Grain Boundary Character Distribution Optimization Research Articles

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.

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.

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.

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.

Inhibition of Nitride Precipitation in a Fe-Cr-Mn-Mo-N High-Nitrogen Austenitic Stainless Steel through Grain Boundary Character Distribution Optimization

The precipitation behavior of the second phase in Fe-18Cr-17Mn-2Mo-0.85N high nitrogen and nickel-free austenitic stainless steel after grain boundary character distribution (GBCD) optimization was investigated. The results show that the fraction of low Σ coincidence site lattice (CSL) boundaries increases from 47.9% for the solid solution treated specimen to 82.25% for the specimen cold-rolled by 5% and then annealed at 1423 K for 72 h (r5%-a1423 K/72 h). Compared with the solution treated sample, nitride precipitation is obviously restrained and the fraction of precipitates is the least in r5%-a1423 K/72 h sample with the highest proportion of SBs. The appearance of a high proportion of CSL boundaries with low energy inhibits the nitrides precipitation through GBCD optimization.

New perspectives on twinning events during strain-induced grain boundary migration (SIBM) in iteratively processed 316L stainless steel

Characterization of microstructure and grain boundary character distribution (GBCD) during iterative thermo-mechanical processing (TMP) of 316L stainless steel were done using electron backscatter diffraction (EBSD). Results from EBSD scans were analyzed in terms of the fraction of special boundaries, their deviation from ideal misorientation, connectivity of random high-angle grain boundaries, and grain size. In order to understand the efficacy of the iterative processing route leading to a well-connected twin boundary network, additional analysis was done in terms of twin-related domain statistics and triple junction distribution. Results from these analyses show that initial iteration results in a microstructure with insufficient twin boundary density, and a poor twin boundary network. Although the next iteration leads to a small increase in twin statistics, it also does not significantly improve the statistics of favorable grain boundaries and their network. However, the fourth iteration of the processing results in a large improvement in both the population and distribution of favorable grain boundaries. All the subsequent iterations were found to result in microstructural deterioration in terms of both population and network of these grain boundaries. Based on the analysis, role of underlying mechanisms such as strain-induced grain boundary migration and static recrystallization in the effective optimization of GBCD was analyzed. The present research gives important insights into these mechanisms and their control during TMP in order to achieve an engineered microstructure.

Grain boundary character distribution in the CoCrMo alloy processed by selective laser melting and post-heat treatment

The evolution of grain boundary character distribution (GBCD) in the CoCrMo alloys processed by SLM and post-heat treatments (aging and solid solution) has been investigated by the electron backscatter diffraction (EBSD) and transmission electron microscope techniques. A high fraction of Σ3n (n = 1, 2, 3) grain boundaries and the corresponding clusters occur in the solid solution-treated samples at 1150 °C, which indicates GBCD optimization is somewhat realized. The coherent and incoherent Σ3 boundaries were distinguished via the single-section trace analysis method based on EBSD measurements. Σ3 boundaries in the SLMed and aged samples were determined as incoherent, while those in the solid solution-treated sample are coherent. Regular e precipitation did not occur in the solid solution-treated samples as in the aged samples. Instead, stacking fault bands associated with coherent Σ3 twin boundaries are dominant. It suggests that post-heat treatment has a significant influence on the GBCD, especially the Σ3 grain boundary characteristics.

Ordered Structure, Dislocation, and Grain Boundary Character Distribution and Their Effects on Warm Deformation in Soft‐Magnetic Fe–6.9Si–0.01B Alloy

The ordered structure, dislocation, and grain boundary character distribution (GBCD) in warm‐rolled Fe–6.9Si–0.01B alloy are characterized, and their effects on plastic deformation are investigated. The study reveals that some {110} recrystallized grains preferentially formed at {111} shear bands as the main nucleation sites at lower rolling temperature, whereas more {111} recrystallized grains occur at 680 °C rolling temperature. The content and size of B2 ordered phases increase with increasing deformation temperatures, meanwhile majority of superdislocations are changed into single dislocations, relieving the stress through cross‐slip. The plastic deformability of Fe–6.9Si–0.01B alloy improves by increasing deformation temperature is attributed to a contribution combining decrease in order degree and recrystallization softening with optimization of GBCD.

Effect of the grain boundary character distribution on the self-organization of adiabatic shear bands in 1Cr18Ni9Ti austenitic stainless steel

The self-organization of adiabatic shear bands (ASBs) in the steel with a different grain boundary character distribution (GBCD) was investigated using the radial collapse of thick-walled cylinders. GBCD of 1Cr18Ni9Ti austenitic stainless steel was optimized by thermo-mechanical treatment. The results showed that the proportion of special grain boundaries, especially the Σ3 grain boundaries, were increased from 27.8, 18.9 to 72.5, 55.8%, respectively. And the high-angle grain boundaries were replaced or interrupted by the low-angle grain boundaries and special grain boundary clusters. The results of optical microscopic examination showed that the shear bands mainly propagated along the clockwise direction, and the multiple shear bands influenced each other along with bifurcation and merging. Also, the number and spacing of the shear bands of the solution treatment specimen (ST specimen) and thermo-mechanical treatment specimen (TMT specimen) were the same, which meant that GBCD optimization had no obvious effect on the nucleation of shear bands. However, the lengths of ASBs in TMT specimen were much shorter than those of the ST specimen under the same deformation conditions. The slower propagation rate of ASBs in TMT specimen suggested that by introducing a large number of special grain boundaries, the GBCD optimization hindered the propagation of ASBs and improved the damage resistance under high-strain-rate loading. Also, voids were nucleated preferentially in the ASBs and coalesced along ASBs, and cracks formed finally.

Effect of two-step thermomechanical processing on grain boundary character distribution of CoCrFeMnNi high-entropy alloy

The influence of annealing time and secondary-rolling reduction ratio on grain boundary character distribution (GBCD) in an FCC CoCrFeMnNi high-entropy alloy were investigated via electron backscatter diffraction (EBSD). The results show that the processing of 10% cold-rolling reduction subsequently with annealing at 1073 K for 10 h induces a high frequency of low coincident site lattice (CSL) boundary with >70%, thereby effectively achieving GBCD optimization. As annealing time increases, low-Σ CSL (Σ ≤29) boundary frequency first decreases and then remains constant at approximately 60%. The secondary-rolling reduction ratio followed by annealing at 1073 K for 10 h also exerts a substantial influence on GBCD. As the reduction ratio increases from 6% to 40%, the low-Σ CSL (Σ ≤29) boundary frequency first increases and then decreases.

Evolution of Grain Boundary Character Distribution in Pure Copper during Low-Strain Thermomechanical Processing

In order to get optimal grain boundary character distribution (GBCD) and grain boundary properties, thermomechanical processing (TMP) is usually adopted in grain boundary engineering. However, the mechanism behind the TMP treatments and GBCD optimization is still unclear. The present study has conducted a series experiments involving low-strain TMPs to study the relationship between TMP parameters and the behind microstructural evolution. The experimental results indicate that in the scope of low-strain TMP, strain induced boundary migration (SIBM) is the most effective process for GBCD optimization. Besides, SIBM and grain growth would gradually transfer to recrystallization with the increase of pre-deformation level and annealing temperature. Further quasi in-situ EBSD results infer that SBIM is activated locally in some region with high stored energy, and further gradual initiation of SIBM from one region to another contributes to the gradual increase of special boundaries with annealing time.

Characterization of texture and grain boundary character distributions of selective laser melted Inconel 625 alloy

Select laser melting (SLM) is one of the practical metal additive manufacturing processes to fabricate complex structures. The inhomogeneous microstructure and high residual stress are typically resulted from a non-uniform melting and rapid solidification during a SLM process, which has the significant impact on part dimensional instability, corrosion resistance, and mechanical properties. The bulk of current research is limited to the general characterization of grain size, texture, and mechanical properties. However, grain boundary character distribution (GBCD) and its relationship with texture are yet to be understood. This study has shown that the preferred 〈110〉 along building direction and 〈100〉 along laser scanning direction were produced. The high density of low angle grain boundary (LAGB) can be an indicator of residual stress levels in the as-SLMed samples. The newly-developed ∑3, ∑9 and ∑27 grain boundaries were heavily populated, and the cluster of grains interfaced by ∑3n (n = 0, 1, 2…) was introduced at the expense of LAGB only at high recrystallization annealing (1150°C or over). The research results suggest that GBCD optimization be possible in the SLMed Inconel 625 alloy via suitable post annealing, which allows improving intergranular corrosion resistance and enhancing mechanical properties.

Improving intergranular corrosion resistance in a nickel-free and manganese-bearing high-nitrogen austenitic stainless steel through grain boundary character distribution optimization

Grain boundary character distribution (GBCD) and the effect of GBCD optimization on the intergranular corrosion (IGC) of a cold rolled and subsequently annealed nickel-free and manganese-bearing high-nitrogen austenitic stainless steel were investigated. The results show that the fraction of low Σ coincidence site lattice boundaries increases from 47.3% for the solid solution treated specimen to 83.3% for the specimen cold-rolled by 7% and then annealed at 1423K for 10min. Σ3 boundaries of high fraction effectively interrupt the connectivity of non-corrosion-resisting boundaries (special boundaries like Σ9, Σ27, etc. and general high angle boundaries) network, thus improving the IGC resistance.

Optimization of grain boundary character distribution in Fe-18Cr-18Mn-0.63N high-nitrogen austenitic stainless steel

Grain boundary engineering (GBE) is a practice of improving resistance to grain boundary failure of the material through increasing the proportion of low Σ coincidence site lattice (CSL) grain boundaries (special grain boundaries) in the grain boundary character distribution (GBCD). The GBCD in a cold rolled and annealed Fe-18Cr-18Mn-0.63N high-nitrogen austenitic stainless steel was analyzed by electron back scatter diffraction (EBSD). The results show that the optimization process of GBE in the conventional austenitic stainless steel cannot be well applied to this high-nitrogen austenitic stainless steel. The percentage of low ΣCSL grain boundaries could increase from 47.3% for the solid solution treated high-nitrogen austenitic stainless steel specimen to 82.0% for the specimen after 5% cold rolling reduction and then annealing at 1423 K for 10 min. These special boundaries of high proportion effectively interrupt the connectivity of conventional high angle grain boundary network and thus achieve the GBCD optimization for the high-nitrogen austenitic stainless steel.

Grain Boundary Engineering: Progress and Challenges

The progress of grain boundary engineering (GBE) is overviewed and the challenges for further investigations emphasized. It points out that, the electron backscatter diffraction (EBSD) reconstruction of grain boundaries, which gives the information of connectivity interruption of general high angle boundaries (HABs), is more significant than purely pursuing high frequency of so-called special boundaries. The criterion for the optimization of grain boundary character distribution (GBCD) needs to be established. The energy spectrum and the degradation susceptibility of grain boundaries of various characters including HABs and low Σ（Σ≤29） coincidence site lattice (CSL) needs to be studied and ascertained. And finally, the newly proposed model of non-coherent Σ3 interactions for GBCD optimization are discussed.

The control of brittleness and development of desirable mechanical properties in polycrystalline systems by grain boundary engineering

Grain boundaries can be effectively controlled to produce or enhance their beneficial effects and also to diminish or reduce their detrimental effects on bulk properties in polycrystalline materials. Particular attention has been paid to the control of intergranular brittleness which remains a serious problem of material processing and development. Recent studies are presented and discussed, which have been successfully performed to control intergranular brittleness of “intrinsically brittle” materials such as the refractory metal molybdenum and the ordered intermetallic alloy Ni 3Al and to produce superplasticity in an Al–Li alloy, by grain boundary engineering through controlling a new microstructural factor termed the grain boundary character distribution (GBCD). The optimization of GBCD and the grain boundary connectivity has been found to be a key to produce desirable bulk mechanical properties in both structural and functional polycrystalline materials.

Toward Optimization of the Grain Boundary Character Distribution in OFE Copper

The authors used a two-step (low and high temperature) strain-annealing process to evolve the grain boundary character distribution (GBCD) in fully recrystallized oxygen-free electronic (OFE) Cu bar that was forged and rolled. Orientation imaging microscopy (OIM) has been used to characterize the GBCD after each step in the processing. The fraction of special grain boundaries, special fraction, was {approximately} 70% in the starting recrystallized material. High, moderate, and low temperature processing conditions were employed. The high-temperature process resulted in a reduction in the fraction of special grain boundaries while both of the lower temperature processes resulted in an increase in special fraction up to 85%. Further, the lower temperature processes resulted in average deviation angles from exact misorientation, for special boundaries, that were significantly smaller than observed from the high temperature process. Results indicate the importance of the low temperature part of the two-step strain-annealing process in preparing the microstructure for the higher temperature anneal and commensurate increase in the special fraction. The authors approach is aimed at testing a hypothesis for the mechanism for GBCD optimization in Cu, i.e., it is possible to optimize the GBCD in Cu by selectively removing random grain boundaries through a strain-annealing process. Theymore » begin with a fully recrystallized material, deform it by a crucial amount, then apply a specific heat treatment schedule. The deformation is not intended to be sufficient to induce full recrystallization upon heating, but is intended to localize the deformation energy at random boundaries (since random boundaries are expected to be less efficient at transmitting dislocations than special boundaries). Upon heating, the stored energy near these random boundaries is expected to provide sufficient driving force to rearrange these boundaries into special types.« less

Recrystallization of electron beam refined tantalum-tungsten alloy : F. Siciliano, H.R.Z. Sandim (McGill University, Montreal, Canada). Int. J. Refractory Metals Hard Mater., vol 14, no 4, 1996, 257–262

Grain boundary character distribution (GBCD) and the effect of GBCD optimization on the intergranular corrosion (IGC) of a cold rolled and subsequently annealed nickel-free and manganese-bearing high-nitrogen austenitic stainless steel were investigated. The results show that the fraction of low Σ coincidence site lattice boundaries increases from 47.3% for the solid solution treated specimen to 83.3% for the specimen cold-rolled by 7% and then annealed at 1423 K for 10 min. Σ3 boundaries of high fraction effectively interrupt the connectivity of non-corrosion-resisting boundaries (special boundaries like Σ9, Σ27, etc. and general high angle boundaries) network, thus improving the IGC resistance.