Σ3 Boundaries Research Articles

Dislocation-assisted particle dissolution: A new hypothesis for abnormal growth of Goss grains in grain-oriented electrical steels

The physical mechanisms that lead to the abnormal growth of Goss-oriented grains in grain-oriented electrical steel (GOES) are still not well understood, despite almost a century of research. The present paper reviews the existing hypotheses on the formation of Goss-oriented grains by abnormal grain growth and provides more insights into the underlying mechanism for Goss texture formation by proposing a new hypothesis named “Dislocation-assisted particle dissolution”. Abnormally grown Goss-oriented grains in fully-processed industrial GOES samples are shown to contain a fine network of internal subgrain boundaries with very low angle (0.03° - 0.18°) each consisting of regular arrays of dislocations. These subgrain boundaries form a branched ray-like pattern from the Goss grain center towards its perimeter, i.e. they seem to have evolved with the grain during its growth. Structural and compositional analysis of these dislocations by controlled electron channelling contrast imaging (cECCI) and atom probe tomography (APT) show that these dislocations are enriched with solutes such as Sn, Cu, C, and more importantly, with Al, N and Mn, which all build the composition of the inhibitor particles that assist the abnormal growth of Goss-oriented grains. Additionally, molecular statics (MS) calculations are employed to compare the segregation tendencies of Al atoms on dislocations and on Σ9 boundaries. It is found that Al prefers to segregate to dislocations rather than to the boundaries. The origin and the role of subgrain boundaries are discussed based on the experimental and simulation results. The results indicate that, after the dissolution of inhibitors along the grain boundaries, solutes are absorbed by the subgrain dislocations. As a result, grain boundaries surrounding Goss grains become less decorated by solutes and precipitates and more mobile compared to the boundaries of matrix grains.

Grain boundary engineering activated by residual stress during the laser powder bed fusion of Inconel 718 and the electrochemical corrosion performance

Conventional grain-boundary engineering (GBE) is not directly compatible with the complicated geometric parts fabricated via laser powder bed fusion (LPBF) owing to the multiple thermomechanical processes involved therein. In this study, GBE was activated by the inherent residual stress of an Inconel 718 superalloy produced via LPBF. The grain boundary character distribution and corresponding electrochemical corrosion behaviors were systematically investigated in a 3.5 wt% NaCl solution. The results indicate that numerous Σ3 boundaries (twin boundary, 64.89%) can be obtained via a simple heat treatment, and a favorable grain size can be maintained owing to the segmentation effect of the twin boundary. The corrosion resistance is closely related to the fraction of low special low-coincidence site lattices because the random boundary networks are broken. In addition, samples fabricated using GBE with the highest fraction of the twin boundary exhibited more compact and stronger passive films. The main components (oxide and metallic states) of thicker passive films provided intrinsic protection for the matrix. This study demonstrates a feasible method for GBE in additive-manufactured superalloys for fabricating complicated geometric parts with unique properties.

Mixing enthalpy near grain boundaries in Fe-Cr alloy: The results of atomistic simulation

Energy characteristics of symmetric tilt (twin) grain boundaries (GBs) Σ5(210) and Σ5(310) with the tilt axis [001] and boundaries Σ3(111) and Σ3(112) with the tilt axis [11¯0] in a bcc random binary FeCr alloy have been studied using atomistic simulation. The simulation has been carried out based on DFT calculations and two-band semi-empirical potential. We have proposed the formula for calculating the mixing enthalpy near GBs. The relationship between the mixing enthalpy and the energy of the boundary in the alloy has been revealed. Both simulation methods have shown a decrease in the mixing enthalpy near a GB compared to the ideal crystal. Moreover, DFT has predicted a stronger decrease in the mixing enthalpy compared to estimates based on semi-empirical interaction. Based on the obtained dependences of the mixing enthalpy on the alloy composition, phase diagrams for the considered GB regions have been constructed. It has been shown that the GB region is characterized by a higher equilibrium concentration of chromium, and the precipitates of the α'-phase have a lower concentration of chromium compared to the bulk. The smallest difference in equilibrium compositions has been observed for the Σ5(310) and Σ3(112) boundaries, and the largest one for the Σ3(111) boundary, which has the highest energy among considered GBs.

Research on influence of grain boundaries on the mechanical properties of 460 MPa refractory steel used for high-strength building structures

In this paper, the microstructure of Mo-type seismic refractory steel for building g, as well as different boundary densities and boundary ratios, are combined with elevated-temperature mechanical properties analysis to explore the laws of boundary for high-temperature performance. The results show that salt water cooling (SWC) and water cooling (WC) can obtain lath bainite with a higher content, and oil cooling (OC) with a lower cooling rate can obtain the microstructure of multiphase bainite + bulk ferrite. The boundary characterization results show that when the sample contains more high angle grain boundaries (Block and High-Packet boundaries), and the dislocation density is high, it can make it have better mechanical properties at room temperature. When the content of low angle boundary and low interfacial energy twin boundary (Σ3 boundary, which is mainly composed of V1/V2 variant pair) is high, it will have better microstructure stability after high temperature tempering, and the boundary density and dislocation density will decrease slightly, ensuring that it has better refractory performance.

The influence of hafnium removal on the microstructure and properties of 8YSZ ceramics

The effects of hafnium removal on the sinterability, phase composition, and microstructural, mechanical, and electrical properties of 8YSZ (8 mol% yttrium stabilized zirconia) were investigated using SEM, XRD, Raman spectroscopy, EBSD, three-point bending, Vickers indentation, and impedance spectroscopy. The 8YSZ and 8YSZ0 (8 mol% yttrium-stabilized hafnium-free zirconia) ceramics were prepared via dry pressing and atmospheric sintering, respectively. The overall mechanical properties of the 8YSZ0 ceramic were poor. However, at a sintering temperature of 1450°C, the relative density of 8YSZ and 8YSZ0 ceramics was almost identical. 8YSZ0 had a slightly smaller grain size and activation energy, and its electrical properties were slightly better than those of the 8YSZ ceramics. The presence of tetragonal secondary phases in the cubic structure of 8YSZ ceramics inhibited crack propagation and led to an increase in the mechanical properties and a decrease in the ion conductivity. In terms of the crystal structure, the increase in the cubic phase lattice parameters and tetragonal phase c/a values of the 8YSZ0 ceramics was attributed to the larger Zr4+radius, reduced local lattice distortion, and increased matrix oxygen vacancy concentration and cubic phase content. The EBSD analysis results indicated that there was no significant difference in grain orientation between the two types of ceramics, but the content of 8YSZ ceramics in large angle grain boundaries was slightly higher, especially in special grain boundaries Σ3 and Σ9. Therefore, this material can be used as a solid-state electrolyte candidate.

The Effect of B on the Co-Segregation of C-Cr at Grain Boundaries in Austenitic Steels

In austenitic steels, the co-segregation of C and Cr at grain boundaries can result in the formation of Cr23C6. However, the addition of B to steels can effectively reduce the amount of Cr23C6 formed and inhibit its ripening in experiments, simultaneously transforming it into Cr23(BC)6. Therefore, the effect of B on the co-segregation of C and Cr at the Σ5(210), Σ9(221) and Σ11(113) grain boundaries in austenitic steels was investigated using density functional theory. The results indicate that B, C, and Cr all tend to segregate at the three grain boundaries, with B and C showing a stronger segregation tendency. Furthermore, co-segregation of C and Cr with short distances occurs readily at the Σ5(210), Σ9(221) and Σ11(113) grain boundaries. The presence of B at grain boundaries can impede the segregation of Cr, particularly at the Σ9(221) and Σ11(113) grain boundaries. When B is pre-segregating at the Σ5(210) grain boundary, B, C, and Cr tend to co-segregate at the grain boundary. The grain boundary B hinders the accumulation of Cr near it for most grain boundaries, thereby inhibiting the co-segregation of Cr and C, making it difficult for B, C, and Cr to aggregate at most grain boundaries. This is beneficial for controlling the nucleation of Cr23(BC)6.

A different type of intergranular corrosion facilitated by hydrogen in austenitic stainless steel

This research aims to investigate the distribution of hydrogen and hydrogen-facilitated intergranular corrosion (IGC) in an austenitic stainless steel, which comprises mainly Σ3 and random grain boundaries. The coherent twin boundaries Σ3, which routinely exhibited excellent corrosion resistance, were observed to be prone to IGC after hydrogen charging. Furthermore, the results demonstrated that the inhomogeneous distribution of absorbed hydrogen finally induced IGC at grain boundaries in both the solution-treated and sensitized specimens. The mechanism of hydrogen-facilitated IGC differs from the conventional chromium-depleted theory. A model was established to describe this different type of IGC induced by hydrogen.

Effect of double tempering process on sulfide stress cracking susceptibility in API-5CT-C110 casing steel

The sulfide stress cracking (SSC) of the API-5CT-C110 grade casing steel in this work was evaluated by double cantilever beam (DCB) testing, two thermal treatments were used to investigate microstructures and properties. Compared to one, long-term tempering treatment, the study showed that precipitate size and dislocation density significantly decreased if two, short-term tempering treatments were used instead. DCB testing showed that SSC resistance is improved by double tempering with no degradation of strength. Specimens treated by double tempering have a lower proportion of Σ3 boundary, high-angle grain boundaries and high Taylor factor values, reduced susceptibility to crack initiation and propagation.

Misorientation Development at Σ3 Boundaries in Pure Copper: Experiments and MD Simulations

Misorientation Development at Σ3 Boundaries in Pure Copper: Experiments and MD Simulations

Investigation of the Mechanical Properties and Microstructure of the Co40NiCrMo Alloy Used for STACERs and Prepared by the CSPB Process and the Winding and Stabilization Method.

The Co40NiCrMo alloy, used for STACERs fabricated by the CSPB (compositing stretch and press bending) process (cold forming) and the winding and stabilization (winding and heat treatment) method, was investigated with regard to its tensile property, residual stress, and microstructure. The Co40NiCrMo STACER prepared by the winding and stabilization method was strengthened with lower ductility (tensile strength/elongation: 1562 MPa/5%) compared to that prepared by CSPB (tensile strength/elongation: 1469 MPa/20.4%). The residual stress of the STACER prepared by winding and stabilization (τxy = -137 MPa) showed consistency with that obtained through CSPB (τxy = -131 MPa). Combined with the driving force and pointing accuracy performances, the optimum heat treatment parameters for the winding and stabilization method were determined as 520 °C + 4 h. The HABs in the winding and stabilization STACER (98.3%, of which 69.1% were Σ3 boundaries) were much higher than those in the CSPB STACER (34.6%, of which 19.2% were Σ3 boundaries), while deformation twins and h.c.p ε-platelet networks were present in the CSPB STACER, and many more annealing twins appeared in the winding and stabilization STACER. It was concluded that the strengthening mechanism in the CSPB STACER is the combined action of deformation twins and h.c.p ε-platelet networks, while for the winding and stabilization STACER, annealing twins play the dominant role.

Relationship between Σ3 Boundaries, Dislocation Slip, and Plasticity in Pure Nickel.

This study investigated the relationship between the Σ3 boundaries, dislocation slip, and plasticity in pure nickel wires after grain boundary (GB) modification. Both quasi in situ tensile tests and simulations were employed. During plastic deformation, twins surrounded by Σ3 boundaries may exhibit a good deformation coordination. With an increase in strain, the slip systems corresponding to the maximum Schmid factor and the actual activated slip systems remain unchanged. Even sub-grains can maintain the dominant slip system of their origin matrix grains. Slip systems with slip planes (111) and (1-1-1) are the most active. Moreover, random boundaries have strong hindering effects on dislocations, and the nearby stress accumulates continuously with an increase in strain. In contrast, Σ3 boundaries demonstrate weak blocking effects and can release the nearby stress due to their unique interfacial structures, which is favorable for improving plasticity. They are more penetrable for dislocations or may react with the piled dislocations. In addition, some Σ3 boundaries can improve their geometrical compatibility factor with an increase in the strain, which enhances the deformation coordination of the grains. The research results provide a better understanding of the plasticizing mechanism for face-centered cubic (fcc) materials after grain boundary modification.

Synergistic effects of Ta and Mo on the hydrogen embrittlement resistance in ultra-high strength hot stamping steel

Hydrogen embrittlement (HE) is a major problem that restricts the application of ultra-high strength hot stamping steels. This paper proposes a novel strategy for against HE via Ta–Mo multi-microalloying, and systematically analyzes the synergistic effects of Ta and Mo. The results show that a major fraction of Ta and a small fraction of Mo generated high-density semicoherent nanosized (Ta, Mo) C precipitates and further refined the prior-austenite-grain/martensite structure. Ta–Mo multi-microalloying resulted in significantly higher precipitate and grain boundary (GB)-induced H trap densities and a higher H trapping capacity of the precipitates than Ta alloying, thereby reducing the H diffusivity. Additionally, Ta–Mo alloying significantly increased the HE resistance through the following mechanisms: (i) Ta–Mo hindered H enrichment at the GBs by providing numerous additional H traps and increasing the GB cohesive strength through Mo segregation, inhibiting hydrogen-enhanced decohesion (HEDE); and (ii) Ta reduced the proportion of Σ3 boundaries, Mo increased the binding force of Σ3 boundaries, and (Ta, Mo) C precipitates hindered the H–dislocation interactions, suppressing hydrogen-enhanced localized plasticity (HELP). In summary, Ta–Mo multi-microalloying combines the advantages of Ta and Mo and synergistically hinders the HELP and HEDE, thereby significantly increasing the HE resistance.

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.

Study on deviation distribution of Σ3 and Σ9 boundaries in Hastelloy C-276 superalloy with different hot rolling reductions

In this study, the deviation distributions of Σ3 and Σ9 grain boundaries in a Hastelloy C-276 sheet were measured using electron backscatter diffraction technology (EBSD) and the grain boundary plane orientation was analyzed using a five-parameter technique after hot rolling at 900 °C with different thickness reductions. The results show that the relative areas of Σ3 grain boundary planes reveal a strong peak at the (111) twist position, and the corresponding intensity significantly decreases as the reduction increases. The hot rolling generates dislocations, resulting in significant deviation of Σ3 grain boundary orientations and the loss of special properties. However, the Σ9 grain boundary plane distribution (GBPD) becomes more diffused with increasing deformation, and the multiples of random distribution (MRD) value is ∼2. In addition, the large deviation of the Σ9 grain boundary orientation varies across the different strain samples. In low-strain samples, the deviation of the Σ9 grain boundary orientation may be affected by the Σ3 grain boundary deviation with construction of the Σ3–Σ3–Σ9 triple junction. The deformation microstructures mainly determine the deviation of the Σ9 grain boundary orientation in high-strain samples. Moreover, the distorted grain boundaries act as potent nucleation sites of dynamic recrystallization (DRX) during the hot rolling process.

Microstructure and microtexture evolution of dynamic recrystallization of a Ni-based superalloy under high-throughput gradient thermal compression

The GH4169 alloy with high through-put gradient distribution of strain and microstructure was obtained by thermal compression of double-frustum specimens at 1050 °C, 0.4 s−1 deformation condition. The effect of strain on the evolution of the microtexture and recrystallization of the alloy was investigated using electron backscatter diffraction (EBSD). The strain gradually increases from the edge to the center of the compressed sample, accompanied by increased recrystallization. As the strain increases, new Σ3 boundaries are generated within the dynamic recrystallization (DRX) grains, affecting the deformation microstructure in the subsequent deformation process and gradually decreasing the overall maximum intensity of the microtexture. The deformed grains are oriented in the <101> direction parallel to the compression direction (ND, equivalent to normal direction), and the orientation of the recrystallized grains is in the <001> direction with low pole density. With increasing strain, Brass and Goss of α-fiber textures characterized by {110}<uvw> textures gradually weaken, and the volume fraction of the Rotated Cube increases under the effect of shear stress.

An Investigation of Microstructure Characteristic and Sulfide Stress Cracking Behavior of 110 ksi Cr-Mo Grade Casing Steel

The sulfide stress cracking (SSC) behavior of three micro-alloying 110 ksi grade Cr-Mo casing steels were evaluated according to the A-Method tensile test specified in the NACE TM0177 standard (NACE A-method). The fractures of SSC specimens and the micro-structures of the casing steels were characterized by scanning electron microscopy (SEM) with electron backscattering diffraction (EBSD) and transmission electron microscopy (TEM) in the BF and STEM mode. The results show that on the premise of 110 ksi, the yield strength of Co-Mo steel was satisfactory, finer spherical carbides increased the number of irreversible traps; widened polygonised martensitic laths, lowered dislocation density, and lowered the proportion of high-angle grain boundaries and the Σ3 boundaries decreased hydrogen permeability and diffusivity. The SSC resistance of Co-Mo steel with above micro structure characteristics was enhanced.

Interface selection behavior during the growth of δ-Fe grains in liquid Cu

In this study, the selection process of low-energy grain boundaries of δ-Fe grains (>25 μm) suspended in the liquid copper was studied based on the characteristics of Cu-Fe monotectic alloy which suggests that liquid Cu-rich phase and solid Fe grains can coexist over a wide temperature range. The distribution of grain boundaries was analyzed by EBSD, and it was found that Σ3 and Σ9 high angle grain boundaries with low energy were preferentially formed between δ-Fe grains at 1100 and 1150 °C. After heat treatment at 1200 °C, the δ-Fe grains agglomerated into a spherical shape with a ``bud'' structure, in which Σ1 grain boundaries dominated. Thus, the selective attachment of low-energy interfaces between floating grains in the liquid phase was elucidated.

Ion diffusion across/along symmetric tilt grain boundaries in yttria-stabilized zirconia investigated by molecular dynamics simulations

Atomic arrangements around symmetric tilt grain boundaries (GBs) Σ3[110]/{111}, Σ9[110]/{221} and Σ11[110]/{113} in yttria-stabilized zirconia (YSZ) and influence of these GBs on oxygen ion diffusion were investigated by molecular dynamics (MD) simulations. The models including those GBs along which yttrium ions are segregated were used for simulations. The atomic arrangements obtained by MD simulations were consistent with the structure obtained by using advanced electron microscopy techniques. MD simulations predict that Σ11[110]/{113} GB reduce oxygen ion diffusion both across and along GB, while Σ9[110]/{221} GB does not give large influence on the diffusion. The origin of such difference can be explained systematically based on the variation of nearest neighbor OO distance which may change the probability of oxygen ion hopping from one site to another. The simulated results also predict the emergence of two-dimensional honeycomb-shaped path networks of oxygen ion diffusion along the cores of Σ3[110]/{111} symmetric tilt GB in YSZ. The unique honeycomb networks have the potential for enhancing the ion diffusion along GBs.

Advancing analytical electron microscopy methodologies to characterise microstructural features in superalloys

Electron backscatter diffraction (EBSD) generally links crystallographic orientation to the microstructure of crystalline materials. EBSD datasets are now commonly used to identify phases, grains, and their orientations using off-the-shelf software, although substantial additional information may be extracted. Due to the lack of commercially available software, advanced analyses are often done manually and provide only localised information, lacking statistical significance. Here we introduce novel automated methodologies for advanced analyses of microstructural features in Ni-based superalloys. Our methodologies provide additional insights into the characteristics of these features and their underlying physical phenomena. We showcase how to correct wrongly indexed γ/γ’ interface artefacts in combined EBSD and energy-dispersive X-ray spectroscopy (EDS) measurements, how to classify recrystallised grains based on their location, how to assess and visualise grain boundary planes, and how to study the evolution of Σ3 twins during hot deformation. We further demonstrate how phase fractions and grain sizes are more accurately determined in combined EBSD-EDS measurements. The classification of recrystallised grains into different groups enables individual analyses, facilitating the straightforward identification of the underlying recrystallisation mechanism. Our grain boundary plane analysis provides insights into the coherence of Σ3 twins and the potential boundary planes of incoherent Σ3 boundaries. The current paper is a tutorial-style guide for these methodologies. The algorithms are made freely available and, although demonstrated here on Ni-based superalloys, can also be applied to other systems.

Effect of solution treatment on static recrystallization of a post-deformed GH4586 superalloy

Solution treatment was carried on a post-deformed GH4586 superalloy in the solution temperature range of 1020°C to 1100°C and solution time of 2 h and 4 h. The results showed that the static recrystallization (SRX) occurred due to the residual distortion energy caused by high temperature deformation. With increasing solution temperature from 1020°C to 1100°C, the average grain size of γ matrix firstly decreased and then increased. With increasing solution time from 2 h to 4 h, the average grain size of γ matrix gradually increased from 24.2±3.2 μm to 30.9±3.8 μm. Grain boundaries and twinning boundaries are the main nucleated site for SRX. And when solution treated at 1020°C for 2 h, carbides can also become the preferential nucleation position. In addition, by analysing the relative deviation (v/vm ) distributions of Σ3 boundaries, it could be found that the higher solution temperature would lead to the increasing fraction of coherent Σ3 boundaries and the decreasing fraction of incoherent ones. Besides, the annealing twins nucleated and grew gradually during the migration of recrystallized grain boundaries, and their length fraction increased from 10.0% to 54.9% with increasing solution temperature.