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.     

Ductility and fracture of copper bicrystals with boundary SiO2 particles

Abstract

High temperature deformation andfracture ofCu-SiO₂ bicrystals with [001] twist boundaries of various misorientation angles were investigated under the condition of non-activation of grain boundary sliding. As the misorientation angle increases, the bicrystals became more susceptible to intergranular brittle fracture. Clear intermediate temperature embrittlement was observed in bicrystals with a random high angle boundary. The boundary segregation of O atoms was found to enhance intergranular fracture. Although the boundary SiO₂ particles provide stress concentration sites which cause early formation of boundary cavities, the boundary dependent deformation and fracture behaviour is essentially determined by inherent boundary strength, which is afunction of the misorientation angle.

MST/1969     

Effect of grain boundary on martensite transformation behaviour in Fe–32 at.%Ni bicrystals

Two types of Fe–32 at.%Ni bicrystals containing a 90°{211} tilt or a 90°<211= twist grain boundary were prepared to investigate the effect of grain boundary character on the martensitic transformation behaviour. The martensite-start temperature (Ms) of bicrystals with the tilt boundary was significantly higher than that of single crystals, while Ms of bicrystals with the twist boundary showed no significant difference from that of single crystals. Near the tilt boundary, the coarse lenticular martensites were symmetrically formed in the neighbouring grains. In contrast, the tiny martensites were homogeneously distributed in bicrystals with the twist boundary, similar to those in single crystals. In the vicinity of the tilt and the twist boundaries, some variants with the habit plane almost parallel to the boundaries were preferentially selected among 24 variants; moreover, the equivalent variants in neighbouring grains were adjoined at the tilt boundary. As a result, the compatibility of shape strains across the boundary was maintained in the case of the tilt boundary, resulting in increasing the Ms. Such characteristic nucleation of martensites can be regarded as an example of self-accommodation across the boundary, which is called cooperative nucleation (C–N). From a crystallographic viewpoint, C–N can occur only at the symmetric tilt boundary. Effects of pre-deformation and applied stress on the heterogeneous nucleation at the boundary were also examined; C–N was always confirmed to occur at the tilt boundary, and the advantage of the boundary for nucleation of martensites did not change even under pre-deformation or applied stress. Furthermore, the martensite-start stress (σ_M) and the morphology of martensites in the stress-assisted transformation were strongly influenced by C–N.     

Relationship between coincidence site lattice,boundary plane indices,and boundary energy in nickel

Abstract

This paper reports an analysis both by coincidence site lattice (CSL) categorisation and interface–plane categorisation of a large data set of grain boundary geometry in nickel. The analysis showed that whereas two-thirds of Σ3 and Σ9 CSLs were classed as having ‘special’ geometry (i.e. related to low energy), very few other CSLs had ‘special’ geometries. It was found that for Σ3s there was an empirical relationship connecting the frequency of occurrence of specific planes (mostly asymmetrical tilt types on the 011 zone), the average interplanar spacing at the boundary (d(eff)), and the boundary energy. Σ3 boundaries having a planar coincidence site density (PCSD) of unity were much more poorly represented in the data set than Σ3s having PCSD = 3, indicating the far greater importance of boundary planes than the PCSD. The occurrence of ‘special’ planes at Σ9 boundaries is considered to be partly a geometrical consequence of their contiguity with Σ3s.     

Effect of the grain boundary on the evolution of deformation in a bicrystal

The role of grain boundary constraint in strain localization and concomitant constitutive response was examined by performing a series of uniaxial compression tests on a tantalum bicrystal. Tantalum single crystals were diffusion bonded to form a (011) 90^∘ twist boundary that was compressed along the common [011] direction. The plastic deformation resulted in the creation of deformation bands away from the highly constraining grain boundary, resembling those bands known from single crystal plastic deformation. Near the grain boundary, such deformation band formation could not be detected. Instead a distinctive pattern of crystal lattice rotation was observed that filled a rather large volume (several millimeters in size) around the bicrystal grain boundary. The internal deformation band structure as well as the crystal lattice rotation pattern near the bicrystal grain boundary were characterized and found to give greater rates of work hardening in the neighborhood of the grain boundary.     

Accumulation of coincidence site lattice boundaries during grain growth

Abstract

Monte Carlo simulations were used to investigate the effect of grain growth on the coincidence site lattice (CSL) boundary content of randomly textured polycrystals. Each grain was assigned an orientation, and grain boundary properties were dependent on both the boundary misorientation and the CSL character. While low misorientation angle boundaries (LABs) increase during growth, the fraction of CSL boundaries does not change with time. Decreasing CSL boundary energy and mobility did not alter these results. In contrast with LABs, which are characterised by a scalar misorientation angle, a particular combination of three independent rotation variables is required to create a low energy CSL boundary; thus, these boundaries are unlikely to form or to persist in a random polycrystal. While texture influences boundary formation, a texture that can enhance CSL boundaries is not apparent. Boundary plane effects should not increase CSL fraction during grain growth.     

Effect of two-step cold rolling and annealing on texture,grain boundary character distribution and r-value of Nb + Ti stabilized ferritic stainless steel

In order to improve the r-value of ferritic stainless steel, the Nb + Ti stabilized ferritic stainless steel is subjected to two-step cold rolling and annealing treatment. The results show that two-step cold rolling and annealing could improve the uniformity of grain size and increase the fraction of low-angel grain boundaries and coincidence site lattice boundaries, especially ∑3 and ∑13b. The uniform and high density texture of the two-step specimens is associated with the uniform grain size distribution, low-angel grain boundaries and coincidence site lattice boundaries, which will lead to the increase of the average r-value.     

Grain boundary motion and grain growth in zinc in a high magnetic field

The motion of grain boundaries in zinc bicrystals (99.995 %) driven by the “magnetic” driving force was measured. An in situ technique for observations and continuous recording the boundary migration was applied. Planar symmetrical and asymmetrical $ \left\langle {10\overline{1} 0} \right\rangle $ tilt grain boundaries with rotation angles in the range between 60° and 90° were studied. The boundary migration was measured in the temperature regime between 330 and 415 °C. The mobility of $ \left\langle {10\overline{1} 0} \right\rangle $ tilt boundaries in zinc and its temperature dependence were found to depend on the misorientation angle and the inclination of the boundary plane. An application of a magnetic field during the annealing of cold rolled (90 %) zinc–1.1 % aluminum alloy sheet specimens substantially affected the texture and microstructure evolution. This effect is attributed to the additional magnetic driving force for grain growth arising due to the magnetic anisotropy of zinc.     

Migration of specific planar grain boundaries in bicrystals: application of magnetic fields and mechanical stresses

Recent research on the dynamics of planar grain boundaries is reviewed. Novel measuring techniques developed for in situ observation and recording of magnetically and stress driven grain boundary migration are presented. The results of migration measurements obtained on bismuth, zinc and aluminum bicrystals are addressed. The experiments revealed that the inclination of a 〈112〉 tilt boundary in Bi has a very strong influence on its mobility. The migration of planar tilt grain boundaries with different misorientation angles was measured in situ in bicrystals of high purity zinc. The results proved that there is a pronounced misorientation dependence of grain boundary mobility in the investigated angular range. The shear stress induced migration of planar symmetric 〈100〉 tilt boundaries in aluminum bicrystals was observed to be accompanied by a lateral translation of the adjacent grains. The coupling between boundary motion and shearing is not confined to low angle and some low Σ high angle boundaries, but occurs also for non-coincidence high angle 〈100〉 tilt boundaries. It has been found that also for stress induced grain boundary motion there is a misorientation dependence of the migration activation parameters. Lower values of the activation enthalpy and the pre-exponential mobility factor can be associated with boundaries with tilt angles close to low Σ CSL orientation relationships.     

Effect of the pre-existing carbides on the grain boundary network during grain boundary engineering in a nickel based alloy

Grain boundary engineering was carried out on an aging-treated nickel based Alloy 690, which has precipitated carbides at grain boundaries. Electron backscatter diffraction technique was used to investigate the grain boundary networks. Results show that, compared with the solution-annealed samples, the aging-treated samples with pre-existing carbides at grain boundaries need longer duration or higher temperature during annealing after low-strain tensile deformation for forming high proportion of low-Σ coincidence site lattice grain boundaries (more than 75%). The reason is that the primary recrystallization is inhibited or retarded owing to that the pre-existing carbides are barriers to grain boundaries migration.     

Migration of specific planar grain boundaries in bicrystals: application of magnetic fields and mechanical stresses

Recent research on the dynamics of planar grain boundaries is reviewed. Novel measuring techniques developed for in situ observation and recording of magnetically and stress driven grain boundary migration are presented. The results of migration measurements obtained on bismuth, zinc and aluminum bicrystals are addressed. The experiments revealed that the inclination of a 〈112〉 tilt boundary in Bi has a very strong influence on its mobility. The migration of planar 〈10$ \bar 1 $ \bar 1 0〉 tilt grain boundaries with different misorientation angles was measured in situ in bicrystals of high purity zinc. The results proved that there is a pronounced misorientation dependence of grain boundary mobility in the investigated angular range. The shear stress induced migration of planar symmetric 〈100〉 tilt boundaries in aluminum bicrystals was observed to be accompanied by a lateral translation of the adjacent grains. The coupling between boundary motion and shearing is not confined to low angle and some low Σ high angle boundaries, but occurs also for noncoincidence high angle 〈100〉 tilt boundaries. It has been found that also for stress induced grain boundary motion there is a misorientation dependence of the migration activation parameters. Lower values of the activation enthalpy and the pre-exponential mobility factor can be associated with boundaries with tilt angles close to low Σ CSL orientation relationships.     

Texture evolution and probability distribution of Goss orientation in magnetostrictive Fe–Ga alloy sheets

Texture evolution and the distribution of Goss orientation in polycrystalline Fe–Ga alloy were investigated as a series of rolling and subsequent annealing processes were used to develop highly textured rolled sheet. A dramatic change from the random nature of the as-rolled and primary recrystallized texture is observed when careful control of atmosphere and temperature during anneal leads to development of a sharp Goss orientation over up to 98 % of the surface of a sample during secondary recrystallization. In this work, grain boundary properties in local areas surrounding Goss grains are investigated and the evolution of Goss orientation is traced through the different stages of alloy processing using electron backscatter diffraction analysis. To evaluate the evolution of grains with Goss orientation, {011} grains are selected and separated from other texture components at each processing step and statistical analysis used to correlate the structural inheritance chain of Goss-oriented grains. The four processing stages considered are the alloy after hot rolling, the as-rolled alloy (i.e., after subsequent warm and cold rolling), the alloy after an initial anneal during which primary recrystallization occurs, and the alloy after final anneals in which secondary recrystallization with abnormal grain growth occurs. Analysis of Goss grain orientation probability distribution functions after primary and secondary recrystallization convincingly demonstrates that the orientation of the abnormally grown Goss texture that develops during secondary recrystallization is determined by the orientation of Goss components that develop during the primary recrystallization stage of alloy processing.     

Effect of grain boundary on martensite transformation behaviour in Fe–32 at.%Ni bicrystals

Two types of Fe–32 at.%Ni bicrystals containing a 90°〈211〉 tilt or a 90°{211} twist grain boundary were prepared to investigate the effect of grain boundary character on the martensitic transformation behaviour. The martensite-start temperature (Ms) of bicrystals with the tilt boundary was significantly higher than that of single crystals, while Ms of bicrystals with the twist boundary showed no significant difference from that of single crystals. Near the tilt boundary, the coarse lenticular martensites were symmetrically formed in the neighbouring grains. In contrast, the tiny martensites were homogeneously distributed in bicrystals with the twist boundary, similar to those in single crystals. In the vicinity of the tilt and the twist boundaries, some variants with the habit plane almost parallel to the boundaries were preferentially selected among 24 variants; moreover, the equivalent variants in neighbouring grains were adjoined at the tilt boundary. As a result, the compatibility of shape strains across the boundary was maintained in the case of the tilt boundary, resulting in increasing the Ms. Such characteristic nucleation of martensites can be regarded as an example of self-accommodation across the boundary, which is called cooperative nucleation (C–N). From a crystallographic viewpoint, C–N can occur only at the symmetric tilt boundary. Effects of pre-deformation and applied stress on the heterogeneous nucleation at the boundary were also examined; C–N was always confirmed to occur at the tilt boundary, and the advantage of the boundary for nucleation of martensites did not change even under pre-deformation or applied stress. Furthermore, the martensite-start stress (σ_M) and the morphology of martensites in the stress-assisted transformation were strongly influenced by C–N.     

Misorientation dependence of grain boundary migration in advanced ultra-supercritical Ni-based superalloy

ABSTRACT

The microstructures of a Ni-based superalloy used in the advanced ultra-supercritical condition were investigated after creep deformation. The grain boundary migrated during tertiary creep. Accompanied by the migration of grain boundaries, the coarsened γ′ phase with rodlike shape was formed and the precipitate-free zones emerged around this coarsened γ′. The distributional misorientation angle of these grain boundaries was from 45° to 65°. After the examination of the distribution characteristics of the cracks, it was found that the intergranular cracks did not propagate through the precipitate-free zones with the coarsened γ′.     

Non-linear current–voltage characteristics related to native defects in SrTiO3 and ZnO bicrystals

SrTiO₃ and ZnO bicrystals with various types of boundaries were fabricated in order to examine their current—voltage characteristics across single grain boundaries. Their grain boundary structures were also investigated by high-resolution transmission electron microscopy. In Nb-doped SrTiO₃, electron transport behaviors depend on the type of boundaries. Random type boundaries exhibit highly non-linear current—voltage characteristics, while low angle boundaries show a slight non-linearity. On the contrary, undoped ZnO does not exhibit non-linear current—voltage characteristics in any type of boundaries including random ones. It is suggested that the differences observed in current—voltage properties between the two systems are mainly due to the difference in the accumulation behavior of acceptor-like native defects at grain boundaries. A clear non-linearity is obtained by means of Co-doping even for the highly coherent ∑ 1 boundary in a ZnO bicrystal. This is considered to result from the production of acceptor-like native defects by Co-doping.     

Influence of primary annealing on Goss development in grain oriented silicon steels

Abstract

Two different primary annealing conditions (continuous heating and discontinuous heating) on conventional oriented silicon steel were employed, and the evolution of microstructure and Goss frequency, as well as orientations of Goss neighbourhood from recovery to secondary recrystallisation, was investigated by means of optical microscopy and advanced electron backscatter diffraction (EBSD) technique. It could be concluded that high Goss frequency before secondary recrystallisation possibly did not contribute to sharp Goss orientation, even excellent magnetic property and that no grains with less deviation from ideal Goss first began to grow. As for coincidence site lattice (CSL) grain boundary and high energy grain boundary theories, the latter can explain the development of Goss due to its high frequency compared with CSL boundary of low frequency.     

Interaction of Hydrogen Atoms with Grain Boundaries in Palladium Bicrystals

Hydrogen diffusion in palladium bicrystals containing a small-angle twist or tilt boundary or a large-angle boundary similar to a special boundary is investigated using molecular dynamics simulation. We assess the effect of grain boundaries on the hydrogen diffusion process. The types of grain boundaries considered here are shown to differ in their absorption activity for hydrogen. The temporal grain-boundary segregation of hydrogen atoms can be accounted for in terms of their coordination, which differs significantly from that in the grain bulk.     

Grain boundary engineering for improving stress corrosion cracking of 304 stainless steel

Grain boundary engineering (GBE) via low strain tension and annealing was used to enhance the resistance to stress corrosion cracking of a 304 stainless steel. Electron backscattered diffraction (EBSD) analysis exhibited that the GBE steel had a higher fraction of low-∑ coincidence site lattice (CSL) boundaries, larger grain-clusters, longer twin boundary chains, and fewer paths of connected non-twin boundaries with a more zigzag shape. Slow strain rate tests in high-temperature water showed that the GBE steel performed better plasticity, higher tensile strength, and similar yield strength compared to conventional steel. The low fraction of random boundaries in GBE steel resulted in a lower frequency of intergranular crack initiation, and the zigzag paths of non-twin boundaries made the intergranular crack propagation more difficult.     

Simulation on Grain Boundary Sliding during Superplastic Deformation Using Molecular Dynamics Method

Grain growth and grain boundary sliding are the two main superplastic deformation mechanisms. In the paper, simulation work is focused on the sliding of a∑3 (111) symmetric twist coincidence grain boundary, a ∑13 (110) asymmetric tilt coincidence grain boundary, and a ∑3 (110) symmetric tilt coincidence grain boundary in AI, and the energies of grain boundary for each of equilibrium configurations are computed. An embedded atom method (EAM) potential was used to simulate the atomic interactions in a bicrystal containing more than 2000 atoms. At 0 K, the relationships between total potential energy and time steps for ∑3 (111) symmetric twist coincidence grain boundary and ∑3 (110) symmetric tilt coincidence grain boundary during sliding at 2 m/s represent the periodic characteristic. However, the relationship between total potential energy and time steps for ∑13 (110) asymmetric tilt coincidence grain boundary represents the damp surge characteristic. It is found that grain boundary sliding for ∑3 (110) symmetric tilt coincidence grain boundary is coupled with apparent grain boundary migration.