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Abstract: Ultrafine grain sized (∼0.4μm) Cu samples having different fractions of nanotwinned grains (∼5%, ∼60% and >90%) were synthesized in order to evaluate the effect of the nanotwins on the overall microstructure. It was observed that nanotwins have a fundamental effect on altering the grain boundary network and texture of sputtered Cu samples. Increasing the fraction of nanotwinned grains increases both the fraction of special (Σ3) grain boundaries and the intensity of the {111} texture. Polarization and immersion corrosion tests using a 3.5% NaCl solution were performed on the nanotwinned Cu samples and compared to microcrystalline Cu (grain size∼9μm). It was found that highly nanotwinned Cu (nanotwinned grains>90%) had the highest corrosion resistance among all samples, which is attributed to its unique grain boundary network and {111} texture. In addition, the passive layer formed on highly nanotwinned Cu during corrosion achieved the highest pitting resistance when compared with the other samples. [Copyright &y& Elsevier]

High density and skin effect restrict the research progress of metal predominated electromagnetic wave absorbing (EMA) materials. Although some works try to solve it, they do not focus on the metal itself and do not involve the optimization of the active site of the inherent defects of the metal. In this work, the modulation of morphology, composition, interface, defects, and conductivity is achieved by adjusting the ratio of copper salt to reducing agent chitosan. Uniquely, the appearance of twin boundaries (TBs) accelerates the ability of the homogeneous interfaces to transfer charges, resists the oxidation of metal Cu 0 , keeps the high electric conductivity of Cu 0 nanoparticles, and enhances the conduction loss, which provides a boost for electromagnetic wave dissipation. As a result, the metal Cu 0 predominated absorber (Cu-NC (N-doped carbon)-10,) exhibits an ultra-width effective absorption band of 8.28 GHz (9.72-18.00 GHz) at a thickness of 2.47 mm and the minimum reflection loss (RL) value of -63.8 dB with a thickness of 2.01 mm. In short, this work explores the EM regulation mechanism of TBs compared with grain boundaries (GBs), which provides a new insight for the rational design of metal predominated EMA materials., (© 2022 Wiley-VCH GmbH.)

• The isothermal phase transformation of Co-Cr-Mo alloy can be controlled by changing grain boundary characteristic. • The high density of stacking faults and low-angle boundaries accelerated the phase transformation of γ-Co to ε-Co and Laves phase precipitation. • The twin boundaries retard the isothermal phase transformation and improve the resistance to Laves phase precipitation. • The GBE Co-Cr-Mo alloy maintains high mechanical performance after isothermal treatment. In this study, we investigate the influence of grain boundary character on isothermal phase transformation and mechanical properties of selective laser melting (SLM) fabricated Co-Cr-Mo alloy. The grain boundary engineering (GBE) was used to manipulate the isothermal phase transformation and precipitation. The As-SLM Co-Cr-Mo and GBE Co-Cr-Mo alloys were isothermal treatment at 800 ℃ for 6 h in a muffle furnace, followed by air cooling, respectively. It was found that the high density of stacking faults and low-angle boundaries in As-SLM Co-Cr-Mo alloy accelerated the phase transformation of γ-Co to ε-Co and Laves phase precipitation, which improved the tensile strength and drastically deteriorated the ductility. While, the high fraction of twin boundaries in GBE Co-Cr-Mo alloy retard the isothermal phase transformation and improve the resistance to Laves phase precipitation, which nearly maintain the high mechanical performance after the same isothermal treatment. The results of this study can be a guide to fabricate high-performance Co-Cr-Mo alloy according to work conditions. [ABSTRACT FROM AUTHOR]

A higher yield strength of parent phase in shape memory alloys is necessary for achieving a higher recovery stress. Fe alloying combined with grain boundary engineering (GBE) were investigated to improve the yield strength of ductile Cu17Al11Mn alloy. Results showed that the precipitation of low strength ductile α phase through the GBE remarkably enhanced the ductility while not obviously reducing the yield strength in the step-quenched Cu17Al11Mn4.5Fe alloy due to the precipitation of nanoscale κ phase inside them. Compared to the Cu17Al11Mn alloy, a 200 MPa higher yield strength and better ductility could be realized in the Cu17Al11Mn4.5Fe alloy step-quenched into two-phase region between 993 K and 1003 K. The recovery strains of ∼6.5% could be guaranteed in the step-quenched Cu17Al11Mn4.5Fe alloy. The Fe alloying combined with the GBE through the step-quenching will be a direction for developing ductile CuAlMn based alloys with both high strength and recovery strain. [ABSTRACT FROM AUTHOR]

• Seven groups of TiNiSn powders, which contained a wide range of particle sizes were fabricated. • Energy filtering effect leads to an increase in the Seebeck coefficient and power factor. • Lattice thermal conductivity has decreased largely due to the grain boundary scattering. • Significant improvement has been achieved in the thermoelectric properties of intrinsic TiNiSn. [Display omitted] Half-Heusler compounds MNiSn (M = Ti, Zr, Hf) have attracted increasing attentions in the field of thermoelectrics, since their constituent elements are non-toxic and abundant. Meanwhile, it possesses good thermal stability as well as high mechanical strength. Herein, a substantial enhancement to the thermoelectric performance of intrinsic TiNiSn is achieved by passing the pulverized powders through the sieve prior to sintering. The Seebeck coefficient has increased sharply owing to the energy filtering effect, which is induced by the reduction in the average grain sizes, combined with a slightly decreased electrical conductivity, leading to a significant improvement in power factor. Moreover, the lattice thermal conductivity can be further reduced via the enhancement of the grain boundary scattering. Consequently, a maximum figure-of-merit (zT) value of 0.84 at 820 K is achieved for un-doped TiNiSn. This work indicates that the grain boundary engineering may serve as an effective method for improving the thermoelectric performance of other half-Heusler compounds. [ABSTRACT FROM AUTHOR]

• Nb and Mo absorption by micron-TiC particles reduces grain boundary segregation. • Micron-TiC addition facilitates nano-sized carbonitride precipitates formation. • Nano-precipitates increase microhardness, except when they exist as co-precipitate. • Grain coarsening at high solutionizing temperature was reduced. • Isotropic microstructure was obtained without segregation and hot cracking. In this study, micron-TiC particles were added into an Inconel 718 matrix to fabricate a metal matrix composite by selective laser melting. The interactions between the micron-TiC particles and the precipitates in the matrix were studied through solutionization treatment. Four holding temperatures 975, 1075, 1175, and 1275 °C were chosen while the holding time was kept constant at 1 h. It was found that the addition of micron-TiC particles contributed to the reduction of solute segregation at the grain boundary and the susceptibility to hot cracking. The results indicated that the solute materials (Nb and Mo) were absorbed by micron-TiC particles through Ostwald ripening mechanism instead of diffusing to the grain boundaries. Moreover, grain coarsening at high solutionizing temperature was also reduced substantially due to the Zener pinning mechanism. The addition of micron-TiC also facilitated the formation of nano-sized carbonitride precipitates in as-printed samples. These precipitates were still observed at the highest solutionization temperature and contributed mainly to the higher microhardness of the reinforced Inconel 718, except when they formed co-precipitate with Laves or δ phases at 1175 °C. A more isotropic microstructure was obtained at 1275 °C without significant grain growth, solute segregation, or hot cracking. [ABSTRACT FROM AUTHOR]

Co-Cr alloy fabricated by selective laser melting was used to investigate the recrystallization behavior and grain boundary character evolution from As-SLM to isothermal annealed at 1200 °C for 5 min to 60 min. The As-SLM Co-Cr alloy has a prominent residual strain, high densities of dislocations and stacking faults, and solute segregation along the subgrain boundaries. The driving force for recrystallization of As-SLM Co-Cr alloy is mainly provided by phase transformation rather than crystal defects, which also offer plenty of routeways for both low- and high-angle grain boundary diffusion. In addition, the stacking faults in As-SLM Co-Cr alloy can generate morphologically complex twins by extending and gliding. The obtained optimal Co-Cr alloy has a large fraction (more than 80%) of special boundaries (Σ3, Σ9, and Σ27) after complete recrystallization, which enables high designing freedom and near-net-shape production towards additive manufacturing (AM) compatible grain boundary engineering (GBE) processing strategy. [Display omitted] • The phase transformation can provide enough driving force for complete recrystallization of As-SLM Co-Cr alloy. • After fully recrystallization, the obtained GBE Co-Cr alloy had more than 80% special grain boundaries. • The twins were formed by extending and gliding the stacking faults in original As-SLM Co-Cr alloy during recrystallization process. [ABSTRACT FROM AUTHOR]

In this work, we demonstrate a novel machining solution for controlling microstructures with special grain boundaries and nanotwins in composites during laser powder bed fusion (LPBF) that is based on a composite design of TiC/316L stainless steel (TiC/316LSS). Moreover, a microscopic mechanism is proposed. Gradient element segregation toward the amorphous TiC interface induces a low stacking fault energy (SFE), driven by the residual stress generated during LPBF thermal cycling, and 9R formed at the TiC interface with a low SFE and transformed to nanotwins. The migration of the incoherent twin boundary regenerates a high proportion of special grain boundaries. Based on the mechanistic analysis, the evolution of the microstructure, including TiC nanoparticle agglomeration, grain refinement and special grain boundaries in the microstructure, is well explained by optimizing the laser power, scanning speed and hatch spacing. TiC/316LSS composite with improved characteristics are observed, including a local residual stress decrease and grain refinement, which are notably superior to those of conventional thermomechanically treated 316LSS. The composite possesses a high proportion of special grain boundaries and nanotwins that maintain wall strengthening via an ultrafine dislocation cell structure produced during LPBF. Microstructural control is realized in additive manufacturing (AM) by predictably adjusting the process parameters. Thus, additive manufacturing grain boundary engineering (AM-GBE) shows great potential due to growth twinning and special grain boundary regeneration by optimizing the LPBF parameters and segregation-induced design. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: Triple junctions are recognized as important microstructural features in the context of grain boundary network topology and grain boundary sensitive materials properties. In spite of this prominence, mathematical tools for quantifying distributions of misorientations around triple junctions have been limited to special cases. Here we derive a general formula for the uncorrelated triple junction distribution function (TJDF) for materials of arbitrary texture. Additionally, we provide the mathematical link between TJDF and the orientation distribution function, which should facilitate the use of materials design paradigms on the grain boundary network. [Copyright &y& Elsevier]

The evolution of grain boundary microstructure during annealing in sputtered gold thin films was investigated on the basis of FEG-SEM/EBSD/OIM analyses of nanocrystalline microstructure, in order to find a clue to the precise control of grain boundary microstructure for development of high performance polycrystalline thin films. Remarkably high fractions of coincidence site lattice (CSL) boundaries with specific Σ values such as Σ1, Σ3, Σ7, Σ13, Σ19 and Σ21 occurred in the gold thin film specimens on Pyrex glass substrate by annealing in air. The occurrence of higher fraction of these specific low-Σ boundaries is probably attributed to the evolution of a very sharp {111}-textures of different degrees which results from the preferential growth of {111}-oriented grains due to surface energy-driven grain growth. The fraction of low-Σ CSL boundaries increased with increasing area fraction of {111}-texture. The grain boundary character distribution in the gold thin film specimens was strongly affected by the annealing atmosphere and substrate materials. The sharpness of {111}-texture in the specimen annealed in low-vacuum was weaker than that in the specimen annealed in air, and an extraordinarily high fraction of Σ3 CSL boundaries occurred. The grain growth of gold thin film specimens on SiO2glass substrate was much slower than that of specimens on Pyrex glass substrate. The fraction of low-Σ CSL boundaries observed for the gold thin film specimens on SiO2glass substrate was lower than that in the specimens on Pyrex glass substrate. The inverse cubic root Σ dependence of low-Σ CSL boundaries in the gold thin film specimens was discussed in connection with the process of the evolution of grain boundary microstructure. [ABSTRACT FROM PUBLISHER]

Control of grain boundary microstructure for grain boundary engineering (GBE) is effective to prevent intergranular corrosion by disconnection of corrosive random boundary network resulting from the introduction of coincidence site lattice boundaries during the frequent formation of annealing twins in bulk austenitic stainless steels. Since the practical use of austenitic stainless steels often includes straining and heating processes, such as forming, annealing and welding, the processes after the GBE treatment could degrade the optimised grain boundary microstructure and the high corrosion resistance of grain boundary engineered (GBEed) austenitic stainless steels. Therefore the present study examined the effects of post-GBE strain-sensitisation on grain boundary microstructure and corrosion resistance of a GBEed 304 austenitic stainless steel produced by a single-step thermomechanical processing for GBE. The results showed that the grain boundary microstructure was changed apparently and the corrosion resistance gradually decreased in the GBEed steel with an increase in post-GBE strain followed by sensitisation, but the GBEed steel maintained significantly higher corrosion resistance during the post-GBE strain-sensitisation than the equivalently strain-sensitised base steel. [ABSTRACT FROM PUBLISHER]

Abstract: The effect of grain boundary microstructure on the fracture resistance of sulfur-doped polycrystalline nickel was investigated using specimens with different grain boundary microstructures to reveal the usefulness of grain boundary engineering for control of segregation-induced intergranular brittle fracture of polycrystalline materials. The sulfur-doped polycrystalline nickel specimen with more homogeneous fine-grained structure and a higher fraction of low-Σ coincidence site lattice (CSL) boundaries shows higher fracture resistance than the specimen with coarse-grained structure and a lower fraction of low-Σ CSL boundaries. It was found that high-energy random boundaries play a key role as the preferential crack path in fracture processes. The resistance to sulfur segregation-induced intergranular brittle fracture was evaluated by analyzing the fractal dimension of random boundary connectivity in the polycrystalline nickel specimens studied. The fractal dimension of random boundary connectivity decreases with increasing fraction of low-Σ CSL boundaries, resulting in the generation of a higher fracture resistance by restricting more frequent branching and deflection of propagating crack path along random boundaries from the main crack. [Copyright &y& Elsevier]

Abstract: Microstructure evolution has been studied in the nickel-based superalloy PER®72 subjected to hot torsion, to annealing below the primary γ’ solvus temperature and to annealing at a supersolvus temperature, with a special emphasis on grain size and twin content. Dynamic abnormal grain growth occurs before the onset of dynamic recrystallization. The resulting bimodal grain size distribution affects the grain-coarsening kinetics at the supersolvus temperature, so that the final microstructures depend on the former straining stages. As a consequence, the twin content does not follow a univocal relationship with the average grain size. The grain boundary velocity history before reaching the final grain size is a contributing factor, and this is notably related to the initial grain size distribution width. Dynamically recrystallized microstructures are by nature more homogeneous and thus give rise to lower rates in supersolvus grain coarsening, and accordingly lead to relatively lower twin densities. [Copyright &y& Elsevier]

Abstract: We present experimental validation of a method for estimating three-dimensional (3-D) relative numerical populations of grain boundaries from measurements of individual two-dimensional (2-D) cross-sections. Such numerical populations are relevant to network topology and the modeling of intergranular failure modes in grain boundary engineered materials, and are distinct from geometrical population measures such as area per volume. We examine 3-D reconstructions of stainless steel and copper, with varying populations of twin-related boundaries, generated by serial-section electron backscatter diffraction and high-energy X-ray diffraction microscopy. We show that 2-D length fractions, 2-D number fractions and 3-D number fractions are all distinct quantities when grain boundary type is correlated with grain boundary size. We also demonstrate that the last quantity may be reliably inferred from the first two, provided the experimental spatial resolution is much finer than the grain size, eliminating the need to use 3-D experimental methods to access at least some information about 3-D network properties. Many of the Σ3 boundaries are extremely complex, with highly re-entrant shapes that can intersect a sample plane many times, giving a false impression of multiple separate boundaries. [Copyright &y& Elsevier]

AISI 304 stainless steel was subjected to grain boundary engineering by applying cycles of calibre rolling and subsequent heat treatment. After three cycles the grain size started to decrease, and after the fourth cycle a very fine grained material having high fraction of special grain boundaries was produced. Due to the short heat treatment at 850°C, only partial recrystallization occurred after the first three cycles, which was proven by the large amount of low angle boundaries. The stored elastic strain energy helped the grain boundary movement and the formation of annealing twins in the fourth cycle, which caused the formation of very fine grained structure with a large amount of special grain boundaries. [ABSTRACT FROM AUTHOR]

Abstract: The roles of grain boundary microstructure in fatigue crack propagation during cyclic deformation were investigated in order to evidence the utility of grain boundary engineering for control of fatigue crack propagation in SUS304 austenitic stainless steel. It was found that the polycrystalline specimen with a higher fraction of low-∑ boundaries shows a lower crack propagation rate than the specimen with a lower fraction of them. Grain boundary microstructures play important role in the deflection, path and the local propagation rate of fatigue crack. The introduction of a higher fraction of {111}/∑3 boundaries leads to a lower contribution of intergranular fracture to the total crack length and a lower local propagation rate of fatigue crack in SUS304 stainless steel. [Copyright &y& Elsevier]

Abstract: The structure, microstructure and low-temperature electrical properties of core–shell-type mixed conductors based on lanthanum gallate with Fe-doped grain boundaries are analyzed in depth. Electron probe microanalysis revealed that the iron concentration in the grain-boundary regions (shell) is below 1at.% and their thickness is no more than 1.5μm. The low-temperature (<400°C) electronic conductivity is enhanced by up to 2–3 orders of magnitude with respect to the corresponding undoped ceramics, as revealed by the analysis of impedance spectra combined with microstructural information. The electronic transport numbers lie in the range between 0.35 and 0.1 at 275 to 400°C, decreasing at higher temperatures, where the influence of grain boundaries on the overall transport properties vanishes and the ionic conductivity increases. [Copyright &y& Elsevier]

Grain boundaries (GBs) in high-temperature superconductors suppress the critical current density (Jc) dramatically, with the Jc decreasing exponentially with GB angle, especially when GB misorientation exceeds 4°. To reduce the number of high-angle GBs, fabrication of biaxially textured, superconducting wires via heteroepitaxial growth on cube-textured metals has been widely investigated worldwide. Such wires exhibit very high Jc in applied magnetic fields despite having a majority of GBs with total misorientations in the range 4-8°. Here, we show that GB networks in these wires have numerous GBs with out-of-plane misorientations greater than 4° but few boundaries having in-plane misorientations greater than 4°. Transport measurements on bicrystal GBs show that GBs with out-of-plane tilts between 4° and 8° are well linked. Together, these results explain the high performance of superconducting films on cube-textured metals. [ABSTRACT FROM AUTHOR]

By employing low-strain one-step thermo-mechanical processing (OTMP) and iterative thermo-mechanical processing (ITMP) we developed grain boundary engineered microstructure in a Ti-modified austenitic stainless steel (alloy D9). In OTMP, small amount of strain (0, 5, 10 and 15%) was imparted on solution annealed sample and subsequently annealed at various temperatures (1173-1273 K) for different time periods (0.5, 1 and 2 h). A pre-strain of 10-15% followed by annealing at 1273 K for 0.5-2 h has been found to be the suitable OTMP to increase the fraction of Σ3 boundaries significantly. ITMP employing 10% thickness reduction followed by annealing at 1273 K for 0.5 h revealed fluctuations in the evolution of Σ3s. ITMP employing 2.5% thickness reduction per iteration, on the other hand, resulted in continuous increase in Σ3 boundary fraction and a moderate increase in Σ9 and Σ27 boundaries. The role of Σ3 boundaries on the mechanical properties of GBE processed specimen was studied by correlating the hardness with grain size evaluated with and without considering twin boundaries. The paper also discusses the micro-mechanisms involved during low-strain GBE processes in austenitic stainless steel. [ABSTRACT FROM AUTHOR]

Abstract: A duplex surface treatment consisting of High Temperature Gas Nitriding (HTGN) followed by Low Temperature Plasma Nitriding (LTPN) was carried out in an UNS S31803 duplex stainless steel. The HTGN treatment was intended to produce a relatively thick and hard fully austenitic layer giving mechanical support to the thinner and much harder expanded austenite layer. HTGN was performed at 1200°C for 3h, in a 0.1MPaN2 atmosphere while LTPN, was carried out in a 75% N2 +25% H2 atmosphere, at 400°C for 12h, under a 250Pa pressure, and 450V. An expanded austenite γN layer, 2.3μm thick, 1500 HV0.025 hard, was formed on top of a 100μm thick, 330 HV 0.1 hard, fully austenitic layer, containing 0.9wt%N. For comparison purposes LTPN was carried out with UNS S30403 stainless steel specimens obtaining a 4.0μm thick, 1500 HV 0.025 hard, expanded austenite layer formed on top of a fully austenitic matrix having 190 HV 0.1. The nitrided specimens were tested in a 20kHz vibratory cavitation–erosion testing equipment. Comparison between the duplex treated UNS S31803 steel and the low temperature plasma nitrided UNS S30403 steel, resulted in incubation times almost 9 times greater. The maximum cavitation wear rate of the LTPN UNS S30403 was 5.5g/m2h, 180 times greater than the one measured for the duplex treated UNS S31803 steel. The greater cavitation wear resistance of the duplex treated UNS S31803 steel, compared to the LTPN treated UNS S30403 steel was explained by the greater mechanical support the fully austenitic, 330 HV 0.1 hard, 100μm layer gives to the expanded austenite layer formed on top of the specimen after LTPN. A strong crystallographic textured surface, inherited from the fully austenitic layer formed during HTGN, with the expanded austenite layer showing {101} crystallographic planes//surface contributed also to improve the cavitation resistance o f the duplex treated steel. [Copyright &y& Elsevier]

There is an increasing amount of interest in grain boundary engineering (GBE) to improve the properties of the grain boundary network. In general, research has concentrated on use of a grain boundary misorientation based approach, namely, the coincidence site lattice (CSL), to categorise 'special' boundaries. The present paper presents the case that the specialness of grain boundaries is based on having at least one low index plane at the boundary, rather than because the boundary is a CSL type. It is shown that grain boundary planes play a pivotal role in GBE, and hence, an approach which focuses on the plane is advocated. Some potential routes for 'grain boundary plane engineering' are discussed. [ABSTRACT FROM AUTHOR]

Abstract: Thermo-mechanical processes involving both single and multiple cycles of low level (5%) strain and annealing were applied to specimens of a type 304 austenitic stainless steel in order to encourage grain boundary engineering (GBE). As a result of the GBE processing the total length proportion of Σ3 n coincidence site lattice (CSL) boundaries was increased from 43% up to a maximum of 75% in conjunction with moderate grain growth. The increases in Σ3 and Σ9 boundaries resulted in significant decreases in the degree of sensitisation following exposure at 650°C for up to 4h and assessment through Double Loop-Electrochemical Potentiokinetic Reactivation (DL-EPR) tests. Over 97% of Σ3 boundaries were immune to sensitisation and approximately 80% of Σ9 boundaries were either immune or partially resistant to sensitisation, whereas all other CSL boundaries and general boundaries did not resist sensitisation. Therefore, only Σ3 and Σ9 boundaries were ‘special’. Deformation applied by cold rolling was more effective than tensile deformation in bringing about GBE. In summary, the results presented here show that increasing the fraction of Σ3 and Σ9 boundaries through GBE processing, accompanied by only moderate grain growth, provides an effective route to protection from sensitisation and intergranular corrosion. [Copyright &y& Elsevier]

Microstructures, and especially grain boundary populations, are characterized in a γ/γ′ nickel-base superalloy submitted to various thermomechanical treatments involving hot deformation and subsequent supersolvus annealing. The resulting microstructures consist of equiaxed grains, including twins. Both the grain size and the amount of twin boundaries are dependent on the processing parameters, which demonstrates that grain boundary engineering is applicable to those materials. The mechanisms involved are discussed, taking into account the hot-working conditions. [Copyright &y& Elsevier]

The distribution of grain boundaries of particular crystallographic character can provide descriptive information on the properties of engineering materials. For example, the fraction and connectivity of corrosion susceptible grain boundaries typically correlates with the extent of intergranular corrosion and stress corrosion cracking resistance in sensitised austenitic stainless steels. A parameter defining the cluster compactness is proposed to describe the breakup of the network of corrosion susceptible grain boundaries. It may therefore provide a measure of intergranular stress corrosion cracking resistance. The cluster compactness of the network of random grain boundaries (>Σ29) in electron backscatter diffraction assessments of microstructure is shown to decrease with increasing fraction of Σ3 boundaries. However, the cluster compactness of the network of corroded grain boundaries identified after electrochemical testing is less sensitive to changes in microstructure obtained by thermomechanical processing. [ABSTRACT FROM AUTHOR]

Control of brittleness associated with segregation of sulfur to grain boundaries in ultrafine-grained nickel was investigated. The brittleness could be successfully improved by controlling the grain boundary microstructure. The specimens with a higher fraction of special boundaries can have higher fracture toughness even in sulfur-bearing condition. It is evident that grain boundary engineering is applicable to controlling segregation-induced embrittlement in ultrafine-grained materials. [Copyright &y& Elsevier]

In 1984, 'grain boundary design', later known as 'grain boundary engineering (GBE)', was proposed. The central premise of GBE is that specific thermomechanical treatments, mainly on face centred cubic materials which readily form annealing twins, can be used to improve resistance to various forms of intergranular degradation such as corrosion, cracking or embrittlement. Engagement with the concept has accelerated in recent years. This overview charts the progress of GBE from its inception 25 years ago to the present day, including suggestions of key topics for ongoing or future research. These topics comprise confirmation of which boundaries are 'special' in terms of crystallography and properties, optimisation of processing regimes, new approaches to GBE in systems without annealing twinning and incorporation of connectivity metrics, especially in three dimensions. [ABSTRACT FROM AUTHOR]

Specimens of a UNS S31803 steel were submitted to high temperature gas nitriding and then to vibratory pitting wear tests. Nitrided samples displayed fully austenitic microstructures and 0.9 wt. % nitrogen contents. Prior to pitting tests, sample texture was characterized by electron backscattering diffraction, EBSD. Later on, the samples were tested in a vibratory pit testing equipment using distilled water. Pitting tests were periodically interrupted to evaluate mass loss and to characterize the surface wear by SEM observations. At earlier pit erosion, stages intense and highly heterogeneous plastic deformation inside individual grains was observed. Later on, after the incubation period, mass loss by debris detachment was observed. Initial debris micro fracturing was addressed to low cycle fatigue. Damage started at both sites, inside the grains and grain boundaries. The twin boundaries were the most prone to mass-loss incubation. Grains with (101) planes oriented near parallel to the sample surface displayed higher wear resistance than grains with other textures. This was attributed to lower resolved stresses for plastic deformation inside the grains with (101) ∥ surface texture. [ABSTRACT FROM AUTHOR]

The evolution of fibre textured structures is simulated in two dimensions using a generalised phase field model assuming two forms for the misorientation dependence of the grain boundary energy. In each case, a steady-state regime is reached after a finite amount of grain growth, where the number and length weighted misorientation distribution functions (MDF) are constant in time, and the mean grain area A as a function of time t follows a power growth law A - A0 = ktn with n close to 1 and A0 the initial mean grain area. The final shape of the MDF and value of the prefactor k in the power growth law clearly correlate with the misorientation dependence of the grain boundary energy. Furthermore, a mean field approach is worked out to predict the growth exponent for systems with non-uniform grain boundary energy. The conclusions from the mean field approach are consistent with the simulation results. In previous studies on grain growth in anisotropic fibre textured systems, this steady-state regime was often not reached, which resulted in wrong conclusions on the growth exponent n and evolution of the MDF. [ABSTRACT FROM AUTHOR]

Abstract: The roles of plastic strain, annealing temperature, and annealing time in affecting the fraction of special boundaries (Fsp), and twin densities of thermo-mechanically processed commercially pure nickel are examined. Different strain levels were achieved by cold rolling the material at different amounts. One-step low strain-recovery processing with strain levels in the range of 3.0–7.5% and annealing temperatures in the range 800–1000°C were conducted in order to ensure that recrystallization did not occur. From orientation image microscopy analysis it was found that the fraction of special boundaries increased from about 30% for the as-received material to almost 80% for plastically deformed and annealed material. This showed that material strained in the range from 3.0% to 7.5% and annealed at 800°C for different times all reached Fsp values in the range 75–80%, a considerable increase over the as-received material. Various multi-processing cycle treatments did not increase the fraction of special boundaries to above the value of 80% achieved in single processing cycles. TEM observations indicated dislocation tangles/cells occurred near grain boundaries in material strained at 6%. The density of these dislocation tangles decreased with annealing time at 800°C and was reduced considerably after 20min. Experimental results for the variation of twin density with grain size showed that the twin density decreased with increase in grain size. However, there was a tendency for the twin density to decrease more slowly with increase in grain size in the more highly strained samples. The experimental data for twin density were compared with values calculated from the equation suggested by Pande et al. . With appropriate choice of the two variable parameters in the equation, a good “average” fit for all the data was obtained. However, the effect of plastic strain on the twin density could not be accommodated in the model. The experimental twin density–grain size relationship was also compared to values calculated from the formulation developed by Gleiter modified to accommodate the effects of prior plastic train on the twin densities of annealed samples. Calculations from the modified model followed the observed trend that twin densities for the more highly deformed samples decreased more slowly with grain size than those for lesser strained samples. Good quantitative agreement between the experimental values of twin density and those calculated from the modified Gleiter formulation was achieved. [Copyright &y& Elsevier]

Grain boundary engineering, which increases the special boundary fraction, may improve microstructural stability during annealing. Different processing routes are undertaken to establish the effectiveness of each and to better understand which microstructural features determine the resulting stability. We find that multiple cycles of grain boundary engineering result in a material that resists abnormal grain growth better than other processing routes despite similarities in special boundary fraction, grain size, and general boundary connectivity among as-processed materials. [Copyright &y& Elsevier]

Abstract: A high nitrogen austenitic stainless steel (0.9wt% N) and an ordinary 304 austenitic stainless steel were submitted to cavitation-erosion tests in a vibratory apparatus operating at a frequency of 20kHz. The high nitrogen stainless steel was obtained by high temperature gas nitriding a 1-mm thick strip of an UNS 31803 duplex stainless steel. The 304 austenitic stainless steel was used for comparison purposes. The specimens were characterized by scanning electron microscopy and Electron Back Scatter Diffraction. The surface of the cavitation damaged specimens was analyzed trying to find out the regions where cavitation damage occurred preferentially. The distribution of sites where cavitation inception occurred was extremely heterogeneous, concentrating basically at (i) slip lines inside some grains and (ii) Σ-3 coincidence site lattice (CSL) boundaries (twin boundaries). Furthermore, it was observed that the CE damage spread faster inside those grains which were more susceptible to damage incubation. The damage heterogeneity was addressed to plasticity anisotropy. Grains in which the crystallographic orientation leads to high resolved shear stress show intense damage at slip lines. Grain boundaries between grains with large differences in resolved shear stress where also intensely damaged. The relationship between crystallite orientation distributions, plasticity anisotropy and CE damage mechanisms are discussed. [Copyright &y& Elsevier]

Two different grain boundary engineering processing routes for type 304 austenitic stainless steel have been compared. The processing routes involve the application of a small level of strain (5%) through either cold rolling or uni-axial tensile straining followed by high-temperature annealing. Electron backscatter diffraction and orientation mapping have been used to measure the proportions of Σ3n boundary types (in coincidence site lattice notation) and degree of random boundary break-up, in order to gain a measure of the success of the two types of grain boundary engineering treatments. The distribution of grain boundary plane crystallography has also been measured and analyzed in detail using the five-parameter stereological method. There were significant differences between the grain boundary population profiles depending on the type of deformation applied. [ABSTRACT FROM AUTHOR]

Grain boundary engineering (GBE) approaches involving small deformation and annealing to modify grain boundary networks have been widely used to improve grain boundary-related properties of polycrystalline materials. However, most GBE approaches are designed by trial-and-error method since the mechanism of GBE is unclear. An important issue still under debate is whether GBE is achieved by strain induced boundary migration (SIBM) or recrystallization. Also, the evolution of grain boundary structure during GBE process is unclear. a series of strains and annealing treatments covering SIBM and recrystallization were applied to pure copper in the current study. The result indicates that SIBM is more effective in the optimization of grain boundary character distribution compared with recrystallization. Then a quasi in-situ heating electron backscatter diffraction method was employed to the 10% compression/500 °C annealing copper to study the microstructural evolution during SIBM. SIBM was observed to be activated consecutively at high residual stress regions and then sweep into surrounding deformed areas until almost the whole material was covered by SIBM. The major procedure of SIBM involves the formation of numerous new Σ3 boundaries behind the migrating grain boundary front to enhance the fraction of special boundaries and the introduction of low energy segments to interrupt the connectivity of random high-angle boundary networks. A schematic model is proposed to understand the SIBM controlled GBE process. Our results provide the underlying insights needed to guide the design of GBE routes and parameters. • Strain induced boundary migration can effectively optimize grain boundary structure. • Numerous annealing twins were formed behind the migration front. • Low energy segments were introduced to break grain boundary networks. • Strain induced boundary migration is consecutively activated at high strain regions. [ABSTRACT FROM AUTHOR]

[Display omitted] • Directed energy deposition and single point incremental forming are combined to develop a novel hybrid process. • The hybrid process produces alloys with gradient or "sandwich" microstructures and improved properties. • Our method may open the path to grain boundary engineering of neat-net-shape parts. Owing to the relatively lower dimensional accuracy and poorer surface finish compared to other additive manufacturing (AM) technologies, directed energy deposition (DED) yields parts that often require extensive post-processing. Thus, it is well suited to be combined with subtractive or deformation processes into hybrid manufacturing strategies, which may enable microstructure engineering of near-net-shape parts directly upon production. In this work, we use a custom-made machine that combines DED and single point incremental forming (SPIF) processes to produce samples of stainless steel 316L which are amenable to undergo recrystallization upon heat treatment. After recrystallization, the microstructure exhibits a high density of twin boundaries, which are known to enhance the physical and mechanical properties of alloys. We investigate how different SPIF parameters affect the extent of recrystallization and find that our strategy may be used to produce both gradient and "sandwich" microstructures, which integrate dissimilar grain boundary character distributions and grain structures. We assess the corrosion resistance and mechanical properties of such samples and discuss the resulting performance enhancement. Our results showcase new opportunities for microstructure engineering of DED components and lay the groundwork for the design of AM processes that enable grain boundary engineering of metal alloys. [ABSTRACT FROM AUTHOR]

Exploiting grain boundary engineering in the design of alloys for extreme environments provides a promising pathway for enhancing performance relative to coarse-grained counterparts. Due to its attractive properties as a plasma facing material for fusion devices, tungsten presents an opportunity to exploit this approach in addressing the significant materials challenges imposed by the fusion environment. Here, we employ a ternary alloy design approach for stabilizing W against recrystallization and grain growth while simultaneously enhancing its manufacturability through powder metallurgical processing. Mechanical alloying and grain refinement in W-10 at.% Ti-(10,20) at.% Cr alloys are accomplished through high-energy ball milling with transitions in the microstructure mapped as a function of milling time. We demonstrate the multi-modal nature of the resulting nanocrystalline grain structure and its stability up to 1300 °C with the coarser grain size population correlated to transitions in crystallographic texture that result from the preferred slip systems in BCC W. Field-assisted sintering is employed to consolidate the alloy powders into bulk samples, which, due to the deliberately designed compositional features, are shown to retain ultrafine grain structures despite the presence of minor carbides formed during sintering due to carbon impurities in the ball-milled powders. [ABSTRACT FROM AUTHOR]

• Deformation twins were observed in 15% cold-rolled specimens. • 5% reduction and annealing at 1223 K for 1 h yielded GBE microstructure. • SIBM led to prolific multiple twinning resulting in disconnected RHAGBs connectivity. • GBE specimen showed superior corrosion resistance in 0.1 M and 0.6 M NaCl solution. • EIS studies revealed the formation of more resistant passive film on GBE specimen. In the present study, strain-annealing based thermo-mechanical processing was employed to achieve a grain boundary engineered (GBE) microstructure in an equiatomic CoCrFeMnNi high entropy alloy (Cantor alloy) with a single phase, fcc structure. Cast, homogenized and recrystallized strips were cold rolled to 5%, 10% and 15% thickness reductions and annealed at temperatures from 1173 K to 1373 K for 1–6 h duration. Deformation twins were observed following cold rolling to 15%. From the deformed and annealed specimens, GBE microstructure was identified based on coincident site lattice (CSL) (Σ ≤ 29) boundary length fraction, number of twins per grain, triple junctions (TJs) character distribution and grain boundary plane orientations. Specimens rolled to 5% and annealed at 1223 K for 1 h exhibited GBE microstructure. The Σ3 fraction was enhanced from ~44% to ~62% in the GBE specimen with concurrent increments in TJs containing at least two CSL boundaries from ~20% to ~40% compared to as-recrystallized (AR) specimen. Potentiodynamic polarization studies revealed that the GBE specimen exhibited lower corrosion rate in both 0.1 M and 0.6 M NaCl solutions as compared to AR counterpart. The GBE specimen also displayed better passive film resistance due to higher polarization and charge transfer resistance, as evaluated from electrochemical impedance spectroscopy studies. [ABSTRACT FROM AUTHOR]

[Display omitted] Grain boundary engineering (GBE) plays an important role in the design of new polycrystalline materials with enhanced mechanical properties. This approach has been shown to be very effective in design of Ni-base alloys, where grain boundary segregation is expected to play a central role in defining their mechanical behavior. In the present work, we apply a powerful combination of advanced experimental and theoretical methods to reveal the grain boundary chemistry of the 725 Ni-base alloy at the atomic level. The methods of investigation comprise atom probe tomography (APT) measurements and density functional theory (DFT) calculations. We also propose a way to cross-validate DFT and APT results in a DFT-based model approach for evaluation of the interfacial excess as a function of the heat treatment history of the material and its chemistry. Both theoretical and experimental methods are applied to a detailed analysis of the GB chemistry of three modifications of the 725 alloy and the results of this investigation are presented and discussed in detail. [ABSTRACT FROM AUTHOR]

Abstract: Due to the projected requirement for highly enhanced efficiency in power generation, the environment in advanced high temperature water-cooled nuclear systems becomes more rigorous compared to conventional fossil and nuclear power plants. The corrosion behavior of candidate alloys Inconel 617, 625, and 718 exposed to supercritical water was studied by a variety of analytical techniques. Grain boundary engineering (GBE) was performed on the alloy 617 to compare its corrosion behavior with the as-received samples. The results indicate that the GBE-treated alloy 617 samples showed the best response and the alloy 718 samples showed the worst corrosion resistance. Pitting observed on the alloy 718 samples was more serious than that on the alloy 625 samples. The effect of the alloying elements and the GBE treatment on the corrosion behavior is discussed. [Copyright &y& Elsevier]

The influence of cold roll reduction history on grain boundary network development in an austenitic stainless steel has been investigated. A dominant effect of the final thermomechanical process cycles on grain boundary character development was observed. The application of low-strain processing cycles with 5% cold reduction was found to increase the fraction of Σ3 and Σ3-related variants on solution annealing. Two-step processing treatments with final reductions between 15% and 82% showed no significant differences to their single-step processed equivalents. [Copyright &y& Elsevier]

Abstract: Roughness scaling laws for intergranular cracks deviate from self-affine (fractal-like) behavior at length scales related to the polycrystalline microstructure. We consider two versions of the same alloy material with many of the same microstructural length scales but differing in their processing history: one conventional and one grain boundary engineered. The engineered material, processed to contain a high fraction of “special” grain boundaries, fails more slowly and more isotropically. We present evidence that the difference is determined by processes related to clusters of twin-related grains, shown through analysis of scales of the fracture roughness measured with confocal microscopy and the special grain boundary network determined by electron backscatter diffraction. Above the cluster scale, the fracture roughness exponents in the two materials are nearly indistinguishable (confirming theoretical predictions); below this scale conventional cracks exhibit correlations indicating consistently weak paths for crack propagation, suggesting percolation of “random” boundaries. [Copyright &y& Elsevier]

Grain boundary engineering of austenitic stainless steel, through the introduction of plastic strain and thermal annealing, can be used to develop microstructures with improved resistance to inter-granular degradation. The influence of low-strain thermo-mechanical processing on grain boundary network development, with systematic variations of annealing treatments, has been investigated. Three stages of the microstructure development during grain boundary engineering in low-strain processing conditions are identified, and correlated with changes in grain boundary character and deviation distributions. Low-energy connected length segments at triple junctions, which have been proposed to be responsible for crack bridging during inter-granular stress corrosion cracking, can be influenced by the choice of the annealing treatment parameters. The development of individual grain boundary length segments of different character showed consistent trends with increasing grain size. Crack length predictions are consistent with the beneficial effect of designing microstructures with high fractions of twin grain boundaries and smaller grain size. [ABSTRACT FROM AUTHOR]

Abstract: A method is presented to determine the misorientation probability distribution function in polycrystalline materials based on a known, analytical or numerical, representation of the associated orientation probability distribution function, i.e., texture. The proposed formulation incorporates the local grain-to-grain orientation correlations by combining local or macroscopic statistical information, and finds a natural interpretation through the well-known stereographic projection (pole-figure) representation. The proposed formulation distinguishes between antiparallel crystallographic orientations, as well as cone-angle and polar angle misorientations. For fiber-textured samples, it is quantitatively shown that highly oriented samples are equivalent to polycrystals with a high density of low-angle misorientations, while completely random (untextured) materials are equivalent to microstructures with a high probability of large-angle misorientations. [Copyright &y& Elsevier]

Austenitic stainless steel (AISI 304L), which was thermomechanically processed by introducing cold deformation followed by annealing, was sensitized. The percentage of coincident site lattice boundaries did not correlate well with the degree of sensitization (DOS). The material with grain sizes below 10μm was found to be the least sensitized, while specimens with larger grains showed an increase in DOS with sensitization time. The DOS is shown to have an inverse correlation with the grain boundary area, decreasing exponentially with increasing grain boundary surface area. [Copyright &y& Elsevier]