Coincident Site Lattice Boundaries Research Articles

Clarifying the role of Ce in the corrosion behavior of Fe–36Ni Invar alloy

The effect of Ce on the corrosion behavior of Fe–36Ni Invar alloy was investigated in a 3.5 wt% NaCl solution. The proper addition of Ce enhanced the corrosion resistance. The second-phase particles were gradually transformed in type, size, and number following the addition of Ce, which alleviated the localized corrosion behavior. The appropriate Ce content increased the fraction of low coincident site lattice boundaries. Furthermore, the enhancement of protectiveness and compactness of the passive film was achieved. However, overadded Ce led to the degradation of corrosion resistance. 0.0170 wt% Ce was advised for the sake of the enhancement of corrosion resistance.

Deformation assisted [formula omitted]-[formula omitted] double twins with Σ23b and Σ15a coincident site lattice boundaries in magnesium

This work investigates the activation of 101̅1-(101̅2) double twins (DT) in the fatigue pre-cracked tip vicinity during fracture toughness tests in magnesium. Primarily, among the six {101̅1}101̅2 contraction twin (CT) variants, the high Schmid factor (SF) ones evolved by ∼56° lattice rotation about a specific 2̅110 axis. Within the CT domain, only two among the six {101̅2}101̅1 extension twins (ET) variants evolved with the same 2̅110 rotation axis as their primary CT, irrespective of their SF. Among these two ETs, the one with the highest SF evolved. The ET within CT with the same or opposite ∼86.4° lattice rotation about 2̅110 axis creates DT with ∼38°2̅110 i.e., Σ23b and ∼30°2̅110 i.e., Σ15a coincident site lattice boundaries, respectively.

Structural transformation behavior of oxide scale during coiling of 0.9 wt% Cr-containing high-strength steel

The phase transformation mechanism of oxide scale on the surfaces of chromium (Cr)-free carbon steel and 0.9 wt% Cr-containing steel at different coiling temperatures and cooling rates was studied using thermal simulation and electron backscattering diffraction experiments. The results showed that the eutectoid transformation of both the steel followed the “C” curve relationship. The Cr-containing steel had a larger eutectoid transformation area fraction than the Cr-free carbon steel. The nose tip temperature of the eutectoid transformation of the oxide scale on the surfaces of Cr-free carbon steel and 0.9 wt% Cr-containing steel were 420∼510 °C and 420–480 °C, respectively. For the 0.9 wt% Cr-containing steel, the proportion of Σ3 and Σ13b grain boundaries in cubic Fe3O4 increased, followed by a decrease, when the coiling temperature decreased from 610 to 410 °C. The Σ3 and Σ13b grain boundaries of the Fe3O4 low-dimensional coincident site lattice had the highest proportion for 510 °C coiling temperature. The low-dimensional coincident site lattice grain boundaries could be used to enhance crack resistance and improve the oxide scale on the steel plate surface.

Evolution of Grain Boundary Character Distribution in B10 Alloy from Friction Stir Processing to Annealing Treatment.

In this study, the grain boundary character distribution (GBCD) of a B10 alloy was optimized, employing thermomechanical processing consisting of friction stirring processing (FSP) and annealing treatment. Using electron backscatter diffraction, the effects of rotational speed of FSP and annealing time on the evolution of GBCD were systematically investigated. The GBCD evolution was analyzed concerning various parameters, such as the fraction of low-Σ coincidence site lattice (CSL) boundaries, the average number of grains per twin-related domain (TRD), the length of longest chain (LLC), and the triple junction distribution. The experimental results revealed that the processing of a 1400 rpm rotational speed of FSP followed by annealing at 750 °C for 60 min resulted in the optimum grain boundary engineering (GBE) microstructure with the highest fraction of low-Σ CSL boundaries being 82.50% and a significantly fragmented random boundary network, as corroborated by the highest average number of grains per TRD (14.73) with the maximum LLC (2.14) as well as the highest J2/(1 - J3) value (12.76%). As the rotational speed of FSP increased from 600 rpm to 1400 rpm, the fraction of low-Σ CSL boundaries monotonously increased. The fraction of low-Σ CSL boundaries first increased and then decreased with an increase in annealing time. The key to achieving GBE lies in inhibiting the recrystallization phenomenon while stimulating abundant multiple twinning events through strain-induced boundary migration.

The effects of grain boundary structure and bulk Cr content on grain boundary degradation behavior of Ni-Cr binary alloys in high temperature CO2

The effects of grain boundary (GB) structure and bulk Cr content (from 15 wt.% to 30 wt.%) on GB oxidation behavior of Ni-Cr binary alloys were investigated in 650°C/1atm CO2 gas. The degradation behaviors of GBs, which varies greatly (including preferential intergranular oxidation, general intergranular oxidation and GB passivation), is dictated by the available Cr source which depends on the bulk Cr content and the Cr diffusion flux to the reaction front. The Cr diffusivity along GB is dependent on GB structure and decreases in the order of random high angle GB (RHAB), low angle GB (LAGB), incoherent twin boundary (ITB) and coherent twin boundary (CTB). Interestingly, other coincident site lattice boundary (CSLB (Σ>3)) shows a Cr diffusivity between ITB and CTB. More importantly, the threshold bulk Cr content for achieving GB passivation is also closely related to GB structure. To be specific, LAGB completely passivates when the bulk Cr content is between 20 wt.% and 25 wt.%, while ITB completely passivates at a bulk Cr content of 25 wt.% and CSLB (Σ>3) completely passivates when the bulk Cr content is between 25 wt.% and 30 wt.%. In contrast, CTB is subject to general intergranular oxidation while RHAB shows passivation regardless of the bulk Cr content.

High strength, ductility and sheet formability by normalizing and quenching of low carbon microalloyed dual-phase steel

This work investigates the possibility of developing high strength, ductility, and normal-anisotropy in cold rolled (CR, ∼84%) 0.05 wt% carbon microalloyed dual-phase (DP) steel sheets by normalizing (N, 920°C, 30 min) followed by intercritical annealing and quenching (IA&Q, 80 0°C, 3 min). CR + N produces ferrite with ∼6.9μm grain size and ∼0.23 bainite fraction with ∼5.3μm packet size. A high engg. (true) strength of ∼605(∼798)MPa; uniform engg. (true) strain eU≈0.32 (εU≈0.28); engg. fracture strain, ef≈0.48; with high normal average anisotropy (r‾) of ∼1.84 and low planar anisotropy, Δr of ∼0.30 could be obtained. Further, IA&Q produced ferrite with ∼6.3μm average grain size, ∼0.25 martensite fraction (Ceq≈0.16) with ∼3.2μm island size. A high engg. (true) strength of 690(864)MPa; engg. (true) uniform strain of ∼0.25 (∼0.22); engg. fracture strain ∼0.39; and high r‾ of ∼1.51, low Δr of ∼0.31, suitable for sheet metal formability, were obtained. The high strength could be attributed to (a) fine ferrite grain size, (b) secondary phase fraction (∼0.23B/∼0.25M), packet size and island morphology, and (c) geometrically necessary boundaries (GNB). High ductility could be due to (a) fine ferrite grains with secondary phase morphology, (b) GNB, coincident site lattice boundaries (CSLB), and random high angle grain boundary (RHAGB) networking. Better anisotropic properties could be associated with strong γ-fiber, and suppression of α-fiber and θ-fiber texture. Loci in the stress and strain space indicate that the hydraulic bulge and plane-strain biaxial strength-ductility improves to 950MPa-0.41 and 1030MPa-0.67 after CR + N, and to 968MPa-0.30 and 1070MPa-0.58 after CR + N + IA&Q.

Interdependency between Electron Backscatter Diffraction-Based Misorientation Parameters and Coincidence Site Lattice Boundaries in 316LN Stainless steel

Interdependency between Electron Backscatter Diffraction-Based Misorientation Parameters and Coincidence Site Lattice Boundaries in 316LN Stainless steel

Grain boundary assembly in a 316 L steel produced by selective laser melting and annealing

The effect of recrystallization annealing on the grain boundary assembly of a 316 L-type austenitic stainless steel produced by selective laser melting was studied. The primary recrystallization readily developed upon heating to 1373–1473 K. Using the orientation imaging microscopy, the recrystallization development could be evaluated by means of the grain orientation spread and/or the kernel average misorientation. The recrystallization kinetics was mainly controlled by the grain nucleation rate rather than the grain growth rate. The recrystallized microstructures were characterized by large fractions of special Σ3n CSL (coincident site lattice) grain boundaries above 0.7 that was associated with a high number density of annealing twins. The change in the fraction of Σ3n SCL boundaries could be expressed by unique linear function of the recrystallized fraction irrespective of annealing temperature. The development of Σ3n CSL boundaries interrupted the network of ordinary grain boundaries, when the number of triple junctions with 3 ordinary boundaries decreased and that with 1 ordinary and 2 Σ3n CSL boundaries increased. An increase in the fraction of triple junctions with 1 ordinary and 2 Σ3n SCL boundaries clearly correlated with the recrystallized fraction.

Characterisation of Microstructure and Special Grain Boundaries in LPBF AlSi10Mg Alloy Subjected to the KoBo Extrusion Process

Grain boundary engineering (GBE) enhances the properties of metals by incorporating specific grain boundaries, such as twin boundaries (TB). However, applying conventional GBE to parts produced through additive manufacturing (AM) poses challenges, since it necessitates thermomechanical processing, which is not desirable for near-net-shape parts. This study explores an alternative GBE approach for post-processing bulk additively manufactured aluminium samples (KoBo extrusion), which allows thermo-mechanical treatment in a single operation. The present work was conducted to examine the microstructure evolution and grain boundary character in an additively manufactured AlSi10Mg alloy. Microstructural evolution and grain boundary character were investigated using Electron Back Scattered Diffraction (EBSD) and Transmission Electron Microscopy (TEM). The results show that along with grain refinement, the fraction of Coincidence Site Lattice boundaries was also increased in KoBo post-processed samples. The low-Σ twin boundaries were found to be the most common Coincidence Site Lattice boundaries. On the basis of EBSD analysis, it has been proven that the formation of CSL boundaries is directly related to a dynamic recrystallisation process. The findings show prospects for the possibility of engineering the special grain boundary networks in AM Al–Si alloys, via the KoBo extrusion method. Our results provide the groundwork for devising GBE strategies to produce novel high-performance aluminium alloys.

Effect of misorientation of grain boundaries on the discontinuous precipitates of Cu–Ni–Si alloy

In this study, the effects of differences in grain size, grain boundary type and misorientation on the initiation of discontinuous precipitates are investigated for the Cu–5Ni–1.25Si alloy. Samples with different grain sizes are obtained by thermomechanical treatment, and the smaller the grain size, the higher the percentage of discontinuous precipitate at the same aging time, while there is no significant difference in the growth rate of the reaction front of discontinuous precipitates. EBSD is used to study the effect of grain boundary misorientation on discontinuous precipitates, the characteristics of discontinuous precipitates initiation on different grain boundaries are investigated by multivariate statistical analysis. It is found that discontinuous precipitates only initiate from random grain boundaries instead of coincident site lattice boundaries, where random grain boundaries with misorientation angles of 30–50° are more prone to discontinuous precipitates. And the initiation of DPs at different misorientation axes is different. There is a clear borderline for the axes such as <102>, <103>, and <203>. The grain boundaries below this borderline do not initiate DP, while all grain boundaries below this angle initiate DPs. The cases for axes such as <112>, <114>, and <214> are more complicated. On these misorientation axes, grain boundaries with lower or higher misorientation angles do not initiate DPs, while grain boundaries at intermediate range misorientation angles do initiate DPs.

High strength-ductility combination by quenching and partitioning of a low carbon microalloyed dual-phase steel

This work investigates the quenching and partitioning of hot and cold rolled (HR & CR) low carbon micro-alloyed (Fe-0.05C-1.3Mn-0.4Si-0.6Cr) dual-phase (DP) steel. The quenching (Q) step involved intercritical annealing (IA) at 840°C for 60min and water quenching (200°Cs−1). This is followed by partitioning (P) treatment comprising of below critical annealing at 600°C for 15min and water quenching. The final microstructure after Q and Q&P contains ∼33% martensite within fine ferrite grains with lath for HR and island type martensite morphology for CR sheets. A high strength, ductility and isotropy combination of ∼544±4MPa UTS; 44%(n=0.30) &49% (n=0.32) and Lankford parameter (r‾)=∼1, respectively were obtained for the HR and CR conditions. The strength could be attributed to the equiaxed fine ferrite grain structure (4.3−6.5μm avg. grain size), secondary martensite phase, and dislocation density. The ductility could be a combined effect of the fine ferrite grain size, low dislocation density, geometrically necessary boundaries networking inside the grains, increased co-incidence site lattice boundaries (CSLB), TRIP effect, and texture weakening. ‘P’ treatment successfully stabilized the retained austenite in this lean alloy and induced the TRIP effect. The martensitic transformation during quenching led to ∑3, ∑11, ∑19b, ∑25b, ∑31b and ∑33c CSLB formation (predominantly ∑3), and partitioning treatment could further increase the CSLB density. The presence of CSLB led to the development of weak and partial α-, β-, γ- and θ- fiber texture in the final microstructure. The weak texture led to isotropy (r‾∼1).

Atomistic investigation of the impact of phosphorus impurities on the tungsten grain boundary decohesion

In the present work, we have generated a new second-nearest neighbour modified embedded atom method potential (2NN-MEAM) for the W–P system to investigate the impact of P impurity segregation on the strength of symmetric 〈110〉 tilt coincident site lattice grain boundaries (GBs) in tungsten. By incorporating the impurity-induced reduction of the work of separation in the fitting strategy, we have produced a potential that predicts decohesion behaviour as found by ab initio density functional theory (DFT) modelling. Analysis of the GB work of separation and generalized stacking fault energy data derived from DFT and the 2NN-MEAM potential show that P-impurities reduce the resistance to both cleavage and slip. Mode I tensile simulations reveal that the most dominant mode of GB failure is cleavage and that pristine GBs, which are initially ductile, on most accounts change to brittle upon introduction of impurities. Such tendencies are in line with experimentally observed correlations between P-impurity content and reduced ductility.

The vibrational entropy spectra of grain boundary segregation in polycrystals

The vibrational contributions to the energetics of grain boundary segregation are calculated for binary Ni polycrystals with solutes of Pd, Pt, Ag, Au and Cu. The computational challenge of the problem is to assess each different grain boundary site and produce full spectra of both site energies and entropies, which is addressed through a multiscale approach in which a full harmonic calculation is completed only in the vicinity of the solute atom, and a buffer region based on a local harmonic calculation effects connectivity to the bulk polycrystal. The resulting segregation free energy spectra illustrate the artificial nature of segregation temperature dependencies when the vibrational entropy is not considered, and highlight the importance of including the vibrational entropy spectrum for rigorous calculations. For a 5% solute loading level, for example, the magnitude of the artificial temperature dependence calculated ranges from 31 to 43% of the total effective segregation entropy for the various systems explored. Finally, a strong linear correlation between site segregation energy (which is not much affected by vibrational effects within the harmonic approximation) and vibrational entropy is observed in polycrystals, in agreement with prior works of coincident site lattice boundaries; this correlation promises significant simplification of segregation calculations in polycrystals generally.

Post-hot-deformation microstructure development in austenitic stainless steel

In the present work, austenitic stainless steel has been thermo-mechanically processed to study the post-deformation microstructure evolution at elevated temperatures. The samples were axially compressed (at a strain rate 0.1 s−1) and isothermally held at temperatures 900, 1000 and 1100 °C, for different time spans (2–1000 s). The electron back-scattered diffraction technique has been used to study the microstructure of the thermo-mechanically processed samples. The result shows for temperatures of 900 °C and 1000 °C, the resultant microstructures are comprised of deformed grains for samples held for 2 s. However, at 1100 °C sample isothermally held for 2 s shows a nearly complete recrystallization. The grain average misorientation approach has been used to estimate the softening fraction caused by recrystallization. The average grain size of the thermo-mechanically processed samples reveals grain refinement for the samples processed at 900 °C and 1000 °C. In addition to the grain refinement, the samples processed at 1000 °C shows a more consistent grain size. At all three temperatures, the increase in the holding time leads to a gradual decrease in the fraction of low angle boundaries and a simultaneous increase in high angle grain boundaries (HAGBs). The HAGBs are mainly contributed by Σ3 coincident site lattice boundaries. At the end, a discussion on post-deformation softening, considering strain-induced boundary migration, has been presented.

Grain Boundary Engineering and Its Effect on Intergranular Corrosion Resistance of a Ni-Cr-Mo Based C276 Superalloy

A Ni-Cr-Mo-based C276 superalloy was cold rolled to 5–40% and annealed at 1050 °C for 30 min. The microstructure and grain boundary character distribution after cold rolling and annealing were characterized. Grain refinement and a certain amount of coincident-site lattice (CSL) boundaries were obtained through recrystallization. The fraction of CSL boundaries reached peak at the cold rolling of 15% and annealing at 1050 °C for 30 min, which was the critical condition for completed recrystallization. In addition, sensitization treatments and double-loop electrochemical potentiokinetic reaction (DL-EPR) tests were applied to the cold rolled and annealed samples. The samples with a high fraction of CSL boundaries showed higher intergranular corrosion resistance as compared to the ones with a low fraction of CSL boundaries. It implies that the intergranular corrosion resistance of C276 superalloy can be enhanced by optimizing the grain boundary structure through cold rolling and annealing.

Beam oscillating parameters on pore inhibition, recrystallization and grain boundary characteristics of laser-arc hybrid welded AZ31 magnesium alloy

Oscillating laser-arc hybrid welding of AZ31B magnesium alloy was carried out, the effects of beam oscillation parameters on pore inhibition, microstructure, grain boundary characteristics and tensile properties were investigated. The results showed that the pore formation can be inhibited with oscillating frequency higher than 75 Hz and radius smaller than 0.5 mm. The columnar grains neighboring the fusion line can be broken by the beam oscillation behavior, while the grain growth was promoted with the increase of frequency or radius. It should be noted that the coincidence site lattice (CSL) boundaries were mainly Σ13b and Σ29 boundaries, which were contributed by {101¯2} tensile twins and {112¯2} compression twins, respectively. The total fraction of CSL boundaries reached maximum at radius of 0.25 mm and frequency of 75 Hz, which was also confirmed as the optimized parameters. In this case, the elongation rate increased up to 13.2%, 12.8% higher than that of the weld without beam oscillation. Finally, the pore formation and inhibition mechanisms were illustrated according to the state of melt flow and keyhole formation, the abnormal growth was discussed basing on secondary recrystallization, and the relationship among the pore formation, grain size, boundary characteristics and weld toughness were finally established.

Effect of Deep Cryogenic Time on Martensite Multi-Level Microstructures and Mechanical Properties in AISI M35 High-Speed Steel.

High-speed steel is widely used for cutting tools due to its convenience of preparation and cost-effectiveness. Previous research has shown that deep cryogenic treatments improve the mechanical properties of high-speed steel, due to the transformation of the residual austenite and the precipitation of carbide, while few studies have researched martensitic changes. The variations in martensite multi-level microstructures in AISI M35 high-speed steel, treated over different deep cryogenic time periods, were investigated in this study. Meanwhile, the effect of these variations on the mechanical properties of the selected steel was discussed. It was found that prolonging deep cryogenic time facilitated an increase in dislocation, low-angle grain boundary, and the coincident-site lattice boundary (especially the twin boundary) of martensite. The size of the martensite block (db) and lath (dl) decreased with deep cryogenic time. However, the effect on the microstructure was limited when the cryogenic treatment time exceeded 5 h. The increase in dislocation decreased the temperature for carbide precipitation and promoted fine carbide precipitation during tempering. The refinement of martensite multi-level microstructures and the greater precipitation of fine carbides gave the tempered specimens excellent impact toughness. The impact toughness of the tempered samples undergoing deep cryogenic treatment for more than 5 h was about 32% higher than the sample without deep cryogenic treatment.

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.