Measurements of spacing of sliding grain boundaries

The spacing of grain boundaries at which grain boundary sliding (GBS) had occurred during superplastic (SP) deformation was determined by measuring the length of segments of marker lines inscribed on the pre-polished surface in Pb-62%Sn after superplastic deformation in shear. Statistical distribution of this segment length (L) was bimodal at low strain levels, but became unimodal at high strain levels. The concept of cooperative GBS, i.e. sliding of groups of grains as an entity, has been invoked to explain the evaluation of the L-distribution with strain. This investigation suggests that the real spacing of sliding grain boundaries should be taken into account for modelling of SP flow.On leave from Ufa Aviation Institute, Ufa 450025, Russia.     

Grain Boundary Sliding at Serrated Grain Boundaries

A constitutive model is developed for grain boundary sliding (GBS) at serrated grain boundaries. Based on a previously developed GBS model, using the dynamics of grain boundary dislocation pile-up, the present model takes the average of the sliding rate over the characteristic dimensions of grain boundary serrations. Thus, a geometric factor is introduced to account for the effects of serration wave length and amplitude on the GBS rate, as compared to the GBS rate at planar boundaries. By considering the role of grain boundary shear stress in stress balancing, the proposed model removes the singularity at planar boundaries which exists in the diffusion-controlled GBS model at serrated grain boundaries. The modified model describes very well the transient creep of complex Ni-base superalloys with and without grain boundary serrations and should be suitable for other engineering alloys (with the exception of columnar grained and single crystal alloys).     

Studying grain boundary regions in polycrystalline materials using spherical nano-indentation and orientation imaging microscopy

In this article, we report on the application of our spherical nanoindentation data analysis protocols to study the mechanical response of grain boundary regions in as-cast and 30% deformed polycrystalline Fe–3%Si steel. In particular, we demonstrate that it is possible to investigate the role of grain boundaries in the mechanical deformation of polycrystalline samples by systematically studying the changes in the indentation stress–strain curves as a function of the distance from the grain boundary. Such datasets, when combined with the local crystal lattice orientation information obtained using orientation imaging microscopy, open new avenues for characterizing the mechanical behavior of grain boundaries based on their misorientation angle, dislocation density content near the boundary, and their propensity for dislocation source/sink behavior.     

Quantitative evaluation of high temperature deformation mechanisms: a specific microgrid extensometry technique coupled with EBSD analysis

A microgrid extensometry method has been developed and used to obtain information about intragranular and intergranular creep mechanisms. An oxide grid was deposited on a creep specimen using an electron lithography technique. This oxide grid offers high backscattered electron contrast and can withstand long duration creep tests under vacuum in the 700–850 °C range without degradation. Specific methods were used to measure in-plane displacements at the grid nodes or at the grain boundaries using correlation of grid images taken before and after the creep test. The local strain and grain boundary sliding (GBS) data were then calculated. Combined information about grain boundary crystallography and GBS has been obtained by superimposing the electron backscattered diffraction (EBSD) map on the deformation maps. To illustrate the potential of this set of processes, two examples of application on a nickel-base disc superalloy are presented. The first one concerns the influence of the creep temperature on the local strain and the GBS. The second application quantitatively shows the influence of grain boundary character on GBS of this material.     

Inconel 600合金中不同类型晶界处铬的浓度

应用TEM、EDS和EBSD等技术研究了Inconel 600合金在715℃时效过程中不同类型晶界处碳化物的结构、形貌和晶界附近Cr浓度的分布。结果表明,不同类型晶界处碳化物的结构和形貌有较大的不同：在Σ3_c晶界析出的碳化物很少,在Σ3_i晶界析出不规则形状的M₂₃C₆碳化物,在Σ9晶界析出较大的M₂₃C₆碳化物颗粒,在Σ27晶界随机析出粗大的M₇C₃碳化物颗粒。富Cr碳化物在晶界的析出使晶界附近贫铬,在相同的时效条件下晶界的Σ值越高其附近贫铬越严重。随着时效时间的延长各晶界附近贫铬区的宽度不同程度地增大,时效15 h贫铬区的深度最大,时效50 h后深度不同程度地减小。     

Grain boundary migration in Fe-3mass%Si alloy bicrystals with twist boundary

Abstract

The characteristics of grain boundary migration in Fe-3mass%Si alloy bicrystals with ∑3(011), ∑5(001) and ∑9(011) coincidence twist boundaries and random twist boundaries were examined to obtain an information on the development of {110}(001) (Goss) texture. The bicrystals were annealed at 1223 K for an appropriate time and the grain boundary migration speed was evaluated.

The ∑5 001l and ∑9 011l twist boundaries showed higher migration speed than ∑3(011) twist boundaries, and the random twist boundaries migrated faster than other boundaries. The migration speed decreased with increasing annealing time due to an increase in the edge components of the lattice misfits in the migrated boundaries. The grain boundary migration was also sensitive to the deviation angle (?θ) from the ideal orientation relationship for a coincidence boundary. The increase of ?θ accelerated the boundary migration. The motion of the grain boundary was influenced by plastic strain. Migration of the ∑9 twist boundary was more suppressed by plastic strain than that of the random boundary. On the basis of characteristics of the grain boundary migration, the effect of inhibitor on the Goss texture was discussed. © 2000 Elsevier Science Ltd. All rights reserved.     

Plastic deformation and fracture in coarse-grained aluminum

Plastic deformation and fracture in aluminum polycrystalline aggregate were investigated experimentally. A series of tensile specimens with a single edge crack were made of coarse-grained aluminum plates. The in-plane moiré technique was used to quantitatively obtain the deformation field around the crack tip. The strain field ahead of the crack tip prior to crack growth, as well as grain rotations during the course of plastic deformation, were evaluated from the corresponding moiré fringe patterns. The results of this study show that for small plastic deformation, grain rotation starts to take place at the very beginning of the plastic deformation and increases proportionally with plastic strain. The plastic strain ahead of the crack tip prior to crack growth drops significantly with decreasing average grain size of the specimen. Grain boundary sliding was also observed at some of the grain boundaries where the resolved shear stress had reached a critical value. The results also show that the crack propagated with maximum velocity at the center of a grain and assumed much slower velocity near grain boundaries or grain boundary junctions. The influence of the deformation rate is also discussed in terms of the stress relaxation.     

Slip behaviour near a grain boundary in high-cycle fatigue of poly-crystal copper

In order to investigate the characteristic slip behaviour near a grain boundary in high‐cycle fatigue, a high‐cycle fatigue test is carried out using a copper poly‐crystal specimen, which consists of several tens of grains. Seventeen persistent slip bands (PSBs) are observed along the grain boundaries. Their location and the activated slip system are different from those expected by the Schmid factor. After the fatigue test, the crystalline orientation and the three‐dimensional shape of each grain are specified by the repetition of polishing and observation by means of an orientation‐imaging microscope (OIM). A finite‐element method (FEM) analysis is also conducted for the specimen with the same orientation and shape of grains taking into account the anisotropy. This analysis reveals that the shear stress concentrates near the grain boundaries where the PSBs are formed. The activated slip systems predicted by the maximum resolved shear stress agree well with those observed in the fatigue experiment. Thus, the characteristic slip near the grain boundary in the poly‐crystal is governed by the concentrated resolved shear stress on the specific slip system due to the deformation constraint by neighbouring crystals.     

Effect of Transverse Grain Boundary on Microstructure,Texture and Mechanical Properties of Drawn Copper Wires

In the present study, microstructure and texture of drawn copper wires with a large number of transverse grain boundaries have been characterized and their mechanical properties have been analyzed. The results show that the texture evolution is accelerated by transverse grain boundary and the saturation value 60% of volume fraction of 〈111〉 fiber texture component is reached rapidly with increasing strain. For the microstructure of drawn wires with a large number of transverse grain boundaries, the critical strain, where lamellar boundaries form, is less than that for wires with equiaxed grains or columnar grains （all grain boundaries parallel to axis direction）. Since transverse grain boundary accelerates grain subdivision and dislocation density increases rapidly in drawn wires with a large number of transverse grain boundaries, there are a higher flow stress and a higher work hardening rate.     

Grain boundary migration in Fe-3mass%Si alloy bicrystals with twist boundary

The characteristics of grain boundary migration in Fe-3mass%Si alloy bicrystals with Σ3〈011〉, Σ5〈001〉 and Σ9〈011〉 coincidence twist boundaries and random twist boundaries were examined to obtain an information on the development of {110}〈001〉 (Goss) texture. The bicrystals were annealed at 1223 K for an appropriate time and the grain boundary migration speed was evaluated.The Σ5〈001〉 and Σ9〈011〉 twist boundaries showed higher migration speed than Σ3〈011〉 twist boundaries, and the random twist boundaries migrated faster than other boundaries. The migration speed decreased with increasing annealing time due to an increase in the edge components of the lattice misfits in the migrated boundaries. The grain boundary migration was also sensitive to the deviation angle (Δθ) from the ideal orientation relationship for a coincidence boundary. The increase of Δθ accelerated the boundary migration. The motion of the grain boundary was influenced by plastic strain. Migration of the Σ9 twist boundary was more suppressed by plastic strain than that of the random boundary. On the basis of characteristics of the grain boundary migration, the effect of inhibitor on the Goss texture was discussed.     

Cobalt intergranular segregation in WC-Co composites

Interfaces between carbide grains in the tungsten carbide-cobalt composite have been considered. Different techniques, such as transmission electron microscopy and energy dispersive X-ray analysis have been used to characterize the orientation relationship, the nature of the planes and the chemical composition of the grain boundaries.The cobalt concentration at WC-WC grain boundaries was determined by X-ray energy selective analysis in the TEM. Cobalt profiles were performed across low-angle grain boundaries, coincidence and general grain boundaries. Cobalt segregation was found whenever dislocations were imaged in the grain-boundary plane of a low-energy grain boundary. The segregation value was compared with the segregation ratio measured in special grain boundaries characterized by a coincidence site lattice.     

Structure of special grain boundaries in SiAlON ceramics

The microstructure of hot-pressed samples of the 15 R polytype phase in the Si-Al-O-N system was studied by means of TEM. Emphasis was put on studies of high angle grain boundaries. In this material high angle boundaries of arbitrary orientation usually possess a vitreous grain boundary phase. However, special grain boundaries were found, which were free of any vitreous grain boundary phase. From the orientation relation of the adjacent grains Σ=1 (coherent reflection twin boundary), Σ=7 and Σ=13 boundaries were found. For their explanation a coincidence site lattice (CSL) model was developed for (0001) twist boundaries. By assuming an exponential form for the potential of atomic interaction, the calculation of minimum grain boundary energies for special twist angles was in accordance with the experimental observations.     

Microstructure Evolution During High-Temperature Deformation of 8090 Al-Li Alloy

The microstructure evolution of an as-processed 8090 Al-Li alloy during high temperature deformation has been investigated with emphasis on the dynamic grain size refinement and the formation of high-angle grain boundaries. Tensile tests were conducted at temperatures 470-560°C and initial strain rates of 10^-2-10^-6 s^-1. The starting and deformed samples were characterized using Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and Electron Backscattered Diffraction (EBSD). The material showed a maximum elongation to failure of 660% at 530°C and strain rate of 10^-3 s^-1. A microstructural transformation from coarse grains to uniform fine microstructure through dynamic recrystallization (DRX) was observed. The DRX process was characterized by (1) a bimodal microstructure, (2) a gradual increase in average boundary misorientation angles, and (3) a gradual decrease of microtexture. The development of high angle boundaries was attributed to the absorption of dislocations into subboundaries and the grain boundary sliding (GBS)-induced subgrain rotation. The microstructural evolution was suggested to be responsible for the superplastic behavior observed in this as-processed material.     

Effect of grain boundary characteristics on mechanical behaviour of quenched aluminium

Abstract

Tests concerning the mechanical properties of quenched aluminium were carried out on samples having different grain boundary characteristics. In comparison with aluminium of equilibrium structure, the quenched aluminium was found to be characterised by: a greater value of yield point in samples having a low fraction of special grain boundaries, and a lower value of yield point and a reduction of the Lüders strain in samples having a high fraction of special grain boundaries. The effects can be ascribed to the activity of dislocation loops as sources of dislocations or to a change in grain boundary characteristics as a result of annihilation of non-equilibrium vacancies on the grain boundaries.

MST/965     

Direct observation and quantification of grain boundary shear-migration coupling in polycrystalline Al

Rapid grain boundary motion under stress has been observed during in-situ TEM straining experiment in polycrystalline Al at moderate temperatures. Crystallographic orientation of both sides of moving grain boundaries (GBs) was simultaneously recorded. A shear produced by the grain boundary migration has been estimated using fiducial markers and image correlation yielding a coupling factor (i.e., ratio of the shear strain over the migration distance) of around 7%. This result is compared to existing models based on shear coupled migration theories, and is discussed within the new Shear Migration Geometrical model (SMIG).     

Effects of Deformation (Strain) and Heat Treatment on Grain Boundary Sensitization and Precipitation in Austenitic Stainless Steels

Sensitization, particularly the degree of sensitization (DOS) in type 316 stainless steel pipe is critically dependent upon the solution anneal of the mill-annealed or commercial material, and is particularly sensitive to low-temperature aging when the starting material is solution annealed between about 1,000°C and 1,100°C. It is observed that when the DOS is above about 10 C/cm² (quantitative electrochemical potentiokinetic reactivation units in Coulombs/cm²), noticeable carbide precipitation occurs in the grain boundaries and increases with increasing DOS. Transmission electron microscopy (TEM) examination of precipitation occurring in type 316 stainless steel pipe grain boundaries has shown them to exhibit many microstructural features that seem to be coincident with grain boundary microstructures, particularly ledges. Uniaxial, tensile strain prior to aging of solution annealed pipe was observed to have a dramatic effect on DOS of type 316 stainless steel pipe; DOS increases with increasing strain. This effect appears to be correlated with the propensity of dislocations to be generated near grain boundaries, and for grain boundary microstructural changes to be related with strain, as observed in type 304 stainless steel. The effects of strain on DOS, combined with instabilities connected with annealing of type 316 stainless steel, indicate that in the practical utilization of pipe, it is imperative to reduce deformation prior to service and during service applications, to avoid sensitization and, thus, minimize the probability of grain boundary cracking.     

Experimental prediction of deformation mechanism after continuous dynamic recrystallization in superplastic P/M7475

The deformation mechanism in high-strain-rate superplastic P/M7475 before and after continuous dynamic recrystallization (CDRX) was investigated. The recrystallization process in P/M7475 differed from that in conventional superplastic material, I/M7475. In I/M7475, the fine-grained microstructure was obtained by static recrystallization before deformation. On the other hand, the substructure in P/M7475 evolved into fine grains during deformation by CDRX. The percentage of high-angle and random boundaries was low at an initial stage of deformation. However, it increased with strain in P/M7475. The microstructural change in P/M7475 influenced a deformation mechanism and affected grain boundary sliding (GBS). The ratio of contribution of GBS to total elongation was low at an early stage of deformation in P/M7475. However, it increased with deformation progressed. It is suggested that the deformation behavior in P/M7475 changed from dislocation creep to superplasticity as the dominant deformation mechanism changed to GBS. The activation energy for superplastic flow in P/M7475 was close to that for lattice self-diffusion in pure aluminum. It is therefore concluded that the dominant deformation mechanism after CDRX in P/M7475 is GBS accommodated by dislocation movement controlled by lattice self-diffusion, similar to that in I/M7475.     

全层状TiAl基合金断裂中晶界的双重作用

通过SEM原位拉抻技术和双晶体压缩实验研究了全片层TiAI基合金晶界断裂行为。研究表明,在全层状组织的断裂行为中,晶界具有双重作用。一方面,微裂纹首先萌发于晶界区,其扩展方式取决于晶界两侧片层的取向。另一方面,不同类型的晶界对裂纹扩展的阻力不同,因而对全层状TiAI基合金韧性的作用不同,纵向晶界有助于断裂韧性的提高,而横向晶界对合金韧性不利。     

Grain boundary sliding revisited: Developments in sliding over four decades

It is now recognized that grain boundary sliding (GBS) is often an important mode of deformation in polycrystalline materials. This paper reviews the developments in GBS over the last four decades including the procedures available for estimating the strain contributed by sliding to the total strain, ξ, and the division into Rachinger GBS in conventional creep and Lifshitz GBS in diffusion creep. It is shown that Rachinger GBS occurs under two distinct conditions in conventional creep depending upon whether the grain size, d, is larger or smaller than the equilibrium subgrain size, λ. A unified model is presented leading to separate rate equations for Rachinger GBS in power-law creep and superplasticity. It is demonstrated that these two equations are in excellent agreement with experimental observations. There are additional recent predictions, not fully resolved at the present time, concerning the role of GBS in nanostructured materials.     

Characteristics of hydrogen-assisted intergranular fatigue crack growth in interstitial-free steel: role of plastic strain localization

The behavior of intergranular fatigue crack growth in an interstitial-free (IF) steel in a hydrogen environment was investigated at different frequencies. Focusing on the plastic strain localization, we observed details of the striation-like feature on the intergranular fracture surface, slip behavior around microvoids, and crystallographic orientation gradient underneath the fracture surface. It was determined that the intergranular fatigue crack growth mechanism in the IF steel is microvoid formation at the crack tip and subsequent coalescence with the crack. Moreover, it was found that the grain boundaries, acting as propagation paths, suffer from pre-damage arising from plastic strain localization near the grain boundaries even before the main crack propagates to a certain location. Therefore, fatigue cracks in a hydrogen environment easily propagate to the grain boundaries. The frequency dependence of fatigue crack growth in the hydrogen environment is significantly smaller than that in a low carbon steel, probably because of the frequency dependence of the pre-damage evolution behavior.