Σ3 Boundaries Research Articles

AB INITIO МОДЕЛИРОВАНИЕ ВЗАИМОДЕЙСТВИЯ P, H, C, S С ГРАНИЦАМИ ЗЕРЕН В α-ЖЕЛЕЗЕ

The results of modeling from the first principles of interaction of nonmetallic impurities of interstitial (H, C) and substitutional (P, S) with grain boundaries in α-iron are presented. The modeling has been conducted within the framework of the density functional theory (DFT) by the full-potential linearized augmented plane waves (FP LAPW) method with consideration to the generalized gradient approximation (GGA'96) in the WIEN2k software package. Three grain boundaries of the slope Σ3 (111), Σ5 (210) and Σ5 (310) are studied. The supercells of the tilt grain boundaries using the coincidence site lattice model is constructed. The values of the energy characteristics of various grain boundaries with impurities are influenced by a number of factors, namely, the volume of the Voronoi polyhedron per impurity, magnetic moments, and the symmetry of the surrounding matrix. The results show that symmetric grain boundaries Σ3 (111) and Σ5 (310) are embrittled by phosphorus, hydrogen, and sulfur, while carbon strengthens interatomic bonds at the grain boundary, which coincides with the data available in the work. In the case of an asymmetric grain boundary Σ5 (210), phosphorus and hydrogen weaken bonds at the grain boundary, while sulfur strengthens them. This is primarily explained by the geometry of the surrounding matrix. The magnetic moments at the impurity atoms are very small and, in most cases, are antiparallel to the magnetic moments at the neighboring Fe atoms.

First-principles study of boron segregation in fcc-Fe grain boundaries and its influence on interface adhesive strength

Boron, a commonly used microalloying element in steel, is distributed mainly at the grain boundary of stainless steel and plays an important role in regulating the mechanical, corrosion resistance and grain boundary structure of stainless steel. Owing to the small amount of boron added into the steel, it is difficult experimentally to detect the traces of boron segregation at the grain boundary, not to mention analyzing the structural characteristics of the boron segregation grain boundary. First-principles density functional theory (DFT) provides convenience in analyzing the existence mode and mechanism of boron in austenitic steel from the atomic level. Combining with the actual grain boundary structure types in austenitic stainless steel, Fcc-Fe Σ3(112), Σ5(210), Σ5(310), Σ9(114), Σ9(221) and Σ11(113) symmetric tilt grain boundaries are constructed based on DFT, and the segregation behaviors of boron atoms at the six grain boundaries are studied to reveal the segregation mechanism from the atomic and electronic level. The results show that boron segregation occurs mostly at Σ5(210), Σ5(310) and Σ9(114) grain boundaries, while a relatively weak segregation tendency is observed at Σ9(221), Σ3(112) and Σ11(113) grain boundaries; boron atom preferentially occupies the pentahedral or hexahedral segregation position with the largest coordination number; the interface adhesive strength at grain boundaries is improved by the segregation of boron according to the tensile test, which complies with the calculation results of Rice-Wang thermodynamic model; the chemical effect caused by the increase of local charge density after boron segregation at Σ9(114) grain boundary outstrips the adverse effect of structural changes, and the strong interaction between B-p electrons and Fe-s electrons improves the interface adhesive strength. The results provide a reference for using boron to optimize the interface structure of austenitic stainless-steel.

Structure and Grain Boundaries of Ultrafine-Grained Nickel after Rolling and Forging at Cryogenic Temperature

The structure and misorientations of grain boundaries of ultrafine-grained nickel subjected to rolling and forging at liquid nitrogen temperature are studied. It is shown that as a result of rolling in UFG nickel obtained by the ECAP the forming of a band fragmented structure with the formation of special twin boundaries Σ3 is observed. An increase in the strain rate (forging) leads to the appearance of localized deformation bands in which the formation of new small grains is observed through dynamic recrystallization. The development of recrystallization results in increase up to 7% in UFG nickel the fraction of special twin boundaries Σ3 which are similar in nature to annealing twins.

The effect of grain boundary structure on chromium carbide precipitation in alloy 600

The aim of the present work is to study the effect of grain boundary structure on the precipitation of chromium carbides in Alloy 600. The material was processed by thermo-mechanical treatment consisting of hot rolling (at 1000 °C) and a short-time annealing treatment (for 10 min) to generate a high fraction of twins and disrupted random boundaries network. These samples were then given sensitization heat treatment at 704 °C for various times in the range 0.5–100 h, in order to introduce varying degrees of carbide precipitation at the interface regions, depending on the structure of grain boundaries and triple junctions. The characteristics of chromium carbides (identified as Cr23C6) precipitation on different kinds of grain boundaries and triple junctions were investigated using field emission scanning electron microscopy (FESEM) and electron backscatter diffraction (EBSD) based orientation imaging microscopy (OIM) techniques. The propensity for chromium carbides precipitation increased with increasing aging time. However, it was largely dependent on the structure of grain boundaries. Low angle grain boundaries (LAGBs) and low-Σ coincidence site lattice (CSL) boundaries were observed to retard the precipitation of carbides. In contrast, on random high angle grain boundaries (HAGBs), coarse and discrete carbides were observed. Amongst the low-Σ CSL boundaries, the deviation in misorientation was observed to have a profound effect on the precipitation of chromium carbides. Low-deviation twin (Σ3) boundaries (having misorientation deviation within 1°) were found to be strongly resistant against carbide precipitation. However, on high deviation twin and Σ9 boundaries, needle-like carbides evolved. Based on the results, it can be concluded that among the special (i.e., low-Σ CSL) boundaries, only low-deviation twin boundaries have sufficiently low interfacial free energy to completely subdue the precipitation of chromium carbides.

Effects of different processes on microstructure and mechanical properties of H65 brass

Electron backscatter diffraction (EBSD) is used to study the effect of rolling reductions and annealing temperatures on texture evolution and special boundaries (SBs) of H65 brass. The results indicate that the texture components of H65 brass after annealed is mainly {110} (<110>//ND), which is due to the existence of a large number of high-angle boundaries (HABs) with high mobility and a small amount of incoherent Σ3 boundaries around the {110} component. Besides, the number of Σ3 boundaries and the ability to produce its family boundaries such as Σ9 and Σ27 might be influenced by the reduction rate and annealing temperature. The textures of annealed specimens are mainly Brass{110}<112>, S{123}<634> and {011}<111>, and the formation of a large number of annealing twins decreases the intensity of Cube{100}<001>. As annealing temperature increases, {110} texture grows and twin density increases, resulting in a decrease in strength but an increase in conductivity and plasticity.

Pitting behavior of friction stir repair-welded 304L stainless steel in 3.5% NaCl solution at room temperature: role of grain and defect structures

Stainless steel (SS) canisters used for storing spent nuclear waste are prone to chloride-induced stress corrosion cracking. Friction stir welding (FSW), being a low heat input process, can be used for repair welding of these canisters. In this study, an artificial crack was introduced on the 304L SS coupons by an electric discharge machining process. The artificial crack was repair-welded by FSW by maintaining the tool temperature a constant at two levels (725 and 825 °C). Friction stirring significantly reduced the grain size of the stirred zone (SZ) from about 47 to 2–4 μm. The fraction of low-angle grain boundaries increased in the SZ from 2 to 37–43%. On the other hand, the fraction of special grain boundaries (Σ3 and Σ9) that was ~ 50% in the base metal reduced to < 10% in the SZ. During friction stirring, the oxide layer of artificial crack was broken and aligned to form a spiral defect called lazy-S structure. All these microstructural changes affected the corrosion behavior of the FSW specimens when tested in 3.5% NaCl at room temperature using cyclic polarization, chronoamperometry, and electrochemical impedance spectroscopy. The FSW specimens showed lower polarization resistance and lower transpassive potential than those of the base metal specimens. However, the pitting protection potentials of the FSW specimens were higher than that of the base metal. The pitting behavior of the FSW specimens was influenced more by the preferential attack on the lazy-S region than by passive film breakdown. The flat band potentials of the passive film formed on the FSW specimens were more positive than that of base metal. The charge carrier density of passive film formed on the FSW specimens was higher than that of the base metal. The higher fraction of low-angle grain boundaries present in the FSW specimens could supply more number of misorientation dislocations at the metal/film interface which could form anion vacancies by a climb process leading to formation of oxide at these locations without stressing the substrate. Therefore, low-angle boundaries are considered helpful for formation of stress-free passive films that will be highly stable and enhance both pitting and stress corrosion cracking resistance.

Alloying induces directionally-dependent mobility and alters migration mechanisms of faceted grain boundaries

Faceted grain boundaries exhibit unusual segregation and migration tendencies. To gain a deeper understanding of how solute atoms interact with faceted interfacial structures during migration, this study probes the migration behavior of a faceted Σ11 boundary in Cu doped with Ag atoms. The solutes are found to segregate to the facet with more free volume and strongly reduce boundary velocity in one migration direction, but not the other, due to the presence of a directionally-dependent motion mechanism that can escape solute pinning and therefore speed up migration. Hence, a new mechanism of chemically-induced anisotropy in grain boundary mobility is uncovered by these simulations.

Recrystallization and Associated Twinning Evolution in Nickel-Based GH4141 Superalloy During Annealing

The GH4141 superalloy with 20, 40 and 55% cold deformations is annealed at 1080, 1120 and 1180 °C for 3-25 min to study the recrystallization and twinning behavior. The migration rate of grain boundaries increases significantly as the temperature increases in the range of 1080-1180 °C. After recrystallization, the hardness of the alloy is obviously reduced, and the recrystallized grains are randomly oriented without preferred orientation. The orientation imaging microscopy maps indicate that large amounts of Σ3 twin boundaries caused by the stacking faults appear in recrystallized grains. On one hand, the fraction of Σ3 boundaries after short time annealing has increased with the increase of temperature, meaning that the higher temperature contributes to the faster migration rate of grain boundaries and thus promotes the formation of stacking faults and twin boundaries before full recrystallization. On the other hand, the density of Σ3 boundaries has gradually decreased with the increase of grain size due to the annihilation and rare formation of twin boundaries after full recrystallization. The twinning evolution can be explained using the classical growth accident model.

Shuffling mode competition leads to directionally anisotropic mobility of faceted Σ11 boundaries in fcc metals

Faceted grain boundaries can migrate in interesting and unexpected ways. For example, faceted {\Sigma}11 <110> tilt grain boundaries were observed to exhibit mobility values that could be strongly dependent on the direction of migration. In order to understand whether this directionally-anisotropic mobility is a general phenomenon and to isolate mechanistic explanations for this behavior, molecular dynamics simulations of bicrystals evolved under an artificial driving force are used to study interface migration for a range of boundary plane inclination angles and temperatures in multiple face centered cubic metals (Al, Ni, and Cu). We find that directionally-anisotropic mobility is active in a large fraction of these boundaries in Ni and Cu and should therefore impact the coarsening of polycrystalline materials. On the other hand, no such anisotropy is observed in any of the Al boundaries, showing that this behavior is material-dependent. Migration of the faceted boundaries is accomplished through transformation events at facet nodes and incommensurate boundary plane facets, which are termed shuffling modes. Three major shuffling modes have been identified, namely Shockley shuffling, slip plane shuffling, and disordered shuffling. A shift from the first two ordered modes to the third disordered mode is found to be responsible for reducing or removing directionally-anisotropic mobility, especially at the highest temperatures studied.

Neural Network Based Modeling of Grain Boundary Complexions Localized in Simple Symmetric Tilt Boundaries Σ3 (111) and Σ5 (210)

A method is proposed for the neural network based analysis of the existence and stability of grain boundary complexions formed at high-symmetry tilt boundaries Σ3 (111) and Σ5 (210) in a polycrystalline Ni(Bi) solid solution. This method is based on the use of reference interparticle interaction potentials constructed within the framework of the density functional theory in combination with the structural capabilities of an artificial two-level self-learning neural network. The absolute error in determining potential energy by the neurosystem analysis is 0.012 eV/atom. The values of the formation enthalpy of grain boundary complexions for Σ3 and Σ5 boundaries are in rather good agreement with the published results of simulating this system and experimental data.

Grain boundary character distribution in an additively manufactured austenitic stainless steel

The grain boundary character distribution (GBCD) in an austenitic stainless steel produced by additive manufacturing (AM) in both as-built and annealed conditions was studied. Relatively fine grains and a non-fibre texture was achieved by AM, and as-built structure showed a high population of Σ3 boundaries. A five-parameter GBCD analysis revealed that the microstructure is mostly dominated by highly incoherent Σ3 boundaries. The grain boundary network also consisted of random high angle, coherent Σ3s terminating on (111) planes with a pure twist character, and tilt Σ9 boundaries. The findings show prospects for the possibility of engineering the grain boundary network of materials in-situ, via the stress and heat induced by the thermal cycles during AM.

Effect of iron bicrystal orientation on mechanical properties and dislocation density using molecular dynamics simulations of nanoindentation

This study concerns the effect of bicrystal spatial orientation on the iron Σ5 boundary under nanoindentation tests. The orientation of the bicrystal is changed relative to the indenter by the parameter “nanoindentation angle” to create dislocation collisions with grain boundary in different directions and to determine the possibility of transmission of these dislocations through the grain boundary. Simulations were carried out for nanoindentation angles between −90° and 90° with a resolution of 18°. Molecular dynamics with EAM atomic potential functions are employed for the simulations. The effect of nanoindentation angle on the crystals Young’s modulus, shear stress, hardness, and Poisson’s ratio were determined. Due to the periodicity which has been observed in all of the aforementioned quantities of the rotated crystal, analytical equations were extracted by adapting the Fourier series expansion on the calculated data. Some explanations were provided for the dislocation behaviors based on the physics of the problem and also by the results reported in the literature. Simultaneous effects of Poisson’s ratio and pile-ups on each other were also investigated and interpreted.

Grain boundary character distribution in the CoCrMo alloy processed by selective laser melting and post-heat treatment

The evolution of grain boundary character distribution (GBCD) in the CoCrMo alloys processed by SLM and post-heat treatments (aging and solid solution) has been investigated by the electron backscatter diffraction (EBSD) and transmission electron microscope techniques. A high fraction of Σ3n (n = 1, 2, 3) grain boundaries and the corresponding clusters occur in the solid solution-treated samples at 1150 °C, which indicates GBCD optimization is somewhat realized. The coherent and incoherent Σ3 boundaries were distinguished via the single-section trace analysis method based on EBSD measurements. Σ3 boundaries in the SLMed and aged samples were determined as incoherent, while those in the solid solution-treated sample are coherent. Regular e precipitation did not occur in the solid solution-treated samples as in the aged samples. Instead, stacking fault bands associated with coherent Σ3 twin boundaries are dominant. It suggests that post-heat treatment has a significant influence on the GBCD, especially the Σ3 grain boundary characteristics.

Characterization of Grain Boundary‐Engineered Aluminum–Magnesium Alloys

Herein, the effect of sputtering rate on the fraction of special grain boundaries in 5xxx series Al–Mg alloys is explored. Samples are synthesized via interrupted direct current (DC) magnetron sputtering with varying deposition rates, and the grain size and grain boundary character are evaluated with electron backscatter diffraction (EBSD). The highest sputtering rate (7 nm s−1) leads to an increase in the total number of special grain boundaries, ≈1.5× greater than that of the lower rates. Increased thermal energy and enhanced stress relaxation during film growth promote the formation of Σ3 and Σ7 boundaries.

Nucleation Analysis of Variant Transformed from Austenite with Σ3 Boundary in High-Strength Low-Alloy Steel

We elucidate here the effects of annealing twins on phase transformation products based on electron back-scatter diffraction analysis and corresponding microstructure visualization and quantification methods. Martensite and bainite were obtained by rapid continuous cooling and isothermal processing at different temperatures, respectively, which were designed to study variants formed in austenite with Σ3 boundary. The isothermal transformation near martensite start (Ms) temperature was most conducive in obtaining the highest content of twin-related V1/V2 variant pair. Based on the classical nucleation theory of martensite and bainite, respectively, the role of austenite Σ3 boundary in martensite and bainite transformation is illustrated.

Hot Deformation Characteristics and Dynamic Recrystallization Mechanisms of a Novel Nickel‐Based Superalloy

The hot deformation characteristics of a novel nickel‐based superalloy is investigated via the isothermal compression test in temperature range of 1000–1150 °C and strain rate of 0.001–10 s−1 under the true strain of 0.8. The hot deformation characteristics of GH4065 alloy are studied here for the first time. Based on the flow stress data, it is observed the typical features of flow curves exhibit the occurrence of dynamic recrystallization (DRX) during the hot deformation process. The constitutive equation in the Arrhenius‐type model is established, and activation energy (Q) is determined as 844.787 kJ mol−1. The microstructure evolution and DRX mechanism are investigated by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) technique. The results reveal that the fraction of low angle grain boundaries (LAGBs) decrease gradually with the increase in deformation temperature, whereas the fraction of Σ3 boundaries increase first and then decrease. For γ + γ′ dual‐phase region, the particle‐induced DRX (PIDRX), characterized by the generation of sub‐grains accelerated by γ′ precipitates pinning dislocations, and discontinuous dynamic recrystallization (DDRX) are the dominant nucleation mechanism of DRX. For γ quasi‐phase region and γ single‐phase region, the occurrence of bulged grain boundaries with twins further illustrates that DDRX plays a more significant role.

Hot Deformation Behavior of a High-Mn Austenitic Steel for Cryogenic Liquified Natural Gas Applications

Hot deformation behavior of a high-Mn austenitic steel was investigated employing hot compression tests at different temperatures and strain rates. The flow behavior related to deformation temperature and strain rate was analyzed. Microstructures and grain boundary characteristics of the deformed specimens quenched at selected conditions were examined using SEM-EBSD. It was observed that the flow stress and critical characteristic parameters were sensitive to deformation temperature and strain rate. Grain boundary bulging was the main nucleation mechanism which signified discontinuous dynamic recrystallization played a vital role in microstructure evolution. Strain rate had a complex influence on DRX kinetics and the formation of Σ3 boundaries. At high strain rates, the higher stored energy and adiabatic temperature rise induced the boundary to migrate at a higher velocity, thus accelerating the nucleation of DRX grains and increasing the frequency of twinning. At low strain rates, longer time was available for grain boundary migration which facilitated the growth of DRX grains and the nucleation of annealing twins. However, at intermediate strain rates, sluggish recrystallization kinetics and annealing twins evolution were observed as the stored energy was not sufficiently high and the time available for grain boundary migration was also fairly short.

Fatigue behavior of biomedical Co–Cr–Mo–W alloy fabricated by selective laser melting

Co–Cr–Mo based alloy was widely used biomaterials for metallic implants fields. Recently, selective lase melting (SLM), one of the additive manufacturing technologies, is broadly studied because it can produce metallic implants with complex geometries and high degree of personalization. However, the possibility of mechanical failure must be avoided during the working period of SLM produced biomaterials. Therefore, in this paper we investigated the fatigue behaviors of the SLM Co–Cr–Mo–W alloy and their corresponding microstructure and fracture morphology. The sand mold casting fabricated (Cast) Co–Cr–Mo–W alloys with the same chemical composition as SLM specimens were also investigated and compared. The results showed that the SLM Co–Cr–Mo–W alloy exhibits higher fatigue life, tensile strength, and ductility than Cast Co–Cr–Mo–W alloy. The high fraction of Σ3 boundaries and precipitates in SLM Co–Cr–Mo–W alloy lead to secondary cracks and deflected fatigue crack path, which can alleviate the stress concentrations of the main fatigue cracks and decrease the fatigue crack growth rate. However, the dendritic structure of Cast Co–Cr–Mo–W alloy cause cleavage fracture mode and the large island-like precipitates can generate stress concentration, which result in lower fatigue life. We believed that the results of this work can provide a new perspective towards microstructure regulation for additive manufacturing technology and paved way for in-depth application of SLM fabricated biomedical alloy.

Atomistic mechanisms of twin–twin interactions in Cu nanopillars

Twinning is an important mode of plastic deformation in metallic nanopillars. When twinning occurs on multiple systems, it is possible that twins belonging to different twin systems interact and forms a complex twin–twin junctions. Revealing the atomistic mechanisms of how twin–twin interactions lead to different twin junctions is crucial for our understanding of mechanical behaviour of materials. In this paper, we report the atomistic mechanisms responsible for the formation of two different twin–twin interactions/junctions in Cu nanopillars using atomistic simulations. One junction contains two twin boundaries along with one Σ9 boundary, while the other contains five twin boundaries (fivefold twin). These junctions were observed during the tensile deformation of [100] and [11¯0] Cu nanopillars, respectively.

Evolution of Annealing Twins and Recrystallization Texture in Thin-Walled Copper Tube During Heat Treatment

Thin-walled copper tubes are usually produced by multi-pass float-plug drawing deformation. In general, the annealing treatment subsequently is necessary to release the stored energy and adjusts the microstructure. In this study, an investigation on the evolution of annealing twins as well as textures in the thin-walled (Ф6 mm × 0.3 mm) copper tube underwent holding time-free heat treatment was reported. Electron backscattered diffraction analysis reveals that a large number of Σ3 boundaries (60° 〈111〉 twin relationship) are produced at the early stage of heat treatment, which is due to the lower boundary energy. With the recrystallization proceeding, the migration rate of grain boundaries decreases on account of the grain growth; meanwhile, the unique Σ9 boundaries (38.9° 〈110〉 relationship) are formed due to the interaction of the Σ3 boundaries. As a result, the number fractions of Σ3 boundaries and high-angle grain boundaries decrease rapidly. During the grain growth stage, a strong recrystallization texture was formed due to the fact that the grains of Goss orientation have a growth advantage over the others. As a result, the initial copper texture was transferred into the Goss texture in domination.