Σ3 Boundaries Research Articles

Effect of cold rolling and annealing on microstructure and properties of a new resource-saving duplex stainless steel Fe–19Cr–0.6Al–12Mn

An Fe–19Cr–0.6Al–12Mn duplex stainless steel with an optimized alloy composition of Ni replaced by Mn and Cr partially replaced by Al was developed to avoid the edge cracking, which is a common defect in the hot rolling of traditional two-phase stainless steels. The newly developed duplex stainless steel could be hot-rolled in the single-phase ferrite (α) region by controlling rolling temperature and the single-phase ferrite microstructure was retained on water cooling. To obtain the two-phase stainless steel product with ferrite and austenite (γ) microstructure, cold rolling and annealing were carried out, and appropriate cold rolling reduction and annealing process parameters were determined. The significant impact of annealing on microstructure, mechanical properties and pitting resistance of the experimental steel was studied. It was observed that with the increase in cold rolling reduction, the number of γ nucleation points was dramatically increased leading to the precipitation of more γ at α grain boundaries after annealing. During annealing at 800 °C and with the increase in annealing time, the austenite fraction was increased with a lower rate and remained almost constant when the annealing time was greater than 4 h. With the increase in annealing temperature, the austenite fraction decreased gradually in the temperature range of 750–860 °C. Good combination of strength, ductility and excellent pitting resistance was obtained by cold rolling to 80% reduction and annealing at 800 °C for 4 h. Grain refinement and the existence of Σ3 boundaries played a vital role in improving the pitting resistance of the experimental steel. With good combination of strength, ductility and corrosion resistance, the newly developed duplex stainless steel is expected to be a new resource-saving dual-phase stainless steel.

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.

Hydrogen induced vacancy clustering and void formation mechanisms at grain boundaries in palladium

Hydrogen has a significant impact on the formation of vacancies, clusters and voids in palladium and other metals. The formation of vacancy-hydrogen complexes in bulk Pd and at Σ3 and Σ5 grain boundaries was investigated using first-principles calculations and thermodynamic models. Equilibrium vacancy and cluster concentrations were evaluated as a function of temperature and hydrogen partial pressure based on the Gibbs energy of formation including vibrational and configurational entropies. Vacancies were found to be significantly stabilized by association with interstitial hydrogen, leading to enhanced concentrations by several orders of magnitude. Vacancy clusters were further stabilized at grain boundaries, with equilibrium concentrations reaching site saturation for clusters comprising up to three vacancies. Nanovoids were investigated based on Wulff constructions from calculated surface energies of the (0 0 1) and (1 1 1) terminations as a function of temperature and coverage of hydrogen adsorbates. The most stable termination changed from (1 1 1) in vacuum to (0 0 1) in H2, and the surface energies were lowered due to hydrogen adsorbates. Consequently, voids were also stabilized in the presence of hydrogen. Coalescing of vacancies into nanovoids was found to be thermodynamically unfavorable due to the loss of configurational entropy. It was therefore concluded that enhanced concentrations of vacancies and clusters does not directly cause the formation of voids. The formation of voids in Pd-based membranes was discussed in terms of microstructural characteristics, and strain due to chemical expansion and plastic deformation.

Enhancing the Intergranular Corrosion Resistance of High-Nitrogen-Containing 316L Stainless Steels by Grain Boundary Engineering via Thermomechanical Treatment

High-nitrogen-containing Type 316L stainless steels (SS) with 0.12% to 0.22% N are being developed as future structural material of fast breeder reactors because of their improved hardness and resistance to localized corrosion. However, stainless steels with higher nitrogen content are prone to intergranular corrosion (IGC) due to their tendency to get sensitized by enhanced precipitation of Cr2N. Thermomechanical treatment (TMT) of 6.5% cold-work and heat-treatment (1,323 K for 30 min) is evaluated in this study to enhance IGC resistance of 0.07%, 0.12%, 0.14%, and 0.22% nitrogen-containing Type 316L SS. The frequency of coincident site lattice (CSL) boundaries is found to increase with increase in nitrogen content in Type 316L SS. A maximum CSL increase of 35% was seen in 0.22% nitrogen containing stainless steel, as compared to samples containing 0.07% to 0.12% N. The effective grain boundary energy was the least (<0.1 μm−1) for Type 316L SS containing 0.22% N, which is attributed to the higher percentage of Σ3 boundaries. Double-loop electrochemical potentiokinetic reactivation (DL-EPR) tests conducted on the sensitized as-received and TMT samples showed a clear decrease in sensitization for TMT samples. The improved resistance to IGC visualized in the post-DL-EPR optical micrographs of TMT samples is attributed to the breakdown in the connectivity of attacked boundaries. The role of nitrogen in austenitic SS on twinning and generation of CSL boundaries is also discussed.

Suppression of liquid-metal-embrittlement by twin-induced grain boundary engineering approach

Grain boundary engineering (GBE) has lately been recognized as a viable approach to manipulate grain boundary characteristics to improve resistance against intergranular degradation. One of the most challenging intergranular degradation phenomena is liquid-metal-embrittlement (LME), where a reactive liquid metal penetrates along random grain boundary network. In contrast to random grain boundaries, special coincidence site lattice (CSL) boundaries has been shown to be resistant against the embrittlement. The present study investigates the feasibility of using a GBE technique to arrest LME in a Fe (FCC)-Zn couple. Two sets of low-strain-heat-treatment processing routes were used to optimize grain boundary characteristics based on: (a) Σ3 and Σ3n boundaries frequency, and (b) the material's texture and grain size evolution. The optimum characteristics resulted in significantly improved resistance against LME. The mechanism of the embrittlement arrest is discussed based on the random-grain boundary network continuity and the grain boundary triple junctions distributione.

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.

Dynamic recrystallization in AlXCoCrFeNi duplex high entropy alloys

Most of the applications planned for high entropy alloys (HEAs) entail thermomechanical processing. In the current study, the hot deformation and dynamic restoration mechanisms of two duplex high entropy alloys (Al0.6CoCrFeNi and Al0.9CoCrFeNi) are revealed through a detailed electron back-scattered diffraction and transmission electron microscopy study. A standard homogenization of the cast microstructures results in face-centred cubic/body-centred cubic (FCC/BCC) duplex structures for both alloys. Al0.6CoCrFeNi has an FCC matrix containing dispersed BCC particles surrounded by interdendritic BCC. Al0.9CoCrFeNi has a BCC matrix with allotriomorphic and intragranularly dispersed FCC particles. Hot deformation at 1030 °C under the strain rate of 0.1s−1 is mainly accommodated through the matrix phase in both alloys. In Al0.6CoCrFeNi alloy, the FCC matrix undergoes discontinuous dynamic recrystallization mechanism (DDRX) while there are also some local observations of subgrain coalescence. DDRX mainly occurs at the interphase boundary mantle regions through the formation of Σ3 boundaries and the propagation of multiple twinning chains. Dislocations within the hot deformed FCC matrix have a large screw component. In the Al0.9CoCrFeNi alloy, the BCC matrix softens through continuous dynamic recrystallization (CDRX), characterised by the progressive conversion of low-misoriented subgrains into (sub)grains delineated partly by low-angle and partly by high-angle boundaries. Dislocations within the BCC matrix have a mixture of edge and screw character. The dispersed second phases in both alloys are mechanically fragmented during hot deformation and are quite evenly distributed throughout the microstructure at a strain of 1.5.

Application of Grain Boundary Engineering to Improve Intergranular Corrosion Resistance in a Fe–Cr–Mn–Mo–N High-Nitrogen and Nickel-Free Austenitic Stainless Steel

Optimization of grain boundary engineering (GBE) process is explored in a Fe–20Cr–19Mn–2Mo–0.82 N high-nitrogen and nickel-free austenitic stainless steel, and its intergranular corrosion (IGC) property after GBE treatment is experimentally evaluated. The proportion of low Σ coincidence site lattice (CSL) boundaries reaches 79.4% in the sample processed with 5% cold rolling and annealing at 1423 K for 72 h; there is an increase of 32.1% compared with the solution-treated sample. After grain boundary character distribution optimization, IGC performance is noticeably improved. Only Σ3 boundaries in the special boundaries are resistant to IGC under the experimental condition. The size of grain cluster enlarges with increasing fraction of low ΣCSL boundaries, and the amount of Σ3 boundaries interrupting the random boundary network increases during growth of the clusters, which is the essential reason for the improvement of IGC resistance.

Crystallographic Peculiarities of β-α Transformation in Brass Induced by Hot Extrusion

Structure-texture states in brass rods after hot extrusion and air-cooling have been investigated with the orientation microscopy (EBSD). In the examined samples, a significant concentration of β-phase with the lattice, close to bcc and fcc α-phase, has been detected. The β-phase texture consisted of the main components: two close to {110}<110> and {001}<110>. The α-phase texture consisted of the main components: close to {001}<100> and two close {110}<111>. The analysis of crystallographic relationship of the texture components of β-and α-phases demonstrates that they may all be obtained, in accordance with the orientation relations, which are intermediate between the Kurdjumov-Sachs and Nishiyama-Wasserman types It is assumed that β-α transformation began in β-phase at coincident site lattice Σ3 and Σ33a boundaries.

Hydrogen Induced Vacancy Clustering and Void Formation Mechanisms at Grain Boundaries in Palladium

Hydrogen has a significant impact on the formation of vacancies, clusters and voids in palladium and other metals. The formation of vacancy-hydrogen complexes in bulk Pd and at Σ3 and Σ5 grain boundaries was investigated using first-principles calculations and thermodynamic models. Equilibrium vacancy and cluster concentrations were evaluated as a function of temperature and hydrogen partial pressure based on the Gibbs energy of formation including vibrational and configurational entropies. Vacancies were found to be significantly stabilized by association with interstitial hydrogen, leading to enhanced concentrations by several orders of magnitude. Vacancy clusters were further stabilized at grain boundaries, with equilibrium concentrations reaching site saturation for clusters comprising up to three vacancies. Nanovoids were investigated based on Wulff constructions from calculated surface energies of the (0 0 1) and (1 1 1) terminations as a function of temperature and coverage of hydrogen adsorbates. The most stable termination changed from (1 1 1) in vacuum to (0 0 1) in H2, and the surface energies were lowered due to hydrogen adsorbates. Consequently, voids were also stabilized in the presence of hydrogen. Coalescing of vacancies into nanovoids was found to be thermodynamically unfavorable due to the loss of configurational entropy. It was therefore concluded that enhanced concentrations of vacancies and clusters does not directly cause the formation of voids. The formation of voids in Pd-based membranes was discussed in terms of microstructural characteristics, and strain due to chemical expansion and plastic deformation.

Microstructure and microtexture evolution of dynamic recrystallization during hot deformation of a nickel-based superalloy

Effect of strain rate route on the microstructure and microtexture evolution during the dynamic recrystallization (DRX) of a nickel-based superalloy subjected to the isothermal compression tests was investigated using electron back-scatter diffraction. The evolutions of microtexture components and fiber textures and the role of Σ3 boundaries in microstructure on the pole density in the partially and fully DRX processes were explored. The α-fiber in Euler space is regarded as the compression microtexture. The locally organized substructures in the matrix grains are formed on certain {111} slip planes with the high Schmid factor. Although the dislocation-free DRX grains show the random orientated distribution, the weak recrystallization 〈001〉 fiber parallel to the compression axis (ND) is developed at the high strain in the fully DRX processes. Whereas the compression 〈101〉 fiber parallel to ND in the partially DRX processes is sharpened. Moreover, the instantaneous decrease of strain rate leads to the highest fraction of Σ3 grain boundaries, indicating the lowest pole density. Cube-Twin component adjacent to Cube component contributes partly to the formation of Σ3 grain boundaries. The findings provide insights into the microtexture characteristics and enhance the understanding of the orientation dependence of the mechanical behavior of nickel-based superalloys.

Study on the dynamic recrystallization mechanisms of Inconel 740 superalloy during hot deformation

The dynamic recrystallization (DRX) mechanisms of Inconel 740 superalloy were studied by electron backscatter diffraction (EBSD) analysis of the superalloy microstructures obtained under different deformation conditions. The results show that the discontinuous dynamic recrystallization (DDRX) occurs together with the continuous dynamic recrystallization (CDRX), while CDRX is not active during hot deformation. CDRX easily happens at the low deformation temperature, high strain rate and medium strain. Under the testing conditions, there are five mechanisms, i.e., the bulging of pre-existing grain boundaries, the nucleation assisted by Σ3 boundaries, the nucleation assisted by dynamic recovery at pre-existing grain boundaries, and the progressive subgrain rotation and the increase of the heterogeneous misorientation in the deformed grain interiors, to facilitate the occurrence of DRX. The first three are classified as DDRX mechanism and the latter two belong to CDRX mechanism. The illustration of different DRX mechanisms validated via observation of DRX process helps establish an in-depth understanding and insight into the microstructural evolution of this relatively new superalloy during hot forming.

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.

Study on texture and dynamic recrystallization behavior of high purity copper during reverse extrusion

The dynamic recrystallization (DRX) behavior of high purity copper was studied by the combination of numerical simulation and experiment. Specifically, extrusion experiments were performed in the temperature range of 200 °C∼800 °C. The effects of temperature and deformation on the microstructure of the deformation zone were studied. It was found that the grain size decreased with the decrease of temperature and the increase of deformation. The optimized extrusion temperature of 500 °C∼550 °C was determined based on grain size and uniformity of microstructure. The special grain boundaries, grain orientation and recrystallization texture during extrusion were studied by electron backscattered diffraction (EBSD) technique. It was found that the increase of the special grain boundary Σ3 and the texture of 〈111〉 parallel to the ED direction have a positive effect on grain refinement and microstructure uniformity. As the temperature increases, the proportion of recrystallized grains increases initially, and finally decreases. A cellular automaton (CA) method that couples the developed DRX model was implemented to investigate the evolution of microstructure during high-purity copper extrusion using DEFORM-3D software, and the results were verified by experiments. The microstructure evolution and average grain size distribution predicted by numerical simulation agreed well with the experimental results.

“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.

Effect of Heat Treatment Temperature on the Spinning Structure and Properties of a Cu-Sn Alloy.

A thin-walled copper (Cu)-tin (Sn) alloy cylinder was treated after spinning at 200-400°C for 0.5 h. The characteristics of the alloy microstructure under different temperatures were analyzed through electron back-scattered diffraction. The results were as follows. The grain size at 200-300°C decreases as the heat treatment temperature rises, but the grain size at 400°C increases. At 200-300°C, the microstructure primarily consists of deformed grains. It is found that the main reason for the formation of high-angle grain boundaries (HAGBs) is static recrystallization. For the grain boundary orientation differential, the low-angle sub-grain boundary gradually grows into the HAGB, and multiple annealing twin Σ9 boundaries appear. Grain orientation is generally random at any temperature range. The mechanical property test indicated that, at the upper critical recrystallization temperature of 300°C, the elongation of the Cu-Sn alloy gradually increases, and its yield strength and ultimate tensile strength rapidly decrease.

Effect of grain boundary character distribution on soft magnetic property of face-centered cubic FeCoNiAl0.2 medium-entropy alloy

The impact of grain boundary character distribution on soft magnetic properties of as-cast, deformed and recrystallized FeCoNiAl0.2 medium-entropy alloys (MEAs) with single face-centered cubic (FCC) phase was investigated. Electron backscatter diffraction (EBSD) results show that the existence of a large number of random grain boundaries (71.3%) and Σ3 boundary (55.2%) is responsible for good soft magnetic properties of as-cast and recrystallized FeCoNiAl0.2 MEAs, respectively, due to effectively decreasing the concentration of strain. In contrast, the formation of low angle boundaries (64.6%) and the serious concentration of strain significantly deteriorate magnetic property of cold-rolled FeCoNiAl0.2 alloy. The reduction of grain size has no obvious effect on improving magnetic properties. The compositional distribution at the sub-grain boundary appears to have marginal effect on magnetic properties of studied samples. It indicates that the grain boundary characteristics have predominant effect on soft magnetic properties.

Size dependent strength, slip transfer and slip compatibility in nanotwinned silver

Perfect slip transfer through single coherent Σ3 twin boundaries is known to be a cross-slip-like mechanism occurring at low stresses, which is expected to strongly depend on material properties like stacking fault energy. In the present study, we extend the argument of perfect slip transfer to (i) multiple closely spaced coherent twin boundaries in a nanotwinned thin film and (ii) to materials with very low stacking fault energy. The slip transfer is indicated by the continuity of slip steps and observed across up to 100 coherent Σ3 boundaries. The study addresses size scaling due to multiple weak obstacles for dislocation motion and discusses the underlying deformation mechanisms. The importance of strain compatibility is further extended to incoherent twin boundaries.

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.