Incoherent Σ3 Research Articles

Relationships Between the Σ9 and Σ27 Boundaries and the Connectivity of Random Boundary in Hastelloy C-276 Alloy

Hastelloy C-276 was subjected to 10% deformation and annealed at 1100 °C for different time. The fractions of Σ3n (n=1, 2, 3) boundaries and the connectivity of random boundary networks were characterized by electron back-scatter diffraction (EBSD). The results show that the Σ3 boundary fraction is hardly increased during the annealing period. Instead, the fractions of Σ9 and Σ27 boundaries have a dramatic change with annealing time, whose peaks are arisen after annealing for 15 min. Meanwhile, the corresponding connectivity of random boundary network is effectively disrupted. This can be attributed to the incoherent Σ3 boundary producing more Σ9 and Σ27 boundaries and incorporating those boundaries into the random boundary networks. Actually, the connectivity of random boundary networks can be definitely identified by Σ(9+27)/Σ3 radio.

Evolution of Σ3n CSL boundaries in Ni-Cr-Mo alloy during aging treatment

The evolution of Σ3n (n=1,2,3) boundaries in a Ni-Cr-Mo alloy during an aging treatment at 600°C was investigated based on electron backscatter diffraction (EBSD) measurements. Incoherent Σ3 (Σ3ic) and coherent Σ3 (Σ3c) boundaries were identified using the single-section trace analysis method. The results show that the Σ3n boundary distribution characteristics are changed and that a Pt2Mo-type Ni2(Cr,Mo) superlattice phase precipitates during the aging treatment. Through the analysis and comparison of the average grain diameters measured under the different conditions, the migration behaviors of both the Σ3ic boundaries and random high-angle boundaries (HABs) are found. The migration of the Σ3ic boundaries provides the probability of interaction between Σ3n boundaries. This interaction is responsible for some different types of triple junctions, the increase in Σ9 and Σ27 boundary fractions and interrupted connectivity of the network of random HABs. However, the Σ3c boundaries' consumption caused by the migration of random HABs and the difficulty of forming new Σ3c boundaries lead to the decreased fraction of Σ3c boundaries. The further discussion about the increase in the fraction of Σ3ic boundaries indicates that the generation mechanism of the Σ3ic boundaries during the aging treatment differs from the models applied to recrystallization and subsequent grain growth processes.

Experimental quantification of mechanically induced boundary migration in nanocrystalline copper films

A new experimental technique is presented that combines in situ straining with transmission electron microscope-based automated crystal orientation mapping to document microstructural evolution with nanoscale resolution at sequential stages of deformation. Orientation maps of freestanding annealed nanocrystalline Cu films have been collected, and the resultant datasets provide direct measures of grain size, shape and orientation as well as local grain boundary character and position at various stages of applied strain. Numerous examples of stress-driven grain boundary and twin boundary migration were recorded and studied. Detailed analysis of the misorientation of mobile and immobile grain boundaries provided clear experimental evidence that a broad range of grain boundaries are susceptible to stress-assisted migration; no general correlation between grain boundary misorientation and mobility was detected. Incoherent Σ3 boundaries were observed to be significantly more mobile than coherent Σ3 twin boundaries. Nevertheless, deformation twins were observed to nucleate and grow from grain boundaries and triple points.

Effect of deformation parameters on twinning evolution during hot deformation in a typical nickel-based superalloy

The twinning evolution of a typical nickel-based superalloy was investigated by means of isothermal compression tests in the temperature range of 960–1160°C and strain rate range of 0.001–1s−1. During hot deformation, the original Σ3 boundaries lost their Σ3 misorientation due to crystal rotations, while lots of new Σ3 boundaries were formed mainly by growth accidents. With the increasing temperature and decreasing strain rate, the distributions of twin boundaries became more and more uniform, while the length and width of twin boundaries increased in size. In particular, the existing twins promoted the formation of more additional stacking error, and resulted in the occurrence of twinning adjacent to the existing twins, which coalesced with the existing twins and increased the twin width. Moreover, the fraction and density of Σ3 boundaries increased with the increasing deformation temperature and decreasing strain rate firstly, and then they decreased again. At the higher temperature and lower strain rate, the accelerating grain growth inhibited the occurrence of twinning, leading to the decreasing fraction and density of Σ3 boundaries. On the other hand, the higher deformation temperature and lower strain rate would also lead to the lower fraction of incoherent Σ3 boundaries, the reason of which was also discussed.

Morphology Evolution of Grain Boundary Carbides in Highly Twinned Inconel Alloy 600

The effects of grain boundary characters on the morphology evolution of grain boundary carbides in Inconel Alloy 600 with high proportional low Σ coincidence site lattice (CSL) boundaries aged at 715 oC for 1-100 h were investigated by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). During the aging process, the carbides precipitated at coherent twin (Σ3) boundaries are very few and finest within all the aging time. Bar like carbides precipitated near both sides of the incoherent twin (Σ3) boundaries, and bigger carbides than that of coherent Σ3 boundaries had been found on the incoherent Σ3 boundaries. Bar like carbides precipitated near only one side of Σ9 boundaries, and much bigger carbides than that of Σ3 boundaries have been found on the Σ9 boundaries. The morphology of carbides precipitated at Σ27 and random grain boundaries are similar, and is bigger than that of precipitated at other grain boundaries. The carbides precipitated at grain boundaries with all types grow bigger with the aging time prolonging, but their growth rates are different.

Anti-thermal mobility in the Σ3 [111] 60° {11 8 5} grain boundary in nickel: Mechanism and computational considerations

Synthetic driving force molecular dynamics simulations were performed to examine anti-thermal mobility in the incoherent Σ3 [111] 60° {11 8 5} grain boundary in a model FCC metal. This boundary is faceted, and motion occurs by the movement of the {110} facets via the glide of sets of three Shockley partial dislocations. We explore several consequences of this motion mechanism, and show how additional factors must be taken into consideration when simulating this boundary.

Grain size dependence of the twin length fraction in nanocrystalline Cu thin films via transmission electron microscopy based orientation mapping

Abstract

Monte Carlo simulation of primary recrystallization and annealing twinning

The formation of annealing twins has been studied from the beginning of the 20th century and a variety of mechanisms have been suggested. Molecular dynamics simulations on the atomic scale have also been performed. This paper reports a microscale simulation of primary recrystallization and twinning of a nickel alloy based on the Monte Carlo approach. Different twin morphologies were simulated. A possible dependence of grain growth direction on twin formation during annealing was demonstrated. The formation of incoherent Σ3 and Σ9 boundaries is verified as the indirect outcome after coherent Σ3 formation.

Investigations on residual strains and the cathodoluminescence and electron beam induced current signal of grain boundaries in silicon

Cathodoluminescence (CL) and electron beam induced current (EBIC) measurements were used to investigate the optical behavior and electrical activity of grain boundaries (GBs) in coarsely grained silicon. Electron backscatter diffraction (EBSD) was applied for a comprehensive characterization of the structural properties of the high angle and low angle GBs (HAGBs and LAGBs) in the sample. It was found that not only the EBIC but also the panchromatic (pan) CL contrast of Σ3 HAGBs strongly depends on the hkl-type of the boundary plane. At room temperature coherent Σ3 GBs exhibit no significant contrast in the CL or EBIC images, whereas at low temperatures the pan-CL contrast is strong. For incoherent Σ3 GBs, a strong pan-CL and EBIC contrast was observed in the entire temperature range. Only on a LAGB (misorientation angle 4.5°) CL investigations at low temperatures revealed a line with peak position at about (0.82 ± 0.01) eV, usually related to the dislocation associated D1 transition. Cross-correlation EBSD was applied to analyze the strain fields of Σ3 HAGBs as well as of the LAGB. All the components of the local strain tensors were quantitatively determined. The relationship between the extension of the strain field at the LAGB and the spatial D1 intensity distribution is discussed.

Study on grain boundary character and strain distribution of intergranular cracking in the CGHAZ of T23 steel

Intergranular reheat cracking in the coarse-grained heat-affected zone of T23 steel was produced by strain to fracture tests on a Gleeble 3500 thermal–mechanical simulator. Then the grain boundary character, as well as the strain distribution after reheat crack propagation, was studied by electron backscatter diffraction technique. The results showed that incoherent Σ3 boundaries were seldom found on the prior austenite grain boundaries. Therefore, only the type of random high-angle boundaries played a crucial role in the intergranular cracking. Microstructurally cavities and small cracks were preferentially initiated from high-angle grain boundaries. Low-angle grain boundaries and high-angle ones with misorientation angles less than 15° were more resistant to the cracking. More importantly, the fraction of high-angle grain boundaries increased with the plastic strain induced by both temperature gradient and stress in the coarse-grained heat-affected zone, which contributed to the crack initiation and propagation. Furthermore, the strain distributions in the vicinity of cavities and cracks revealed the accommodation processes of plastic deformation during stress relaxation. It also reflected the strength differences between grain interior and grain boundary at different heat-treated temperatures, which had a large influence on the cracking mechanism.

Effects of recovery treatment after large strain on the grain boundary character distributions of subsequently cold rolled and annealed Pb–Ca–Sn–Al alloy

Electron backscatter diffraction (EBSD) measurements showed that recovery treatment of 250 °C/15 min after large strain of 90% cold rolling (thickness reduction) can promote the formation of special boundaries in Pb–Ca–Sn–Al alloy during subsequent moderate cold rolling followed by high temperature annealing, recovery treatments of shorter or longer duration at this temperature have no such effect. Orientation distribution function (ODF), general misorientation and kernel average misorientation (KAM) analyses indicated very strong texture of single S component and specific dislocation configuration in recovered microstructure are probably the key factors of the effect. High-resolution electron backscatter diffraction (HR-EBSD) analysis revealed that the formation, migration and interactions of incoherent Σ3 boundaries might be the mechanism of the effect.

Corrosion Behaviors of Random and Special Grain Boundaries in a Sensitized 304 Stainless Steel

Abstract The corrosion behaviors of random and special grain boundaries were compared in a sensitized type 304 stainless steel sheet previously treated by particular grain boundary engineering process. In order to evaluate the corrosion susceptibility of various grain boundaries, the samples were corroded (etched) in a particular solution and then the characters of grain boundaries were determined by electron backscatter diffraction (EBSD), associating with the microstructure acquired by optical microscopy. The results show that random boundaries are susceptible to corrosion whereas most of CSL grain boundaries (mainly Σ3 and Σ9) exhibit better resistance to corrosion. The incoherent Σ3 boundaries, located in the random grain boundary network, play a more significant role in arresting the development of corrosion along the random grain boundary path than coherent ones. The symmetrical incoherent Σ3 boundaries are more resistant to corrosion than asymmetrical ones. In addition, the more the boundary plane deviates from the exact CSL position, the more susceptible to corrosion it is.

Effects of thermo-mechanical iterations on the grain boundary character distribution of Pb–Ca–Sn–Al alloy

Recrystallized Pb–0.05%Ca–1.5%Sn–0.026%Al (mass fraction) alloy, with an averaged grain size of 20–30 μm, special grain boundary (Σ1–Σ29) fraction of less than 40% and the general high angle boundary (HAB) network of fully connected, was subjected to 1–4 cycles of thermo-mechanical processing (TMP) of rolling at ambient temperature followed by annealing at 270 °C (0.9 T m). Electron back-scatter diffraction (EBSD) techniques were employed to determine the grain boundary character distribution (GBCD) of the processed samples. The results indicated that 1-, 3- and 4-cycle TMP has very strong but nearly identical effects on the GBCD, the fraction of special boundaries enhanced to 80%, and the connectivity of general high angle boundary (HABs) network is interrupted sufficiently by the so-called special boundaries; However, 2-cycle TMP exerts slight impacts on the GBCD, the fraction of special boundaries is only increased to 59.2% and the connectivity of HABs network is not interrupted substantially. Further discussion pointed out the effects of 1-, 3- and 4-cycle TMP might be attributed to the migration and interactions of incoherent Σ3 (Σ3 ic) and its variants Σ9 and Σ27 boundaries, while that of 2-cycle TMP may be caused by an enhanced recrystallization.

Ruddlesden–Popper Planar Faults and Nanotwins in Heteroepitaxial Nonstoichiometric Barium Titanate Thin Films

Nonstoichiometric 10‐mol%‐excess‐BaO–BaTiO3 (Ba1.1TiO3.1) thin film grown on a (100) SrTiO3 substrate consisted of heteroepitaxial c‐axis‐oriented BaTiO3 perovskite crystals including Ruddlesden–Popper planar faults and nanometer‐scale multiple (111) twin lamellae. High‐resolution electron microscopy and electron energy‐loss spectroscopy revealed that nanotwins with coherent (111) Σ3 coincident site lattice boundaries were terminated in the BaTiO3 matrix to form incoherent (211) Σ3 boundaries accommodating excess barium ions. Both Ruddlesden–Popper planar fault and incoherent (211) boundary formation were proposed as possible accommodation mechanisms of excess barium ions in the perovskite film.

Structure modelling of 3 and 9 coincident boundaries in CVD diamond thin films

High-resolution electron microscopy (HREM) was used to investigate the structure of σ3 and σ9 coincident-site-lattice (CSL) boundaries in chemical vapour deposited (CVD) polycrystalline diamond thin films. In the incoherent σ3 boundaries, such as {112} S3, {111}/{115} S3, and {001}/{221}S3 boundaries, no rigid-body translation parallel to >111< direction was identified. The structural units in their atomic models consisted of symmetrical five and seven membered rings, which contained non-tetra-coordinated sites. The σ9 CSL boundary was always parallel to a {114} plane. Its structural units were composed of irregular five- and seven-membered rings, which also contained non-tetra-coordinated sites. Computer image simulations based on multislice calculations were performed using unrelaxed atomic models containing dangling bonds. The simulation results revealed that the models could describe the atomic structure of the boundaries quite well, except for the {112} S3 boundary, where a rigid-body translation perpendicular to the boundary plane is necessary to reproduce the experimental image. Although the reconstruction structure could not be observed along (110) axis directly, the possible influence of the non-tetra-coordinated sites on the boundary stability was discussed in relation to the electronic structure of the boundaries.

Faceting transformation and energy of a Σ3 grain boundary in silver

Grain boundary facets forming at the intersection between a grain boundary and the free surface in diffusion bonded Σ3〈011〉Ag bicrystals during prologed annealing have been characterized crystallographically by metallographic methods. It is shown that the observed faceting has qualitatively the same character as that in Σ3〈011〉 grain boundaries in Cu. The energy of an incoherent Σ3 grain boundary in Ag (210 mJ/m 2) is determined from the dihedral angle of the thermal groove and the extrapolated literature data on the surface tension of Ag. The facet geometry is discussed with respect to computer simulation data on the inclination dependence of the energy of Σ3 grain boundaries in Cu. The geometrical stability of a grain boundary near the free surface is considered.

Quantitative high-resolution transmission electron microscopy of the incoherent Σ3 (211) boundary in Cu

The incoherent (211) twin boundary in Cu was imaged by high-resolution transmission electron microscopy (HRTEM), using a newly designed high-voltage high-resolution microscope. An innovative procedure of automatic structure refinement by off-line digital image processing is applied to determine the atomistic structure of the grain boundary at a well defined level of confidence. The result verifies molecular statics (MS) calculations, which predict the formation of a thin slab with 9R structure in the boundary.

Diffusion induced grain boundary migration of symmetric and asymmetric 〈011〉 {011} tilt boundaries during the diffusion of Zn into Cu

Diffusion induced grain boundary migration (DIGM) of symmetric and asymmetric 〈011〉{011} tilt boundaries during the diffusion of Zn into Cu bicrystals having misorientation angles θ ranging from 10° to 172° was studied over the temperature range from 623 to 773 K using light microscopy and electron probe microanalysis. All boundaries except for the low angle ( θ < 15°) and coherent Σ3 twin boundaries exhibited DIGM with the migration being randomly distributed on both sides of the originally symmetric boundaries and in preferred directions for the asymmetric boundaries. Symmetric Σ19a boundaries exhibited a unique faceted form of DIGM in which all boundary migration resulted in triangular shaped alloyed regions having {111} and {113} habit planes in the neighboring grains. At 693 K the average migration rates of the grain boundaries during DIGM as well as calculated grain boundary diffusivities exhibited a strong dependence on misorientation angle with some coincident site boundaries such as the Σ9 boundaries exhibiting high migration rates and others such as the incoherent Σ3 twin boundaries exhibiting low migration rates. For asymmetric Σ19a boundaries the migration rate at 693 K was observed to increase with increasing asymmetry. Grain boundary diffusivity values calculated for Σ19a, Σ9 and 45° high angle grain boundaries over the temperature range from 627 to 773 K agreed well with the values reported for Zn tracer diffusion in stationary Cu grain boundaries.

Dissociated grain boundary dislocations in some incoherent twin boundaries

It is shown that partial grain boundary dislocations (GBDs) form two of the three independent arrays of secondary GBDs in incoherent Σ3 boundary facets on {421}/{1085} planes in a f.c.c. Cu+6at%Si ...

A coincidence — Ledge — Dislocation description of grain boundaries

In the last few decades, it has been recognized that a single grain boundary (GB) may exist in several different stable and metastable states, which differ in their atomic structure. However, experimental insights at the atomic structure level are rarely reported. In this study, two different microstates of incoherent Σ3 [111] (112¯) GBs from two different orientation relationships (ORs) in an aluminum thin film grown on sapphire, comprised of slightly different structural units, are reported. The structural units in ORII exhibit hexagonal units while in ORI the hexagonal units are slightly distorted. Molecular statics simulations are utilised to understand the difference in excess properties of both states, which suggest that strain could potentially contribute to the stability of the ORII GB microstate over the ORI microstate. In addition, the atomic structure of the two ORs along the 〈110〉 zone axis reveal the rigid body microscopic translations of different magnitude of {111} planes across the GB experimentally. Furthermore, in the case of asymmetric variants of the same GBs, different types of Σ3 [111] disconnections with Burgers vectors (b1 = 1/6[1¯1¯2]), (b2 = 1/2[1¯01]) and varying step heights (h = 2adsc, 5adsc) are investigated and their implications on the GB mobility are discussed.