Boundary Misorientation Angle Research Articles

Conditions of Strain-Induced Facet Formation during Interaction between a Lattice Dislocation Pile-Up and a Grain Boundary

A model describing the formation of a strain-induced facet at a high-angle boundary during its interaction with a dislocation pile-up is proposed in this work. The model of continuous dislocation distribution in a pile-up has been used to analyze the conditions under which a facet appears and to calculate its length as a function of the total Burgers vector of the dislocation pile-up and the boundary misorientation angle. The formation of a facet at a grain boundary is possible only if the total Burgers vector exceeds a certain critical value that depends on the external stress and boundary misorientation. The facet length as a function of the initial distance between a dislocation source and a grain boundary at various external stresses and misorientation angles has been calculated in the case of a pile-up replenished by a dislocation source. It is found that the material grain structure refinement and a shift of grain boundaries misorientation spectra into the region of large angles suppress strain induced grain boundary faceting. The facet formation has been shown to be accompanied by the formation of a shear band in the body of the adjacent grain.

Phase-field modeling of crack propagation in polycrystalline materials

A phase-field model based on a modified form of the regularized formulation of Griffith’s fracture theory is presented to investigate intergranular and transgranular crack propagations in polycrystalline brittle materials. Grains and grain boundaries are incorporated in the crack initiation and propagation model based on a phase-field model for grain growth, in which the elastic anisotropy varies based on the grain orientation angle, and the grain boundary energy is related to the misorientation angle of the adjacent grains. Correction parameters are utilized in the total free energy functional and mechanical equilibrium equations to consider the effect of material strength on crack nucleation and propagation independent of the regularization parameter. This allows controlling the strength and crack surface energy along the grain boundaries as a function of the misorientation angle in order to mediate intergranular and/or transgranular crack propagation. To demonstrate the capability of the proposed model, intergranular and transgranular crack propagation in ZrB2 bicrystal systems under tensile loading are studied in detail. The effects of grain boundary misorientation angle, grain boundary inclination with respect to initial crack direction, and grain boundary strength (and/or crack surface energy) on the crack propagation path are investigated. Intergranular crack propagation can be promoted by specific combinations of grain boundary strength and crack surface energy, which can contribute to the fracture toughness of polycrystalline materials.

Influence of variable Ca-doping on the critical current density of low-angle grain boundaries in YBa2Cu3O7−d

The rapid decrease in the critical current density Jc with grain boundary (GB) misorientation angle θ strongly limits the current-carrying capability of yttrium–barium–copper-oxide (YBCO), and the residual low-angle GB distribution is the most important current-limiting mechanism in biaxially textured YBCO coated conductors. To deepen the understanding of the Ca doping in low-angle GBs in YBCO, transport characteristics of low-angle GBs in YBa2Cu3O7−d bicrystals with different Ca contents x (Y1−xCaxBa2Cu3O7−d) were examined to verify the influence of the carrier density at the GB due to the Ca doping level. Y1−xCaxBa2Cu3O7−d (x = 0, 0.10, 0.15, and 0.30) epitaxial films were deposited by pulsed laser deposition on single crystal SrTiO3 (STO) substrates and 7° and 9° symmetric [001]-tilt bicrystal STO substrates. We found that 15% Ca doping was the most effective at increasing the Jc behavior across the 7° GB in intermediate fields, completely eliminating the GB dissipation signature in the V–I characteristics. For the 9° GB, 10% Ca doping yielded the highest depairing current Jd across the GB, calculated from the flux flow resistivity, even though a significant GB dissipation signature remained in the V–I curves.

Continuous growth and coarsening mechanism of the orientation-preferred Cu6Sn5 at Sn-3.0Ag/(001)Cu interface

Orientation-preferred Cu6Sn5 usually offers solder interconnections superior mechanical and physical performances. In this work, morphology and orientation evolutions of Cu6Sn5 at Sn-3.0Ag/(001)Cu interface were investigated in the 300 °C–300 °C, 230 °C–230 °C and 300 °C–230 °C reflow processes. Results demonstrated that the continuous growth of orientation-preferred Cu6Sn5 was realized in the temperature-decreased reflow process of 300 °C–230 °C and the fraction of 0° boundaries in preferred Cu6Sn5 decreased from 0.81 to 0.60 with time while the fraction of 90° boundaries increased from 0.19 to 0.40; both curvature difference and misorientation angle of grain boundaries should be valued in the growth of preferred Cu6Sn5; the coarsening of preferred Cu6Sn5 includes two methods: coalescence and annexation, induced by the disappearance and migration of grain boundaries respectively. These results are of great significance in the orientation and size control of Cu6Sn5 crystals in 3D packaging.

New perspectives on the grain boundary misorientation angle dependent intergranular corrosion of polycrystalline nickel-based 625 alloy

Low-angle grain boundaries and high-angle grain boundaries with different corrosion depths of percolation were revealed by the in-situ FIB cutting. The grain boundary misorientation angle, grain boundary structure, and the orientations of the grains adjacent to the grain boundary were analyzed by a high-resolution transmission electron microscope from the atomic level. It was found that the low-angle grain boundary was more resistant to intergranular corrosion due to the same orientation of the abutting grains of the grain boundary, rather than the state of precipitates on the grain boundary.

Characterization of heterogeneous–nano structure in austenitic stainless steel: crystal orientations and hardness distribution

Hardness distribution in the heterogeneous–nano (HN) structure developed in a heavily cold-rolled SUS316LN austenitic stainless steel was systematically investigated. The HN structure consisted of twin domains surrounded by shear bands, which were further embedded in the conventional low-angle lamellae. The twining planes of {111} in the twin domains were nearly parallel to the rolling direction (RD). The average twin spacing was 20–40 nm. The longitudinal direction of the lamellae was also nearly parallel to the RD, and the average interboundary spacing was about 100 nm. Rather shortened ultra-fine grains with an average size of 100 nm were well developed within the shear bands. Microstructural observations using the transmission Kikuchi diffraction technique revealed that both of the disorientation and the value of the kernel average misorientation in the twin domains were relatively low compared to those in the shear bands and lamellar grains, and the misorientation angle of twin boundaries within the twin domain still remained 60°. On the other hand, the misorientation angles among grains within the shear bands were considerably high. The intense strain localization within the shear bands during cold rolling would result in the instant formation of ultra-fine grains surrounded by high-angle boundaries. These results suggested that dislocation density within twin domains was lower than that of the shear bands and lamellar grains. Although the twin domains possess a smaller interboundary spacing than that in the shear bands and lamellar grains, nanoindentation tests revealed that hardnesses were almost identical among the component nanostructures. The nearly identical hardness among the component nanostructures would be ascribed to a lower amount of dislocation strengthening in the twin domains than that in the shear bands and lamellae.

Hot cracking behavior and mechanism of a third-generation Ni-based single-crystal superalloy during directed energy deposition

Hot cracking is a frequent and severe defect that occurs during the directed energy deposition (DED) of single-crystal superalloys. Understanding the cracking behavior and mechanism is key to avoiding these defects. Thus, in this study, a third-generation single-crystal superalloy, CMSX-10, was investigated. Hot cracking occurred at high-angle grain boundaries and especially at low-angle grain boundaries. Cracks were formed beyond a critical misorientation angle of 6.9°. Hot cracking was determined to be caused by a stable liquid film, stress concentration, and Re-rich precipitates. The stability of the liquid film depended on dendrite coalescence undercooling which was related to the misorientation angle. The dendrite coalescence undercooling at low-angle grain boundary (misorientation angle 6.9°) was 178 K, which was far higher than the vulnerable temperature interval 38 K for hot cracking within a single dendrite. Stress concentration provided the driving force for crack initiation and propagation. Re-rich precipitates promoted crack initiation by a pinning effect on the liquid feed. These findings provide technical support for achieving high-quality additive manufacturing and repair of non-weldable Ni-based single-crystal superalloys.

On the role and alteration of grain boundaries in/during accommodating plasticity in magnesium

The deformation behavior of two specifically oriented grains in a magnesium tricrystal, and the corresponding changes in the character of their 90°〈101¯0〉 tilt boundary were investigated. Identical samples with grain boundaries aligned parallel to the direction of loading were deformed by plane strain compression at room temperature up to select strains. Orientation measurements showed that character/geometry of the grain boundary changed drastically during plastic straining, which was accompanied by lattice rotation of the adjoining crystallites. The boundary misorientation angle decreased from 90° to 71° and all the way down to 10° at strains of ɛt = – 0.15 and – 0.4, respectively. This was interpreted in terms of grain boundary - dislocation interaction, where lattice dislocations due to favored basal slip collided with the boundary and caused a gradual annihilation of grain boundary structural units. When deformation in the two grains became clearly non-uniform at larger strains of – 0.3 and – 0.4 due to the presence of the boundary, the adjacent regions of the latter underwent a pronounced lattice curvature of about 0.005 deg/µm persisting over a 4 mm distance from the boundary.

Mechanical property and characterization of 7A04-T6 aluminum alloys bonded by friction stir welding

The 7A04-T6 aluminum (Al) alloys were bonded via a friction stir welding (FSW) technique. The tensile strength of this FSW joint was the highest when the welding speed was 120 mm/min and the rotation speed was 1000 r/min, which reached 77.93 % of that of the base material (BM). The nugget zone (NZ) consisted mainly of the fine grains; the grain boundary misorientation angle in NZ was about 15°, indicating that the propagation of micro-crack was hindered to a certain degree, improving the toughness of this joint. The S-N curve of this joint showed that the fatigue strength decreased with an enhancement of the fatigue life when N was below 105, also N was above 106. The fatigue limit (141 MPa) of the joint was calculated through an up-and-down test method, the fatigue fracture position occurred in HAZ, exhibiting the ductile fracture. Then a laser local heat treatment (LLHT) process was carried out on this weld seam, which refined the microstructure, strengthening the mechanical property of this FSW joint.

Geometrically necessary dislocations distribution in face-centred cubic alloy with varied grain size

The evolution of geometrically necessary dislocation (GND) distribution in the deformed 6061 alloy is studied with electron back scatter diffraction (EBSD) and transmission electron microscope (TEM). In dynamic recrystallized state, the average GND density is 8.70 × 1014 m−2 with grain size ~30 μm. In annealed state, the average GND density decreases by an order of magnitude, from 6.81 × 1015 to 7.32 × 1013 m−2 (annealing temperature from 200 to 500 °C) accompanied by increasing grain size. Moreover, the sample has greatest ductility with lowest GND density. According to the tendency of dislocations in TEM, GND gradually migrate from near (200) plane to near (220) plane with increasing annealing temperature. Additionally, GND mainly concentrate on the grain boundaries in the deformed state, and the GND distribution is more homogenous after annealing. It is shown that the GND density is related to the grain boundary type and misorientation angle between adjacent grains. The relationship between average GND density and grain size is approximately parabolic.

Crystallographic study of liquid-metal-embrittlement crack path

Compared to other intergranular degradation phenomena, liquid metal embrittlement (LME) is less understood. The main gap in knowledge has been the lack of fundamental information of how grain-boundary characteristics (e.g. grain boundary misorientation angle and grain boundary crystallographic plane) affects LME susceptibility. In this study, the recently developed trace analysis has been applied to the grain boundaries exhibiting LME cracking. Despite the presence of a diverse range of low- and high-index planes in the entire population of the grain boundaries, liquid metal selectively penetrated high-index random grain boundary planes such as (5 3 1), (5 1 1), and (7 3 3).

Hardness testing and why it matters

In this work we concentrate on the in situ dynamics of interphase boundary motion during transformations. In-situ high temperature electron-back scatter diffraction (HT EBSD) was employed to study the ferrite-austenite-ferrite transformation in low carbon steel. A novel method was designed to derive the velocity of the interphase boundaries from the EBSD phase maps. It is concluded that the motion of the transformation front occurs in a jerky-type motion, i.e. not continuous in time on a microscale, as the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation would predict on the macro-scale. It is shown that HT EBSD is capable of providing relevant supplementary insight in the ferrite-austenite phase transformations with adequate spatiotemporal resolution, which would remain hidden in volume averaging experimental techniques such as X-Ray diffraction. In particular, ferrite-austenite phase boundary velocities between 1.4 ± 0.3 and 4.0 ± 0.2 nm/s were detected during isochronal heating with 0.5 °C/min. The mean interphase boundary velocity was ranging between 5.0 ± 0.2 and 6.4 ± 0.2 nm/s for austenite-ferrite transformation with a cooling rate of 1 °C/min. Ledge growth at isothermal conditions, resulted in velocity of 23 nm/s along phase boundary. A strong dependency of interphase boundary mobility and parent-daughter phase boundary misorientation angles was not found.

Effect of cutting parameters on surface integrity characteristics of Ti-6Al-4V in abrasive water jet machining process

Experiments were conducted on Ti-6Al-4V alloy using abrasive water jet machining (AWJM) by varying process parameters such as water jet pressure, stand-off distance and abrasive flow rate. Surface integrity characteristics like surface roughness, surface hardness and microstructural evolution were studied. Effect of machining parameters on the surface and sub-surface hardness was determined. The grain size distribution, phase fractions, grain boundary deformation and grain boundary misorientation angle of the machined titanium (Ti6Al4V) alloy were studied using electron backscattered diffraction (EBSD) images. XRD images of machined Ti-6Al-4V samples were utilized to assess the peak shift and formation of new phases such as oxides. Crystallite size, dislocation density and lattice strain of the machined samples were evaluated using XRD results. Average grain size, mis-orientation angle, α and β phase fraction were evaluated with respect to AWJM parameters.

Radiation induced grain boundary segregation in ferritic/martensitic steels

Abstract The radiation induced segregation of Cr at grain boundaries (GBs) in Ferritic/Martensitic steels was modeled assuming vacancy and interstitialcy diffusion mechanisms. In particular, the dependence of segregation on temperature and grain boundary misorientation angle was analyzed. It is found that Cr enriches at grain boundaries at low temperatures primarily through the interstitialcy mechanism while depletes at high temperatures predominantly through the vacancy mechanism. There is a crossover from Cr enrichment to depletion at an intermediate temperature where the Cr:Fe vacancy and interstitialcy diffusion coefficient ratios intersect. The bell-shape Cr enrichment response is attributed to the decreasing void sinks inside the grains as temperature rises. It is also shown that low angle grain boundaries (LAGBs) and special Σ coincidence-site lattice (CSL) grain boundaries exhibit suppressed radiation induced segregation (RIS) response while high angle grain boundaries (HAGBs) have high RIS segregation. This different behavior is attributed to the variations in dislocation density at different grain boundaries.

Mechanical behavior of (Ni, Fe)Cr2O4 spinel grain boundaries studied by molecular dynamics simulations.

Using molecular dynamics simulations, we study the fracture initiation of grain boundaries in NiCr2O4 and FeCr2O4 of spinel structure. These compounds are representative of corrosion layers of nickel-chromium-iron austenitic stainless alloys. Uniaxial deformation is applied to several symmetric tilt-, twist-, and random-grain boundaries until complete decohesion is reached, in order to measure the critical cleavage stresses. We find that this mechanical quantity depends on the chemical composition and the structure of the grain boundaries. A correlation between the critical stress and the misorientation angle of the grain boundaries can be established. It is also found than the twist- and the random-grain boundaries exhibit the weakest resistance. Furthermore, these simulations show a localization of the elastic response in the vicinity of the grain boundary plane. Therefore, we were able to compare our simulation results with those provided by the theoretical model based on the universal binding energy relation that relates the critical stress to the cleavage grain boundary energy. It shows that this model provides similar critical stresses than the molecular dynamics simulations if the localization distance parameter is defined as the distance related to the deformation of the grain boundary thickness during the deformation.

High thermal stability and excellent mechanical properties of ultrafine-grained high-purity copper sheets subjected to asymmetric cryorolling

For most ultrafine-grained metals, the yield stress increases with finer grain size, but the thermal stability reduces. In this study, high purity (99.999%) copper sheets were fabricated using three different techniques: symmetric rolling, asymmetric rolling and asymmetric cryorolling. In each case, the sheets were annealed at a temperature ranging from 50 °C to 125 °C for 1 h. Their mechanical properties were tensile-tested using dog-bone samples, and their microstructure evolution was examined using electron backscatter diffraction and transmission electron microscopy. The results show that the asymmetric-cryorolled copper sheets have finer grains and higher tensile strength, and better thermal stability compared with the copper sheets subjected to symmetric rolling and asymmetric rolling and low-temperature annealing. The finer grains in copper sheets subjected to asymmetric cryorolling result from the additional shear strain and severe plastic deformation at low temperature. The improvement in the thermal stability may be due mainly to the vacancy clusters, small laminate thickness, low-angle grain boundary and high misorientation angle in asymmetric cryorolled samples. These results can provide significant insights into the development of ultrafine-grained metal sheets with both excellent mechanical properties and high thermal stability.

Texture control of Inconel 718 superalloy in laser additive manufacturing by an external magnetic field

Texture control is a major challenge in laser-assisted additive manufacturing of metal part due to the focused heat source and rapid cooling. In this study, a novel magnetic-field-assisted laser additive manufacturing method is proposed to obtain highly oriented texture. Inconel 718 samples are deposited under a transverse magnetic field of various intensities and different levels of laser power. The elemental distribution, grain morphology, and texture of as-built material are carefully analyzed using various material characterization techniques, including, SEM, EDS, and EBSD. The results indicate that the magnetic field changes the interdendritic microsegregation pattern from a continuous chain to discrete morphology. The direction of deposited material tends to align with the transverse magnetic field. The orientation of stray crystals is more strictly constrained by both vertical heat flux and transverse magnetic field, resulting in a highly oriented texture on horizontal cross-section and texture on the vertical cross-section. It is also observed that by applying the transverse magnetic field, the average grain boundary misorientation angle significantly decreases from 32° to 19°, and the maximum multiple of uniform density increases from 3.0 to 5.3. The effects of laser power as well as magnetic field strength are investigated, and the underlying orientation effect is discussed.

Effect of ion implantation on the grain boundary strength of heat treated Inconel X750

Bending tests performed with notched cantilever micro-beams were used to measure the initiate of plastic deformation from grain boundaries in Ni+ and He+ implanted Inconel X750. Whereas the local grain boundary yield stress followed a complex nonlinear dependence upon grain boundary misorientation angle up to about 60⁰, it showed a simpler continuous decreasing dependence upon grain boundary energy. This suggests that the grain boundary strength of the heat treated Inconel X750 alloy, in all conditions tested, is related to the grain boundary energy and decreases as the energy increases. Both Ni+ and He+ ion implantation increased the local grain boundary yield stress over the complete range of boundary misorientation investigated. We attribute this to the strengthening effect resulting from segregation of ion-induced crystal defects and helium bubbles/cavities to the grain boundaries. Our data suggest that the strengthening effect from 5000 appm He+, implanted at 25 °C, is significantly higher than what is produced by 1 dpa crystal damage associated with Ni+ implantation at 25 °C. Analysis of these findings in terms of a dispersed barrier hardening model illustrate that accumulated helium is more effective than irradiation-induced crystal defects, such as stacking faults tetrahedra and small dislocation loops, at preventing dislocation nucleation and/or movement from grain boundaries in the heat treated Inconel X750 alloy.

The impact toughness of novel keyhole TIG welded duplex stainless steel joints

Abstract Keyhole tungsten inert gas (K-TIG) welding is a novel, high-efficiency welding method. In this study, 10.8-mm-thick duplex stainless steel (DSS) plates were welded via K-TIG welding, without groove preparation or filler metals. The microstructure, grain size and grain boundary misorientation angle distribution (GBMAD) of the weld metal (WM) were analyzed to characterize its impact toughness. Charpy impact tests were carried out, and the results proved that the impact absorbed energy (IAE) of the WM increases with the increase in heat input. This finding indicates that the austenite volume fraction and the GBMAD have an influence on the impact property of the WM.

Effect of grain boundary misorientation on the apparent diffusivity in nanocrystalline aluminum by atomistic simulation study

To obtain a fundamental understanding of the effect of structure and geometry of grain boundary on the diffusion kinetics in nanocrystalline materials, the influence of grain boundary misorientation on the effective diffusion coefficient (apparent diffusivity) in nanocrystalline aluminum was investigated using molecular dynamics simulations. Nine series of [001] symmetric tilt grain boundaries, including high and low symmetric boundary planes, were studied. The apparent diffusivity in the samples was calculated in the temperature range from 423 K to 823 K by monitoring the mean square displacement of atoms as a function of simulation time. A temperature dependence of the effective diffusion coefficient according to the Arrhenius law was obtained for all samples. It is found that the apparent diffusivity is anisotropic and it is a strong function of grain boundary misorientation at low and high temperatures. At all temperatures, Σ29 [001]/(520) symmetric tilt grain boundary with misorientation angle of 43.68° exhibits the highest effective diffusion coefficient among the investigated grain boundaries. The simulation results show that the activation energy and pre-exponential factor are affected significantly by the grain boundary misorientation angle. Moreover, the results indicated that the misorientation dependence of activation energy for diffusion exhibits two local maxima, which correspond to two symmetric tilt grain boundaries. Additional calculation of misorientation dependence of the pre-exponential factor shows two local minima at the same symmetric tilt grain boundaries. The misorientation dependence of the effective diffusion coefficient was explained on the basis of grain boundary energy and the crystallographic structure of grain boundary.