Σ3 Twin Research Articles

New insights into selective leaching and ferritization in 15-15Ti austenitic steel in lead-bismuth eutectic through parent phase reconstruction

Selective leaching accompanied by ferritization is widely observed in the dissolution corrosion of austenitic steels exposed to lead-bismuth eutectic. This study proposes a refined mechanism for selective leaching involving solid-state diffusion followed by dissolution-reprecipitation processes. Pole figure analysis reveals that ferritization follows the Nishiyama-Wassermann orientation relationship, independent of corrosion environment or corrosion behavior. Moreover, austenite reconstruction analysis shows that the growth of ferrite is hindered by the misfits at random high-angle grain boundaries. Hence the above results suggest that ferritization is initially driven by solid-state diffusion. As LBE channels open within the grains, further growth of ferrite is assisted by the dissolution-reprecipitation mechanism. Additionally, the formation of dual-oriented ferrite grains at Σ3 twin boundary is observed, and the selectivity of this formation with respect to grain boundary and ferrite orientation is discussed.

Grain size effect on strain localization, slip-grain boundary interaction and damage in the Alloy 718 Ni-based superalloy at 650 °C

Grain size effects on the early plastic strain localization and slip transfer at grain boundaries were investigated for the Alloy 718 Ni-based superalloy at 650 °C. Three microstructures with different grain sizes underwent monotonic tensile tests at 650 °C, both in air and under vacuum, until rupture. All the microstructure variants exhibit fully intragranular fracture under vacuum and partially intergranular fracture in air. In this latter case, predominant intergranular fracture mode was found in the fine-grain microstructures. Interrupted tensile tests were also conducted under vacuum with ex-situ SEM high-resolution digital image correlation (HR-DIC) measurements to assess in-plane kinematics fields at the microstructure scale. Out-of-plane displacement jumps were obtained using laser scanning confocal microscopy. Both crystallographic slip within grains and near Σ3 twin boundaries (TBs) and morphological sliding happening at grain boundaries (GBs) were documented. Statistical analysis of all plastic events aimed at quantifying strain localization distribution as a function of the microstructure. The fine-grain microstructure was found to have extensive strain localization at grain boundaries, while the coarse-grain microstructure is more prone to intragranular slip development and slip localization near TBs. Different scenarios of slip band/grain boundary interactions were evidenced.

Extended defects-enhanced oxygen diffusion in ThO[formula omitted]

Oxygen self-diffusion is key to understanding stoichiometry and defect structures in oxide nuclear fuels. Experimentally, low activation-barrier oxygen migration was found in ThO2, a candidate nuclear fuel, possibly due to short-circuit diffusion mechanisms. Here, we perform extensive molecular dynamics simulations to show that various types of extended defects can enhance oxygen self-diffusion with a much-reduced activation barrier in ThO2. In this work, we consider extended defects including 1D (dislocation), 2D (grain boundary), and 3D (void) defects. Due to the distinct characteristics of each type of extended defect, the modulation of oxygen diffusion varies. These results provide a quantitative description of oxygen transport, which is significantly enhanced within a close distance (nanometer scale) from the extended defects. Among all these considered defects, grain boundary, particularly the low-energy Σ3 twin boundary, exhibits the strongest effect on increasing oxygen transport.

Microstructure evolution of high-temperature deformation behavior of nickel based alloys for advanced ultra-supercritical applications

The microstructure evolution of nickel based alloy used in advanced ultra-supercritical power plants was studied in the strain range of 0.22–0.91 at 1000–1100 °C/0.01 s−1 by using the electron backscattered diffraction technique. The interaction of the dislocation with the Σ3 twin boundary causes the Σ3 twin boundary to deviate from its specific misorientation. The Σ3 twin boundary is stripped of its signature by the interaction between the dislocation and the Σ3 within the deformed grain. In the process of recrystallization, the Σ3 twin boundaries are affected by both recrystallization fraction and the size of recrystallization grain. Three recrystallization mechanisms, including DDRX, CDRX and TDRX, play their role in the deformation of the alloy. Leading to the recrystallized grains with high misorientation, CDRX tends to grow and promote twin nucleation, and the mechanism of CDRX becomes less significant with the increase of deformation temperature. Due to deformation, the orientation of the grain shifts to <101> direction. At 1000 °C and 1050 °C, the recrystallization texture inherits the deformation texture due to the effect of CDRX and the slow diffusion of the recrystallization process, which causes the recrystallization texture to show a sharp <101> fiber texture. At 1100 °C, the enhancement of the DDRX mechanism and the high diffusivity of the recrystallization process lead to the randomization of the texture, and the oriented growth of recrystallized grains at 1100 °C/0.91 enhances the <001> texture. In addition, both recrystallization and twinning are contributory to the randomization of texture.

High-temperature chromium diffusion in austenitic stainless steel: Ab initio molecular dynamics simulations

Chromium self-diffusion through stainless steel (SS) matrix and along grain boundaries is an important mechanism controlling SS structural materials corrosion. Cr diffusion in austenitic SS was simulated using canonical ab initio molecular dynamics with realistic models of type-316 SS bulk, with and without Cr vacancies, and a low-energy Σ3 twin boundary typically observed at active corrosion sites. Cr self-diffusion coefficients at 750 and 850 °C calculated using Einstein’s diffusion equation are 4.2 × 10−6 and 8.1 × 10−6 Å2 ps−1 in pristine bulk, 3.8 × 10−3 and 5.5 × 10−3 Å2 ps−1 in bulk including Cr vacancies, and 9.5 × 10−2 and 1.0 × 10−1 Å2 ps−1 at a Σ3[111]60° twin boundary.

Microstructure evolution and constitutive analysis of nuclear grade AISI-316H austenitic stainless steel during thermal deformation

Compression experiments were performed on AISI-316H austenitic stainless steel using Gleeble-3800 at temperatures ranging from 900 °C and 1200 °C and strain rates ranging from 0.01 and 10 s−1, up to the actual strain of 0.69. The tests aimed to examine the material’s microstructure evolution and flow stress behavior. Based on OM and EBSD studies, it was found that thermal deformation mostly induces discontinuous dynamic recrystallization (DDRX). The proportion of recrystallization nucleation increases steadily with increasing deformation temperature, while the impact of strain rate on recrystallization is complex. At the same deformation temperature, the recrystallization volume fraction initially declines and rises as the strain rate rises. In low strain rate regime, the longer (deformation) time available for grain boundary migration, the higher recrystallization volume fraction. In high strain rate regime, the higher stored energy (and thus the increased boundary velocity) raises the probability of nucleation events, stimulating twin formation. As a result, the twin promotes a dynamic recrystallization (DRX) process. An abundance of Σ3 twins was notably observed in uniformly refined recrystallized grains at a true strain of 0.69, at a temperature of 1200 °C, and a strain rate of 10 s−1. As a result, it was discovered that DRX occurs at higher strain rates and deformation temperatures. In addition, the flow stress curves were modified to account for adiabatic heating at strain rates exceeding 1 s−1. The findings demonstrated that adiabatic heating increased when strain level and strain rate increased and deformation temperature decreased. The strain compensation Arrhenius model is developed following the given stress–strain curve while considering strain. The model exhibits high accuracy, with a correlation value of 0.986. According to a kinetic study, the average activation energy for hot deformation of the tested steel was 444.994 kJ/mol. These findings provide fundamental insights into the microstructure control technology and the outstanding mechanical properties of the austenitic stainless steel AISI-316H.

The formation mechanisms and mechanical effects of lattice defects in carbon nanotube reinforced 2024Al composite

Lattice defects significantly affect the mechanical performance of metallic materials. In this study, lattice defects of a carbon nanotube reinforced 2024 aluminum alloy (CNT/2024Al) composite were systematically investigated and the contribution of carbon nanotubes was illustrated. The results showed that dislocations and grain boundaries were closely related to the deformation behavior and lead to discontinuous yielding phenomena. Moreover, the carbon nanotubes reduced the stacking fault energy, which was conducive to forming stacking faults and a small number of twins in the composite. The stacking faults, twin boundaries, and intragranular Al4C3 phases hindered dislocation motion, thus enhancing the mechanical properties of the composite. In addition, the novel formation mechanism of the 9R structure was revealed: the Σ3(11‾1)/(151) incoherent twin boundary may dissociate into two tilt walls bounding a 9R structure (zone axis: [1‾01]), which enriches understanding of the relationship between Σ3 twin boundaries and the 9R structure.

Plastic deformation behavior and strengthening mechanism of SLM 316L reinforced by micro-TiC particles

In this work, the integrated study on the microstructure evolution, plastic deformation behavior, and strengthening mechanism of Selective Laser Melting (SLM) 316L austenitic stainless steel (ASS) reinforced with micro-TiC particles was investigated. The results indicated that there were lots of small angle grain boundaries in SLM 316L ASS, and the high dislocation density and component segregation of the cellular dislocation structure resulted in a good combination of high strength and good elongation in performance. The tensile strength, yield strength, and elongation of SLM 316L ASS were 682 MPa, 571 MPa, and 49.9%, respectively. The addition of TiC reinforced particles induced microstructure evolution, including grain refinement, reduction of epitaxial columnar dendrites, reduction of low angle grain boundary (LAGB) ratio and increase of Σ3 (60°/<111>) twin boundaries. Through the effects of grain refinement strengthening, Orowan strengthening, and dislocation strengthening, SLM 316L-2TiC showed a higher tensile strength of 1021 MPa and higher yield strength of 809 MPa, while maintaining an elongation of 30.8% due to the suppression of plasticity by twinning induced plasticity (TWIP) effect. Although nano-reinforced particles were commonly used to improve mechanical properties, in our work, we chose to add micrometer-sized TiC particles, and the strength of the SLM 316L-2TiC metallic matrix composites (MMCs) exceeded the previous reports of adding nanoparticles. The reason was that the small contact angle between TiC and 316L provides good wettability between the particles and the melt, and the selection of laser processing parameters ensures good bonding between TiC particles and the substrate.

Stable Strain State of Single‐Twinned AgPdF Nanoalloys under Formate Oxidation Reaction

Surface reconstruction as common phenomenon during catalysis complicates the prediction and modeling on catalytic activity of the nanoalloy, hence developing a stable structure to be resistant to surface restructuring would provide an ideal prototype for substantial and reliable mechanism analysis. Herein, the single‐twinned structure in inverse AgPdF catalyst is constructed to enhance the catalytic activity and stability for the formate oxidation reaction (FOR). The single‐twinned AgPdF nanoalloy (t‐AgPdF) catalyst exhibits an enhanced peak current density of 4.6 A mgPd−1, a reduced onset potential of 0.44 V, a higher activity retention of 55.7% after 600 cycles, and a longer activity retention time of 55.9 h. Additionally, the t‐AgPdF catalyst presents a higher hydrogen generation rate of 1.11 mL mgPd−1 than that of single‐crystalline AgPd nanoalloy (AgPd) catalyst, and density functional theory calculations reveal that t‐AgPdF(111) surface exhibits a reduced activation energy of 0.59 eV for formate decomposition reaction. Impressively, the t‐AgPdF maintains compressive and tensile strain state along the Σ3 twin boundaries before and after the FOR, in contrast to AgPd. This is the first time to reveal that the nanotwinned structures contribute inverse t‐AgPdF catalysts the catalytic active sites with stable strain state since starting reaction for the FOR.

Role of precipitates-decorated Σ3 twin boundaries on hydrogen-induced crack initiation and propagation in Ni-based superalloy 718

The role of ∑3 twin boundaries (∑3s) on hydrogen embrittlement behavior of Ni-based superalloy 718 was investigated by uniaxial tensile tests. The hydrogen resistance is improved with the fraction of ∑3s increasing after grain boundary engineering treatment. A high fraction of ∑3s can effectively reduce the nucleation sites of cracks and suppress the further propagation. ∑3s also act as potential sites for hydrogen-induced crack initiation. A distinct morphology of γ′′/γ′ co-precipitates and γ′ precipitates with one-to-one correspondence along the ∑3s is observed. The mechanism of hydrogen-induced ∑3s cracking is extensively discussed in terms of precipitates and hydrogen distribution.

The Effects of Cold Rolling and Annealing on the Microstructure Evolution of Ordered C-2000 Alloy during Metallic Wire Preparation

When using well-designed multiple-stage heavy-drawn processes, i.e., cold rolling, drawing and cluster drawing to fabricate a metallic wire or fiber in steps, cold rolling and annealing are critical steps due to their effect on the initial microstructure before the heavy-drawn process. Understanding the relationship between microstructure evolution and cold rolling followed by annealing is required for smoothly implementing the heavy-drawn process. In this work, the evolution behavior in terms of the microstructure during cold rolling followed by annealing was investigated in a novel C-2000 alloy that is a promising candidate material for the fabrication of high-performance metallicwire. The investigation encompassed parameters including the grain size, grain boundaries, recrystallization texture, and short-range ordered (SRO) structure. Results show that the grain size distribution of the cold-rolledC-2000 alloy followed by annealing at 900 °C is quite uneven. The low-angle grain boundaries induced by cold rolling are more frequently transformed into the Σ3 twin boundaries during recrystallization. At the initial stage of annealing at 900 °C after cold rolling, the contents of different texture components are significantly different, but the differences tend to decrease with the extension of the annealing time. In addition, cold rolling destroys SRO domains formed during solid solution water quenching, and the destruction of SRO affects the precipitation of the long-range ordered phase during annealing. Incoherent Σ3ic with curved grain boundaries play an important role in the recrystallization of nucleation sites in the process of static recrystallization by nucleation–growth.

Transmission Kikuchi Diffraction Mapping Induces Structural Damage in Atom Probe Specimens.

Measuring local chemistry of specific crystallographic features by atom probe tomography (APT) is facilitated by using transmission Kikuchi diffraction (TKD) to help position them sufficiently close to the apex of the needle-shaped specimen. However, possible structural damage associated to the energetic electrons used to perform TKD is rarely considered and is hence not well-understood. Here, in two case studies, we evidence damage in APT specimens from TKD mapping. First, we analyze a solid solution, metastable β-Ti-12Mo alloy, in which the Mo is expected to be homogenously distributed. Following TKD, APT reveals a planar segregation of Mo among other elements. Second, specimens were prepared near Σ3 twin boundaries in a high manganese twinning-induced plasticity steel, and subsequently charged with deuterium gas. Beyond a similar planar segregation, voids containing a high concentration of deuterium, i.e., bubbles, are detected in the specimen on which TKD was performed. Both examples showcase damage from TKD mapping leading to artefacts in the distribution of solutes. We propose that the structural damage is created by surface species, including H and C, subjected to recoil from incoming energetic electrons during mapping, thereby getting implanted and causing cascades of structural damage in the sample.

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.

Precipitation characteristics of γ' precipitates of the GH4742 nickel-based superalloy at a slow cooling rate

The double cone samples of the predeformed GH4742 superalloy were treated by subsolid solution heat treatment (Sub-SHT) at 1080 ℃/8 h and then cooled at a cooling rate of 22 ℃/min. The evolution of γ' precipitates by coupling hot deformation and heat treatment was studied by in situ electron backscatter diffraction + scanning electron microscopy. This research explains the morphological variations between the recrystallized and deformed grains of the γ' precipitates, as well as the relationship between the characteristics of the grain boundaries (GBs) and γ' precipitates at the GBs. The results show that when the predeformed samples were subjected to an 8 h Sub-SHT and then cooled at 22 ℃/min, the γ' precipitates exhibit a tri-modal size distribution, and the primary γ' precipitates are dendritic. The morphology of γ' precipitates within deformed grains is related to the accumulation of dislocations and other defects. The maximum size of columnar γ' precipitates is obtained at the high-angle grain boundary, where the misorientation ranges from 30° to 40°. The length of columnar γ' precipitates results from the comprehensive effect of the GB energy determined by the GB misorientation and the GB mobility determined by the stored energy difference. Owing to the low energy and low mobility of the Σ3 twin boundary, the γ' precipitates at the Σ3 twin boundaries remain stable, and the morphology and size are no different from those within the grains. These findings may be helpful for the design and optimization of Ni-base superalloys.

Grain boundary engineering process for nano reinforced aluminum matrix composites

It is challenging to obtain a balance of strength and ductility for alloys and their composites. They are expected to exhibit good comprehensive properties by adjusting the grain boundary character and/or introducing twin structures. However, for coarse grained aluminum with high stacking fault energies, twin structures are rarely found. In this study, a grain boundary engineering process was carried out on coarse grained aluminum containing nano-reinforcements via an alternated compression deformation, followed by an annealing treatment. The effects of the thermo-mechanical process and alloy elements on the microstructures of the composites were analyzed. The mechanical properties and inter-granular corrosion properties of the composites were evaluated. Twin structures could be generated in these alloys. After thermo-mechanical process, the composites exhibited good plasticity as well as higher strength. A Σ3 twin boundary hindered inter-granular corrosion crack propagation. The formation mechanism of the twin structures and the strengthening mechanism are discussed.

On the origin of grain refinement and twin boundaries in as-fabricated austenitic stainless steels produced by laser powder bed fusion

316 L austenitic stainless steel samples have been fabricated by laser powder bed fusion using two powder batches of slightly different compositions but keeping the same processing parameters. Compared to the first powder batch, the second powder batch led to a finer grain structure, a more random texture, and an extremely high density of Σ3 twin boundaries. A review of the literature shows that these two kinds of microstructures had been observed in previous works, but separately and not connected to changes in composition. Several hypotheses are reviewed to explain these microstructure differences. We conclude that local liquid ordering, also described as icosahedral short range order (ISRO) can have an effect on the microstructure formation during solidification. ISRO in the liquid is thought to be stronger in the second powder batch. Finding ways to enhance the influence of the ISRO in the liquid on the microstructure formation can be seen as a promising strategy to design alloys for additive manufacturing.

A supersaturated Cu-Ag nanoalloy joint with ultrahigh shear strength and ultrafine nanoprecipitates for power electronic packaging

Ag-Cu bimetallic nanoalloy, integrating the advantages of reducing migration and cost of nano-Ag and alleviating oxidation of nano-Cu, is a prospective bonding material for power electronic packaging. The Ag-coated Cu nanoparticles (Cu@Ag NPs) paste can execute bonding with high quality at 250 °C, and the achieved supersaturated Ag-Cu nanoalloy joint with ultrahigh shear strength (152 MPa) dramatically exceeds most nano-paste joints. The interstitial solid solutions with atomic-level metallurgical bonds at the interface dominantly promoted the shear strength. Besides, the numerous ultrafine nanograin, high proportion of low angle grain boundaries (7.44%) without deformation, and the Cu nanoprecipitates in the joint would improve subordinately. Furthermore, the high content (16.8%) of Σ3 twin boundaries would contribute to the electrical and thermal conductivity. Thus, the multiple strengthening mechanisms with the solid solution, the second precipitated phase, and ultrafine nanograin can dramatically enhance shear strength and electro-thermal conductivity of joints for high-temperature device packaging.

Dynamic recrystallisation via nucleation at distorted twins in a Ni-based superalloy

Annealing twins of Σ3 character are common special grain boundaries in face centred cubic metals with low-to-medium stacking fault energy such as Ni-based superalloys. Despite their prevalence, little is known about the detailed evolution of these interfaces during hot deformation, then often referred to as distorted twins, and how their presence contributes to nucleation during dynamic recrystallisation. Here, we developed an automated methodology to systematically analyse large numbers of distorted Σ3 twins in microstructures after hot compression tests using electron backscatter diffraction. We show that annealing twins lose their Σ3 character via a deviation in misorientation angle from the ideal 60°. This is accompanied by a rotation axis shift from 〈111〉 towards 〈110〉. These distorted twins then facilitate discontinuous dynamic recrystallisation. Newly recrystallised grains generally form via twinning phenomena and maintain a special orientation relationship to their neighbouring deformed grains by sharing a close to common 〈111〉 direction. At high deformation temperatures above 1130 °C, most pre-existing twin boundaries are fully transformed into recrystallised grains. This highlights the importance of twin boundaries as nucleation sites for recrystallisation. Furthermore, we demonstrate that automated methodologies are required to enable systematic studies of the interplay between recrystallisation and microstructural features such as Σ3 twins.

Multi-scale simulation of deformation and recrystallization of austenitic stainless steels with solidified columnar crystal structures

Columnar grain structures (CCS) often distinct in steel ingots, which have to be refined and homogenized during forging. In this investigation, simulation of deformation and dynamic recrystallization of austenitic stainless steels with CCSs were carried out by macroscopic, mesoscopic (microscopic) and nanoscale simulation techniques. (1) Using molecular dynamics simulation method, the nano-CGSs model with different loading directions was simulated. The results show that the deformation stresses are anisotropic with variation of angles between the loading direction and the columnar crystal growth direction. The higher stresses present at the 0° and 90° angles due to higher dislocation density; However, the lower stresses present at the angles from 30° to 60° due to higher stacking faults and twins. (2) The cellular automata (CA) fractal rules were proposed to simulate nucleation and grain growth of dynamic recrystallization by introducing weighted variables considering Σ3 twin nucleation rate for the twinning-promoted recrystallization. The CA method of nucleation at primary columnar crystal boundaries, secondary dendrites and deformation bands was proposed to simulate the joint nucleation. (3) The coupling simulation of macroscopic thermal parameters by finite element method, solidification of columnar structures and hot deformation dynamic recrystallization by CA was realized.

Effect of twin-related boundaries distribution on carbide precipitation and intergranular corrosion behavior in nuclear-grade higher carbon austenitic stainless steel

Based on different character distribution of twin-related boundaries (Σ3n, 1 ≤ n ≤ 3) introduced by special thermomechanical processing, the effect on carbide precipitation and the resulting intergranular corrosion (IGC) behavior was investigated in depth in nuclear-grade 316H stainless steel. It was worked out that the Σ3 twin boundaries with single distribution was difficult to suppress the IGC percolation along random high-angle grain boundaries, while the interconnected Σ3 twin boundaries could fully block the percolation channel and thus greatly improve the IGC resistance. Among which, the precipitation of M23C6 carbides at different boundaries plays key role to affect their ability of resisting IGC attack.