Behavior Of Individual Grains Research Articles

Micro-mechanics investigation of heterogeneous deformation fields and crack initiation driven by the local stored energy density in austenitic stainless steel welded joints

This work investigates the heterogeneous deformation and failure of HR3C austenitic stainless steel welded joints at room and typical service temperatures. Such types of welded joints are widely used in the new generation of fossil fuel power stations and are known to suffer from premature high temperature failure. Observation of in-service failures revealed that cracks may nucleate either in the heat affected zone or in the weld metal. The main objective of this work is to identify the local microstructural conditions and stress–strain fields responsible for both ductile failure and micro-crack nucleation within the weldment at low and high temperatures, respectively. To that purpose, a novel multi-scale micromechanics-based modelling framework is proposed. It relies on representative weld microstructure models digitally reconstructed from electron backscatter diffraction measurements, and dislocation density-based crystal plasticity models to describe the behaviour of individual grains in each weld region. A novel thermo-dynamically consistent relation for the configuration stored energy per unit volume is derived from the crystal plasticity framework.The single crystal models are calibrated and validated from a combination of uniaxial tensile tests on cross-weld specimens using high resolution digital image correlation techniques, representative volume elements of the polycrystals at the macro-scale, as well as micropillar compression tests at the scale of the individual grains. Predictions of heterogeneous inelastic deformation fields within the weldment at both 22 °C and 665 °C, and of micropillar compression behaviour of the individual weld single crystal phases are consistent with experimental data. The experimentally observed ductile failure of the weld metal at room temperature is successfully predicted using a Gurson-type void nucleation, growth and coalescence constitutive model. The suitability of the stored strain energy density and the accumulated plastic strain as crack initiation/creep failure indicators is investigated. It was found that predicted creep lifetimes with either indicator were very similar, and that they were relatively accurate for low stress levels.

Investigation of diffusive grain interactions during equiaxed dendritic solidification

Grain growth during the solidification of an alloy is accompanied by the enrichment of the surrounding liquid phase in chemical species. The resulting diffusion fields induce interactions between the grains that are strongly dependent on their spatial arrangement. The influence of these diffusive interactions is not limited to the shapes and sizes of grains but also controls the overall kinetics of the transformation. In this study we investigate the role of the spatial arrangement of equiaxed grains in the average growth kinetics of an ensemble of grains. We perform full-field simulations of different grain ensembles using the mesoscopic grain envelope model (GEM). We show that the growth kinetics of randomly arranged ensembles is fundamentally different from that of periodic arrangements. We provide an explanation of this difference through analyses of the behaviour of individual grains in their local neighbourhood, defined by a Voronoi tessellation. Finally, we compare the full-field GEM simulations to state-of-the-art mean-field models and we point out the limitations of the latter in the prediction of dendritic growth.

The heterogeneous nature of mechanically accelerated grain growth

AbstractWhile grain growth is traditionally viewed as a purely thermally driven process, nanocrystalline metals can undergo grain growth under mechanical loads, even at room temperature. We performed a detailed atomistic study of the heterogeneous nature of mechanically accelerated grain growth in a polycrystalline Pt nanowire. Using molecular dynamics simulations, we compared the grain-growth behavior of individual grains during tensile and shear cyclic loading, for three different equivalent strain levels, and at two temperatures. Pure thermal grain growth with no mechanical loading provided a baseline reference case. On average, grains that were already susceptible to thermal grain growth were stimulated to grow faster with mechanical loading, as expected. However, when analyzed on a grain-by-grain basis, the results were far more complex: grains that grew fastest under one stimuli were less accelerated under other stimuli. Even when the magnitude of loading changed, the relative growth of individual grains was distorted. We interpret this complexity from the perspective of superimposed growth mechanisms.

Multimodal assessment of mechanically induced transformation in metastable multi‐phase steel using X‐ray nano‐tomography and pencil‐beam diffraction tomography

A combination of X-ray nano-tomography and pencil-beam diffraction tomography was utilized for multimodal assessment of the mechanically induced transformation of individual retained austenite grains during tensile deformation in a 0.1C-5Mn-1Si multi-phase steel. In the present study, a newly developed high energy (20 - 30 keV) and high resolution (spatial resolution of 0.16 µm in this study) X-ray nano-tomography technique was applied for the first time to the in-situ observation of a steel under external loading. The gradual transformation, plastic deformation, and rotation behaviour of the individual austenite grains were clearly observed in 3D. It was revealed that the early stage of the transformation was dominated by the stress-assisted transformation that can be associated with measured mechanical driving force, whilst the overall transformation was dominated by the strain-induced transformation that is interrelated with measured dislocation multiplication. The transformation behaviour of individual grains was classified according to their initial crystallographic orientation and size. Noteworthy was the high stability of coarse austenite grains (i.e., 2.5 μm or larger in diameter), contrary to past reports in the literature. Characteristic rotation behaviour and wide data dispersion were also observed in the case of the individual austenite grains. It was conclusively demonstrated that such characteristic behaviour partly originated from interactions with surrounding soft and hard phases. The origins of these characteristics are discussed by combining the image-based and diffraction-based information.

A Grain-Scale Study of Mojave Mars Simulant (MMS-1).

Space exploration has attracted significant interest by government agencies and the scientific community in recent years in an attempt to explore possible scenarios of settling of facilities on the Moon and Mars surface. One of the important components in space exploration is related with the understanding of the geophysical and geotechnical characteristics of the surfaces of planets and their natural satellites and because of the limitation of available extra-terrestrial samples, many times researchers develop simulants, which mimic the properties and characteristics of the original materials. In the present study, characterization at the grain-scale was performed on the Mojave Mars Simulant (MMS-1) with emphasis on the frictional behavior of small size samples which follow the particle-to-particle configuration. Additional characterization was performed by means of surface composition and morphology analysis and the crushing behavior of individual grains. The results from the study present for the first time the micromechanical tribological response of Mars simulant, and attempts were also made to compare the behavior of this simulant with previously published results on other types of Earth and extra-terrestrial materials. Despite some similarities between Mars and Moon simulants, the unique characteristics of the MMS-1 samples resulted in significant differences and particularly in severe damage of the grain surfaces, which was also linked to the dilation behavior at the grain-scale.

Influence of temperature on crystallographic orientation induced anisotropy of microscopic wear in an AZ91 Mg alloy

Herein, the temperature-dependent wear behavior of individual grains of solution treated AZ91 alloy with different crystallographic orientations was investigated. Nanoindentation was employed to measure the hardness and wear of differently oriented grains at 25 °C (RT), 100 °C and 150 °C. Scratching at RT showed strong anisotropy in wear for different crystallographic orientations where the wear resistance of basal orientation was found to be highest, followed by pyramidal and prismatic orientations at RT. At 100 °C, the wear anisotropy reduced significantly and at 150 °C, the anisotropic behavior was absent. The decrease in anisotropy with temperature can be rationalized by considering the variation in active deformation mechanisms. Additionally, at 100 °C and 150 °C, dynamic recrystallization was observed and the evolved texture was independent of initial orientation.

Calibration of discrete-element-method model parameters of bulk wheat for storage

The discrete element method (DEM) parameters of wheat were calibrated for the modelling of a grain storage system. In addition, the effects of material parameters on the accuracy of DEM modelling of the oedometric compaction and unloading of bulk wheat were analysed. The bi-stiffness Hertz-like contact model was applied to incorporate the hysteretic behaviour of individual grains. The parameters of the contact model were calibrated based on experimental data of the loading–unloading cycle of a single wheat grain. The results of DEM simulations of the bulk behaviour of grain were validated using experimental data from oedometric tests. The observed trends indicate good correspondence between the experimental data and DEM simulations using the calibrated parameters. Simulations performed assuming spherical particles provide the best approximation of the loading–unloading cycle. The application of a special procedure for performing DEM simulations allows for the accurate reproduction of experimental values of the initial, compacted, and unloaded porosities of bulk wheat grains in the considered moisture-content range considered. • Bi-stiffness contact model is positively verified for the storage of wheat. • Calibration of the DEM parameters of wheat for storage is performed. • Bulk properties of wheat grain are modelled and verified experimentally. • The effect of moisture content on bulk properties is examined.

Semi in-situ observation of crystal rotation during cold rolling of commercially pure aluminum

In this research, the validity of the deformation texture simulation based on the full-constraint (FC) Taylor model is assessed by a semi in-situ observation of crystal rotation during rolling. In order to study the deformation behavior of individual grains, successive rolling reductions (up to 24%) were applied to a split-sample of a commercially pure aluminum specimen. The electron backscattering diffraction (EBSD) technique was used to collect the crystallographic orientations and to investigate the microstructural evolution. The results indicate that as the deformation proceeds, the orientation spread inside the grains increases. Local variation of lattice rotation inside the grains results in the formation of a fragment boundary. As the deformation proceeds, the fragment boundary gradually moves through the original grain while it becomes narrower and carries a larger misorientation. However, up to the maximum rolling strain of 24%, the microstructural evolution did not give rise to massive generation of new high angle grain boundaries (HAGB). In general, the results of the texture predictions by the FC-Taylor model were found to be in reasonable agreement with the experimentally measured texture at low deformation. However, more plastic deformation resulted in a larger deviation. Study of individual grains rotation revealed large discrepancies in the simulated results of near cube orientations, which was attributed to the high symmetry of these orientations with respect to the deformation axis.

The effects of surface-layer grain size and texture on deformation-induced surface roughening in polycrystalline titanium hardened by ultrasonic impact treatment

The ultrasonic impact treatment (UIT) of titanium specimens leads to the grain refinement and formation of a strong basal texture in the surface layer, affecting the mechanical properties of the UIT titanium parts. In this paper, we aim at investigating numerically the effects which the UIT-modified microstructure has on the micro- and mesoscale deformation behavior of titanium specimens subjected to uniaxial tension. For this purpose, three-dimensional polycrystalline models taking an explicit account of the grain morphology and crystallographic orientations of the base material and UIT modified surface layer are generated by the method of step-by-step packing and implemented in the finite-element calculations. The constitutive models describing the nonlinear behavior of individual grains are implemented in terms of anisotropic elasticity and crystal plasticity theories. The boundary value problem is solved within a dynamic approach using the ABAQUS/Explicit package. This study allows distinguishing between the grain size and texture effects and drawing conclusions on their roles in the stress-strain evolution, plastic strain localization and deformation-induced surface roughening in UIT titanium specimens under loading.

Plastic strain localization in surface-hardened titanium polycrystals

In this paper, features of plastic strain localization in commercial purity titanium surface-hardened by ultrasonic impact treatment (UIT) are numerically investigated. Three- dimensional polycrystalline structures inherent in unhardened and UIT surface-hardened titanium specimens are generated by a step-by-step packing method. For taking into account the deformation mechanisms at the micro- and mesoscales, constitutive equations describing a nonlinear behavior of individual grains are constructed using crystal plasticity theory. Quasi-static boundary value problems are solved in a dynamic statement using ABAQUS\\Explicit finite-element package to simulate uniaxial tension of the model polycrystalline specimens. Based on the calculation results, conclusions are drawn on the effect of the UIT surface- hardened layer on the plastic strain localization and deformation-induced surface roughening in model polycrystals.

Strain partitioning in host rock controls light rare earth element release from allanite-(Ce) in subduction zones

AbstractCombined microstructural, mineral chemical, X-ray maps and X-ray single-crystal diffraction analyses are used to reveal the behaviour of individual grains of magmatic allanite relicts hosted in variably deformed metagranitoids at Lago della Vecchia (inner part of the Sesia-Lanzo Zone, Western Alps, Europe), which experienced high-pressure and low-temperature metamorphism during the Alpine subduction. X-ray single-crystal diffraction shows that none of the allanite crystals, irrespective of the strain state of the host rock, record any evidence of plastic deformation (i.e. intracrystalline deformation), as indicated by the shape of the Bragg diffraction spots, the atomic site positions, and their displacement around the centre of gravity. On the contrary, strong plastic deformation affected matrix minerals, such as quartz, white mica and feldspar of the hosting rocks, during the development of the Alpine eclogitic- and blueschist-facies metamorphism. Despite the strain-free atomic structures of allanite, different patterns of chemical zoning, as a function of strain accumulated in the rock matrix, are observed. As allanite occurs in magmatic and metamorphic rocks and it is stable at high-pressure and low-temperature conditions, we infer that allanite could behave as one of the main carriers of light rare earth elements into the mantle wedge during subduction of continental crust. In particular, the release of light rare earth elements from allanite, under high-pressure conditions in subduction zones, is facilitated by high strain accumulated in the host rock.

In situ observations of single grain behavior during plastic deformation in polycrystalline Ni using energy dispersive Laue diffraction

Energy dispersive X-ray Laue diffraction is applied to investigate the deformation behavior of individual grains in a polycrystalline nickel wire under tensile loading. 38 Laue spots are identified in the Laue pattern which originate from 9 separate grains. The simultaneous measurements of the Laue spot's position and energy obtained by using a 2D energy dispersive detector, allows to track the evolution of the 9 grains through multiple stages of deformation. Angular and spectral elongation (streaking) of the Laue spots increases as tensile loading is increased and is attributed to macroscopic texture changed and strain due to defect accumulation. On the single grain level, a correlation between crystallographic orientation and strain is investigated. Moreover, spatially resolved anisotropic deformation within a single grain is measured to increase at the grain boundaries. Comparison of the grain specific responses allow for development of a deformation scenario for the whole specimen. The presented experiment demonstrates an alternative protocol for the investigation of deformation mechanisms in polycrystalline materials.

Comparison of single-aliquot and single-grain MET-pIRIR De results for potassium feldspar samples from the Nihewan Basin, northen China

In this study we compared De results obtained from single-aliquot (SA) and single-grain (SG) measurements on potassium rich feldspars (K-feldspars) from the Nihewan Basin, north China, based on the multiple-elevated-temperature post-infrared infrared (MET-pIRIR) stimulated luminescence. Discrepancy was observed between the SA and SG De results. Potential reasons for such discrepancy, including residual doses, sensitivity changes, fading rates, internal dose rates, and/or the combination of these possibilities, were investigated. Our results indicate that the disagreement between the SA and SG De results are mainly caused by the different fading rates and, to a small extent, by the internal K contents variations of different grains. Our study highlights the importance to investigate the relationship of single-grain De values and the luminescence behavior of individual grains, such as brightness. For partial-bleached or post-depositional mixed K-feldspar samples, statistical analysis should be conducted only on the grains with the same fading rate.

Analysis of Grain-Resolved Data from Three-Dimensional X-Ray Diffraction Microscopy in the Elastic and Plastic Regimes

The combination of experimental data and polycrystal plasticity modeling is a powerful tool to advance our understanding of the behavior of individual grains and how these interact. The present paper illustrates how much information a single three-dimensional x-ray diffraction experiment contains and how this may be analyzed and compared with model predictions. The data cover the elastic and plastic regimes (0.1%, 1%, and 5% strain) of a tensile experiment on a fully recrystallized stainless-steel sample. The number of bulk grains analyzed is on the order of 200. Specific phenomena considered are stress correlations across twin boundaries, stress states, and resolved shear stresses in the elastic and plastic regimes along with lattice rotations.

Modeling Macroscopic Material Behavior With Machine Learning Algorithms Trained by Micromechanical Simulations

Micromechanical modeling of material behavior has become an accepted approach to describe the macroscopic mechanical properties of polycrystalline materials in a micro- structure-sensitive way. The microstructure is modeled by a representative volume ele- ment (RVE), and the anisotropic mechanical behavior of individual grains is described by a crystal plasticity model. Such micromechanical models are subjected to mechanical loads in a finite element (FE) simulation and their macroscopic behavior is obtained from a homogenization procedure. However, such micromechanical simulations with a discrete representation of the material microstructure are computationally very expensive, in par- ticular when conducted for 3D models, such that it is prohibitive to apply them for process simulations of macroscopic components. In this work, we suggest a new approach to de- velop microstructure-sensitive, yet flexible and numerically efficient macroscopic material models by using micromechanical simulations for training Machine Learning (ML) algo- rithms to capture the mechanical response of various microstructures under different loads. In this way, the trained ML algorithms represent a new macroscopic constitutive relation, which is demonstrated here for the case of damage modeling. In a second application of the combination of ML algorithms and micromechanical modeling, a proof of concept is presented for the application of trained ML algorithms for microstructure design with respect to desired mechanical properties. The input data consist of different stress-strain curves obtained from micromechanical simulations of uniaxial testing of a wide range of microstructures. The trained ML algorithm is then used to suggest grain size distributions, grain morphologies and crystallographic textures, which yield the desired mechanical response for a given application. For validation purposes, 1the resulting grain microstructure parameters are used to generate RVEs, accordingly and the macroscopic stress-strain curves for those microstructures are calculated and compared with the target quantities. The two examples presented in this work, demonstrate clearly that ML methods can be trained by micromechanical simulations, which capture material behaviour and its relation to microstructural mechanisms in a physically sound way. Since the quality of the ML algorithms is only as good as that of the micromechanical model, it is essential to validate these models properly. Furthermore, [...]

Micromechanical Fatigue Modelling of the Size Effect in Micro-Scale 316L Stainless Steel Specimens

For many years, computational modelling and simulation studies have been used by developers to advance device design and have been reported in regulatory medical device submissions. However, cardiovascular stent materials in such computational models are typically assumed to behave as a continuum. This approach assumes that bulk material properties apply to the micro-sized structure, i.e. material behavior is scale independent. However, as size is reduced, mechanical size effects arise as the grain size to specimen width ratio drops below a critical value. These size effects cause material behavior to deviate significantly from bulk material behavior. If such a deviation in material behavior is to be captured within computational models, it is necessary to represent the crystalline structure of a metal and to capture the anisotropic behavior of individual grains within these models. This paper describes the development of such a modelling methodology to investigate the phenomenon of strain localization within grains of a 316L stainless steel specimen under fatigue loading conditions.

Importance of outliers: A three-dimensional study of coarsening in α -phase iron

Grain coarsening behavior in an $\ensuremath{\alpha}$-phase iron sample is studied in three dimensions using high energy x-ray diffraction microscopy. 4971 grains that are entirely inside the sample are segmented in the initial state and 3905 remain after annealing. A matching procedure was used to track 3299 grains between the two states while the remainder were either consumed by neighbors or the tracking algorithm failed to correlate them. During the single annealing treatment, the average grain volume increased by 13%. Statistical analysis in each state yields subtle changes in the grain size and nearest neighbor number distributions. Correlating topological features with volume changes between states, the average behavior is seen to be consistent with an isotropic model of curvature driven coarsening, but the dispersion of volume changes in each topological class is comparable to the overall trend in the average behaviors. Thus, the topological characterizations used here are not predictive of the behavior of individual grains under the isotropic assumption. Examination of anecdotal cases allows understanding of some outliers but others appear counter to an isotropic theory.

Heterogeneous grain-scale response in ferroic polycrystals under electric field.

Understanding coupling of ferroic properties over grain boundaries and within clusters of grains in polycrystalline materials is hindered due to a lack of direct experimental methods to probe the behaviour of individual grains in the bulk of a material. Here, a variant of three-dimensional X-ray diffraction (3D-XRD) is used to resolve the non-180° ferroelectric domain switching strain components of 191 grains from the bulk of a polycrystalline electro-ceramic that has undergone an electric-field-induced phase transformation. It is found that while the orientation of a given grain relative to the field direction has a significant influence on the phase and resultant domain texture, there are large deviations from the average behaviour at the grain scale. It is suggested that these deviations arise from local strain and electric field neighbourhoods being highly heterogeneous within the bulk polycrystal. Additionally, the minimisation of electrostatic potentials at the grain boundaries due to interacting ferroelectric domains must also be considered. It is found that the local grain-scale deviations average out over approximately 10–20 grains. These results provide unique insight into the grain-scale interactions of ferroic materials and will be of value for future efforts to comprehensively model these and related materials at that length-scale.

Characterization of Deformation Behavior of Individual Grains in Polycrystalline Cu-Al-Mn Superelastic Alloy Using White X-ray Microbeam Diffraction

White X-ray microbeam diffraction was applied to investigate the microscopic deformation behavior of individual grains in a Cu-Al-Mn superelastic alloy. Strain/stresses were measured in situ at different positions in several grains having different orientations during a tensile test. The results indicated inhomogeneous stress distribution, both at the granular and intragranular scale. Strain/stress evolution showed reversible phenomena during the superelastic behavior of the tensile sample, probably because of the reversible martensitic transformation. However, strain recovery of the sample was incomplete due to the residual martensite, which results in the formation of local compressive residual stresses at grain boundary regions.

Mechanical properties for irradiated face-centred cubic nanocrystalline metals.

In this paper, a self-consistent plasticity theory is proposed to model the mechanical behaviours of irradiated face-centred cubic nanocrystalline metals. At the grain level, a tensorial crystal model with both irradiation and grain size effects is applied for the grain interior (GI), whereas both grain boundary (GB) sliding with irradiation effect and GB diffusion are considered in modelling the behaviours of GBs. The elastic-viscoplastic self-consistent method with considering grain size distribution is developed to transit the microscopic behaviour of individual grains to the macroscopic properties of nanocrystals (NCs). The proposed theory is applied to model the mechanical properties of irradiated NC copper, and the feasibility and efficiency have been validated by comparing with experimental data. Numerical results show that: (i) irradiation-induced defects can lead to irradiation hardening in the GIs, but the hardening effect decreases with the grain size due to the increasing absorption of defects by GBs. Meanwhile, the absorbed defects would make the GBs softer than the unirradiated case. (ii) There exists a critical grain size for irradiated NC metals, which separates the grain size into the irradiation hardening dominant region (above the critical size) and irradiation softening dominant region (below the critical size). (iii) The distribution of grain size has a significant influence on the mechanical behaviours of both irradiated and unirradiated NCs. The proposed model can offer a valid theoretical foundation to study the irradiation effect on NC materials.