Fraction Of Low Angle Grain Boundaries Research Articles

Texture and grain-boundary misorientation distributions in Y123 ceramics deformed by torsion under pressure

Texture and grain-boundary misorientation distributions in different parts of the sample of the high-Tc superconductor Y123 deformed by torsion under pressure at 1008 °C to a twist angle of 30° were investigated. The average levels of the basal plane texture (the Lotgering factor F) of undeformed and deformed samples were F = 0.25 and 0.965, respectively. In the undeformed sample, the fraction of low-angle grain boundaries (LAGBs) with misorientation angles from 2° to 10° does not exceed 5%. The deformation led to the formation of a strong, but non-uniform texture along the radius of the sample. In the center of the sample, a strong fiber texture is formed with the [001]-axis parallel to the axis of compression / torsion, and the fraction of LAGBs is about 20%. At a distance of 2 mm from the center of the sample there is a ring with a non-basal texture, where the fraction of LAGBs reaches 30%. A biaxial texture is formed at the edge of the sample: the [001] axis is parallel to the axis of compression / torsion, and the [110] axis is oriented along the radius of the sample. The fraction of LAGBs at the edge of the sample is 56%.

The effect of grain boundary structure on sensitization behavior in a nickel-based superalloy

The present work discusses the evolution of grain boundary structure during thermomechanical processing and its effect on sensitization behavior in a nickel-based superalloy. Alloy 600 was deformed to varied degrees of strain (4–25%) using hot rolling followed by annealing at 1000 °C for 10 min followed by water quenching. Structure of the grain boundary was analyzed with reference to various parameters, such as grain boundary character distribution, twin-related domains, misorientation, and triple junction distribution. Each thermomechanically processed sample was heat-treated at 650 °C for 24 h before studying its sensitization behavior. The effect of structure of the grain boundary on sensitization was assessed through double loop electrochemical potentiokinetic reactivation test and measured in terms of degree of sensitization (DOS). DOS was found to be in a direct relation with the fraction of random high-angle grain boundaries and their connectivity, while it was inversely related to the fraction of low-Σ coincidence site lattice boundaries and special triple junctions. Residual strain and the fraction of low-angle grain boundaries were found to be weakly related to DOS. We show that a simple parameter can be used to predict the combined effect of all these factors on sensitization behavior.

Influence of Porosity on Mechanical Behavior of Porous Cu Fabricated via De-Alloying of Cu–Fe Alloy

We report on a study of the mechanical behavior of porous Cu containing micron-sized pores and fabricated by de-alloying of a Cu–Fe precursor alloy. Our results show that the minimum volume fraction of pores that can be obtained by using an approach that involves de-alloying of a Cu–Fe precursor alloy is approximately 40 vol%. Moreover, the average pore size formed by de-alloying Cu–Fe of varying compositions is in the range of 1.5–4.0 μm. Our mechanical behavior results reveal that the yield stress increases from 3.9 to 58.6 MPa as the volume fraction of porosity decreases from 78.9% to 39.3%. Moreover, our data shows that the influence of porosity on the relative yield stress and relative Young’s modulus conforms to the scaling equations of Gibson and Ashby as formulated for open-cell porous metals. The pore cell characteristics and deformation modes of porous Cu produced by de-alloying Cu–Fe alloys were discussed in the context of the observed fluctuations in the value of the constants C and n in the Gibson-Ashby scaling equation. The evolution of microstructure during compressive deformation of porous Cu was studied and the results reveal an increase in the fraction of low-angle grain boundaries, an increase in the number of twins and a decrease in the average grain size with increasing strain from 0% to 70%.

Linear friction welding of a solid-solution strengthened Ni-based superalloy: Microstructure evolution and mechanical properties studies

A solid-solution strengthened Ni-based superalloy was welded with linear friction welding (LFW) under different process parameters. Microstructural examination of the sound joints showed a refined microstructure in the weld zone (WZ) and the thermo-mechanically affected zone (TMAZ), with limited oxide presence at the edge of the weld for certain welding conditions. The flow line feature was present in the WZ and TMAZ due to carbide rearrangement. However, the mechanical properties of the joints, i.e. microhardness and tensile strength, were higher than those of the base metal. The weld microstructure underwent a discontinuous dynamic recrystallization during LFW, which was accompanied by limited continuous dynamic recrystallization and static recrystallization, which lead to a remarkable increase in the fraction of low-angle grain boundaries (LAGBs) and a dramatic decrease of the Σ3 twin boundaries in WZ and TMAZ. In comparison to WZ, the TMAZ had a higher fraction of LAGBs because of the presence of stronger microtexture and less recrystallization.

Microscopic origin of highly enhanced supercurrent in 122 pnictide superconductor

By a combination of microstructure analysis techniques, we reveal the structural origin of the extremely high supercurrent (up to the practical level of 0.1 MA/cm2 at 10 T, 4.2 K) in Sr0.6K0.4Fe2As2 (122) tape. Transmission Kikuchi diffraction analysis reveals that hot pressing promotes a very high fraction of low-angle grain boundaries and texturing of the crystals, which is beneficial for the intergrain physical properties. Moreover, the unique characteristics of low-angle grain boundaries favor both long-range dislocations and short-range dislocations that totally change the pinning mechanism of the bulk 122 system. These defects combined with the grain texturing are not only effective for pinning vortices in the superconducting state, but also improve inter-granular supercurrent degradation, leading to substantially enhanced supercurrent over a wide range of magnetic fields.

Microstructural evolution and strain hardening behavior of AISI 316L type austenitic stainless steel

Abstract The microstructural evolution of AISI 316L austenitic stainless steel subjected to interrupted uniaxial tensile testing is investigated in relation to the strain hardening behavior. The microstructural evolution is characterized by means of scanning electron microscopy equipped with electron backscattered diffraction, optical profilometry and atomic force microscopy. The experimental results show that the increase in level of deformation causes increased surface roughness, extrusion height, plastic strain value, fraction of low-angle grain boundaries and geometrically necessary dislocations. The tensile work hardening analysis reveals a three-stage behavior, where the deformation mechanism is dominated by localization of plastic strain in slip bands due to dislocation–dislocation interactions in stage II and by rotation of austenite grains and dynamic recovery which is due to dislocation annihilations in stage III.

Effect of strain level on the evolution of microstructure in a recently developed AD730 nickel based superalloy during hot forging

Design and control of microstructure of engineering parts made from nickel based superalloys with superior mechanical properties for high temperature applications, require the parts to be subjected to certain thermo-mechanical processing during forging. This often includes sequential straining and annealing at elevated temperatures followed by subsequent aging heat treatments at lower temperatures. In this study, the effect of strain magnitude on the evolution of microstructure during hot forging of a recently developed AD730 nickel based superalloy has been investigated. Microstructural heterogeneity was observed in a forged material manifested in a form of large non-recrystallized grains within the recrystallized matrix that is observed to be dependent on the level of deformation (i.e. strain magnitude). Analyses of microstructure indicated significant reduction in the fraction of low-angle grain boundaries and sub-structures with an increase in the applied strain, suggesting higher fraction of recrystallization with higher levels of strains. It was concluded that the lower strain levels were insufficient to provide enough driving force for complete recrystallization throughout the entire microstructure of the forged material.

Enhanced grain refinement and microhardness by hybrid processing using hydrostatic extrusion and high-pressure torsion

An investigation was conducted to examine the microstructure and mechanical properties of an Al-5483 aluminium alloy subjected to a hybrid severe plastic deformation (SPD) process consisting of hydrostatic extrusion (HE) followed by high-pressure torsion (HPT) for up to 10 revolutions. The results are compared with those for samples processed separately by HE or by HPT. Microhardness measurements were taken on cross-sectional planes of the HE billets and on the HPT disks and in addition the microstructures were examined using transmission electron microscopy. The results demonstrate that the hybrid process of HE+HPT induces additional grain refinement when compared with HPT with average grain sizes of ~ 60 and 90nm, respectively. Also, a significantly higher fraction of high-angle grain boundaries (HAGBs) was present after HE+HPT and the beneficial role of HE pre-processing was also apparent in the microhardness measurements. After the hybrid process, the microhardness saturated at Hv ≈ 255 which is higher than after either HPT (Hv ≈ 235) or HE (Hv ≈ 160). A linear Hall-Petch relationship was maintained for coarse-grained and SPD-processed samples with high fractions of HAGBs (above 70%) while samples with higher fractions of low-angle grain boundaries showed a significant deviation from linearity.

Evolution of the microstructure during annealing of ultrafine-grained Ni with different Mo contents

Research was undertaken to evaluate the thermal stability of ultrafine-grained (UFG) Ni alloys with two different molybdenum (Mo) contents. The UFG microstructures in Ni samples with low (~0.28at.%) and high (~5.04at.%) Mo concentrations were achieved by the consecutive application of cryorolling and high-pressure torsion (HPT). The evolution of the microstructure during annealing up to ~1000K was studied by differential scanning calorimetry (DSC) at the peripheral regions of the disks processed by 20 turns of HPT. The grain size and the fraction of low-angle grain boundaries (LAGBs) were monitored by electron backscatter diffraction while the dislocation density was determined by X-ray line profile analysis as a function of temperature. It was found that the recovery of the UFG microstructures started at ~400K irrespective of the Mo content but recrystallization occurred at a much higher temperature for the Ni alloy with higher Mo content. During recovery, the LAGB fraction increased due to the arrangement of dislocations into low energy configurations. In the recrystallization process, the fraction of LAGBs decreased. After annealing up to ~1000K, the grain size remained much smaller for the sample with higher Mo concentration. Moreover, the larger Mo content yielded a separation of recovery and recrystallization processes in the DSC thermogram. It is concluded that the higher Mo concentration has a more pronounced hindering effect on recrystallization than on recovery.

Microstructure and texture evolution of TA32 titanium alloy during superplastic deformation

A study based on uniaxial tension tests were conducted in this paper to understand the hot deformation behaviors of TA32 alloy during superplastic deformation at a deformation temperature of 915°C and an initial strain rate of 6.64×10−3s−1. In the test, the maximum fracture elongation reached 1065%, exhibiting good superplastic deformation ability. Electron back-scattered diffraction (EBSD) was used to analyze and study evolution of microstructures and textures in the process of deformation with the following findings: During tensile deformation, the fraction of low-angle grain boundaries (<15°, LAGBs) decreased drastically, while that of high-angle grain boundaries (≥15°, HAGBs) increased from 45% to 96.2%. As the further straining, the role of continuous dynamic recrystallization (CDRX) was weakened, while that of discontinuous dynamic recrystallization (DDRX) was strengthened. Dynamic recrystallization helped refine coarse grains in the initial structures and reduced deformation defects in grains. The superplastic deformation mechanism of TA32 alloy was dominated by boundary sliding of high-angle grains, coordinated by dislocation motion at the softening stage and by grain rotation at the steady deformation stage respectively. In addition, the drawing force contributed to both dynamic recrystallization and rotation of grains.

Microstructure Stability of a Fine-Grained AZ31 Magnesium Alloy Processed by Constrained Groove Pressing During Isothermal Annealing

In this study, an AZ31 magnesium alloy plate was processed by constrained groove pressing (CGP) under three deformation cycles at temperatures from 503 to 448 K. The process resulted in a homogeneous fine grain microstructure with an average grain size of 1.8 μm. The as-processed microstructure contained a high fraction of low-angle grain boundaries (LAGB) of subgrains and dislocation boundaries that remained in the structure due to incomplete dynamic recovery and recrystallization. The material's yield strength was found to have increased from 175 to 242 MPa and with a significant weakening of its initial basal texture. The microstructure stability of the CGP-processed material was further investigated by isothermal annealing at temperature from 473 to 623 K and for different time. Abnormal grain growth was observed at 623 K, and this was associated with an increased in nonbasal grains at the expense of basal grains. The effect of annealing temperature and time on the grain growth kinetics was interpreted by using the grain growth equation, Dn+D0n=kt, and Arrhenius equation, k=k0 exp (−(Q/RT)). The activation energy (Q) was estimated to be 27.8 kJ/mol which was significantly lower than the activation energy for lattice self-diffusion (QL = 135 kJ/mol) and grain boundary diffusion (Qgb = 92 kJ/mol) in pure magnesium. The result shows that grain growth is rapid but average grain size still remained smaller than the as-received material, especially at the shorter annealing time.

Thermal stability and superplastic behaviour of an Al-Mg-Sc alloy processed by ECAP and HPT at different temperatures

An Al-3%Mg-0.2%Sc alloy was processed by ECAP and HPT at different temperatures. Afterwards, samples subjected to 10 turns of HPT at 300 and 450 K, 8 passes of ECAP at 300 K and 10 passes of ECAP at 600 K were annealed for 1 hour at 523 K and their mechanical properties and microstructure were examined using microhardness measurements and EBSD analysis. In addition, tensile specimens with similar dimensions were machined from the HPT and ECAP-processed materials and further tensile tested at 523 K. The results demonstrate that the Al alloy processed by HPT at 450 K exhibits higher microhardness values (~138 Hv) and a smaller average grain size (~0.28 µm) after annealing at 523 K among all SPD processing conditions. Accordingly, the material subjected to HPT at an elevated temperature displays superior superplastic properties such that an elongation of ~1020 % was attained after testing at 523 K at 1.0 × 10−3 s−1. Furthermore, detailed EBSD analysis revealed a significant fraction of low-angle grain boundaries (LAGBs > 33 %) in the ECAP-processed material after annealing which may be responsible for the inferior superplastic behaviour by comparison with the HPT-processed samples (LAGBs < 13 %) tested at 523 K.

Laser surface modification of 316L stainless steel.

Medical grade 316L stainless steel was laser surface melted (LSM) using continuous wave Nd-YAG laser in argon atmosphere at 1 and 5 mm/s. The treated surfaces were characterized using electron backscatter diffraction to study the influence of top surface crystallographic orientation and type of grain boundaries on corrosion resistance, wettability, and biocompatibility. The laser scan velocity was found to have a marginal influence on the surface roughness and the type of grain boundaries. However, the crystal orientation density was found to be relatively high in 1 mm/s samples. The LSM samples showed a higher concentration of {101} and {123} planes parallel to the sample surface as well as a higher fraction of low-angle grain boundaries. The LSM samples were found to exhibit better surface wettability and enhanced the viability and proliferation of human fetal osteoblast cells in vitro when compared to the untreated samples. Further, the corrosion protection efficiency of 316L stainless steel was improved up to 70% by LSM in as-processed condition. The increased concentration of {101} and {123} planes on surfaces of LSM samples increases their surface energy, which is believed to be responsible for the improved in vitro cell proliferation. Further, the increased lattice spacing of these planes and high concentration of low-energy grain boundaries in LSM samples would have contributed to the better in vitro corrosion resistance than untreated 316L stainless steel. Our results indicate that LSM can be a potential treatment option for 316L stainless steel-based biomedical devices to improve biocompatibility and corrosion resistance. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 569-577, 2018.

In-situ grain structure and texture evolution during friction stir welding of austenite stainless steel

The in-situ grain structure and texture evolution during the whole process of friction stir welding (FSW) are clarified. As the base material approached the probe, grains were compressed, forming compression zone (CPZ). The grains evolved to fine equiaxed grains mainly through discontinue dynamic recrystallization (DDRX) and twining as it approached the tool. As material in MFZ rotated around the probe, the fraction of low-angle grain boundaries (LAGBs) was reduced, the ratio of twin boundaries increased, and the grain size distribution became wider, while the average grain size was fairly constant despite the increase in strain. After the material was deposited behind the probe, the grains began to grow. Deformation caused by tool shoulder disrupted the twin boundaries and generated some segments of LAGBs. In the post-weld zone, some LAGBs disappeared and new twin boundaries developed as a result of elevated temperatures. The B component {112} 〈110〉 was developed and was sustained in the MFZ with its shear direction being consisted with the local probe rotation. The shear plan of B component maintained approximately 30deg. away from the probe surface. In the weld zone behind the probe, the B component was weakened and evolved to a C component {001}〈110〉.

Asymmetric microstructure and fracture behaviour of friction stir welded joints of 2024 aluminium alloy under cyclical load

In this study, the effect of asymmetric microstructure on fracture behaviour of friction stir welded joints of 2024-O aluminium alloy under cyclical load was investigated via optical microscopes, electron backscattering diffraction, microhardness and infrared thermography, and digital imaging correlation (DIC). The results demonstrate that the thermo-mechanical affected zone (TMAZ) at retreating side possesses a relatively higher fraction of low-angle grain boundaries and coincidence site lattice boundaries in comparison to that at the advancing side (AS), which results in the strain localisation and fracture path along the region between nugget zone and TMAZ at the AS under cyclical load. The profile of microhardness presents the asymmetric characteristic as well. The temperature evolution of the joint obtained by infrared camera is in good agreement with strain distribution measured by DIC. The thermography proves to be generally sufficient to predict the fracture path before failure under cyclical load.

Microstructure and Property Evolution for Hot-Rolled and Cold-Rolled Austenitic Stainless Steel 316L

Grain boundary character distribution plays an important role in determining the functional and mechanical properties of polycrystalline materials. The aim of this work was to achieve improved coincident site lattice (CSL) fraction without increasing low angle grain boundary (LAGB) proportion. We utilized single-step thermo-mechanical processing route involving rolling followed by short heat treatment and compared the effect of rolling temperature. Our results indicated that rolling at elevated temperature led to significant increase in the fraction of special boundaries while keeping the fraction of LAGB very low, as desired. We conducted thermal stability of our sample-conditions at elevated temperatures for various lengths of time and found the microstructure of the samples to be stable up to 1000 °C. This study showed that even commercially suitable process (single step processing with short heat treatment duration) could lead to microstructure with considerable increase in CSL boundaries fraction, improved hardness values and good thermal stability.

Precession electron diffraction for SiC grain boundary characterization in unirradiated TRISO fuel

Precession electron diffraction (PED), a transmission electron microscopy-based technique, has been evaluated for the suitability for evaluating grain boundary character in the SiC layer of tristructural isotropic (TRISO) fuel. This work reports the effect of transmission electron microscope (TEM) lamella thickness on the quality of data and establishes a baseline comparison to SiC grain boundary characteristics, in an unirradiated TRISO particle, determined previously using a conventional electron backscatter diffraction (EBSD) scanning electron microscope (SEM)-based technique. In general, it was determined that the lamella thickness produced using the standard focused ion beam (FIB) fabrication process (∼80nm), is sufficient to provide reliable PED measurements, although thicker lamellae (∼120nm) were found to produce higher quality orientation data. Also, analysis of SiC grain boundary character from the TEM-based PED data showed a much lower fraction of low-angle grain boundaries compared to SEM-based EBSD data from the SiC layer of a TRISO-coated particle made using the same fabrication parameters and a SiC layer deposited at a slightly lower temperature from a surrogate TRISO particle. However, the fractions of high-angle and coincident site lattice (CSL)-related grain boundaries determined by PED are similar to those found using SEM-based EBSD. Since the grain size of the SiC layer of TRSIO fuel can be as small as 250nm (Kirchhofer et al., 2013), depending on the fabrication parameters, and since grain boundary fission product precipitates in irradiated TRISO fuel can be nano-sized, the TEM-based PED orientation data collection method is preferred to determine an accurate representation of the relative fractions of low-angle, high-angle, and CSL-related grain boundaries. It was concluded that although the resolution of the PED data is better by more than an order of magnitude, data acquisition times may be significantly longer or the number of areas analyzed needs to be significantly greater than the SEM-based method to obtain a statistically relevant distribution. Also, grain size could be accurately determined but significantly larger analysis areas would be required than those used in this study.

Superplastic deformation mechanisms of high Nb containing TiAl alloy with (α2 + γ) microstructure

In this paper, superplastic deformation behaviour of a high Nb containing TiAl alloy with fine (α2 + γ) microstructure, Ti–43.5Al–8Nb–0.2W–0.2B (at.%), has been examined and studied by means of hot tension from 850 °C to 1050 °C under an initial strain rate of 10−4 s−1. The mechanical behaviour and microstructure evolution have been characterized and analyzed. Besides, to gain insight into deformation mechanisms, the texture evolution during deformation at ordinary (non-superplastic) and superplastic conditions has been systematically studied. The results showed that, the alloy exhibited impressive superplastic elongation at 1000 °C with a strain-rate sensitivity exponent (m) of about 0.5 and an apparent activation energy (Qapp) value of about 390 kJ/mol. The microstructural characterization showed that, when the alloy was deformed at ordinary condition (850 °C), severe grain refinement occurred and the fraction of low-angle grain boundary notably increased. Meanwhile, the textures were characterized by <100> and <111> double-fiber components parallel to the tensile direction. All these observations suggested a dislocation slip and twinning mechanism. However, if deformed at the superplastic condition (1000 °C), it was found that the microstructure was fairly stable in terms of grain size, morphology and grain boundary characteristics during tension, but a continuous weakening of the initial <110> fiber texture (resulted from canned-forging) was observed. This was believed to be an indication of grain boundary sliding mechanism. Moreover, the deformation texture (<100> + <111>)—though is very weak—was simultaneously appeared. According to a detailed discussion on the deformation kinetics and microstructure evolution, it was believed that the slip/twinning-accommodated grain boundary sliding was responsible for superplastic deformation and the dislocation climb inside of γ grains was the rate-controlling step.

The Microstructure Evolution and Processing Map of Ni-18.3Cr-6.4Co-5.9W-4Mo Superalloy During Hot Deformation

The processing map and microstructure evolution of Ni-18.3Cr-6.4Co-5.9W-4Mo superalloy were investigated in the temperature range of 1010-1160 °C and strain rate range of 0.001-1 s−1 by means of hot compression tests. According to the processing map, the optimum processing parameters were located in the temperature range of 1100-1160 °C and strain rate range of 0.01-0.3 s−1. The carbides in the alloy contributed to form a fine grain microstructure. In addition, four different instability criteria of Prasad, Gegel, Malas, and Murty were used to predict the unstable domains in the processing map. Meanwhile, the microstructure observations revealed that the unstable domain predicted by Murty’s instability criterion was very effective at high strain rate, which was the best choice for the studied alloy. Furthermore, the electron backscattered diffraction technique was used to detect the Σ3 boundaries and the adiabatic shear bands in the alloy. The results revealed that the fractions of Σ3 boundaries in the alloy deformed at 1100 °C/0.01 s−1 and 1160 °C/0.1 s−1 reached about 25 and 27%, respectively. Meanwhile, the fraction of low-angle grain boundaries and dislocation density were relatively high in the adiabatic shear band.

Microstructure and strength of nickel subjected to large plastic deformation at very high strain rate

A polycrystalline coarse-grained Ni with 98.4wt% purity was deformed plastically to the strain of 1.2 at the strain rate of ~104s−1 using a Dynamic Hopkinson Plastic Deformation technique. The microstructure investigations indicate the ability of this loading method to process very fine microstructure with high dislocation density and large fraction of low-angle grain boundaries. It turned out that there is a transition in the mechanisms which determine the flow stress during the impact test. In the beginning of deformation viscous dislocation drag dominates, while at the end of the impact test the velocity of the individual dislocations is low due to the high dislocation density, therefore the obstacle hardening determines the flow stress. Accordingly, the flow stress measured in-situ during impact test is in good agreement with the values determined ex-situ after deformation. The main contribution to the flow stress is given by dislocations.