Local Variations In Microstructure Research Articles

Untangling competition between epitaxial strain and growth stress through examination of variations in local oxidation

Understanding corrosion mechanisms is of importance for reducing the global cost of corrosion. While the properties of engineering components are considered at a macroscopic scale, corrosion occurs at micro or nano scale and is influenced by local microstructural variations inherent to engineering alloys. However, studying such complex microstructures that involve multiple length scales requires a multitude of advanced experimental procedures. Here, we present a method using correlated electron microscopy techniques over a range of length scales, combined with crystallographic modelling, to provide understanding of the competing mechanisms that control the waterside corrosion of zirconium alloys. We present evidence for a competition between epitaxial strain and growth stress, which depends on the orientation of the substrate leading to local variations in oxide microstructure and thus protectiveness. This leads to the possibility of tailoring substrate crystallographic textures to promote stress driven, well-oriented protective oxides, and so to improving corrosion performance.

Influence of Local Microstructural Variations on the Bendability of Aluminum Extrusions: Experiments and Crystal Plasticity Analyses

Abstract The bendability of extruded profiles of an age hardenable aluminum alloy is investigated using mechanical tests on flat tensile specimens and bending specimens. Two profile geometries are considered, where the profiles exhibit different grain structures. The microstructure of the profiles in terms of the crystallographic texture and constituent particles is otherwise comparable. While the tensile properties are not that different for the two profiles, their bendability is strongly dependent on the grain structure and is about twice as high for one profile than for the other. A newly proposed coupled damage and single crystal plasticity model is used in finite element analyses of the mechanical tests to investigate the influence of the grain structure on the bending behavior, and the numerical results are compared to the experimental tests. The crystallographic texture and the grain morphology of the profiles, found by the electron back-scatter diffraction technique, are explicitly represented in the finite element models. The crystal plasticity simulations capture the difference in the bendability of the two profiles, and in agreement with the experiments predict a considerably higher bendability for one of the profiles. It is found that the grain structure affects the shear band formation in these profiles, but also the local texture where the shear bands are located is important for crack initiation and propagation as grains with certain crystallographic orientations may have a higher fracture resistance.

Cavity evolution and void swelling in dual ion irradiated tempered martensitic steels

Microstructural evolutions in two variants of the F82H 8 wt%Cr tempered martensitic steel under dual (He+ plus Fe3+) ion irradiations at 500 °C have been characterized by transmission electron microscopy. Emphasis is on understanding the combined effects of displacements per atom (dpa) and the helium/dpa ratio (He/dpa) on cavity evolution and void swelling (fv). The dual ion database includes 231 alloy-dpa-He/dpa conditions for irradiations up to ∼ 82 dpa and 3700 appm helium. We hypothesize that the large “scatter” observed in the fv data is primarily due to local microstructural variations, especially those that shield regions at shallower depths from the effects of injected interstitial atoms, which would otherwise reduce fv. More generally, our hypothesis is that the highest fv data, at a specified dpa and He/dpa, provides the most appropriate estimate of swelling for fusion neutron irradiation conditions. The dose dependence of fv can be described by an incubation dpai, marking the onset of significant void growth, and a post-incubation swelling rate, fv’ = Δfv/Δdpa. Both dpai and fv’ decrease with increasing He/dpa. The fv’ data are quantitatively consistent with a simple model for defect partitioning between evolving bubble, void, and dislocation sinks strengths and biases. Most notably, the model correctly predicts both the incubation dpai, and the subsequent fv’, in spite of the fact that they reflect somewhat different physics. Our analysis also suggests that defect recombination is modest at 500 °C. Along with other considerations, this supports the hypothesis that, even at high dpa rates, dual ion irradiations can be used to reasonably emulate void nucleation and growth for a range of fusion-relevant neutron irradiation service conditions. A companion paper incorporates the results of this study in a reduced-order, data-driven void swelling model.

Unraveling compacted graphite evolution during solidification of cast iron using in-situ synchrotron X-ray tomography

In spite of many years of research, the physical phenomena leading to the evolution of compacted graphite (CG) during solidification is still not fully understood. In particular, it is unknown how highly branched CG aggregates form and evolve in the semi-solid, and how local microstructural variations at micrometer length scale affect this growth process. We present here the first time-resolved synchrotron tomography combined with a bespoke high-temperature environmental cell that allows direct observation of the evolution of CG and relates this dynamic process to the local surrounding microstructures in a cast iron sample during repeated melting and solidification. Distinct processes are identified for the formation of CG involving the nucleation, growth, development of branches and interconnection of graphite particles, ultimately evolving into highly branched graphite aggregates with large sizes and low sphericities. CG is found to nucleate with a spheroidal or a plate-like shape, developing branches induced by high carbon concentration, e.g. thin melt channels. Additionally, CG grows much faster than spheroidal graphite during subsequent cooling in solid state. The direct visualization of the dynamic solidification process provides unprecedented new insights into formation mechanisms of CG and correlating factors such as local microstructural variations, and guides the development of CG iron solidification models.

Crystal plasticity modeling for mechanical property prediction of AA2195-T6 friction stir welded joints

The gradient material exposure to frictional heat and plastic flow during friction stir welding leads to local variations in microstructure and mechanical properties among different joint zones. This study aims to predict strengthening contributions of local microstructural evolutions including precipitate distribution, crystal morphologies, grain size, and dislocation density for friction stir welded joints of AA2195-T6. These effects are included by a unified microstructure-sensitive physics-based constitutive modeling approach into joint local variations in yield strength, plastic hardening, and ultimate tensile strength. Local precipitate phase distributions are observed by the transmission electron microscopy method. X-ray diffraction process is conducted to measure joint local variations in dislocation density. Electron backscattered diffraction mapping is utilized to observe joint local crystal morphologies. Evolutions in the crystal slip strength due to each of mentioned microstructure attributes are inserted into a crystal plasticity finite element frame work. Modeling parameters are calibrated for AA2195-O and T6 conditions, and simulated outputs are validated by comparing with experimentally observed joint local tensile properties using the digital image correlation method. Subsequently, obtained local tensile properties in the grain scale are inserted into a macroscale global model containing all friction stir welded joint areas. An acceptable match is observed between simulated and experimentally obtained data on local and global friction stir welded joint tensile stress-strain curves and tensile failure location.

Interface strength and mechanical properties of Inconel 718 processed sequentially by casting, milling, and direct metal deposition

Inconel 718 is the most commonly used nickel-based superalloy, mainly because it exhibits good weldability and can be processed by various types of manufacturing technologies. The combination of these processes with AM, typically referred to as hybrid manufacturing, can overcome limitations that exist in conventional process chains. However, different cooling rates of the material during hybrid manufacturing cause local variations in microstructure and mechanical properties within the components. Thus, quantification of the impact of individual process steps on the resulting properties could reveal most suitable process combinations. The present study focuses on the fabrication and repair of parts by casting, interface milling, and direct metal deposition (DMD). Four processing routes are investigated where heat treatment and interface conditions are varied before applying the DMD process. The cast components are either solution annealed or without heat treatment, and the interface to the DMD part remains either as-cast or it is milled. The results show that all conditions allow dense bonding between the cast section and the additively manufactured structure. The tensile properties of the test specimens exceed the level of conventionally cast parts and can be predicted by numerical simulation. The proposed combination of casting, milling, and DMD may therefore be applied to hybrid manufacturing process chains to increase the level of material efficiency and design flexibility.

Nanofatigue behaviour of single struts of cast A356.0 foam: cyclic deformation, nanoindent characteristics and sub-surface microstructure

Struts are the main load carrying elements in cyclically loaded open cell metal foams. Little is known about the local fatigue behaviour and the influence of the microstructure on nanoscale deformation mechanisms. Different to the bulk counterpart, the millimetre-sized struts in precision-cast AlSi7Mg0.3 foams contain only 1–2 Al-dendrites, Si-Al-eutectic and intermetallic phases. We applied cyclic nanoindentation to N = 105 to assess nanofatigue. The change in minimum depth per cycle and the ratio of minimum to maximum indentation depths versus the number of cycles correspond to cyclic plastic processes. These and the indent and pile-up morphologies were correlated with the microstructure and dislocation formations revealed by phase-contrast-enhanced micro-computed tomography and transmission electron microscopy. Our results reveal that Si-particles affect deformation within 5 to 10 μm from the indent, and that they favour the formation of fatigue induced dislocation cells in the affected volume. We believe that this interaction is mediated through residual stresses. Furthermore, local variations in microstructure strongly influence the cyclic deformation behaviour and the indent pile-up size and morphology. Interestingly, the results well coincide with observations during fatigue of the bulk alloy reported in the literature.

Microstructure and Properties of SLM AlSi10Mg: Understanding the Influence of the Local Thermal History

Selective Laser Melting (SLM) is an Additive Manufacturing technique that is widely used to produce AlSi10Mg parts with a good strength-to-weight ratio. Indeed, strongly refined microstructures are obtained due to the ultra-fast cooling rates reached in this process, conferring high strength to the parts, even in the as-built state. However, microstructural heterogeneities at the scale of the melt pool may exert a detrimental influence on the mechanical properties e.g. by causing a loss in ductility. This study thus aims at a better understanding of the influence of the local thermal history on local variations of microstructure and mechanical properties. Microscopy (i.e. SEM+EDS) and nanoindentation have been combined to reach a detailed knowledge of the local microstructure and properties. In particular, the solute Si content in the α-Al matrix, the volume fraction and the size of Si precipitates have been quantified by microscopy analysis. These local microstructural parameters are correlated with the matrix hardness as revealed by nanoindentation. Finally, the results of this detailed characterization are linked with the local thermal history that is approached in two different ways i.e. (i) an analytical description of thermal gradients inside the melt pool based on Rosenthal’s and Matyja’s equations and (ii) a simple Finite Element model for the deposition of a few layers in the SLM process.

Preface

This year the Risø International Symposium on Materials Science is celebrating its 40th year jubilee. During this 40-year period, significant progress has taken place and new discoveries and understanding in the field of material science and engineering have materialized in advanced metals and alloys with wide applications in society. In parallel, there has been an unprecedented development of 2D, 3D and 4D characterization techniques operating on multiple length scales down to the atomic dimension. The development has led to revision and foundation of fundamental principles and models for example in the areas of deformation, recovery and recrystallization of metals and alloys. This in-turn has led to new and demanding challenges that are addressed in the Proceedings. The challenges include:• Transitional stages in the evolution of deformation microstructure and strengthening mechanisms.• Effects of local microstructural variations on the evolution of microstructures during annealing• 2D, 3D and 4D characterization techniques based on for example x-rays and electron microscope techniques in the laboratory and at international facilities.• Microstructural variations at different length scales which is of critical importance for the properties and performance of metals and alloys for engineering applications.• Advanced and new processing techniques of bulk materials and by application of powder metallurgy in additive manufacturing and spark plasma sintering processes.• Experimental findings in 3D and 4D used as input to as well as validation of analytical and numerical models.The metals and alloys investigated are of fcc, bcc and hcp structure, in bulk form or present as gradients or multilayer structures.The Proceedings contain 15 keynote, 11 invited and 34 contributed presentations covering the above-mentioned key themes. It is our hope that Proceedings can inspire the future development of this field.

Microstructural response to compression deformation of ultrafine-grained aluminum with various microstructures

Ultrafine-grained aluminum having a variety of microstructural features such as fibrous grain morphology, strong or random texture and a dominance of low- or high-angle grain boundaries was compressed to 4% of true strain. This made it possible to measure yield strength and begin a microstructure evolution to accommodate an external load. The level of deformation was high enough to provide changes in the microstructure, but relatively low in order to make it possible to determine the deformation mode in grain types recognized in the pre-compressed condition, before they become unrecognizable. As a next step, a detailed EBSD examination of these samples before and after compression was performed to track those changes. These observations revealed that, in the samples with a fibrous microstructure, the role of particular fibers changes when the grain refinement is more pronounced within them. In the sample with a relatively uniform microstructure and a grain size of below 800 nm, the post-deformation observations indicated there had been significant grain rotations with a blurring of the initial texture. Based on our observations, we can state that the heterogeneity of plastic deformation mechanisms in ultrafine-grained aluminum is dependent on local variations in microstructure – the arrangement of defects and the crystallographic orientation.

On the use of heterogeneous thermomechanical and thermophysical material properties in finite element analyses of cast components

Cast components generally show a heterogeneous distribution of material properties, caused by variations in the microstructure that forms during solidification. Variations caused by the casting process are not commonly considered in structural analyses, which might result in manufacturing of sub-optimised components with unexpected in-use behaviour. In this paper, we present a methodology which can be used to consider both thermomechanical and thermophysical variations using finite element analyses in cast components. The methodology is based on process simulations including microstructure modelling and correlations between microstructural features and material properties. Local material data are generated from the process simulation results, which are integrated into subsequent structural analyses. In order to demonstrate the methodology, it is applied to a cast iron cylinder head. The heterogeneous distribution of material properties in this component is investigated using experimental methods, demonstrating local variations in both mechanical and physical behaviour. In addition, the strength-differential effect on tensile and compressive behaviour of cast iron is considered in the modelling. The integrated simulation methodology presented in this work is relevant to both design engineers, production engineers as well as material scientists, in order to study and better understand how local variations in microstructure might influence the performance and behaviour of cast components under in-use conditions.

Atom probe examinations of Zircaloy irradiated at nominally 358 °C

Abstract Zircaloy-2 and Zircaloy-4 were neutron irradiated at 358 °C in the high flux isotope reactor (HFIR) to a neutron fluence of 29.6 × 1024 n/m2 (E > 1 MeV) and the resultant irradiation hardening was measured by tensile testing. Irradiation at the relatively high temperature of 358 °C resulted in about 25–45% less hardening than observed in the literature for irradiations at 260–326 °C. The lower irradiation hardening was shown to result from a coarser loop distribution. The measured irradiation hardening was closely related to the 〈a〉 loop size and number density (ND) using an Orowan hardening model when the loops were assumed to be strong barriers like voids. In this work Atom Probe Tomography (APT) was used to detect the clustering of Fe, Sn, and Cr at features consistent with loops. The tendency for clustering of Fe, Sn, and Cr at loops could result in the apparent high barrier strength for these features. These clusters are also an additional hardening barrier. However, local variation in the distribution of clusters is observed from region to region in the irradiated material that appears to be related to factors such as the proximity to precipitates, local variations in microstructure, and/or microchemistry.

Microstructural analysis of mass transport phenomena in gas diffusion media for high current density operation in PEM fuel cells

Cost reduction is a key issue for commercialization of fuel cell electric vehicles (FCEV). High current density operation is a solution pathway. In order to realize high current density operation, it is necessary to reduce mass transport resistance in the gas diffusion media commonly consisted of gas diffusion layer (GDL) and micro porous layer (MPL). However, fundamental understanding of the underlying mass transport phenomena in the porous components is not only critical but also not fully understood yet due to the inherent microstructural complexity. In this study, a comprehensive analysis of electron and oxygen transport in the GDL and MPL is conducted experimentally and numerically with three-dimensional (3D) microstructural data to reveal the structure–transport relationship. The results reveal that the mass transport in the GDL is strongly dependent on the local microstructural variations, such as local pore/solid volume fractions and connectivity. However, especially in the case of the electrical conductivity of MPL, the contact resistance between carbon particles is the dominant factor. This suggests that reducing the contact resistance between carbon particles and/or the number of contact points along the transport pathway can improve the electrical conductivity of MPL.

Reconstruction of the 3D representative volume element from the generalized two-point correlation function

This paper presents the first application of three-dimensional (3D) cross-correlation microstructure reconstruction implemented for a representative volume element (RVE) to facilitate the microstructure engineering of materials. This has been accomplished by developing a new methodology for reconstructing 3D microstructure using experimental two-dimensional electron backscatter diffraction data. The proposed methodology is based on the analytical representation of the generalized form of the two-point correlation function—the distance-disorientation function (DDF). Microstructure reconstruction is accomplished by extending the simulated annealing techniques to perform three term reconstruction with a minimization of the DDF. The new 3D microstructure reconstruction algorithm is employed to determine the 3D RVE containing all of the relevant microstructure information for accurately computing the mechanical response of solids, especially when local microstructural variations influence the global response of the material as in the case of fracture initiation.

An investigation into microstructure and mechanical properties of cold sprayed 7075 Al deposition

Cold spray deposition is well known to produce a highly deformed microstructure with large strain gradients across the individual particles composing the deposited layer. This study investigates the relationship between the as-deposited cold spray microstructure and the resulting local and coating-scale mechanical properties of 7075 aluminum powder deposited onto wrought 7075 aluminum substrates via high pressure cold spray. Local mechanical property variations were probed using nanoindentation and correlated with microstructural characterization conducted via various microscopy techniques. 7075 Aluminum powder particles experienced high deformation rates during impact, resulting in local microstructural variations, specifically grain size and dislocation densities, between particle interfaces and their interiors. Consequently, an average 0.5 GPa increase in nanohardness was observed in particle interface regions. This increase in hardness was concluded to be primarily the result of grain refinement promoted by local dynamic recrystallization, rather than from an increase in local dislocation density causing strain hardening behavior. Examination of the coating revealed a decrease in hardness and an increase in local grain size with increasing distance from the substrate. The effects of these microstructural variations on the quality of the deposition were also evaluated by tensile pull-off and three lug shear testing.

Closed Chain Simulations of a Cast Aluminium Component - Incorporating Casting Process Simulation and Local Material Characterization into Stress-strain Simulations

The coupling between simulations of solidification, microstructure and local mechanical behaviour and simulation of stress-strain behaviour is studied by applying a recently developed simulation strategy to a high pressure die cast aluminium component. In the simulation strategy, named a closed chain of simulations for cast components, the mechanical behaviour throughout the component is determined locally by a casting process simulation. The entire casting process, including mould filling and solidification, is simulated to predict the formation of microstructure and residual stresses throughout the component, and material characterization models are applied to relate microstructural features to local elastic and plastic mechanical material behaviour. The local material behaviour is incorporated into a finite element method (FEM) stress-strain simulation of a realistic load case of the component in service.In the current contribution the influences of local variations in mechanical behaviour and residual stresses on the component behaviour are investigated. The simulation results for local microstructure and mechanical behaviour are compared to experimental results, and the predicted local mechanical behaviour is incorporated on an element level into the FEM simulation. The numerical effect of the variations in mechanical behaviour is quantified by comparing the results achieved using local behaviour and homogeneous behaviour. The influence of residual stresses predicted by the casting process simulation on the component behaviour is also studied.The casting process simulation is found to accurately predict the local variations in microstructure throughout the component, and the local variations in mechanical behaviour are well described. The numerical results show that casting process simulation and modelling of microstructure formation, material behaviour and residual stresses are important contributions to correctly predict the behaviour of a cast aluminium component in service. This motivates the use of the proposed simulation strategy, and show the importance of incorporating materials science and casting process simulations into structural analyses of cast components. It is discussed that integration of these areas, e.g. using the closed chain of simulations, is important in order to increase the accuracy of FEM simulations and the product development efficiency in the future.

The effect of intravertebral heterogeneity in microstructure on vertebral strength and failure patterns

The goal of this study was to determine the influence of intravertebral heterogeneity in microstructure on vertebral failure. Results show that noninvasive assessments of the intravertebral heterogeneity in density improve predictions of vertebral strength and that local variations in microstructure are associated with locations of failure in the vertebral body. The overall goal of this study was to determine the influence of intravertebral heterogeneity in microstructure on vertebral failure. Trabecular density and microarchitecture were quantified for 32 thoracic vertebrae using micro-computed tomography (μCT)-based analyses of 4.81 mm, contiguous cubes throughout the centrum. Intravertebral heterogeneity in density was defined as the interquartile range and quartile coefficient of variation of the cube densities. The vertebrae were compressed to failure to measure stiffness, strength, and toughness. Pre- and post-compression μCT images were analyzed using digital volume correlation to quantify failure patterns in the vertebrae, as defined by the distributions of residual strain. Failure patterns consisted of large deformations in the midtransverse plane with concomitant endplate biconcavity and were linked to the intravertebral distribution of bone tissue. Low values of connectivity density and trabecular number, and high values of trabecular separation, were associated with high strains. However, local microstructural properties were not the sole determinants of failure. For instance, the midtransverse plane experienced the highest strain (p < 0.008) yet had the highest density, lowest structure model index, and lowest anisotropy (p < 0.013). Accounting for the intravertebral heterogeneity in density improved predictions of strength and stiffness as compared to predictions based only on mean density (strength: R(2) = 0.75 vs. 0.61, p < 0.001; stiffness: R(2) = 0.44 vs. 0.26, p = 0.001). Local variations in microstructure are associated with failure patterns in the vertebra. Noninvasive assessments of the intravertebral heterogeneity in density--which are feasible in clinical settings--can improve predictions of vertebral strength and stiffness.

Digital Volume Imaging of the PEFC Gas Diffusion Layer

The gas diffusion layer (GDL) plays a key role in the overall performance/durability of a polymer electrolyte fuel cell (PEFC). Of profound importance, especially in the context of water management and flooding phenomena, is the influence of the underlying pore morphology and wetting characteristics of the GDL microstructure. In this article, we present the digital volumetric imaging (DVI) technique in order to generate the 3-D carbon paper GDL microstructure. The internal pore structure and the local microstructural variations in terms of fiber alignment and fiber/binder distributions are investigated using the several 3-D thin sections of the sample obtained from DVI.

Macroregion Size Measurements in Bimodal Titanium Forgings Using Two-Dimensional Autocorrelation Method

Etching patterns displayed on the surfaces of near-alpha titanium forgings (alloy IMI834) were quantified in terms of sizes and orientations using a two-dimensional (2-D) autocorrelation method. These patterns, which can be associated with local variations of microstructure and further related to regions of specific crystallographic orientations, are known to play a significant role in fatigue and dwell fatigue life predictions. It is then necessary to quantify their dimensions in a manufactured part in order to build a better statistical approach for life prediction in titanium forgings. These distributions of macroregion size and shape were examined on forging cross sections. A data analysis methodology based on a 2-D autocorrelation was used to process sample image data and quantify the macroregion characteristics. The results are more precise than those obtained using a mean linear intercept (MLI) method and additional useful information can be gathered.

High temperature fatigue of friction welded joints in dissimilar nickel based superalloys

The high temperature fatigue performance of two dissimilar material joints made by inertia friction welding and linear friction welding is assessed in the present study. The fatigue strength of the welded joints is found to be comparable to that of the weaker parent material for the plainsided specimens. However, the fatigue crack growth resistance within the weld zone of both material systems is found to be reduced compared to that of parent material. This effect is more pronounced for high temperature Ni alloys than for the lower temperature nickel alloy. Environmental attack is confirmed to be the controlling failure mechanism for acceleration of crack growth, but the extent to which it is observed is sensitive to local microstructure, i.e. to grain size, redistribution of the main strengthening phase γ′, and to the grain boundary carbide morphology. Such local microstructural variations are the result of welding and subsequent post-weld heat treatment procedures.