Misoriented Grains Research Articles

Crystallographic study of liquid-metal-embrittlement crack path

Compared to other intergranular degradation phenomena, liquid metal embrittlement (LME) is less understood. The main gap in knowledge has been the lack of fundamental information of how grain-boundary characteristics (e.g. grain boundary misorientation angle and grain boundary crystallographic plane) affects LME susceptibility. In this study, the recently developed trace analysis has been applied to the grain boundaries exhibiting LME cracking. Despite the presence of a diverse range of low- and high-index planes in the entire population of the grain boundaries, liquid metal selectively penetrated high-index random grain boundary planes such as (5 3 1), (5 1 1), and (7 3 3).

Nanostructural characterization of EuBa2Cu3Oy layers containing 3.5 mol%BaHfO3 nanorods grown by pulsed laser deposition growing in both vapor–solid and vapor–liquid–solid modes

We characterize nanostructures of two types of EuBa2Cu3Oy (EuBCO) layers containing 3.5 mol%BaHfO3 (BHO) nanorods fabricated by pulsed laser deposition (PLD) process using scanning electron microscopy (SEM), transmission electron microscopy and three-dimensional reconstructions that used a series of cross-sectional SEM images taken in a focused ion beam (FIB)-SEM system. One type of PLD layer was grown in a vapor–solid (V–S) mode, and the other was grown in a vapor–liquid–solid (V–L–S) mode. Relatively large polycrystalline of CuO and Ba–Cu–O phases were found on the surface of EuBCO layer grown in the V–L–S mode, consistent with precipitation from liquid phase. Misoriented EuBCO grains with different orientations to those of the matrix grains nucleated on CuO grains (or clusters of CuO/Ba–Cu–O grains) in both EuBCO layers. Both the self-field and in-field critical current values of the EuBCO layers containing BHO nanorods depend on the volume fraction of the misoriented grains, and thus the V–L–S mode layer exhibits better superconductive properties. The volume fraction of the misoriented EuBCO grains in the matrix of c-axis oriented EuBCO grains grown in the V–L–S mode was smaller than that of those grown in the V–S mode.

Dislocation interactions in near-alpha Titanium alloy Ti6242Si under LCF

Dislocation interactions were investigated in near-alpha titanium alloy Ti6242Si after low cycle fatigue. Samples from the gauge section and the crack initiation site on the fracture surface were studied. Grain pairs with different crystallographic orientations were analysed from the gauge section to understand the dislocation interactions near the boundary. Deformation was primarily caused by planar slip, localized into slip bands in primary alpha (αp) grains. Direct slip transfer was observed within grains having similar orientations. In contrast, slip transfer resulted in a different kind of dislocation nucleation in the neighboring hard grain where there was misorientation between the grains, with the observation of cross-slip. Strain transfer was observed between highly misoriented grain pairs. Crack nucleation occurred on an αp grain by basal plane splitting, due to the large tensile stress developed by a double ended pile-up. This kind of pile-up is suggested to result from the incomplete reversibility of dislocation motion during load reversals. The observation of superjogs on the basal dislocations in the crack nucleated αp grain provides a rationale for why cracks nucleate near, rather than on, the basal plane.

Single-crystal height extension by Laser Metal Deposition of CMSX-4

The introduction of directionally solidified and single-crystal nickel-based superalloys have led to a higher thermal efficiency of engines by allowing for an increase in turbine inlet temperatures from 1000°C to 1400°C. These temperatures however lead to higher thermal stress and corrosive environments, resulting in a shorter life span. Previous work has shown the applicability of laser metal deposition (LMD) for the regeneration of these high value components, while maintaining primary crystalline orientation of single-crystal high pressure turbine blades. However, the processing of these materials by LMD poses several challenges, such as susceptibility to cracking, the formation of misoriented grains and low weldability. This paper examines deposition strategies to extend single-crystal height. While a continuous, meandering strategy shows the most promising percentage single-crystallinity results, an additional remelting step increases epitaxial deposition height. Apart from percentage single-crystallinity and deposition height, this study quantifies the effect of these strategies on cracking and the formation of misorientations.

Texture evolution during Isothermal compression process of Ti-22Al-25Nb alloy in B2 phase region

In the present work, the hot deformation behavior, dynamic recovery, dynamic recrystallization and texture evolution of Ti-22Al-25Nb alloy on the conditions of 1100°C with four different thickness reductions (35%, 50%, 65% and 80%) are investigated by isothermal compression testing on Gleeble-3500 thermo-mechanical simulator. The strain rate is 0.1mm/s-1. Subsequently, metallographic observation and EBSD analysis are carried out. The results show that during the hot deformation, the dynamic recovery (DRV) and dynamic recrystallization (DRX) strongly affect the microstructure and texture evolution. It is observed that with the strain increasing, the intensity of ηbcc-fiber increases firstly (crystallographic fiber axis <100> parallel to the compression direction). When the thickness reduction reaches to 80%, the intensity of <001> pole becomes stronger expectedly. Whereas the ηbcc-fiber transform into cube components ({100} <001>) unexpectedly. In addition, as the strain increases through 35%-80%, the fraction of large misorientation grain boundaries and fraction of DRX grains gradually increase due to continuous recrystallization. The evolution mechanism of grain orientations and texture during the DRX process will be discussed.

Effects of cooling condition on microstructural evolution and mechanical properties of friction stir processed 2A14 aluminum alloy

Detailed analysis on the microstructural evolution and mechanical properties of 2A14 aluminum alloy subjected to friction stir processing (FSP) under different cooling conditions has been conducted. SEM, EBSD and TEM were used to reveal the microstructure characteristic, while the microhardness and tensile tests were also performed. The results showed that microstructures formed by fine, equiaxed and highly misoriented grains with the average grain size in the range of 1.19∼3.1 μm were obtained in the stirred zone (SZ). Compared with FSP under air cooling, rapid cooling can significantly reduce grain size, second phase particles size and fraction of high angle grain boundaries in the SZ. Besides, recrystallized grains with random orientation are formed throughout SZ during FSP. The microtexture analysis also demonstrated the dominance of shear texture ({hkl}〈110〉 and {111}〈uvw〉 fibers), with C and A1* components are produced through the geometrically dynamic recrystallization mechanism. And the texture intensity in SZ of the samples is slightly increased with accelerated cooling method, indicating that another mechanism of grain refinement through discontinuous dynamic recrystallization might be involved. Additionally, the strength and microhardness of friction stir processed samples were improved with increasing the cooling rate due to grain refinement, the increase of dislocation density and fine and uniformly distributed second phase particles.

Effect of prior austenite on reversed austenite stability and mechanical properties of low carbon medium manganese steel heavy plate

AbstractThe effect of prior austenite on reversed austenite stability and mechanical properties of Fe‐0.06C‐0.2Si‐5.5Mn‐0.4Cr (wt.%) annealed steels was elucidated. With the decrease of austenitizing temperature from 1250 °C to 980 °C, the prior austenite changed from complete recrystallization to partial recrystallization, and the average austenite size was reduced. The volume fraction of reversed austenite was increased from 26.32 % to 30.25 % because of high density of grain boundaries and dislocations. The martensite transformation temperature of annealed steels was increased from ∼115 °C to ∼150 °C, and both of thermal and mechanical stability of reversed were reduced. There was no significant different in tensile properties, however, the impact toughness was enhanced from 100 J to 180 J at −60 °C. The excellent impact toughness in annealed steel (austenitized at 980 °C) was obtained because of higher density of high misorientation grain boundaries, more volume fraction of reversed austenite and reduced segregation at grain boundaries.

Determination of representative volume elements for small cracks in heterogeneous, linear-elastic domains

A method is introduced for determining the size of a representative volume element for microstructurally small cracks (RVEMSC) in heterogeneous, linear-elastic domains. RVEMSC is defined as the minimum volume required around a crack front such that crack-front parameters are converged with respect to volume size. This method uses finite element models of idealized microstructures, where microstructural constituents are represented by varying the elastic modulus (E). RVEMSC is expressed using two parameters: d1,MSC (the required distance from a crack front to the sides of a volume) and d2,MSC (the required distance from a crack to the top and bottom of a volume), both expressed in terms of the number of microstructural constituents (e.g. grains). RVEMSC is found for cases involving different crack sizes, error tolerances, and boundary conditions. For a given type of boundary condition, a conservative estimate of RVEMSC is provided as a closed-form function of crack size, average grain size, and desired error tolerance. The solutions for RVEMSC are validated numerically using models of synthetic, realistic microstructures, whose volumes are determined from the RVEMSC equations. The equations are shown to be applicable for a variety of crack sizes and error tolerances. The convergence rates for macroscale parameters are compared with convergence rates of crack-front parameters, and RVEMSC is found to lead simultaneously to converged macroscale parameters. Furthermore, a correlation analysis is performed between microstructural features and required volumes for converged crack-front parameters, and the results indicate that highly misoriented neighboring grains (implicitly represented by large differences in E) located near a crack front lead to larger required volumes to guarantee crack-front parameter convergence. A combination of constraint and microstructural effects are found to contribute to RVEMSC, where smaller cracks tend to be more sensitive to microstructural effects while larger cracks tend to be more sensitive to constraint effects.

Microstructure evolution and mechanical properties of 2196 Al-Li alloy in hot extrusion process

Al-Li alloys are considered as promising materials for the aircraft and aerospace industries. As an effective way of producing complex profiles, the hot extrusion process has become more attractive. Here, isothermal extrusion of homogenized 2196 Al-Li alloys under the temperature of 703–783 K and extrusion speed of 0.1–1.0 mm/s was implemented. The microstructure and mechanical properties of extruded profiles under different processing conditions were evaluated. The results showed that the homogenization could eliminate the dendritic structure caused by casting and reduce segregation, creating a uniform element distribution. The insoluble intermediate particles composed of Al, Cu, and Fe were distributed on the grain boundaries with blocks and dots. After extrusion, the grains were flattened and elongated into thin fibers. With increasing extrusion temperature and speed, the misorientation of grains was increased, the degree of dynamic recovery and recrystallization were promoted, and the grain size was decreased. Increasing the extrusion speed properly is beneficial to grain refinement and dynamic recrystallization. The main texture components of the extruded profiles were Y, Cube textures with the orientations of <111>, <100>, and other {111} fibers. With the increasing temperature and speed, the texture types were changed from shear texture to recrystallization texture. The intensity of texture was decreased, while the texture diversity was increased. The hardness, tensile strength and yield strength of extruded profile were greatly enhanced, but the elongation was decreased. The tensile fracture morphology exhibited a large number of dimples, which was considered to be ductile fracture.

Magnetic in-plane components of FePt nanogranular film on polycrystalline MgO underlayer for heat-assisted magnetic recording media

A systematic study is conducted on the effect of MgO substrate/underlayer in terms of the surface roughness, grain boundary, and crystallographic texture on the magnetic in-plane components (medium noise) of the FePt-based heat-assisted magnetic recording (HAMR) media. Cross-sectional transmission electron microscope observations revealed that good (001) texture can be developed in FePt nanogranular films even when they are grow on a rough surface of a single crystal-MgO (001) substrate or across the grain boundaries of a (001)-textured polycrystalline-MgO underlayer. The misoriented FePt grains with magnetic easy axes (c-axes) tilted away from the film normal direction mainly originate from their epitaxial growth on the initially misoriented MgO underlayer grains. Micromagnetic simulation results revealed that the FePt grains with misorientation angle larger than ∼20° are attributed to the detected magnetic in-plane components. Based on these experimental results, we discuss how to improve the (001) texture of the polycrystalline underlayer as well as to develop FePt-based HAMR media with enhanced signal-to-noise ratio.

TiN film growth on misoriented TiN grains with simultaneous low-energy bombardment: Restructuring leading to epitaxy

We perform large-scale molecular dynamics simulations of TiN deposition at 1200 K on TiN substrates consisting of under-stoichiometric (N/Ti = 0.86) misoriented grains. The energy of incoming Ti atoms is 2 eV and that of incoming N atoms is 10 eV. The simulations show that misoriented grains are reoriented during the early stages of growth, after which the film grows 001 epitaxially and is nearly stoichiometric. The grain reorientation coincides with an increase in film N/Ti ratio. As the grains reorient, additional nitrogen can no longer be accommodated, and the film composition becomes stoichiometric as the overlayer grows epitaxially.

Characterization of Microstructures and Fatigue Properties for Dual-Phase Pipeline Steels by Gleeble Simulation of Heat-Affected Zone.

To increase transmission efficiency and reduce operation cost, dual-phase (DP) steels have been considered for pipeline applications. Welding has to be involved in such applications, which would cause a localized alteration of materials and cause many potential fatigue issues to arise under cyclic loading. In this work, the fatigue crack propagation and fatigue life of simulated heat-affected zone (HAZ) were examined. Results indicate that when the maximum stress is at the same magnitude, the fatigue life at a peak temperature of 1050 °C is very close to that of a peak temperature of 850 °C, and both of them are higher than that of a peak temperature of 1350 °C. The changes in da/dN with ΔK for HAZ subregions are attributed to the variation of crack path and fracture mode during the crack propagation. The fatigue cracks may propagate along the bainite lath preferentially in coarse-grained HAZ (CGHAZ), and the prior austenite grain boundaries can change the crack growth direction. A considerable amount of highly misoriented grain boundaries in fine-grained HAZ (FGHAZ) and intercritical-grained HAZ (ICHAZ) increase the crack growth resistance. The difference of fatigue crack propagation behavior in HAZ subregions between actual and simulated welded joints was also discussed.

Reducing the grain density in semipolar (11-22) AlGaN surfaces on m-plane sapphire substrates

The growth mechanisms during metalorganic vapor phase epitaxy (11-22) oriented AlxGa1-xN with x ∼ 0.80 on m-plane sapphire are studied with the intention of mitigating the expansion of misoriented grains, composed of the (1-10-3) crystal orientation and achieving a flat surface with only the (11-22) orientation. An increase in reactor pressure, metalorganic supply, and V/III ratio led to a decrease in the grain density from 1.0 × 109 cm−2 to 1.5 × 107 cm−2. By comparing different growth regimes, we found that the main factor suppressing the growth of the (1-10-3) orientation and decreasing the grain density in the AlGaN layers is the growth rate, which decreased with increasing reactor pressure, MO supply, and V/III ratio due to increasing pre-reactions in the gas phase. To mitigate pre-reactions even with lower growth rates, growth conditions with higher total flow and lower TMAl flow were chosen, yielding low growth rates of 0.13 μm h−1 and a grain density of 3.0 × 107 cm−2 at an aluminum mole fraction of 84%. To allow the growth of thick LED heterostructures we demonstrated that such a buffer can be overgrown with higher growth rate AlGaN, yielding a low grain density of 1.0 × 106 cm−2 and a smooth morphology with a rms roughness of 2.5 nm by avoiding misoriented crystal propagation during nucleation.

Material Parameter Extraction for Complex AlScN Thin Film Using Dual Mode Resonators in Combination with Advanced Microstructural Analysis and Finite Element Modeling

AbstractAccurate property determination of the piezoelectric thin film material Al(1−x)Sc(x)N is necessary for designing the next generation of radio frequency resonators in mobile communication, and for testing results of ab initio calculations. Sound velocity and piezoelectric coupling of both longitudinal and shear mode are evaluated from a single dual mode resonator. This assures a compatible set of coefficients. It is observed that AlScN thin films grew differently on small, isolated bottom electrodes. The investigated film starts growing with a slightly tilted, c‐textured microstructure, and switches after 200 nm to a polycrystalline film with irregularly oriented grains having c‐axis tilt angles in the range of 35°–70°, as revealed by transmission electron microscope nanodiffraction mapping. Based on this information, a finite element model (FEM) is constructed that properly reproduces the resonance behavior of the resonator. The relevant elastic and piezoelectric constants are derived by curve fitting and yield somewhat lower stiffness and higher piezoelectric coefficients than ab initio calculations published in the literature. The FEM modeling results show that the upper film part with the abnormally oriented grains is overall piezoelectric, i.e., the misoriented grains maintain the polarity projected onto the growth direction from the starting layer.

Strain rate and temperature dependence of low temperature superplastic deformation in a nanostructured microalloyed steel

The low temperature plasticity behavior of a nanostructured microalloyed steel was studied at varied deformation temperatures and strain rates. Tensile condition of 500 °C (0.33 melting point) and 0.00005 s−1 led to superplastic flow with a low peak stress of 140 MPa and a high elongation of 202%. The ultra-fine equiaxed ferrite kept high misorientation grain boundaries and absent of dislocations subjected to superplastic strain, accounted for grain boundary sliding mechanisms. Slip deformation was required at strain rate of 0.00025 s−1 or deformation temperature of 450 °C based on a relatively high dislocation density. Low temperature superplasticity in NG/UFG steel was highly strain rate and temperature dependent.

Effect of Temperature on Grain Size in AA6063 Aluminum Alloy Subjected to Repetitive Corrugation and Straightening

The influence of processing temperature on grain size reduction in AA 6063 aluminum alloy subjected to repetitive corrugation and straightening (RCS) is investigated in this work. The aluminum alloy was processed by RCS at different temperatures (room temperature, 100 °C, 200 °C and 300 °C) till the maximum number of passes possible before failure and the mechanical properties such as tensile strength and hardness were measured. The grain size and their misorientation of grains of the processed samples were analyzed using the electron backscattered diffraction. The results indicated that the transformation of low-angle grain boundaries to high-angle grain boundaries and dislocation tangles were highly dependent on the strain imparted, which could be controlled by selecting the proper processing temperature. As a result, the mechanical properties are affected. In particular, the room temperature tensile strength and hardness values of the processed material decrease with increasing processing temperature.

Observations of Twinning and Detwinning in Magnesium Alloy by Synchrotron Radiation DCT and EBSD

Recently, three-dimensional grain mapping techniques for polycrystalline materials called X-ray diffraction contrast tomography (DCT) have received considerable attention. DCT can simultaneously provide information on the three-dimensional grain arrangement of a sample, such as the shapes, locations, and crystallographic orientations of grains, together with the misorientation of grains. In the present study, the twinning and the detwinning in tension and compression loading in a magnesium alloy was observed by the DCT using ultra-bright synchrotron radiation X-ray, and the electron backscatter diffraction (EBSD) analysis. The shape of each grain in a polycrystalline magnesium alloy was successfully reconstructed and misorientation of each crystallographic plane was successfully measured by DCT. With increasing applied compression stress, the average value of the misorientation was decreased, while it is usually increased with plastic strain for materials without twinning. The value of misorientation increased with unloading process, indicating that the twinning was occurred under the compressive applied stress. The shape of diffraction spots of some grains also shows the twinning under the compressive stress and the detwining under the following tensile stress. The EBSD analysis of specimen surface also shows that the twinning was occurred under the compressive applied stress, and the following applied tension stress brought detwinning.

Superplastic GBS Constitutive Equation Incorporating Average Grain Misorientation Dependence

Recent Al7075 severe friction stir processing (FSP) data gave new insights regarding the relationship among processing, microstructure and high temperature behaviour. Grain boundary sliding, GBS, usually operates with fine, equiaxed and highly misoriented grains although, so far, the variable misorientation is missing from the constitutive equation. A collection of very fine microstructures comprising various grain size and misorientation values is employed to evidence the relative importance of grain size vs misorientation in the superplastic behaviour of the processed alloy. This relationship is included into a new GBS constitutive equation incorporating the average misorientation as a variable.

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.

Superplasticity of Aerospace 7075 (Al-Zn-Mg-Cu) Aluminium Alloy Obtained by Severe Plastic Deformation

The 7075 (Al-Zn-Mg-Cu) aluminium alloy is the reference alloy for aerospace applications due to its specific mechanical properties at room temperature, showing excellent tensile strength and sufficient ductility. Formability at high temperature can be improved by obtaining superplasticity as a result of fine, equiaxed and highly misoriented grains prone to deform by grain boundary sliding (GBS). Different severe plastic deformation (SPD) processing routes such as ECAP, ARB, HPT and FSP have been considered and their effect on mechanical properties, especially at intermediate to high temperatures, are studied. Refined grains as fine as 100 nm and average misorientations as high as 39o allow attainment of high strain rate superplasticity (HSRSP) at lower than usual temperatures (250-300oC). It is shown that increasing misorientations are obtained with increasing applied strain, and increasing grain refinement is obtained with increasing processing stress. Thus, increasing superplastic strains at higher strain rates, lower stresses and lower temperatures are obtained with increasing processing strain and, specially, processing stress.