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For understanding the improvement of intergranular stress corrosion cracking (IGSCC) propagation in grain boundary engineering (GBE)-processed metals exposed to a simulated pressurized water reactor (PWR) environment, characteristics of the grain boundary network of 316L stainless steel before and after GBE were investigated and compared, including proportions both in length and in number of ∑3n boundaries, sizes, and topology of grain clusters (or twin-related domains), and connectivity of random boundaries. The term through-view random boundary path (TRBP) was proposed to evaluate the random boundary connectivity. A TRBP is a chain of end-to-end connected crack-susceptible boundaries that passes through the entire mapped microstructure. The work provides the following key findings: (I) the length fraction of ∑3n boundaries was increased to approximately 75% after GBE, but the number fraction was only approximately 50%; (II) a connected non-twin boundary network still existed in the GBE sample due to the formation of grain clusters; (III) the GBE sample exhibited a higher resistance to IGSCC; and (IV) as the twin boundary fraction increased, the number of TRBPs decreased and the normalized length of the minimum TRBP increased monotonically, leading to a higher resistance to IGSCC.

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.

Realizing application specific manufacture with fusion-based additive manufacturing (F-BAM) processes requires understanding of the physical phenomena that drive evolution of microstructural attributes, such as texture. Current approaches for understanding texture evolution in F-BAM are majorly considerate of the phenomena occurring only during solidification. This hinders the comprehensive understanding and control of texture during F-BAM. In this perspective article, we discuss several physical phenomena occurring during and after solidification that can determine texture in F-BAM processed stainless steels (SS). A crystal plasticity-coupled hydrogen adsorption-diffusion modeling framework is also leveraged to demonstrate the prospects of grain boundary engineering with F-BAM for enhanced hydrogen embrittlement resistance of SS. Implications of varying thermokinetics in F-BAM for solidification behavior of SS are discussed. Additionally, microstructural attributes that are key to high temperature mechanical performance of SS are highlighted. Considerations as outlined in this perspective article will enable grain boundary engineering and application specific microstructural design of SS with F-BAM.

The change in the grain boundary network during recrystallization and grain growth was studied in a 316L austenitic stainless steel subjected to 5% cold rolling reduction. The primary recrystallization rapidly developed upon heating to 1000 °C, resulting in the development of relatively coarse-grained microstructure with a grain size about 100 μm. The recrystallized microstructures contained large fractions of annealing twins with their ∑3n SCL boundaries. The latter ones served as interrupters of the ordinary grain boundary network. The fraction of ∑3n CSL boundaries increased with increasing the grain size during prolonged annealing. On the other hand, the number of interruptions per unit area remained nearly the same during annealing. Hence, the number of interruptions per a grain increased in accordance with a power law function of the grain size with an exponent of 2. The relationships obtained for the grain boundary network evolution can be used to predict the microstructure evolution in austenitic stainless steels during primary recrystallization followed by grain growth.

Realizing application specific manufacture with fusion-based additive manufacturing (F-BAM) processes requires understanding of the physical phenomena that drive evolution of microstructural attributes, such as texture. Current approaches for understanding texture evolution in F-BAM are majorly considerate of the phenomena occurring only during solidification. This hinders the comprehensive understanding and control of texture during F-BAM. In this perspective article, we discuss several physical phenomena occurring during and after solidification that can determine texture in F-BAM processed stainless steels (SS). A crystal plasticity-coupled hydrogen adsorption-diffusion modeling framework is also leveraged to demonstrate the prospects of grain boundary engineering with F-BAM for enhanced hydrogen embrittlement resistance of SS. Implications of varying thermokinetics in F-BAM for solidification behavior of SS are discussed. Additionally, microstructural attributes that are key to high temperature mechanical performance of SS are highlighted. Considerations as outlined in this perspective article will enable grain boundary engineering and application specific microstructural design of SS with F-BAM. Graphical abstract

In this work, the coherent component of twin boundaries (TBs) in AISI 321 austenitic stainless steel (SS) via grain boundary engineering (GBE) is statistically measured by the method of grain boundary inter-connection (GBIC) which is a parameter describing the grain boundary character intrinsically, and the influence of coherent TBs on the low-cycle fatigue property at 623 K is investigated. The GBIC results show that the content of {1 1 1}/{1 1 1} GBICs which is the coherent component in ∑3 boundaries is increased via GBE treatment. The fatigue investigation reveals that coherent TBs of a high fraction eliminate the cyclic softening phenomenon in 321 austenitic stainless steel, which results in a progressive strengthening effect in the process of cyclic loading. Microstructural analysis demonstrates coherent TBs effectively improve fatigue crack propagation resistance as they can restrain the growth of small cracks. It is evidenced that enhancing the length fraction of coherent TBs via GBE is feasible to controlling fatigue crack propagation for AISI 321SS.

Grain boundary engineering (GBE) has been widely used to modify grain boundary (GB) networks to improve GB-related properties in polycrystalline materials. With the development of miniaturized and lightweight terminal connectors comes a greater demand for phosphorus bronze. A fine grain size and excellent GB characteristics are the keys to synergistically enhancing mechanical strength and bending workability. In this study, the effects of the annealing temperature on the grain boundary character distribution (GBCD) optimization and the bending properties of phosphorus bronze were studied by means of electron backscatter diffraction and a 90° bending test. The results show that the deformed microstructure of the as-received material recrystallizes upon annealing at 400 °C for 1 h. The average grain size is 1.6 μm, and a large number of special boundaries (SBs) are present, accounting for 71.5% of all GBs. Further, the incoherent Σ3, Σ9, and Σ27 boundaries are the most abundant, effectively disrupting the network connectivity of random high-angle grain boundaries. The grain size gradually increases with the annealing temperature increase. The fractions of the Σ9 and Σ27 boundaries gradually decrease. Although the proportion of SBs further increases at higher temperatures, most SBs at these temperatures are coherent Σ3 boundaries that do not contribute to the direct optimization of GBCD. Moreover, in the absence of a significant difference in tensile strength, the GBCD-optimized fine-grained sample demonstrates smooth surfaces without orange peel effects when bent at 90° with R/t = 0 in the bad way. This improvement is attributed to the uniform deformation of fine grains and special boundaries, which enhances the bending workability of the GBCD-optimized fine-grained strips.

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.

Grain growth is an unavoidable phenomenon occurring during processing of multicomponent high-entropy alloys (HEAs) and medium-entropy alloys (MEAs), which significantly affects the properties and performance of these compositionally complex alloys during service. Accordingly, the present overview article is dedicated to summarize the state of the art of the grain coarsening behavior and its kinetics for HEAs and propose opportunities and scope for future research, with special attention to the Cantor-based compositions. Firstly, the primary recrystallization and grain growth of equiatomic and non-equiatomic CrMnFeCoNi alloys with face-centered cubic (FCC) crystal structure in the single-phase regime and with the presence of intermetallics such as the Cr-rich sigma (σ) phase is considered, which was accompanied by the powerful utilization of the solute drag effect, Zener pinning, and grain boundary segregation engineering via introducing carbon, nitrogen, boron, titanium, and niobium. Subsequently, the quaternary and ternary alloys based on Cr, Mn, Fe, Ni, and Co are taken into account. In these systems, the novel concept of grain boundary high-entropy effect (via the addition of molybdenum, niobium, and zirconium), the formation of pinning insoluble oxide and thermally stable carbide particles during mechanical alloying, the simultaneous solute drag and Zener pinning effects via Mo and C addition, and the formation of Cu-rich phase and the strong segregation effect of Cu atoms to grain boundaries in the FeCoNiCu HEA are critically discussed. Afterward, the thermal stability of Al x FeCoCrNi(Mn) HEAs is treated, where the formation of the Ni–Al-rich B2 phase and the corresponding Zener pinning effect, the influence of aluminum addition on the temperature range for the stability of the σ phase, and cluster drag mechanism of Al atoms are explained. What is more, the grain growth of refractory HEAs with body-centered cubic (BCC) crystal structure is also summarized, where the high grain growth activation energy is attributed to the presence of elements with very low self-diffusivity such as tantalum and niobium. Moreover, the possible effects of short-range ordering on the mobility of grain boundaries are discussed. Finally, the prospects for future research work are presented, including the development of precipitation-temperature-time (PTT) diagrams, systematic investigation of cold rolling and annealing for prior grain refinement, studying abnormal grain growth (secondary recrystallization), and assessment of grain growth behavior of HEAs manufactured by advanced methods such as additive manufacturing (3D printing).

On the basis of reiterating the concept of grain boundary engineering (GBE), the recent progress in the theoretical models and mechanisms of twin-related GBE optimization and its effect on the mechanical properties is systematically summarized in this review. First, several important GBE-quantifying parameters are introduced, e.g., the fraction of special grain boundaries (GBs), the distribution of triple-junctions, and the ratio of twin-related domain size to grain size. Subsequently, some theoretical models for the GBE optimization in face-centered cubic (FCC) metals are sketched, with a focus on the model of “twin cluster growth” by summarizing the in-situ and quasi-in-situ observations on the evolution of grain boundary character distribution during the thermal-mechanical process. Finally, some case studies are presented on the applications of twin-related GBE in improving the various mechanical properties of FCC metals, involving room-temperature tensile ductility, high-temperature strength-ductility match, creep resistance, and fatigue properties. It has been well recognized that the mechanical properties of FCC materials could be obviously improved by a GBE treatment, especially at high temperatures or under high cyclic loads; under these circumstances, the materials are prone to intergranular cracking. In short, GBE has tremendous potential for improving the mechanical properties of FCC metallic materials, and it is a feasible method for designing high-performance metallic materials.