Showing papers on "Grain boundary strengthening published in 1970"

Grain boundary sliding as a deformation mechanism during creep

Abstract: A model for grain boundary sliding is developed in which sliding occurs by the movement of dislocations along, or adjacent to, the boundary by a combination of climb and glide. Under these conditions the strain rate due to sliding is proportional to [sgrave]2/d, where [sgrave] is the applied stress and d is the average grain diameter. It is shown that reports in theliterature of enhanced creep rates at low stresses and/or small grain sizes may be explained by assuming that the various deformation mechanisms, including sliding, operate independently.

The role of dislocations in the flow stress grain size relationships

Abstract: Calculations involving pile ups of dislocations, both analytical and numerical, using either discrete dislocations or continuous distribution of dislocations of infinitesimal Burgers vectors, are reviewed in the light of their effects on the relation between yield or flow stress and grain size. The limitations of the pileup models are discussed and some nonpileup theories of yielding are critically reviewed also. More critical experiments are still needed to reveal the fundamental mechanicm of yielding.

The influence of polycrystal grain size on several mechanical properties of materials

Abstract: The ductile-brittle transition, hardness, fatigue, and creep behavior of polycrystalline materials are known to be influenced under certain conditions by the polycrystal grain size. These properties have been correlated, historically, with the material stress-strain behavior. The (Hall-Petch) stress-grain size relations are useful for describing the complete stress-strain behavior for polycrystals and, therefore, these relations provide a reference for understanding the manner in which these other properties should also depend on the grain size. In some cases, the grain size dependence of a particular property follows directly from this connection.

The Hall–Petch Relation and High-Temperature Subgrains

Abstract: Although originally proposed for high-angle boundaries, a relation of the Hall–Petch type has been applied by a number of authors to low-angle boundaries produced by high-temperature deformation. Investigations published to date include data on commercially pure Al and Fe and on silicon steel; in these studies room-temperature testing showed that the following relation applies: S = S 0′ + K′d−1/2. Here S represents the mechanical property being measured, d is the mean sub grain size, and S 0′ and K′ are empirical constants. However, this relation has previously been applied only to small ranges of sub grain size, and neither S 0′ nor K′ has any fundamental significance. A rationalization is proposed in which the published observations are reinterpreted in terms of sub-boundary strengths that depend on sub-boundary misorientation and perfection and therefore, indirectly, on sub grain size. According to this view, the relatively perfect sub-boundaries formed at creep strain rates have low strengths;...

Grain size effects in the strain hardening of polycrystals

Abstract: The degree to which the flow stress of a polycrystal is sensitive to grain size is discussed in terms of the distribution of slip and dislocation structure that develops in the vicinity of grain boundaries as deformation proceeds. The point of view is taken that the two principal classes of grain boundary hardening models, namely, those based on dislocation pile-ups and those based on dislocation density concepts respectively represent special cases of a single rationale developed in this paper. Grain boundary strengthening is intimately related to strain hardening which is affected by slip mode,i.e., the number of slip systems and the ability to cross slip. The effects of substitutional solute elements on grain boundary strengthenings is considered to be a consequence of their influence on slip modes rather than on their interaction with dislocation sources.

Grain boundaries as sinks for dislocations

Abstract: A thermal grooving technique has been used to measure the relative grain boundary energy of deformed and annealed copper. The relative grain boundary energy after annealing at 600°c is a function of amount of previous strain, but is always higher than that of material annealed at 1000°c. Annealing at 900°c leads to a return of the energy towards the equilibrium value. On the basis of the above results a mechanism is proposed for the action of grain boundaries as a sink for dislocations.

Structure and energy of grain boundaries: Application to symmetrical tilt boundaries around [100] in aluminium and copper

Abstract: On the basis of the classical assumption of interatomic central forces, a method is proposed for the computation of the structure and energy of a grain boundary: It is applied to the particular case of symmetrical tilt boundaries around [100] in Al and Cu. The resulting curve shows that the interfacial energy is almost independent of the angle θ except for values near 0° and 90° and for high density coincidence boundaries.

Grain boundary resistivity of aluminium

Abstract: Residual resistivity at 4 K has been studied as a function of grain boundary area per unit volume in aluminium of two differing purities. A positive correlation between residual resistivity and boundary area was observed above a minimum boundary area per unit volume. The resistivity attributable to the presence of grain boundaries was found to reflect both the defect structure of the boundaries and the solute redistribution due to boundary segregation. Elimination of the solute effect yielded a specific boundary resistivity of 1·35±0·5 × 10−12 ohm-cm2 for pure boundaries. Evidence of a purification effect due to grain boundary segregation was noted.

Structure of grain boundaries

Abstract: The present discussion is concerned with the more relevant experimental studies, performed since about 1952, that provide evidence on the structure of large angle boundaries. These experiments include diffusion, mobility, energy, and field ion studies of grain boundaries. It is shown that the results of these experiments provide support for a model that can be described quite accurately in terms of coincidence in the boundary plane, a dislocation array, and a ledge structure.

Surface and interior measurements of grain boundary sliding during creep

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.

Observation of Grain Boundaries with the Field‐Ion Microscope

Abstract: The information which can be gained from field‐ion micrographs showing grain boundaries is considered critically. An attempt is made to quantify the factors governing the ``ring'' configuration across both low‐angle and high‐angle grain boundaries. It is shown that spiral configurations can be generated in the absence of grain boundary dislocations. Some potential applications of the grain boundary contrast theory are indicated.

The grain size dependence of the yield, flow and fracture stress of commercial purity silver

Abstract: Heavily cold-rolled sheet material of 99.9 pct purity Ag has been recrystallized at varying temperatures to give average grain diameters,l, in the range between 1 and 60 μ. For this material, the yield stress, flow stress at several strain values, and fracture stress follow the Hall-Petch relation: $$\sigma _\varepsilon = \sigma o_\varepsilon + k_\varepsilon l^{ - 1/2} $$ whereσ e is the flow stress at a particular value of strain, e, ands o e andk e are the experimental constants appropriate to a particular strain value. The range in grain size obtained for this material was sufficiently large to determine that silver can be appreciably strengthened by grain size refinement and that several other relations previously suggested to relate the stress and grain size could be discounted. The finest grain sizes were measured from replicas of etched specimens as viewed with the electron microscope. It is proposed that this type of grain size strengthening may be responsible for the exceptional strength which occurs in certain films of silver fabricated by vapor deposition techniques.

Annealing Kinetics of the Induced Anisotropy in Ni–Fe Films: The Role of Atom Self‐Diffusion in Grain Surface Regions

Abstract: A theory is given which seems to explain the main aspects of the annealing properties of the induced anisotropy in thin Permalloy films. The theory concerns the main anisotropy component which is very likely due to directional atom pair ordering only (the other being of magnetostrictive origin). The theory is based on the high atom mobility existing usually, in thermal equilibrium conditions, in the vicinity of the grain boundaries and grain surfaces. A kinetic law is derived for the case of annealing not producing grain growth. The phenomenon involved is in this case the evolution of the directional atom pair order in the vicinity of immobile grain boundaries and surfaces. A good agreement is found with experimental data. A second important phenomenon is the effect of the grain boundary migration during grain growth on the induced anisotropy of the film regions swept by the boundaries. This effect has been also computed in a more qualitative way and seems able to explain the irreversible anisotropy evolu...

Influence of cold work and recovery on the strength and toughness of iron

Abstract: In this work some of the structures typical of those found in thermomechanically processed steels were reproduced by cold work, cold work and recovery, and recrystallization treatments of vacuum-melted iron single-crystals and polycrystals. The mechanical properties of the microstructural features such as subgrain formation, texture development, and grain elongation were recorded. It was shown that although the dislocation subboundaries produced on recovery add an increment of strength to that produced by grain boundaries, they are less effective strengtheners than high-angle grain boundaries. Further, the data suggests that yield strength is related not only to subgrain size but also to the angle of misfit of the subgrain boundaries. Although strength increased with subgrain boundary formation, toughness remained constant. Consequently, the introduction of subgrain boundaries offers a means of improving strength while maintaining toughness. The ductile-to-brittle transition temperature of cold-worked as well as cold-worked and recovered polycrystalline iron varied with specimen orientation relative to the direction of deformation. These variations were primarily a function of the anisotropy of grain dimension that is produced by cold deformation. Toughness was not influenced by the preferred orientations produced by the various processing techniques.

Geometry of grain boundaries

Abstract: The first part of this paper deals with the problem of describing and measuring microstructure in exact terms. The Euclidean parameters: volume, area, length, and angle can be measured and expressed only in terms of the total of each in unit volume. Average properties, such as average grain diameter, are accessible only through the topological parameters, specifically number in unit volume, which can be measured only by serial sectioning. The parameters which have been used to represent the concept of grain size are analyzed and shown in most cases to represent a function of grain boundary area. In a second section of this paper the geometric problem of plastic slip through a grain boundary is analyzed. A method is proposed by which all of the components of the deformation, as well as the crystallographic directions, can be manipulated simultaneously through the use of a stereographic projection. The third section of this paper is concerned with the geometry of grain growth. The polycrystalline state is described as a grain boundary network, which must respond to the requirements of surface tension. The several topological changes in a network which can contribute to grain growth are described.

On the influence of grain boundaries on mechanical properties

Abstract: A grain boundary area is simulated in a computer and the model is subjected to a shear strain parallel to the boundary, which makes the boundary slide. A model for grain boundary sliding is proposed, which in some ways is similar to the movement of edge dislocations in a perfect lattice. The model predicts that the rate of grain boundary sliding is decreased as the boundary becomes symmetrical, which is borne out in practice.