Showing papers on "Grain boundary strengthening published in 1982"
TL;DR: Grain boundary conductivities were determined by complex impedance measurements (1-106 Hz) on high-purity ceramics prepared by the alkoxide synthesis and on less pure Ceramics obtained from a commercial powder as discussed by the authors.
403 citations
TL;DR: In this paper, the authors proposed that the grain-size dependence of the yield stress of metals is due to the elastic incompatibility stresses at the grain boundaries, and the generation and motion of these "geometrically necessary" dislocations attenuated the stress concentrations; a work-hardened grain-boundary layer is formed.
Abstract: It is proposed that the grain-size dependence of the yield stress of metals is due to the elastic incompatibility stresses at the grain boundaries. For clarity, the process of yielding is divided into three stages. In the first stage (prior to microyielding), the differences in elastic properties arising from the elastic anisotropy of adjacent grains establish localized stress concentrations at the grain boundaries. In the second stage, the stress concentrations at the grain boundaries result in localized plastic flow; this is the microyield region. The generation and motion of these 'geometrically necessary' dislocations attenuates the stress concentrations; a work-hardened grain-boundary layer is formed. As the stress is increased, the polycrystalline metal acts as a composite material consisting of a continuous network of a work-hardened grain-boundary material and discontinuous bulk material. The bulk is prevented from flowing plastically because the continuous network of work-hardened grain-...
269 citations
TL;DR: In this paper, the authors presented tensile stress strain data for 99.999% copper at room and liquid nitrogen temperature as a function of grain size with some microstructural observations made by transmission electron microscopy.
267 citations
TL;DR: In this paper, the motion of pore attached to two grain interfaces has been analyzed and it is shown that pore motion varies with the dihedral angle and the onset of non-steady state pore motions results in grain boundary convergence and separation of the pore from the grain boundary.
194 citations
01 Dec 1982-Metallurgical and Materials Transactions B-process Metallurgy and Materials Processing Science
TL;DR: In this article, the authors reviewed the current knowledge of the atomistic mechanisms responsible for grain boundary diffusion phenomena and concluded that the atomic migration occurs by a point defect exchange mechanism which, in at least the vast majority of boundaries in simple metals, most likely involves grain boundary vacancies.
Abstract: In recent years it has become well established that fast diffusion along grain boundaries plays a key role in many important metallurgical processes including cases where net mass is transported along boundaries which act as sources and/or sinks for the fluxes of atoms. In addition, considerable advances have been made in understanding grain boundary structure, and new techniques have become available for studying kinetic phenomena in grain boundaries. This lecture will attempt to review our current knowledge of the atomistic mechanisms responsible for these grain boundary diffusion phenomena. Relevant aspects of the structure of grain boundaries and the point and line defects which may exist in grain boundaries are described first. The important experimental observations are then discussed. Diffusion models are then taken up, and it is concluded that the atomic migration occurs by a point defect exchange mechanism which, in at least the vast majority of boundaries in simple metals, most likely involves grain boundary vacancies. The grain boundary sources and/or sinks required to support divergences in the atomic (vacancy) fluxes are grain boundary dislocations. Phenomena therefore occur which resemblethe Kirkendall Effect in the bulk lattice in certain respects. Additional topics are discussed which include effects of boundary structure on boundary diffusion and the question of whether or not boundary diffusion is faster along migrating than stationary boundaries.
191 citations
TL;DR: In this article, the grain size dependence of the yield stress in hot-rolled 99.87 pct magnesium sheets and rods was measured in the temperature range 77 K to 420 K.
Abstract: The grain size dependence of the yield stress in hot rolled 99.87 pct magnesium sheets and rods was measured in the temperature range 77 K to 420 K. Hot rolling produced strong basal textures and, for a given grain size, the hot rolled material has a higher strength than extruded material. The yield strength-grain size relation in the above temperature range follows the Hall-Petch equation, and the temperature dependencies of the Hall-Petch constants σ0 and k are in support of the theory of Armstrong for hcp metals that the intercept σ0 is related to the critical resolved shear stress (CRSS) for basal slip (easy slip) and the slope k is related to the CRSS for prismatic slip (difficult slip) occurring near the grain boundaries. In the hot rolled magnesium, σ0 is larger and k is smaller than in extruded material, observations which are shown to result from strong unfavorable basal and favorable {1010} textures, respectively. Texture affects the Hall-Petch constants through its effect on the orientation factors relating them to the CRSS for the individual slip systems controlling them.
127 citations
TL;DR: In this article, the fracture strength of polycrystalline ice was found to decrease with increasing grain size, d, at least for aggregates slowly strained (10−6 s−1) at high temperatures (− 10°C or 0.96 Tm).
107 citations
TL;DR: In this paper, a model for diffusion induced grain boundary migration in which the migration occurs by the climb of grain boundary dislocations and their associated steps as a result of an inequality in the boundary diffusivities of solute and solvent atoms was proposed.
86 citations
TL;DR: In this paper, the mechanical properties of fine-grained polycrystalline TiC were studied using both four-point bending and compression tests and two rate-controlling mechanisms were found to be in operation during deformation at different test temperatures.
Abstract: The mechanical properties of fine-grained polycrystalline TiC were studied using both four-point bending and compression tests The ductile-brittle transition (D-B) temperature in compression was determined to be =800°C and was found to depend on grain size Yield-point behavior was observed for the first time in fine-grained TiC deformed in compression and was found to depend on grain size and test temperature The yield stress as a function of grain size can be described by a Hall-Petch type of relation, ie yield stress α (grain size)-1/2 The dislocations resulting from deformation in compression at lower temperatures were predominately screw in character, with edge dipoles and dislocation loops being present As the temperature of deformation was increased, the dipoles and loops were gradually annihilated by climb and the dislocations were observed in the form of hexagonal networks with a much-reduced dislocation density A plot of log yield stress vs 1/T showed a change in slope, which suggests that two rate-controlling mechanisms are in operation during deformation at different test temperatures Thermal activation analysis at T = 1050° to 1500°C suggested that the rate controlling mechanism during deformation in this temperature range is associated with cross slip
80 citations
TL;DR: In this paper, the effect of various grain structures, produced by thermomechanical treatments, on the monotonie tensile properties of the Al-Cu-Li-Mn-Cd alloy 2020 was investigated.
Abstract: The effect of various grain structures, produced by thermomechanical treatments, on the monotonie tensile properties of the Al-Cu-Li-Mn-Cd alloy 2020 was investigated. Materials having a completely or partially recrystallized structure exhibit elongations between 4 and 8 pct when aged to peak strength. For both cases the low ductility is associated with (a) planar deformation, (b) random texture, (c) the presence of large intermetallic compounds along the recrystallized grain boundaries, and (d) precipitate free zones. The first three enhance crack nucleation at high angle grain boundaries, and subsequent crack propagation occurs along the precipitate free zones. The completely unrecrystallized materials have elongations between 10 and 13 pct in both longitudinal and transverse directions. The high ductility is associated with a sharp texture and a transgranular fracture mode. The maximum ductility is obtained by reducing the unrecrystallized grain size. The results of this study suggest that improved properties of a 2020-type alloy may be obtained by lowering the Fe and Si contents to remove coarse constituent phases, eliminating Cd, and replacing Mn with Zr in order to obtain a highly textured, unrecrystallized structure.
78 citations
TL;DR: In this paper, the authors reviewed the properties of micrograin superplasticity in the context of the dominant microstructural characteristics, i.e., grain boundary sliding and migration, grain rotation and rearrangement, and dislocation activity.
Abstract: The phenomenon of micrograin superplasticity is critically reviewed in the context of the dominant microstructural characteristics,i.e., grain boundary sliding and migration, grain rotation and rearrangement, and dislocation activity. Existing theoretical models consider the accommodation process for grain boundary sliding to be either purely diffusional or due to dislocation motion. The latter can be in the form of individual dislocations or dislocations in the pile-up arrays in the interior of the grains or in grain interfaces. The mechanical properties,i.e., stress, strain-rate, activation energy, threshold stress, and so forth, are compared with the prediction of these models. The extreme sensitivity of the activation energy for superplasticity (which usually equals that for grain boundary diffusion) to alloy or impurity content is emphasized. The very large influence to prior thermal and mechanical history of the specimens on the mechanical data is discussed in the context of alteration of the significant details of grain boundary substructure.
TL;DR: Using transmission electron microscopy and equations from the dislocation theory, an experimental method was developed to determine quantitatively the shear stress concentrations at the grain boundaries which are produced by dislocation pile-ups and cause crack nucleation.
Abstract: In an aged Ti-8.6 wt pct Al alloy macroscopic embrittlement occurs with increasing grain size and degree of age hardening. The influence of the grain sizeL on the true fracture strain can be described by eF ∼L-1 Tensile crack nucleation is caused microscopically by strong dislocation pile-ups which crack the grain boundaries. Using transmission electron microscopy and equations from the dislocation theory, an experimental method was developed to determine quantitatively the shear stress concentrations at the grain boundaries which are produced by the dislocation pile-ups and cause crack nucleation. The experimental results show that for all investigated grain sizes and degrees of age hardening a critical local stress t*
C
≈ 38 GPa leads to crack nucleation. Based on this result equations were derived which describe the combined influence of grain size and age hardening on the true fracture strain and on the true fracture stress. These equations show a good agreement with the tensile test results.
TL;DR: In this paper, the resistance to normal grain growth offered by a dispersion of spherical particles is calculated and an allowance is made for grain boundary flexibility and for the tendency for some particles to assist the motion.
TL;DR: In this article, a microhardness measurement of oriented niobium bicrystals with symmetric tilt boundaries was performed and the boundary hardening was found to be misorientation-dependent and related to the boundary energy.
Abstract: Grain-boundary hardening of oriented niobium bicrystals with symmetric tilt boundaries has been determined using microhardness measurements. The boundary hardening was found to be misorientation-dependent and related to the boundary energy. For bicrystals with low-angle boundaries, the hardening varies with misorientation with a linear relationship of the Read-Shockley type. For bicrystals with high-angle boundaries, hardening cusps (minima) were observed near the coincidence-site-lattice boundaries.
TL;DR: In this paper, the step vectors associated with perfect grain boundary dislocations in cubic crystals are determined for coincidence systems up to Σ37a, and certain trends in the data are identified and rationalized.
TL;DR: In this paper, the authors review the atomistic model proposed for the equilibrium structure of grain boundaries in metals and present the present understanding of the structure of dislocation and vacancies (non-equilibrium defects) in grain boundaries.
TL;DR: In this paper, the effects of heating rate on recrystallized grain size have been determined and explained, and it has been shown that heating rates greater than about 5 K. s-1 should be employed in the final stage in order to obtain maximum benefit from the fine grain processing technique.
Abstract: A method has previously been described whereby a fine and stable grain size may be achieved in conventional, heat-treatable aluminum alloy sheet by thermomechanical processing. The present work has examined the final recrystallization stage more closely. In particular, the effects of heating rate on recrystallized grain size have been determined and explained. It has been shown that heating rates greater than about 5 K . s-1 should be employed in the final recrystallization stage in order to obtain maximum benefit from the fine grain processing technique. The coarser recrystallized grain sizes obtained with slower heating rates are mainly due to early activation of the most highly favored nucleation sites. Thermal recovery of the matrix defect structure below the recrystallization temperature is an additional, though less significant, effect. The influence of the degree of cold work and the volume fraction of insoluble particles on recrystallized grain size is discussed in relation to the heating rate.
TL;DR: In this article, it is suggested that this is caused by an acceleration of the rate controlling processes, namely, particle dragging or particle coarsening, resulting in material which cannot be subsequently recrystallized.
Abstract: An experimental charge of the oxide dispersion and γ′ precipitation strengthened superalloy MA 6000 was hot worked in the as-extruded, fine grained condition. The recrystallization during post-deformation annealing was followed by optical and electron microscopy. Coarse grain formation occurs by secondary rather than primary recrystallization (abnormal or discontinuous grain growth) as indicated by the driving forces involved, the inhibition of normal grain growth, the well-defined coarsening temperature, and the dependence of secondary grain size on temperature. Straining at high temperatures and low strain rates produces material which cannot be subsequently recrystallized. The reason is the loss of driving force during straining due to normal grain growth. The influence of deformation rate and deformation temperature can be properly accounted for by using the diffusion compensated strain rate for description of deformation conditions. Normal grain growth is accelerated by straining. It is suggested that this is caused by an acceleration of the rate controlling processes, namely, particle dragging or particle coarsening.
TL;DR: In this article, it was shown that upon annealing, warm-worked NiAl recrystallizes and undergoes subsequent grain growth in a manner typical of metals and alloys.
Abstract: Experiments have established that upon annealing, warm-worked NiAl recrystallizes and undergoes subsequent grain growth in a manner typical of metals and alloys. The recrystallization and grain growth kinetics, respectively, can be described by the expressionsX v = 1 - exp(-Bt k ) and −d =Ct n where Xv is the fraction recrystallized,t is time, −d is the average grain size, andB, k, C, andn are parameters whose magnitudes depend upon temperature.
TL;DR: In this paper, the correlation between the corrosivity and the atomic structure of grain boundaries in copper was investigated by measuring the corrosion rate of about 10,000 boundaries of different structures in an aqueous solution of HC1 and FeCl3 as a function of the orientation relationship.
TL;DR: In this article, the effect of initial grain size on cavitation during superplastic deformation in two commercially available Zn-Al eutectoid alloys has been studied using metallography and precision density measurements.
Abstract: The effect of initial grain size on cavitation during superplastic deformation in two commercially available Zn-Al eutectoid alloys has been studied using metallography and precision density measurements. Cavitation was found to be minimal for initial grain sizes below about 5 μm. Superplastic deformation caused grain growth in both alloys under all testing conditions, and when the grain size exceeded about 8 μm a significant level of cavitation was produced. The grain size and extent of cavitation increased with increasing strain along the specimen gauge length, with cavities concentrated in regions adjacent to the fracture tip. Although never very large, the cross-sectional area at fracture increased with increasing levels of cavitation. It was concluded that cavitation in Zn-Al eutectoid results from incomplete accommodation of grain-boundary sliding when excessive grain growth leads to restricted grain-boundary diffusion and/or to restricted grain-boundary migration.
TL;DR: In this article, the effects of second phase particles, e.g. M23C6, MC and M(C, N) carbides on the grain growth phenomenon of FeNiCr alloys have been determined.
TL;DR: In this article, the authors studied the stress relaxation of a two-dimensional solid with the assumption that defects have been trapped in the sample and calculated the effective shear modulus and stress-strain relaxation rate for a variety of defect configurations, including large and small-angle grain boundaries and dislocation pairs.
Abstract: The stress relaxation of a two-dimensional solid is studied, with the assumption that defects have been trapped in the sample. The effective shear modulus and stress-strain relaxation rate are calculated for a variety of defect configurations, including large- and small-angle grain boundaries and dislocation pairs. Effects of dislocation climb on the long-term stability of the configurations are considered. The viscosity resulting from moving free dislocations and/or flow at the boundaries is described in a particular geometry. The response to a finite applied shear is discussed, in particular, nucleation of free dislocations and sliding of grain boundaries. The theory is applied to free-standing smectic-B films, and it is suggested that stress relaxation observed in these films may result from a dilute ''random neutral array'' of dislocations or from small-angle grain boundaries.
TL;DR: In this paper, the authors summarized the results of some recent research on a thermomechanical method of refining the grain size in precipitation hardenable aluminum alloys and illustrates the infuence of grain refinement on several material properties.
Abstract: This paper summarizes the results of some recent research on a thermomechanical method of refining the grain size in precipitation hardenable aluminum alloys and illustrates the infuence of grain refinement on several material properties. Grain refinement is achieved by deliberately introducing a large number of nucleation sites for recrystallization and by controlling grain growth after recrystallization. Recrystallization to a relatively small and equiaxed grain size has been achieved in a number of commercial aluminum alloys using these concepts. The influence of the fine recrystallized grain size on such properties as superplastic deformation, room temperature tensile properties, fatigue life, and exfoliation corrosion resistance is discussed. The results show that refinement to a grain size of 8–14 µm is sufficient to develop extensive superplasticity and to yield a small increase in tensile properties in alloys such as 7075 and 7475.
TL;DR: In this paper, the fracture of grain boundaries in electron beam zone refined bi-crystals of tungsten has been investigated by propagating a crystallographic crack towards twist boundaries of controlled misorientation.
TL;DR: In this paper, large local misorientations arise due to the sequence of slip systems which are active in a given region during deformation, which is referred to as local slip misorientation.
TL;DR: In this article, the term pseudo-intercrystalline brittleness is proposed to describe a fracture mechanism which can occur in poly-crystaline alloys which contain a fine dispersion of a second phase.
Abstract: The term pseudo-intercrystalline brittleness is proposed to describe a fracture mechanism which can occur in poly-crystalline alloys which contain a fine dispersion of a second phase. If narrow particle-free zones develop along grain boundaries, separation can occur after large amounts of plastic strain, which is highly localized to the vicinity of grain boundaries. Since the hardened grain interior does not contribute to plastic deformation the total plastic deformation to fracture and fracture toughness remain small. Quantitative models are proposed to interpret the micromechanism of fracture and to describe the grain-size dependence of fracture toughness. The fracture of precipitation hardening aluminum alloys, creep resistant and structural steels are discussed in terms of the models. Finally an interpretation of the mechanism of stress-relief cracking in steel weldments is given.
TL;DR: In this article, the effects of rapid solidification on martensitic transformations were studied in Cu-Zn-AI samples prepared by the method of melt-spinning, with an estimated cooling rate of about 106 K per second near the freezing point.
Abstract: The effects of rapid solidification on martensitic transformations were studied in Cu-Zn-AI samples prepared by the method of melt-spinning, with an estimated cooling rate of about 106 K per second near the freezing point. A diffusionless solidification reaction L → β occurs, and a very fine-grained β structure is obtained, with highly structured grain boundaries. The average β grain diameter (∼5 µm) is about two orders of magnitude smaller than that obtained by conventional solid state solution and quench treatment. The β:β grain boundaries contain extraordinary features such as large steps, and the matrix dislocation density is abnormally high. The Ms temperature is depressed significantly in as-melt-spun ribbon material, but as the martensitic transformation is cycled, it shifts upward in temperature and obtains a more narrow hysteresis loop. The martensite has the usual 9R structure (ABCBCACAB stacking) found in bulk alloys, and while the morphology is similar to that in bulk alloys, it is finer in scale. It is suggested that the β → 9R transformation is affected through the combined influence of rapid solidification on parent β grain size, disorder, β:β grain boundary structure, internal stresses, and dislocation substructure. Shape memory behavior is qualitatively similar in the rapidly solidified alloys.