Coincident Site Lattice Boundaries Research Articles

On the grain boundary character evolution in non equiatomic high entropy alloy during hot rolling induced dynamic recrystallization

Grain boundary character evolution during dynamic recrystallization (DRX) in a non-equiatomic Fe40Mn40Co10Cr10 high entropy alloy has been examined systematically through single pass hot rolling of 30, 50 and 70 pct reduction. It was observed that the extent of DRX is highly dependent upon the hot rolling reduction. The recrystallization area fraction was considerably higher at 50 pct as compared to 30 pct and 70 pct rolling reduction. In recrystallized regions, a significant proportion of coincidence site lattice (CSL) boundaries (Ʃ≤29) were observed regardless of the rolling reductions. Interestingly, a higher proportion of CSL boundaries (particularly Ʃ3 and Ʃ9) were observed amongst the grain boundaries between recrystallized grains of last and second last necklace layer (L-S); and grain boundaries between recrystallized grains of the last necklace layer (L-L). However, most of the grain boundaries between last necklace layer and the parent deformed matrix (L-D) have been observed to be random high angle grain boundaries (HAGBs). Generation of significant fraction of second order twin boundaries (Σ9) along with first order twin boundaries (Σ3) adjacent to the recrystallization front (i.e. behind the migratinggrain boundary) is a consequence of multiple twinning processes expected to occur during DRX. Here, role of Σ3 twin boundaries on the recrystallization process has been discussed. Grain boundary character evolution and its influence on DRX in the presently studied High-Entropy Alloy (HEA) has been inferred to be similar to previously reported observations on conventional low stacking fault energy materials.

Effect of thermomechanical processing via rotary swaging on grain boundary character distribution and intergranular corrosion in 304 austenitic stainless steel

In the present work, the thermomechanical processing consisting of rotary swaging deformation and annealing treatment was performed to optimize the grain boundary character distribution (GBCD) of 304 austenitic stainless steel. To systematically investigate the effect of GBCD evolution on intergranular corrosion (IGC), double loop electrochemical potentiokinetic reactivation and electrolytic oxalic acid etch tests were employed. The experimental results show that the fraction of low-Σ coincident site lattice (CSL) boundaries increased from 58.1% to 74.0% for the specimen swaged to 0.06 true strain and then annealed at 1050 °C for 5 min duration. By characterizing the evolution of GBCD as a function of strain level in terms of low-Σ CSL boundaries fraction, average twin-related domain (TRD) size, average number of grain per TRD and fractal dimension of the maximum random boundary connectivity, the grain boundary engineering (GBE) microstructure was realized by the occurrence of prolific multiple twinning events during strain-induced boundary migration while static recrystallization has a detrimental effect on optimizing GBCD for the prolific of new strain-free grains with random boundaries. The IGC resistance of the GBE-treated 304 austenitic stainless steel is enhanced by inhibiting the nucleation and propagation of IGC cracks, resulting from the increase in the fraction of low-Σ CSL boundaries, especially Σ3 boundaries and the disruption of random boundary network connectivity.

The Effects of Microstructure Refinement and Coincidence Site Lattice Boundaries on the Plasticity of FeCrAlY Alloys

FeCrAlY alloys are widely used as thin‐gauge catalyst carriers for automotive exhaust gas purification. However, they exhibit low ductility due to coarse Fe–Y phases in the alloys, which decreases the carrier yield. To improve the plasticity of the alloys, a novel method for fabricating FeCrAlY alloys using twin‐roll strip casting (TRSC) is proposed. It is found that the plasticity of the FeCrAlY alloy is significantly improved by TRSC compared to conventional ingot casting. Then, to understand the mechanism of plasticity improvement, the microstructures of the alloys are investigated. The Fe–Y phases and grains are significantly refined by TRSC due to its characteristics of subrapid solidification. In addition, the TRSC samples produce several low‐Σ coincidence site lattice (CSL) boundaries, and some of these CSL grain boundaries are inherited by the cold‐rolled sheets. The impact characteristic of steel liquid in the TRSC is the major reason for the formation of CSL grain boundaries. The finer microstructure and greater number of low‐Σ CSL boundaries in the TRSC samples result in strong resistance to fracture, which improves the plasticity of the FeCrAl alloys.

A quasi-in-situ EBSD study of the thermal stability and grain growth mechanisms of CoCrNi medium entropy alloy with gradient-nanograined structure

The thermal stability and mechanical properties of a gradient-nanograined structure (GNS) CoCrNi medium entropy alloy (MEA) processed by ultrasonic surface rolling were studied by using isothermal/isochronal annealing tests combined with quasi-in-situ electron backscatter diffraction (EBSD) characterization and Vickers micro-hardness (HV) measurements. A layer by layer high-throughput investigation method was used to quantitatively study the grain growth kinetics and grain boundary evolution with different initial grain sizes, which could effectively save specimen and time costs. The grain nucleation and growth, as well as shrink and disappearance process through Σ3 coincidence site lattice boundary migration with slightly lattice rotation during annealing were directly revealed. The layer by layer grain growth kinetics and calculated activation energy indicate that the thermal stability of nano-grained top surface layer is relatively higher than that of nano-twined subsurface layer for the gradient CoCrNi MEA processed by ultrasonic surface rolling. Further analysis show that the grain boundary relaxation and dynamic recrystallization of the topmost nano-grains led to the decrease of grain boundary energy, thus improving their thermal stability. The present work provided theoretical basis for the application of CoCrNi MEA at high temperatures. Moreover, the high-throughput method on the investigation of grain stability by using gradient structure can be easily extended to other materials and it is of great significance for understanding the microstructural evolution of gradient materials.

A study on the creep behavior of alloy 709 using in-situ scanning electron microscopy

In this research, an experimental evaluation of creep properties of Alloy 709 in the temperature range of 750–850 °C was undertaken. Alloy 709 is a novel austenitic stainless steel with 20% Cr and 25% Ni by wt% that was developed for application in structural components of nuclear power plants. Creep rupture tests were conducted in an in-situ heating-loading and Scanning Electron Microscope (SEM) unit equipped with Electron Backscatter Diffraction (EBSD) detector and Energy Dispersive Spectroscopy (EDS). “Real-time” creep damage mechanisms of Alloy 709 at various stresses and temperatures using a flat, un-notched sample with continuously reducing cross-section is studied so that the failure and maximum creep damage occurred at the center of the sample where the in-situ SEM imaging could be focused. Accelerated creep tests at temperatures and stresses above service conditions were performed by employing multiple blocks of constant loads where the loads were increased once the sample attained constant creep rate, indicating a secondary creep regime. This technique ensures multiple data points can be obtained from the same test, saves the time required for an otherwise long-term creep test and usage of SEM. Coincident Site Lattice (CSL) boundary maps were collected as control maps before testing, and the grain boundaries were observed during the creep test to understand the effect of grain boundary character on the creep damage mechanism. Void growth, grain boundary separation, and sliding were found to be the main creep mechanisms whose rate is dependent on stress and temperature. Failure mechanisms studied on the fracture surface using SEM fractography were correlated to the sample surface observations to create complementary information to better understand the underline creep mechanism of Alloy 709.

Effect of nitrogen protection on weld metal microstructure and intergranular behavior of S32101 duplex stainless steel 15 m water depth hyperbaric laser underwater welding

In this study, 16-mm-thick S32101 duplex stainless steel were welded using hyperbaric underwater laser welding system in 0.15 MPa. The misorientation angle distribution of grain boundaries in the weld metal was analyzed using the electron backscatter diffraction technique. The ionic currents near the surface of metallography were measured by the Scanning Vibrating Electrode Technique (SVET) and performed to evaluate the intergranular corrosion (IGC) resistance based on corrosion current density. The volume fractions of ferrite and austenite and the grain boundary misorientation angle affect the IGC sensitivity of the welded joints. The relationship between the grain boundary misorientation angle of the austenite and corrosion resistance was analyzed. When the weld metal was shielded with nitrogen, and the volume fraction of austenite and the frequency of Σ3 coincidence site lattice boundary in the weld was increased. The misorientation angle distribution in the austenite and the frequency of Σ3 coincidence site lattice boundary influence the IGC resistance of the welded joints.

A comparative study of additive manufactured and wrought SS316L: Pre-existing dislocations and grain boundary characteristics

The additively manufactured (AM) SS316L shows superior mechanical properties compared to their conventionally manufactured (CM) counterparts. A quantitative comparison is conducted based on the pre-existing dislocations and grain boundary characteristics of AM and CM SS316L, found that the AM possesses more dislocations and significantly larger portion of low angle grain boundaries, resulting in different effective strengthening mechanisms. Coincident site lattice boundaries, yielding higher strength and better ductility, show much diverse distributions in the AM samples, with only very little Σ3. The AM grain boundaries show very distinct behaviors under tensile strain compared to the CM sample. This work conducted in-depth investigations on the strengthening mechanisms and shed a light on grain boundary engineering to further enhance the mechanical properties of AM materials.

Grain boundary plane rotation analysis for FCC bicrystal structures using MD simulation

Grain boundary (GB) plane rotation, one of the GB engineering mechanisms, was investigated using the activation–relaxation technique by molecular dynamics simulation. The simulation systems considered in this work are bicrystals with Lennard-Jones-type interatomic potential. The systems of four GB types established are symmetric (SYM), asymmetric (ASYM), symmetric zigzag (SZ), and high-angle zigzag (HZ) models. Of the first two models, [Formula: see text]5 (310) for SYM and [Formula: see text] tilted for ASYM particularly provided reference atomic potential energy distributions and structures at minimum energy state at 0 K. The characteristic of SYM is the discrete atomistic potential distributions which are distinct from ASYM. The other two models based on zigzag-like GBs were created by rotating GB planes about [001] at the center of a [Formula: see text]5 (310)[Formula: see text]GB system for the SZ case and a high-angle GB system for the HZ case. Simulation results show that the initially tilted GBs kinetically transferred to relaxed states for shorter-length GBs through a series of curved GBs. The GBs consist of different combinations of order defect segments, amorphous regions, and defect-free regions. A mechanism proposed is the GB plane rotation, the rate of which is structure-dependent. A low-[Formula: see text] coincidence site lattice boundary section can stabilize the systems at a specified metastable state.

Investigation of non-classical creep behavior of Inconel 617 alloy at 700 °C and 800 °C through interrupted tests and microstructural characterizations

The ‘non-classical creep behavior’ of the Inconel 617 alloy consisting of the miniscule primary and secondary creep regimes followed by a prolonged tertiary creep regime, has been investigated through interrupted tests at 700 °C/275 MPa and 800 °C/95 MPa, having nearly similar time to rupture (∼3500 h) to understand the role of microstructural evolution. A steady state tertiary creep region present at 800 °C/95 MPa, is found to be absent at 700 °C/275 MPa. The microstructural changes including formation of γ′ and fine secondary carbide precipitates, and twin boundary generation observed on interrupting the creep tests at both the aforementioned conditions after varying durations, have been investigated using optical, scanning and transmission electron microscopy, electron back scattered diffraction (EBSD) and thermal analysis. The formation of ample fine γ′ precipitates along with the fine secondary carbides in the samples subjected to creep tests at 700 °C with interruptions after varying fractions of time to rupture lead to increase in hardness, which is found to be lower for similar tests at 800 °C due to the absence of γ′ precipitates. Moreover, the EBSD analysis has confirmed a higher length fraction ratio of twin boundary (TB) to high angle grain boundary (HAGB) in the samples exposed to the steady state tertiary creep regime (almost 95% of total rupture time) at 800 °C. The increased TB to HAGB length fraction ratio is responsible for improving the creep resistance by delaying the damage accumulation, as the TBs are typical coincident site lattice boundaries with lower energy and greater packing density.

Superior strength-ductility synergy in a novel tailored nanoparticles-strengthened medium-entropy alloy

A novel f.c.c. (CoCrNi) 94 Al 3 Ti 3 medium-entropy alloy strengthened with an L1 2 phase was designed using CALPHAD. Hot rolling, cold rolling, and heat treatment were used to produce a fine grain size (0.61 μm) and a high volume fraction (0.37) of coherent 23.2 nm diameter L1 2 nanoparticles . This tailored microstructure exhibited an excellent strength-ductility synergy (yield stress∼1203 MPa, ultimate tensile strength∼1577 MPa, elongation ∼24%) and a high working hardening rate (3700 MPa at strain of 5%). The contributions from grain boundary strengthening and precipitation strengthening to the yield strength were estimated to be ∼61%. After the tensile tests, the fraction of LAGBs sharply increased and that of Σ3 n coincidence site lattice boundaries decreased: inverse pole figures indicate the formation of a {101}<111> texture. Such an ultrafine grain size suppressed deformation twinning, while applied stress led to a higher density of geometrically-necessary dislocations at the grain boundaries compared to the grain interiors.

Grain boundary engineering and its implications on corrosion behavior of equiatomic CoCrFeMnNi high entropy alloy

In the present study, strain-annealing based thermo-mechanical processing was employed to achieve a grain boundary engineered (GBE) microstructure in an equiatomic CoCrFeMnNi high entropy alloy (Cantor alloy) with a single phase, fcc structure. Cast, homogenized and recrystallized strips were cold rolled to 5%, 10% and 15% thickness reductions and annealed at temperatures from 1173 K to 1373 K for 1–6 h duration. Deformation twins were observed following cold rolling to 15%. From the deformed and annealed specimens, GBE microstructure was identified based on coincident site lattice (CSL) (Σ ≤ 29) boundary length fraction, number of twins per grain, triple junctions (TJs) character distribution and grain boundary plane orientations. Specimens rolled to 5% and annealed at 1223 K for 1 h exhibited GBE microstructure. The Σ3 fraction was enhanced from ~44% to ~62% in the GBE specimen with concurrent increments in TJs containing at least two CSL boundaries from ~20% to ~40% compared to as-recrystallized (AR) specimen. Potentiodynamic polarization studies revealed that the GBE specimen exhibited lower corrosion rate in both 0.1 M and 0.6 M NaCl solutions as compared to AR counterpart. The GBE specimen also displayed better passive film resistance due to higher polarization and charge transfer resistance, as evaluated from electrochemical impedance spectroscopy studies.

Influence of Welding Speeds on the Morphology, Mechanical Properties, and Microstructure of 2205 DSS Welded Joint by K-TIG Welding.

In this paper, 8.0 mm thickness 2205 duplex stainless steel (DSS) workpieces were welded with a keyhole tungsten inert gas (K-TIG) welding system under different welding speeds. After welding, the morphologies of the welds under different welding speed conditions were compared and analyzed. The microstructure, two-phase ratio of austenite/ferrite, and grain boundary characteristics of the welded joints were studied, and the microhardness and tensile properties of the welded joints were tested. The results show that the welding speed has a significant effect on the weld morphology, the two-phase ratio, grain boundary misorientation angle (GBMA), and mechanical properties of the welded joint. When the welding speed increased from 280 mm/min to 340 mm/min, the austenite content and the two-phase ratio in the weld metal zone (WMZ) decreased. However, the ferrite content in the WMZ increased. The proportion of the Σ3 coincident site lattice grain boundary (CSLGB) decreased as the welding speed increased, which has no significant effect on the tensile strength of welded joints. The microhardness of the WMZ and the tensile strength of the welded joint gradually increased when the welding speed was 280–340 mm/min. The 2205 DSS K-TIG welded joints have good plasticity.

Specific features of crystallographic texture formation in BCC-FCC transformation in extruded brass

Crystallographic features of body-centered cubic to face-centered cubic transformation were studied in an extruded brass product. X-ray phase analysis, scanning electron microscopy with electron microprobe analysis, and orientation microscopy based on diffraction of back-scattered electrons were used. A sample after treatment consisted of the β′-phase (Cu0.55Zn0.45, structural type B2) and α-phase (Cu0.65Zn0.35, structural type A1) at 30:70. Integral textures (within the product) of phases were axial with axes parallel to the elongation direction in extrusion:< 110 >for β’ and< 100 >for α. Local textures of phases are described as limited and consisting of a set of components. For β′ – {001}< 110 >, with scattering up to± 25° around the axis< 110 >; and {110}< 110 >consisting of three components – one {110}< 110 >and two {110} ± 24° around the axis< 110 >; for α – {001}< 100 >(strong) and two of {110}< 111 >(weak). It was shown that components of local textures of the β′- and α-phases were associated by orientation relationships close to Kurdjumov-Sachs orientation relationships. A small number of limited textural components formed as a result of β(BCC)→α(FCC) transformation realized as per multi-variant orientation relationships is explained by the formation of nuclei of a new phase at special boundaries close to coincidence site lattice boundaries Σ3 and Σ33.

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

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

A single grain boundary parameter to characterize normal stress fluctuations in materials with elastic cubic grains

A finite element analysis of intergranular normal stresses is performed in order to identify a possible statistical correlation between the intergranular normal stresses and the corresponding grain boundary type within a polycrystalline aggregate. Elastic continuum grains of cubic lattice symmetry are assumed in the analysis. Meaningful results are obtained by analyzing the first two statistical moments of grain boundary normal stresses obtained on several grain boundary types.Among the five macroscopic parameters (5D) describing a grain boundary, the orientation of the grain boundary plane relative to the two adjacent crystal lattices (4D) is found to be the most important property influencing the normal stresses. To account for it, a single new (1D) parameter E12 is introduced, which combines the geometrical aspect of grain boundary with its material properties and measures the average stiffness of grain boundary neighborhood along the grain boundary normal direction. It is demonstrated that E12, in combination with Zener elastic anisotropy index A, is able to accurately predict normal stress fluctuations on any grain boundary type in a material with cubic lattice symmetry. It is argued that largest normal stresses most likely form on grain boundaries whose normals are oriented along the stiffest direction in both adjacent grains (〈111〉 for crystals with A>1 or 〈001〉 for crystals with A<1). Moreover, it is shown that classification of grain boundaries according to their propensity to exhibit large normal stresses can be trivially reduced to the (analytical) calculation of the corresponding effective stiffness parameter E12.A few practical implications are discussed relevant to intergranular stress-corrosion cracking of Coincidence Site Lattice grain boundaries. For example, it is highlighted that in face-centered cubic materials the coherent Σ3 twin grain boundaries, which are known for their very high cracking resistance, nevertheless exhibit largest intergranular normal stresses, indicating that cracking resistance is associated with high grain boundary strength.

Microstructure Evolution and Texture Characteristics of Pure Nickel N6 During Cold Rolling Process

A single-pass large-deformation rolling method was used to study the effect of cold rolling degree on the microstructure, microtextural evolution, and grain orientation during the cold rolling of pure nickel N6. The results show that the microstructure evolves from micro-bands to lamellar-bands with the increase of cold rolling reduction, and the fraction of coincidence site lattice boundaries always decreases. Starting from an initially low imposed strain (20% of the cold rolling reduction), the deformed microstructure is mainly composed of Goss, rotated Goss, P, and Brass orientations. When cold rolling reduction exceeds 50%, the shear textures appear and other textures remain but their intensities decrease. The strong shear stress on the surface of the rolled plate causes the normal cold-rolled texture dominated by Cu{112} , S{123} , Brass{011} to rotate around the transverse direction to form a shear texture composed of shear4{112} and {110}∥RD fiber textures.

In situ scanning tunneling microscopy study of 2-mercaptobenzimidazole local inhibition effects on copper corrosion at grain boundary surface terminations

New insight on local inhibition effects of 2-mercaptobenzimidazole (MBI) on early stage intergranular corrosion of copper in hydrochloric acid solution is reported from in situ analysis at the nanometer scale and comparison with 2-mercaptobenzothiazole (MBT) effects in the same pre-adsorption and corrosion testing conditions. Macroscopic cyclic voltammetry analysis, including grains and grain boundary (GB) network, showed a passivation-like behavior in the Cu(I) oxidation range, specific to MBI since not observed with MBT and assigned to the anodic formation of a surface film of Cu(I)-MBI reaction products protecting against dissolution. Electrochemical scanning tunneling microscopy analysis revealed net intergranular dissolution, mitigated by the imperfect protection provided by the anodically formed MBI layer. It also showed local accumulation of reaction products in the GB surface regions, blocking preferential dissolution. For random GBs, blocking by local accumulation of reaction products was dominant, in agreement with the expected higher reactivity of these GBs generating more Cu(I) ions under anodic polarization and thus less efficiently protected by the anodically formed MBI layer. For Coincidence Site Lattice (CSL) boundaries, mitigated net dissolution was more frequently observed. Coherent twins showed equally efficient inhibition in the GB surface region than on adjacent grains. MBI inhibition was less efficient than MBT inhibition with more Cu(I) reaction products generated on the grains to form a surface film and their preferential local accumulation more frequently observed in the GB surface regions.

Improving mechanical anisotropy and corrosion resistance of extruded AA7075 alloy by warm cross rolling and annealing

The anisotropy of mechanical properties and poor corrosion resistance are important problems for high strength aluminum plate. In this study, the combined process of hot extrusion and warm cross rolling was conducted on AA7075 alloy, and both the extruded and rolled plates were annealed at various temperatures. The effects of warm cross rolling and annealing on the microstructure, mechanical properties, and corrosion behavior were studied. The results indicated that both cross rolling and annealing weakened the intensity of main textures of brass, Dillamore, and S. Compared to the extruded plate, the cross rolled ones exhibited higher hardness, higher conductivity, and lower anisotropy of tensile strength. Both cross rolling and annealing made the tensile strength of a plate along different directions more uniform. Moreover, annealing greatly enhanced the corrosion resistance by consuming the distortion energy, reducing the number of crystalline defects and increasing the fraction of low-Σ coincident site lattice grain boundaries. After annealing, the rolled AA7075 plates exhibited better corrosion resistance than the extruded ones, since they possessed lower fractions of brass texture, finer grains, and higher fraction of low-Σ coincident site lattice grain boundaries.

Microstructure and texture evolution during multi-direction forging of titanium

Abstract Wrought Titanium is used as load-bearing aircraft and automobile components due to superior specific strength, low density, and corrosion resistance. In this work, Titanium billets were subjected to open-die forging successively along mutually perpendicular axes at ambient temperature. Microstructure and texture evolution was investigated using optical microscopy and electron backscattered diffraction in a scanning electron microscope to understand deformation behaviour. The analysis revealed (i) the grain refinement through dislocation activities with the increase in geometrically necessary dislocations, (ii) evolution and interaction of considerable 10 1 - 2 extension and 11 2 - 2 contraction twins leading to the formation of coincident site lattice boundaries, and (iii) unusual but desirable texture. Strength of the material improved considerably after the ambient temperature multi-direction forging.

Evolution of Connectivity Among CSL Boundaries During Hot Plate Rolling of 316H Stainless Steel and the Effect on Intergranular Corrosion Resistance

Effects of the topological grain boundary network in hot rolled plates of 316H on intergranular corrosion (IGC) propagation were studied, which demonstrated that the connectivity among low-Σ coincidence site lattice (CSL) boundaries, instead of the CSL fraction, played the dominant role in arresting the IGC propagation. It was worked out that original dynamic recrystallized structures with homogeneous CSL boundaries and larger grain size were the efficient way to improving the CSL connectivity, which can be realized by optimizing the processing parameters during hot plate rolling.