Fraction Of Σ3 Boundaries Research Articles

Microstructure and texture characterisation of friction stir welded CoCrNi and CoCrFeMnNi multi-principle element alloys

This work investigates the microstructure formed in friction stir welds of FCC alloys, focused on two multi-principal alloys: a CoCrFeMnNi high-entropy alloy (HEA) and a CoCrNi medium-entropy alloy (MEA). A commercial stainless steel AISI 304 is used for comparison. The largest nugget was formed in the MEA, while the smallest was formed in the HEA. Grain refinement occurs in the stirred zone in all welds. Discontinuous dynamic recrystallisation is the predominant restoration mechanism during friction stir welding of the three investigated alloys. A sharp decrement in the Σ3 boundary fraction occurs in the stirred zone of the AISI 304 and HEA welds, while comparable values with the base metal are found for the MEA weld. The peak in the maximum index of crystallographic texture is observed on the advancing side of the stirred zone of the AISI 304 weld. A strong < 001 > θ-fibre texture is formed in the advancing side of the nugget in the AISI 304 from a well-established {123}< 634 > S-type texture in the base metal. Multiple crystallographic texture components without specific fibres are identified in most regions of the welds, indicating the complex shear path history during friction stir welding.

Discrimination of Copper Molten Marks through a Fire Reproduction Experiment Using Microstructure Features.

The copper molten marks at a fire site provide important clues for determining the causes of fire. Four factors have been presented to quantitatively discriminate copper molten marks, namely the fraction of (001) component perpendicular to the demarcation line, the grain aspect ratio, the fraction of Σ3 boundaries, and the fraction of maximum grain size. However, only laboratory-level results of these parameters have been presented, and their applicability in actual fires is yet to be verified. In this study, a fire reproduction experimental system was configured to generate molten marks similar to those in actual fire sites. The molten marks were measured by electron backscatter diffraction and applied to the four discriminant factors. The results obtained similar characteristics to those of the laboratory unit, confirming the applicability of the four discriminant factors. Discriminant equations and processes that can distinguish the primary and secondary arc beads were derived using the molten marks generated in the laboratory and reproduction experiments. Furthermore, a probabilistic discrimination method and classification model developed by machine learning were proposed. Therefore, the use of the discriminants in actual fires can improve the reliability of the statistics and prevent the recurrence of similar fires.

Electrochemical Corrosion Behavior of CrFeCoNi and CrMnFeCoNi Coatings in Salt Solution

CrFeCoNi and CrMnFeCoNi coatings were prepared by laser remelting method. The grain boundary characteristics distribution of both coatings was investigated by electron backscattered diffraction technique. The results showed that the CrFeCoNi coating exhibited higher fraction of low angle grain boundaries and Σ3 boundaries compared to the CrMnFeCoNi coating. The corrosion properties of the CrFeCoNi and CrMnFeCoNi coatings in 3.5 wt% NaCl solution were identified by electrochemical method and localized electrochemical impedance spectroscopy technique. The results indicated that the CrFeCoNi coating had a higher corrosion resistance than the CrMnFeCoNi coating. The better corrosion resistance of the CrFeCoNi coating could be attributed to the high fraction of Σ3 boundaries and low fraction of the high angle boundaries.

Potential Method to Distinguish Copper Molten Marks Using Boundary and Grain Characteristics.

The microstructure of molten marks changes according to ambient temperatures, when a short circuit occurs. Investigation of microstructural changes is important for understanding the properties of copper and examining the cause of a fire. In this study, the boundary characteristics and grain-size distribution of molten marks—primary-arc beads (PABs), which short-circuited at room temperature (25 °C), and secondary-arc beads (SABs), which short-circuited at high temperatures (600 °C, 900 °C)—were compared using electron backscatter diffraction. The distribution of Σ3 boundaries was compared, and it was found that SABs have a higher fraction of Σ3 boundaries than PABs. Moreover, it was confirmed that the ratio of maximum grain size (area) to the total area of the molten mark in SABs is larger than that in PABs. Thus, reliable discriminant factors were suggested, such as the fraction of Σ3 boundaries and normalized maximum grain size, which can distinguish PABs and SABs. The four discriminant factors, such as the (001)//LD, GAR, fraction of Σ3 boundaries, and fraction of maximum grain size to the total molten-mark area, were verified using the machine learning of t-SNE and Pearson correlation analyses.

Effect of deformation mode on hot deformation characteristics and microstructural evolution in Cu-Cr-Zr-Ti alloy

Uniaxial (UC) and plane strain compression (PSC) tests were performed on the Cu-Cr-Zr-Ti alloy specimens to investigate the effect of the deformation mode on the stress-strain response, work hardening behaviour and microstructural evolution in the alloy. These tests were performed at a strain rate of 1 s−1 and in the temperature range of 700–800 °C to impart von Mises equivalent strains of 0.15, 0.45 and 0.69. The alloy showed a regime of constant work hardening rate in the recovery stage before transitioning to the steady state under UC as opposed to PSC in which the alloy exhibited a smooth transition from recovery stage to the steady state stage. The microstructures evolved under UC consisted of randomly oriented equiaxed grains delineated by high angle boundaries (HABs) with high fraction of Σ3 boundaries. On the other hand, under PSC a very strong {001}〈100〉 cube texture evolved with increase in strain. This strong cube texture was ascribed to the evolution of and the increased stability of cube-oriented elongated substructures observed in the deformed matrix. The difference in the work hardening behaviour was thus attributed to the higher evolved Σ3 fraction under UC and the cube-oriented elongated substructures evolved under PSC. The dynamically recrystallized grains fraction calculated from area of grain orientation spread (GOS) map with GOS < 2o was found to be lower in the alloy under PSC owing to the presence of high orientation gradients within the evolved cube-oriented elongated substructures.

On the three-dimensional twin-limited microstructure

Abstract Grain boundary networks consisting entirely of Σ3 and Σ9 boundaries (twin-limited microstructures) are analyzed. Analyses of thought examples and theoretical considerations show that there are fundamental differences between two-dimensional and three-dimensional networks. The notion of the upper limit of the fraction of Σ3 boundaries is reassessed, and implications for the concept of grain boundary engineering are discussed.

Heat Input Effect on Microstructure and Mechanical Properties of Electron Beam Additive Manufactured (EBAM) Cu-7.5wt.%Al Bronze.

Electron beam additive wire-feed deposition of Cu-7.5wt.%Al bronze on a stainless-steel substrate has been carried out at heat input levels 0.21, 0.255, and 0.3 kJ/mm. The microstructures formed at 0.21 kJ/mm were characterized by the presence of both zigzagged columnar and small equiaxed grains with 10% of Σ3 annealing twin grain boundaries. No equiaxed grains were found in samples obtained at 0.255 and 0.3 kJ/mm. The zigzagged columnar ones were only retained in samples obtained at 0.255 kJ/mm. The fraction of Σ3 boundaries reduced at higher heat input values to 7 and 4%, respectively. The maximum tensile strength was achieved on samples obtained with 0.21 kJ/mm as tested with a tensile axis perpendicular to the deposited wall’s height. More than 100% elongation-to-fracture was achieved when testing the samples obtained at 0.3 kJ/mm (as tested with a tensile axis coinciding with the wall’s height).

Mitigation of hydrogen embrittlement in ultra-high strength lath martensitic steel via Ta microalloying

Hydrogen embrittlement (HE) is a key challenge limiting the utilization of ultra-high strength martensitic steel. In this work, we reported a novel method for dramatically improving HE resistance by Ta microalloying, and the significant effect of Ta on the HE susceptibility of lath martensitic steel was elucidated from the perspectives of hydrogen-enhanced decohesion (HEDE) and hydrogen-enhanced localized plasticity (HELP). As the Ta content increased, numerous dispersed nano-sized TaC precipitates were generated and the effective areas of martensite/prior austenite grain boundaries also increased, which increased both the irreversible/reversible H trap densities, impeded localized H aggregation at defects and weakened HEDE. A quantitative analysis regarding each type of H trap revealed that, compared with reversible traps provided by microstructural refinement, TaC precipitate-induced irreversible traps exhibited a dominant role in weakening the HEDE process. Additionally, in Ta-bearing steels, the resistance to hydrogen-assisted crack propagation was enhanced through the increased Σ11 boundary, increased low-angle grain boundary fraction and the reduced Σ3 boundary fraction, which combined with the suppressing role of TaC precipitates on H-dislocation interaction, impeded the HELP process. This study provided new, deep insights into the impact of Ta on HE, which has important implications for developing steels with high HE resistance.

Evolution of grain boundary character distribution in near-surface regions of a cold-rolled nickel-based superalloy during induction heating process

The influence of induction heating on the grain boundary character distribution (GBCD) in near-surface regions of a cold-rolled Nickel-based superalloy was researched. After induction heating, most of low-Σ coincidence site lattice (CSL) boundaries were Σ3 boundaries, which were mainly formed via the growth accident model. Moreover, the grain structures evolution during induction heating had a great influence on the evolution of GBCD. At the low strain of 0.1, both the fraction of Σ3 boundaries and grain size increased with the increasing temperatures, while the former was closely related to the better development of grain-clusters at the higher temperature. In addition, the coherent Σ3 boundaries were easier to be formed at the higher temperature during induction heating, owing to their low interface energy and mobility. At the large strain of 0.5, the fraction of Σ3 boundaries also increased with the increasing temperatures, but the grain size exhibited the opposite trend, which was closely related to the well development of static recrystallization (SRX) behaviors. Meanwhile, there was a “symbiotic relationship” between the SRX grains and Σ3 boundaries during induction heating. Through the electrochemical corrosion tests, it was proved that induction heating can contribute to the improvement of corrosion properties of superalloys via increasing the fraction of Σ3 boundaries, while the best corrosion resistance appeared in the samples treated at 800 °C with the strain of 0.5. Moreover, the evolution of both Σ9 and Σ27 boundaries was also closely related to strains and induction heating temperatures, but their fractions were less than 4%.

Microstructure and Texture Development in Thermomechanically Processed Leaded Brass

Leaded brass alloys with high zinc content are classified as dual-phase (α-β′) brass alloys. For these alloys, α-β′ phase transformation induced by thermomechanical processing (TMP) is expected to play a significant role not only in the formation of α and β′ phases and uniformity of lead particles but also in the development of Σ3 boundaries and their variants in the α phase. To assess such impact, the current study has employed two TMP schemes, with each scheme consisting of four cycles of compression and annealing. Samples of the first scheme (TMP-1) were annealed at 670 °C to promote phase transformation from α to β′, as compared to the second scheme (TMP-2) samples annealed at 525 °C. Microstructure examination indicated that TMP-1 scheme fostered a remarkable growth in the area fraction of β′ phase, accompanied by grain coarsening of α and β′ phases. This resulted in the isolation of α grains and less interaction of Σ3 boundaries at trip junctions, and therefore, a reduction in the fraction of Σ3 boundaries occurred in the α phase. On the other hand, the TMP-2 scheme successfully enhanced cluster formation of α grains, enabling further generation of Σ3 boundaries via strain-induced boundary migration mechanism and ultimately leading to breakup of the random boundary network. In addition, the two schemes resulted in limited coarsening of lead particles. Most lead particles were located at the α-βʹ interfaces, which suppressed grain boundary migration via the particle-dragging effect. For both TMP schemes, the initial texture of tension fibers was largely preserved after four cycles for both α and βʹ phases. Nevertheless, the TMP-2 scheme produced texture randomization of the α phase due to increasing the fraction of Σ3 boundaries.

Grain Orientation Spread in Dynamically Recrystallized Austenitic Steel

Some feature of discontinuous dynamic recrystallization (DRX) in an Fe-0.4%C-18%Mn austenitic steel during isothermal compression tests at temperatures of 973-1373 K and strain rates of 10-3-10-1 s-1 were studied. The DRX microstructures consisted of various grains, i.e., DRX nuclei, growing DRX grains, and work-hardened DRX grains, which differentiated with the grain orientation spread (GOS). DRX was commonly promoted by a decrease in temperature-compensated strain rate, i.e., Zener-Hollomon parameter (Z), corresponding to an increase in deformation temperature and/or a decrease in strain rate. In contrast, the GOS distribution varied non-monotonously with Z. The large area fraction of DRX grains with small GOS below 1° appeared at definite temperature/strain rate conditions. The large fraction above 0.6 of DRX grains with small GOS was observed in DRX microstructures with a large ratio of CSL Σ3 boundary fraction to low-angle subboundary fraction. The GOS distribution in the DRX microstructures is discussed in terms of the DRX grain nucleation and growth rates.

Recrystallization and Associated Twinning Evolution in Nickel-Based GH4141 Superalloy During Annealing

The GH4141 superalloy with 20, 40 and 55% cold deformations is annealed at 1080, 1120 and 1180 °C for 3-25 min to study the recrystallization and twinning behavior. The migration rate of grain boundaries increases significantly as the temperature increases in the range of 1080-1180 °C. After recrystallization, the hardness of the alloy is obviously reduced, and the recrystallized grains are randomly oriented without preferred orientation. The orientation imaging microscopy maps indicate that large amounts of Σ3 twin boundaries caused by the stacking faults appear in recrystallized grains. On one hand, the fraction of Σ3 boundaries after short time annealing has increased with the increase of temperature, meaning that the higher temperature contributes to the faster migration rate of grain boundaries and thus promotes the formation of stacking faults and twin boundaries before full recrystallization. On the other hand, the density of Σ3 boundaries has gradually decreased with the increase of grain size due to the annihilation and rare formation of twin boundaries after full recrystallization. The twinning evolution can be explained using the classical growth accident model.

Hot Deformation Characteristics and Dynamic Recrystallization Mechanisms of a Novel Nickel‐Based Superalloy

The hot deformation characteristics of a novel nickel‐based superalloy is investigated via the isothermal compression test in temperature range of 1000–1150 °C and strain rate of 0.001–10 s−1 under the true strain of 0.8. The hot deformation characteristics of GH4065 alloy are studied here for the first time. Based on the flow stress data, it is observed the typical features of flow curves exhibit the occurrence of dynamic recrystallization (DRX) during the hot deformation process. The constitutive equation in the Arrhenius‐type model is established, and activation energy (Q) is determined as 844.787 kJ mol−1. The microstructure evolution and DRX mechanism are investigated by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) technique. The results reveal that the fraction of low angle grain boundaries (LAGBs) decrease gradually with the increase in deformation temperature, whereas the fraction of Σ3 boundaries increase first and then decrease. For γ + γ′ dual‐phase region, the particle‐induced DRX (PIDRX), characterized by the generation of sub‐grains accelerated by γ′ precipitates pinning dislocations, and discontinuous dynamic recrystallization (DDRX) are the dominant nucleation mechanism of DRX. For γ quasi‐phase region and γ single‐phase region, the occurrence of bulged grain boundaries with twins further illustrates that DDRX plays a more significant role.

Fatigue behavior of biomedical Co–Cr–Mo–W alloy fabricated by selective laser melting

Co–Cr–Mo based alloy was widely used biomaterials for metallic implants fields. Recently, selective lase melting (SLM), one of the additive manufacturing technologies, is broadly studied because it can produce metallic implants with complex geometries and high degree of personalization. However, the possibility of mechanical failure must be avoided during the working period of SLM produced biomaterials. Therefore, in this paper we investigated the fatigue behaviors of the SLM Co–Cr–Mo–W alloy and their corresponding microstructure and fracture morphology. The sand mold casting fabricated (Cast) Co–Cr–Mo–W alloys with the same chemical composition as SLM specimens were also investigated and compared. The results showed that the SLM Co–Cr–Mo–W alloy exhibits higher fatigue life, tensile strength, and ductility than Cast Co–Cr–Mo–W alloy. The high fraction of Σ3 boundaries and precipitates in SLM Co–Cr–Mo–W alloy lead to secondary cracks and deflected fatigue crack path, which can alleviate the stress concentrations of the main fatigue cracks and decrease the fatigue crack growth rate. However, the dendritic structure of Cast Co–Cr–Mo–W alloy cause cleavage fracture mode and the large island-like precipitates can generate stress concentration, which result in lower fatigue life. We believed that the results of this work can provide a new perspective towards microstructure regulation for additive manufacturing technology and paved way for in-depth application of SLM fabricated biomedical alloy.

Characterization of Microstructure and Stress Corrosion Cracking Susceptibility in a Multi-pass Austenitic Stainless Steel Weld Joint by Narrow-Gap TIG

In the present study, correlation of the microstructure and stress corrosion cracking (SCC) susceptibility in the top, middle and root regions of a multi-pass austenitic stainless steel weld joint fabricated by narrow-gap tungsten inert gas welding (NG-TIG) was investigated by slow strain rate tests (SSRT) following a microstructure characterization. The results show that from the top to root regions in both the heat-affected zone (HAZ) and weld metal (WM), the dislocation density and length fraction of the Σ3 boundaries present a decreased trend, while the grain size, length fraction of LAB and residual strain all increase. By comparing the microstructure of WM with HAZ, δ-ferrite in WM is excessive and continuous while that in HAZ is island-shaped and discontinuous. In addition, larger grain size, lower dislocation density, higher residual strain, lower length fraction of Σ3 boundaries and higher length fraction of RGB are observed in WM. As a result, the SCC susceptibility in various regions of the weld joint follows the order of root regions > middle regions > top regions and WM > HAZ.

Controlling the corrosion behavior of CoNiFe medium entropy alloy by grain boundary engineering

CoNiFe medium entropy alloys (MEAs) with different fractions of low-Σ coincidence site lattice boundaries (CSL) are synthesized by thermal-mechanical processing and modified by grain boundary engineering (GBE). The effects of the grain boundary characteristic distributions (GBCD) on the corrosion behavior is investigated. A larger fraction of the low-ΣCSL boundaries, especially Σ3, improves the corrosion resistance of the CoNiFe MEAs significantly. The HT900-10 specimen with the largest Σ3 boundary fraction of 90% shows the best corrosion resistance and the Σ3 boundary has a close relationship with the Σ9 and Σ27 boundaries.

Evolution of grain boundary character distributions in a cold-deformed Nickel-based superalloy during electropulsing treatment

The evolution of grain boundary character distribution in a cold-deformed nickel-based superalloy during the electropulsing treatment (EPT) was studied. The results revealed that both coherent Σ3 boundaries (Σ3c) and incoherent Σ3 boundaries (Σ3ic) were formed in the recrystallized structures mainly by growth accidents during EPT, while the fraction of Σ3ic boundaries became much lower at the higher EPT temperatures. Meanwhile, the fraction of Σ3 boundaries increased with the increasing EPT temperatures till 850 °C, and decreased again at 900 °C. In addition, the high mobility of grain boundary during EPT could promote the formation of stacking errors at some locations of high angle grain boundaries (HAGBs), and thus promoting the nucleation of annealing twins. With the increasing EPT time, the fraction of Σ3 boundaries increased slightly, and the unstable Σ3ic boundaries were transformed into stable Σ3c boundaries, in which intersecting migrating SRX fronts were easier to be extended. On the other hand, a lot of low-Σ CSL grain boundaries were formed in the pre-deformed alloy with the strain of 0.1 during EPT, which was mainly attributed to the development of large grain-clusters. As the strain was large enough, the energy supplied by electropulsing could enable the cold-rolled alloy to effectively utilize the stored energy on the formation of Σ3 boundaries. Besides that, the evolution of Σ9 and Σ27 boundaries was analyzed in detail as well as the corrosion resistance of the studied alloy after different treatments.

Effect of Nb–Ti multi-microalloying on the hydrogen trapping efficiency and hydrogen embrittlement susceptibility of hot-stamped boron steel

Multi-microalloying technique has been widely used to improve strength and ductility in steels, however, the effect of multi-microalloying on the hydrogen diffusion and hydrogen embrittlement (HE) behavior of steels is still not completely understood. This study evaluates the effect of Nb–Ti multi-microalloying on the hydrogen trapping efficiency and HE of hot-stamped steel by a combination of hydrogen permeation test, slow strain rate tensile (SSRT) test and quantitative analysis. The microstructural examination and hydrogen permeation tests showed that with increasing Nb + Ti content, more nano-sized (Nb, Ti) C precipitates were formed, and the martensite grain boundaries areas increased, thus increasing the irreversible and reversible hydrogen trap sites and decreasing the hydrogen diffusion coefficient in the hot-stamped steel. A quantitative analysis of the hydrogen traps demonstrated that the number of hydrogen traps induced by the (Nb, Ti) C precipitates was larger than that of the grain boundaries and dislocations in the Nb–Ti bearing steels. In addition, the SSRT tests showed that the HE susceptibility of the test steel decreased with increasing Nb + Ti content. This was because the additional hydrogen traps generated by the Nb–Ti addition hindered localized hydrogen accumulation at prior austenite grain boundaries, the cracking resistance was improved by a lower Σ3 boundary fraction, and the pinning role of the (Nb, Ti) C precipitates on the movable dislocation could inhibit H-dislocation interaction.

“Strain-annealed” grain boundary engineering process investigated in Hastelloy-X

The mechanisms responsible for microstructure evolution during “strain-annealed” grain boundary engineering in Hastelloy-X have been investigated. GBE-quantifying parameters such as Σ3n density and fraction, triple junction density and fraction have been utilized to quantify the extent of GBE in the processed microstructures. “Strain-annealed” GBE process favors the formation of Σ3 boundaries through dissociation mechanism. When the amount of strain/cycle is high, the change in microstructure is limited and most of the Σ3 boundaries are remnant annealing twins from the initial microstructure. When the amount of strain/cycle decreases, more grain boundary migration takes place and majority of Σ3 boundaries start integrating into the GB network through dissociation mechanism. AGG is found to take place under special conditions: when (i) the dislocation density stored in the microstructure is low and (ii) the fraction of Σ3 boundaries that are integrated into the general grain boundary network is high. It is the preferential growth advantage of CSL boundaries that leads to abnormal grain growth.

Dual role of nanosized NbC precipitates in hydrogen embrittlement susceptibility of lath martensitic steel

The effect of dispersed nanosized spherical NbC precipitates (∼10 nm) on the hydrogen embrittlement (HE) of martensitic steel was investigated by slow strain rate tests, hydrogen permeation tests, atom probe tomography and microstructural examination. HE susceptibility decreases as the amount of NbC increases. NbC plays dual roles in HE. Primarily, NbC alleviates hydrogen accumulation in other areas via hydrogen traps which are induced by NbC directly and increased by the additional grain boundaries resulting from NbC, inhibiting the hydrogen-enhanced decohesion. Moreover, NbC impedes the hydrogen-enhanced localized plasticity by pinning hydrogen-dislocation atmospheres and reducing Σ3 boundary fraction. Secondarily, NbC weakens the beneficial {111}//ND texture, reduces the crack-propagation resistance, and plays a harmful role.