Σ3 Boundaries Research Articles

Evolution of grain boundary character distribution in near-surface regions of a cold-rolled nickel-based superalloy during induction heating process

The influence of induction heating on the grain boundary character distribution (GBCD) in near-surface regions of a cold-rolled Nickel-based superalloy was researched. After induction heating, most of low-Σ coincidence site lattice (CSL) boundaries were Σ3 boundaries, which were mainly formed via the growth accident model. Moreover, the grain structures evolution during induction heating had a great influence on the evolution of GBCD. At the low strain of 0.1, both the fraction of Σ3 boundaries and grain size increased with the increasing temperatures, while the former was closely related to the better development of grain-clusters at the higher temperature. In addition, the coherent Σ3 boundaries were easier to be formed at the higher temperature during induction heating, owing to their low interface energy and mobility. At the large strain of 0.5, the fraction of Σ3 boundaries also increased with the increasing temperatures, but the grain size exhibited the opposite trend, which was closely related to the well development of static recrystallization (SRX) behaviors. Meanwhile, there was a “symbiotic relationship” between the SRX grains and Σ3 boundaries during induction heating. Through the electrochemical corrosion tests, it was proved that induction heating can contribute to the improvement of corrosion properties of superalloys via increasing the fraction of Σ3 boundaries, while the best corrosion resistance appeared in the samples treated at 800 °C with the strain of 0.5. Moreover, the evolution of both Σ9 and Σ27 boundaries was also closely related to strains and induction heating temperatures, but their fractions were less than 4%.

Grain boundary energy function for α iron

Polycrystalline α iron has been used in various applications, yet its microstructure design via grain boundary engineering (GBE) is not well established. One limiting factor is that while there are many different grain boundaries in the five-dimensional space of grain boundary types, relatively few of the energies have been determined. In this study, a piece-wise continuous grain boundary energy function for α iron is constructed to fill the entire five-dimensional space of grain boundary types using scaffolding subsets with lower dimensionality. Because the energies interpolated from the grain boundary energy function are consistent with the 408 boundaries that have been calculated using atomistic simulations, the energy function is then employed to generate a larger set of grain boundary energies. Comparisons between the interpolated energies and the measured grain boundary population indicate that they are inversely correlated for the high-energy anisotropy misorientations (those for which the difference between the maximum and minimum grain boundary energies is greater than 0.4 J/m2). The results suggest that GBE in the α iron should consider the high-energy anisotropy misorientations, rather than the twinning-related grain boundaries (Σ3, Σ9, Σ27a, and Σ27b) as in the case of fcc metals.

Non-destructive determination of phase, size, and strain of individual grains in polycrystalline photovoltaic materials

The non-destructive structural characterization of individual grains in thin-films photovoltaics based on polycrystalline materials is a powerful tool for revealing important details of the microstructure of solar cell absorbers. Here, we use three-dimensional X-ray diffraction (3DXRD) to obtain statistics on the phase, size, orientation and strain tensors of the grains, as well as their twin relations in Cu2ZnSnS4 (CZTS) absorbers. Moreover, this powerful approach allows the non-ambiguous determination of phases with distinct optical and electrical properties but similar lattice parameters, such as CZTS and ZnS. Our analysis over cumulative statistics of nearly 600 grains in polycrystalline CZTS reveals that a fraction of 2.5% corresponds to the ZnS secondary phase. Statistics of the strain distribution in the polycrystalline CZTS layer indicate an average tensile stress in the plane of the film of ~70 MPa and a compressive stress along the normal to the film of ~145 MPa. We found that 41% of the total number of grains in CZTS absorbers are Σ3 twins. We calculate the frequency of the six types of Σ3 boundaries, revealing that the 180° rotation along the axis<221>is the most frequent. Accessing the microstructure opens the possibility to study its influence on the properties of the film, such as bandgap variations due to strain or the role of twin boundaries in the charge transport mechanisms.

Investigation on thermo-mechanical processing and intergranular corrosion of TP347H stainless steel

The grain boundary character distributions of TP347H stainless steel after different thermo-mechanical processes in terms of grain boundary engineering (GBE) were investigated by EBSD. DL-EPR and electrolytic oxalic acid etch tests were carried out to evaluate the improvement of GBE treatment on intergranular corrosion. The low-Σ CSL grain boundary proportion of the TP347H can be increased to about 90% through a single-step thermo-mechanical treatment consisted of a slight tensile deformation (3%) and subsequent annealing process at 1150°C for 1 h. Compared with conventional TP347H, the optimum GBE microstructure suppressed intergranular corrosion due to the high proportion of low-Σ CSL grain boundaries, particularly the twin (Σ3) boundaries which is almost immune to intergranular corrosion.

Achievement of forging without canning for β-solidifying γ-TiAl alloy containing high content of niobium

ABSTRACT The processes of forging without canning were optimized, such that forging could be successfully performed and forged pancakes without cracks could be obtained. The maximum height reduction could reach 80%. The as-forged microstructure consisted of γ and β/B2 phases. The volume fraction of the β/B2 phase rose slightly compared to the pre-forging state. Near the top or bottom of the pancake, residual lamellae were found, while full dynamic recrystallization was completed at the center and lateral parts. Equiaxed γ grains illustrated that dynamic recrystallization mainly occurred in the γ phase, while the softer β phase coordinated the deformation by adjusting morphology. Two types of Σ3 boundaries, [110] 70° and [111] 60°, were found in the γ phase. At room temperature, the fracture was brittle, and the elongation was 0.07%. With increasing temperature, the yield strength and ultimate strength decreased, while the elongation increased. The brittle–ductile transition temperature was between 800°C and 850°C.

Microstructure and Texture Development in Thermomechanically Processed Leaded Brass

Leaded brass alloys with high zinc content are classified as dual-phase (α-β′) brass alloys. For these alloys, α-β′ phase transformation induced by thermomechanical processing (TMP) is expected to play a significant role not only in the formation of α and β′ phases and uniformity of lead particles but also in the development of Σ3 boundaries and their variants in the α phase. To assess such impact, the current study has employed two TMP schemes, with each scheme consisting of four cycles of compression and annealing. Samples of the first scheme (TMP-1) were annealed at 670 °C to promote phase transformation from α to β′, as compared to the second scheme (TMP-2) samples annealed at 525 °C. Microstructure examination indicated that TMP-1 scheme fostered a remarkable growth in the area fraction of β′ phase, accompanied by grain coarsening of α and β′ phases. This resulted in the isolation of α grains and less interaction of Σ3 boundaries at trip junctions, and therefore, a reduction in the fraction of Σ3 boundaries occurred in the α phase. On the other hand, the TMP-2 scheme successfully enhanced cluster formation of α grains, enabling further generation of Σ3 boundaries via strain-induced boundary migration mechanism and ultimately leading to breakup of the random boundary network. In addition, the two schemes resulted in limited coarsening of lead particles. Most lead particles were located at the α-βʹ interfaces, which suppressed grain boundary migration via the particle-dragging effect. For both TMP schemes, the initial texture of tension fibers was largely preserved after four cycles for both α and βʹ phases. Nevertheless, the TMP-2 scheme produced texture randomization of the α phase due to increasing the fraction of Σ3 boundaries.

Effect of grain boundary engineering on corrosion behavior of nickel-based alloy 825 in sulfur environment

Grain boundary engineering (GBE) by appropriate deformation and heat treatment processes was applied to nickel-based alloy 825 due to its excellent corrosion resistance. The microstructure of nickel-based alloy was analyzed by electron backscatter diffraction (EBSD) and the effect of GBE on the corrosion resistance in high sulfur-containing environments was studied by electrochemical method. The results show that the ratio of low Σvalue coincidence site lattice (CSL) grain boundary is obviously improved from 47.1% to 65.5%. The special angle grain boundaries Σ3, Σ9, Σ27 and so on are randomly distributed on the network of the large angle grain boundaries, which destroy the connectivity of the original grain boundary network, and effectively block the corrosion cracking of the material along them. The corrosion resistance of nickel-based alloy 825 in high sulfur environments is enhanced verified by polarization curve and scanning electrochemical microscope, providing references for corrosion protection during the exploitation of high sulfur gas reservoirs.

Grain Boundary Energy Function for α Iron

Polycrystalline α iron has been used in various applications, yet its microstructure design via grain boundary engineering (GBE) is not well established. One limiting factor is that while there are many different grain boundaries in the five-dimensional space of grain boundary types, relatively few of the energies have been determined. In this study, a piece-wise continuous grain boundary energy function for α iron is constructed to fill the entire five-dimensional space of grain boundary types using scaffolding subsets with lower dimensionality. Because the energies interpolated from the grain boundary energy function are consistent with the 408 boundaries that have been calculated using atomistic simulations, the energy function is then employed to generate a larger set of grain boundary energies. Comparisons between the interpolated energies and the measured grain boundary population indicate that they are inversely correlated for the high-energy anisotropy misorientations (those for which the difference between the maximum and minimum grain boundary energies is greater than 0.4 J/m2). The results suggest that GBE in the α iron should not only be considered the twinning-related grain boundaries (Σ3, Σ9, Σ27a, and Σ27b) as in the case of fcc metals, but also other high-energy anisotropy misorientations.

Specific features of crystallographic texture formation in BCC-FCC transformation in extruded brass

Crystallographic features of body-centered cubic to face-centered cubic transformation were studied in an extruded brass product. X-ray phase analysis, scanning electron microscopy with electron microprobe analysis, and orientation microscopy based on diffraction of back-scattered electrons were used. A sample after treatment consisted of the β′-phase (Cu0.55Zn0.45, structural type B2) and α-phase (Cu0.65Zn0.35, structural type A1) at 30:70. Integral textures (within the product) of phases were axial with axes parallel to the elongation direction in extrusion:< 110 >for β’ and< 100 >for α. Local textures of phases are described as limited and consisting of a set of components. For β′ – {001}< 110 >, with scattering up to± 25° around the axis< 110 >; and {110}< 110 >consisting of three components – one {110}< 110 >and two {110} ± 24° around the axis< 110 >; for α – {001}< 100 >(strong) and two of {110}< 111 >(weak). It was shown that components of local textures of the β′- and α-phases were associated by orientation relationships close to Kurdjumov-Sachs orientation relationships. A small number of limited textural components formed as a result of β(BCC)→α(FCC) transformation realized as per multi-variant orientation relationships is explained by the formation of nuclei of a new phase at special boundaries close to coincidence site lattice boundaries Σ3 and Σ33.

Synergetic Deformation Mechanism in Hierarchical Twinned High-Entropy Alloys

The mechanical properties of crystalline materials can be efficiently optimized using a hierarchical twinned structure. Conventional deformation mechanisms for coherent Σ3 boundaries generally involve three basic models: cross-slip, partial dislocation step, and full dislocation step. In this study, we report a novel deformation mechanism that allows the co-existence of de-twinning, phase transformations, grain rotation, and cracking, around a triple junction of twin boundaries in a hierarchical twinned high-entropy alloy. The deformation mechanisms in the reference high-entropy alloy (Fe-30Mn-10Co-10Cr at. %) were investigated using LAADF-STEM. The triple junction of the hierarchical twinned structure gradually deformed during in-situ strain and showed mechanisms significantly different from that observed in the purely twinned structures. These new mechanisms are referred to as “novel synergetic deformation mechanisms of hierarchical twin boundaries.” Understanding the fundamental mechanisms of the hierarchical twin boundaries under deformation could assist the design of strong and ductile bulk materials with hierarchical twinned structure.

Effect of Microstructures on the Mechanical Behavior of Fire‐Resistant Dual‐Phase Steels at Elevated Temperature

Herein, the start laminar cooling temperature to obtain different volume fractions of ferrite and bainite/martensite is optimized. Lower start laminar cooling temperature yields a higher volume fraction of ferrite and also decreases the transformation temperature of bainite/martensite. The yield strength of the experimental dual‐phase steel at 600 °C increases from ≈484.8 to ≈526.2 MPa by decreasing the start laminar cooling temperature from 650 to 600 °C. To understand the effect of microstructure, the dislocation density, volume fraction of precipitates, and Young's modulus at elevated temperature are statistically analyzed. A machine learning method, known as gradient boosting decision tree, is used to classify the grain boundaries. The statistical results suggest that the steel subject to lower start laminar cooling temperature shows higher Young's modulus and larger volume fraction of precipitates at elevated temperature. Furthermore, at elevated temperature, the boundary is stable and the proportion of Σ3 boundary is greater, when lower start laminar cooling temperature is adopted. Theoretical calculations results suggest that the lower transformation temperature of bainite/martensite improves the fire resistance of dual‐phase steels.

Correlative statistical microstructural assessment of precipitates and their distribution, with simultaneous electron backscatter diffraction and energy dispersive X-ray spectroscopy

Modern engineering alloys have bespoke microstructures, where features such as precipitates are used to control properties. In many Ni-based alloys, carbo-nitride precipitates are introduced to strengthen and improve performance. These precipitates can be distributed throughout the microstructure and niobium rich carbides are often found at grain boundaries. In this work, we used combined energy dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD) to characterise the population of these precipitates. Processing of the EDS signal is used to label the Mo/Nb-rich precipitates, and their size and location are measured from maps using a circular Hough transform. This label map is combined with the grain boundary network (from EBSD analysis). Statistical analysis, using ANOVA testing, reveals differences in chemistry between carbides found in Ni-rich matrix grain interiors, on random high angle boundaries and on special boundaries (Σ3 and Σ9). These results are compared between wrought and power metallurgy product forms. These distributions are discussed in the context of their performance within demanding environments, such as reactor core internals.

Effects of aging temperature and grain boundary character on carbide precipitation in a highly twinned nickel-based superalloy

ABSTRACT The precipitation of carbides at grain boundaries in nickel-based superalloys and austenitic stainless steels can significantly affect the grain boundary related properties of the bulk material. This study evaluated the growth and dissolution of carbides in grain boundary engineering (GBE) treated nickel-based superalloy (Inconel 690) during aging treatment using scanning electron microscopy and transmission electron microscopy. The grain boundary network was stable during aging below 1000°C. At lower aging temperatures of 715 and 800°C, a small number of tiny carbide precipitates were observed on Σ3c grain boundaries. In contrast, plate-like carbides precipitated near both sides of Σ3i grain boundary and on one side of Σ9 grain boundary, and large carbide precipitates formed on Σ27 and random grain boundaries. At higher aging temperatures of 900 and 1000 °C, large carbide precipitates with a more consistent morphology precipitated at Σ9, Σ27, and random grain boundaries. Carbide dissolution was common during aging at these higher temperatures, which promoted the growth and redistribution of other carbides. These experimental findings provide valuable insights into the effect of aging temperature and grain boundary character on carbide precipitation behaviours.

Control of deformation and annealing process to produce incoherent Σ3 boundaries in Hastelloy C-276 alloy

Hastelloy C-276 samples were cold-rolled with their thickness reduction of 7.5% and 15%, and then annealed at 1100 °C for 5–60 min in solid solution. The grain boundary character distributions (GBCDs) were characterized by a technique of electron back scatter diffraction (EBSD). Results indicated that, more convoluted Σ3 boundaries (i.e. incoherent Ʃ3 boundaries) are generated by reasonable annealing time in low strain (7.5%) sample during strain-induced boundary migration (SIBM) processing. They can induce higher Ʃ3n fraction and activate more interactions between special boundaries for GBE. Instead, more coherent Ʃ3 boundaries were produced by moderate strain (15%) and annealing treatment with the occurrence of static recrystallization (SRX). The recrystallized fine grains possess fewer incoherent Ʃ3 boundaries cannot promote to get more multiple twinning for optimization of GBCD. Therefore, reasonable deformation and annealing treatment parameters lead to more incoherent Ʃ3 boundaries would play an important role in the optimization of GBE.

The evolution of Σ3 grain boundaries in the strip-cast Hi-B grain orientated Fe-6.5% Si alloy

A novel high magnetic induction Fe-6.5%Si alloy with B800 that reached up to 1.77 Telsa was prepared by twin-roll strip casting. Electron backscattered diffraction (EBSD) investigations demonstrated that Σ3 grain boundaries formed in the body-centered cubic (BCC) metal solidified microstructure without phase transition where the ratio of grain boundaries was greater than 20%. Transmission electron microscopy (TEM) revealed that the partial Σ3 grain boundaries are BCC structured twinning. A certain amount of Σ3 boundaries were inherited from the process of warm rolling, cold rolling and annealing, which acted to stabilize the matrix grain boundaries, resulting in the formation of a perfect Goss microstructure with excellent magnetic properties.

Microstructure Evolution and Corrosion Behavior of Deformed Austenitic Stainless Steel Manufactured by Selective Laser Melting

The effect of room temperature deformation on the microstructure and corrosion behavior of selective laser-melted (SLM) 316L steel in 0.9 wt.% NaCl medium was explored by electrochemical tests, microstructural analysis, and passive film characterization. The irregular evolution of corrosion behavior was first found in the deformed SLM 316L steel. The corrosion resistance of SLM 316L steel improved with increasing room temperature deformation level reached the maximum at 20% deformation, and then weakened with further increasing deformation below 40%. Microstructural analysis indicated that the deformation twins passed through the molten pool boundaries when the deformation was 20%, but not when the deformation was 40%. The dislocation density, Σ3, and low-angle grain boundaries increased slightly with increasing deformation below 20%, but they increased distinctly with deformation level greater than 20%. Besides, room temperature deformation modified the oxide content of the surface passive film, especially for the chromium oxide, which affected the corrosion resistance of passive film. Above results indicate that low room temperature deformation level of SLM 316L stainless steel is feasible to improve its corrosion resistance and optimize the passive film.

Effect of nitrogen on grain boundary character distribution in 316 stainless steel

Influence of nitrogen content and percentage of prior rolling deformation on evolution of grain boundary character distribution in 316L(N) stainless steel, during grain boundary engineering type thermo-mechanical processing, have been studied in the present work. 10% rolling deformation followed by annealing produces highest fraction of low energy Σ3 boundaries and high fraction of low energy grain boundary network in high nitrogen SS, leading to improved corrosion resistance.

Cavitation erosion behavior of high‑nitrogen austenitic stainless steel: Effect and design of grain-boundary characteristics

An understanding of the cavitation erosion behavior and mechanisms of high‑nitrogen austenitic stainless steel is of interest for scientific and economic reasons. The original microstructure of 18Mn18Cr0.6 N steel was examined using electron backscatter diffraction analysis. Specimens were eroded using an ultrasonic cavitation erosion facility. The damaged surfaces were observed using scanning electron microscopy. Coherent Σ3 boundaries had the worst cavitation erosion resistance; the cavitation damage was located primarily at the same sides of the twins with a low Taylor factor. The cavitation erosion resistance decreased in the order: random high-angle grain boundaries and Σ27 boundaries; incoherent Σ3 boundaries; Σ9 boundaries; and coherent Σ3 boundaries. The Taylor factor was important in the analyses of metal material cavitation erosion. A large difference between the two grains or twins in the Taylor factor yielded a higher cracking risk of the boundary between them. Grain rotation led to a step formation at a boundary and the cavitation damage was accelerated. Three types of cavitation erosion were identified. A cavitation mechanism model for coherent Σ3 boundaries was proposed. Finally, the design of grain-boundary characteristics to improve the cavitation erosion resistance was discussed.

Three-Dimensional Characterization of Morphology and Crystallography of a Gold Nanoisland Film

The morphological and crystallographic characteristics of noble metal nanoisland films play an important role in determining their properties, performance, and reliability. In this work we have applied a rapid three-dimensional orientation mapping technique in the transmission electron microscope (3D-OMiTEM) in the characterization of a gold nanoisland film. A volume of 200×1024×1024 nm3 has been analyzed, generating a 3D orientation map composed of more than 500 nanoislands and 7000 grains constituting the islands. The 3D shapes and sizes of individual islands and grains have been analyzed, revealing their true 3D morphological features and the correlation between the number of grains within individual islands and the size of the islands. The crystallographic orientations of the grains and the misorientations across the grain boundaries have been quantified, revealing a weak texture but a preferential presence of Σ3 and Σ9 grain boundaries in the gold nanoisland film.

Grain Orientation Spread in Dynamically Recrystallized Austenitic Steel

Some feature of discontinuous dynamic recrystallization (DRX) in an Fe-0.4%C-18%Mn austenitic steel during isothermal compression tests at temperatures of 973-1373 K and strain rates of 10-3-10-1 s-1 were studied. The DRX microstructures consisted of various grains, i.e., DRX nuclei, growing DRX grains, and work-hardened DRX grains, which differentiated with the grain orientation spread (GOS). DRX was commonly promoted by a decrease in temperature-compensated strain rate, i.e., Zener-Hollomon parameter (Z), corresponding to an increase in deformation temperature and/or a decrease in strain rate. In contrast, the GOS distribution varied non-monotonously with Z. The large area fraction of DRX grains with small GOS below 1° appeared at definite temperature/strain rate conditions. The large fraction above 0.6 of DRX grains with small GOS was observed in DRX microstructures with a large ratio of CSL Σ3 boundary fraction to low-angle subboundary fraction. The GOS distribution in the DRX microstructures is discussed in terms of the DRX grain nucleation and growth rates.