Grain Boundary Character Distribution Research Articles

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.

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.

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 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.

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.

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.

Effect of annealing twins, strain-recrystallization processing and δ-phase fraction on microtexture and evaluation of mechanical properties of nickel-based superalloy 718

The Nickel-based superalloy 718 is a precipitation-hardened material, especially by the ordered and coherent phases γ” (Ni3Nb) and γ″ (Ni3 (Al, Ti)). In addition to these phases, precipitation of carbides and δ-phase is possible. This material has high strength, creep, and corrosion resistance at temperatures up to 650 °C. However, despite the alloy’s outstanding performance under harsh environments, research effort has been put into optimizing its properties and increasing the operation temperature. Manipulation of microstructural parameters such as grain boundary character distribution (GBCD), microtexture, second phase fraction, etc. is frequently betaken in order ot achieve better properties. In this context, this work aimed to assess the influence of Thermomechanical Processing on the recrystallization texture of the material and its influence on the alloy’s mechanical behavior. The material was submitted to four iterative processing routes, composed of deformation-heat treatment cycles. The alloy’s microtexture data were obtained through the Electron Backscattered Diffraction technique. Data processing was performed using the MTEX toolbox, where Orientation Density Functions were obtained for microtexture analysis. It was observed that the deformation imposed during thermomechanical processing significantly affects the recrystallization texture of the material so that smaller deformations induce the formation of typical recrystallization texture components; on the other hand, high degrees of deformation lead to a retained deformation texture after annealing. Mechanical properties were evaluated by depicting the influence of different strengthening mechanisms and δ-phase fraction on yield strength and ductility, respectively. It was observed that the main strengthening parameters for the alloy were grain boundaries and intrinsic resistance (Pierls-Nabarro and solid solution), meanwhile ductility was greatly influenced by δ-phase and ∑3n boundaries fractions.

A Review on Controlling Grain Boundary Character Distribution during Twinning-Related Grain Boundary Engineering of Face-Centered Cubic Materials

Grain boundary engineering (GBE) is considered to be an attractive approach to microstructure control, which significantly enhances the grain-boundary-related properties of face-centered cubic (FCC) metals. During the twinning-related GBE, the microstructures are characterized as abundant special twin boundaries that sufficiently disrupt the connectivity of the random boundary network. However, controlling the grain boundary character distribution (GBCD) is an extremely difficult issue, as it strongly depends on diverse processing parameters. This article provides a comprehensive review of controlling GBCD during the twinning-related GBE of FCC materials. To commence, this review elaborates on the theory of twinning-related GBE, the microscopic mechanisms used in the optimization of GBCD, and the optimization objectives of GBCD. Aiming to achieve control over the GBCD, the influence of the initial microstructure, thermo-mechanical processing (TMP) routes, and thermal deformation parameters on the twinning-related microstructures and associated evolution mechanisms are discussed thoroughly. Especially, the development of twinning-related kinetics models for predicting the evolution of twin density is highlighted. Furthermore, this review addresses the applications of twinning-related GBE in enhancing the mechanical properties and corrosion resistance of FCC materials. Finally, future prospects in terms of controlling the GBCD during twinning-related GBE are proposed. This study will contribute to optimizing the GBCD and designing GBE routes for better grain-boundary-related properties in terms of FCC materials.

Microstructure and Mechanical Properties of Dissimilar Friction-Welded Commercially Pure Ti and Ti–6Al–4V Alloy

Here, we investigated the microstructure and mechanical properties of dissimilar joints between grade 2 commercially pure titanium (CP-Ti-2) and Ti–6Al–4V alloy welded via friction welding. Accordingly, specimens with 15 mm diameter and 50 mm length were prepared, and friction welding was performed at a constant rotation speed (1600 rpm) and different upset lengths: 1, 2, and 3 mm. Electron backscattered diffraction (EBSD) method was introduced to systematically analyze grain boundary characteristic distributions (GBCDs), such as grain size, grain orientation, phase distribution, and misorientation angle distribution. In addition, mechanical properties of dissimilar joints were evaluated using the micro-Vickers hardness and tensile test machine. The microstructures at both sides of the CP-Ti-2 and Ti–6Al–4V alloy were refined materials, compared with each base material. These grain refinements were determined via dynamic recrystallization accompanied by frictional heat and severe plastic deformation. In addition, the increase in the upset length distinctively affected the change trend of the grain size of the CP-Ti-2 and Ti–6Al–4V alloy. The hardness values in regions around the weld interface were higher than those of base materials. Furthermore, yield and tensile strengths were maintained at a similar level to those of CP-Ti-2, and the tensile specimens were fractured at the base material zone of CP-Ti-2, not at the weld interface and its periphery. Consequently, friction welding to dissimilar joints between the CP-Ti-2 and Ti–6Al–4V alloy was successfully conducted and sound welded materials without welding defects were obtained.

Effect of Heat Treatment Process on the Optimization of Grain Boundary Character Distribution in Heavy Gage Austenitic Stainless Steel

The optimization of grain boundary character distribution (GBCD) is of great significance to improve the GB-related properties for heavy-gauge austenitic stainless steels worked in harsh environments such as reactors of nuclear power, which can usually be realized by regulating the thermomechanical process. In this paper, special solution annealing processes for a hot-rolled nuclear grade 316H plate were designed to introduce different character distribution of Σ3n boundaries (1 ≤ n ≤ 3) and random high-angle GBs (RHAGBs), and the regulation principle among them were clarified. It was worked out that the optimized GBCD by characterization of large twin related domains, abundant interconnected Σ3n boundaries and interrupted topology network of RHAGBs could be effectively facilitated through solution annealing with a long time period at lower temperature or short time period at higher temperature, in which the recrystallization, grain growth and GB migration during heat treatment process played key roles. Moreover, the length fraction of Σ3n boundaries were found to be hardly changed when they reached about 77%, but their character distribution could be continuously optimized.

Effects of grain boundary character distribution on hydrogen-induced cracks initiation and propagation at different strain rates in a nickel-saving and high-nitrogen austenitic stainless steel

The effect of grain boundary character distribution (GBCD) on hydrogen-induced cracks (HICs) initiation and propagation in nickel-saving austenitic stainless steel was investigated at different strain rates with electrochemical hydrogen charging. The fracture strength and elongation of specimens decreased as the strain rates decreased due to more hydrogen atoms being transported by mobile dislocations. HICs are more likely to initiate at random grain boundaries (RGBs) because of poor deformation coordination of adjacent grains at RGBs after hydrogen charging. Special boundaries (SBs) deflected HICs and hindered the propagation of HICs owing to a more stable boundary structure and less strain localization during slow strain rate tensile (SSRT) deformation with hydrogen charging. Furthermore, highly efficient distributions of SBs contribute to great resistance to hydrogen embrittlement.

Effect of Short-Range Ordering on the Grain Boundary Character Distribution Optimization of FCC Metals with High Stacking Fault Energy: A Case Study on Ni-Cr Alloys

The critical roles of short-range ordering (SRO) in the grain boundary character distribution (GBCD) optimization of Ni-Cr alloys with high stacking fault energies were experimentally studied by thermomechanical treatments. It is found that, with the enhancement of the SRO degree (or the increase in Cr content), the dislocation slip mode changes from wavy slip to planar slip, and even deformation twins (DTs) appear in the cold-rolled Ni-40at.%Cr alloy. Within the lower level of Cr content (≤20 at.%), the optimized result of GBCD is conspicuous with the increase in Cr content. As the Cr content is higher than 20 at.%, the GBCD optimization of Ni-Cr alloys cannot be further enhanced, since the cold rolling induced DTs would hinder the growth of twin related domains during subsequent annealing.

Influence of grain boundary energy anisotropy on the evolution of grain boundary network structure during 3D anisotropic grain growth

In this paper, we present the results of grain growth simulations in three-dimensions using an existing level set method. Most of the previous grain growth studies have been either isotropic or, if they are not isotropic, consider simplified models of anisotropy. We perform these simulations using three different grain boundary (GB) energy functions (a realistic 5D GB energy function, a Read–Shockley energy model that depends only on disorientation angle, and an isotropic GB energy model). We compare the results in terms of several statistical microstructural descriptors (crystallographic texture, GB character distribution, triple junction distribution (TJD), etc.). In addition to considering different energy models, we also compare the evolution of microstructures that have different initial crystallographic textures (fiber texture and random texture). We find that the morphological evolution of individual grains can be completely different depending on the GB energy function employed. However, we also find that certain spatially independent microstructural statistics (e.g. orientation distribution function, misorientation distribution function, and TJD) are similar at steady state for all of the tested GB energy functions.

Relative grain boundary energies from triple junction geometry: Limitations to assuming the Herring condition in nanocrystalline thin films

Grain boundary character distributions (GBCD) are routinely measured from bulk microcrystalline samples by electron backscatter diffraction (EBSD) and serial sectioning, and this data can be used to reconstruct relative grain boundary energy distributions (GBED) based on the 3D geometry of triple lines, assuming that the Herring condition of force balance is satisfied. These GBEDs correlate to those predicted from molecular dynamics (MD); furthermore, the GBCD and GBED are found to be inversely correlated. For nanocrystalline thin films, orientation mapping via precession enhanced electron diffraction (PED) has proven effective in measuring the GBCD, but the GBED has not been extracted. Here, the established relative energy extraction technique is adapted to PED data from four sputter deposited samples: a 40 nm-thick tungsten film and a 100 nm aluminum film as-deposited, after 30 and after 150 min annealing at 400 °C. These films have columnar grain structures, so serial sectioning is not required to determine boundary inclination. Excepting the most energetically anisotropic and highest population boundaries, i.e. aluminum Σ3 boundaries, the relative GBED extracted from these data do not correlate with energies calculated using MD nor do they inversely correlate with the experimentally determined GBCD for either the tungsten or aluminum films. Failure to reproduce predicted energetic trends implies that the conventional Herring equation cannot be applied to determine relative GBEDs and thus geometries at triple junctions in these films are not well described by this condition; additional geometric factors must contribute to determining triple junction geometry and boundary network structure in spatially constrained, polycrystalline materials.

B2-precipitation induced optimization of grain boundary character distribution in an Al0.3CoCrFeNi high-entropy alloy

The grain boundary character distribution (GBCD) of Al0.3CoCrFeNi high entropy alloy (HEA) subjected to 5% cold rolling and subsequent annealing at 1000 °C from 1 h to 100 h was investigated. The results show that the microstructure of Al0.3CoCrFeNi HEA is composed of FCC structure and ordered BCC (B2) structure. With the increase of annealing time, the frequency of low coincident site lattice (low-ΣCSL (3 ≤ Σ ≤ 29)) is divided into three stages, i.e., increases slowly in stage I (1–10 h), increases sharply in stage II (10–50 h), and remains stable in stage III (>50 h). The HEA annealed for 100 h possesses the highest low-ΣCSL fraction of ~71.34% and the biggest special boundary cluster, indicating that GBCD is effectively optimized. To this end, GBCD optimization mechanisms are proposed to explain this evolution: strain-induced boundary migration (SIBM) mechanism in stage I, non-coherent Σ3 boundary migration reaction model in stage II, and maintain stability in stage III. In this process, B2-precipitation has a key influence on the evolution of mechanism. B2-precipitation pinning induces the SIBM mechanism in stage I to generate numerous non-coherent Σ3 boundaries, which provides a basis for the non-coherent Σ3 boundary migration reaction in stage II. Therefore, the introduction of precipitation in HEAs provides a new and effective pathway to optimize GBCD.

Influence of solution temperature on microstructure and creep behaviors of a Ni–Co base superalloy for turbine disc

The influence of solution temperature on the microstructure evolution and creep behavior of a new Ni–Co base superalloy for turbine discs, including the grain morphology, grain boundary character distribution (GBCD), and grain boundary (GB) carbide morphology, was investigated using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). The results showed that the creep fracture life first increased and then decreased with increasing solution temperature. The alloy solution treated at 1140 °C exhibited the best creep properties, with a creep life of 209 h. The enhanced creep performance of this alloy can be explained as follows: the volume fraction of GB carbides increased with increasing solution temperature, the diffuse fine granular carbides relieved stress concentration and strengthened GBs, the coarse primary γ′ phases rapidly dissolved, and the volume fraction of the fine and dispersed secondary and tertiary γʹ phases significantly increased. The narrower channels in the γ matrix increased the resistance to dislocation motion. The coherent Σ3 twin boundaries (TBs) with low interface energy hindered the dislocations and stacking faults intersecting the TBs on different slip planes. The relative frequency of Σ3 TBs increased to 53.6% when the solution temperature was increased to 1140 °C. The high proportion of Σ3 TBs played a key role in improving the creep resistance of the alloy.

The effect of thermomechanical treatment on the evolution of the grain boundary character distribution in a Cr0.8MnFeNi high-entropy alloy

We have employed thermomechanical processing to optimize the grain boundary character distribution (GBCD) of a grain boundary engineered (GBE) microstructure in a face-centered cubic Cr0.8MnFeNi high-entropy alloy (HEA). The influence of the annealing time and the degree of tensile deformation on the GBCD were investigated through electron backscatter diffraction (EBSD). The results show that a 5% tensile deformation followed by annealing at 1000 °C for 40 min led to the formation of a high fraction of low Σ (Σ ≤ 29) coincident site lattice (CSL) boundaries (81.9%), a high (Σ9 + Σ27)/Σ3 ratio (0.17), and a high twin-related domain size to grain size ratio (4.49), demonstrating that the optimization of the GBCD had been achieved. Extending the annealing time further led to a slight decrease in the fraction of low-ΣCSL boundaries. The degree of tensile deformation employed before annealing at 1000 °C/40 min also had a significant influence on the GBCD. As the degree of deformation was increased from 3% to 20%, the low-ΣCSL (Σ ≤ 29) boundaries fraction first increased and then decreased. Any strain-induced boundary migration (SIBM) occurred mainly within low deformation (≤10%) specimens, while any extensive recrystallization occurred within high deformation (20%) specimens. Thus, the optimization of the GBCD could be attributed to sufficient SIBM rather than to significant recrystallization in Cr0.8MnFeNi HEA.

Effects of Grain Boundary Engineering on the Microstructure and Corrosion Fatigue Properties of 316L Austenitic Stainless Steel

Grain boundary engineering (GBE) treatment was performed through thermomechanical processing (TMP) to optimize the grain boundary character distribution (GBCD) of 316L austenitic stainless steel. The effects of TMP on the GBCD and corrosion fatigue properties in high-temperature and high-pressure water were investigated. The results indicated that a high fraction (about 74%) of special boundaries as well as the interrupted network of random high-angle grain boundaries were obtained through 5% strain followed by annealing at 1,273 K for 90 min. The Σ9 and Σ27 boundaries were generated by the reaction of special boundaries. The highest corrosion fatigue life for 3,187 cycles was obtained when the TMP parameters of the 316L ASS were of 5% strain, annealing temperature of 1,273 K, and annealing time of 45 min. The low-energy special boundaries had strong intergranular corrosion resistance, but the strength of these boundaries was not enough to resist the propagation of transgranular fatigue cracks.

On the critical strain of thermal-mechanical processing to tailor grain boundary character distribution in INCOLOY 925 alloy

Through a series of thermal-mechanical processing (TMP) routes with various ratios of cold rolling and subsequent high-temperature annealing, the samples with different critical strain states were prepared to explore the evolution of grain boundary character distribution (GBCD) in INCOLOY 925 during grain boundary engineering (GBE). The GBE microstructure was characterized using the electron backscatter diffraction to study the influence of TMP parameters on the evolution of GBCD and the penetration of random high angle grain boundaries. The results show that the different states of pre-strain correspond to the variational formation mechanism of grain boundary network (GBN). The strain induced boundary migration is the predominant mechanism with the rolling reduction lower than critical strain. With the slight increase of strain, the recrystallization nucleation can be initiated to generate the strain-free grains at a certain annealing temperature. Moderate strain is needed to ensure adequate growth for twin related domains (TRDs). Besides, with the increase of strain and temperature, the dominant mechanism of GBCD gradually transfers from the new twinning mechanism to the Σ3 regeneration mechanism. Specifically, the TMP sample with 10% rolling reduction followed by annealing at 1075 °C for 10min shows relatively higher grain size difference ratio and average grains numbers in the TRDs, indicating that a larger initial grain size can increase critical strain and delay the nucleation of strain-free grains during TMP treatment. The moderately higher grain size shows an opposite effect on the formation of GBN. Finally, an estimation model of critical strain, as a function of initial grain size and annealing temperature, was constructed based on the artificial neural network, and the predicted results show a high consistency with the actual microstructure evolution.