Distribution Of Grain Orientations Research Articles

Influence of compound field-assisted on the mechanical properties of 316L stainless steel fabricated by laser powder bed fusion

The advantages of field-assisted laser additive manufacturing in optimizing microstructure and performance are increasingly recognized. Recently, we investigated the effects of acoustic field (AF) assisted laser powder bed fusion (LPBF) on the mechanical properties of 316L stainless steel (SS) and 316L SS/WC composite materials. Due to the effects of acoustic streaming and cavitation, acoustic field-assisted LPBF achieved optimized mechanical properties. Building on our previous research on AF-assisted LPBF, we introduced magnetic field (MF) and compound field (MF+AF) to study their effects on the microstructure and mechanical properties of 316L SS, 1 wt% and 3 wt% WC/316L SS. Application of the MF induces induced currents in the melt pool due to the Thomson-Seebek effect. The interaction between the MF and induced currents generates thermoelectric magnetic force (TEMF) and thermoelectric magnetic convection (TEMC), which inhibit the growth of columnar grains and enhance mass and heat transfer in the melt pool. Compound field-assisted LPBF reduced the dislocation density of 316L SS, refined grain size, alleviated the concentration distribution of grain orientation, and the ultimate tensile strength of the samples reached 777 MPa and the elongation at break reached 57.5%. Interestingly, for WC/316L SS, both MF and compound field resulted in a decrease in tensile performance, with ultimate tensile strengths as low as 677 MPa and elongations at break as low as 25.1%. This is because the MF affects the uniform distribution of WC particles, causing significant agglomeration and severe unmelted defects in the composite material, leading to a decrease in tensile performance.

Application research on ultrasonic phased array detection algorithm for austenitic stainless steel with V-groove weld

Conventional ultrasonic detection methods are commonly used for detecting austenitic stainless steel weld structure. The anisotropy and inhomogeneity of the weld produces a strong scattering and energy attenuation effect on the acoustic wave. Controlling the accurate focusing of synthetic beams by ultrasonic transducer with multi-array element can be challenging, and difficult in detecting internal defects. A multi-group sequence grain model was established according to the distribution of grain orientation (DGO) in the weld, and then the “Velocity Coefficient K” algorithm was explored and developed, which is capable of quickly determining the acoustic wave propagation in the austenitic stainless steel weld with V-groove weld. By establishing an acoustic ray-tracing model with its analysis, the focusing law can be obtained quickly. The experimental results showed that the proposed algorithm has great effectiveness in detecting the signal-to-noise ratio (SNR) and imaging quality of defects.

Texture components and magnetic properties of laser powder bed fusion fabricated near grain-oriented and near non-oriented silicon steel

Silicon steel is a widely used soft magnetic material that requires different texture components for different applications, typically classified as grain-oriented or non-oriented. However, the methods of fabricating such types of silicon steel via laser-powder bed fusion (LPBF) have not been fully investigated. In this study, near grain-oriented and near non-oriented Fe-3.5 wt.%Si silicon steel is fabricated using LPBF by controlling processing parameters. Different textures are investigated using electron backscatter diffraction (EBSD), and the morphology of the molten pool is characterized by optical microscopy (OM) and scanning electron microscopy (SEM). Magnetic properties are measured with alternating current (AC) method. The results show that reducing both the linear energy density (LED) and laser power leads to a change in the side morphology of the molten pool from large, flat, and well-overlapped to small, protuberant, and less-overlapped, resulting in an extremely strong θ-fiber texture or a random distribution of grain orientations, respectively. Additionally, reducing both the laser power and scanning speed causes the top morphology of the molten pool to change from teardrop to elliptical shape at the trailing edge, resulting in a shift in the angle between the 〈001〉 of grains in the θ-fiber texture and the scanning direction from 45° to 30°. Samples with fewer defects (i.e., larger grain size and fewer pores) and a larger area fraction of 〈001〉//H exhibit higher permeability, although this superiority is not so significant due to residual stress and high dislocation in the as-built samples. This study provides insight into the relationship between processing parameters, texture evolution, and magnetic properties in LPBFed silicon steel.

Mechanical ordering of pigment crystallites in oil binder: can electron paramagnetic resonance reveal the gesture of an artist?

Is it possible to reconstruct the gesture of an ancient artist applying a paint layer, considering that the orientation distribution of crystallites of an inorganic pigment remains definitively imprinted on the support after drying of the layer? If the pigment contains paramagnetic transition metal ions whose magnetic interactions are themselves anisotropic, then the shape of the electron paramagnetic resonance (EPR) spectrum should reflect the distribution of grain orientations. We have demonstrated this effect in the case of Egyptian blue CaCuSiO, a pigment used for at least 3 millennia in antiquity, by reconstructing the probability density of crystallite orientations under various modes of application, such as brush painting, dabbing and droplet deposition.

Microstructure sensitive fatigue life prediction model for SLM fabricated Hastelloy-X

A microstructure-sensitive fatigue life prediction framework based on CP-FFT is proposed to study SLM fabricated Hastelloy-X. The microstructure enters in the model through the shape, size and orientation distributions of grains in the RVEs, which are generated from experimental EBSD data. The framework has been applied to specimens built in two different directions with different polycrystalline microstructures and is able to accurately predict their fatigue life, using just two experiments for calibration. The model reproduces the better fatigue performance found experimentally at high stresses for samples built in transverse direction, identifying the origin of this anisotropy in grain aspect ratios.

Influence of arc mode on the microstructure and mechanical properties of 5356 aluminum alloy fabricated by wire arc additive manufacturing

In this paper, two arc modes of gas shielded metal arc welding (GMAW), a new pulsed mode (microMIG) and a non-pulsed mode (microMIG-CC), were used to melt ER5356 aluminum alloy wire for thin-wall fabrication. The influence of both arc modes on the melt–drop transition pattern, thin–wall properties, porosity, microstructure and mechanical properties was investigated at the same heat input. The microMIG mode adds SKS-based current time based on conventional pulsed GMAW for short-circuiting the transition, which leads to a faster spread of melt pool temperature. The rapid cooling of the melt pool contributed to the increase of grain nucleation rate and inhibited grain growth, resulting in smaller grain size in the microMIG mode specimens. In addition, the low porosity of microMIG specimens is attributed to the high voltage of the micro-MIG mode resulting in an unfocused heat source, which leads to a shallow penetration depth of the specimens and a reduced hydrogen escape distance. Compared with the microMIG-CC mode, the microMIG mode specimens have smaller grain size and lower porosity, therefore higher tensile strength is obtained. The isotropic property of thin walls is due to the existence of equiaxed grains and the random distribution of grain orientation. However, the aggregation of dislocations and the increase of precipitates make microMIG-CC have higher hardness. Overall, the mechanical properties of the thin wall specimens of WAAM 5356 aluminum alloy are higher than those of the as-cast specimens.

Modeling the micromechanical behaviors of Zircaloy-2 alloy under large deformation

The orientation distribution of grains (so-called texture) and its evolution almost decisively govern the in-service performance of a polycrystalline material, especially for materials with low crystallographic symmetry. As an important nuclear material, zirconium and its alloys have a hexagonal-close-packed (hcp) crystallographic structure and multiple plastic deformation mechanisms. Among them, deformation twinning, reorienting the basal poles 85.22°, could result in an abrupt and drastic change in the texture. As simulation plays a more and more important role in the design, manufacturing, and service of an end-product, a reliable twinning model is essential for zirconium alloys. Two twinning models, namely the Twinning and De-twinning (TDT) and Predominant Twin Reorientation (PTR) models, were implemented in an Elastic Viscoplastic Self-consistent (EVPSC) framework with the thermal effect accounted for. The EVPSC-TDT and EVPSC-PTR models were applied to study the deformation behavior of a Zircaloy-2 slab under different strain paths. A predictive capability of the two twinning models was assessed by both the macro-mechanical behavior of stress-strain response and Lankford coefficient and the micro-mechanical behavior of texture evolution in terms of both pole figure and texture coefficient. It was found that the TDT model exhibits superior performance to the PTR model, especially in predicting texture development.

Particle size dependent microstructure evolution of inert gas atomized nickel base superalloy powders

Powder of an experimental Nickel base superalloy produced through Inert Gas Atomization technique is characterized for its physical properties, chemical composition, morphology, micro-segregation and orientation distribution of grains. The atomized powders exhibits near spherical morphology with average particle size of 61 µm. Micro-chemical analysis reveal, elements like Ti, Mo, Ta and Hf partition at inter-dendritic regions, irrespective of particle size.The extent of micro-segregation however is found to be intimately related to the cooling rate and particle size. The EBSD analysis reveals random texture in the particles with the number of grains per particle decrease with decreasing particle size. The nucleation event is found to be initiated on particle surfaces as evidenced from the dominant presence of the Low Angle Grain Boundaries (LAGBs) near the particle surfaces. The selective growth of favourably oriented nuclei are observed towards the core of the particle and characterized by presence of High Angle Grain Boundaries (HAGBs) in the particle interior. A mechanistic understanding correlating evolution of grain size distribution and micro-segregation with droplet solidification process, has been elucidated through the present study.

A Geometry Optimization of Spiral Grain Selector During Directional Solidification of Nickel‐Based Superalloy

A spiral grain selector with excellent geometric parameters can ensure qualified single-crystal (SX) is selected while decreasing lesser thermal gradient (G) during casting of SX nickel-based superalloy . In this paper, three configurations of spiral grain selectors with different height of starter block (H = 18, 25, 35 mm) and same spiral passage are adopted. Based on the quantitative analysis of orientation distribution of grains in starter block, it is found that the deviation of <001> oriented grains in the projection zone of spiral passage inlet (circle with a diameter of 4 mm) are smaller than grains of overall cross-section (circle with a diameter of 10 mm), which verify that starter block with smaller diameter have better optimization effect of grain orientation. Moreover, the average deviation of <001> oriented grains decreases to a range of 6.6–10.8° when the height of starter block is 18–27 mm, and SX selection is completed on the position of less than half turn in spiral passage regardless of the number of grains in the inlet position. Finally, a modified spiral grain selector is designed from the perspective of practical production, of which total height is reduced by at least 15 mm, and can decrease lesser G at some extent during casting .

Dislocations, texture and stress development in hydrogen-cycled Pd thin films: An in-situ X-ray diffraction study

For Pd thin films, microstructural changes involved during hydrogen cycling provide the information needed to predict and optimize the film's mechanical strength. In this paper, a systematic study of the morphology, microstructure, texture, and stress has been performed on Pd thin films during hydrogen loading and deloading cycles at room temperature. Pd thin films of similar morphology were prepared by magnetron sputtering on substrates of different compliances, i.e., Si-oxide, Titanium (Ti) and Polyimide (PI). The evolution of the morphology, grain-orientation distribution (texture), state of stress, and dislocation densities are analyzed for each of the film substrate types for 20 hydrogen loading/deloading cycles. The lattice expansion and contraction caused by the transition from Pd to Pd-hydride and back result in a strong stress increase. This stress increase stabilizes after a few cycles by grain boundary motion that leads to a gradual enhancement of the (111) texture and changes in the dislocation density for Pd films that are strongly clamped on to an oxidized Si(100) wafer substrate with an intermediate layer (Ti or PI). For Pd on PI, the stress is also partly released by a crack-based (crack widening/growth/propagation) pathway. Pd films on Ti and PI do not buckle or blister after 20 hydrogen cycles. By providing a sufficiently compliant substrate the traditional problems of buckle-delamination of a film on a stiff substrate are mitigated. • Pd thin films: microstructure and mechanical strength during hydrogen cycling studied. • Cyclic deformation resulted in irreversible texture changes − sharpening of (111)-fiber texture. • An open morphology restricts internal stress from amplifying during hydrogen uptake. • All Pd films contain similar kinds of defects, that differ only by their concentration. • The flexible PI inter-layer provides enhanced adhesive support, preventing strain localization and film delamination.

A deep learning method for predicting microvoid growth in heterogeneous polycrystals

In heterogeneous polycrystals, microvoid growth presents inherent randomness and dispersion, which generally follows a statistical law. This statistical characteristic intrinsically arises from randomly distributed grain-orientations around microvoids. It remains a huge challenge to explicitly depict the inherent correlation between microvoid growth and grain-orientation distribution by conventional deterministic damage models. In recent years, deep learning has gained in popularity in materials science and has been demonstrated to exhibit excellent data-mining abilities. To our best knowledge, deep learning has not been applied to investigate the statistical damage-evolution issues hitherto. In this work, a novel microvoid growth model based on deep neural network is creatively designed, incorporating both convolutional and long short-term memory components. The former extracts the spatial grain-orientation information, and the latter captures the causal effect of strain history on the microvoid growth. Moreover, to train and test the deep learning-based model, a microvoid-growth database is generated through a large number of crystal plasticity-based finite element simulations, incorporating randomly-oriented grains and different void locations. All the sample data (i.e., the grain-orientation distributions, microvoid locations and microvoid-growth curves) are processed by specific methods (e.g., the pixel-based method) to be amenable for the training process. Our results show that this novel model well captures the statistical characteristic of the microvoid growth in heterogeneous polycrystals. It is expected that the deep learning-based method can provide a new way to predict the microvoid growth at the grain-level.

COMPUTATIONAL STUDY OF EVOLUTION AND FATIGUE DISPERSITY OF MICROSTRUCTURES BY ADDITIVE MANUFACTURING

In order to predict the correlation among the processing parameters, microstructures, and mechanical properties for additive manufacturing, a computational framework integrating discrete element method, phase-field simulation, crystal plasticity finite element method, and extreme value statistics is proposed. The framework is applied to reveal the influence of laser scanning velocity on the microstructure evolution, yield stress, fatigue indicator parameter (FIP) distribution, and fatigue dispersity, in order to show its capability in simulating additive manufacturing process and the resultant mechanical properties. Firstly, discrete element method simulations are carried out to spread the powder bed layer by layer with the consideration of powder size distribution. The spreading is performed on the curved surface of the previously solidified layer. Secondly, the heat-melt-microstructure coupled non-isothermal phase-field simulations are performed to obtain the temporal and spatial evolution of melt, pore, grain boundary, grain distribution/orientation, etc., as well as the final polycrystal microstructure. Thirdly, crystal plasticity finite element method is utilized to attain the macroscopic mechanical response and stress/strain distribution of the additively manufactured polycrystal microstructure (AMPM) and FIP which is a surrogate measure for the driving force to form fatigue cracks. Fourthly, extreme value statistics are carried out to analyze the extreme value distribution of FIPs and the fatigue dispersity of the AMPM. Comprehensive simulations are put into practice for the selective laser melting based additive manufacturing of a typical metallic material 316L stainless steel. The simulation results indicate that the macroscopic yield stress of the AMPM is anisotropic and decreases with the increasing laser scanning velocity. The extreme value of FIPs from the AMPM with random distribution of grain orientations correlates well with the Gumbel extreme value distribution. The increase of laser scanning velocity could decrease the fatigue dispersity of the AMPM, but increase the FIP extremum and the associated driving force for fatigue crack initiation and thus notably decrease the fatigue life.

Microstructural differences between naturally-deposited and laboratory beach sands

The orientation of, and contacts between, grains of sand reflect the processes that deposit the sands. Grain orientation and contact geometry also influence mechanical properties. Quantifying and understanding sand microstructure thus provide an opportunity to understand depositional processes better and connect microstructure and macroscopic properties. Using x-ray computed microtomography, we compare the microstructure of naturally-deposited beach sands and laboratory sands created by air pluviation in which samples are formed by raining sand grains into a container. We find that naturally-deposited sands have a narrower distribution of coordination number (i.e., the number of grains in contact) and a broader distribution of grain orientations than pluviated sands. The naturally-deposited sand grains orient inclined to the horizontal, and the pluviated sand grains orient horizontally. We explain the microstructural differences between the two different depositional methods by flowing water at beaches that re-positions and reorients grains initially deposited in unstable grain configurations.

Prediction of Grain Formation and Defects during Solidification in Single Crystal Superalloy Castings

Numerical simulation and prediction of grain formation and defects, including the stray grain and high angle orientation deviation during directional solidification process of a single crystal superalloy hollow turbine blade are experimentally conducted by means of commercial software ProCAST and backscattering scanning electron microscope. The results show that the initial nucleation amount at the beginning section of the starter block is 104 of magnitude, and the number of grains decreases gradually with the competitive growth, and the number is about 100 at the spiral of the selector. And the orientation distribution of grains is close to &lt;001&gt; direction, with the orientation deviation between 10° and 15°. Moreover, with the increase of withdrawal rate, the curvature of isoline of liquidus of single crystal blade increases, and the tendency to form stray grains defects increases. The grain with a large deviation from orientation blocks the growth of other grains at the first rotating transition site of the selector, and then gradually grows and solidifies to form the final blade.

Grain-scale strain heterogeneity as a function of crystallographic texture in phase-change electro-mechanical ceramics

A reduction in the orientation distribution of grains, via crystallographic texturing, is known to significantly enhance the electro-mechanical response in polycrystalline piezoelectrics. Through calculations of the electric-field-induced strain in phase-changing ceramics, the local strain heterogeneity was also found to increase with crystallographic texture. The increase in grain-scale strain heterogeneity potentially indicates greater stress concentrations within the polycrystal and may degrade the expected lifetime and fatigue behavior in these ceramics. In crystallographic symmetries with higher numbers of polarization directions, it was found that the magnitude of heterogeneity in the response was lower.

Analysis on the microstructure regulation based on the pulsed laser oscillating molten pool in Laser-PTA additive manufacturing

A technological method of pulse laser assisted plasma transferred arc (Laser-PTA) additive manufacturing was proposed. The characteristics of microstructure such as austenite morphology, grain size and grain orientation were investigated through Scanning Electron Microscope (SEM) and Electron Backscattered Diffraction (EBSD) test. In particular, the mechanisms of pulsed laser affecting the molten pool and regulating the microstructure were revealed by the analysis on the temperature and dynamic response of molten pool. The experimental results show that an appropriate intensity of pulsed laser promotes the distribution uniformity of austenite, grain refinement and porosity reduction. The microstructure is transformed from lath-shaped austenite into skeleton-shaped austenite in deposition layer manufactured by Laser-PTA additive manufacturing. Furthermore, distribution of grain orientations in deposition layer became more random compared with those manufactured by PTA additive manufacturing. Under the excitation of pulsed laser, the molten pool oscillates in the boiling phase and oscillating phase, which strengthens the molten fluid flow. The enhanced fluid flow could reduce the temperature gradient and change the direction of the maximum temperature gradient, furthermore break the dendrites to form new heterogeneous nucleation particles. Therefore, the main factor causing the microstructure change is the oscillation of molten pool induced by pulsed laser.

Electron Backscatter Diffraction for Investigating Lithium-Ion Electrode Particle Architectures

Summary The performance and degradation of layered cathode materials for lithium-ion batteries depend on their morphological, surface, and crystallographic properties. A comprehensive tool to spatially characterize grain geometry and orientation in electrode particles is needed in order to understand solid-state lithium transport and the efficacy of particles to avoid lithiation heterogeneities and strain-induced degradation. Here we apply electron backscatter diffraction on LiNi0.5Mn0.3Co0.2O2 particle cross-sections to spatially describe intra-particle grain architectures. A method for segmenting, labeling, and quantifying morphological properties of distinct grains is developed and applied. Crystallographic orientations are measured for each grain, and the spatial distribution of grain orientations is quantified with a specific focus on describing lithium transport and accounting for inter-grain lithiation barriers. The necessary extension from two-dimensional to three-dimensional descriptions of grain morphologies and orientations to predict lithiation inefficiencies is discussed, as well as a method to quantify the ideality of grain architectures for high-rate and efficient operation.

Mechanical properties of the solution treated and quenched Al–Cu–Li alloy (AA2195) sheet during high strain rate deformation at room temperature

In this paper, mechanical properties of the solution treated and quenched Al–Cu–Li alloy (AA2195) sheet were obtained by means of quasi-static tensile tests and split Hopkinson bars tests at room temperature. The mechanical properties results showed that the break elongations were 32.3%, 43.2%, 59.4% corresponding to the strain rates of 2000 s−1, 3500 s−1 and 4500 s−1, which was 18.6% at the strain rate of 0.001 s−1. With the increase of strain rate, the yield strength increased gradually and reached 302 MPa at the strain rate of 4500 s−1, which was 150 MPa at the strain rate of 0.001 s−1. The tensile strength were 341 MPa, 360 MPa, 369 MPa corresponding to the strain rates of 2000 s−1, 3500 s−1 and 4500 s−1, which was 359 MPa at the strain rate of 0.001 s−1. Significant thermal softening behaviors were observed with the softening ratio increased from 1.4% to 23.3% when the strain rate ranged from 0.001 s−1 to 4500 s−1, resulting in an increasing break elongation and a little change in ultimate tensile strength. Microstructure characteristics, including the crystallographic orientation distribution of grains and the average Schmid factor, were observed by electron backscattering diffraction (EBSD) under the condition of the same elongation with different strain rates, revealing the reasons of the plastic improvements during high strain rate deformation. The crystallographic orientation distribution of grains showed that an evident <001>-orientation was observed at the strain rate of 0.001 s−1, which was not changed obviously at the strain rate of 2000 s−1 under the same elongation. The average Schmid factor was 0.436 at the strain rate of 0.001 s−1, which was 0.462 at the strain rate of 2000 s−1 under the same elongation.

An Improved Ultrasonic Imaging Method for Austenitic Welds Based on Grain Orientation Distribution Inversion Algorithm

Traditional ultrasonic imaging inspection does not perform well for austenitic welds, in which the defect locating accuracy is insufficient due to the distortion of propagation paths caused by the weld’s anisotropy and inhomogeneity. The distribution of grain orientation (DGO) contributes to the velocity distribution in austenitic welds and influences the ultrasonic propagation paths. An improved ultrasonic imaging method for austenitic welds based on a DGO inversion algorithm is proposed in this paper. A DGO model is established using an iterative disturbance method of local parameters. The model is optimized by an evolution strategy based on the model error evaluated by propagation times of known reflectors. Inspection experiments were carried out, in which a reference specimen was used to acquire the DGO inversion result and the inspection images of a target specimen were generated based on the optimized DGO model. Experimental results have proved that the improved ultrasonic imaging method can obviously increase the defect locating accuracy and effectively improve the imaging performance for austenitic welds.

The Application of Electron Backscatter Diffraction for Investigating Intra-Particle Grain Architectures and Boundaries in Lithium Ion Electrodes

The performance and degradation of layered cathode materials for lithium-ion batteries are largely governed by their morphological and crystallographic properties. A comprehensive tool to spatially characterize grain architectures in electrode particles is needed to understand solid-state lithium transport and the efficacy of particles to avoid lithiation heterogeneities and strain-induced degradation. Here, we apply electron backscatter diffraction on LiNi0.5Mn0.3Co0.2O2 particle cross-sections to spatially describe intra-particle grain architectures. A method for segmenting, labelling, and quantifying morphological properties of distinct grains is developed and applied. Crystallographic orientations are measured for each grain and the spatial distribution of grain orientations is quantified with a specific focus on describing the plane along which lithium transports, accounting for inter-grain lithiation barriers and for favorable inward lithiation of the particle. The extension from 2D to 3D descriptions of grain morphologies and orientations to predict lithiation inefficiencies is discussed, as well as a method to quantify the ideality of grain architectures for high-rate and efficient operation.