphasis on the influence of localized boundary structure. Intrinsic grain boundary defects are found to have a significant effect on the precipitation sequence in that they assist the emerging precipitates in establishing a low energy habit plane relationship with at least one bordering grain. Under more extreme conditions of unavailable habits or unfavorable intrinsic structures, extrinsic defects dominate the precipitation reaction.
TL;DR: In this article, a high strength low alloy steel at temperatures ranging from 123 to 300 K and grain sizes ranging from 10 to 123 μm was used to study the fatigue threshold and fatigue crack propagation.
TL;DR: In this article, the atomic structure of vacancies in high angle grain boundaries was investigated by means of computer simulations, and the results indicated that vacancies in short-periodic grain boundaries (boundaries of good atomic fit) are localized similarly to vacancies in crystal lattices.
TL;DR: A detailed examination of the grain structure of commercial multifilamentary N3Sn-wire has been carried out by as mentioned in this paper, where an analytical description of grain growth versus reaction time and reaction temperature was determined.
Abstract: A detailed examination of the grain structure of commercial multifilamentary N3Sn-wire has been carried out This examination as well as the various electron microscopic preparation techniques used are discussed Ion-milling was found to be a suitable method especially when combined with electrolytic pre-thinning This yielded almost 100% acceptable samples The grain size of the samples examined ranged from 30 to 200 nm Based on the theory of normal grain growth an analytical description of grain growth versus reaction time,t R, and reaction temperature,T R, was determined and thus important parameters such as apparent activation energy, nucleation grain size and ultimate grain size of grain growth of Nb3Sn were obtained The grain structure itself was found to be free of dislocations or other distortions whereas outstanding grain growth textures were evident, resulting in a certain grain size distribution over the Nb3Sn area Strong evidence of abnormal grain growth of Nb3Sn grains was found depending on the heat treatment, for which some possible mechanisms are discussed Based on the results of the analytical description of grain growth versus reaction conditions and an examination of the corresponding superconducting properties a “two-stage” annealing process is proposed which hypothetically should result in improved superconducting properties in the high-field range Initial orientation examinations seem to confirm this hypothesis
TL;DR: In this paper, the authors observed that twin boundary sliding is a direct result of the mobility of extrinsic boundary dislocations collected at these interfaces through interactions with matrix dislocation, and that twin boundaries contributed approximately 1middot;6% of the total strain during steady-state creep.
Abstract: Offsets due to grain boundary sliding have been observed at previously coherent annealing twin boundaries after high-temperature power-law creep of an austenitic stainless steel (Alloy 800). On average the magnitude of these offsets was 0middot;2-0middot;3 of that measured at 'general' grain boundaries. Sliding at twin boundaries contributed approximately 1middot;6% of the total strain during steady-state creep, while sliding at 'general' grain boundaries contributed 12-22%. Observations of dislocation structures in twin boundaries after creep suggest that twin boundary sliding is a direct result of the mobility of extrinsic boundary dislocations collected at these interfaces through interactions with matrix dislocations.
TL;DR: In this article, the authors considered the problem of estimating the concentration of vacancies about a moving grain boundary and found that the vacancy concentration can be increased by grain boundary motion, regardless of whether or not the grain boundary movement occurs by means of vacancies.
TL;DR: In this article, the movement of grain boundary dislocations in a recrystallizing boundary in aluminium has been shown by the observation of oblique slip traces left behind a migrating interface.
Abstract: The movement of grain boundary dislocations in a recrystallizing boundary in aluminium has been shown by the observation of oblique slip traces left behind a migrating interface. This indicates that the moving grain boundary retains the structure of the stationary boundary. The observed movement cannot, however, completely account for the migration using a dislocation mechanism for migration.
TL;DR: In this paper, the flow stress was measured on recrystallized zinc with pronounced texture for grain sizes from 0,6 to 502 μm, both parallel and perpendicular to the rolling direction.