Material Grain Research Articles

Influence of testing mode on the fatigue behavior of austenitic grain at the nanometric length scale for TRIP steels

Metastable austenitic stainless steels, and in particular the TRansformation Induced Plasticity steels, rely on a phase transformation from a ductile austenite to a slightly harder martensite. These materials under fatigue testing conditions at the macrometric length scale can induce a softening or hardening effect as a function of the underlying deformation feature activated. Thus, given the interaction of single grains in polycrystalline materials, the collective response to macroscopic fatigue testing is not trivial to interpret. Within this context, small scale tests are required to obtain a more in detail understanding of the fatigue properties at the local level of those materials. In this regard, cyclic nanoindentation tests represent a suitable technique to give insight on the local fatigue of metastable stainless steels for a certain crystallographic orientation. In this experimental work, the influence of the testing mode (loading and/or displacement control mode) on the fatigue behavior of <111> austenitic grains as a function of their micromechanical properties as well as their deformation features was investigated in detail. It was found that the experiments done under loading control mode could be compared to conventional low cycle fatigue tests. In contrast when experiments were performed under displacement control mode they may be compared to high cycle fatigue tests. Furthermore, the microstructural observation by transmission electron microscopy allowed to observe the formation of shear bands. This phenomenon preceded the apparition of martensitic laths during the cyclic indentation process.

Cracks in heterogeneous materials with rotating constituents – Small and Intermediate scale Cosserat continua

Cosserat continuum modelling of crack propagation in particulate materials with microrotations and in layered materials with sliding requires the analysis of the Cosserat characteristic lengths in their relation with the characteristic length of the material microstructure. The Cosserat lengths that are of the order of the microstructural characteristic length should be set to zero to derive the main asymptotic terms. When all Cosserat characteristic lengths are of the microscopic length the obtained continuum is termed the Small scale Cosserat continuum. Cracks of Modes I, II, III have conventional stress singularity, as well as the moment stress singularity. Moment stresses can cause bending failure of the bonds connecting the grains in the particulate material or layers in the layered material. Cases are identified when the bending stress induced by moment stress singularity exceeds tensile stresses induced by the conventional stress singularity by an order of magnitude. Then the main mechanism of crack propagation is bending failure produced by the moment stress, which explains, in a unified fashion the often-observed in-plane propagation of cracks of the classical modes. Another interesting case is when some Cosserat characteristic lengths are large. This gives what we term the Intermediate scale Cosserat continuum. An example is given by layered material with sliding layers that keep full contact during sliding. In this case the crack normal to the layering also propagates by bending failure produced by the moment stress singularity.

Mechanically Driven Grain Boundary Formation in Nickel Nanowires.

Metallic nanomaterials are widely used in micro/nanodevices. However, the mechanically driven microstructure evolution in these nanomaterials is not clearly understood, particularly when large stress and strain gradients are present. Here, we report the in situ bending experiment of Ni nanowires containing nanoscale twin lamellae using high-resolution transmission electron microscopy. We found that the large, localized bending deformation of Ni nanowires initially resulted in the formation of a low-angle tilt grain boundary (GB), consisting of randomly distributed dislocations in a diffuse GB layer. Further bending intensified the local plastic deformation and thus led to the severe distortion and collapse of local lattice domains in the GB region, thereby transforming a low-angle GB to a high-angle GB. Atomistic simulations, coupled with in situ atomic-scale imaging, unravelled the roles of bending-induced strain gradients and associated geometrically necessary dislocations in GB formation. These results offer a valuable understanding of the mechanically driven microstructure changes in metallic nanomaterials through GB formation. The work also has implications for refining the grains in bulk nanocrystalline materials.

Simulation of the plastic response of Ti–6Al–4V thin sheet under different loading conditions using the viscoplastic self-consistent model

In this article, the capacity of the Visco-Plastic Self-Consistent model (VPSC) as a constitutive model for the simulation of cold-rolled Ti–6Al–4V under a diverse set of loading conditions is investigated. The model uses information about the material crystallographic texture and grain morphology obtained with EBSD together with a grain constitutive model that is sensitive to strain rate and temperature. The ability of the model to capture the macroscopic response and underlying microstructural phenomena is critically assessed considering various sets of hardening parameters, obtained applying different fitting procedures to tensile experiments in different directions on the sheet plane. In order to validate the fitted parameters, the obtained model is applied to the simulation of experiments in a wide range of testing conditions, including uniaxial, plane strain and simple shear loading modes at strain rates ranging from 10−4s−1 to 103s−1 and temperatures between −10°C and 70°C. High strain rate experiments were performed using a Split Hopkinson Bar setup, with specimens designed to achieve the desired loading mode, while isothermal experiments were performed with the help of a fluid cell to keep a constant temperature. A good agreement between experimental and simulation results is obtained.

Investigating mechanical degradation due to fire exposure of aluminum alloy 5083 using crystal plasticity finite element method

Fire exposure causes significant mechanical property degradation of metallic materials. This paper investigates the influence of fire exposure on mechanical response of AA5083 through in situ neutron diffraction tensile tests and crystal plasticity finite element simulations. The Mechanical Threshold Stress (MTS) model is utilized, with a focus on the lattice strain response under tensile loading before and after exposure. A rigorous material parameter calibration procedure is proposed for this MTS model directly based on yield stress, plastic flow, hardening and rate dependency as a function of temperature, requiring only the macroscopic stress–strain curve. Material parameters of virgin material and fire-exposed material with full recrystallization are calibrated separately using this procedure. To initiate the investigation of mechanical degradation caused by fire exposure, a fire-exposed specimen with partial recrystallization is simulated by assigning material parameters and texture of recrystallized grains to selected grains in the virgin material according to the recrystallization fraction. Experimentally measured stress–strain curves and lattice strain evolution are employed to validate the simulations. For the first time, the local mechanical response of AA5083 is compared between in situ testing and finite element simulations, and the discrepancies of lattice strain evolution between virgin and fire-exposed material are discussed. Finally, the influence of fire-exposure on stress and lattice strain distribution is investigated to provide insight into mechanical property degradation.

Degradation of the Crystalline Structure of ZnS Ceramics under Abrasive Damage

Stability of optical elements based on ZnS ceramics to dust and rain erosion is usually estimated from the loss of material mass in a directional flow of solid particles or atmospheric precipitates. In this case, the mechanism of degradation and fracture of the surface layer of an optical element is not considered. The photoluminescence (PL) method was used for investigating the crystal lattice response to the abrasive action and the formation of cleavage in ZnS ceramics, which differ in manufacturing technology and, accordingly, in the grain size by two orders of magnitude. It is shown that during abrasive treatment of samples, their spectra exhibit changes typical of degradation of the crystal lattice of material grains. The PL spectra of cleavage surfaces reveal almost complete degradation of the structure of crystallite grains with a size from 1–2 to 100–200 μm.

Nanocrystalline gradient engineering: Grain evolution and grain boundary networks

We study materials with spatial gradients in nanoscale grain size (5–120nm), and quantitatively examine the effect of spatial gradient on microstructure evolution and thermal stability using mesoscale Monte Carlo modeling and statistical analysis. The spatial grain size gradient weakens and the grain size distribution widens at elevated temperatures, accompanied by grain rounding and movement of grains along the gradient direction. Introducing heterogeneous grain boundary networks into gradient materials leads to better preservation of the spatial grain size gradient but less equiaxed grains. Coarsening in small grain regions is accompanied by an increase in the local fraction of low-energy grain boundaries, as these are competing mechanisms for reducing total energy, and spatial gradients in grain boundary character distribution and triple junction character distribution develop in the material. We further compare concave, linear, and convex gradient materials with increasing grain size gradient for small grains. Grains in convex gradient materials have the highest grain growth rate compared to grains of the same size in linear and concave gradient materials. The accelerated grain growth in the presence of a steeper grain size gradient is attributed to a change in local grain neighbor environment that promotes grain boundary curvature (and pressure) and enhances the driving force for grain growth.

Biological effects and the Grain-Shearing model of wave propagation in unconsolidated sediments

The Grain-Shearing (GS) model of wave propagation in unconsolidated sediments is embodied in two sets of dispersion relations, one for the compressional wave and the other for the shear wave. Besides frequency, these expressions for the wave speed and attenuation depend on the physical properties of the sediment, notably the bulk density and porosity. They also involve a material exponent, n, and grain shearing coefficients that are associated with the sliding of grains against one another, which is a non-linear, stick-slip type of process that is controlled by micro-asperities on the surface of the grains. Bioturbation is known to affect both the bulk density and porosity of sediments, while biogenic coatings of mineral grains modify the surface roughness, which alters the grain-shearing coefficients. The magnitude of the effects of bioturbation and biogenic coatings on the wave speeds and attenuations predicted by the GS theory will be discussed, based on available biological information in the literatu...

A 3D finite element modelling of crystalline anisotropy in rolling contact fatigue

Rolling contact fatigue (RCF) is one of the primary modes of failure in bearings and in this study it is hypothesized and demonstrated that RCF is strongly influenced by inhomogeneity in polycrystalline material microstructure. This paper presents a three-dimensional modeling approach to include the effects of microstructure topology and material anisotropy in a polycrystalline microstructural bearing steel subject to RCF loading. A randomly generated 3D Voronoi tessellation is used to represent the microstructural topology of the material. A cubic material definition with random spatial orientation is specified for the material grains to simulate the polycrystalline anisotropy. The size of the RVE was chosen such that it represented the macroscopic linear elastic material properties of a polycrystalline aggregate. The semi-infinite domain is then subjected to a moving Hertzian pressure to simulate a load cycle. Due to inhomogeneity in the polycrystalline material local stress risers occur at the grain boundaries, however, in general, the locations of the maximum shear stresses compare well to the experimental RCF results readily available in literature. Elemental shear stresses averaging scheme was employed to illustrate that the finite element model is mesh independent. The estimated fatigue life scatter obtained from the inhomogeneous polycrystalline material model corroborates well with the fatigue life scatter obtained from the RCF experiments.

Simulation of dynamic recrystallization in AZ80 magnesium alloy using cellular automaton

The deformation behaviors of AZ80 magnesium alloy were systematically investigated by the combination of simulations and experiments. Specifically, the isothermal hot compression test of AZ80 magnesium alloy was performed on Gleeble-3500 thermo-simulation machine in the temperature range of 573–723K with strain rates from 0.001 to 1s−1. It is found that the flow stress is very sensitive to deformation temperatures and strain rates. On the basis of microstructure evolution, a DRX kinetics model was proposed by using regression analysis in microstructural evolution of AZ80 alloy with a consideration of effects of dislocation density and nucleation rate on the DRX. The developed DRX model was coupled into the finite element software, DEFORM-3D, along with cellular automaton (CA) based on physical model, to simulate the microstructural evolution in the deformation zone. The developed CA model with DRX is able to predict the nucleation and growth of DRX grains (R-grains) of the testing material in hot working process. Microstructural evolution and the distribution of average grain size in final microstructure show a good agreement between the simulated results and experimental ones. It suggests that the present CA model with DRX is feasible in studying the DRX process of AZ80 alloy.

Force Transmission Modes of Non-Cohesive and Cohesive Materials at the Critical State.

This paper investigates the force transmission modes, mainly described by probability density distributions, in non-cohesive dry and cohesive wet granular materials by discrete element modeling. The critical state force transmission patterns are focused on with the contact model effect being analyzed. By shearing relatively dense and loose dry specimens to the critical state in the conventional triaxial loading path, it is observed that there is a unique critical state force transmission mode. There is a universe critical state force distribution pattern for both the normal contact forces and tangential contact forces. Furthermore, it is found that using either the linear Hooke or the non-linear Hertz model does not affect the universe force transmission mode, and it is only related to the grain size distribution. Wet granular materials are also simulated by incorporating a water bridge model. Dense and loose wet granular materials are tested, and the critical state behavior for the wet material is also observed. The critical state strength and void ratio of wet granular materials are higher than those of a non-cohesive material. The critical state inter-particle distribution is altered from that of a non-cohesive material with higher probability in relatively weak forces. Grains in non-cohesive materials are under compressive stresses, and their principal directions are mainly in the axial loading direction. However, for cohesive wet granular materials, some particles are in tension, and the tensile stresses are in the horizontal direction on which the confinement is applied. The additional confinement by the tensile stress explains the macro strength and dilatancy increase in wet samples.

Cu2SnS3 thin film: Structural, morphological, optical and photoelectrochemical studies

Cu2SnS3 (CTS) thin film has been successfully deposited on the cost effective stainless steel substrate by simple and inexpensive chemical bath deposition method. The film is studied in term of their structural, morphological, optical and photoelectrochemical properties. The XRD study shows that CTS films consists a nanocrystalline structure with triclinic phase Raman analysis confirms the formation of CTS compound thin film. The SEM images show small crystallite grains of CTS material. The optical absorption study shows that the films have direct transitions with band gap energies of 1.35 eV. Moreover, the photoconversion efficiency of CTS thin film-based photoelectrochemical cell (PEC) is studied and found 0.12% efficiency.

Corrosion fatigue crack growth of laser additively-manufactured 316L stainless steel in high temperature water

Corrosion fatigue crack growth of laser additively-manufactured 316L stainless steel was studied in oxygenated 288°C water. Effects of heat treatment, crack orientation, and applied cold work were evaluated. Stress-relieved material showed higher crack growth rate than annealed material. Their difference was not significant when the loading frequency was high. When the loading frequency decreased, the effects of material conditions created by heat treatment, crack orientation and applied cold work slightly increased. Retained unrecrystallized grains in an annealed material did not affect significantly the cracking susceptibility. High-temperature recrystallization treatment of an AM 316L component may be necessary for nuclear applications.

The history of Taxus baccata L. in the Wierzchlas (N Poland) on the basis of palynological research

Wierzchlas forest reserve forms the biggest natural stand of the Taxus baccata in Poland. The pollen analysis of biogenic sediments from Mukrz Lake registers the history of the plant cover of area surrounding Mukrz Lake starting from Vistulian Late Glacial period through the Holocene until the contemporary times and indicates that Taxus baccata may be native from the beginning of the Subboreal to the present time. Additional research was made because of the small representation of Taxus in the fossil samples. The fossil spectra were supplemented with the results of surface samples taken from the bottom of the lake and from the forest, together with the results of annual pollen deposition. The presence of yew pollen grains in surface materials from the reserve in Wierzchlas depends, thus, on both the distance between the sample taken and a male yew individual as well as the stand density. The comparison of the surface spectra from different habitats of this reserve (lake, forest floor and peatbog) and material from Tauber’s traps reflected a large discrepancy in the representation of yew. These differences are related to both the habitat from which the material originated, and also to the distance between the sampling point and the location of a male yew tree.

Influence of hydraulic loading rate on ion precipitations and performance of a vertical-flow soil filter treating contaminated groundwater under cold climatic conditions

Long-term investigations were carried out in a pilot-scale vertical-flow soil filter (VFSF) system to characterize the fate of different precipitating ions (Fe2+, Mn2+, Ca2+) during the remediation of heavily contaminated groundwater under different hydraulic loading rates (HLRs) and to assess MTBE and BTEX removal efficiencies under low winter temperatures. Two specially designed VFSF, a roughing filter (RF) packed with expanded clay material (grain size: 3–6mm) and a polishing filter (PF) with zeolite material (grain size: 0–5mm) were operated in both single-stage and two-stage configuration. During single-stage operation, a stepwise increase of HLRs from 60 to 120, 240, 480Lm−2 d−1 in both RF and PF were applied. During two-stage operation, a highly efficient Fe2+ removal with a mean removal efficiency of 99% was already observed in the effluent of the first treatment step (RF; HLR=960Lm−2 d−1) and protected the second filter (PF; HLR=480Lm−2 d−1) from potential clogging due to iron hydroxide precipitates. The removal of Ca2+ decreased remarkably from 28% to 7% in the RF and from 54% to 10% in the PF as the HLR increased stepwise in single-stage operation. During two-stage operation with a high HLR in the RF resulted in only 3% removal of the mean Ca2+ inflow concentration (211±18mgL−1). This clearly showed that at high HLR, Ca2+ was rapidly flushing out of the VFSF system instead of carbonate precipitations and proved to be very effective to protect the filter from potential clogging. During winter, when daily mean outside air temperature was as low as −20°C, a relatively low water temperature in the range of 5.1–8°C in the effluent and a low MTBE and benzene mean removal rate of 0.41±0.07 and 2.08±0.33gm−2 d−1, respectively, was observed at a low HLR (120Lm−2 d−1). Whereas, at a high HLR (960Lm−2 d−1), a mean temperature of 13.0±1.3°C was maintained inside the filter resulting in a high MTBE and benzene removal rate of 2.37±0.02 and 14.41±0.65gm−2 d−1, respectively. Our results with high removal efficiencies for MTBE and benzene together with highly efficient Fe2+ and Mn2+ removal suggest that a two-stage VFSF can be a very promising technology for the remediation of heavily contaminated groundwater in oil and gas industries. Furthermore, it was shown that at high HLRs, filter clogging due to Ca-precipitation was prevented and treatment efficiency was not affected even under cold climate conditions with atmospheric air temperature as low as −20°C.

Modelling of flow stress by correlating the material grain size and chip thickness in ultra-precision machining

Abstract Inducing compressive stress on the machined surface is a desirable practice in ultra-precision metal cutting to improve the part quality. An important parameter to analyse the machining performance is the material flow stress, which plays a crucial role in the material deformation characteristics in machining. As the dimensions of the workpiece descend from macro to micro scale in metal forming, the flow stress reduces accordingly. Hence, based on the ‘surface model’ analogy developed for metal forming coupled with the crystal plasticity effect, an attempt has been made in this study to construct a flow stress model for ultra-precision machining process. While developing the model, for the first time, the size factor (η, d/ho) is introduced to incorporate the material ‘grain size effect’ in ultra-precision machining. The proposed model is validated with experimental results of Al6082 alloy of different grain sizes. Orthogonal turning experiments were conducted on an ultra-precision machine by utilizing the concept of ‘cutting edge radius effect’, which is identified as the relative tool sharpness (RTS) and quantified as the ratio of undeformed chip thickness (hc) to edge radius (rn). In this paper, the investigation on the material flow stress is carried out by considering the phenomenon of the shifting material flow separation (cutting) from primary deformation zone to material deformation (ploughing and rubbing) at tertiary deformation zone. The distribution of contact stresses along the tool rake and flank faces at the minimum value of RTS (hc/rn) of 0.01 substantiates the ploughing effect (compressive stresses are induced into the machined layer) rather than chip separation. Moreover, the distinct variation of the machined surface quality and μ-chip morphology at the extreme low and high RTS conditions distinguishes the material ploughing effect from the cutting effect. Additionally, for the same RTS value, it is found that different grain materials (Cu and Mg alloy) exhibited variations in flow stress, chip morphology and surface quality. Therefore, material grain size is an influential factor for analysing machining performance with material flow stress at ultra-precision level.

Towards a Physics Based Foundation for the Estimation of Bearings RUL

Rolling element bearing (REB) prognosis is the process of forecasting the remaining operational life, future condition, or probability of failure based on the acquired condition monitoring data. One of the common reasons for rolling element bearings failure is the rolling contact fatigue (RCF). Complete understanding of the fatigue process is critical for estimation of the bearing remaining useful life (RUL) and allows planning maintenance actions. In the current work, it is assumed that the spall generation, on the surface of the raceway, is a result of RCF. However, after the first spall formation, the bearing might be fully operational for millions of cycles. Thus, for the estimation of the bearing RUL it is also important to understand the damage propagation process. The proposed method of RUL prediction is separated into two steps: diagnostics and prognostics. The diagnostics includes characterization of the defect in terms of location, type, and extent. The prognostics includes estimation of the defect propagation as a function of time, using its characterization derived from the diagnostics step. It is expected that results of the current study will provide an estimation of the bearing’s RUL: from first spall formation to the unoperational bearing. The spall generation process, as a result of RCF, is modeled based on continuum damage mechanics with representation of material grain structure and implemented using a Finite Element software. The results of the model are in a good agreement with published theoretical and experimental data. The paper also includes a discussion on the ongoing research and the methodology that will be implemented as part of it.

Efficiency of Mn Removal of Different Filtration Materials for Groundwater Treatment Linking Chemical and Physical Properties

This paper presents research on Fe and Mn removal from groundwater. In treatment systems of aeration followed by rapid filtration (no chemical dosage), manganese removal is possible due to the manganese dioxide catalyst present on the grains of filtration material. The goal of the presented research was to find a correlation between the catalyst layer’s composition as well as its internal porosity and the effectiveness and stability of manganese removal in the filtration process. In order to establish the influence of catalyst characteristics on manganese removal effectiveness, the filtration experiment was conducted using filtration materials with catalytic contact layers of different origin. Oxide coated auto-activated silica sand and Gabon manganese ore were tested. Inactive silica sand was used as reference. The results of filtration experiments were combined with analyses of chemical composition, internal porosity, and crystalline parameters of catalyst contact layers of grains. For the determination catalyst contact layer parameters, the following methods were used: Raman spectroscopy, X-ray powder diffractometry (XRD), Scanning Electron Microscope – Energy dispersive spectroscopy (SEM-EDAX), nitrogen adsorption. Pilot scale research on the filtration process demonstrated that auto-activated filtration material was characterized by the highest efficiency of manganese removal and stability of effects during the whole research. The effectiveness of Gabon manganese ore dropped from 90% and stabilized on the level of ca. 60% within 15 days of the experiment.

Comet 67P outbursts and quiescent coma at 1.3 au from the Sun: dust properties from Rosetta/VIRTIS-H observations

We present 2-5 $\mu$m spectroscopic observations of the dust coma of 67P/Churyumov-Gerasimenko obtained with the VIRTIS-H instrument onboard Rosetta during two outbursts that occurred on 2015, 13 September 13.6 h UT and 14 September 18.8 h UT at 1.3 AU from the Sun. Scattering and thermal properties measured before the outburst are in the mean of values measured for moderately active comets. The colour temperature excess (or superheat factor) can be attributed to submicrometre-sized particles composed of absorbing material or to porous fractal-like aggregates such as those collected by the Rosetta in situ dust instruments. The power law index of the dust size distribution is in the range 2-3. The sudden increase of infrared emission associated to the outbursts is correlated with a large increase of the colour temperature (from 300 K to up to 630 K) and a change of the dust colour at 2-2.5 $\mu$m from red to blue colours, revealing the presence of very small grains ($\leq$ 100 nm) in the outburst material. In addition, the measured large bolometric albedos ($\sim$ 0.7) indicate bright grains in the ejecta, which could either be silicatic grains, implying the thermal degradation of the carbonaceous material, or icy grains. The 3-$\mu$m absorption band from water ice is not detected in the spectra acquired during the outbursts, whereas signatures of organic compounds near 3.4 $\mu$m are observed in emission. The H$_2$O 2.7-$\mu$m and CO$_2$ 4.3-$\mu$m vibrational bands do not show any enhancement during the outbursts.

A Study on Microstructural Parameters for the Characterization of Granular Porous Ceramics Using a Combination of Stochastic and Mechanical Modeling

To correlate the mechanical properties of granular porous materials with their microstructure, typically porosity is being considered as the dominant parameter. In this work, we suggest the average coordination number, i.e., the average number of connections that each grain of the porous material has to its neighboring grains, as additional — and possibly even more fundamental — microstructural parameter. In this work, a combination of stochastic and mechanical modeling is applied to study microstructural influences on the elastic properties of porous ceramics. This is accomplished by generating quasi-two-dimensional (2D) and fully three-dimensional (3D) representative volume elements (RVEs) with tailored microstructural features by a parametric stochastic microstructure model. In the next step, the elastic properties of the RVEs are characterized by finite element analysis. The results reveal that the average coordination number exhibits a very strong correlation with the Young’s modulus of the material in both 2D and 3D RVEs. Moreover, it is seen that quasi-2D RVEs with the same average coordination number, but largely different porosities, only differ very slightly in their elastic properties such that the correlation is almost unique. This finding is substantiated and discussed in terms of the load distribution in microstructures with different porosities and average coordination numbers.