Crystallographic Orientation Of Grains Research Articles

Effect of hot isostatic pressing and heat treatment on the evolution of precipitated phase and mechanical properties of GH3230 superalloy fabricated via selective laser melting

In this paper, the microstructure and mechanical properties of SLM-GH3230 superalloy treated by different post treatments (single hot isostatic pressing and heat treatments after hot isostatic pressing) were studied. The micro-morphologies show that the grains along building direction after HIP + HT are mainly columnar grains, which is consistent to that after single HIP. The granular precipitates distribute intergranularly and intragranularly after HIP, but gradually re-dissolve into matrix with the increase of the temperature of the post HT processes. XRD and EDS analyses results indicate that the matrix mainly consists of austenitic γ phase and the granular precipitates are mainly M6C carbides enriched in W element. The average grain size decreases after HT compared to that after HIP, and the crystallographic orientation of grains tends to be more uniform. The Vickers hardness decreases after HIP and HIP + HT processes within a certain degree. According to tensile test results, the sample shows a better combination of strength and plastic performance under HIP+1250 °C/30 min processing, which can also be verified by the characteristic of the fracture morphology with more tearing dimples.

Twinning-assisted static recrystallization and texture evolution in a Mg-Gd-Y-Zr alloy

The formability and mechanical properties of magnesium (Mg) alloys are strongly related to the crystallographic basal texture. Twins play critical roles in adjusting crystallographic orientation of grains during both deformation and annealing treatment via deformation twinning and twinning-assisted recrystallization. In this study, cold rolling and subsequent annealing were conducted on a Mg-5.9Gd-3.3Y-0.5Zr alloy to investigate the recrystallization behavior and texture evolution. Electron backscatter diffraction and transmission electron microscopy techniques were applied to characterize the nucleation of recrystallized grains, especially the twinning-assisted recrystallization, at multi-scales. The results indicated that a large number of {101‾2}, {101‾1} twins and {101‾1}‐{101‾2} double twins were introduced after cold rolling. The {101‾1}‐{101‾2} double twins, double twin - grain boundary intersections and dense twin-twin intersections acted as the preferential nucleation sites for recrystallization during annealing treatment, while the coarse and parallel {101‾2} twins were unfavorable for the nucleation of recrystallized grains. Although {101‾2} tension twins are the most common twins in Mg alloys, the interface of this type of twin has strong mobility and is easy to expand. Therefore, it is generally difficult for a single {101‾2} twin to recrystallize. However, the {101‾1} compression twins and {101‾1}‐{101‾2} double twins are generally difficult to expand and can store high deformation energy, so they are conducive to becoming nucleation sites for recrystallization. During the recrystallization process, the texture type (basal texture) of the cold rolled sample remained unchanged, but the overall texture intensity was significantly reduced due to the dispersion of grain orientations brought by new grains generated by twinning recrystallization.

Fitting the grain orientation distribution of a polycrystalline material conditioned on a Laguerre tessellation

The description of distributions related to grain microstructure helps physicists to understand the processes in materials and their properties. This paper presents a general statistical methodology for the analysis of crystallographic orientations of grains in a 3D Laguerre tessellation dataset which represents the microstructure of a polycrystalline material. We introduce complex stochastic models which may substitute expensive laboratory experiments: conditional on the Laguerre tessellation, we suggest interaction models for the distribution of cubic crystal lattice orientations, where the interaction is between pairs of orientations for neighbouring grains in the tessellation. We discuss parameter estimation and model comparison methods based on maximum pseudolikelihood as well as graphical procedures for model checking using simulations. Our methodology is applied for analysing a dataset representing a nickel–titanium shape memory alloy.

Factors governing surface exchange kinetics in undoped and strontium/iron co-substituted La2NiO4+δ

Surface oxygen exchange in the La2NiO4+δ and La1.5Sr0.5Ni1-yFeyO4+δ (y = 0.3, 0.4) oxides is analyzed using the data on oxygen permeability through the membranes with different thicknesses measured under various oxygen partial pressure P(O2) gradients in the 800–1000 °C range. The increase in P(O2) gradient induced surface limitations in La2NiO4+δ leading to a predominant role of surface exchange in the overall oxygen flux. The origin of surface exchange limitations in La2NiO4+δ is ascribed to a relatively fast decrease in oxygen excess and Ni3+ concentration with P(O2) reduction compared to La1.5Sr0.5Ni0.7Fe0.3O4+δ and La1.5Sr0.5Ni0.6Fe0.4O4+δ, which retained an oxygen excess. Faster surface exchange kinetics for La1.5Sr0.5Ni0.6Fe0.4O4+δ in comparison with that for La1.5Sr0.5Ni0.7Fe0.3O4+δ is interpreted on the basis of surface microstructure obtained by electron backscatter diffraction (EBSD). It is suggested that the observed changes in size, shape and crystallographic orientation of grains in La1.5Sr0.5Ni0.6Fe0.4O4+δ (compared to La1.5Sr0.5Ni0.7Fe0.3O4+δ) could result in a higher amount of 3d-metal cations in surface layers of the oxide.

Challenges related to tomographic reconstruction of 3D intragranular orientation fields in the presence of orientation relationships

X-ray diffraction contrast tomography (DCT) is a near-field diffraction imaging technique to characterize the 3D shape and crystallographic orientation of grains within polycrystalline samples. The presence of orientation relationships as such encountered between Σ3n annealing twins found in Cu and Ni lead to systematic diffraction spot overlap, since a significant fraction of the lattice planes is shared between the parent and twin crystal lattices. If not correctly addressed, these overlaps will lead to artifacts in the individual grain reconstruction. In this paper we introduce a strategy for joint, tomographic reconstruction of Twin Related Domains (TRD) and cost function weights into DCT to improve the grain reconstruction quality. A large-grained polycrystalline sample made from pure Ni is used for testing this approach and its ability to reveal intragranular orientation gradients related to plastic strain localization at the onset of plastic deformation in this type of materials.

Study on the micro-structure and the nano-indentation responses of laser solid formed epitaxial Ni-based superalloy

Laser solid forming (LSF) is one of the most promising technologies which has been extensively applied for the reshape and fabrication of the thermal parts in gas turbine and aero-engine. In this paper, the Ni-based single crystal superalloy thin-walled parts were prepared by using the LSF technology. Then, the electron back-scatter diffraction (EBSD) and the scanning electron microscope (SEM) technologies were employed to characterize the micro-structure morphology and crystallographic orientation of grains. The dislocation density of Ni-based superalloy thin-walled parts was analyzed. The results showed that a remarkable micro-structure gradient with respect to different heights of repaired Ni-based superalloy thin-walled parts was observed after the LSF process while the micro-structure distribution in width direction is comparatively more uniform. In addition, the misorientation of crystallographic orientation can be attributed to the dislocation density. A higher dislocation density contributes to aggregation of massive low angle grain boundaries (LAGBs) and formation of high angle grain boundaries (HAGBs), which in return conversely reduces dislocation density within the material. Finally, the mechanical properties of the LSF epitaxial Ni-based superalloy were investigated by the cyclic creep and Nano-indentation tests. The results showed that with the increasing of repair height, the residual indentation depth increased, while the elastic modulus increased first and then decreased. Meanwhile, a lower hardness of the stray grain region was obtained comparing with that of the substrate. Within the stray grain region, the hardness decreases along the building direction.

Crystal plasticity-based finite element modeling of twin transmission across grain boundaries in magnesium

For polycrystalline materials with the deformation mechanism of twinning, the interaction between the twins and grain boundaries are substantial to their deformation behavior. In the current work, the twin transmission across the grain boundaries in a magnesium bicrystal is investigated with the aid of crystal plasticity finite element model. The influencing factors, such as the crystallographic orientation of grains, GB misorientation, critical resolved shear stress (CRSS), dislocation slip, and twin-induced stress relaxation are considered. The commonly used geometric factors of macroscopic Schmid factor (MSF), geometric compatibility factor (GCF), and the lately proposed composite Schmid factor (CSF) are assessed by comparing the simulations and available experiments. The CSF can interpret the twin transmission better than either the MSF or the GCF. Moreover, the available twin transmission ability characterized in terms of the fraction of twin transmitted GBs is well captured by the CSF.

Influence of Si content on the microstructure and mechanical properties of silicon stainless steel

The influence of Si addition on the microstructure and mechanical behavior of cast silicon stainless steel alloys was investigated. It was recognized that after incorporation of Si in the range of ∼ 2–6 wt%, microstructure changes significantly. It transforms from single-phase austenitic to duplex microstructure containing ferrite and austenite. In addition, TiC precipitates with angular and spherical morphology were observed in all the sample conditions. Uniaxial tensile tests suggested that the incorporation of Si improved the strength and ductility as long as the microstructure remains primarily austenitic. However, at much higher silicon content (∼6 wt%), microstructure transformed to duplex structure, followed by the drastic reduction in ductility and toughness. Overall, the alloy with ∼4 wt% Si exhibited comparatively better strength, ductility as well as toughness. The mechanical properties of the individual phases, characterized using nanoindentation, indicated that the hardness and modulus of the austenite phase improved significantly after Si incorporation up to ∼6 wt%. Concomitantly correlation between the mechanical properties and crystallographic orientation of grains was also established by performing post-nanoindentation EBSD analysis. The results indicated that the average hardness and modulus of each phase are significantly related to the crystallographic orientation of grains, i.e., it is highly anisotropic. Finally, the nanomechanical properties of the individual phases were utilized to predict and compare the bulk mechanical properties using the ECM model. A reliable quantitative comparison between nanomechanical and bulk mechanical properties of alloys with single-phase microstructure was obtained, whereas a more refined ECMs are required for envisaging the bulk characteristics of the materials with duplex microstructure.

A comparison of the corrosion behaviour of copper materials in dilute nitric acid

The corrosion of a number of coarse- and fine-grained copper materials has been studied in an aerated aqueous nitric acid (100 mM) solution. The materials properties and the consequences of corrosion were characterized by scanning electron microscopy, electron back scatter diffraction and confocal laser scanning microscopy. The corrosion of the coarse-grained materials was shown to be strongly influenced by the crystallographic orientations of grains. For fine-grained materials grain boundary etching at high angle grain boundaries and grain boundary pores (for cold sprayed copper) overrode the influences of texture on the corrosion rate.

Influence of thermal cycling on the optical and the electrical properties of atmospheric pressure chemical vapor deposited tin oxide films grown using water and methanol vapors as oxidizers

Tin oxide films were chemically vapor deposited at atmospheric pressure at temperatures near 400 °C using SnCl4 as metal precursor and water or methanol vapors as oxidizers. The optical and electrical properties of these films were studied with spectroscopic ellipsometry (SE) and resistivity measurements while morphology and structure by scanning electron microscopy (SEM) and x-ray diffraction (XRD) measurements. It was found that both kinds of samples exhibited granular morphology with grain sizes of 10 and 80-100 nm for water and methanol deposited samples respectively. The crystallographic orientation of grains was similar for both kinds of samples. SE and resistivity measurements were simultaneously performed on samples up to 400 °C in air and at two thermal cycles. Using the Tauc-Lorentz combined with the Drude physical models the refractive index (real and imaginary parts) and the Tauc gap of samples were extracted from the SE measurements. Films morphology proved to be the decisive factor that governs optical and electrical properties of SnO2 films. So, large gained methanol vapor deposited SnO2 films were more than 80% transparent within the visible and an order of magnitude more conductive (resistivity of the order of 9-10 mΩ.cm) than the water deposited small-grained ones. It was also found that thermal cycling causes slight atomic re-arrangements in films that are invisible in XRD and SEM measurements but which influence significantly their optical and electrical properties.

Nanoscale Characterization of Stress Corrosion Cracking in a Failed Alloy C-276 Component

A failure analysis was performed on an Alloy C-276 pull rod which underwent unexpected brittle, intergranular fracture after exposure to 280°C to 300°C aqueous solutions designed to replicate secondary side environments in nuclear energy systems: Pb-containing alkaline (pH300°C 8.5 to 9.5), and sulfate-containing acidic solutions (pH280°C 3 to 5). The component was characterized using advanced electron microscopy methods to demonstrate the benefits of these techniques for determining the nanoscale chemical, mechanical, and material factors contributing to failure, and to provide insight into the mechanisms of stress corrosion cracking (SCC) responsible for failure. Site-specific transmission electron microscopy specimens containing crack tips were prepared using focused ion beam. Nanoscale chemical characterization methods revealed that Pb was present in some oxidized regions of cracks, suggesting that the element may be inhibiting or impairing the passivity of the Cr-rich oxide. Complementary nanoscale microstructural analysis was performed. At an intergranular to transgranular cracking mode transition, it was observed that the transgranular crack (and corrosion process) propagated along the (110) crystallographic plane. Also, the cracking mode was highly dependent on the tensile stress direction relative to grain boundary orientation, the crystallographic orientation of grains, and geometrically necessary dislocation structures. A comparison of results with proposed mechanisms for SCC of Ni alloys in similar environments are discussed; the highly directional nature of cracking is consistent with a slot-tunnel corrosion mechanism.

Texture-related biological properties of severely deformed titanium

Pure titanium manufactured by hydrostatic extrusion has been investigated with the aim to find the clear origin of superior biological properties observed in the materials processed by various severe plastic deformation techniques. In so doing, physicochemical properties of the surface were characterised as well as protein adsorption tests and cell culture examinations were thoroughly carried out. It has been found that, irrespective of the grain size, the basal planes exposed to the surface favour protein adsorption, whereas it is prismatic planes that experience pronounced cell activity on the surface of hydrostatically extruded titanium substrates. Biological behaviour of severely deformed titanium-based materials should be attributed to crystallographic orientation of grains, chemical composition of the surface as well as the presence of surface irregularities in the form of various nano-peaks or nano-grooves, while the effect of grain size might be of less importance.

Fundamental Investigation of Diamond Cutting of Micro V-Shaped Grooves on a Polycrystalline Soft-Brittle Material

Fabricating micro-structures on optical materials has received great interest in recent years. In this work, micro-grooving experiments were performed on polycrystalline zinc selenide (ZnSe) to investigate the feasibility of surface micro-structuring on polycrystalline soft-brittle material by diamond turning. A photosensitive resin was coated on the workpiece before cutting, and it was found that the coating was effective in suppressing brittle fractures at the edges of the grooves. The effect of tool feed rate in groove depth direction was examined. Results showed that the defect morphology on the groove surface was affected by the tool feed rate. The crystallographic orientation of grains around the groove was characterized by electron backscatter diffraction (EBSD), and it was found that the formation of defects was strongly dependent on the angle of groove surface with respect to the cleavage plane of grain. The stress distribution of the micro-grooving process was investigated by the finite element method. Results showed that the location of tensile stresses in the coated workpiece was farther from the edge of the groove compared with that in the uncoated workpiece, verifying the experimental result that brittle fractures were suppressed by the resin coating.

Magnetic Field-Assisted Chemical Vapor Deposition of UO2 Thin Films.

Chemical vapor deposition (CVD) of UO2 thin films from in situ reductive decomposition using a U(VI) precursor ([U(OtBu)6]) was performed under applied magnetic fields (up to 1 T). The molecular mechanism responsible for the formation of U(IV) oxide was determined by nuclear magnetic resonance (NMR) analysis of gaseous byproducts revealed a reductive transformation of uranium hexakis-tert-butoxide into urania. Thin films were grown under zero-field and applied magnetic field conditions that clearly showed the guiding influence of the magnetic field on altering the morphology and crystallographic orientation of grains in UO2 deposits produced under an external magnetic field. Application of magnetic fields was found to reduce the grain size. Whereas films with a ⟨111⟩ preferred orientation were observed under zero-field conditions, the application of magnetic fields (500 mT to 1 T) promoted a polycrystalline growth. X-ray photoelectron spectroscopy confirmed the formation of UO2 films with traces of U(VI) centers present on the surface, which was evidently due to the surface oxidation of coordinatively unsaturated U(IV) centers, which was found to be significantly reduced in the field-assisted process. These findings emphasize the positive effect of magnetic fields on controlling the texture and chemical homogeneity of CVD-grown films. The availability of a magnetic field as an extrinsic parameter for the CVD process adds to the conventional parameters, such as temperature, deposition time, and pressure, and expands the experimental space for thin-film growth.

Texture-Governed Cell Response to Severely Deformed Titanium.

The phenomenon of superior biological behavior observed in titanium processed by an unconventional severe plastic deformation method, that is, hydrostatic extrusion, has been described within the present study. In doing so, specimens varying significantly in the crystallographic orientation of grains, yet exhibiting comparable grain refinement, were meticulously investigated. The aim was to find the clear origin of enhanced biocompatibility of titanium-based materials, having microstructures scaled down to the submicron range. Texture, microstructure, and surface characteristics, that is, wettability, roughness, and chemical composition, were examined as well as protein adsorption tests and cell response studies were carried out. It has been concluded that, irrespective of surface properties and mean grain size, the (101̅0) crystallographic plane favors endothelial cell attachment on the surface of the severely deformed titanium. Interestingly, an enhanced albumin, fibronectin, and serum adsorption as well as clearly directional growth of the cells with preferentially oriented cell nuclei have been observed on the surfaces having (0001) planes exposed predominantly. Overall, the biological response of titanium fabricated by severe plastic deformation techniques is derived from the synergistic effect of surface irregularities, being the effect of refined microstructures, surface chemistry, and crystallographic orientation of grains rather than grain refinement itself.

Microvoid growth mechanism in FCC polycrystals and a statistical damage model

In polycrystalline materials, microvoid size is usually of the same order of magnitude as grain size. However, in most previous studies, matrix around microvoid is generally assumed to be isotropic and homogeneous, ignoring local heterogeneous polycrystalline microstructure. In this paper, microvoid growth under finite deformation in a face centered cubic (FCC) polycrystal is studied, focusing on the effect of local heterogeneous polycrystalline microstructure. Crystal plasticity finite element simulation (CPFEM) of polycrystalline representative volume element (RVE) is performed, with special attention to three important factors influencing the microvoid growth, i.e., external macroscopic stress triaxiality, internal crystallographic orientation of grains and ratio of microvoid size to grain size. The results indicate that the crystallographic orientation of grains has significant influences on the microvoid growth, especially when the stress triaxiality is not high. The local heterogeneous polycrystalline microstructure in polycrystals is found to retard the microvoid growth, suggesting that the traditional microvoid growth models based on the assumption of homogeneous isotropic matrix can significantly overestimate the microvoid growth. Besides, the size ratio of microvoid to the voided grain can heavily influence the microvoid growth, showing a strong size effect due to the first order heterogeneous deformation effect around the microvoid in heterogeneous polycrystals. Due to random grain-orientation distributions in polycrystals, the microvoid growth rate exhibits randomness and dispersion. To assess this dispersion under different stress triaxialities, a statistical microvoid growth model is proposed, which well envelops all the dispersed CPFEM results.

Observation of semisolid deformation by using 4D-CT and 3DXRD

Time-resolved X-ray tomography (4D-CT) and three-dimensional X-ray diffraction (3DXRD) were performed to investigate semisolid deformation in Al–10mass%Cu alloy. The configurations of solid grains were observed in-situ by 4D-CT and the crystallographic orientations of grains were also measured by 3DXRD. Translation and rotation of the solid grains in the semisolid are quantitatively analysed by the reconstructed 3D images and the diffraction spots. Based on the in-situ observation, we discuss about the motion of the solid grains and the formation of the shear band during the semisolid deformation.

Cellular automata modeling of the kinetics of static recrystallization during the post-hydroforming annealing of steel tube

A modeling setup based on the cellular automata (CA) method was developed to model the kinetics of static recrystallization (SRX) in hydroformed steel tubes undergoing the annealing process. In addition, the impact of multiaxial deformations on the kinetics of SRX within the hydroformed steel tube was investigated. First, hoop and axial strains developing at the pole of the steel tube during hydroforming were obtained from the digital image correlation measurements. Then, an exact analytical solution was employed to calculate the corresponding hoop and axial stresses at the pole of the bulged tube. Using these biaxial stress–strain curves, the stored energy was calculated for the hydroformed tube specimen. Second, the actual grain topology and crystallographic orientation of grains in the deformed specimen were obtained with the electron backscatter diffraction. Third, the calculated stored energy as well as the microstructural and crystallographic data was incorporated into the CA model as initial conditions to predict the kinetics of SRX in the specimen during annealing. Finally, to assess the accuracy of the CA model, experimental and predicted results were compared in terms of grain topology data including the grain size and aspect ratio distributions, as well as the rate of recrystallization during annealing. A reasonable agreement between the experimental and CA predictions in partial and fully recrystallized specimens was achieved inferring the validity of the developed algorithm and the modeling setup to predict the SRX kinetics during the post-tube hydroforming annealing process.

Anisotropic dislocation nucleation in ZrB2 grains and deformation behaviour of constituents of ZrB2-SiC and ZrB2-B4C composites during nanoindentation

Anisotropic slip activation of ZrB2 grains and deformation behaviour of constituents of ZrB2-SiC and ZrB2-B4C composites were investigated by nanoindentation. Samples were prepared by spark plasma sintering; crystallographic orientation of grains was determined by electron backscatter diffraction and deformation around indents in constituents was studied by scanning electron microscopy. Based on the Hertzian stress analysis of the measured strain bursts, so called pop-ins, the critical resolved shear stresses for slip system families of 101¯0112¯0, 101¯00001 and 101¯0112¯3 were determined for ZrB2. Homogeneous dislocation nucleation was revealed with similar critical resolved shear stresses (∼35 GPa) for each slip system close to the theoretical value. Based on the analysis of the orientation dependent maximal resolved shear stress, anisotropic dislocation nucleation was inferred in ZrB2 grains with single and simultaneous activation of the investigated slip systems. Macromechanical properties of the composites were correlated to nanohardness and indentation modulus of grains and grain boundaries.

Effect of microstructure on crack behavior in nanocrystalline nickel using molecular dynamics simulation

The crack growth process in columnar nanocrystalline samples is simulated using the molecular dynamics method. The effects of grain size, grain boundary, crystallographic orientation and crack tip position on the crack growth behavior are investigated. Different sets of samples with mean grain sizes ranging from 4 nm to 14 nm are prepared. Samples with a similar number of grains and identical dimensions are considered for examining the impact of grain boundary and crystallographic orientation. To assess the effect of the grain boundary, no constraint is considered on the position and orientation of grains, while only the grain orientations are changed to examine the effect of crystallographic orientation. Also, samples with scaled dimensions are created to study the effect of grain size on the crack growth process. In such samples, the relative position of seeds and crystallographic orientation of grains remain constant. The results show that as the mean grain size is reduced, a higher level of stress is required for the initiation of crack growth. Moreover, the reduction of grain size from 14.24 nm to 11.65 nm increases the critical stress intensity factor. Further reduction of grain size decreases the critical stress intensity factor. For grain sizes smaller than 11.65 nm, the volume of stacking fault in the grains near the crack-tip is not significant. Grain size and grain boundary volume have a simultaneous impact on the initial slope which can be observed around the grain size of 11.65 nm. As a result, in nanocrystals with mean grain sizes of less than 11.65 nm, the crack grows in a brittle manner to reach the grain boundary, then progresses intergranularly with high speed. Consequently, grain boundaries and crystallographic orientations are not effective on the crack growth mechanism in nanocrystals with grain sizes less than 11.65 nm, whereas these factors can blunt or arrest the crack in nanocrystals with mean grain sizes of larger than 11.65 nm.