Intragranular Slip Research Articles

Stimulation and Suppression of Grain Boundary Sliding by Intragranular Slip in Zinc Bicrystals

The influence of intragranular slip on grain boundary sliding is studied in originally compatible zinc bicrystals with symmetric tilt boundary. The experiment is designed to separate different effects of intragranular slip on the boundary sliding and establish their mechanisms. Grain boundary sliding with and without development of intragranular slip is observed. The rate of sliding accompanied by slip is more than five times of that without slip. A good correlation between the boundary sliding and intragranular slip prior to slide hardening is established. Slide hardening followed by the negative sliding near one end of the boundary and strain hardening in the boundary vicinity, are observed at the last stages of deformation. For the case of formation of slip induced glissile grain boundary dislocations of opposite signs the possibility of their contribution to total grain boundary sliding, is analyzed. The effect of the increase in the rate of sliding is explained in terms of the accommodation of sliding by slip and appearance of additional glissile grain boundary dislocations of one sign due to strain incompatibility. Contribution of these different dislocation mechanisms to the increase in the sliding rate is determined for the stage of deformation preceding slide hardening. It is supposed that the effect of slide hardening and negative sliding as well as boundary curving is created by non-smooth boundary and small degree of incompatibility caused by straining.

Deformation of electrodeposited nanocrystalline nickel

The mechanisms of deformation and damage evolution in electrodeposited, fully dense, nanocrystalline Ni with an average grain size of ~30 nm and a narrow grain size distribution were investigated by recourse to (i) tensile tests performed in situ in the transmission electron microscope and (ii) microscopic observations made at high resolution following ex situ deformation induced by compression, rolling and nanoindentation. Particular attention was also devoted to the characterization of the structure in grain interiors and in the vicinity of grain boundaries at Angstrom-level resolution in the as-deposited material and following compression, and to the real-time video-imaging of the evolution of dislocation activity and damage during deformation; these images are presented in this paper and in the web sites provided as supplementary material to this paper. These observations clearly reveal that dislocation-mediated plasticity plays a dominant role in the deformation of nanocrystalline Ni examined in this study. Fracture surface examination confirms dimpled rupture with the scale of the dimples being several times larger than the grain size. Dislocation emission at grain boundaries together with intragranular slip and unaccommodated grain boundary sliding facilitate the nucleation of voids at boundaries and triple junctions. Individual monocrystal ligaments, formed by the growth/linking of these voids, undergo extensive local plasticity to the extent that many of them neck down to a chisel point. These voids as well as those that may have existed prior to deformation can act as nucleation sites for dimples leading to fracture that does not occur preferentially along grain boundaries. The transmission electron microscopy observations of in situ and ex situ deformed specimens are synthesized to formulate a mechanistic framework that provides new insights into the mechanisms of flow and fracture in nanostructured metals.

Cooperative processes during plastic deformation in nanocrystalline fcc metals: A molecular dynamics simulation

Large scale molecular dynamics (MD) simulations are used to simulate the plastic deformation of a nanocrystalline model Ni sample with an average grain size of 5 nm containing 125 grains at 800 K up to 4.0% plastic strain. For the first time in MD simulation, emerging shear planes involving several grain boundaries have been observed indicating cooperative plastic deformation activity between grains. It is found that three mechanisms have been involved in the development of such shear planes: grain-boundary migration, continuity of shear plane via intragranular slip, and rotation and coalescence of grains.

Superplasticity and grain boundary sliding characteristics in two stage deformation of Mg–3Al–1Zn alloy sheet

A ‘Two-Stage Deformation Method’ was proposed to enhance the superplasticity of Mg–3Al–1Zn (AZ31) alloy sheet. This method exploited the capability of the material to undergo dynamic recrystallization (DRX) at optimum DRX conditions of 250 °C and constant strain rate of 1×10 −4 s −1. Stage I was aimed at refining the coarse microstructure of the as-received alloy to result in fine equiaxial grains measuring less than 10 μm, which deformed by grain boundary sliding accommodated by intragranular slip. Subsequently, Stage II was performed at a higher deformation temperature, whereby viscous glide mechanism accommodated by lattice diffusion was predominant. By altering the deformation mechanisms at different strain levels, elongation-to-failure of 320 and 360% was attained at 400 and 450 °C, respectively.

Influence of elastic and plastic anisotropy on the flow behavior in a duplex stainless steel

The load partitioning between two phases in a cold-rolled duplex stainless steel has been experimentally studied in situ by X-ray diffraction, for different loading directions. It was found that the load partitioning between the two phases is dependent on the loading direction. For loading in the rolling direction, both phases deform plastically to the same degree, while more plastic deformation occurs in the austenitic phase during loading in the transverse direction. For loading in the 45-deg direction, more plastic deformation occurs in the ferritic phase. The strong crystallographic texture in the ferritic phase makes the material anisotropic, with a higher stiffness and yield strength in the transverse direction compared to the rolling direction. The measured texture was used as input to theoretical predictions of both elastic and plastic anisotropy. The plastic anisotropy was predicted by assuming intragranular slip as the main deformation mechanism. The predicted anisotropic material properties were then used in finite-element simulations to study the flow behavior of the material in different directions. The predicted flow behavior was found to be in good agreement with the experimentally observed load partitioning between the phases for loading in the rolling and transverse directions. However, the yield strength of the ferritic phase during loading in the 45-deg direction was found to be lower than what was predicted. The reason for this is the difference in slip characteristics in different sample directions, because of the morphological texture.

Characterization of room-temperature plastic deformation of ß-Si3N4 by atomic force microscopy and transmission electron microscopy

Dislocation microstructures induced by room-temperature microhardness tests have been investigated in silicon nitride. Surface analysis of the residual indent by atomic force microscopy reveals intragranular slip bands and demonstrates that room-temperature plastic deformation involves dislocation motion as well as cross-slip events. Cross-slip events have been found to occur between {1010} prismatic planes. Transmission electron microscopy shows that dislocations have a Burgers vector b = [0001] and are located along the screw direction. Based on these observations, specific dislocation core configurations are discussed.

On the Independent Behavior of Grain Boundary Sliding and Intragranular Slip During Superplasticity

Operation of grain boundary sliding is examined for conditions of plastic strain incompatibility that is the most frequent case for deformation of polycrystals. Two coexisting components of grain boundary sliding: dependent and independent on intragranular slip are distinguished. Theoretical estimate of a ratio between slip induced sliding and intragranular slip is obtained. It is concluded that at the beginning of deformation intragranular slip and grain boundary sliding behave independently. However, at high strains they can be viewed to some extent as interdependent processes.

Deformation Characteristics of a 3Y-TZP/20%Al<sub>2</sub>O<sub>3</sub> Composite in Tensile Creep

Creep tests were performed over a range of stresses and temperatures on a 3Y-TZP/20% Al 2 O 3 composite termed 3Y20A. After creep, measurements were undertaken to determine the average shapes and sizes of the zirconia and alumina grains. The experimental data are compared with creep models including interface-controlled diffusion creep. The results suggest that interface-controlled diffusion creep is important at the lower stress levels but grain boundary sliding may be accommodated by intragranular slip within the alumina grains at the higher stresses.

On the mechanism of the influence of crystallographic slip on grain-boundary sliding at similar deformation of grains

This letter presents a geometrical study of grain-boundary sliding stimulated by intragranular slip in a compatible zinc bicrystal with a symmetric 90o,<1010> tilt boundary. Assuming that the interaction of lattice dislocations with the boundary produces glissile grain-boundary dislocations of two different signs, an analysis of the possibility of their contribution to sliding has been made. It is shown that these grain-boundary dislocations can induce only local grain-boundary sliding and hence cannot contribute to usual macroscopic grain-boundary sliding. It is suggested that the reason for the increase in the grain-boundary sliding rate in the presence of intragranular slip is the facilitation of the accommodation of grainboundary sliding at boundary irregularities by the emission of lattice dislocations to the grain interior.

An examination of the flow process in superplastic yttria-stabilized tetragonal zirconia

Extensive high temperature creep data are available for a superplastic yttria-stabilized tetragonal zirconia containing 3 mol% of Y 2O 3 (termed 3Y-TZP). These data are analyzed and it is shown that, contrary to superplastic metals, intragranular slip cannot occur to accommodate grain boundary sliding at the temperatures and stresses examined experimentally. It is proposed instead that flow occurs by Coble diffusion creep controlled by movement of the Zr 4+ ions and, because of the small grain sizes in the 3Y-TZP materials, the creep rates are reduced through interface-controlled diffusion creep. A detailed analysis demonstrates this approach predicts the experimental creep rates to within one order of magnitude over the range of grain sizes and temperatures examined experimentally and it accounts also for the reported values for the stress exponent and the exponent of the inverse grain size at high and low stress levels.

Superplastic Deformation without Relative Grain Translation?

It is now almost universally accepted that the mechanism by which superplastic deformation occurs involves relative grain translation, by grain boundary sliding, together with dislocation creep and/or diffusional creep. While there has been much debate on the fine points of the process - principally the so-called accommodation mechanisms, the primary role of the relative translation of grains via grain boundary sliding is virtually undisputed. Although this mechanism appears to be consistent with several aspects of microstructural evolution in superplastic deformation, the direct evidence for a dominant grain translation mechanism is mainly drawn from work using surface markers, not withstanding the fact that the extra degree of freedom at the surface of a superplastically deformed sample may make it somewhat unrepresentative of the bulk material. Research carried out recently on Ti-6Al-4V and AA5083+Cr, two well known superplastic materials, using both internal and surface markers throws into some doubt the importance of relative grain translation in superplastic deformation. The behaviour of aligned grain structures - acting as internal markers - is also difficult to reconcile with relative grain translation mechanisms The fact that such microstructures occur in materials which give a high strain rate sensitivity is inconsistent with mechanisms in which grain boundary sliding is a principal, rather than a subservient or even accommodating, mechanism. Evidence from transmission electron microscopy and plastic anisotropy suggest that intragranular slip may have a fundamental role in superplasticity. Certainly, some consideration must be given to mechanisms other than relative grain translation, though these must include the role of grair boundaries in leading to high strain rate sensitivity.

On the Relationship Between Grain Boundary Sliding and Intragranular Slip During Superplastic Deformation

AbstractThis paper examines grain boundary sliding under the conditions of plastic strain incompatibility that is the most frequent case in polycrystalline materials. Two components of grain boundary sliding: dependent and independent on intragranular slip are distinguished. Theoretical estimate of a ratio between slip induced sliding and intragranular slip is obtained. It is concluded that slip and sliding are rather independent than interrelated processes.

Sliding Characterization of Coincidence, Near-Coincidence and General Boundaries in Zinc

Observations of grain boundary sliding along symmetric tilt coincidence Σ=9{1122} and three boundaries deviated at different angles from coincidence misorientation have been made in zinc bicrystals. The coincidence boundary and boundaries whose deviation does not exceed ∼6 degrees slide and migrate in a coupled manner predicted by the DSC-dislocation model. The boundary whose deviation exceeded ∼6 degrees slides without migration and with a significantly lower rate in contrast to the previous boundaries except the coincidence one. The sliding along the coincidence boundary is activated at higher stresses only in the presence of intragranular slip. These results enable to distinguish special, near-special and general boundaries. It is emphasized that the ability of a boundary to slide and migrate in a fixed ratio is an important criterion for its specialness.

The model of grain-boundary sliding stimulated by intragranular slip

The model of grain-boundary sliding stimulated by intragranular slip

The model of grain-boundary sliding stimulated by intragranular slip

Abstract The theoretical model of grain-boundary sliding (GBS) stimulated by intragranular dislocation slip has been suggested on the basic of experiments on Zn and Cd bicrystals. Lattice dislocations of two grains dissociate in boundary during the spreading time t s and form two families of mobile gain-boundary dislocations (GBDs) with the opposite sign. The motion of GBDs containing a non-conservative component is the main mechanism of GBS. The GBS rate is determined by the GBD density and by the rate of their annihilation or sink to the surface. The increase in GBD density forms additional elastic fields in the vicinity of boundary, which inhibits the entering of lattice dislocations into the boundary and weaken the source efficiency. The GBD density balance determined by the ratio of influx of lattice dislocations and annihilation of the GBDs is established. Thus the boundary demonstrates the behaviour of system with negative feedback. The model allows one to describe the GBS-time dependence in Cd and Zn bicrystals in the whole volume and to extend the understanding of the interaction of the above-mentioned mechanisms on deformation of the polycrystal.

Sliding behaviour of ∑ = 9{1 [formula omitted]12} symmetric tilt boundary in zinc bicrystals

Sliding behaviour of the 57° 〈101̄0〉 ∑ = 9{1 2 12} symmetric tilt boundary has been studied in zinc bicrystals. The bicrystals have been tested under creep at two different rates of intragranular slip (IS). For all the bicrystals, coupled grain boundary sliding (GBS) and migration is observed, the rates of these processes being similar along the boundary. In the vicinity at one end of the boundary, at a slower rate of IS there is no GBS or migration, and at a greater rate of IS they have negative values. Also, the increasing values of GBS and migration from one end of the boundary to the other is observed. The results obtained have been interpreted in terms of behaviour of grain boundary dislocations, produced as a result of interaction between lattice dislocations and the grain boundary.

PLASTIC ANISOTROPY IN A SUPERPLASTIC DUPLEX STAINLESS STEEL

Measurements of the plastic anisotropy in uniaxial tension of the duplex stainless steel, SAF2304, have been made at room temperature and under conditions where the material was superplastic. There was significant plastic anisotropy in both types of deformation and there were some similarities between the low and high temperature variations with tensile axis orientation. Although it was possible to model the high temperature anisotropy using a grain boundary sliding model, the assumed distribution of sliding boundaries was considered to be unrealistic. This, together with aspects of microstructural and textural development, indicated that deformation was principally occurring by intragranular slip with a significant contribution caused by mechanical inhomogeneity in the two-phase material. © 1997 Acta Metallurgica Inc.

Development of microstructure in the high-temperature deformation of ice

Microstructural changes in three sets of experiments involving crystallographic slip in anisotropic polycrystalline ice are described and interpreted with the aid of computer models. The development of microstructure was followed using time-lapse photography and transmitted light observations with deformation undertaken in plane strain and at a temperature of approximately –1°C. The deformation within a grain aggregate that accompanies axial shortening is always heterogeneous on a grainscale. The extent of inhomogeneity varies depending on the pre-existing grain structure and the way it can accommodate intragranular slip. Grain interactions are extremely important in determining the bulk deformation and the degree of grain-boundary migration. A consequence of shortening of the aggregate is the formation of high stresses between neighbouring grains and under the appropriate conditions there may be either grain-boundary migration or melting at these sites. Where a sample undergoes translation and shear during deformation, anisotropic grains in the appropriate orientation undergo bending. A buckle instability may then develop and much of the strain is accommodated by grains in easy-glide orientations. In such situations, the ice undergoes extensive recrystallization and grain growth that is concentrated in the areas of greatest buckling.

Development of microstructure in the high-temperature deformation of ice

Microstructural changes in three sets of experiments involving crystallographic slip in anisotropic polycrystalline ice are described and interpreted with the aid of computer models. The development of microstructure was followed using time-lapse photography and transmitted light observations with deformation undertaken in plane strain and at a temperature of approximately –1°C. The deformation within a grain aggregate that accompanies axial shortening is always heterogeneous on a grainscale. The extent of inhomogeneity varies depending on the pre-existing grain structure and the way it can accommodate intragranular slip. Grain interactions are extremely important in determining the bulk deformation and the degree of grain-boundary migration. A consequence of shortening of the aggregate is the formation of high stresses between neighbouring grains and under the appropriate conditions there may be either grain-boundary migration or melting at these sites. Where a sample undergoes translation and shear during deformation, anisotropic grains in the appropriate orientation undergo bending. A buckle instability may then develop and much of the strain is accommodated by grains in easy-glide orientations. In such situations, the ice undergoes extensive recrystallization and grain growth that is concentrated in the areas of greatest buckling.

On the influence of intragranular slip on grain boundary sliding in bicrystals

On the influence of intragranular slip on grain boundary sliding in bicrystals