Coarse-grained Polycrystalline Research Articles

Tension-compression-tension tertiary twins in coarse-grained polycrystalline pure magnesium at room temperature

Using electron backscatter diffraction, the microstructural features of tension–compression–tension (T–C–T) tertiary twins are studied in coarse-grained pure polycrystalline magnesium subjected to monotonic compression along the extrusion direction in ambient air. T–C–T tertiary twins are developed due to the formation of a compression–tension double twin inside a primary tension twin. All the observed T–C–T twin variants are of TiCjTj type. TiCi+1Ti+1 (or TiCi−1Ti−1) variants are observed more frequently than TiCi+2Ti+2 (or TiCi−2Ti−2) variants. The number of tertiary twin lamellae increases with the applied compressive strain.

Calorimetric analysis of coarse-grained polycrystalline aluminum by IRT and DIC

A novel method is presented in this paper which aims at achieving grain scale energy balances at finite strain in mechanically-loaded polycrystalline metallic specimens. For this purpose, two complementary imaging techniques were used to investigate material thermomechanical behaviour: Digital Image Correlation and InfraRed Thermography to separately reach the kinematic and the thermal responses of the material. A calorimetric analysis can be conducted by combining these techniques. The aim of this paper is to present and to validate a novel IR data processing method that can be used to perform local thermal field measurements. The procedure was validated on numerical data associated with the response of aluminum polycrystalline aggregates.

The nature of the brittle-to-ductile transition of ultra fine grained tungsten (W) foil

The aim of this work is to answer the question of whether an ultra fine grained (UFG) microstructure has an impact on the nature of the brittle-to-ductile transition (BDT) of tungsten. Therefore, four-point bend tests were performed using unnotched 100μm UFG tungsten foil and annealed coarse-grained polycrystalline tungsten foil (2073K for 2h in vacuum) with exactly the same chemical composition and dimensions. The reported results for coarse-grained tungsten foil show a strain-rate dependent BDT that occurs between 350 and 450K. The activation energy of the BDT, EBDT, for the coarse-grained material was deduced to be 2.9 (+2.6/−0.9)eV. The results obtained from UFG tungsten foil indicate a strain-rate independent BDT at relatively low temperatures (≈77K). The strong disparity in the nature of the BDT can be attributed solely to the presence of microstructural differences. The possible mechanism controlling the BDT in tungsten for both tungsten foil conditions will be discussed against this background and in the light of previous studies performed on the nature of the BDT of tungsten reported in pertinent literature.

Localized strain and heat generation during plastic deformation in nanocrystalline Ni and Ni–Fe

Room temperature tensile testing was performed on a coarse-grained polycrystalline Ni (32 μm), a nanocrystalline Ni (23 nm) and two nanocrystalline Ni–Fe (16 nm) electrodeposits at two strain rates of 10−1 and 10−2/s. Strain localizations and local temperature increases were simultaneously recorded during tensile testing. For all materials, higher loads or higher strain rate generally resulted in higher peak temperature with the highest temperatures recorded in the fracture regions. The maximum temperature for the nanocrystalline materials was just over 80 °C, which is significantly below the reported temperatures for the onset of thermally activated grain growth. Therefore, the previously reported grain growth observed on similar materials after tensile deformation is likely not thermally activated but a stress-induced phenomenon. Despite the wide grain range from 16 nm to 32 μm, all samples exhibited similar strain localization behavior. Local strain variations initiated in the early stage of macroscopic uniform deformation, subsequent necking and fracture took place in the region of initial strain localization. While the coarse-grained polycrystalline Ni exhibited little strain rate sensitivity, gradually increased strain rate sensitivity was observed for the 23 nm Ni and the two 16 nm Ni–Fe samples, suggesting that both dislocation-mediated and grain-boundary-controlled mechanisms were operative in the deformation of the nanocrystalline Ni and Ni–Fe samples.

Critical behaviour of nanocrystalline gadolinium: evidence for random uniaxial dipolar universality class

We report on how nanocrystal size affects the critical behaviour of the rare-earth metal Gd near the ferromagnetic-to-paramagnetic phase transition. The asymptotic critical behaviour of the coarse-grained polycrystalline sample (with an average crystallite size of L≅100 μm) is that of a (pure) uniaxial dipolar ferromagnet, as is the case with single crystal Gd, albeit the width of the asymptotic critical region (ACR) is reduced. As the grain size approaches ∼30 nm, the ACR is so narrow that it could not be accessed in the present experiments. Inaccessibly narrow ACR for L ∼ 30 nm and continuous increase in the width of the ACR as L decreases from 16 to 9.5 nm basically reflect a crossover to the random uniaxial dipolar fixed point caused by the quenched random exchange disorder prevalent at the internal interfaces (grain boundaries).

Micromechanical Modeling of Creep Damage in a Copper-antimony Alloy

A micromechanical model of creep induced grain boundary damage is proposed, which allows for the simulation of creep damage in a polycrystal with the finite element method. Grain boundary cavitation and sliding are considered via a micromechanically motivated cohesive zone model, while the grains creep following the slip system theory. The model has been calibrated with creep test data from pure Cu single crystals and a coarse-grained polycrystalline Cu-Sb alloy. The test data includes porosity measurements and estimates of grain boundary sliding. Finally, the model has been applied to Voronoi models of polycrystalline structures. In particular the influence of grain boundary sliding on the overall creep rate is demonstrated.

Fracture analysis of U-notched disc-type graphite specimens under mixed mode loading

Fracture phenomenon was investigated both experimentally and theoretically for a type of coarse-grained polycrystalline graphite weakened by a U-shaped notch under mixed mode loading. First, 36 disc-type graphite specimens containing a central U-notch, so called in literature as the U-notched Brazilian disc (UNBD), were prepared for four different notch tip radii and the fracture tests were performed under mode I and mixed mode I/II loading conditions. Then, the experimentally obtained fracture loads and the fracture initiation angles were predicted by using the U-notched maximum tangential stress (UMTS) and the newly formulated U-notched mean stress (UMS) fracture criteria. Both the criteria were developed in the form of the fracture curves and the curves of fracture initiation angle, in terms of the notch stress intensity factors (NSIFs). The results showed that while the criteria could predict successfully the experimental notch fracture toughness values, the UMS criterion provides slightly better predictions than the UMTS criterion, particularly for shear-dominant deformations. Also, found in this research was that the curves of fracture initiation angle were almost identical for the two criteria which both could predict well the experimental results.

Fracture Assessment of Blunt V-Notched Graphite Specimens by Means of the Strain Energy Density

The main aim of the present work is to check the suitability of the brittle fracture model, namely the local strain energy density (SED), in predicting the experimental results on mode I fracture of blunt V-notched graphite components. For this purpose, a wide range of test results reported in the recent literature on brittle fracture of V-notched test specimens characterized by different geometries is considered. The specimens are made of the same type of coarse-grained polycrystalline graphite. The fracture assessment is carried out predicting theoretically the fracture loads by means of the SED criterion. The SED parameter is evaluated by averaging the local energy over a well-defined control volume which embraces the notch edge. It is found that the SED criterion allows assessing the fracture behavior of graphite specimens characterized by different notch angles and tip radii.

Micromechanical investigations and modelling of a Copper–Antimony-Alloy under creep conditions

In many practical applications, creep damage is the limiting factor of a component’s lifetime. A micromechanical model of creep induced grain boundary damage is proposed, which allows for the simulation of creep damage in a polycrystal within the framework of finite element analysis. The model considers grain boundary cavitation and sliding according to a micromechanically motivated cohesive zone model while creep deformation of the grains is described following the slip system theory. The model can be applied to idealised polycrystalline structures, such as a Voronoi tessellation or, like demonstrated here, to real grain structures of miniature creep specimens. Creep tests with pure Cu single crystals and with a coarse-grained polycrystalline Cu-1wt.% Sb alloy at 823K have been performed and used to calibrate the polycrystal model. The grain structure of the polycrystalline Cu–Sb specimens has been revealed by the EBSD method. Extensive grain boundary sliding and cavitation has been observed in the crept specimens. Grain boundary sliding has been found to promote wedge-type damage at grain boundary triple junctions and to contribute significantly to the total creep strain. Furthermore, the assumed stress sensitivity of the models grain boundary cavity nucleation rate strongly influences the development of wedge-type damage.

Tensile fracture in coarse-grained polycrystalline graphite weakened by a U-shaped notch

U-notched Brazilian disc specimens made of a type of commercial graphite were used to measure experimentally the mode I notch fracture toughness of material. The experimental results were estimated by means of the mean stress and the point stress fracture criteria. An excellent agreement was found to exist between the results of the mean stress criterion and the experimental results for different notch tip radii. Also, found in this research was that the point stress criterion provides weaker estimates compared to the mean stress model except when one deals with larger values of the notch tip radius.

Boundary identification in EBSD data with a generalization of fast multiscale clustering

Electron backscatter diffraction (EBSD) studies of cellular or subgrain microstructures present problems beyond those in the study of coarse-grained polycrystalline aggregates. In particular, identification of boundaries delineating some subgrain structures, such as microbands, cannot be accomplished simply with pixel-to-pixel misorientation thresholding because many of the boundaries are gradual transitions in crystallographic orientation. Fast multiscale clustering (FMC) is an established data segmentation technique that is combined here with quaternion representation of orientation to segment EBSD data with gradual transitions. This implementation of FMC addresses a common problem with segmentation algorithms, handling data sets with both high and low magnitude boundaries, by using a novel distance function that is a modification of Mahalanobis distance. It accommodates data representations, such as quaternions, whose features are not necessarily linearly correlated but have known distance functions. To maintain the linear run time of FMC with such data, the method requires a novel variance update rule. Although FMC was originally an algorithm for two-dimensional data segmentation, it can be generalized to analyze three-dimensional data sets. As examples, several segmentations of quaternion EBSD data sets are presented.

Single-crystal structure determination of (Mg,Fe)SiO3 postperovskite.

Knowledge of the structural properties of mantle phases is critical for understanding the enigmatic seismic features observed in the Earth's lower mantle down to the core-mantle boundary. However, our knowledge of lower mantle phase equilibria at high pressure (P) and temperature (T) conditions has been based on limited information provided by powder X-ray diffraction technique and theoretical calculations. Here, we report the in situ single-crystal structure determination of (Mg,Fe)SiO3 postperovskite (ppv) at high P and after temperature quenching in a diamond anvil cell. Using a newly developed multigrain single-crystal X-ray diffraction analysis technique in a diamond anvil cell, crystallographic orientations of over 100 crystallites were simultaneously determined at high P in a coarse-grained polycrystalline sample containing submicron ppv grains. Conventional single-crystal structural analysis and refinement methods were applied for a few selected ppv crystallites, which demonstrate the feasibility of the in situ study of crystal structures of submicron crystallites in a multiphase polycrystalline sample contained within a high P device. The similarity of structural models for single-crystal Fe-bearing ppv (~10 mol% Fe) and Fe-free ppv from previous theoretical calculations suggests that the Fe content in the mantle has a negligible effect on the crystal structure of the ppv phase.

Recrystallization of Cold-Rolled Zr Single Crystals

Recrystallization of rolled Zr single crystals is considered in comparison with analogous recrystallization processes in rolled coarse-grained iodide Zr and polycrystalline plates of commercially pure Zr. Diffractometric X-ray methods were used by texture and X-ray line profile measurements. The treatment of obtained data included construction of correlation diagrams, connecting as-rolled and recrystallized conditions of samples. A number of recrystallization mechanisms, operating in rolled α-Zr under annealing, were revealed on the basis of found regularities of texture changes.

Fabrication and thermoelectric properties of c-axis oriented nanocrystalline Bi2Sr2Co2Oy thin films

c-axis oriented nanocrystalline Bi2Sr2Co2Oy thin films with the average grain size of 25nm have been achieved by chemical solution deposition combined with a rapid annealing process in oxygen. The resistivity and Seebeck coefficient of the nanostructured films are about 7.0mΩ·cm and 171μV/K at 873K, resulting in a power factor 2–3 times larger than that of the best reported value in the coarse-grained Bi2Sr2Co2Oy polycrystalline bulks. Since the thermal conductivity can be greatly reduced due to the increased phonon scattering at the grain boundaries of nanocrystalline films as well as at film surface and at film/substrate interface, great improvements in the ZT are expected in the nanocrystalline Bi2Sr2Co2Oy thin films.

Study of grain growth and thermal stability of nanocrystalline RuAl thin films deposited by magnetron sputtering

Special properties of RuAl regarding temperature stability and resistance against corrosion and oxidation, make this intermetallic material suitable to be used under high-temperature and chemically aggressive working conditions. In the present work, the microstructure, thermal stability and grain growth are studied by means of scanning transmission electron microscopy, peak broadening analysis of X-ray diffraction and electron backscatter diffraction. Isothermal kinetics analysis revealed that annealing at temperatures as low as 650°C does not activate the grain growth, whilst at 700°C and times lower than 6h, the growth is induced by a 3D curvature-driven evolution characterised by a monomodal grain size distribution. Higher temperatures and/or annealing times show also abnormal grain growth characterised by a bimodal distribution of the grain sizes, which is correlated with the presence of impurities and grain-orientation-specific driving forces. The corresponding grain boundary activation energy is 145±14kJ/mol, indicating that the growth is highly dominated by grain-boundary diffusion due to the high volume fraction of grain boundaries compared with coarse-grained polycrystalline materials.

Preparation and phase stability of nanocrystalline Li2C2 alloy

Single-phase ultrafine nanocrystalline Li2C2 alloy was prepared using a simple and efficient technique that combines the ball milling process and spark plasma sintering. The crystal structure of the nanocrystalline Li2C2 alloy was constructed by the Rietveld refinement based on the X-ray diffraction pattern of the prepared nanocrystalline sample. Furthermore, the phase stability in the nanocrystalline Li2C2 alloy was quantitatively studied using the developed thermodynamic model for the nanocrystalline alloys. It has been found that in a certain low temperature range, the β-Li2C2, which is a metastable phase in the coarse-grained polycrystalline system, can be stabilized due to the nanograin size effect.

MULTIPHASE EQUILIBRIUM, PHASE STABILITY AND PHASE TRANSFORMATION IN NANOCRYSTALLINE ALLOY SYSTEMS

A nanoscale thermodynamic approach is developed to quantify the multiphase equilibrium in nanocrystalline alloy systems. The nanoscale multiphase equilibrium calculations indicate that the activities and mole numbers of phases in alloy systems change diversely as a function of the nanograin size at constant temperature, which results in distinctly different characteristics of phase stability and phase transformation behavior in nanocrystalline alloys with respect to coarse-grained polycrystalline counterparts. Take the nanocrystalline Sm–Co alloy system as an example, the diverse phase stabilities and phase transformations are demonstrated. To verify the thermodynamic predictions, a series of experiments on preparation and characterization of nanocrystalline SmCo3, Sm2Co7 and SmCo2 alloys have been performed. The experimental results are in agreement with the model calculations, which validate the present thermodynamic approach for nanoscale multiphase equilibrium.

The interplay between osteoblast functions and the degree of nanoscale roughness induced by grain boundary grooving of nanograined materials

From the perspective of osseointegration, nanograined/ultrafine-grained (NG/UFG) metals provide surfaces that are different from conventional coarse-grained (CG) polycrystalline metals because of the high fraction of grain boundaries. We describe here the interplay between the cellular response and grain boundary grooving as a potential approach to enhance osteoblast functions and facilitate the biomechanical interlocking and anchorage. This is accomplished by making a relative comparison of osteoblast response of NG/UFG grains electrochemically grooved to different depths to induce different degree of nanoscale roughness with planar NG/UFG surfaces, under identical biological environment. Electrochemically grooved NG/UFG structures indicated significant attachment and proliferation, and consequently enhanced modulation of cellular response that was significantly different from planar (non-grooved) NG/UFG substrate. Consistent with cell attachment and proliferation, immunofluorescence microscopy and computational analysis indicated stronger vinculin signals associated with actin stress fibers in the outer regions of the cells and cellular extensions on electrochemically-grooved NG/UFG substrates. These observations are indicative of accelerated response of cell–substrate interaction and activity. The behavior is attributed to average nanoscale roughness and high surface hydrophilicity of the nanoengineered surface.

Pressure-Induced Nano-Crystallization of Y2O3

ABSTRACTA reversible-phase transformation process to convert coarse-grained polycrystalline cubic-Y2O3 directly into the nanocrystalline state is being developed. The process involves a forward cubic-to-monoclinic phase transition under high pressure and a backward transformation from monoclinic-to-cubic under a lower pressure. The process has been used to reduce the grain size of fully dense cubic-Y2O3 from 300 μm to 0.1 μm. A surface modification effect, comprising a columnar-grained structure, has also been observed. Preliminary work indicates that the surface structure is modified, apparently formed by interaction between the graphite heater and sample.

Thermodynamic study on metastable phase: From polycrystalline to nanocrystalline system

First an approach based on the Debye model is developed to quantify the thermodynamic parameters of metastable phase in the conventional coarse-grained polycrystalline systems. Subsequently, by combining the experimental measurements on heat capacity with the nanothermodynamic calculations for nanocrystalline alloys, a method is established to determine the fundamental thermodynamic functions of the metastable phases in both polycrystalline and nanocrystalline alloy systems. Taking the typical metastable-phase SmCo7 alloy as an example, the thermodynamic properties of polycrystalline and nanocrystalline systems are studied, and good agreement between model calculations and experimental results is achieved.