Local Microstructure Properties Research Articles

Simulation methods for local microstructure evolution-cooling and time–temperature transformation behavior in heat treatment of tool steels

This paper presents the development of a simulative workflow capable of predicting microstructural evolution during heat treatment processes. It represents a meaningful advance in this field by extending existing simulation models previously published by the authors. In this previous work, the software solutions MatCalc®, MATLAB®, and Abaqus FEA® were coupled to calculate several local microstructural properties: the carbide content, the type, the distribution, and the chemical composition of the matrix. In addition, the model could determine the proportions of microstructural components such as martensite and retained austenite within the matrix. The hardening treatment was simplified by assuming a fast quenching, leading to complete martensitic phase transformation. However, this assumption may not be valid for larger components, leaving room for optimization. Therefore, the simulation model in this publication has been successfully extended to include local solution-state dependent time–temperature transformation behavior. In addition, an automated microstructure simulation of the entire component is now possible. As an application example, two tool geometries of different sizes were simulated with an identical furnace heat treatment. The same furnace temperature (T = 1050 °C) and the same holding time (t = 60 min) were simulated with a slow air cooling (Tair = 25 °C). The austenitizing temperature and holding time were chosen to dissolve a sufficient amount of carbides during austenitization, and the slow cooling rates were chosen to form diffusion controlled phases such as bainite or pearlite. To validate the simulation model, the simulated time–temperature sequences were reproduced experimentally in a quenching dilatometer. The resulting real microstructures were compared with the simulated ones.

Insight into layer formation during friction surfacing: Relationship between deposition behavior and microstructure

Friction surfacing (FS) is a solid state layer deposition technique with a simple setup, presenting advantages compared to fusion-based approaches. Previous investigations showed microstructural gradients along layer width and thickness. The current study provides new insight into the FS layer formation for aluminum and its relation with the microstructure evolution. Special consumable studs containing two different aluminum alloys were used to visualize the different materials in the resulting deposit. The investigation was performed at different process parameters, revealing some fundamental material flow characteristics. The layer center presents inner stud material, where advancing side and top are formed by outer stud material. The bottom and retreating side present a mixture of inner and outer stud material. The part of the layer that is formed by the outer material, presumably undergoes higher strain rates during deposition, presenting finer grains. The top of FS layers shows a pronounced texture, i.e. shear texture components, compared to the other parts with random texture. This phenomenon can be related to the shearing of the stud material between already deposited material below and the stud at its rear edge. Overall, the FS layer formation characteristics revealed in this study are directly related to local microstructural properties.

Joint Distributions of Local Pore Space Properties Quantitatively Explain Simulated Air Flow Variations in Paper

The gas flow through sheet-like porous materials such as paper can show marked lateral variations due to a heterogeneous, locally varying microstructure. Hence, reliable predictions of such lateral flux variations require an appropriate consideration of local variations in the microstructure. The flow through such sheet-like materials is commonly described with Darcy’s law in which permeances are formulated in terms of microstructure properties, such as porosities, tortuosities, or hydraulic radii. This work proposes an extension of existing permeance models that directly considers the variation and the cross-dependence between local microstructure properties. The extended model is applied to local air fluxes through a paper sheet to exemplarily reveal the joint impact of local porosities and local tortuosities on the air flux. The key extension is to consider a joint distribution of porosity and tortuosity. The latter is constructed from the univariate property distributions using a copula approach and yields local tortuosities including their variation for any encountered local porosity. These values jointly enter any permeance model that qualitatively captures the dependence of the air flux on the porosity. To assess the merit of the model, variations in the air flux and in the pore space properties are independently determined from the same measured microstructure of paper. Air flux variations are provided by computational fluid dynamics simulations on multiple, nonoverlapping segments taken from the microstructure. A statistical analysis of the entire microstructure provides the distribution of local porosity, tortuosity, and thicknesses. Our model quantitatively explains that porosity-dependent variations in the tortuosity, in particular the ones associated with high-volume pathways, decisively determine air flux variations.

Effect of local microstructure on fatigue and related failure mechanisms in AlSi-castings

Mobility applications often demand shape-optimized designs. Casting of AlSi-alloys is an efficient bulk production technology featuring parts with strongly varying wall thickness. The local cooling rate is process controlled, implying severe changes in mechanical properties such as degree of porosity and secondary dendrite arm spacing.This study provides a brief overview about the effect of local microstructural properties, and intrinsic defect distribution, on associated fatigue strength for alloy EN AC 42100 and alloy 46200. Results from extensive fractographic investigations and computed tomography revealed microporosity as the primary failure mechanism for high-cycle-fatigue design, but dependent on the load level and casting process condition other crack initiating imperfections such as oxide films, slip planes and inclusions get more pronounced. A statistical assessment of fatigue strength considered the local manufacturing conditions and the associated varying failure mechanisms.

Phase field mechanics of residually stressed ceramic composites

ABSTRACT A phase field theory for crystalline solids accounting for thermoelasticity, fracture, twinning, and limited slip is presented. Residual stresses are incorporated via referencing thermoelastic strain to a reference state that is not always stress-free. Rate dependence, dissipated energy, and residual strain energy (possibly degraded by local fracture) are included in the theory, with physically valid predictions first verified for homogeneous stress states. A variational form of the model is implemented in finite element (FE) calculations of three-dimensional (3-D) polycrystalline aggregates consisting of up to two different crystal constituents and a binding matrix along grain and phase boundaries. Model specifics correspond to constituents of boron carbide-titanium diboride (BC-TiB) ceramic composites. Effects of thermal-residual stresses incurred during processing, as well as other local microstructure properties and physical features, on deformation and failure mechanisms are revealed. Peak aggregate strengths observed for different boundary conditions demonstrate a pressure-dependent failure surface.

Investigation of the statistical and geometrical size effect on the fatigue strength of EN-GJL-300 under consideration of the graphite morphology

Grey cast iron (GJL) is one of the most important materials for numerous components in car, utility vehicle and ship engines, power generation units and machine beds in various branches of mechanical engineering. In order to optimize the design of grey cast iron components for applications subjected to cyclic loading, a precise knowledge of the fatigue mechanisms is necessary. In the framework of a research project, the statistical and geometrical size effect on the fatigue strength of EN-GJL-300 is investigated. The statistical size effect describes the relationship between the fatigue strength and a changing stressed volume or surface and can be expressed with the highly stressed volume (HSV) of the component. The geometrical size effect instead takes into account the influence of a stress gradient e.g. due to notches on fatigue strength. Knowledge of the statistical and geometrical size effect allows the transfer of the fatigue strength from small specimens with different notch factors to complex large-scale components. For the investigation, unnotched and notched specimens with different highly stressed volumes are removed from cast samples as well as from a gearbox housing to determine the cyclic material behavior by stress controlled axial fatigue tests. The investigations are complemented by metallographic analyses to describe the influence of the graphite morphology on fatigue strength. The data obtained in this study serve as a contribution to the development of a design and lifetime assessment concept enabling an estimation of the local lifetime of the component as a function of the highly stressed volume and the local microstructural properties.

Numerically simulated prediction of residual stresses in welding considering phase transformation effects

This work deals with investigating the effects of solid-state phase transformation on residual stresses during the welding of low carbon and high carbon steels. In this study, depending on MSC Marc code, the simulation considers the local microstructure properties changes due to the thermal welding cycles. A sequentially coupled thermal and mechanical 2-D finite element model (FEM) was used. In FEM, phase transformation temperature diagrams are used to anticipate the amount of martensite in the fusion zone (FZ), and heat affects zone (HAZ). The simulation results demonstrated that the residual stress in low carbon steel is not affected by the volume change caused bythe austenite-martensite transformation. In contrarily,the residual stressesin the high carbon steel are considerably influenced by the martensitic transformation.

Blanking induced damage in thin 3.2% silicon steel sheets

The cores of electrical motors and transformers are made by blanking, piercing and stacking of thin metallic sheets having various features cut from the original blank. The material experiences local plastic deformation near the cut edge due to the blanking operation. The quality and efficiency of the produced products are directly affected by the mechanical and magnetic properties of the blanks at the cut edge. The effects of the blanking process on deformation evolution in thin sheets of high Si electrical steels was investigated. In-situ blanking experiments together with the digital image correlation (DIC) technique were used to quantify local deformation evolution during thin sheet blanking operations. Magnetic hysteresis losses were measured using a purpose-built single sheet tester and linked to the measured deformation maps. The residual stresses were qualitatively assessed by means of nano-hardness measurements while the local microstructural properties and dislocation generations were determined using EBSD analysis of the blanked parts. The results indicated that for the tested materials with 0.1t blanking clearance, electrical steel sheets with 0.2 mm thickness experiences larger deformation prior to fracture during blanking compared with samples having 0.35 mm thickness. This has a direct relationship with the measured hysteresis losses. However, the dislocation maps indicated that dislocations of GNDs are more pronounced for thicker samples that aligns with the effect of dislocations on magnetic power losses rather than hysteresis losses measured in this research.

Microporosity and statistical size effect on the fatigue strength of cast aluminium alloys EN AC-45500 and 46200

This paper investigates the fatigue strength of two cast aluminium alloys, EN AC-45500 and 46200, dealing with the influence of microporosity and the statistical size effect. Small-scale round specimens are extracted from cylinder heads and crank cases as typical cast components in automotive industry. Uniaxial fatigue tests under alternating tension/compression loading are performed. Local microstructural properties, such as second dendrite arm spacing and microporosity, are characterized by means of metallography, fracture surface analysis utilizing scanning electron microscopy, and X-ray computed tomography. The measurements reveal significant differences in microporosity and microstructure depending on the extraction position and specimen type. These findings are reflected by the experimental test results showing that the microporosity majorly affects the fatigue behaviour with a maximum difference in fatigue resistance at ten million load-cycles of up to 39% in case of the EN AC-45500 specimens. Additional experiments involving two different EN AC-46200 specimen types exhibiting unequal highly-stressed volumes demonstrate a reduction of the high-cycle fatigue strength by 8% caused by the statistical size effect. Fatigue strength assessment incorporates the application of the model by Tiryakioğlu based on the extreme value distribution of the micropore sizes by Gumbel, as well as the √area approach by Murakami. The evaluated results agree well to the fatigue tests enabling a local fatigue strength assessment under consideration of manufacturing process dependent material characteristics.

Residual stresses formation in multi-pass weldment: A numerical and experimental study

In this study, the residual stresses distribution induced by multi-pass arc welding of the steel S355J2+N are investigated experimentally and numerically. An extended approach is used for the simulations, which considers the change of the local microstructure properties due to multiple reheating. Experimental material data obtained from physical welding simulations with Gleeble® are used for the model calibration. The experimental stress study is performed using a neutron diffraction method on a fourier stress diffractometer. Numerical analysis of the welding stresses formation in the weldment is performed and compared to the experimental study. The results explain the influence of the welding thermal history on the resulting local thermo-mechanical properties in the heat-affected zone and, thus, on the residual stress distribution. The consideration of the local microstructure properties in the welding simulation leads to a significant increase in accuracy of the numerical results. The major influence factor on the residual stress formation is the change in the interpass microstructure yield strength. When a root pass with short cooling times is subjected to re-austenitisation in the fine-grained zone, the yield strength increases in this area and affects consequently the residual stress distribution. The influence of the reheating is detectable in the depth of the weldment, but it is less significant for the residual stress formation near the surface of the welded joint.

Modelling the Local Microstructure Properties due to Multi-Pass Welding

The paper presents an advanced simulation approach developed for considering the local microstructure properties variation due to multiple heat treatment. The model describes the resulting microstructure as a function of the peak temperature, austenisation time, cooling time and takes into account the microstructure formed after each thermal cycle. The model is calibrated with experimental material data obtained by repeated thermal loads. It is qualified to calculate the hardness and local microstructure properties in the HAZ of multi-pass welds. Thermo-mechanical simulation of the residual welding stresses and distortions in multi-pass welded joint is performed and validated by measurements.

Diffusion-tensor imaging of major white matter tracts and their role in language processing in aphasia

A growing literature is pointing towards the importance of white matter tracts in understanding the neural mechanisms of language processing, and determining the nature of language deficits and recovery patterns in aphasia. Measurements extracted from diffusion-weighted (DW) images provide comprehensive in vivo measures of local microstructural properties of fiber pathways. In the current study, we compared microstructural properties of major white matter tracts implicated in language processing in each hemisphere (these included arcuate fasciculus (AF), superior longitudinal fasciculus (SLF), inferior longitudinal fasciculus (ILF), inferior frontal-occipital fasciculus (IFOF), uncinate fasciculus (UF), and corpus callosum (CC), and corticospinal tract (CST) for control purposes) between individuals with aphasia and healthy controls and investigated the relationship between these neural indices and language deficits.Thirty-seven individuals with aphasia due to left hemisphere stroke and eleven age-matched controls were scanned using DW imaging sequences. Fractional anisotropy (FA), mean diffusivity (MD), radial diffusivity (RD), axial diffusivity (AD) values for each major white matter tract were extracted from DW images using tract masks chosen from standardized atlases. Individuals with aphasia were also assessed with a standardized language test in Russian targeting comprehension and production at the word and sentence level.Individuals with aphasia had significantly lower FA values for left hemisphere tracts and significantly higher values of MD, RD and AD for both left and right hemisphere tracts compared to controls, all indicating profound impairment in tract integrity. Language comprehension was predominantly related to integrity of the left IFOF and left ILF, while language production was mainly related to integrity of the left AF. In addition, individual segments of these three tracts were differentially associated with language production and comprehension in aphasia. Our findings highlight the importance of fiber pathways in supporting different language functions and point to the importance of temporal tracts in language processing, in particular, comprehension.

A general linear relaxometry model of R1 using imaging data

PurposeThe longitudinal relaxation rate (R1) measured in vivo depends on the local microstructural properties of the tissue, such as macromolecular, iron, and water content. Here, we use whole brain multiparametric in vivo data and a general linear relaxometry model to describe the dependence of R1 on these components. We explore a) the validity of having a single fixed set of model coefficients for the whole brain and b) the stability of the model coefficients in a large cohort.MethodsMaps of magnetization transfer (MT) and effective transverse relaxation rate (R2*) were used as surrogates for macromolecular and iron content, respectively. Spatial variations in these parameters reflected variations in underlying tissue microstructure. A linear model was applied to the whole brain, including gray/white matter and deep brain structures, to determine the global model coefficients. Synthetic R1 values were then calculated using these coefficients and compared with the measured R1 maps.ResultsThe model's validity was demonstrated by correspondence between the synthetic and measured R1 values and by high stability of the model coefficients across a large cohort.ConclusionA single set of global coefficients can be used to relate R1, MT, and R2* across the whole brain. Our population study demonstrates the robustness and stability of the model. Magn Reson Med, 2014. © 2014 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. Magn Reson Med 73:1309–1314, 2015. © 2014 Wiley Periodicals, Inc.

Progress in inferring desmoplastic stromal tissue microstructure noninvasively with ultrasound nonlinear elasticity imaging

Desmoplasia is the abnormal growth of fibrous tissue that is often associated with malignancy in breast and other solid tumors. The collagen comprising the extracellular tissue matrix tends to be more dense than in normal breast tissue, and to exhibit altered microstructure. The distinctive mechanical properties of solid tumors are often attributed to the abnormal density and microstructure of their collagen extracellular matrices. For example, a higher concentration of collagen implies a larger linear elastic modulus, while a decrease in fiber tortuosity would mean a smaller “toe” region in the stress-strain curve. This talk describes progress of inferring local tissue microstructural properties from noninvasive, in vivo measurement of nonlinear elastic properties of breast tissue. An ultrasound elasticity-imaging based approach for determining the average local micro-structural properties of tissue was developed and implemented. It utilizes multiscale homogenization methods to determine effective macroscopic properties from assumed microstructural arrangements of collagen fibers. The resulting nonlinear constitutive equation is then used in a model-based elastic inverse problem solver to obtain local microstructural tissue properties from macroscopic observations of tissue deformation. Its potential in diagnosing malignant breast tumors in a small set of patients was also examined. [NSF Grant 50201109; NIH NCI-R01CA140271.]

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.