General High-angle Boundaries Research Articles

Optimal microstructural design for high thermal stability of pure FCC metals based on studying effect of twin boundaries character and network of grain boundaries

Three nickel electrodeposits with comparable grain size were synthesized by tailoring the electrodeposition conditions. Thorough microstructural characterizations including electron backscatter diffraction, ion channeling contrast imaging, electron channeling contrast imaging, transmission Kikuchi diffraction, transmission electron and high annular dark-field imaging were applied. The deposits contain a high density of twin boundaries with similar microstructures in terms of grain boundary character. These materials were annealed at various temperatures to study the microstructural evolution, and hence, their thermal stability. The differences in the character of twin boundaries and morphology of the grain boundaries in as-deposited state and their influence on the microstructural evolution at elevated temperatures are analyzed. The importance of incoherent twin boundaries, and the interaction of mobile general high angle boundaries with stationary boundaries are discussed. Finally, an optimal design for high thermal stability is proposed, based on the mechanisms that were inferred from the results.

Improving intergranular corrosion resistance in a nickel-free and manganese-bearing high-nitrogen austenitic stainless steel through grain boundary character distribution optimization

Grain boundary character distribution (GBCD) and the effect of GBCD optimization on the intergranular corrosion (IGC) of a cold rolled and subsequently annealed nickel-free and manganese-bearing high-nitrogen austenitic stainless steel were investigated. The results show that the fraction of low Σ coincidence site lattice boundaries increases from 47.3% for the solid solution treated specimen to 83.3% for the specimen cold-rolled by 7% and then annealed at 1423K for 10min. Σ3 boundaries of high fraction effectively interrupt the connectivity of non-corrosion-resisting boundaries (special boundaries like Σ9, Σ27, etc. and general high angle boundaries) network, thus improving the IGC resistance.

Spall behaviors of high purity copper under sweeping detonation

Suites of sweeping detonation experiments were conducted to assess the spall behavior of high purity copper samples with different heat treatment histories. Incipient spall samples were obtained at different sweeping detonation condition. Metallographic and Electron Backscattered Diffraction (EBSD) analyses were performed on the soft-recovered samples. The effects of grain boundaries, grain size, crystal orientation and loading direction on the spall behaviors were discussed. Spall plane branching was found in the main spall plane of the damage samples. For similar microstructure, the area of voids increase with the increase of shock stress, and the coalescence of voids also become more obvious. Results from EBSD analysis show that the grain sizes were decreased and the grains were elongated along the direction of the plate width. Triple junctions composed of two or more general high angle boundaries are the preferred locations for intergranular damage. Voids prefer to nucleate in the grain boundaries composed of grain with high Taylor Factor (TF) than other grains. The damage areas in the grains with high TF are more severe. Boundaries close to perpendicular to the loading direction are more susceptible to void nucleation than the boundaries close to parallel to the loading direction, but the difference of voids nucleated in these two boundaries is less significant than the results obtained by plate impact experiment. It would be caused by the obliquity between the shock loading direction and the plate normal.

Tensile and compressive creep behavior of extruded Mg–10Gd–3Y–0.5Zr (wt.%) alloy

The tensile and compressive creep behavior of an extruded Mg–10Gd–3Y–0.5Zr (wt.%) alloy was investigated at temperatures ranging from 200°C to 300°C and under stresses ranging from 30MPa to 120MPa. There existed an asymmetry in the tensile and compressive creep properties. The minimum creep rate of the alloy was slightly greater in tension than in compression. The measured values of the transient strain and initial creep rate in compression were greater than those in tension. The creep stress exponent was approximately 2.5 at low temperatures (T<250°C) and 3.4 at higher temperatures both in tension and in compression. The compression creep activation energy at low temperatures and high temperatures was 83.4 and 184.3kJ/mol respectively, while one activation energy (184kJ/mol) represented the tensile–creep behavior over the temperature range examined. Dislocation creep was suggested to be the main mechanism in tensile creep and in the high-temperature regime in compressive creep, while grain boundary sliding was suggested to dominate in the low-temperature regime in compressive creep. Precipitate free zones were observed near grain boundaries perpendicular to the loading direction in tension and parallel to the loading direction in compression. Electron backscattered diffraction analysis revealed that the texture changed slightly during creep. Non-basal slip was suggested to contribute to the deformation after basal slip was introduced. In the tensile–creep ruptured specimens, intergranular cracks were mainly observed at general high-angle boundaries.

Quantification of Microstructural Evolution in Grain Boundary Networks

In this work, the effect of grain boundary engineering (GBE) on the structure and connectivity of networks of two types of boundaries was quantified. General high angle boundaries and “special” Σ = 3 and Σ = 9 coincident site lattice boundaries were considered. The effect of GBE processing was to increase the population and length of special boundaries and to disrupt the network of high-angle grain boundaries (HAGBs) in the microstructure. The GBE processing resulted in an increase in the population of special boundaries as determined by line length fraction from approximately 37% to approximately 57%. The connectivity of the special boundaries, as determined by topological analysis, increased by a factor of 4, while the connectivity of HAGBs decreased by an order of magnitude. Cluster sizes in the special boundary network increased across the range of sizes, and the maximum cluster size of HAGBs decreased significantly. The metrics reported here allow for a quantitative analysis of grain boundary connectivity in microstructures, as well as for a quantitative means of comparison of microstructures. These metrics will be used in simulations of diffusional creep, with the aim of quantifying structure–property relationships in grain boundary engineered systems.

Hall&amp;ndash;Petch Tensile Yield Stress and Grain Size Relation of Al&amp;ndash;5Mg&amp;ndash;0.5Mn Alloy in Friction-Stir-Processed and Post-Thermal-Exposed Conditions

Friction stir process (FSP) and post thermal exposure were carried out to a microstructure-thermostable cast Al­5Mg­0.5Mn alloy to examine the grain size dependence of tensile yield stress by using Hall­Petch relation, ·y = ·0 + kyd. The measurement of preferred crystalline orientations and grain boundaries characteristic is used to explain the difference of the Hall­Petch parameters. FSP produces fine grain structure and, for the obtained average grain size from 3.7 to 12.8 μm, near-random crystalline orientations and certain proportion of rigid coincident site lattice boundaries (CSLs) is detected. Two-step post thermal exposure leads to a quick development of rotated ©001a type crystalline orientations and the obtained coarse grains with average size from 229 to 508μm are also commonly surrounded by general high angle boundaries. The calculation of Taylor factor and CSLs proportion reveals that stronger texture and grain boundary hardening effects exist in FSPed specimens than in thermal exposed specimens. These items are the major factors leading to larger ·0 and ky in FSPed specimens than in thermal exposed specimens. [doi:10.2320/matertrans.M2013334]

Inhomogeneous deformation behavior in intercrystalline regions in polycrystalline Ni

We report on three-dimensional spatially resolved X-ray microdiffraction measurements of inhomogeneous deformation within individual polycrystalline grains. These measurements provide new insights into the role of grain boundaries. Particularly striking is the qualitatively different mechanical behavior of subgrain volumes near grain boundaries and triple junctions compared to bulk-grain mechanical behavior. These differences are studied by characterizing the evolution of the local orientation distribution in a polycrystalline Ni sample using polychromatic synchrotron X-ray microdiffraction. Dependence of deformation on grain boundary types and triple junctions is determined. Quantitative distinctions in deformation behavior between low-angle boundaries, special low-energy high-angle boundaries, general high-angle boundaries and triple junctions are characterized in terms of spatially resolved misorientation development and extensions of grain boundary effects. In general, larger lattice misorientations are observed near boundaries with higher grain boundary energy. Special high-coincidence grain boundaries exhibit smaller deformation misorientations than general high-angle grain boundaries. These observations are consistent with the observed increases in ductility in grain boundary engineered materials.

Topographic Changes in Ni-5at.%W Substrate After Annealing Under Conditions of Buffer Layer Crystallization

Abstract Topographic changes have been studied in an annealed Ni-5at.%W substrate with a strong cube texture before and after additional annealing reproducing conditions of buffer layer crystallization. It was found that during this additional annealing the microstructure slightly coarsened and that the average depth of grain boundary grooves increased considerably for certain boundary types. Grooves at general high angle boundaries and Σ3 boundaries with large deviations from the ideal twin relationship were found to be more sensitive to the additional heat-treatment than grooves at low angle and true twin boundaries. Average groove widths increased for all boundary types. Despite the observed changes in the extent of grain boundary grooving, the mean surface roughness was almost identical before and after the additional annealing.

Effect of thermomechanical processing on the creep behaviour of Udimet alloy 188

Udimet alloy 188 was subjected to grain-boundary engineering involving thermomechanical processing in an attempt to improve the creep performance and determine the effects on creep deformation processes. The as-received sheet was cold-rolled to either 10, 25 or 35% reduction per pass followed by a solution treatment at 1191°C for 1 h plus air cooling. This sequence was repeated four times and the resultant microstructure and grain-boundary character distribution were described using electron backscatter diffraction. The fraction of general high-angle grain boundaries tended to increase with increased cold rolling. The 10 and 25% cold-rolled materials exhibited lower creep rates than the 35% cold-rolled material. The measured creep stress exponents and activation energies suggested that dislocation creep with lattice self-diffusion was dominant at 760°C for stresses ranging between 100 and 220 MPa. A transition in the creep exponent below the applied stresses of 100 MPa indicated that a different secondary creep mechanism was rate-controlling at low stresses. A significant amount of grain-boundary cracking was observed both on the surface and subsurface of deformed samples, but surface cracks were greater in number and size than those within the bulk. The cracking behaviour was similar in both vacuum and air environments, indicating that grain-boundary cracking was not caused by environment. To assess the mechanisms of crack nucleation, in situ scanning electron microscopy was performed during elevated-temperature (T ≤ 760°C) tensile-creep deformation. Sequential secondary electron imaging and electron backscatter diffraction orientation mapping were performed in situ to allow the evolution of crack nucleation and linkage to be followed. Cracking occurred preferentially along general high-angle grain boundaries and less than 15% of the cracks were found on low-angle grain boundaries and coincident site lattice boundaries. A fracture initiation parameter analysis was performed to identify the role of slip system interactions at the boundaries and their impact on crack nucleation. The parameter was successful in separating the population of intact and cracked general high-angle boundaries at lower levels of strain, but not after crack coalescence dominated the fracture process. The findings of this work have significant implications regarding grain-boundary engineering of this alloy and potentially for other alloy systems.

Grain Boundary Engineering: Progress and Challenges

The progress of grain boundary engineering (GBE) is overviewed and the challenges for further investigations emphasized. It points out that, the electron backscatter diffraction (EBSD) reconstruction of grain boundaries, which gives the information of connectivity interruption of general high angle boundaries (HABs), is more significant than purely pursuing high frequency of so-called special boundaries. The criterion for the optimization of grain boundary character distribution (GBCD) needs to be established. The energy spectrum and the degradation susceptibility of grain boundaries of various characters including HABs and low Σ（Σ≤29） coincidence site lattice (CSL) needs to be studied and ascertained. And finally, the newly proposed model of non-coherent Σ3 interactions for GBCD optimization are discussed.

Developing realistic grain boundary networks for use in molecular dynamics simulations

A Voronoi/Delaunay technique which allows for the construction of three-dimensional grain boundary networks with special boundaries for use in molecular dynamics simulations is presented. Using this technique, we make a series of samples containing clusters of grains with given configurations of CSL, low-angle tilt, and general high-angle boundaries and demonstrate the importance of including special grain boundaries in three-dimensional grain boundary networks in the study of the mechanical properties of nanocrystalline systems.

Grain Boundaries with High Misorientation and Low Mobility

Island grains have been studied in iron samples that had been treated by critical-strainannealing and in commercial silicon iron alloy sheets after incomplete secondary recrystallisation. Such islands remain behind because their boundaries have such a low mobility that the grains cannot shrink away in the time available during annealing. Misorientations of these islands in relation to the grains surrounding them were measured using EBSD. Similar results were observed in both materials. A small number of low angle boundaries were found and also many twin boundaries. The most remarkable observation, however, was the presence of many general high angle boundaries that did not correspond to any evident coincidence relation.

Mechanical Behaviour of Nanocrystalline Copper Related to Grain-boundary Structure

AbstractUnderstanding the mechanical behaviour of metallic nanostructures is a key issue for their development. On the one hand, knowledge of the plastic behaviour at various temperatures is essential to control the synthesis, forming, and machining of such materials. Equally, a clear understanding of atomic and mesoscopic mechanisms, involving defects and their interactions, is essential for the control of ageing and functional properties. Regarding plastic deformation at room temperature, there is now evidence for unusual behaviour in nanostructured metals. In addition to high resistance and ductility, tensile testing reveals peculiar elasto-plastic deformation. Such behaviour was initially attributed to grain-boundary sliding. However, intergranular areas (including triple junctions) may possess special properties compared to their microcrystalline counterparts. For example, low activation energies have been measured for grain-boundary diffusion and it has been observed that grain-boundaries may act as dislocation sources and nucleation sites for deformation twinning.In this paper, we report on analysis on bulk copper nanostructures. Grain-boundaries are studied, by cross-correlating information from mechanical tensile testing and structural analysis, including X-ray diffraction (XRD) and transmission electron microscopy (TEM). Macroscopic bulk specimens (with grain size of about 80 nm) are prepared by powder metallurgy techniques, modified to fit to the special properties of nanocrystalline powders. Processing includes coldisostatic pressing, sintering and differential extrusion. The powders used (grain size of 40 nm) are synthesised by evaporation and cryo-condensation of a metallic vapour within liquid nitrogen. Results on mechanical testing and structural analysis will be reported. Emphasis will be placed on the structure of grain-boundaries (type of grain-boundary, grain-boundary thickness) studied by TEM and high resolution TEM image analysed using the geometric phase technique. The nanostructure was revealed to be consist in agglomerate of nano-size grains separated by low angle grain-boundaries. Agglomerates are themselves separerated by general high angle boundaries. These observations will then be related to the unusual mechanical true stress-true strain curves of the metallic nanostructures.

Measurements of grain boundary mobility during recrystallization of a single-phase aluminium alloy

A combination of in situ annealing and electron backscattered diffraction in the SEM has been used to determine the mobility of high angle grain boundaries in a deformed single-phase Al–Si alloy. It is found that the boundary velocity is directly proportional to the driving pressure and that the activation energy for boundary migration over all the conditions investigated is consistent with control by lattice diffusion of the solute. It is confirmed that tilt boundaries of recrystallized grains misoriented by 40±10° about axes within ±10° of 〈111〉 have an increased mobility compared to other high angle boundaries, whereas the mobilities of 40°〈111〉 twist boundaries are similar to those of general high angle boundaries. The mobility maximum for the 40°〈111〉 tilt boundaries is very broad, which is in contrast to the sharp mobility peaks reported for curvature-driven grain growth, and possible reasons for these differences are discussed.

Recrystallization of electron beam refined tantalum-tungsten alloy : F. Siciliano, H.R.Z. Sandim (McGill University, Montreal, Canada). Int. J. Refractory Metals Hard Mater., vol 14, no 4, 1996, 257–262

Grain boundary character distribution (GBCD) and the effect of GBCD optimization on the intergranular corrosion (IGC) of a cold rolled and subsequently annealed nickel-free and manganese-bearing high-nitrogen austenitic stainless steel were investigated. The results show that the fraction of low Σ coincidence site lattice boundaries increases from 47.3% for the solid solution treated specimen to 83.3% for the specimen cold-rolled by 7% and then annealed at 1423 K for 10 min. Σ3 boundaries of high fraction effectively interrupt the connectivity of non-corrosion-resisting boundaries (special boundaries like Σ9, Σ27, etc. and general high angle boundaries) network, thus improving the IGC resistance.

TEM study of grain boundaries in Ag-clad Bi2Sr2CaCu2Ox tapes

Abstract Grain boundaries in Ag-clad 2212 tapes have been characterized extensively by TEM and HREM. All tapes show a colonial microstructure. The grains in each colony are plate-like, and their c-axes are parallel to each other. Most of the grain boundaries in each colony are low-angle [001] twist boundaries. The equilibrium structure of the twist boundaries consists of the perfect crystal structure plus a square network of the primary grain boundary dislocations. A low-energy twist boundary in the high-angle region was also observed. Observation of a grid of the secondary grain boundary dislocations indicates that the twist boundary consists of the corresponding order structure and substantial periodic strain fields. The colony boundaries are in general high-angle mixed-grain boundaries. The c-axes between colonies are tilted by a large angle that ranges from 10 to 30°. Most of the colony boundary planes are parallel to the (001) lattice plane in one of the grains that form the boundaries. Non-parallel boundaries are strongly faceted into a number of segments. which are parallel to the (001) plane. Large triangular holes were observed along a large part of the colonial boundaries in the cross-sectional samples, indicating that the impurity phases or voids could exist at colonial boundaries.

The Role of grain boundary misorientation in intergranular cracking of Ni-16Cr-9Fe in 360 °C argon and high-Purity water

The effect of grain boundary misorientation on the intergranular cracking behavior of pure Ni-16Cr-9Fe was assessed by determining if low-angle boundaries (LABs) or coincident site lattice boundaries (CSLBs) are more crack resistant than general high-angle boundaries (GHABs) in argon and high-purity water. Cracking susceptibility of boundary types was determined using constant extension rate tensile tests (CERTs) in 360 °C argon and in deaerated, high-purity water. Annealed samples contained 12 to 20 pct CSLBs, while CSLB-enhanced samples contained 27 to 44 pct CSLBs; GHAB proportions varied accordingly. Cracked boundary fractions for CSLB-enhanced samples tested in either environment ranged from 0.01 to 0.08, while those for annealed samples ranged from 0.07 to 0.10, indicating that samples with increased proportions of CSLBs are more crack resistant. No LABs cracked in either environment. In annealed samples, the proportion of CSLBs that cracked in water was 6.7 pct compared to 1.5 pct in argon; the proportion of GHABs that cracked in water was 9.3 pct compared to 6.6 pct for argon. Thus, CSLBs are more crack resistant than GHABs in either environment, and both are more crack resistant in argon than in water. The higher amounts of cracking and the higher CSLB cracking susceptibility in high-purity water indicate the presence of an environmental effect on cracking behavior. The beneficial effect of LABs and CSLBs is likely due to the ability of these boundaries to induce slip in neighboring grains by either transmitting or absorbing and re-emitting lattice dislocations, thereby reducing grain boundary stresses and the propensity for crack initiation. The results indicate that control of grain boundary proportions can improve the intergranular stress corrosion cracking susceptibility of pure Ni-16Cr-9Fe.

Grain boundary kinetics—II. In situ observations of the role of grain boundary dislocations in high-angle boundary migration

The capillary-induced motion of near-Σ5 grain boundaries in Au was directly observed and video-recorded by means of hot-stage in situ transmission electron microscopy. The observed boundary motion differed from the boundary “displacement” caused by the stress-induced passage of secondary grain boundary dislocation (SGBD)-steps along the boundary which was studied in Part I of this work in the following respects. It occurred in a jerky fashion, no translation of one abutting crystal with respect to the other was observed (proving that the motion occurred by the growth of one crystal at the expense of the other), and quantative evidence proved that SGBDs were not involved. These results ruled out the SGBD-Step Model for the observed motion which was identified as common long-range boundary migration. In situ observations of the migration of more general high-angle boundaries in polycrystalline Al films revealed similar jerky motion and an apparent lack of any significant contribution to migration by observed SGBDs. Although it was impossible to rule out absolutely the SGBD-Step Model for these boundaries because all possible SGBDs could not be imaged, its applicability seems highly unlikely. On the basis of these results, it is suggested that the migration of high-angle boundaries, in general, occurs mainly by the local conservative shuffling of atoms across the boundary. These shuffles take place either at pure steps or at other favorable sites, the details of which depend upon the boundary type.

Relationships between grain boundary structure and energy

The Read-Shockley equation describing the dependence of the grain boundary energy on misorientation of the grains, has been reformulated using the recently developed structural unit model of the atomic structure of grain boundaries, so that it is applicable to general high angle boundaries. The boundary energy then consists of the energy of a reference structure, and of the core and elastic energies of the corresponding DSC dislocations. While the latter part of the boundary energy is determined using the elastic theory of dislocations the former two parts are determined on the basis of atomistics of grain boundaries employing the structural unit model. In the framework of this analysis the minimum number of reference structures in a given misorientation range as well as the extent of misorientations related to a given reference structure are determined unambiguously. Hence, the positions of cusps in the energy vs misorientation dependence and their relative depths and extents are also determined uniquely. The validity of this model is demonstrated for the [001] and [111̄] symmetrical tilt boundaries which have been studied by atomistic computer simulations.

An analysis of diffraction from intrinsic structure in general grain boundaries

Abstract The fine structure in diffraction patterns from general grain boundaries and the way it changes with boundary normal are used to analyse the intrinsic structure of two boundaries in 99.999% Cu with misorientations of ∼6° and ∼11°. It is shown in both cases that the total misorientation is accommodated by three arrays of dislocations with non-coplanar Burgers vectors. For the 6° boundary the Burgers vectors are simple 1/2〈111〉 lattice vectors from either grain, but for the 11° boundary the Burgers vectors required involve linear combinations of 1/2〈110〉 lattice vectors. It is shown how the geometrical method of analysis developed and tested here can be applied to the case of general high-angle boundaries in polycrystals.