Effect Of Grain Boundary Character Research Articles

Effect of grain-boundary character on segregation of vacancies: Thermodynamic aspects

Effect of grain-boundary character on segregation of vacancies: Thermodynamic aspects

Ferroelectric domain percolation in polycrystals

Ferroelectric materials possess the ability to switch between polarization states on application of external fields. This switching is facilitated via movement of domains walls and is a critical factor in the performance metrics of these materials. Hence, the interaction of domain walls with microstructural features such as grain boundaries can influence their performance. Experimentally, domain continuity has only been observed over a few grains with no information of the domain percolation length in larger polycrystals. For domain continuity to be energetically feasible, conditions of ferroelectric polarization continuity and domain wall plane matching need to be satisfied at the grain boundary. In this work, we have studied the extent of continuity of domains within modelled polycrystals. It is shown that under tighter conditions of plane matching and uncompensated polarization charge, favorable grain boundary character can have considerable impact on the domain percolation. However, when the conditions of domain continuity are relaxed, due to higher charge and geometric tolerance, domain percolation throughout the polycrystal is a likely phenomenon. It is shown that percolation of domains through a polycrystalline ferroelectric could be tailored by microstructural control of the grain boundary types and defect chemistry control of the charge compensation mechanisms. The ability to manipulate the length-scale of correlated domain wall motion may lead to new opportunities in ferroelectric functional device design. Additionally, these results can be extended to study the effect of grain boundary character on other material phenomena that involve planar interactions at grain boundaries, including ferroelastic twin boundaries and slip plane continuity.

Towards the dependence of radiation damage on the grain boundary character and grain size in tungsten: A combined study of molecular statics and rate theory

Grain boundary (GB) character and radiation conditions restrict the in-depth understanding of the effectiveness of tailoring grain size in controlling the radiation damage and promoting the radiation resistance of materials. Here, we constructed 46 [0 0 1] W symmetric tilt GBs with tilt angle varying from 6.73° to 82.37° Then, molecular statics (MS) was used to investigate the impact of GB character (tilt angle, GB energy, Σ) on the defect energetics. The present results suggested that it is energetically favorable for Vs/SIAs to segregate into the GB and GBs have the biased absorption of SIAs over Vs. Some high-angle and high-energy GBs could provide larger energetic driving force for the V/SIA to segregate. Multiple Vs/SIAs tended to be bound together inside the GB, with a varying binding strength. Using steady-state rate theory (RT) calculations, we explored the Vs accumulation in the W systems with different grain sizes (50–1000 nm) under the typical radiation conditions of temperature (300–1800 K) and dose rate (10−8–102 dpa/s). In particular, based on the results of MS calculations, the effect of GB character on the Vs accumulation was examined by varying V segregation energies (0.99–2.04 eV). In the process, two extreme values (2.04 and 0.99 eV) of V segregation energy were extracted to investigate its impact, with other values being in this range. The results suggested that nanocrystalline (NC) W would exhibit better radiation tolerance than coarse-grained (CG) W under the conditions of a relatively high temperature and low dose rate. A high V segregation energy led to a low damage accumulation in the grain interior. Some high-angle and high-energy GBs with a large V segregation energy and a small V–V binding energy were expected to improve the radiation resistance of NC.

A study on the creep behavior of alloy 709 using in-situ scanning electron microscopy

In this research, an experimental evaluation of creep properties of Alloy 709 in the temperature range of 750–850 °C was undertaken. Alloy 709 is a novel austenitic stainless steel with 20% Cr and 25% Ni by wt% that was developed for application in structural components of nuclear power plants. Creep rupture tests were conducted in an in-situ heating-loading and Scanning Electron Microscope (SEM) unit equipped with Electron Backscatter Diffraction (EBSD) detector and Energy Dispersive Spectroscopy (EDS). “Real-time” creep damage mechanisms of Alloy 709 at various stresses and temperatures using a flat, un-notched sample with continuously reducing cross-section is studied so that the failure and maximum creep damage occurred at the center of the sample where the in-situ SEM imaging could be focused. Accelerated creep tests at temperatures and stresses above service conditions were performed by employing multiple blocks of constant loads where the loads were increased once the sample attained constant creep rate, indicating a secondary creep regime. This technique ensures multiple data points can be obtained from the same test, saves the time required for an otherwise long-term creep test and usage of SEM. Coincident Site Lattice (CSL) boundary maps were collected as control maps before testing, and the grain boundaries were observed during the creep test to understand the effect of grain boundary character on the creep damage mechanism. Void growth, grain boundary separation, and sliding were found to be the main creep mechanisms whose rate is dependent on stress and temperature. Failure mechanisms studied on the fracture surface using SEM fractography were correlated to the sample surface observations to create complementary information to better understand the underline creep mechanism of Alloy 709.

Understanding the Effect of Grain Boundary Character on Dynamic Recrystallization in Stainless Steel 316L

Dynamic recrystallization (DRX) occurs during high-temperature deformation in metals and alloys with low to medium stacking fault energies. Previous simulations and experimental research have shown the effect of temperature and grain size on DRX behavior, but not the effect of the grain boundary character distribution. To investigate the effects of the distribution of grain boundary types, experimental testing was performed on stainless steel 316L specimens with different initial special boundary fractions (SBF). This work was completed in conjunction with computer simulations that used a modified Monte Carlo method which allowed for the addition of anisotropic grain boundary energies using orientation data from electron backscatter diffraction (EBSD). The correlation of the experimental and simulation work allows for a better understanding of how the input parameters in the simulations correspond to what occurs experimentally. Results from both simulations and experiments showed that a higher fraction of so-called “special” boundaries (e.g., Σ3 twin boundaries) delayed the onset of recrystallization to larger strains and that it is energetically favorable for nuclei to form on triple junctions without these so-called “special” boundaries.

SUS430フェライト系ステンレス鋼の粒界疲労き裂形成に及ぼす粒界性格の影響

SUS430フェライト系ステンレス鋼の粒界疲労き裂形成に及ぼす粒界性格の影響

Capillarity-driven shrinkage of grains with tilt and mixed boundaries studied by molecular dynamics

Molecular dynamics simulations of isolated cylindrical grains in aluminum shrinking under capillary forces were carried out to systematically study the effect of grain boundary character on grain shrinkage behavior. Grains with pure tilt and various mixed tilt-twist 14.25°〈100〉 boundaries were examined. The simulation results confirmed that the shrinking grains with pure tilt boundary simultaneously rotated towards higher misorientations. However, the results also showed that even a minor change of the grain boundary character from pure tilt to mixed tilt-twist is enough to avoid grain rotation. The observed kinetics of the shrinking grains was strongly dependent on the character of grain boundaries. The mobility of the mixed boundaries was estimated to be much higher than that of the pure tilt boundary. Also, the shrinkage kinetics of computed grains was found to vary with a decreasing grain size. The advanced stage of shrinkage was characterized by the stagnation or decrease of the rate of grain area change. This behavior of small grains was attributed to a lack of mechanisms for the elimination of the dislocations (boundary structural units) in the grain boundary. It is hypothesized that such shrinkage stagnation of nanosized grains (≤6 nm) can contribute to the thermal stability of nanocrystalline materials.

Effect of grain boundary character of multicrystalline Si on external and internal (phosphorus) gettering of impurities

AbstractWe investigated the effect of the grain boundary (GB) character of multicrystalline Si (mc‐Si) on the efficiency of external and internal gettering of impurities during phosphorus diffusion gettering (PDG). We utilized seed crystals with an artificially designed GB configuration to grow mc‐Si ingots with different artificial GB characters. PDG combined with an originally developed multiple‐cycle gettering technique at low temperature was introduced on intentionally Fe‐contaminated mc‐Si samples to enhance external and internal gettering. A significant positive PDG effect was observed after PDG combined with the multiple‐cycle technique, as evidenced by the increase in lifetimes after PDG. A bright cloud‐like photoluminescence signal around contaminated GBs was observed for artificial Σ5‐GBs and tilt‐GBs after PDG, suggesting the enhancement of the internal gettering efficiency by leaving a cleaner area around the GBs. This result suggests the importance of the control of crystal defect character as well as impurities in mc‐Si ingots, which could strongly affect the PDG efficiency. Copyright © 2016 John Wiley & Sons, Ltd.

Effect of grain boundary character on segregation-induced structural transitions

Segregation-induced structural transitions in metallic grain boundaries are studied with hybrid atomistic Monte Carlo/molecular dynamics simulations using Cu-Zr as a model system, with a specific emphasis on understanding the effect of grain boundary character. With increasing global composition, the six grain boundary types chosen for this study first form ordered complexions, with the local segregation pattern depending on the grain boundary core structure, then transform into disordered complexions when the grain boundary composition reaches a critical value that is temperature dependent. The tendency for this transition to a disordered interfacial structure consistently depends on the relative solute excess, instead of the grain boundary energy or misorientation angle. Grain boundaries with high relative solute excess go through gradual disordering transitions, whereas those with low relative solute excess remain ordered until high global Zr concentrations but then abruptly transform into thick disordered films. The results presented here provide a clear picture of the effect of interface character on both dopant segregation patterns and disordered intergranular film formation, showing that all grain boundaries are not equal when discussing complexion transitions.

Effect of grain boundary character on sink efficiency

The dependence of the width of void-denuded zones (VDZs) on grain boundary (GB) characters was investigated in Cu irradiated with He ions at elevated temperature. Dislocation loops and voids formed near GBs during irradiation were characterized by transmission electron microscopy, and GB misorientations and normal planes were determined by electron back-scatter diffraction. The VDZ widths at Σ3〈110〉 tilt GBs ranged from 0 to 24nm and increased with the GB plane inclination angle. For non-Σ3 GBs, VDZ widths ranged from 40 to 70nm and generally increased with misorientation angle. Nevertheless, there is considerable scatter about this general trend, indicating that the remaining crystallographic parameters also play a role in determining the sink efficiencies of these GBs. In addition, the VDZ widths at two sides of a GB show different values for certain asymmetrical GBs. Voids were also observed within GB planes and their density and radius also appeared to depend on GB character. We conclude that GB sink efficiencies depend on the overall GB character, including both misorientation and GB plane orientation.

Strain-induced grain boundary migration in {112} 〈111〉/{100} 〈001〉 and {123} 〈634〉/{100} 〈001〉 aluminum bicrystals

Abstract {1 1 2} 〈1 1 1〉/{1 0 0} 〈0 0 1〉 and {1 2 3} 〈6 3 4〉/{1 0 0} 〈0 0 1〉 bicrystals were deformed by plane strain compression at a true strain of 0.34 at room temperature and annealed at 400 °C for 180 s. Both bicrystals showed the occurrence of strain-induced grain boundary migration (SIBM) along the original grain boundary. {1 0 0} 〈0 0 1〉 recrystallized areas formed by SIBM invaded the {1 1 2} 〈1 1 1〉 crystal and the {1 2 3} 〈6 3 4〉 crystal, being the adjacent crystals. The migration distance of the SIBM boundary from the original position of the grain boundary was found to be larger in the {1 1 2} 〈1 1 1〉/{1 0 0} 〈0 0 1〉 bicrystal than in the {1 2 3} 〈6 3 4〉/{1 0 0} 〈0 0 1〉 bicrystal. The difference in the migration distance is mainly because of the differences in stored energy between the component crystals, which are the dominant factor in nucleation by bulging of the grain boundary and growth by migration of the SIBM boundary. The effect of grain boundary character on SIBM is small.

Multi-Scale Examination of the Effect of Grain Boundary Character on Irradiation-Induced Defects: A Comparison of In Situ and Ex Situ Techniques

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

Grain Boundary Character of 9Cr Feritic/Martensitic Heat Resistant Steels Strengthened by Nano-Sized MX Precipitates

In this work, two heats of 9Cr ferritic/martensitic heat resistant steels with different carbon and nitrogen contents were prepared. The steels were designed to have much lower carbon content than conventional 9-12Cr heat resistant steels for obtaining dense nano-sized MX precipitates. Microstructure of the two steels in different heat treatment states was analyzed by electron backscatter diffraction (EBSD) method. The results show that grain boundary character is greatly affected by carbon and nitrogen contents. Martensite in the steel with 0.02wt.% carbon and ultra low nitrogen is easier to recrystallize than that in the steel with ultra low carbon and 0.03wt.% nitrogen during tempering treatment. The effect of grain boundary character on stress rupture properties is also discussed.

Reverse Transformation in Fe-Ni Bicrystals

The effect of grain boundary character on the reverse phase transformation was investigated using Fe-Ni bicrystals containing a 90° ‹211› symmetric tilt or a 90° {211} twist boundary, focusing on the transformation temperature during heating, the morphology of reversed austenite and the variant selection. Martensitic transformation behaviour in bicrystals depends strongly on the type of grain boundary. The tilt bicrystals showed a characteristic self-accommodation of martensites across the boundary called cooperative nucleation (C-N) and higher martensite-start temperature (Ms). On the other hand, the reverse transformation-start temperature (As) of bicrystals with the tilt boundary was higher than that of single crystals or bicrystals with the twist boundary. By in-situ observation, a retardation of reverse transformation of cooperatively nucleated martensites was confirmed in the vicinity of the tilt boundary on heating. The C-N reduces a local stress field around martensites which helps the reverse transformation. Therefore, the bicrystals with the tilt boundary containing the C-N show higher than the others. Many martensites near the tilt and the twist grain boundary returned to the austenite phase inheriting the initial orientation. However, numerous sub-boundaries were retained near the tilt boundary. The C-N seems responsible for the lack of information about initial orientation of austenite. Thus, the reverse transformation behaviour in bicrystals was also found to depend strongly on the grain boundary character.

Role of Coincident Site Lattice Boundaries in Creep and Stress Corrosion Cracking

AbstractInterfaces control many properties in engineering materials, several of which are critical to the integrity of the engineering structure. In single phase, solid solution, austenitic alloys, grain boundaries are often the weak link, displaying susceptibility to creep, corrosion and stress corrosion cracking. As such, grain boundary structure control affords the opportunity to improve the overall performance of alloys in a variety of applications. The role of coincident site lattice boundary (CSLB) enhancement and grain boundary connectivity is examined for how it affects the response of an alloy to stress and the environment. Specifically, the effect of grain boundary character on creep, grain boundary sliding, intergranular stress corrosion cracking, and irradiation assisted stress corrosion cracking in austenitic nickel-base (high purity Ni-Cr-Fe and alloy 600) and iron-base (high purity Fe-Cr-Ni and 304 stainless steel) alloys and for ferritic- martensitic alloy T91 is discussed.

Effect of Grain Boundary Character on Selective Growth of Goss Grain in Fe-3%Si Alloy

Effect of Grain Boundary Character on Selective Growth of Goss Grain in Fe-3%Si Alloy

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 (σM) and the morphology of martensites in the stress-assisted transformation were strongly influenced by C–N.

Effect of grain boundary character on the martensitic transformation in Fe–32at.%Ni bicrystals

The effect of grain boundary character on the martensitic transformation was examined in two types of Fe–32at.%Ni bicrystals containing a 90°<211> tilt or a 90°{211} twist boundary focusing on the martensite-start temperature (Ms) during cooling, the morphology of the martensite and the variant selection. The Ms of bicrystals with a tilt boundary was higher than that of single crystals, while bicrystals with a twist boundary showed no significant difference from that of single crystals. Coarse lenticular martensites formed symmetrically in neighbouring grains around the tilt boundary. In contrast, tiny martensites were uniformly distributed in bicrystals with a twist boundary, and in single crystals. In the vicinity of the tilt and twist boundaries, some variants with the habit plane almost parallel to the boundaries were preferentially selected among 24 variants. Moreover, since the equivalent variants in neighbouring grains at the tilt boundary are selected, strain compatibility of the shape strain of martensite across the tilt boundary is satisfied, resulting in an increased Ms. The effect of pre-strain on the heterogeneous nucleation of martensite at the boundaries was also investigated.

Structure-dependent intergranular oxidation in Ni–Fe polycrystalline alloy

The effect of grain-boundary character on intergranular oxidation has been investigated in a Ni–40 at.%Fe polycrystalline alloy. A difference in the extent of intergranular oxidation between coincidence site lattice (hereafter denoted as CSL) boundaries and random boundaries was observed. In general, CSL boundaries were more immune to oxidation than random boundaries. Some specific CSL boundaries (Σ3,11,19) were more resistant to oxidation than other CSL boundaries. This can be explained by a special feature of the grain-boundary energy for f.c.c. crystal structures. It is evident that in addition to the relative orientation relationship some geometrical parameters, such as grain-boundary inclination, are needed for the more precise engineering of grain boundaries to control intergranular oxidation.

Grain boundary control during high temperature low cycle fatigue

Conspicuous microstructural changes were observed for a variety of metallic materials during elevated temperature cyclic deformation, owing to grain growth, grain boundary motion and reorientation of the grain boundaries under 45° with respect to the stress axis. These phenomena are associated with an obvious change of microtexture and orientation correlation between neighbouring grains. The current investigation focused on the evolution of microstructure and microtexture during high temperature low cycle fatigue in pure nickel. The orientations of the individual grains were determined by means of the electron backscattering diffraction technique in a scanning electron microscope. The spatial arrangement of the grain boundaries was measured using optical microscopy. The results are discussed with regard to the effect of grain boundary character on the propensity for grain boundary damage by motion and alignment.