Boundary Misorientation Angle Research Articles

Imaging and Segmenting Grains and Subgrains Using Backscattered Electron Techniques.

We present two new methods of processing data from backscattered electron signals in a scanning electron microscope to image grains and subgrains. The first combines data from multiple backscattered electron images acquired at different specimen geometries to (1) better reveal grain boundaries in recrystallized microstructures and (2) distinguish between recrystallized and unrecrystallized regions in partially recrystallized microstructures. The second utilizes spherical harmonic transform indexing of electron backscatter diffraction patterns to produce high angular resolution orientation data that enable the characterization of subgrains. Subgrains are produced during high-temperature plastic deformation and have boundary misorientation angles ranging from a few degrees down to a few hundredths of a degree. We also present an algorithm to automatically segment grains from combined backscattered electron image data or grains and subgrains from high angular resolution electron backscatter diffraction data. Together, these new techniques enable rapid measurements of individual grains and subgrains from large populations.

Intrinsic mechanism of grain size effect and grain boundary misorientation angle effect on crack propagation in martensitic steels

Grain size and grain boundary misorientation angle as two important parameters in the polycrystalline system have a significant influence on the mechanical properties of advanced martensitic steels. A phase field model is used to investigate the intrinsic mechanism of the effects of grain size and grain boundary misorientation angle on the crack propagation process which considers the coupling damage behavior between grains and grain boundaries in martensitic steels by means of the finite element finite method. The transgranular (through grain boundaries) and intergranular (along grain boundaries) failure can be simulated by this model through constructing a grain boundary energy function and using a specific order parameter to track crack propagation in the grain boundary area. This model can directly display the crack propagation path inside the grain (martensite and austenite phases) and grain boundary area with the stress distribution. The preferential cleavage planes in austenite and martensite phases are 111γ and 100α in grains with different orientations, respectively. Moreover, the simulation results show that the crack mainly bifurcates near the trifurcated grain boundary. The crack tip tends to induce transgranular fracture in low-angle grain boundaries and intergranular fracture in high-angle grain boundaries when it reaches the grain boundary area. Consequently, this model will be a powerful tool to investigate the influence of grain size and grain boundary misorientation angle on the mechanical properties of polycrystalline martensitic steels.

In-situ EBSD tensile revealing the evolution mechanism of high angle grain boundaries in Al–Zn–Mg alloy profile with heterogeneous structures

The evolution of grain boundary (GB) of an Al–Zn–Mg alloy profile with heterogeneous structures is studied based on in-situ EBSD tensile. This evolution is not only related to its grain boundary misorientation angle (GBMA) and slip transfer parameter (m'/(Δb/b)) but also to the behavior of the geometrically necessary dislocation (GND) tensor in the deformation. The effect of GBMA and m'/(Δb/b) for three types of high angle grain boundary (HAGB) on GND density and back stress has been revealed. The increase of GND density is not obvious near the HAGBs with 45°–55° GBMA of neighboring coarse grains or neighboring fibrous grains. The optimal grain boundary for forming back stress is the HAGBs between coarse grain and fibrous grain, which has 35°–45° GBMA and a low m'/(Δb/b). The back stress is hard to form near these HAGBs with 25°–35° GBMA and a low m'/(Δb/b). Combined with the neighboring grain type, slip transfer and GBMA, the new criterion is proposed to predict GND behavior near HAGBs.

Effects of Grain Boundary Misorientation Angle on the Mechanical Behavior of Al Bicrystals

This research article explores the effect of grain boundary (GB) misorientation on the mechanical behavior of aluminum (Al) bicrystals by means of molecular dynamics (MD) simulations. The effect of GB misorientation on the mechanical properties, fracture resistance, and crack propagation are evaluated under monotonic and cyclic load conditions. The J-integral and the crack tip opening displacement (CTOD) are assessed to establish the effect of the GB misorientation angle on the fracture resistance. The simulations reveal that the misorientation angle plays a significant role in the mechanical response of Al bicrystals. The results also evidence a gradual change in the mechanical behavior from brittle to ductile as the misorientation angle is increased.

Role of grain boundary crystallography on void growth in FCC metals

The growth of an isolated spherical void at and near a grain boundary or a triple junction is studied by means of full-field crystal plasticity simulations with explicit representation of the void. To examine the fundamental aspects of void/grain boundary interactions in these cases, a three-dimensional large-strain elasto-viscoplastic fast-Fourier transform (LS-EVP-FFT) model with axisymmetric tensile periodic boundary conditions was developed. We employ this model to reveal the role of crystallography (grain orientations, grain boundary misorientation and inclination angle with respect to the largest component of the applied stress) and type of loading on void growth. It was found that grain orientation was the primary crystallographic feature governing void growth in strain-rate controlled simulations. For strains small enough to not significantly alter the local geometry, the overall growth of the void in bicrystal boundaries was not strongly affected by grain boundary inclination angle. The relative amount of void growth experienced within each grain in a bicrystal or tri-crystal was strongly dependent on the grain boundary inclination angle. We found a subset of orientations that exhibit faster void growth in bicrystal boundaries than inside the constituent single crystals. Last, we show that when placed primarily in one grain but near the grain boundary, void growth was similar to void growth when placed at the grain boundary. The growing void remained in the original grain and did not grow into the other grain, regardless of crystallographic character.

Quantitative investigation on the {10–12} twinning-detwinning behavior and twinning transfer of an extruded Mg-2.8Y sheet during compression-tension loading via quasi-in-situ EBSD observation

Abundant {10–12} twins appeared in an extruded Mg-2.8Y (wt. %) sheet with rare-earth (RE) texture under compression along the transverse direction (TD). Twinning-detwinning behavior during compression-tension loading was quantitatively investigated via quasi-in-situ EBSD observation. The evolution of deformation mechanisms was quantitatively studied by calculation of the plastic strain fraction of twinning, detwinning, and dislocation slip. Twinning and detwinning were the dominant deformation modes in compression and reverse tension, respectively, owing to their relatively higher Schmid factors (SF) and lowest activation stresses (τ). Twin variants with SF ranking 1st and 2nd were activated preferentially while detwinning with low SF ranking showed high activation. The impact of texture and load path on variant selection was systematically discussed. Moreover, short twin chains only passing through two or three grains caused by twinning transfer were observed. Grain boundary misorientation angle (GBMA) and geometric compatibility factor (m’) were introduced to analyze the crystallographic characteristics of twinning transfer. GBMA ≤34.5° or m’ ≥ 0.80 is a necessary condition for most twinning transfer events. RE texture as well as the nearly uniform distribution of GBMA gave rise to short twin chains and restricted the formation of long twin bands. The relationship between GBMA and m’ was also discussed.

Critical Role Played by Interface Engineering in Weakening Thickness Dependence of Superconducting and Structural Properties of FeSe0.5Te0.5-Coated Conductors.

Increasing the thickness of a superconducting layer and simultaneously reducing the thickness effect in iron-based superconducting coated conductors are particularly essential for improving the critical current Ic. Here, for the first time, we have deposited high-performance FeSe0.5Te0.5 (FST) superconducting films up to 2 μm on LaMnO3-buffered metal tapes by pulsed laser deposition. An interface engineering strategy, alternating growth of a 10 nm-thick nonsuperconducting FST seed layer and a 400 nm-thick FST superconducting layer, was employed to guarantee the crystalline quality of the films with thicknesses of the order of micrometers, resulting in a highly biaxial texture with grain boundary misorientation angle less than the critical value θc ∼ 9°. Moreover, the thickness effect, that the critical current density (Jc) shows a clear dependence on thickness as in cuprates, is reduced by the interface engineering. Also, the maximum Jc was found for a 400 nm-thick film with 1.3 MA/cm2 in self-field at 4.2 K and 0.71 MA/cm2 (H∥ab) and 0.50 MA/cm2 (H∥c) at 9 T. Anisotropic Ginzburg-Landau scaling indicates that the major pinning centers vary from correlated to uncorrelated as the film thickness increases, while the thickness effect is most likely related to the weakening of flux pinning by the fluctuation of charge-carrier mean free path (δl) and strengthening of flux pinning caused by the variation of superconducting transition temperature (δTc) due to off-stoichiometry with thickness.

The structure dependence of oxidation behavior of high-angle grain boundaries of alloy 600 in simulated pressurized water reactor primary water

The intergranular degradation of seven different types of high-angle grain boundaries (HAGBs) were investigated on Alloy 600 after exposure to simulated pressurized water reactor primary water. All boundaries are susceptible to preferential intergranular oxidation (PIO) except for ideal coherent twin boundary. Diffusion induced grain boundary migration (DIGM) normally occurs and its depth is positively correlated with the PIO extent. Interestingly, the PIO susceptibility is independent on the grain boundary misorientation angle or Σ value, but related to the grain boundary atom packing density (GBAPD). Grain boundaries with higher GBAPD values show higher PIO resistance as the element diffusion is slower.

Suppressed hydride precipitation in the welding zone of a zirconium-based alloy cladding tube

The difficulty in characterizing mesoscale hydrides in the highly deformed submicron (< 1 μm) scale grain size of the reactor-grade stress-relief annealed (SRA) zirconium-based alloy tube via electron backscatter diffraction (EBSD) prevented, so far, the field from directly observing the mesoscale characteristics of precipitated hydrides in the welding and tube region. This study presents new high-quality EBSD characterization of precipitated hydrides along the axial length of reactor-grade zirconium-based alloy tubing covering the end cap head, welding zone (WZ), heat affected zone (HAZ), and nominal tube region. The result demonstrates that hydride precipitation is suppressed in the WZ, indicating increased resistance of the hydride embrittlement. Extensive EBSD analyses coupled to thermodynamic modeling were conducted to unveil the mechanism of hydride precipitation suppression at the WZ. Suppressed hydride precipitation in the WZ region is caused primarily by the combined effects of a higher critical nucleation Gibbs free energy of the WZ (ΔGWZ*) due to a greater matrix stiffness, increased misfit strain, and greater grain boundary misorientation angles.

Operating slip modes and inhomogeneous plastic deformation of Mg-10Gd-3Y-0.5Zr alloy during compression

To understand the operating slip modes and inhomogeneous plastic deformation quantitatively and statistically for a high-performance cast Mg−10Gd−3Y−0.5Zr (wt.%, GW103) alloy during room-temperature uniaxial compression, detailed slip trace analysis and electron backscatter diffraction (EBSD) based misorientation analysis of this alloy in aged condition were carried out. After 2% plastic strain, according to relative frequency of identified slip traces, the active slip modes were basal <a> slip (73.3%), followed by prismatic <a> slip (15.8%), then second-order pyramidal <c+a> slip (6.9%), and finally first-order pyramidal <a> slip (4%). Although most of the active slip systems exhibited large Schmid factor (m) values (>0.3), it was worth noting that some hard-oriented (m<0.1) slip systems were also active. For most of the grain boundaries exhibiting extremely large geometrically necessary dislocation (GND) density, at least one of the following conditions was satisfied: large grain boundary misorientation angle (GBMA) and/or large deviation of mmax for particular slip mode between neighboring grains. The plastic heterogeneity of grains (magnitude/distribution of GND density) was independent of the visibility of slip traces. The grain orientation speared (GOS) and/or grain average GND density showed no obvious correlation with mmax (for particular slip mode).

Statistical characterization of twin transmission across grain boundaries in magnesium

A detailed statistical analysis of twin transmission (TT) across grain boundaries is performed in rolled, commercial purity magnesium compressed along the rolling direction. EBSD images are acquired from two different cuts: a section containing the rolling (RD) and normal (ND) directions; and a section at 35° to the ND which contains transverse direction (TD). An automated twinning analysis software, METIS, is used to obtain the statistical correlations between deformation twins and other microstructural features by analyzing EBSD microstructures comprising thousands of grains and twins. This detailed statistical analysis reveals that the TT propensity is sensitive to the grain boundary (GB) misorientation angle but not to the GB misorientation axis. Specifically, TT propensity decreases with increasing GB misorientation angle; however, the decreasing trend is not monotonic. Further, the detailed analysis of TT events combining macroscopic Schmid factor and the geometric measures (m’: accounts for the alignment of both twin plane normal and shear directions; m”: accounts for the alignment of only twin shear directions; and m”’: accounts for the alignment of only twin plane normal) helps in understanding and identifying the process of twin-pair formation, i.e., co-nucleation versus transmission, also to identify the role of local stresses at GBs induced by inter-grain interaction versus twinning shear transformation processes. In addition, Bayesian inference is used to draw statistically meaningful conclusions as to the likelihood of transmission given misorientation angles. Lastly, twin chain frequency as a function of chain length is predicted using the probability distribution obtained from the Bayesian inference and compared with the actual data from the EBSD microstructures.

Investigation of microstructure evolution, mechanical and corrosion properties of SAF 2507 super duplex stainless steel joints by keyhole plasma arc welding

Keyhole plasma arc welding (PAW) of the super duplex stainless steel (SDSS) is expected to achieve full-penetration joints of medium-thickness plates in a single pass with higher efficiency. However, the unique key-holing processes due to the special constricted arc behaviors cause much different thermal cycles of the liquid metals as well as changed microstructures. SDSS 2507 with a thickness of 10 mm was joined by PAW to investigate the joints' microstructure and properties. Microstructure characterization reveals substantial differences in the content of austenite, ranging from 68.9% in weld metal (WM) to 32.1% in the heat-affected zone (HAZ). The oxide inclusions in the WM enhance the hardness of the welded joints, but provide the path for crack propagation to decrease its impact toughness. While complicated microstructure is found in HAZ forming ferrite and diverse austenite, such as grain boundary austenite (GBA), intragrain austenite (IGA), and Widmanstätten austenites (WA). A small amount of secondary austenite (γ2) and chromium nitride are also found in HAZ, and they significantly deteriorate its corrosion resistance. The grain boundary misorientation angle distribution of the austenite in WM is found to be dominated by the low angle boundaries (LABs). However, its content decreases from the WM to base metal (BM). Selective corrosion occurs in all samples, where ferrite is preferentially dissolved when the extrinsic voltage is low (<0.5 V) and austenite is preferentially dissolved when the extrinsic voltage is high (>0.5 V), indicating the different electrochemical activity of the two phases.

Periodic microstructure of Al–Mg alloy fabricated by inter-layer hammering hybrid wire arc additive manufacturing: Formation mechanism, microstructural and mechanical characterization

Inter-layer plastic deformation is an effective method to improve the mechanical properties of wire arc additive manufacturing (WAAM) components. In this study, the inter-layer hammering hybrid WAAM process was applied, and a periodic microstructure composed of alternate coarse and fine grain regions was obtained. These regions were defined as the work hardening region (W region) and recrystallization region (R region) according to the respective formation mechanism. The grain size, texture intensity, grain boundary misorientation angle, phase structure, and dislocation density in these two regions were systematically investigated, and their contributions to the mechanical properties were analyzed. The results show that hammering, especially with increase of hammering times, maintains the depth of recrystallization of each layer at 1.0 mm and increases the ratio of the R region. The grain size in the R region was significantly refined, and the dislocation density in the R region was lower than that in the W region due to the dislocation release by recrystallization caused by subsequent deposition heat, and sub-grains were formed in the W region. The best tensile properties were achieved in the horizontal direction when the inter-layer region was hammered 3 times. The ultimate tensile stress (UTS) exceeded 400 MPa and the elongation exceeded 13%. However, the change in pore morphology caused anisotropy in hammered samples.

Correlated subgrain and particle analysis of a recovered Al-Mn alloy by directly combining EBSD and backscatter electron imaging

Correlated analysis of (sub)grains and particles in alloys is important to understand transformation processes and control material properties. A multimodal data fusion workflow directly combining subgrain data from electron backscatter diffraction (EBSD) and particle data from backscatter electron (BSE) images in the scanning electron microscope is presented. The BSE images provide detection of particles smaller than the applied step size of EBSD down to 0.03 μm in diameter. The workflow is demonstrated on a cold-rolled and recovered Al-Mn alloy, where constituent particles formed during casting and dispersoids formed during subsequent heating affect recovery and recrystallization upon annealing. The multimodal dataset enables statistical analysis including subgrains surrounding constituent particles and dispersoids' location with respect to subgrain boundaries. Among the subgrains of recrystallization texture, Cube{001}〈100〉 subgrains experience an increased Smith-Zener drag from dispersoids on their boundaries compared to CubeND{001}〈310〉 and P{011}⟨5¯6¯6⟩ subgrains, with the latter experiencing the lowest drag. Subgrains at constituent particles are observed to have a growth advantage due to a lower dislocation density and higher boundary misorientation angle. The dispersoid size per subgrain boundary length increases as a function of misorientation angle. The workflow should be applicable to other alloy systems where there is a need for analysis correlating grains and grain boundaries with secondary phases smaller than the applied EBSD step size but resolvable by BSE imaging.

Effects of billet heating temperature and extrusion speed on the microstructures and mechanical properties of the longitudinal welds in aluminum alloy profiles with complex cross-section

The effects of billet heating temperature and extrusion speed on the microstructures and mechanical properties of longitudinal welds in the aluminum alloy profiles with complex cross-section were studied. The results show that as the billet heating temperature or extrusion speed gradually increases, the average grain size shows a decreasing trend, the average grain boundary misorientation angle shows an increasing trend, and the density of dislocations increases first and then decreases. The elongation (EL) shows a trend of increasing first and then decreasing. Higher billet heating temperature contributes to increase the ultimate tensile strength (UTS) and yield strength (YS) of the longitudinal welds, while extrusion speed has little effect. Poor welding quality is the main factor leading to the low mechanical properties of the longitudinal welds. When the welding quality is higher, the interaction between dislocations and precipitation phases has an important impact on the mechanical properties of the longitudinal welds. The longitudinal welds of the profiles studied in this work have the highest EL and the largest UTS when the combinations of the billet heating temperature and the extrusion speed are 490 °C × 3.5 mm/s and 520 °C × 3.5 mm/s, respectively.

The effect of neighboring grain orientation on dislocation-grain interaction in Ti-5553 alloy

The accumulated dislocations are widely reported to be easily transmitted at low angle grain boundaries (GBs), while the associated mechanism at high angle GB needs further investigation. In this paper, the mechanisms associated with the interaction between GBs and 〈111〉 dislocations are investigated in Ti-5553 alloy with body-centered cubic structure. Slip traces transmitted into the neighboring grain or blocked by GBs are observed on the surface of the sample via secondary electron imaging. The corresponding slip system is determined with the combined use of EBSD-detected grain orientation and slip traces. The experimental results indicate that most slips can transfer at low angle GBs, while some slips can also transmit into the neighbor at high angle GBs. The misorientation angle of GBs is invalid to evaluate the mechanism induced by the interaction of dislocations with GBs. Crystallographic alignment-based analysis shows that slip transfer happens at GBs with favorable parameters of high geometric alignment ( m ′) and low residual Burgers vector magnitude (|∆ b |). The general relation of GB misorientation angle with m ′ and |∆ b | is established. The results indicate that low angle GBs always possess high m ′ and low ∆ b , while only some high angle GBs meet the requirement of large alignment parameters. This difference contributes to their preference of slip transfer. Finally, the orientations of best neighboring grains that are favorable for slip transfer are calculated, which is in agreement with the experimentally detected examples. • Dislocations can transmit at some high-angle grain boundaries with special orientations. • General effect of grain boundary misorientation angle on slip transmission is studied. • The best orientations of neighboring grains favorable for slip transmission are established for low- and high-angle GBs.

Grain-scale material removal mechanisms of crystalline material micro-cutting

Crystalline material miniature parts are highly required under the development trend of miniaturization in some fields, and micro-cutting is a promising technology to manufacture complex-shaped crystalline material miniature parts. In the micro-cutting process of crystalline materials, the cutter edge radius, cutting parameters and grain size are in the same order of magnitude, and the workpiece has significant microstructure heterogeneity and mechanical property anisotropy. Therefore, the “cutter edge size effect” and “microstructure effect” occur during the micro-cutting process of crystalline materials, which make it difficult to predict the material deformation behavior and machinability, leading to low efficiency and poor accuracy in the manufacturing process of crystalline material miniature parts, and seriously restricting their widespread application. Taking Inconel-718 as the example, the influence of grain orientation, grain boundary and grain size on the micro-cutting machinability of crystalline materials are comprehensively investigated through single-crystal, bi-crystal and polycrystalline orthogonal micro-cutting simulations. Results show that, grain orientation affects the slip system and slip degree during micro-cutting processes, which leads to different material deformation behaviors. The influence of grain boundary on material behaviors in micro-cutting processes can be divided into two aspects i.e., the material response across grain boundary introduced by its presence and the response difference caused by the difference of grain boundaries. Grain boundary features like grain boundary misorientation angle, grain boundary interface orientation, the number deviation of primary slip systems and the Schmid factor deviation of primary slip systems significantly influence the micro-cutting machinability. Besides, grain size affects the micro-cutting machinability by influencing the number of grains, slip systems and grain boundaries involved in the affected zone. Clarified mechanisms can be used as a credible basis to predict the machinability of crystalline materials in micro-cutting processes, and improve the manufacturing efficiency and quality of crystalline material miniature parts.

Multi-scale modeling method for polycrystalline materials considering grain boundary misorientation angle

Grain boundaries (GBs) are microstructures in polycrystalline materials, which influence the mechanical properties of materials significantly. Simulation is an indispensable means to study GBs due to its high flexibility. However, the existing GB simulation models mostly focus on a single simulation scale, lack the consideration of the grain boundary misorientation angle (GBMA) characteristic and fail to describe the coupled elastic–plastic damage behavior between grains and GBs accurately. To describe the influence mechanism of GB on the mechanical behavior of materials accurately, a GBMA-considered multi-scale modeling method for polycrystalline materials is proposed in this paper. The method is based on molecular dynamics (MD), the crystal plasticity finite element method and the cohesive zone model, which considers the GBMA information at grain and atomic scales comprehensively. Firstly, a GB geometric model containing GBMA characteristic is generated at grain scale through EBSD information. Then the GB cohesive parameters are obtained at the atomic scale by MD simulation. Finally, some experiments are performed for verification, which indicates the high accuracy of the proposed method. Furthermore, three models with the same geometric shape and different grain orientation and GBMA are established to study the influence of GBMA on the mechanical properties of polycrystalline materials.

Recovery and recrystallization of warm-rolled tungsten during helium thermal desorption spectroscopy annealing

Tungsten has been perceived as one of the most promising plasma-facing materials for future fusion reactors. Its behavior under helium irradiation has been extensively studied. In this work, we focus on the recovery and recrystallization of helium-implanted surface grains in warm-rolled polycrystalline tungsten. Mechanically polished specimens with and without 40 keV helium implantation were heated to 1650 °C during helium thermal desorption spectroscopy experiment. Our results suggest that recovery and recrystallization occurred via subgrain coarsening and removal of low-angle grain boundaries in as-polished specimen. Its average grain size increased from 1.70 to 8.89 μm, and its fraction of low-angle grain boundary decreased from 0.77 to 0.29. By contrast, little changes of grain size and grain boundary misorientation angle were found in helium-implanted surface grains. Such enhanced thermal stability is attributed to the pinning effect of helium bubbles on grain boundaries, and the underlying mechanism is discussed based on the proposed model of cellular microstructure.

A Two‐Step Thermomechanical Process for Improving High‐Angle Grain Boundary Proportion and Achieving Delamination Toughening in Hot‐Rolled Steel with a Limited Slab‐to‐Plate Ratio

A Two‐Step Thermomechanical Process for Improving High‐Angle Grain Boundary Proportion and Achieving Delamination Toughening in Hot‐Rolled Steel with a Limited Slab‐to‐Plate Ratio