Showing papers on "Grain boundary strengthening published in 1972"
01 Dec 1972
TL;DR: The effect of interfaces on mechanical properties is considered in this paper, with emphasis on dislocation mechanisms and the atomic scale structure of boundaries, and Elastic and plastic compatibilities at boundaries are treated.
Abstract: The effect of interfaces on mechanical properties is considered. Elastic and plastic compatibilities at boundaries are treated. Specific influences at both low and high temperatures are discussed, with emphasis on dislocation mechanisms and the atomic scale structure of boundaries.
562 citations
01 Apr 1972-Metallurgical and Materials Transactions B-process Metallurgy and Materials Processing Science
TL;DR: In this article, a variety of alloy steels with grain sizes in the range 0.3 to 1.1 μ were obtained in manganese and nickel steels by annealing 1 to 400 hr at temperatures between 450° and 650°C (840° to 1200°F).
Abstract: Ultrafine-grained microstructures can be developed in a variety of alloy steels by coldworking followed by annealing in theα +γ region. Because the annealing temperatures are relatively low and the recrystallized structure is two-phase, grain growth is restricted. Specimens with grain sizes in the range 0.3 to 1.1 μ.m (ASTM 20 to 16) were obtained in manganese and nickel steels by annealing 1 to 400 hr at temperatures between 450° and 650°C (840° to 1200°F). The expected improvement in yield strength through grain refinement was observed in almost all alloys. Other tensile properties depend on factors such as grain size, austenite stability, and specimen geometry, that determine which of three types of plastic behavior will occur. Transformation of austenite during straining improves the mechanical properties of ultrafine-grained specimens.
311 citations
TL;DR: The effect of grain growth on stresses in films is discussed in this paper, where grain growth generally leads to stress relaxation when the film is under a compressive stress and further grain growth can occur if plastic flow relieves some of the strain energy.
Abstract: The effect of grain growth on stresses in films is discussed. Grain growth generally leads to stress relaxation when the film is under a compressive stress. When the initial grain size is below a critical value, grain growth can occur by normal boundary migration and generates a tensile stress in the plane of the film. Grain growth proceeds until a particular grain size is reached, at which the sum of the strain energy and surface energy is minimized. Further grain growth can occur if plastic flow relieves some of the strain energy. When the initial grain size is above the critical size, there is no minimum in the energy vs grain size relationship, and grain growth to infinite size can occur by normal boundary migration. In the presence of impurities, apparently anomalous intrinsic stresses in films may be produced.
273 citations
TL;DR: In this article, the diffusion-controlled coarsening behavior of precipitates situated on both high-angle and low-angle grain boundaries has been investigated theoretically, and the effect of volume fraction, φ, is included in the calculations.
262 citations
TL;DR: Extensive grain growth was observed by scanning electron microscopy in very porous Al2O3 compacts, even at densities <40% of theoretical after ∼7% shrinkage at 1700°C, the grain size increased from ∼03 to 051 μm in a compact having a relative green density of 031 as mentioned in this paper.
Abstract: Extensive grain growth was observed by scanning electron microscopy in very porous Al2O3 compacts, even at densities <40% of theoretical After ∼7% shrinkage at 1700°C, the grain size increased from ∼03 to 051 μm in a compact having a relative green density of 031 During grain growth in highly porous compacts, the grains appear initially to be chainlike, then to be oblong, and finally to be equiaxed The proposed mechanism of initial grain growth involves the filling of necks between adjacent grains followed by the movement of the grain boundary through the smaller grain Although grain growth in very porous compacts is quite different from coalescence and ordinary grain growth, the kinetics are similar
207 citations
TL;DR: A review of grain boundary dislocation in thin-film bicrystal specimens containing grain boundaries possessing different controlled geometries is given in this article, where a number of observations of grain boundaries containing excess non-equilibrium GBD's are presented.
199 citations
TL;DR: In this article, it was proposed that a grain boundary may be made of disclinations instead of dislocations which are translational defects, and the excess elastic energy for a symmetric tilt boundary whose orientation is between two low energy orientations was calculated.
171 citations
TL;DR: In this article, the influence of microstructure on room temperature and elevated temperature strength was investigated and it was found that substructure refinement was a much more potent means of strengthening at room temperature than was dispersion hardening.
165 citations
TL;DR: In this paper, it is shown that grain boundary sliding contributes additional axial strain to polycrystalline aggregates due to Lifshitz boundary sliding, where egb(L) is the grain strain due to the sliding, ed is the strain a poly crystaline aggregate would experience if only stress-directed diffusion were to occur without the necessary grain boundary separation would occur.
Abstract: Grain boundary sliding is necessary during diffusion creep to maintain continuity across the grain boundaries. In this paper it is shown, by use of a two-dimensional hexagonal array, that grain boundary sliding will contribute additional axial strain to the specimen as predicted by R. N. Stevens (1971). It is shown, however, that there is a difference between the macroscopic effect of this type of sliding and that of ‘ordinary’ grain boundary sliding, in that the former, named Lifshitz grain boundary sliding, does not increase the number of grains along the length of the specimen, whereas, the latter, named Rachinger grain boundary sliding, does. Considering only diffusion creep, it is concluded that where egb(L) is the strain due to Lifshitz grain boundary sliding, ed is the strain a polycrystalline aggregate would experience if only stress-directed diffusion were to occur without the necessary grain boundary sliding in which case grain boundary separation would occur, egb is the grain strain et...
83 citations
Abstract: Effects of additives Li2CO3, Ag2O, NiO, Cr2O3, Fe2O3, Gd2O3, Bi2O3, TiO2, MnO2, Nb3O5 and WO3 on piezoelectric and related properties of PbTiO3 ceramics are studied. Additives partly dissolve in lattice and the rest segregate at grain boundary. These additives act as grain growth inhibitors and binders. Additions of Li2CO3, NiO, Fe2O3 and MnO2 are most effective for densification. The additives Gd2O3, Bi2O3, Nb2O5 and WO3 give high resistivity probably by compensating Pb vacancies. Many piezoelectric and related properties depend on species and content of additives. Grain size is a main factor influencing these properties. The specimens with Cr2O3 and MnO2 characterized by coarse grain have small e*, small dissipation factor, short poling time, large coupling factors, high mechanical Q and small elastic compliance. These features may be explained by diminishing of stress between grains, decrease of interfacial polarization and large binding force at grain boundary.
01 Dec 1972
TL;DR: In this article, the significance of matrix and grain boundary microstructural characteristics on the mechanical properties and stress corrosion susceptibility of 7075 aluminum alloy has been evaluated and the effect of grain boundary precipitate spacing is most significant to the crack propagation stage of stress corrosion.
Abstract: The significance of matrix and grain boundary microstructural characteristics on the mechanical properties and stress corrosion susceptibility of 7075 aluminum alloy has been evaluated. Maximum strength was found to be associated with a Guinier-Preston zone matrix. The precipitate-free-zone adjacent to high angle grain boundaries had only a slight effect on yield and tensile strength but a greater influence on hardness. Stress corrosion susceptibility was studied in an aqueous chloride environment over a 0.7–3.5 pH range. For material of highest strength, grain boundary precipitate spacing was found to be of primary importance to susceptibility. The effect of grain boundary precipitate spacing is most significant to the crack propagation stage of stress corrosion. These results indicate that improved properties for Al-Mg-Zn type alloys could be attained by a desirable combination of matrix and grain boundary structure.
01 Jul 1972-Metallurgical and Materials Transactions B-process Metallurgy and Materials Processing Science
TL;DR: In this paper, a grain boundary precipitate of hard particles (SiO2 or A12O3) is used to slow down the sliding and render all boundaries identical, and isotropic.
Abstract: Grain boundary sliding can be conveniently studied by loading helical springs wound of wire with a “bamboo” structure. Such tests on pure silver coils show that identically-stressed boundaries slide at widely different rates, and that the sliding rate in any one boundary is very anisotropic. A grain boundary precipitate of hard particles (SiO2 or A12O3) slows down the sliding, and renders all boundaries identical, and isotropic (as far as grain boundary sliding is concerned). This can be understood if the grain boundary particles are thought of as hard pegs, around which silver must diffuse if sliding is to continue.
TL;DR: In this paper, the authors proposed a self-consistent method for calculating the core configuration, energy, and stress-strain field which minimizes the thermodynamic potential of a dislocated crystal.
TL;DR: In this article, a mechanism similar to that of Li for edge dislocations is proposed for formation of screw dislocation from grain-boundary ledges, and several possible consequences with respect to macroscopic plastic deformation are discussed, the most important being incubation of slip at grain boundaries in the microstrain region.
TL;DR: In this article, the authors give a further development of the 0-lattice theory with respect to secondary dislocation networks in high-angle boundaries, and show that such networks can be calculated and constructed in complete analogy to primary dislocations networks of low-angle boundary.
Abstract: This paper gives a further development of the 0-lattice theory particularly with respect to secondary dislocation networks in high-angle boundaries. It is shown that such networks can be calculated and constructed in complete analogy to primary dislocation networks of low-angle boundaries.
01 Sep 1972
TL;DR: In this paper, electron microscopy, quantitative optical microscopy and texture studies were made on swaged and recrystallized titanium wire of three impurity contents: zone refined, a special lot of intermediate purity, and commercial A-70.
Abstract: Transmission electron microscopy, quantitative optical microscopy, and texture studies were made on swaged and recrystallized titanium wire of three impurity contents: zone refined, a special lot of intermediate purity, and commercial A-70. The electron microscopy studies revealed that a) during recrystallization a number of processes overlap, and b) during grain growth there occurs a decrease in the dislocation density within the grains along with the increase in the average grain size. The quantitative microscopy studies indicated that the linear intercept grain size distribution is approximately log normal and that for a given mean grain size the distribution is relatively independent of the combination of annealing time and temperature used to obtain it. Moreover, there exists a range of grain sizes in space, the numbers of grains in each class interval changing with increase in grain size. The so-called grain shape factor decreases with increase in mean grain size (annealing time) at a constant temperature and with decrease in temperature for a constant grain size. The texture of the as-swaged wire and the changes in the texture during grain growth are in qualitative accord with those previously reported for deformed and recrystallized titanium. Impurity content influences the degree of these various structural characteristics but not their substance.
TL;DR: Grain boundaries of polycrystalline copper deformed under creep conditions have been studied in a 1000 kV electron microscope as mentioned in this paper, where two boundaries show dense dislocation arrangements, while high-angle boundaries are almost dislocation-free.
Abstract: Grain boundaries of polycrystalline copper deformed under creep conditions have been studied in a 1000 kV electron microscope. Twin boundaries show dense dislocation arrangements, while high-angle boundaries are almost dislocation-free. Cavities are observed in the very early stages of formation. They are always found associated with obstacles in the grain boundaries such as triple points and kinks. These results are discussed in terms of a previously proposed model for grain-boundary sliding. The implications for the deformation of a polycrystalline material at various temperatures are also considered.
01 Nov 1972-Metallurgical and Materials Transactions B-process Metallurgy and Materials Processing Science
TL;DR: In this article, the critical nucleus is modeled in simple fashion, based upon a rectangular parallelepiped, and the rate of nucleation by the conventional mechanism, which is essentially immobile during the nucleation process, far exceeds that by the Tu-Turnbull "pucker" mechanism, in which the grain boundary is deflected so that its plane is parallel to the habit plane of the nucleus.
Abstract: Both the nucleation process per se and the organization of nearby nuclei or precipitates formed at a given disordered grain boundary into a viable cell structure are considered. When the critical nucleus is modeled in simple fashion, based upon a rectangular parallelepiped, the rate of nucleation by the “conventional” mechanism, in which the grain boundary is essentially immobile during the nucleation process, far exceeds that by the Tu-Turnbull “pucker” mechanism, in which the grain boundary is deflected so that its plane is parallel to the habit plane of the nucleus. A more rigorous model of the critical nucleus, based upon the somewhat specialized assumption that an energy cusp facet forms at only one boundary orientation but developed without a preconceived view of the nucleus morphology, leads to this result only when the facet energy is greater than one-half the energy of a disordered grain boundary. In the reverse energetic situation, the nucleus morphology is effectively that supposed by the pucker mechanism. The initial stage of cellular growth is examined in the framework of the question: why do allotriomorphs form at disordered grain boundaries under some conditions of alloy composition and temperature and cells develop under other conditions? The conditions for the two reaction paths are established on the basis of two key ideas: the direction in which the torque term associated with a facet deflects a grain boundary meeting the edge of the facet (deduced from the considerations of Hoffman and Cahn), and the existence of a driving force for the breakaway of a grain boundary from such a junction resulting from the requirement of continuity of path to another junction located nearby.
01 Feb 1972-Metallurgical and Materials Transactions B-process Metallurgy and Materials Processing Science
TL;DR: In this article, it was shown that P, Sn, Ni, and Cr were segregated at prior austenite boundaries in the steels exposed to 750° and 850°F for times up to 35,000 hr.
Abstract: Four commercial purity Ni-Cr-Mo-V steels of closely comparable bulk chemistry and grain size, but tempered to various strength levels, were embrittled by exposure at 600°, 750°, and 850°F for times up to 35,000 hr. Maximum temper embrittlement occurred at 850°F in all steels. Severe cases of embrittlement resulted in a marked decrease in tensile ductility and an intergranular tensile fracture. Auger electron emission analysis showed that P, Sn, Ni, and Cr were segregated at prior austenite boundaries in the steels exposed to 750° and 850°F. Increased segregation of phorphorus and tin was always accompanied by increased segregation of nickel and chromium. The severity of grain boundary segregation increased with increasing values of fracture transition temperature. Despite comparable bulk chemistry and grain size, the degree of segregation was different in different steels. Under exposure conditions causing severe embrittlement, the FATT values displayed a strong dependence on the strength level of the steel. In a giyen steel, while the composition and morphology of carbides at austenite boundaries were the same as in the matrix, the density and size of carbides were much higher at the austenite boundaries. The preference of these boundaries as fracture sites would seem to arise from two considerations, namely, a high degree of impurity and alloy element segregation and the fact that the density and size of carbides at these boundaries is higher than that in the matrix.
TL;DR: In this article, the effect of second phase particles on grain boundary sliding has been observed in shear tests of bicrystal specimens, and evidence indicating a yield stress for the onset of sliding was obtained and the magnitude of this stress was related qualitatively to the density of grain boundary particle coverage.
IBM1
TL;DR: In this paper, the relationship between grain boundary contributions to electrical resistivity in terms of specific grain boundary models is discussed. And analytical expressions are presented to allow extraction of the boundary contributions.
TL;DR: In this article, it is shown that if a polycrystalline material is to deform in a coherent manner without the development of voids, grain boundary sliding is necessary as well as relative motion normal to the boundaries by diffusion.
TL;DR: In this article, the authors analyzed the long range stress field introduced as a result of grain boundary deformation by the passage of crystal glide dislocations using the concept of grain boundaries.
01 Mar 1972-Metallurgical and Materials Transactions B-process Metallurgy and Materials Processing Science
TL;DR: In this article, the effects of environment and grain size on the steady-state creep and creep rupture properties of Ni-6 pct W solid solution are examined by testing in vacuum and commercial purity argon at 5000 psi and 900°C.
Abstract: The effects of environment and grain size on the steady-state creep and creep rupture properties of a Ni-6 pct W solid solution are examined by testing in vacuum and commercial purity argon at 5000 psi and 900°C. The steady-state creep rate is found to decrease with increasing grain size at small grain sizes, both in vacuum and argon, owing to the effects of grain boundary sliding. At large grain sizes the creep rate is independent of grain size in vacuum and increases with grain size in argon. It is suggested that the increase in creep rate with increasing grain size is associated with fact that large-grained samples tested in argon do not reach steady-state before rupture occurs.
TL;DR: In this article, the experimental observations on the creation and annihilation of point defects and dislocations at high-angle grain boundaries are reviewed and atomistic models that have been proposed to explain the observations are compared with the experimental results.
Abstract: The experimental observations on the creation and annihilation of point defects and dislocations at high-angle grain boundaries are reviewed. The atomistic models that have been proposed to explain the observations are reported and are compared with the experimental results. The interaction between two-dimensional lattice defects (stacking faults and coherent twin boundaries) and high-angle grain boundaries is considered. The theories that have been put forward to explain the formation of coherent twin boundaries (formation of annealing twins) during grain boundary migration are critically reviewed.
TL;DR: The relationship between grain size and flow stress at 1% strain has been examined between 77 and 450 K in copper and in an extensive range of binary copper-base solid solutions as mentioned in this paper, and the strengthening effect of the grain boundaries can be described by the empirical relationship σ = σ 0 + k d − 1 where σ0 and k depend on the type and concentration of solute.
Abstract: The relationship between grain size and flow stress at 1% strain has been examined between 77 and 450 K in copper and in an extensive range of binary copper-base solid solutions. The strengthening effect of the grain boundaries can be described by the empirical relationship σ = σ 0 + k d − 1 where σ0 and k depend on the type and concentration of solute. The stress at infinite grain size, σ0, increases significantly with increasing strain but the slope, k, tends to decrease; k varies only slightly with temperature. The strengthening effects of grain boundaries are qualitatively consistent with a model based on a greater dislocation density in deformed fine-grained, as compared to co...
TL;DR: In this paper, the influence of grain size on the deformation behavior of a-zirconium in the temperature range 77° − 443°K has been investigated, and the flow stress variation with grain size follows the Hall-Petch equation σ = σi + K d−1/2.