Showing papers on "Grain boundary strengthening published in 1968"

The influence of grain size on the nature of portevin-lechatelier yielding

Abstract: When the grain size dependence of dislocation density is applied to Cottrell's theory of the Portevin–Lechatelier effect, the result indicates that the critical strain ∊0, necessary to initiate serrated flow, should depend on the grain size μ, at constant strain rate, according to the relationship ∊0 = (constant)μ p , where p=n/(m + β). The parameters n and (m + β) are evaluated by dislocation density measurements and strain rate experiments respectively to give ∊0 ∝ μ½. Direct experimental determination of ∊0 as a function of grain size is in agreement with that predicted theoretically, indicating that the effect arises from the influence of grain size on the average velocity of mobile dislocations. The period and amplitude of the serrations decrease with increasing grain size. This effect is consistent with the idea that each serration corresponds to a yield point followed by a local Luders strain. The period and amplitude of the serrations decrease with increasing grain size. This effect is co...

Changes Produced by Deformation in Grains and Grain Boundaries of Nickel

Abstract: Polycrystalline nickel (grain size 0.04 mm) has been examined with the electron microscope after annealing and then after plastically straining at room temperature to between 0.4 and 12% elongation. In addition to measuring the grain-interior dislocation density (ρ, cm/cm3), the number (n) per cm3 of three-fold nodes was counted and the arrangement and density (p, cm/cm2) of dislocations in the grain boundaries was examined. The applied tensile stress σ = 1.4 Gb ρ½ = 1.3 Gb n⅓ (where G is the shear modulus, b the Burgers vector). The node density was high and the dislocation network tightly linked at all strains, which is a necessary feature if attractive junctions are to contribute substantially to the flow stress. Straining evidently does not change the shape of the network so much as refine it, and the refinement is presumably the essential feature of strain-hardening. Superimposed on the refinement there developed the normal cellular structure. The grain boundaries trapped dislocations and he...

The Effect of Hydrostatic Pressure on the Cleavage Fracture of Polycrystalline Materials

Abstract: The dislocation pileup model of Stroh and the wall splitting model of Stroh and Friedel have been used to calculate the effects of hydrostatic pressure on the fracture stress of polycrystalline metals. For each model, the three critical stages of fracture, nucleation, propagation through a grain, and the propagation across a grain boundary have been considered in turn. The variation of the critical cleavage‐crack orientation with respect to the tensile axis has been shown to be a function of the hydrostatic pressure. The fracture stress depends upon the grain size and at high pressures where grain boundary propagation is the critical stage, a d−1/2 dependence was found. This analysis both predicts and provides a physical interpretation of the ductility‐transition curve.

Internal strain energy and the strength of brittle materials

Abstract: The theory of Clarke for the formation of grain boundary cracks in anisotropic polycrystalline materials, is re-examined in the light of recent experimental data. The theory predicts correctly the conditions for the formation of grain boundary cracks of length similar to a grain dimension. However, the theory cannot be used to explain the experimentally observed strength/grain size and strength/irradiation dose relationships, for example for BeO. The theory supposes that the process controlling catastrophic fracture is the growth of a crack from a grain boundary pore with an energy absorption rate corresponding to the grain boundary surface energy of ∼103 erg/cm2. In practice, the process controlling catastrophic fracture is the subsequent growth of a crack from a grain dimension, with a higher energy absorption rate corresponding to an effective surface energy of ∼104 erg/cm2.

On the distribution of cavities during creep

Abstract: The true and apparent angular distributions of grain boundary cavities have been measured for OFHC copper deformed over a range of creep rates at 415°C. No effect of strain rate was observed, the f...

Deformation Behaviour of Aluminium Macrocrystals in the Grain Boundary Region

Abstract: Lattice distortions of deformed aluminium macrocrystals were observed by Electron-microprobe-Kossel techniques. Grain boundary effects due to deformation appeared in various ways such as extraordinary separation and branching of Kossel lines, and as intergranular rotation of the grains in pseudo-Kossel X-ray diffraction patterns obtained from the grain boundary regions of the specimens. Such distortions of the patterns peculiar to the grain boundary regions were attributed to the effect of local slipping induced by the active slip systems of the neighbouring crystals. By the interpretation of these patterns and the orientation relationships between neighbouring grains, it was found that the local slipping was induced towards the slip direction of the active slip plane in the neighbouring crystals, or induced by a type of local stress along the slip direction of the active slip planes in the neighbouring crystals. It was also recognised that at an early stage of stressing, deformation bands were produced in the grain boundary region as a result of a grain boundary effect.

Mechanical Effects of Grain Boundaries

Abstract: A summary of previous studies, together with new observations, of the interfacial hardening effect is presented. The topics considered include: hardening associated with vacancies and vacancy-solute interactions, examination by transmission electron-microscopy, vacancy condensation pits, solute-vacancy binding energies, solute clustering and impurity segregation. The results provide support for a solute-clustering model for grain boundary quench-hardening. The general implications of this work to other metallurgical phenomena are briefly considered.

On the Interaction of Inclusions with Migrating Grain Boundaries

Abstract: The interaction energy of a grain boundary with a precipitate particle or inclusion of a second phase is considered in detail When the nature of the inclusion is considered, two regimes emerge: (a) the grain boundary may enter and pass through certain coherent inclusions, leaving them coherent; (b) more commonly, the grain boundary may envelop and bypass the inclusion, causing one which was coherent to lose coherency The maximum pinning force exerted by an inclusion or particle on a boundary depends on its nature

Investigations to understand the deformation and strengthening mechanisms of the solid solution phases of titanium

Abstract: : Mechanical properties data are reported for alpha-titanium of four impurity contents and various grain sizes, tested in the temperature range 4.2K to 800K. Dislocation substructures produced by deformation of commercial purity and high purity alpha-titanium at room temperature were examined by transmission electron microscopy and representative examples are presented and discussed. Work hardening behavior is connected with long-range interactions and is modified by both purity and grain size as well as temperature. The effect of impurity content appears to be related to the degree to which dislocations are restricted to their slip planes. Grain size also influences the yield and flow stresses, the relations observed being approximately of the Hall-Tetch type. The effects of grain size are shown to be athermal in nature and are accounted for in terms of work hardening model. Thermal activation analysis is used to show that the behavior reported is in agreement with the concept that the rate controlling process for low temperature deformation is thermally activated dislocation glide on first order prism planes over barriers associated with individual interstitial impurity atoms. The analysis shows that the nature of barriers is the same in all four materials and that the rate controlling process is unaffected by either grain size or plastic strain. (Author)

Belt-type grain pick-up

Abstract: AN ENDLESS CONVEYOR BELT TYPE GRAIN PICK-UP PROVIDED WITH FLEXIBLE PICK-UP TYNES AND A DRIVEN STRIPPER ROLLER FOR REMOVING GRAIN FROM THE TYNES. THE STRIPPER ROLLER IS LOCATED BELOW THE DISCHARGE END OF THE CONVEYOR AND KEPT A CONSTANT DISTANCE THEREFROM. THE ANGULAR RELATONSHIP OF THE STRIPPER ROLLER AND DISCHARGE END OF THE CONVEYOR RELATIVE TO THE GROUND IS VARIABLE. D R A W I N G

Effect of Grain Size Variations on the Long-Time Stability of Alloy 718

Abstract: Uniform, fine grain size is generally preferred in forgings of Alloy 718 for optimum properties. However, due to limitations imposed by forging equipment, forging size and forging configuration, it is often impossible to produce this desired grain size; instead, some grain duplexing or grain coarsening occurs in certain areas throughout the forging. Stress-rupture tests were conducted before exposure, and after exposing specimens at 1250°F, 50 ksi or 1300°F, 50 ksi for 500 hrs. on material having (1) uniform fine grain size (ASTM 7 or finer), (2) duplex grain size, and (3) uniform coarse grain size (ASTM 5 or coarser). These grain sizes were obtained in forgings with a simple pansake configuration using varying forging techniques. The results of the rupture tests conducted before exposure showed an excellent correlation with grain size and Ni3Cb plate size. After exposure, rupture ductility increased and strength decreased due to overaging. In duplexed samples the controlling factor appears to be the amount and size of the coarse grain. The optimum grain size for the best balance of rupture properties appears to be ASTM 6–7.

The use of a bubble model to study stress-induced migration and sliding of grain boundaries.

Abstract: : Stress-induced grain boundary sliding and migration was studied using a bubble model. Two important results emerge which, we believe, apply to boundaries in real crystals. First, sliding occurs by the propagation of grain boundary dislocations, which may have large burgers vectors, in the plane of the boundary; the magnitude of the burgers vector, and the pre-existing density of these dislocations in the boundary depends on the boundary angle and orientation. Second, migration is very like the process of crystal growth from the vapour: atoms leave or join one side of the boundary at steps; a boundary of which one side is a close-packed plane rarely migrated. The process of sliding periodically places atoms in a position which favours their leaving one side of the boundary and joining the other; this can result in sliding and migration which are proportional, and suggests a model for thermally activated grain-boundary sliding. (Author)

Anomalous porosity in certain aluminum alloys

Abstract: Metallographic examination was used to study the formation of porosity in an aluminum alloy A1-4%Cu with grain size in ranges 1000-200 and 120-85 μ, in technical grade aluminum Al (grain size 80-60 μ), and in type D1 alloy (grain size 30-10 μ) during a thermal cycling treatment, each cycle of which included aging in water at 100 ° C. The character of porosity depends on alloy composition and grain size. In coarsely granular materials the pores are formed inside grains, have clearly defined form, and reach a size of 10–20 μ after 60 cycles. The pore size in finely granular materials is smaller by one order of magnitude; the pores are formed predominantly along grain boundaries with some pores formed in the grain interior. Pores formed in alloy D1 are of an irregular form and grow to sizes exceeding the grain size. Data on changes in the specimen length, diameter, and volume were obtained. The relative increase in specimen volume reaches 0.5–1.0% per cycle. In spite of differences revealed by metallographic examination, the phenomena taking place in specimens with large and small grain size are of the same nature.

Recrystallization processes in steel 4Kh12N8G8MFB

Abstract: 1. An increase in the heating temperature of cold-worked steel 4Kh12N8G8MFB shifts the critical strain toward larger values. 2. A method was proposed for calculating the grain size differential correlated with qualitative observations of differences in grain size. 3. Heating of samples strained after quenching to 1100°C induces the largest difference in grain size at critical strain (1–4%). 4. On recrystallization heating of steel heated at 1050°C before deformation one observes two peaks of grain size on the recrystallization diagrams that correspond to 1 and 4% strain and are due to the difference in grain size.

The effect of grain size on the deformation resistance of nickel

Abstract: The effect of the grain sizel of commerical nickel on the lower yield point, σy, and flow stress, σ f , has been investigated. From the relationship between σy andl −1/2 and between σ f andl −1/2, and also by extrapolation, the parameters σ1(σ i ) and ky(kf), which occur in Petch's well-known expression, were determined. It was found that the values of these parameters depend on the previous history of the samples. It is suggested that the more marked dependence of the deformation resistance of nickel on grain size arising from certain thermal treatments is due to the segregation of impurities to the grain boundaries. It is shown that this is in accord with the presence of grain-boundary hardening and with its dependence on quenching temperature.