Boundary Character Distribution Research Articles

Enhancing mechanical property and corrosion resistance of Al0.3CoCrFeNi1.5 high entropy alloy via grain boundary engineering

In the present study, to improve the performances of Al0.3CoCrFeNi1.5 high entropy alloys (HEAs), grain boundary character distribution (GBCD) of Al0.3CoCrFeNi1.5 HEA has been optimized by an appropriate thermo-mechanical processing. The experiment results showed that the fraction of low-Σ coincidence site lattice (CSL) boundaries could reach approximately 80 % through cold rolling with deformation of 8 % and subsequent annealing at 1050 °C for 5 min. The reason for GBCD optimization could be attributed to sufficient strain-induced boundary migration (SIBM) or grain growth after recrystallization. While recrystallization is not favorable for optimizing GBCD. The mechanical properties and corrosion resistance have been enhanced, with a more pronounced improvement observed in the corrosion resistance. The corrosion current density icorr of the GBEM specimen stands at 0.23 μA∙cm−2, representing a reduction of 66 % in comparison to the BM specimen (0.68 μA∙cm−2). The improvement of corrosion resistance of Al0.3CoCrFeNi1.5 HEA resulted from the discontinuous random grain boundaries (RGBs) broken by the high fraction of low-ΣCSL boundaries, especially Σ3 boundaries suppressed the propagation of corrosion crack.

Effect of Grain Boundary Character Distribution on the Precipitation Behavior: A Comparative Study for 304 Steel and T91 Steel

Effect of Grain Boundary Character Distribution on the Precipitation Behavior: A Comparative Study for 304 Steel and T91 Steel

Effect of thermomechanical processing on the grain boundary character distribution of phosphorus bronze

Grain boundary engineering (GBE) is widely adopted to improve the grain boundary (GB) character distribution (GBCD) of face-centered-cubic (FCC) metals. In phosphorus bronze alloy, achieving an optimized GBCD while maintaining sufficiently small grain size is critical for maintaining high strength and excellent bending workability. However, the precise evolution and formation mechanisms of GBs during GBE remain unclear to date. Leveraging electron backscatter diffraction and transmission electron microscopy analyses, this study examined the effect of thermomechanical processing (TMP) on GBCD optimization and twin-related domain (TRD) evolution in phosphorus bronze. The results revealed that strain-induced boundary migration (SIBM) plays a pivotal role in GBCD optimization. In particular, SIBM facilitates the formation of abundant new Σ3 boundaries behind migrating GB fronts, thus increasing the proportion of special boundaries (SBs) and introducing low-energy segments such as Σ9 and Σ27 boundaries. These boundaries effectively disrupt the connectivity of the random high angle grain boundary (RHAGB) network. Furthermore, the results indicate that the optimal TMP conditions for GBCD optimization include a reduction level of 5 %, annealing temperature of 450 °C, and annealing duration of 1 h. These conditions result in an average grain size being <3 μm and the GB fractions fSBs and fΣ9+Σ27 being 81.3 % and 9.8 %, respectively. Deformation twins formed within the deformed microstructure inhibit TRD growth, thus hindering GBCD optimization. Additionally, the optimal strain threshold for GBCD optimization lies near the strain threshold for the formation of deformation twins. TRDs form during the primary recrystallization process, while high-order twin boundaries form behind the migration front during the growth process.

Effects of the Primary Carbide Distribution on the Evolution of the Grain Boundary Character Distribution in a Nickel-Based Alloy

Grain boundary engineering (GBE) was carried out on a nickel-based alloy (GH3535, Ni-16Mo-7Cr-4Fe), which intrinsically has many strings of primary molybdenum carbides. The strings induce inhomogeneous grain size distributions and increase the difficulties in achieving a GBE microstructure. In this work, the effects of the primary carbide distribution on the grain boundary network (GBN) evolution were investigated. A higher proportion of Σ3n grain boundaries (GBs) associated with extensive multiple twinning events was achieved in the specimen with more dispersive and finer primary carbides, which are the results of cross-rolling, i.e., cold rolling with a changed direction. In a starting microstructure with many strings of primary carbides, the dense and frequent occurrence of particle-stimulated nucleation (PSN) around the carbides induced more general high-angle GBs into the GBN, and the inhibition of GB migrations by the carbide strings suppressed the formation of large-sized highly twinned grain clusters. As a consequence, the Σ3n GBs could not be effectively enhanced.

New insights into ductility improvement of a nickel-based superalloy through grain boundary engineering

Grain boundary engineering (GBE) provides an effective approach for tailoring the properties of metallic materials by optimizing the grain boundary character distribution (GBCD). Using electron backscatter diffraction analysis and uniaxial tensile testing at room temperature, this study systematically investigated the impact of GBE-type thermomechanical processing (TMP) on the ductility of an Incoloy 925 nickel-based superalloy (Alloy 925). The results showed that at 5 % strain, increasing the annealing temperature results in larger twin-related domains through strain-induced boundary migration. The GBE-type TMP can significantly enhance the ductility of Alloy 925, namely, the elongation increases from 64.22 % to 95.86 % as the annealing temperature increases from 1050 °C to 1100 °C. The evolved GBCD introduces more coherent twin boundaries with lower average residual Burgers vectors, enabling efficient dislocation transmission. In addition, extensive twinning can widely generate some soft orientations and simultaneously reduce the average Taylor factor within the twin clusters, leading to easy dislocation activation during plastic deformation. The synergistic effect of the optimized grain boundary network and twin cluster anisotropy promotes uniform plastic flow and effective strain accommodation, thereby enhancing the ductility of Alloy 925. Overall, this work provides novel and valuable insights into ductility optimization through GBE in a nickel-based superalloy.

Intercritically Annealed Medium-Manganese Steel: Insights into Microstructural and Microtextural Evolution, Strain Distribution, and Grain Boundary Characteristics.

Aluminum-incorporated medium-manganese steel (MMnS) has potential for lightweight transport applications owing to its impressive mechanical properties. Increasing the austenite volume fraction and making microstructural changes are key to manufacturing MMnS. However, the grain boundary character and strain distribution of intercritically annealed low-density MMnS have not been extensively scrutinized, and the effects of crystallographic texture orientation on tensile properties remain ambiguous. Therefore, in this study, the microstructure, microtexture, strain distribution, and grain boundary characteristics of a hot-rolled medium-Mn steel (Fe-0.2 C-4.3 Al-9.4 Mn (wt%)) were investigated after intercritical annealing (IA) at 750, 800, or 850 °C for 1 h. The results show that the 800 °C annealed sample exhibited the highest austenite volume fraction among the specimens (60%). The duplex microstructure comprised lath-type γ-austenite, fine α-ferrite, and coarse δ-ferrite. As the IA temperature increased, the body-centered cubic phase orientation shifted from <001> to <111>. At higher temperatures, the face-centered cubic phase was oriented in directions ranging from <101> to <111>, and the sums of the fractions of high-angle grain boundaries and coincidence-site-lattice special boundaries were significantly increased. The 800 °C annealed sample with a high austenite content and strong γ-fiber {111}//RD orientation demonstrated a noteworthy tensile strength (1095 MPa) and tensile elongation (30%).

Investigation on the correlation between grain boundary character distribution and precipitate behavior in T91 ferritic/martensitic steel

In the present study, the relationship between microstructure, grain boundary character distribution (GBCD) and precipitate behavior in T91 steel was investigated via a series of thermomechanical treatments (TMTs). The experimental results revealed that subjecting the steel to cold-rolling (CR) with a deformation of 15–30% followed by tempering at 750 °C for 1 h caused the microstructure of T91 steel to transition from the typical lath martensite structure to a uniform polygonal ferrite, resulting in a change in the GBCD. Specifically, the proportion of random grain boundaries for the ferritic structure was significantly higher than that for the martensitic structure. The M23C6 carbide appeared as irregular polygons and precipitated within the ferrite grains, while it exhibited a rod-like shape and precipitated at the martensitic boundaries. Additionally, the orientation relationship (OR) between M23C6 carbide and the adjoining matrix transformed from the double Kurdjumov-Sachs (DKS) relation to (112) M23C6 // (110) ferrite and 201¯M23C6 // [012] ferrite. The change of OR resulted in a higher coarsening rate of M23C6 carbides precipitated in ferrite compared to those precipitated at martensitic boundaries.

Crystallographic texture evolution in hot-press sintered tungsten heavy alloy with the effect of niobium (Nb) addition

This study examines the impact of niobium on the crystallographic texture evolution and grain boundary distribution of hot press sintered tungsten heavy alloy (W-4.9Ni-2.1Cu). EBSD analysis was used to examine the evolution of crystallographic texture and grain boundary character distribution in sintered compacts. The important findings of this study depict that cubic texture component {001} <111> were evolved in niobium unalloyed compacts due to the solidification of atoms aligned in a specified preferred orientation. Besides, it reveals the weakness in the cubic texture in niobium alloyed compacts due to the recrystallization followed by sub structural recovery occurrence during sintering. In addition, an increase in CSL fraction in niobium alloyed compacts illustrates the dominance of surface diffusion during sintering caused by the grain refinement attributed from the niobium enrichment at the tungsten-binder interface. Metallographic studies showed that the niobium concentration increment in WHAs has concomitant effect with sintered compacts' properties. A W-4.9Ni-2.1Cu-1 Nb alloy achieved the highest values for micro-hardness (432 HV0.5), electrical conductivity (23.74 % of IACS), and relative density (99.90 %).

Micro-mechanical investigation of maraging steel during in-situ tensile test

ABSTRACT The research aimed to examine the development of slip bands in 12Cr-9Ni maraging stainless steel through in-situ tensile testing in scanning electron microscopy (SEM) and atomic force microscopy (AFM). A comprehensive approach was employed, which included assessing strain distribution and the grain boundary character distribution (GBCD) at the intermediate plastic strain steps. During plastic deformation, dislocation pile-up leads to the formation of slip bands in the material. It was discovered that slip band formation, as well as slip line spacing, exhibit crystallographic orientation dependence. Finally, fractography was employed to unveil the likely fracture mechanism and further link it to the formation of slip bands at higher strain levels.

Novel role of thermomechanical grain boundary engineering in the microstructure evolution of austenitic stainless steel

The effect of special grain boundary development via the grain boundary engineering process (GBE) was explored in a bimodal microstructured 316L austenitic stainless steel with an average grain size of 1.92 μm. The GBE process was carried out by the rolling-annealing method via two routes of low and medium applied strain, followed by a short annealing period in a single-step and iterative manner. The grain boundary character distribution was obtained by Electron Backscatter Diffraction (EBSD) analysis, and the intergranular corrosion resistance and the tensile behavior of the samples were examined. Microstructural characterization showed that applying low strain repetitively increased the coincidence site lattice (CSL) and Σ3 boundaries percentage and created large twin-related domains (TRD). The reduced sensitization was attributed to a high proportion of CSLs and large-sized TRDs by applying a low-strain route. In the early stages of the low-strain route, the grain boundary character distribution showed a high number of primary twin nuclei in coarse grains and a continuous network of single twin nuclei in smaller grains, creating a high percentage of Σ3-grain boundaries. The low applied strain and the absence of the necessary driving force for the recrystallization and GBs migration led to the formation of twin nuclei, which would assist with the reduction of microstructure energy. A high percentage of Σ3 boundaries and an increase in the percentage of triple points consisting of low energy boundaries were found to be influential factors in increasing elongation.

Effect of Mn addition in W-Ni-Cu heavy alloy processed through hot press sintering techniques on microstructure, microtexture and grain boundary character evolution

In this current study, the impact of manganese addition to 93 W-4.9Ni-2.1Cu-xMn (where x = 1, 2, 3, and 4 Wt%) on the microstructure, texture development, and grain boundary character distribution of tungsten heavy alloy (WHAs) was examined. The samples were produced through a vacuum hot press sintering, with parameters 50 MPa pressure, 10 °C/min heating rate, and 1450 °C sintering temperature for 20 min holding duration. To study the influence of manganese on the texture development and microstructure in conventional WHAs, electron backscatter diffraction (EBSD) was employed. The important findings from the EBSD analysis reveal that fibre texture (001) 〈111〉 and a higher fraction of low-angle grain boundaries in the Mn unalloyed compact resulted from extensive atomic jumps between the particles at elevated temperatures. Further texture analysis on the Mn-added compacts reveals that manganese oxide (MnO) formation during liquid phase sintering led to lower densification and randomized weak fibre texture development in manganese-alloyed compacts. In addition, the texture degradation in Mn-added compacts is mainly caused by recrystallization followed by sub-structural recovery during liquid phase sintering. This study indicates that increasing manganese content in WHAs stoichiometry led to a heterogeneous distribution of binder matrix (NiCu) in the sintered compact. As a result, W-4.9Ni-2.1Cu-4Mn alloy revealed decreased relative density (90.70%), microhardness (389.5 HV0.5), and electrical conductivity (15.74% of IACS).

Evolution of Grain Boundary Character Distribution in B10 Alloy from Friction Stir Processing to Annealing Treatment.

In this study, the grain boundary character distribution (GBCD) of a B10 alloy was optimized, employing thermomechanical processing consisting of friction stirring processing (FSP) and annealing treatment. Using electron backscatter diffraction, the effects of rotational speed of FSP and annealing time on the evolution of GBCD were systematically investigated. The GBCD evolution was analyzed concerning various parameters, such as the fraction of low-Σ coincidence site lattice (CSL) boundaries, the average number of grains per twin-related domain (TRD), the length of longest chain (LLC), and the triple junction distribution. The experimental results revealed that the processing of a 1400 rpm rotational speed of FSP followed by annealing at 750 °C for 60 min resulted in the optimum grain boundary engineering (GBE) microstructure with the highest fraction of low-Σ CSL boundaries being 82.50% and a significantly fragmented random boundary network, as corroborated by the highest average number of grains per TRD (14.73) with the maximum LLC (2.14) as well as the highest J2/(1 - J3) value (12.76%). As the rotational speed of FSP increased from 600 rpm to 1400 rpm, the fraction of low-Σ CSL boundaries monotonously increased. The fraction of low-Σ CSL boundaries first increased and then decreased with an increase in annealing time. The key to achieving GBE lies in inhibiting the recrystallization phenomenon while stimulating abundant multiple twinning events through strain-induced boundary migration.

Effect of nitrogen content on grain boundary engineering and corrosion resistance of 316LN stainless steel

Nitrogen solid solution can effectively enhance the strength of steels, but its effect on grain boundary engineering and the corrosion mechanism of steels is still unclear and systematic investigation is needed. In this study, 316LN steel specimens with five nitrogen concentrations were melted and subjected to compression and annealing. The grain boundary character distribution was investigated by electron backscatter diffraction. The corrosion behavior was investigated by the electrochemical polarization curves, electrical impedance spectroscopy, and X-ray photoelectron spectroscopy. The results showed that nitrogen promotes the formation of low-Σ boundaries when the pre-strain is not above 10%. The polarization resistance increases with increasing nitrogen content. The quantitative relationship between the nitrogen content and the pitting potential was obtained. Nitrogen favors the formation of oxides, hydroxides, and NH3/NH4+ on the passivated surface. The effect of planar slip on grain boundary engineering and the influence of NH4+ on the corrosion resistance are discussed.

Effect of the grain boundary character distribution on the sulfur corrosion behaviour and mechanisms of copper windings under different high temperatures

AbstractAs a typical failure phenomenon in transformers, sulfur corrosion has garnered significant attention in the field of high‐voltage engineering. Grain boundary character distribution (GBCD) copper windings have been introduced to enhance sulfur corrosion resistance by slowing down intergranular corrosion. In this study, the sulfur corrosion behaviour and mechanisms of the GBCD copper windings under various temperatures were experimentally and theoretically studied. Results show that GBCD can enhance the corrosion resistance of copper in liquid environments. With the increase in temperatures, the insulating properties of oil and papers in traditional copper windings experience notable degradation, while GBCD copper windings show more stable insulating behaviours. In addition, modelling of grain boundary energy indicates that the grain boundary structure of GBCD copper windings has a lower average interface energy of 0.170 eV/Å2. Calculations of reaction thermodynamics show that GBCD copper windings possess a higher failure temperature (135.2°C) and inhibition degree (activation energy) of the sulfur corrosion (32,557.62 J/mol), revealing the stability and enhanced sulfur corrosion resistance at elevated temperatures.

Effect of grain boundary engineering on grain boundary character distribution and deformation behavior of a TRIP-assisted high-entropy alloy

This study carried out two-step thermo-mechanical processing (TMP) in grain boundary engineering (GBE) to optimize the grain boundary character distribution (GBCD) and tensile properties of a transformation-induced plasticity-assisted high-entropy alloy (TRIP-HEA). The evolution and mechanism of GBCD in the HEA were characterized and discussed using electron backscatter diffraction (EBSD). The tensile properties of the HEA after undergoing different TMPs were evaluated through room-temperature uniaxial tensile tests, and the influence of GBE on the deformation mechanisms was investigated using techniques such as EBSD and transmission electron microscopy (TEM). The results indicated that the fraction of low Σ coincident site lattice (CSL, Σ ≤ 29) boundaries exhibited a decreasing trend followed by an increasing trend with an increase of second-step cold rolling strain during TMPs, and the overall fraction of Ʃ9 + Ʃ27 boundaries showed an increasing trend. When the reduction ratio was 10%, the low-ƩCSL boundaries fraction reached ~66.35%. In terms of tensile properties, all GBE specimens showed improvement in strength and ductility. When the reduction ratio of second-step cold rolling was 20%, the yield strength was 506.7 MPa, which was ~59% higher than base material (BM) sample. The ultimate tensile strength was 885.6 MPa, ~26% greater than BM sample. In addition, when the reduction ratio of second-step cold rolling was 3%, the total elongation was 52.6%. The main deformation mechanisms of the HEA were dislocation slip, stacking fault (SF), nano-twining, and phase transformation. The optimization of GBCD had multiple effects on the deformation mechanisms. It manifests in several ways, such as the enhancement of strain uniformity through ΣCSL grain boundaries, improving the ability of dislocation multiplication and stress-induced cracking resistance, as well as the strengthening due to the combined effects of phase transformation and grain refinement.

Topological characteristics of grain boundary networks during severe plastic deformations of copper alloys

The grain boundary character distribution strongly affects the properties of polycrystalline materials. Grain boundaries of similar characters form networks, whose topological invariants can be considered as distribution descriptors. Understanding the evolution of such descriptors during severe plastic deformations (SPD) can elucidate the evolution of properties and underpin the design of processing routes for target behaviour. For topological analysis of grain boundary networks, polycrystalline materials are considered here as polyhedral complexes with grain boundaries classified into two types — low-angle and high-angle. Changes of grain boundary types are calculated using sub-grain rotations, which reflects the physical mechanism of microstructure evolution during SPD. A non-physical approach, by direct conversions of low-angle to high-angle boundaries, is also explored as a reference to demonstrate the impact of the physical constraint imposed by rotations. Reported is the discovery of topological phase transitions in the grain boundary networks which might take place during severe plastic deformations of different copper alloys. Depending on the evolution approach, the transitions correspond to zeros of the Euler characteristic, or of the logarithm of the inverse connectivity, of the grain boundary network. The relations between these transitions and the fraction of high-angle grain boundaries, and between the fraction of high-angle grain boundaries and plastic strain obtainable experimentally, provide new perspectives for grain boundary engineering and network design. Determining the dominant evolution mechanism and critical accumulated strain for a given material and processing route requires further experimental studies of triple junction evolution.

Significantly enhancing elevated-temperature strength and ductility of a FeMnCoCr high-entropy alloy via grain boundary engineering: Exploring multi-deformation mechanisms

The objective of the study is to elucidate the influence of grain boundary engineering (GBE) on the high-temperature properties of a metastable FeMnCoCr high-entropy alloy (HEA). Using thermo-mechanical processing (TMP), the grain boundary character distribution (GBCD) was optimized, which involved the increase of coincidence site lattice (CSL) special boundaries and multiple twinning events, and breaking of random high-angle grain boundaries (RHAGBs) network. Furthermore, the deformation strain of the alloy became more uniform at high temperature, dynamic recrystallization was suppressed, and a number of deformation mechanisms were operative. The ductility of studied HEA was significantly enhanced by over 200% at 1073 K, resulting in a remarkable 1.35-fold increase.

Strategy for suppressing abnormal grain growth of ZK60 Mg alloy during solution by pre-compression: A quasi-in-situ study

Abnormal grain growth (AGG) easily takes place in Mg alloys during high-temperature solutions, resulting in deterioration of mechanical properties. Hence, the compression prior to solution (pre-compression) was conducted to suppress AGG, and the microstructure evolution as well as suppressing mechanisms was investigated based on quasi-in-situ analysis. After compression along the transverse direction, <11–20>//ED grains preferentially nucleated and rapidly grew up, and the initial <10–10>//ED texture was weakened. Two grain growth modes of heat-induced and strain-induced grain boundary migrations were found. The former was attributed to the high interfacial energy of grain boundaries with large curvature. The latter consumed the adjacent grains with high storage energy, forming abnormal grains with irregular shapes. The compression with a reduction > 6% could obviously suppress AGG. The suppressing effects were mainly attributed to weakening the size advantage of <11–20>//ED grains, increasing nucleation, reducing grain boundary character distribution, and redistributing storage energy distribution. After 12% compression along the transverse direction, 30° misorientation of <11–20>//ED grains and high energy grain boundaries were reduced. The {10–12} tensile twins and {10–15} high index twins induced by compression increased the nucleation of static recrystallization. Beside, compression introduces high-density dislocations, which also contributed to suppressing AGG behavior during solution.

Optimization of Grain Boundary Character Distribution of Fe-19Cr-10Mn-1Ni-0.53N High Nitrogen Austenitic Stainless Steel

High nitrogen austenitic stainless steel (HNASS) has outstanding mechanical properties, but in its hot working, welding, and high-temperature use, there will be a precipitation phase, especially nitride precipitation that reduces its intergranular corrosion, stress corrosion, corrosion fatigue, and other grain boundary-related properties. At present, the traditional methods of suppressing precipitation phase precipitation, such as solution treatment and alloying, have drawbacks. Grain boundary character distribution (GBCD) optimization of Fe-19Cr-10Mn-1Ni-0.53N HNASS was characterized by electron backscatter diffraction (EBSD) in this work. This study reveals that after grain boundary engineering (GBE) treatment, the percentage of low Σ coincidence site lattice (CSL) boundaries of the solid solution treated sample rises from 53.94% to 82.41%, because the sample was cold-rolled (CR) by 5%, followed by annealing at 1423 K for 10 min, and the twin related domains (TRD) size increases from 23.99 μm to 169.82 μm and ν (the ratio of TRD size to grain size) raises from 1.74 to 7.52. The connectivity of the random high-angle grain boundary (RHAGB) web is interrupted and the GBCD of the experimental steel is remarkably optimized.

Cryogenic deformation tailors the dislocation accumulation mode to modify grain boundary character distribution of Incoloy 925

Cryogenic deformation was introduced in thermomechanical processing to restrict dislocation mobility and tailor the initial dislocation distribution at the deformation stage, subsequently followed by annealing treatment to alter the grain boundary characteristic distribution with a view to the feasibility of grain boundary engineering in Incoloy 925. The microstructural evolution deformed at cryogenic (77 K) and room temperature (298 K) under various rolling reductions and annealing conditions was systematically studied to optimize the grain boundary network, which is evaluated using multi-source quantitative statistics of the twin related domains. The results showed that cryogenic deformation can adjust the dislocation accumulation mode and plastic deformation mechanism. Geometrically dislocation distributions, reconstructed by HR-EBSD device, with fishbone-like, ribbon-like or lamellar characteristics can be observed for the deformed specimens. During the subsequent annealing, the samples with 5% and 15% pre-strain are incapable of producing the ideal twin related domains, while the sample annealed at 1075°C with 10% pre-strain show the optimization of GBCD with the highest Σ3n (n = 1, 2, and 3) boundary fraction being 69.67%. Compared with the room temperature pre-deformation, cryogenic deformation can effectively reduce the critical temperature of GBE, and also leads to the Σ3n grain boundaries proportion increasing from 31.47% to 67.46% in 10%/1050°C sample. Meanwhile, with the appropriate strain, cryogenic deformation is more conducive to optimizing the GBCD than room-temperature deformation.