Grain Boundary Misorientation Angle Research Articles

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.

Study on Austenite Grain Growth Behavior of GCr15 Bearing Steel

The effects of austenitizing temperature and holding time on the austenite grain growth behavior of GCr15 bearing steel are reflected in this paper. SEM and EBSD deeply reveal the effect of austenite grain size on the misorientation angle of grain boundary. The results show that both the holding time and the austenitizing temperature will make the austenite grain size grow to a certain extent. However, the change of temperature will cause a greater change in grain size. The grain size will also have different growth trends in different austenitizing temperature ranges. The grain size of Gcr15 test steel is combined with the Sellars model to derive its growth kinetics model.

Atomic simulation study of the factors affecting nucleation in deformation-induced martensitic transformation in grains and at grain boundaries in pure iron

Molecular dynamics simulations were performed to investigate the dominant factors affecting the nucleation in deformation-induced martensitic transformation in grains and at grain boundaries in pure iron, and the effectiveness of microstructure designs incorporating the obtained dominant factors was verified. First, the local nucleation stress required for grain-boundary nucleation was lower than that for in-grain nucleation, indicating that the grain boundary is an effective nucleation site for martensitic transformation. Next, we investigated the dominant factors affecting the in-grain nucleation and the grain-boundary nucleation, and found that the dominant factor of the in-grain nucleation is the magnitude of the loading-direction component of the transformation strain induced by lattice deformation (Bain distortion), while the dominant factors of grain-boundary nucleation are the grain-boundary misorientation angle and grain-boundary free volume. Finally, the possibility of controlling the nucleation timing by designing the microstructures based on the obtained nucleation dominant factors was demonstrated by comparing two polycrystalline models with the same texture but different distributions of grain-boundary misorientation angles.

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.

On the co-nucleation of adjoining twin pairs at grain boundaries in hexagonal close-packed materials

This work addresses the formation of adjoining twin pairs (ATPs) at grain boundaries (GBs) in hexagonal close-packed (hcp) metals from the perspective of the co-nucleation (CN) of pairs of deformation twins. A continuum defect mechanics model is proposed to investigate the energetic feasibility of the CN of ATPs resulting from the dissociation of GB dislocations. Application of the model to a set of GBs in Mg reveals that CN is largely preferred over the nucleation of a single twin variant for low misorientation angle GBs, which is consistent with previous experimental analyses. The CN events are subsequently analyzed considering the GB character, and the twin system alignment (m′) and Schmid factors to obtain better insights into the conditions governing the formation of ATPs via CN. The likelihood of CN even at low m′ values reveals that CN events could be responsible for the formation of ATP pairs in these scenarios.

Comparison of the Thermal Stability in Equal‐Channel‐Angular‐Pressed and High‐Pressure‐Torsion‐Processed Fe–21Cr–5Al Alloy

Nanostructured steels are expected to have enhanced irradiation tolerance and improved strength. However, they suffer from poor microstructural stability at elevated temperatures. In this study, Fe–21Cr–5Al–0.026C (wt%) Kanthal D (KD) alloy belonging to a class of (FeCrAl) alloys considered for accident‐tolerant fuel cladding in light‐water reactors is nanostructured using two severe plastic deformation techniques of equal‐channel angular pressing (ECAP) and high‐pressure torsion (HPT), and their thermal stability between 500–700 °C is studied and compared. ECAP KD is found to be thermally stable up to 500 °C, whereas HPT KD is unstable at 500 °C. Microstructural characterization reveals that ECAP KD undergoes recovery at 550 °C and recrystallization above 600 °C, while HPT KD shows continuous grain growth after annealing above 500 °C. Enhanced thermal stability of ECAP KD is from significant fraction (>50%) of low‐angle grain boundaries (GBs) (misorientation angle 2–15°) stabilizing the microstructure due to their low mobility. Small grain sizes, a high fraction (>80%) of high‐angle GBs (misorientation angle >15°) and accordingly a large amount of stored GB energy, serve as the driving force for HPT KD to undergo grain growth instead of recrystallization driven by excess stored strain energy.

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.

Microscale Imaging of Thermal Conductivity Suppression at Grain Boundaries.

Grain-boundary engineering is an effective strategy to tune the thermal conductivity of materials, leading to improved performance in thermoelectric, thermal-barrier coatings and thermal management applications. Despite the central importance to thermal transport, a clear understanding of how grain boundaries modulate the microscale heat flow is missing, owing to the scarcity of local investigations. Here, thermal imaging of individual grain boundaries is demonstrated in thermoelectric SnTe via spatially resolved frequency-domain thermoreflectance. Measurements with microscale resolution reveal local suppressions in thermal conductivity at grain boundaries. Also, the grain-boundary thermal resistance - extracted employing a Gibbs excess approach - is found correlating with the grain-boundary misorientation angle. Extracting thermal properties, including thermal boundary resistances, from microscale imaging can provide comprehensive understanding of how microstructure affects heat transport, crucially impacting the materials design of high-performance thermal-management and energy-conversion devices. This article is protected by copyright. All rights reserved.

Synergistic improvement of strength and ductility via doping cerium into PH13–8Mo stainless steel by laser powder bed fusion

In this study, the PH13–8Mo stainless steel parts doped without and with cerium (Ce) were fabricated via laser powder bed fusion followed by post-heat treatment, and systematically compared in terms of microstructure, phase constituent, and tensile properties. The comparative results show that doping Ce-modified grains with the equiaxed morphology and finer size, increased the mechanical stability of austenite, and enhanced the sphericity of oxide inclusion in the resultant PH13–8Mo. Additionally, the coherency between the newly-formed CeAlO3 inclusion and matrix was effectively improved after doping Ce, as compared to the original aluminum oxide inclusion without doping Ce. The resultant PH13–8Mo parts doped with Ce yielded an ultimate tensile strength of 1446 ± 20 MPa with a fracture elongation of 16.0% ± 1.5%, for the first time meeting the AMS 5629E H1000 standard for the PH13–8Mo made by additive manufacturing (AM). The enhanced strength results from the strengthening effects of nanoscale precipitation (inclusion) and grain refining. Meanwhile, the ductilizing mechanism can be attributed to the enhanced inclusion sphericity and ameliorative coherency between the inclusion and matrix, and improved misorientation angle of grain boundary owing to the modification by Ce, which efficiently reduced the stress concentration and enhanced cracking resistance during deformation. Therefore, doping rare earth elements presents a promising pathway to synergistically improve the strength and ductility of stainless steels by AM.

Dependency of grain boundary dislocation network configuration on generalized stacking fault energy surface in FCC metals

We use a microscopic phase-field (MPF) model, incorporating the generalized stacking fault energy (GSFE) surfaces (i.e., the γ-surface) as inputs, to investigate systematically the evolution of dislocation network configurations at low-angle pure twist grain boundaries (GBs) in six face-centered cubic (FCC) metals (Ag, Cu, Rh, Ir, Pd and Pt) that have different GSFE surfaces. The equilibrium configurations of GB dislocation networks are obtained via the interplay between the crystalline energy and the elastic strain energy during the energy minimization process. It was found that for {111} twist GBs, though the geometrically necessary dislocations (GNDs) of the screw type are observed, they can be either full or partial ones depending on the GSFE surfaces of the above metal systems. The areas of the staking faults regions formed between partial dislocations are quantitatively characterized as a function of the magnitude of the stacking fault energy, the elastic modulus and the GB misorientation angle. A fast-acting model for predicting the geometric characteristics of the GB dislocation networks is developed based directly on the material properties and GB misorientation, which is validated by its applications to another two metal systems (Au and Ni) with the same GB.

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).

An in situ study on the effect of grain boundaries on helium bubble formation in dual beam irradiated FeCrAl alloy

FeCrAl alloy is one of the promising cladding materials in the advanced nuclear reactors. Here we compare the irradiation response of the nanolaminate (NL) and coarse-grained (CG) FeCrAl alloys, by performing He ion irradiation followed by in situ Kr ion irradiation in a transmission electron microscope at 450 °C. NL alloy has reduced He bubble induced swelling and decreased the density of dislocation loops in comparison to the CG alloy. The distribution of He bubbles appears to be dependent on misorientation angles of high angle grain boundaries. A new type of nanoprecipitates was identified in the irradiated CG FeCrAl without noticeable chemical segregation. Chemical segregation occurred along the laminate boundaries in the irradiated NL FeCrAl alloy.

Roles of structural defects in polycrystalline platinum nanowires for enhanced oxygen reduction activity

Nanocatalysts have uneven reactivity and lively change in reactions, challenging revealing catalytic origins. Phenomenologically, Pt-based nanowires, one of the most important nanostructures, have displayed high activity for oxygen reduction reaction (ORR); however, the fundamentally catalytic origin behind such high performance remains elusive and was implicitly ascribed to one-dimensional structure and/or low-index facets. Here, with molecular-level and operando spectroscopic evidence, we reveal that the polycrystalline platinum nanowires with enhanced ORR activity are closely correlated with structural defects including grain boundaries (GBs), atomic steps, and a few amorphous regions, which is previously neglected or simplified. The local micro-strain is induced by GBs and increases with misorientation angles of GBs. Despite the same coordination number between edge sites and atomic steps, the latter together with a few amorphous regions disturb interfacial water networks via preferably hydrogen bonding, which destabilizes ORR intermediates on terraces and presumably promotes proton transfer.

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.

Topologically-Enhanced Thermoelectric Properties in Bi2Te3-Based Compounds: Effects of Grain Size and Misorientation.

Topological insulators (TIs) and thermoelectric (TE) materials seem to belong to distinct physical realms; however, in practice, they both share common characteristics. Introducing concepts from TIs into TE materials to enhance their performance and achieve better understanding of electronic transport requires extensive research. Particularly, grain size, misorientation, and grain boundary (GB) character are of utmost importance to attain effective charge carrier transport in TE polycrystals; these factors, however, have not been thoroughly explored. Herein, we investigate the correlation between grain size, misorientation, and lattice strain in Bi2Te3 and its TI signature, aiming to improve its TE performance. We reveal an unusual behavior showing that electron mobility increases upon the increase of grain size, reaching at a maximum value of 495 cm2/V·s for an optimum grain size of 600 nm and most-frequent GB misorientation angle of 60° and then decreases with increasing grain size. It is also indicated that the combined effects of grain size reduction and point defects induce lattice strain in the Bi2Te3-matrix that is essential to trigger the TI contribution to TE transport. This trend is corroborated by first-principles calculations showing that compressive strains form multiple valleys in the valence band and opens the TI band gap. Such a combination of physical phenomena in a well-known TE material is unique and can promote our understanding of the nature of TE transport with implications for TE energy conversion.

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.

Effect of Grain Boundary Misorientation on Spall Strength in Ta via Shock-Free Simulations with Relatively Few Atoms

A suite of 37 molecular dynamics simulations is conducted at two system sizes to systematically characterize the role of grain boundary (GB) misorientation on spall strength in pure BCC tantalum (Ta). The systems studied consist of bicrystals with a single [110] symmetric tilt grain boundary. Two loading conditions are compared: (i) homogeneous extension under uniaxial strain simulated in this study and (ii) piston/flyer impact of sample, which induces heterogeneous deformation via shockwave propagation along the length of the sample. The piston/flyer impact is taken from the literature and run on the same set of GB misorientation angles using LAMMPS. The major finding here is that both methods result in similar spall strength predictions, but the homogeneous extension method generally requires two to three orders of magnitude fewer atoms and similar reductions in computational costs. Spall strength results systematically overpredict using this method, by about 10% for the dataset three orders of magnitude smaller than piston/flyer simulations, and 5% for the dataset two orders of magnitude smaller. Lastly, the effect of system size and pre-compression magnitude on spall strength is systematically characterized.

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.

Cracking mechanism of Hastelloy X superalloy during directed energy deposition additive manufacturing

Metal additive manufacturing (AM) offers promising potential in the production of components with geometric complex or customized structure and outstanding properties, but severe defect like cracking remains a significant concern. The debate has long prevailed as to the mechanisms that lead to cracking in AM parts of some nickel-based superalloys like Hastelloy X. In this study, we investigated the underlying cracking mechanism of Hastelloy X fabricated via directed energy deposition (DED). The cracks in DED Hastelloy X were confirmed to be solidification cracking based on extensive observations of the inner crack surface and fracture surface. The origins of the solidification cracking were mainly attributed to thermal stress/strain level, grain boundary (GB) characteristics (GB misorientation and GB density), and micro-alloy elements. Our results showed that the plastic strain rate during the terminal stage of solidification is one of the most critical factors affecting cracking susceptibility when AM processing conditions change. We found that the fraction of S-HAGB (crack susceptible - high angle grain boundaries) is another critical factor in affecting cracking susceptibility. More than 75% of cracks occurred preferentially in the range of GB angles of 25°–45° (defined as S-HAGB) owing to its high GB energy rather than high GB misorientation angle. Besides, GB density could affect the cracking susceptibility by adjusting the thermal stress/strain level and S-HAGB fraction. Micro-segregation of C and Mo is a necessary condition for solidification cracking in DED Hastelloy X, which promotes the formation of low-melting liquid films. These new insights gained from this study could be used to instruct the preparation of crack-free metals and alloys during AM.

Atomistic investigation on grain boundary effect on helium segregation and clustering in iron

Molecular dynamics simulation was applied to study the behavior of helium segregation and clustering near different grain boundaries (GBs) in α-Fe. The 〈110〉 and 〈100〉 family symmetric tilt grain boundaries (STGBs) covering a wide range of GB energy and misorientation angle were investigated. The present results indicate that the major factor of GB characteristic effects on helium behavior at the grain boundary are the GB energy and structure. Generally, the proportion of helium that is bound to the grain boundary is higher for high-energy grain boundaries than for low-energy grain boundaries. However, the three grain boundaries with the highest energy lose their original structure units and become disordered under the combined effects of high temperature and helium bubble-induced stress field, forming a diffusion “barrier” for He interstitials to enter the GB plane, thus lead to the slight decrease of the proportion of segregated helium. Four typical He migration patterns at different grain boundaries are found, which depends on the GB structure and energy. In general, with the increase of GB energy, the average diameter of He-V clusters at the GB becomes smaller, and the cluster density becomes larger.