Coincidence Site Lattice Grain Boundaries Research Articles

Intergranular oxidation behaviour of Ni-Cr-Al model alloys in 480 °C hydrogenated steam

The oxidation of Ni-Cr-Al model alloys was investigated after exposure to a 480 °C hydrogenated steam environment. Low angle grain boundaries (LAGBs) and the studied coincidence site lattice grain boundaries (CSLBs) showed intergranular oxidation, while high angle grain boundaries (HAGBs) were protected by an external oxide. The intergranular oxide consisted of a central Al-rich oxide, surrounded by a Cr-rich oxide. Al contribution in intergranular and external oxidation was enhanced in Ni-Cr-Al alloys compared to Ni-Al alloys. Thus, the third element effect is applicable at low homologous temperature along short-circuit diffusion pathways, in this case grain boundaries.

Atomic energy in grain boundaries studied by machine learning

Grain boundaries (GBs) have been studied for decades, but it remains a challenging task to describe characteristic GB properties by simple structural descriptors, especially at the local atomic level. In this paper, we use the atomic descriptor based on the smooth overlap of atomic positions (SOAP) to study the atomic energy at GBs by using machine learning and propose a route to simplify it. It is found that, compared with conventional local atomic descriptors such as the Voronoi index, excess volume, centrosymmetry, or local entropy, the SOAP vector shows excellent predictive performance for the atomic energy among the 172 Al and 388 Ni coincidence site lattice (CSL) GBs as well as general GBs in the nanocrystalline model. Additionally, we successfully used the datasets of GBs and amorphous models to predict the atomic energy of one another, which proves the similarity between the local atomic environments (LAEs) in GBs and the amorphous state. Furthermore, the distribution of local distortion factors based on the SOAP vector shows the transition in atomic pack ordering from special CSL GBs to general GBs and the amorphous structures. The simplified descriptor we propose can reduce the original SOAP vector from >1000 features to only a few yet still shows the superior predictive performance of the atomic energy at GBs in all cases than the conventional descriptors combined. It is expected that the simplification process can be adapted to study more complex GB behaviors. A simple and efficient descriptor of the LAEs should allow us to have a clearer picture of the structure-property correlation in GBs, which is essential for GB engineering.

A brute-force code searching for cell of non-identical displacement for CSL grain boundaries and interfaces

Atomic simulation of coincidence-site lattice (CSL) grain boundaries (GBs) and interfaces is of importance for understanding GBs in polycrystalline materials and for making functional films. A common process of high-throughput simulation for CSL GBs and interfaces is to explore rigid body translation (RBT) of one crystal respect to the other. Cell of non-identical displacement (CNID) is the minimum cell including all non-identical RBTs in the GB or interface plane and is important for effective sampling. This work proposes an algorithm to compute the CNID of any two-dimensional CSL GB or interface based on a reciprocity relation between the displacement shift complete (DSC) and CSL. Program summaryProgram title: cnidcalCPC Library link to program files:https://doi.org/10.17632/wzy67jd56d.1Code Ocean capsule:https://codeocean.com/capsule/1648641Licensing provisions: MIT licenseProgramming language: Python Nature of problemDetermination of atomic structure of crystalline interfaces and grain boundaries is a key issue for understanding the properties of polycrystalline materials, where atomic simulation has made notable contribution in this field. Simulation of crystalline interfaces often applies a bicrystal model of a coincidence-site lattice (CSL) [1, 2] interface comprising two slabs of crystals which coincides in the interface plane. To determine the most stable structure of this interface model, a main task is often to tailor the initial bicrystal model to obtain many non-identical and non-relaxed structures for subsequent relaxation to explore the energy landscape of the system to find the ‘global minimum’ at 0 K, where one of the most important operations to tailor the model is applying rigid body translation (RBT) [1] of one crystal respect to the other by a vector confined in the interface plane. Because of symmetry of the two crystals, there exist periodicity of this translation vector which is reflected by a special cell called ‘cell of non-identical displacement’ (CNID) [1] which is the minimum cell including all non-identical choices of this vector and is key for effective sampling. Although CNID has a straightforward definition that it is a lattice including both the two overlapping two-dimensional lattices, finding a method capable to determine a primitive basis of CNID in general cases can be non-trivial and there has not been a reported programme capable to do so. Solution methodCNID in nature is a displacement shift complete (DSC) lattice [2]. While there has not been effective reported programme to compute DSC, several programmes have been made to compute CSL [3, 4, 5]. Besides, Grimmer [6] has proposed a mathematical relationship between the DSC and the CSL. In this way, based on a previously reported programme computing the CSL by brute-force method [5] and the relationship between DSC and CSL proposed by Grimmer, we generated a python code to compute CNID. This code is available to compute the CNID of a CSL interface comprising any two lattices if only they coincide even including complex cases involving heterogeneous lattices with difference in both shape and size.

Influence of Welding Speeds on the Morphology, Mechanical Properties, and Microstructure of 2205 DSS Welded Joint by K-TIG Welding.

In this paper, 8.0 mm thickness 2205 duplex stainless steel (DSS) workpieces were welded with a keyhole tungsten inert gas (K-TIG) welding system under different welding speeds. After welding, the morphologies of the welds under different welding speed conditions were compared and analyzed. The microstructure, two-phase ratio of austenite/ferrite, and grain boundary characteristics of the welded joints were studied, and the microhardness and tensile properties of the welded joints were tested. The results show that the welding speed has a significant effect on the weld morphology, the two-phase ratio, grain boundary misorientation angle (GBMA), and mechanical properties of the welded joint. When the welding speed increased from 280 mm/min to 340 mm/min, the austenite content and the two-phase ratio in the weld metal zone (WMZ) decreased. However, the ferrite content in the WMZ increased. The proportion of the Σ3 coincident site lattice grain boundary (CSLGB) decreased as the welding speed increased, which has no significant effect on the tensile strength of welded joints. The microhardness of the WMZ and the tensile strength of the welded joint gradually increased when the welding speed was 280–340 mm/min. The 2205 DSS K-TIG welded joints have good plasticity.

Effect of variation in inclination angle of Ʃ5 tilt grain boundary on the shock response of Ni bicrystals

The intensity of damage production during shock wave propagation in polycrystalline metallic systems is mostly dependent on the shock-defect interactions. Traditionally, different coincidence site lattice (CSL) grain boundaries (GBs) are introduced in the polycrystalline structure through various processing techniques to enhance the strength or plasticity. However, their dynamic response to the impulsive loading condition has not been elaborately explored to date, which necessitates a detailed study on the interaction between CSL GBs and shock wave. In this study, we have employed molecular dynamics simulations to investigate the response of both symmetric and asymmetric ∑5[1 0 0] tilt GBs of nickel bicrystal under the influence of shock-wave. We have also analysed the role of piston velocities and inclination angles of the GBs on the shock response and the consequent deformation behaviour. This investigation gives insight into the mechanical response and the underlying mechanisms which are inspected through atomic strain analysis, common neighbour analysis and pressure–time–distance mapping. The results show that the stacking faults formation takes place when specimens are subjected to lower shock velocities, whereas higher velocities facilitate phase transformation along with amorphization. There is a stark contrast between the specimens with symmetric and asymmetric tilt GBs in the manner of plastic deformation behaviour in response to shock-wave and GB failure at lower and higher piston velocities.

Solute segregation to near-coincidence site lattice grain boundaries in α-iron

Coincidence site lattice grain-boundaries (CSL-GBs) are commonly observed in steel alloys and play a major role in controlling their mechanical properties. In practice, CSL-GBs do experience deviations from their ideal configurations, where the deviation from the ideal symmetry plane can be modeled as sub-boundary network of misfit dislocations. In this study, segregation energy of hydrogen and carbon atoms to ∑3 (111), ∑3 (112), and ∑5 (310) CSL-GBs and their deviated configurations within Brandon’s criterion range in α-iron is studied using molecular statics simulations. Thereafter, through utilizing Rice–Wang model the change of the cohesive GB energy is computed and correlated to misfit dislocations structures. The results show significant correlation between the crystallographic aspects of the GBs and the hydrogen/carbon embrittlement/strengthening effect. While the ideal CSL-GBs consistently show the highest resistance to hydrogen enhanced decohesion effect, the deviations from the ideal configurations accompanied by misfit dislocation core structures along the boundaries show high solute carbon strengthening.

Grain boundary wetting of different types of grain boundaries in the Cu–Ag system

Present work is dedicated to the investigation of grain boundary (GB) wetting phase transition on different types of grain boundaries in the Cu–Ag system. The character of GBs in the samples was determined by the electron backscattering diffraction. GB character distribution was based on the coincidence site lattice (CSL) model and on the values of the misorientation angle of GBs. Experimental results show that only the low angle GBs and the Σ3 CSL GBs do not reach complete wetting by the melt. Random high misorientation angle (HA) GBs reach complete wetting at 895 °C. Only Σ11, Σ5 and Σ13 CSL GBs have a higher wetting temperature than the HA GBs, so their GB energy should be lower than the energy of HA GBs.

Correlating the five parameter grain boundary character distribution and the intergranular corrosion behaviour of a stainless steel using 3D orientation microscopy based on mechanical polishing serial sectioning

Using 3D-orientation microscopy consisting of electron backscatter diffraction (EBSD) and serial sectioning by mechanical polishing we investigated the correlation between grain boundary (GB) character expressed by its 5-rotational parameters and the GB precipitation and intergranular corrosion (IC) behaviour on an AISI 304L stainless steel. From the measurements, a volume of approximately 4.8 × 106 μm3 containing 451 grains and approximately 1500 GBs was reconstructed. Dominance of {111} planes for Σ3n (n = 1, 2, 3) GBs was found. In general, the IC behaviour of random high angle grain boundaries (RHAGBs) and low-Σ coincidence site lattice (CSL) boundaries (Σ ≤ 29) does not show a large difference. The corrosion behaviour of low angle grain boundaries (LAGBs) is strongly dependent on the misorientation angles, while the crystallographic GB plane becomes dominant for the behaviour of low-Σ CSL GBs and RHAGBs. It was found that the corrosion resistance is related on the atom packing density (APD) of the GB plane. High corrosion resistance appears for GBs with high APD values (in particular for the (111) planes with a maximum APD value).

强磁场对Al-4.5Cu合金定向凝固过程中织构和晶界的影响

研究了顺磁性Al-4.5Cu合金添加Al-5Ti-1B细化剂后,在强磁场下定向凝固时凝固组织中织构的形成规律和晶界特征分布.结果表明：当温度梯度为27 K/cm,未施加磁场时,细化后晶粒取向杂乱;施加磁场后,随着磁场强度的提高,晶粒位向发生变化,晶粒沿a-Al易磁化轴〈310〉发生取向排布.伴随〈310〉织构的生成,晶粒中重位点阵（CSL）晶界比例提高.熔体中具有磁晶各向异性的a-Al晶粒在磁场下受磁转矩作用发生转动,是织构生成的主要原因.还讨论了磁场下流体流动对织构生成和晶界的影响.

Influence of the grain boundary character on the temperature of transition to complete wetting in the Cu–In system

The incomplete to complete grain boundary (GB) wetting transition is controlled by GB energy and, therefore, by GB character. To find the GB character dependence of the wetting transition, experiments were carried out on polycrystalline Cu–In alloys, which are known to exhibit wetting behaviour between 715 and 986 °C. Electron backscatter diffraction was applied to determine the two-dimensional GB character, using the coincidence site lattice (CSL) model and applying the Brandon criterion. Special wetting behaviour was found in those GBs characterised as low-angle, Σ3 and Σ11. The low-angle and Σ3 GBs were not wetted until the sample melted, while the temperature for the transition to complete wetting of Σ11 GBs was over 100 °C higher than that for complete wetting of all other types of GB, including other CSL GB as well as random GB. It might be that a different Brandon type criterion is needed in the case of GB wetting to show the “special” wetting behaviour of the CSL GBs.

Nickel distribution and recombination activity in as-grown and annealed multicrystalline silicon

To study the impact of annealing on the nickel distribution and recombination activity at Σ3n coincident site lattice grain boundaries (CSL-GBs) in multicrystalline silicon, synchrotron-based X-ray analysis and the electron beam induced current method were performed before and after annealing. For low Σ boundaries, the interfacial symmetry at GBs strongly affects the recombination activity and nickel segregation. High Σ (≥ 81) boundaries are always recombination-active even without nickel segregation. Therefore, nickel is not a dominant factor of recombination activity at GBs. The behaviors of GBs in relation to nickel segregation before and after annealing are found to be affected by other neighboring GBs, triple junctions, or intragrain strain defects.

CSL grain boundary distribution in alumina and zirconia ceramics

The distributions of general and coincidence site lattice (CSL) grain boundaries (GBs) in texture-free alumina and zirconia ceramics sintered at two different temperatures were investigated based on electron backscatter diffraction (EBSD) measurements. Results were compared with the distributions obtained from random 2D spatial models and with calculated random distributions reported in the literature. All alumina samples independent on sintering temperature show the same characteristic deviations of the measured general GB distributions from the random model. No such features can be seen in zirconia. The total fractions of CSL GBs in alumina and zirconia samples are clearly larger, for both sintering temperatures, than those observed in the random simulations. A general GB prominence factor, similar to the twin prominence factor for fcc metals, was defined to simplify the representation of the CSL GB content in zirconia. The observed deviations from the random model show no dependence on sintering temperature nor on lattice geometry. In alumina, however, the change in the CSL GB character distribution with sintering temperature seems to be crystallographically controlled, i.e. directly dependent on the orientation of the CSL misorientation axis.

Germanium Tracer Diffusion in a Series of Symmetrical Near Σ=7, Θ=38.2° [111] Tilt Grain Boundaries of Aluminum

Grain boundary (GB) diffusion was measured in a series of Al bicrystals with symmetrical near Σ=7, Θ=38.2° (12-3) [111] tilt GBs. The actual orientations of the bicrystals were precisely measured with the Kossel technique. The tilt angles Θ ranged from 34.15° to 41.41°. Grain boundary diffusion parallel to the tilt axis was measured in the type-B kinetic regime at five temperatures in the range 518 - 680 K. A characteristic non-monotonic behaviour of the resulting dependence of the GB diffusivity sδD GB on the tilt angle Θ was observed for all five investigated diffusion temperatures. At the ideal Σ=7, Θ=38.2° (12-3) [111] tilt GB the GB diffusion is minimal; with increasing deviation from the CSL (coincidence site lattice) GB the diffusivity increases and reveals a broad maximum at about Θ=38.2°±3°. The opposite dependencies are observed for the Arrhenius parameters (sδD GB ) 0 (pre-exponential factor) and Q GB (effective activation enthalpy of GB diffusion), which were calculated from the temperature dependence of sδD GB . At the ideal Σ=7 CSL GB the Arrhenius parameters are maximal. Due to the rather small GB segregation of Ge in Al, the orientation dependence of sδD GB , (sδD GB ) 0 and Q GB qualitatively reflects the orientation dependence of D GB , D GB0 and H GB . The orientation dependence of the GB diffusion supports the CSL GB diffusion model.

Inelastic microstructural failure modes in crystalline materials: The Σ33A and Σ11 high angle grain boundaries

The initiation and evolution of microstructurally induced failure mechanisms in crystalline materials with coincident site lattice (CSL) high angle grain boundaries (GBs) have been investigated. A constitutive formulation that couples a rate-dependent multiple-slip crystalline formulation, through internal variables, to evolutionary equations for the mobile and immobile dislocation densities, has been developed. This dislocation-density based constitutive formulation is used with specialized computational schemes to investigate failure modes in f.c.c. crystalline materials with Σ11 and Σ33a CSL GBs. The interrelated effects of mobile and immobile dislocation densities, lattice rotation, localized plastic slip, and high angle GB misorientations on the evolution of failure modes that can occur owing to dislocation pile-ups and slip-rate blockage and transmission at the GB have been investigated. Comparisons have been made with experimental results and observations, and results from this study are consistent with experimental observations that transgranular failure can occur owing to single dislocation pile-ups at the GB for the Σ11 and Σ33a CSL GBs.

Inelastic microstructural failure mechanisms in crystalline materials with high angle grain boundaries

Microstructurally-induced failure mechanisms in crystalline materials with coincident site-lattice (CSL) high angle grain boundaries (GBs) have been investigated. A multiple-slip rate-dependent crystalline constitutive formulation that is coupled to the evolution of mobile and immobile dislocation densities and specialized computational schemes have been developed to obtain a detailed understanding of the interrelated physical mechanisms that result in material failure. A transmission scalar has also been introduced to investigate slip-rate transmission, blockage and incompatibility at the GB. The combined effects of high angle GB misorientation, mobile and immobile dislocation densities, strain hardening, geometrical softening, localized plastic strains, and slip-rate transmission and blockage on failure evolution in face centered cubic (f.c.c.) crystalline materials have been studied. Results from the present study are consistent with experimental observations that single dislocation pile-ups result in a transgranular failure mode for the ∑9 CSL GB, and that symmetric double dislocation pile-ups result in an intergranular failure mode for the ∑17b CSL GB.

The effect of carbon and sulphur on the character of the grain boundary population in α-iron

The grain boundary misorientations and CSL frequencies of a population of about 250 boundaries in ultra high purity α-iron doped with small quantities of carbon and sulphur have been determined by a SEM microdiffraction BKD (Backscattered Kikuchi Diffraction) technique. The aim is to examine the effect of these elements on the character of the grain boundary population (CGBP). Samples containing about 75 wt. ppm of sulphur and/or 150 (200) wt. ppm carbon were recrystallized at 725 °C after cold hydrostatic extrusion to ϵ = 1.1 It was found that the addition of sulphur and carbon modifies the CGBP of α-iron. Sulphur clearly increases the density of low angle grain boundaries (LANGB), but only in the pure FeS binary alloy. The fractions of the LANGB and coincidence site lattice grain boundaries (CSLGB) in the FeCS alloy are rather similar to those of FeC and thus the CGBP could be controlled by carbon. As a consequence, the lowest fraction of high energy random grain boundaries (RANGB) appears in the FeS alloy at about two-thirds of the total population. The effect of carbon and sulphur on the CGBP is interpreted in terms of grain boundary selection as a consequence of an impurity-specific dragging effect or a precipitation pinning effect on the migration of boundaries during recrystallization and subsequent grain growth.