Orientation Of Individual Grains Research Articles

Influence of laser powder bed fusion parameters on microstructure and mechanical properties of Co–Cr dental alloy

This research investigates the impact of three process parameters of Laser Powder Bed Fusion (LPBF) - laser power, scanning speed, and base plate preheating temperature on the structure and mechanical properties of the EOS CoCr SP2 dental alloy. The LPBF process was used to fabricate dental Co–Cr alloy specimens for microstructural analysis and mechanical properties testing. Light and electron microscopy were used to determine microstructural parameters, including porosity, inclusions, and cracks. The material's chemical composition was analysed by EDS, while XRD and EBSD methods were used to determine the presence of microstructural phases and the crystallographic orientation of individual grains. The mechanical properties were evaluated through a static tensile test (Rp0.2, ε), a toughness test (KVa), and a three-point bending test to determine the flexural strength (Rms). In the microstructure, differences were observed that reflected statistically significant differences in mechanical properties (one-way analysis of variance (ANOVA) and Scheffé post hoc test (α = 0.05)) Using the base plate preheating temperature ϑp = 310 °C with a constant scanning speed v = 900 mm/s in combination with increasing laser power P from 160 W to 250 W the proportion of porosity decreased while the mechanical properties of toughness (KVa) and flexural strength (Rms) increase to maximum values.

Orientation-dependent plastic flow in nanoscratching of copper surfaces

The characterization of frictional behavior in ductile metals is a crucial aspect of materials science, as it relates to the plastic deformation of the material. However, due to the intricate dislocation evolution at the submicron/nanometer scale, the isolation of the basic plastic mechanisms remains challenging. In this study, nanoscratching technology is employed to investigate submicron/nanometer-level plastic deformation in individual grains of polycrystalline copper. To achieve this, a crystal plastic constitutive model for copper materials is developed by incorporating geometrically necessary dislocation effects. The calibration of the constitutive model is conducted with great precision and specificity, leveraging mechanical responses, surface accumulation morphology, and indentation size effects from nanoindentation experiments of 110-oriented single crystal copper. Through the combination of crystal plastic finite element simulation and nanoscratching experiments, the plastic flow of the material is thoroughly analyzed. The results demonstrate that the anisotropic scratching behavior is influenced by different orientations of individual grains, taking into account both statistical storage dislocations and geometrically necessary dislocations. The insights gleaned from this study offer a valuable contribution to our understanding of the plastic mechanisms of metals at a small scale, enhancing the design and performance of advanced materials for a wide range of applications.

Effect of grain size on deformation and fracture of Inconel718: An in-situ SEM-EBSD-DIC investigation

The effects of grain size on tensile behavior of Inconel718 superalloy have been characterized by an in-situ tensile stage inside a scanning electron microscopy (SEM) combined with electron backscatter diffraction (EBSD) and digital image correlation (DIC) at room temperature. The microstructure evolution and mechanical properties of specimens under different heat treatments were performed and compared. The in-situ tensile test showed that the yield strength, ultimate tensile strength and reduction of cross section decrease with the increasing grain size. An in-situ EBSD study showed that with increasing stress the local plastic deformation become more inhomogeneous. An analysis combined in-situ SEM morphology evolution, DIC strain distribution and EBSD results indicated that the grains deformed coordinately. Geometrically necessary dislocation (GND) density depends on the grain size and the orientation of individual grains. The coordinated deformation ability decreased with increasing grain size. The fracture morphology showed that with the increase of grain size, the fracture mechanism transformed from the ductile transgranular fracture to the brittle intergranular-transgranular mixed fracture.

Study on the Grain Rotation of High-Purity Tantalum during Compression Deformation

A compression experiment with electron backscatter diffraction (EBSD) measurements was designed to characterize the effect of the microtexture on the grain rotation process. The rotation degrees of more than 180 grains before and after the compression were calculated. Results showed that grains with different crystallographic orientations experienced various rotation degrees. Furthermore, grains in certain microtexture regions also had varying degrees of rotation. The compression led to the lattice rotation and change in orientation of individual grains, but the relative misorientation between grains has not changed much in the microtexture region. The microtexture region, as a whole, participated in the compression process. The similar slipping behavior of the grains in the region promoted the slip transmission between the neighboring grains. Thus, the amount of piled-up dislocations at grain boundaries inside the microtexture region are less than that at grain boundaries outside the microtexture region, leading to a small stored energy density for grain boundaries inside the microtexture region.

Влияние напряженно-деформированного состояния на путь распространения трещин квазискола в низкоуглеродистой стали, охрупченной водородом

Сonflicting data on the effect of the stress-strain state on the shape and path of quasi-cleavage cracks in hydrogen-embrittled steels and iron have been reported in the literature recently. However, this issue is important for understanding the nature of hydrogen embrittlement (HE) and, particularly, the role of hydrogen in the mechanism of crack propagation. In this regard, in the present study, smooth and notched specimens of low-carbon steel were tensile tested with in-situ cathodic hydrogen charging. The side surface of the specimens fractured in this way was examined by scanning electron microscopy aiming at quantitative characterization of the length and curvature of secondary side surface cracks. In addition, fractographic analysis was performed. A great number of cracks having a characteristic S-shaped curved geometry are found on the side surface of the notched specimens. Moreover, the cracks can smoothly curve both on the scale of several grains as well as within the individual grain interior. In contrast, in the case of smooth specimens, the cracks generally exhibit relatively straight appearance. Quantitative analysis using a statistically-representative dataset has shown that, on average, the curvature of cracks in the notched specimens is significantly higher and is varied over a wider range of values than in the smooth specimens. Based on the data obtained, it is concluded that the path of quasi-cleavage cracks in hydrogen-embrittled ferritic and ferrite-pearlitic low-carbon steels is mainly determined by the characteristics of the stress-strain state, and not by the microstructure or crystallographic orientation of individual grains.

Kinking facilitates grain nucleation and modifies crystallographic preferred orientations during high-stress ice deformation

Kinking is an important strain-accommodating process during crystal plastic deformation under relatively large stresses and may influence the mechanical properties of the Earth's lithosphere and planetary cryosphere. To better understand the origins, mechanisms, and microstructural effects of kinking, we present detailed microstructural analyses of coarse-grained (∼1300 μm) ice samples deformed under uniaxial compression at -30°C. Deformed samples show elongated (aspect ratio ≥ 4) kink domains developing within or at the tips of remnant original grains (≥300 μm, aspect ratio < 4). Small, equiaxed subgrains also develop along the margins of remnant grains. Moreover, many remnant grains are surrounded by mantles of small, recrystallized grains (<300 μm, aspect ratio < 4). Together, these observations indicate that grain nucleation is facilitated by both kinking and dynamic recrystallization. Low- (<10°) and high-angle (mostly >10°, many >20°) kink bands within remnant grains have misorientation axes that lie predominantly within the basal plane. The c-axes of most kink domains are oriented sub-perpendicular to the sample compression axis, indicating that kinking may produce or modify a crystallographic preferred orientation. Kink band densities are highest within remnant grains that have basal planes sub-parallel to the compression axis—these data are inconsistent with models suggesting that (if kinking is the only strain-accommodating process) there should be higher kink band densities within grains that have basal planes oblique to the compression axis. One way to rationalize this inconsistency between kink models and experimental observations is that kinking and dynamic recrystallization are both active during deformation, but their relative activities depend on the crystallographic orientations of individual grains. For grains with basal planes sub-parallel to the compression axis, strain-induced grain boundary migration (GBM) is inhibited, and large strain incompatibilities can be relaxed via kinking when other processes such as subgrain rotation (SGR) recrystallization are insufficient. For grains with basal planes oblique to the compression axis, strain-induced GBM might be efficient enough to relax the strain incompatibility via selective growth of these grains, and kinking is therefore less important. For grains with basal planes sub-perpendicular to the compression axis, kink bands are seldom observed—for these grains, the minimum shear stress required for kinking exceeds the applied compressive stress, such that kinks cannot nucleate.

Estimation of Lankford Coefficients of Austenitic and Ferritic Stainless Steels using Mean Grain Orientations from Micro-texture Measurements

A new method to calculate the plastic anisotropy r-values of austenitic and ferritic stainless steels has been developed. The mean orientation of individual grains is obtained from SEM-EBSD data and r-values for individual grains are calculated by weighting all slip systems according to their Schmid factors. Calculated and measured r-values are in good agreement for austenitic stainless steels. However, in ferritic stainless steels, which are highly anisotropic, good agreement requires the introduction of a Schmid factor threshold below which slip systems are inactive. The present method can be used to estimate local differences in r-values in ferritic stainless steels showing the local variations in texture responsible for ridging.

Anisotropic thermal conductivity tensor of β-Y2Si2O7 for orientational control of heat flow on micrometer scales

The ability to relate the spatially-varying anisotropy of thermal conductivity to crystal structure would provide a foundational advance in our understanding of length-scale dependent heat flow. The challenge, however, is in determining the thermal conductivity tensor in polycrystalline systems with anisotropic crystal structures. Apparent isotropic thermal conductivity in randomly oriented polycrystals with anisotropic properties break down at length scales approaching the grain size. As a result, a measured isotropic thermal conductivity from the macroscale perspective may differ greatly from that measured at the nano or microscale, and thus microscale anisotropic heat flow could underlie a seemingly isotropic thermal conductivity. We experimentally investigate the anisotropic thermal conductivity in polycrystalline beta-phase yttrium disilicate (β-Y2Si2O7). This is achieved through micrometer-resolution thermal conductivity mapping correlated to the phase and orientation of individual grains, allowing for the determination of the thermal conductivity tensor of β-Y2Si2O7. Our results are the first to reproduce the thermal conductivity tensor of a polycrystalline system with anisotropic crystal structure based on the spatial distribution of thermal conductivity.

The effects of grain size, dendritic structure and crystallographic orientation on fatigue crack propagation in IN713C nickel-based superalloy

The polycrystalline IN713C produced via investment casting is one of the widely-used nickel-based superalloy in automotive and aerospace industries. This alloy, however, has an apparent inhomogeneous microstructure generated during casting and contains dendritic structure that gives rise to strain localisation during loading. Yet, the effect of dendritic structure, grain size and shape as well as crystallographic orientation, which directly influence fatigue property and deformation micromechanism in the components, is rarely studied. In the present study, IN713C cast bars are tailored with three different grain structures, i.e., transition, equiaxed and columnar, with substantial grain size variations. The produced bars were tested under strain controlled LCF (Low Cycle Fatigue) and stress controlled HCF (High Cycle Fatigue) conditions at 650 °C. The results showed that most of fatigue cracks initiated from casting pores and fatigue life extended in the microstructure with a small grain size during both HCF and LCF loadings. It is also demonstrated that fatigue striations were mainly observed within dendritic areas during crack propagation, whereas the higher GND (Geometrically Necessary Dislocation) density were predominantly observed in the interdendritic areas. Here, we propose a concept of ‘Crack Propagation Unit (CPU)’ for better description of deformation mechanism at local scale during fatigue loading by combining fracture surface characteristic methodology and dislocation distribution analyses within the dendritic structural unit. Furthermore, this model to understand the deformation micromechanism can provide a new perspective on the interpretation of Hall-Petch relationship in casting materials that contain dendritic structure. This is further demonstrated via direct correlation of the high crack propagation resistance with the crack path divergence instead of the dislocation pile-up at the grain boundary or in-between the γ/γ′ channels. Moreover, by utilising serial sectioning method followed by layered EBSD scanning, quasi-3-D grain orientation mappings were obtained, and crystallographic texture information were directly correlated with the fracture surface observations. This allowed an investigation of the influence of orientation of individual grains and micro/macro texture on crack propagation rate. The critical stage of crack propagation in fatigue life and its correlations with microstructural features is established, offering potential practical applications by controlling the investment casting process parameters.

Quasi in-situ EBSD characterization of the evolution of microtexture and microstructure in non-oriented electrical steels during cold rolling and annealing

Non-oriented electrical steel sheets are extensively used to manufacture core laminations for electric motors. The microstructure and crystallographic texture of the final thin sheets have a significant effect on the magnetic quality of the lamination core. Controlling the development of microstructure and texture during thermomechanical processing of these steels is of great practical importance. Although the final microstructure and texture of the steel sheets are affected by all the processing procedures employed, cold rolling and final annealing have the most direct effect since these are usually the final steps to process the steel. Although much effort has been made to study the evolution of microstructure and texture in non-oriented electrical steels, the formation mechanisms of the final microstructure and texture during annealing (especially grain growth) are still not completely understood, which is partially due to the fact that it is very difficult to directly investigate these processes using conventional characterization techniques. In this research, a quasi in-situ electron backscatter diffraction (EBSD) technique was presented, which enabled the tracking of the changes of the morphology and orientation of individual grains during cold rolling and the subsequent annealing process. The experiments revealed the rotations of individual grains under plane strain compression, and tracked the formation of nuclei from the deformed matrix and the growth of new grains during the recrystallization process. The results provided valuable insights into the evolution of microstructure and microtexture during cold rolling and annealing of non-oriented electrical steels, and helped understand the mechanisms that govern the nucleation and grain growth during recrystallization.

Deformation Twinning in Polycrystalline Mg Microstructures at High Strain Rates at the Atomic Scales

Large scale molecular dynamics (MD) simulations are carried out to investigate the twinning behavior as well as the atomic scale micromechanisms of growth of tension and compression twins in polycrystalline Mg microstructures at high strain rates. A new defect characterization algorithm (extended-common neighbor analysis (E-CNA)) is developed that allows for an efficient identification of various types of twins in HCP microstructures. Unlike other local orientation analysis methods, the E-CNA method allows for atomic scale characterization of the structure of different types of twin boundaries in HCP microstructures. The MD simulations suggest that the local orientation of individual grains with the loading axis plays a critical role in determining the ability of grains to nucleate either compression twins or tension twins. The twinning behavior is observed through nucleation of a pair of planar faults and lateral growth of the twins occurs through nucleation of steps along the planar faults. The kinetics of migration of steps that determine the rate of growth of twins are investigated at the atomic scales. The twin tip velocity computed at high strain rates compares well with the experimentally reported values in the literature.

The Effects of Grain Size, Dendritic Structure and Crystallographic Orientation on Fatigue Crack Propagation in IN713C Nickel-Based Superalloy

The polycrystalline IN713C produced via investment casting is one of the widely-used nickel-based superalloy in automotive and aerospace industries. This alloy, however, has an apparent inhomogeneous microstructure generated during casting and contains dendritic structure that gives rise to strain localisation during loading. Yet, the effect of dendrite structure, grain size and shape as well as crystallographic orientation, which directly influence fatigue property and deformation micromechanism in the components, is rarely studied. In the present study, IN713C cast bars are tailored with three different grain structures, i.e., transition, equiaxed and columnar, with substantial grain size variations. The produced bars were tested under strain controlled LCF (Low Cycle Fatigue) and stress controlled HCF (High Cycle Fatigue) conditions at 650 °C. The results showed that most of fatigue cracks initiated from casting porosities and fatigue life extended in the microstructure with a small grain size during both HCF and LCF loadings. It is also demonstrated that fatigue striations were mainly observed within dendrite areas during crack propagation, whereas the higher GND (Geometrically Necessary Dislocation) density were predominantly observed in the interdendrite areas. Here, we propose a concept of 'Crack Propagation Unit (CPU)' for better description of deformation mechanism at local scale during fatigue loading by combining fracture surface characteristic methodology and dislocation distribution analyses within the dendrite structural unit. Furthermore, this model to understand the deformation micromechanism can provide a new perspective on the interpretation of Hall-Petch relationship in casting materials that contain dendritic structure. This is further demonstrated via direct correlation of the high crack propagation resistance with the crack path divergence instead of the dislocation pile-up at the grain boundary or in-between the γ/ γ' channels. Moreover, by utilising serial sectioning method followed by layered EBSD scanning, quasi-3-D grain orientation mappings were obtained, and crystallographic texture information were directly correlated with the fracture surface observations. This allowed an investigation of the influence of orientation of individual grains and micro/macro texture on crack propagation rate. The critical stage of crack propagation in fatigue life and its correlations with microstructural features is established, offering potential practical applications by controlling the investment casting process parameters.

Application of the electron backscateered diffraction method in microstructure research for determining of causes of metal structures destruction

The application of the diffraction method for backscattered electrons allows us to take a fresh look at the structural changes in the material as a whole and on the processes of destruction of metal structures in particular. The aim of this work was to apply the method of diffraction of backscattered electrons to reveal the characteristic distinctive features of the structure of the material in areas under the fracture and away from it.The diffraction of backscattered electrons is a method that allows one to determine the orientation of individual grains, the local texture, and also to identify the phases in the sample under research. This method can determine local and general deformations, the number of recrystallized and deformed grains, the size and misorientation of grains, etc.The results of a study of the mast fragment of the unit for drilling and repairing wells with a carrying capacity of 200 tons (APC-200) are presented with the establishment of characteristic structural differences between the sites under the fracture and away from it.The appearance and development of a subgrain structure at the site under the slope is established. It is shown that the material of the mast was made of rolled metal, for which no additional heat treatment was carried out, and destruction could occur at almost any point.

Annealing-Induced Grain Rotation In Ultrafine-Grained Aluminum Alloy

Abstract Results of in situ transmission electron microscopic studies of annealing-induced evolution of grain boundary misorientations in ultrafine-grained aluminum alloy Al-4%Cu-0.5%Zr (wt.%) processed by high pressure torsion are presented. An experimental procedure based on the single reflection technique was applied for the measurements of orientations of individual grains before and after annealing that allowed for determining misorientations between them. The measurements revealed that relaxation processes occurring in non-equilibrium grain boundaries during short-time annealing are accompanied by a change of grain boundary misorientations, grain rotation, grain migration as well as grain growth. Obtained results were compared with theoretical studies considering the kinetics of grain rotation in discrete-dislocation as well as continuum approaches.

Correlation between photoemissive and morphological properties of KBr thin film photocathodes

In the present work, the morphological properties of KBr photocathodes are correlated with their photoemissive behavior using a combination of analyzing techniques including SEM, TEM and AFM. From morphological studies, it is observed that KBr films have granular characteristics with varied average grain size and grain density. Structure and orientation of individual grains have been investigated by selected area electron diffraction technique and found to be the crystalline in nature with a face centered cubic structure. It is evident from the AFM analysis that the root mean square roughness and maximum area peak height have been decreased with the deposition of more KBr layers. The photoemission studies reveal that the resultant photocurrent is enhanced with increasing film thickness and it is directly related to the surface area coverage and grain density.

The microstructural studies of suspension plasma sprayed zirconia coatings with the use of high-energy plasma torches

The presented studies are focused on the microstructure characterization of zirconia-based coatings deposited by two types of high-energy plasma torches: (i) Axial III; and, (ii) hybrid version of Water-Stabilized Plasma (WSP) torch. The suspensions were formulated using solid dispersed phase of: (i) zirconia stabilized with 14wt% of Y2O3 and (ii) zirconia stabilized with 24wt% of CeO2+2.5wt% of Y2O3 and continuous phase of water with ethanol. The spray process parameters were optimized for each plasma set-up individually. The in-flight observations (shadowgraphy) were performed to optimize the injection of the liquid feedstock into the plasma jet. Then the coating's morphology and coating/substrate interface were characterized using conventional light microscopy and scanning electron microscopy (SEM). The results showed that through the change of deposition parameters various coatings microstructures could be obtained, in particular columnar and two-zones structures. The EDS/EDX and XRD studies showed that there was no significant change in chemical/phase composition of zirconia material before and after spraying. Electron backscatter diffraction (EBSD) method allowed to analyze the grain size in the coating microstructure as well as crystallographic orientation of individual grains. The results showed that coatings were characterized by submicrometric microstructure what corresponded to the size of powder particles used to formulate suspension. No texture was observed in the coatings microstructure. The surface topography analysis which was performed by confocal scanning laser microscopy (CSLM) and Shape From Shading (SFS) technique proved the great influence of suspension concentration on the coating structure.

Compound technology of manufacturing and multiple laser peening on microstructure and fatigue life of dual-phase spring steel

The present work proposes an advanced double quenching and tempering heat treatment based laser surface modification process of dual-phase spring steel. Multiple laser peening without coating process utilized the decarburized surface as the protective layer for the further cold working process. The electron backscattering diffraction analysis on crystallographic orientation of individual grains and phase map exhibits a perfect dual-phase steel. Also, the high resolution transmission electron microscopic study explains the high strain induced microstructural grain refinement features and plastic deformation behaviors. The laser peening technique taking an advantage that it induces a large and high magnitude compressive residual stress with good thermal stability. The micro and nano-hardness profile provides better surface and sub-surface mechanical properties. The controlled average surface roughness is achieved in this course of work. The stress-strain characteristics on tensile properties are analyzed through the pre-fatigued specimens. The fully reversed high cycle fatigue test indicates that the current laser peening has substantially improves the fatigue life of the specimens.

Substructural Alignment during ECAE Processing of an Al-0.1Mg Aluminium Alloy

An investigation has been carried out into the microstructures developed during the early stages of equal channel angular extrusion (ECAE) in a polycrystalline single-phase Al-0.13Mg alloy, with emphasis on the substructural alignment with respect to the die geometry and the crystallographic slip systems, which is essentially related to the grain refinement and texture development during deformation. The material was processed by ECAE at room temperature to three passes, via a 90° die. Microstructures were examined and characterized by EBSD. It was found that dislocation cell bands and microshear bands were respectively the most characteristic deformation structures of the first and second pass ECAE. Both formed across the whole specimen and to align approximately with the die shear plane, regardless of the orientation of individual grains. This confirmed that substructural alignment was in response to the direction of the maximum resolved shear stress rather than to the crystallographic slip systems. However, a significant fraction of material developed preferred orientations during deformation that allowed the coincidence between the crystallographic slip systems and the simple shear geometry to occur, which governed texture development in the material. The third pass deformation was characterized with the formation of a fibre structure with a significant fraction of high angle boundaries, being aligned at an angle to the extrusion direction, which was determined by the total shear strain applied.

Elasticity mapping of precipitates in nickel-base superalloys using atomic force acoustic microscopy

The paper reports mapping of elastic properties of various precipitates such as γ′, δ, and carbides in nickel-base superalloys. Atomic force acoustic microscopy is used to measure the elastic properties with a lateral resolution of better than 100 nm in polycrystalline alloy 625 and directionally solidified alloy CM247A. A novel approach of simultaneous acquisition of two contact resonances and using the isotropic indentation modulus of matrix in every scan line as reference is used to circumvent the effect of change in tip condition while mapping elastic properties. Scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy is performed to identify the precipitates and their compositions. Carbides exhibited highest modulus followed by δ precipitates, γ′ and γ matrix, respectively. It is demonstrated that due to the close-packed orientation relationships of precipitates with the matrix, the modulus of a precipitate measured by the methodology described in this study is not affected by the orientation of individual grain in which the measurement is made.

Microtexture of c‐Axis‐Oriented Polycrystalline Lanthanum Silicate Oxyapatite Formed by Reactive Diffusion

We annealed the La2SiO5/La2Si2O7 diffusion couple at 1873 K for 25 h to prepare the grain‐oriented lanthanum silicate oxyapatite (LSO) polycrystal at the interfacial contact boundary. The polycrystalline LSO was characterized using optical microscopy, transmission electron microscopy, scanning electron microscopy, and X‐ray diffractometry (XRD). The external shape line information on individual crystal grains were extracted from the backscattered electron image on the section surface of the polycrystal. We collected the electron backscatter diffraction patterns and XRD patterns to determine, respectively, the crystallographic orientations of individual grains and orientation degrees of the LSO polycrystal. The reaction area was in the form of a layer parallel to the original contact boundary, and consequently categorized, based on the differences in grain size distribution and orientation degree, into three distinct regions. The innermost region with the layer thickness of 30 μm was composed mainly of the rounded and relatively small crystal grains with the size less than ~16 μm in diameter. Furthermore, this region showed the lowest orientation degree among the three regions. On the other hand, the LSO grains on both sides of this region were much larger in size and prismatic in morphology; the LSO crystals were elongated along the c‐axis, and built up of faces {001}, {304}, and {100}. The individual crystal grains were aligned almost along their c‐axes, with their a‐axis directions being randomly distributed around the common c‐axis. The differences in microtexture among the three regions would be induced by the distinct growth behaviors of LSO crystals.