Concurrent Grain Research Articles

Effect of CSET processing on a commercial purity polycrystalline Al sample and numerical simulation of induced plastic deformation

A commercial purity Al sample was processed using complex shearing of the extruded tube (CSET) technique, which enables the preparation of tubes from bulk samples in one step by combining extrusion and two Equal Chanel Angular Pressing - like passes. In the present paper, the CSET process was simulated and analysed using Finite Element Method. The results of the simulations were compared with the experimental findings. Both modelling and experiment revealed a gradual increase in plastic deformation in the material during CSET processing. Simultaneously, microscopic investigations confirmed concurrent grain refinement. Moreover, a slight gradient of plastic deformation predicted by FEM in the main deformation zone was confirmed by microhardness measurements. In the resulting tube wall, electron backscattered diffraction experiments revealed the presence of fine grains and a partially recovered microstructure. The Vickers microhardness was found to be doubled by the CSET processing.

High strain rate superplasticity and secondary strain hardening of Al-Mg-Sc-Zr alloy produced by friction stir processing

There have been many studies on superplasticity of Al-Mg-Sc-Zr alloy with middle magnesium content in friction stir processing (FSP), while few studies on superplasticity of FSPed Al-Mg-Sc-Zr alloy with low-magnesium were researched. In this work, the microstructure of Al-3Mg-0.1Sc-0.1Zr alloy with the grain size of ~1.6 µm and high-angle grain boundaries dominated was obtained by FSP. The high-temperature tensile test was carried out, which showed that Al-3Mg-0.1Sc-0.1Zr alloy had good superplasticity in the range of 400 ℃ ~ 500 ℃ and 3 × 10−3 s−1 ~ 3 × 10−1 s−1, and the optimal elongation reached 1850% at 475 ℃ and 10−2 s−1, showing a high strain rate superplasticity. The FSPed alloy had a significant secondary strain hardening ability with increasing flow stress in the failure fracture stage at higher temperature or relatively low strain rate, showing “H” type (continuous hardening) and “C” type + secondary hardening type (initial hardening then softening + secondary hardening) curves, which was mainly attributed to the concurrent grain growth at higher strain during high-temperature tension. By studying the microstructure evolution and constitutive equation, it was found that the main mechanism of superplastic deformation was grain boundary sliding accommodated by grain boundary diffusion, the coordination mechanism was lattice diffusion. Al3(Sc, Zr) particles hindered the coarsening of grains obviously with good thermal stability, which were conducive to the realization of excellent superplastic properties.

Effect of concurrent grain growth on radiation-induced segregation in nanocrystalline Fe–Cr–Ni alloys

Irradiation of crystalline materials modifies their microchemistry and microstructure. This includes solute segregation toward defect sinks such as grain boundaries (GBs), a phenomenon commonly known as radiation-induced segregation (RIS). Unlike in coarse-grained alloys where GBs are nearly static, RIS is usually accompanied and affected by either thermal or irradiation-induced grain growth in nanocrystalline materials. This work presents a modeling study of concurrent grain growth and RIS in austenitic Fe–Cr–Ni adopting realistic 2D grain structures. RIS can be significantly affected by concurrent grain growth due to (i) increasing grain size, (ii) motion of GBs as defect sinks, and (iii) their combined effect. Consequently, RIS is enhanced by grain growth due to increased grain size and sink motion. More notably, RIS in nanocrystalline materials were found to induce grain-level compositional redistribution in addition to RIS at defect sinks, resulting in grain-size-dependent compositions in individual grains. Without concurrent grain growth, elements depleted at the sinks due to RIS, such as Cr in austenitic steels, will have lower concentrations in smaller grains than in larger grains. The opposite trend becomes true when concurrent grain growth takes place. The compositional difference in individual grains can be significant enough to affect local phase stability. These findings are not discernible with the classical 1D bicrystal model, and they highlight the different effects of RIS in nanocrystalline alloys compared to their coarse-grained counterparts.

Tracing plastic deformation path and concurrent grain refinement during additive friction stir deposition

Owing to melting and solidification, serious issues arise in fusion-based metal additive manufacturing, such as solidification porosity, columnar grains, and large grain sizes. Recently, additive friction stir deposition has been demonstrated to overcome these issues via high-temperature, rapid plastic deformation, which can result in fully-dense as-printed material with equiaxed, fine grains. However, the deformation fundamentals underlying this process—e.g., the strain magnitude, its influence on dynamic microstructure evolution, and material flow details—remain poorly understood. Here, we explore the deformation fundamentals of additive friction stir deposition by employing tracer-based feed material (Al-Cu tracer embedded in Al-Mg-Si matrix). This allows us to unravel: (i) the path of plastic deformation, and (ii) concurrent grain structure evolution along the deformation path. X-ray computed tomography is used to directly observe the plastic deformation paths of center and edge tracers. In both cases, the millimeter-scale cylindrical tracer undergoes extrusion- and torsion-like deformation followed by shear-induced thinning, which eventually results in micro-ribbons piling up along the deposition track. Microstructure mapping along the deformation path reveals significant grain refinement during initial material feeding via geometric dynamic recrystallization but no further grain refinement during steady-state deposition. By analyzing the strain components associated with extrusion, torsion, and shear-induced thinning, we estimate the total strain to be on the order of 101 and establish a quantitative relationship between the strain and tracer grain size. While this work focuses on a specific process, the methodology and findings may provide the basis for developing future deformation processing-based additive technologies.

The defining role of phosphorous on microstructure, nanohardness and thermal stability of pulsed electrodeposited nanocrystalline nickel-phosphorous alloys

Free-standing nanocrystalline (NC) nickel-phosphorous (Ni–P) alloy foils with P content in the range of 0.2–5.0 wt percent are synthesized by pulsed electrodeposition technique. The aim of the present study is to evaluate the effect of P content on the hardness and microstructural evolution in Ni–P alloys during both electrodeposition and thermal treatment. The microstructure, hardness and thermal analysis of the alloys are investigated using electron microscopy, nanoindentation and differential scanning calorimetry (DSC), respectively. This study demonstrates that there is localized structural heterogeneity in conjunction with significant grain refinement in the alloys containing excessive P content beyond its solid solubility. Nanohardness measurements indicate that both the grain refinement and internal strain induced by codeposition of P solute atoms affect the strengthening of Ni matrix. The exothermic peaks observed in the DSC thermograms are found to be strongly dependent on P content in the alloy and these are associated with structural relaxation, formation of metastable and stable nickel phosphide phases along with concurrent grain growth and particle coarsening. The grain growth kinetics study showed that in Ni–P alloys Smith-Zener drag mechanism involving control of grain boundary mobility by Ni3P nanoparticles overtakes the solute drag effect of P with increasing temperature.

Microstructural stability and nanoindentation of an electrodeposited nanocrystalline Ni-1.5 wt. % P alloy

Microstructural stability of nanocrystalline Ni-1.5wt.%P alloy with an initial grain size of 3 nm processed by pulsed electrodeposition was studied using differential scanning calorimetry (DSC) and annealing. Microstructural characterization suggests that the observed exothermic peak during heating in DSC is related to both concurrent grain growth and Ni3P formation. Nanoindentation on samples with grain sizes from 3 to 50 nm revealed a breakdown in Hall-Petch strengthening in nano Ni-P alloy at grain sizes <= 10 nm, consistent with some previous observations. It is concluded that there is a grain boundary weakening regime for grain sizes < 10 nm, based on analysis which show that the data cannot be rationalized in terms of microstrain relaxation, variation in elastic modulus, texture evolution and duplex structure formation.

Understanding of failure mechanisms of the oxide scales formed on nanocrystalline coatings with different Al content during cyclic oxidation

Cyclic oxidation behaviors of three nanocrystalline coatings with phase constitutions of γ/γʹ, single γʹ, and γʹ/β, respectively, tailored by adjusting the Al content, are investigated. The oxide scale on γ/γʹ coating exhibits serious surface rumpling accompanied by frequently cracking at the crests of undulations after 500 cycles at 1100°C. The premature degradation of the coating in microstructure (i.e. γ′ phase dissolution and concurrent grain coarsening), arising from high oxidation rate and low original Al content, not only increases its CTE to introduce larger thermal stress, but also decreases its hardness and elastic modulus, making it susceptible to rumpling and further inducing cracking of oxide scale. Comparatively, benefited from lower oxidation rate and higher Al content, the degradation of γʹ and γʹ/β coatings is less significant. They are subjected to lower thermal stress but possess higher deformation resistance during cycling. However, local spallation of the oxide scale occurs on γʹ/β coating since it is insusceptible to rumpling and thus the thermal stress is difficult to release in time by plastic deformation. The γʹ coating outperforms its counterparts of γ/γʹ and γʹ/β coatings, presenting low oxidation rate, high resistance to scale cracking and spallation, meanwhile, limited elements-interdiffusion with the underlying substrate.

Integrated Modeling Framework for Investigating Multiscale Microstructural Impacts on the Ionic Conductivity of Garnet Solid Electrolytes

Although recent years have seen the emergence of several solid electrolyte candidates with intrinsically high ionic conductivities for all-solid-state batteries, practical processing and synthesis routes often generate grain boundaries and other interfaces that can perturb primary conduction channels. In this talk, we will present our development of a multiscale modeling framework for predicting the effective ionic conductivity through a solid electrolyte with a complex polycrystalline microstructure, which allows us to address challenges associated with directly probing these effects. We first demonstrate an efficient mesoscopic computational method for predicting the effective ionic conductivity through a solid electrolyte with a complex polycrystalline microstructure. By considering the interplay between concurrent grain boundary and bulk conduction modes, this method surpasses simplified circuit descriptions to account for realistic, nontrivial networks of conduction channels. Using the Li7-x La3Zr2O12 (LLZO) garnet solid electrolyte as a technologically relevant model system, we then parameterize our simulation framework using a multiscale approach that connects synthetic microstructures from phase-field simulations with diffusivity tensors from molecular dynamics simulations of ordered and disordered systems. Systematically designed simulations of the effective ionic conductivity of polycrystalline LLZO are applied to elucidate the relationship between relevant microstructural features and overall ionic transport properties, with a focus on topological features of grains and grain boundary networks. In particular, we will discuss atomistic and mesoscopic tunability of grain boundary and microstructure, respectively, controlling mass transport in LLZO. We identify specific instances in which competition between conduction modes results in a high sensitivity to microstructural features, highlighting the need to account for the detailed microstructural information when assessing mass transport in LLZO and related solid electrolyte materials, while simultaneously offering design guidance for achieving desired ionic transport properties in processed polycrystalline LLZO.This work of was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Multiphase-field modelling of concurrent grain growth and coarsening in complex multicomponent systems

Phase-field modelling of microstructural evolution in polycrystalline systems with phase-associated grains has largely been confined to continuum-field models. In this study, a multiphase-field approach, with a provision for introducing grain boundary and interphase diffusion, is extended to analyse concurrent grain growth and coarsening in multicomponent polycrystalline microstructures with chemically-distinct grains. The effect of the number of phases and components on the kinetics of evolution is investigated by considering binary and ternary systems of duplex and triplex microstructures, along with a single phase system. It is realised that the mere increase in the number of phases minimises the rate of concurrent grain growth and coarsening. However, the effect of components is substantially dependent on the respective kinetic coefficients. This work unravels that the disparity in the influence of phases and components is primarily due to the corresponding change introduced in the transformation mechanism. While the raise in number of phases convolutes the diffusion paths, the increase in number of component effects the rate of evolution through the interdiffusion, which introduces interdependency in the diffusing chemical-species. Additionally, the role of phase-fractions on the transformation rate of triplex microstructure is studied, and correspondingly, the interplay of interface- and diffusion-governed evolution is elucidated. A representative evolution of three-dimensional triplex microstructure with equal phase-fraction is comparatively analysed with the evolution of corresponding two-dimensional setup.

Post-FSW Cold-Rolling Simulation of ECAP Shear Deformation and Its Microstructure Role Combined to Annealing in a FSWed AA5754 Plate Joint.

Friction stir welds are considered reliable joints for their lack of voids, cracks and distortions. When compared to the base material, friction stir welding (FSW) joints typically exhibit finer grain structured (especially at the nugget zone, NZ). Similarly, refined grain structure can also be obtained by severe plastic deformation (SPD) techniques, such as equal channel angular pressing (ECAP). In fact, the fine grain structures produced within the NZ of FSW or friction stir processed (FSP) materials are usually coarser than the ones achieved by ECAP. The former is characterized by lower dislocation density, higher high-angle boundary fraction and different mechanical strength, compared to what can be obtained by ECAP. In this study, a dedicated cold-rolling (CR) set-up, specifically designed to simulate an ECAP-equivalent shear deformation, was used to further refine the grain structure of FSW AA5754 sheets. The effect of ECAP-equivalent deformation induced by CR in a 2 mm-thick AA5754-H111 FSW joint was investigated. FSW was carried out at two different rotational (ω) and translational (v) welding speeds, 600 rpm, 200 mm/min and 1800 rpm, 75 mm/min, respectively. FSW sheets were then CR to obtain an equivalent shear strain of ε ~ 1.08, that is equivalent to 1-ECAP pass carried out with an internal die channels intersecting at an angle φ = 90° with a curvature extending over an angle Ψ = 20°. By CR, the sheet thickness reduced only by ~20%. The role of annealing on the FSW and CR plastically deformed AA5754 was also investigated. This was applied either prior or after FSW, and it resulted that whenever it follows the FSW, the mean volume fraction of dispersoids and Mg-rich particles is higher than the case of annealing preceding the FSW process. On the contrary, it was found that the annealing treatment had a minimal role on the dispersoids and particles mean size. The here reported post-FSW ECAP-simulated deformation, obtained by a customized CR process, showed sheet integrity and a significant concurrent grain size refinement.

Regional variation in airborne Alternaria spore concentrations in Denmark through 2012–2015 seasons: the influence of meteorology and grain harvesting

High airborne Alternaria spore concentrations measured in eastern Denmark have been associated with local agricultural sources. However, the density of agricultural areas is highest in western Denmark. This is the first report of airborne Alternaria spore concentrations obtained with Burkard volumetric spore sampler in western Denmark, Viborg. We compared the concentrations of airborne Alternaria spores and the patterns of air mass transport using HYSPLIT model between Copenhagen and Viborg for the seasons 2012–2015, with the main focus on the days with daily average Alternaria spore concentrations ≥ 100 s m−3 (high concentration days). Except for 2012, Annual Spore Integrals (ASIns) were on average 3335 s day m−3 higher in Viborg than in Copenhagen. The high concentration days during 2012–2015 occurred more frequently and with higher values in Viborg (96 days; mean = 381 s m−3) than in Copenhagen (79 days; mean = 270 s m−3). We found increased shares of trajectories coming from South-East on the high concentration days and increased shares of trajectories coming from the West and North-West on the days with concentrations below 100 s m−3 for both stations. July and August had the highest spore integrals matching the periods of grain harvesting in Denmark. The absence of the concurrent grain harvesting in Denmark was associated with the lowest ASIns in 2012. The results of this study support the hypothesis that local sources cause the main load of airborne Alternaria spore concentrations in Denmark; however, the contribution from the remote source areas in northern Germany, Poland and southern Sweden remains unquantified.

Synergistic effects of holding time at intercritical annealing temperature and initial microstructure on the mechanical properties of dual phase steel

The synergistic effects of the initial microstructure and the intercritical annealing time on the microstructure and tensile properties of low carbon steel were studied. It was found that by refining the pre-intercritical annealing microstructure, the resulting duplex DP microstructure becomes finer, which results in the enhancement of the tensile properties and work-hardening behavior. Based on the hardness measurements, three stages were identified for intercritical annealing: initial rise or fall, reaching a plateau, and final fall to a low value. The pre-intercritical annealing microstructure did not show any pronounced effects on these stages, with the exception of the short-lived initial stage. It was shown that the DP steel obtained right after reaching the abovementioned plateau can exhibit improved mechanical properties and work-hardening behavior. During the third stage, the enrichment of manganese in austenite and the concurrent grain growth were found to be the main factors taking part in the change of hardness and tensile strength and reappearance of the yield-point elongation. The impact of the microstructural features (austenite and ferrite) on the grain growth behavior during intercritical annealing and the effect of the grain size on the Lüders strain were also discussed.

Segregation-Induced Nanofaceting Transition at an Asymmetric Tilt Grain Boundary in Copper

We show that chemistry can be used to trigger a nanofaceting transition. In particular, the segregation of Ag to an asymmetric tilt grain boundary in Cu is investigated. Aberration-corrected electron microscopy reveals that annealing the bicrystal results in the formation of nanometer-sized facets composed of preferentially Ag-segregated symmetric Σ5{210} segments and Ag-depleted {230}/{100} asymmetric segments. Our observations oppose an anticipated trend to form coarse facets. Atomistic simulations confirm the nanofacet formation observed in the experiment and demonstrate a concurrent grain boundary phase transition induced by the anisotropic segregation of Ag.

Concurrent grain growth and coarsening of two-phase microstructures; large scale phase-field study

A thermodynamically consistent phase-field model is exploited to demonstrate the influence of relative volume fractions and bulk diffusivity on the grain growth phenomena in two-phase polycrystalline systems. For very small and high volume fractions, the simulated morphology consists of a dispersion of isolated minor phase grains embedded in the matrix of major phase grains. At intermediate fractions, the obtained microstructure resembles an interpenetrating network-like structure. The performed large-scale 2-D simulations elucidate the governing mechanisms for the concurrent two-phase growth at low and high volume fractions, and the continuous transition between interface-controlled and diffusion-limited regimes. While the slowest kinetics is observed for the 50/50 volume fraction case, irrespective of the diffusivity, the fastest kinetics is displayed by the pure systems, with a slight difference, which is due to relative interfacial energies. The relative growth rates of the individual phases and the maximum attainable grain size to mean size ratio are observed to follow the well-known trends for isotropic systems. The obtained results for the concurrent growth of a minor phase with various diffusivities, reveal that it is difficult to reconcile all observed behaviors with a universal Zener relation, in contradiction to the previously made claims in the literature. A need for the full-fledged growth law, which takes into consideration the role of the diffusivity, the volume fractions, and the relative interfacial energies in multiphase polycrystalline systems, is pointed out.

Theoretical analysis of experimental densification kinetics in final sintering stage of nano-sized zirconia

The experimental densification kinetics of 7.8 mol% Y2O3-stabilized zirconia was analyzed theoretically during isothermal sintering in the final stage. By taking concurrent grain growth into account, a possible value of the grain-size exponent n was examined. The Coble’s corner-pore model recognized widely was found not to be applicable for explaining the densification kinetics. The corner-pore model of n = 4 shows a significant divergence in the kinetics at different temperatures. Microstructural observation shows that most pores are not located at grain corners and have a size comparable to the surrounding grains. The observed pore structure is similar to the diffusive model where single pore is surrounded by dense body. The diffusive model combined with theoretical sintering stress predicts n = 1 or n = 2, which shows a good consistence to the measured densification kinetics. During sintering of nano-sized powder, it is found that the densification kinetics can be explained distinctively by the diffusive single-pore model.

Effects of concurrent grain boundary and surface segregation on the final stage of sintering: the case of Lanthanum doped yttria-stabilized zirconia

Dopants play a critical role in tailoring the microstructure during sintering of compacts. These dopants may form solid solution within the bulk, and/or segregate to the grain boundaries (GBs) and the solid-vapor interfaces (free surfaces), each causing a distinct energetic scenario governing mass transports during densification and grain growth. In this work, the forces controlling the dopant distribution, in particular the possibility of concurrent segregation at both surfaces and GBs, are discussed based on the respective enthalpy of segregation. An equation is derived based on the minimum Gibbs energy of the system to determine enthalpy of segregation from experimental interface energy data, and the results applied to depict the role of La as a dopant on the interface energetics of yttria stabilized zirconia during its final stage of sintering. It is shown that La substantially decreases both GB and surface energies (differently) as sintering progresses, dynamically affecting its driving forces, and consequent grain growth and densification in this stage.

Influence of grain size, water, and deformation on dolomite reaction rim formation

Solid-solid mineral reaction rates are influenced by the microfabrics of reactant phases and concurrent deformation. To investigate this interplay in carbonate systems, we performed annealing and deformation experiments on polycrystalline and single-crystal calcite and magnesite, forming dolomite (Dol) and magnesio-calcite (Mg-Cal). At a fixed temperature of T = 750 °C and confining pressure of P = 400 MPa, samples were either annealed for 29 h, or deformed in triaxial compression or torsion for 18 h using a Paterson-type gas deformation apparatus. At the contact interface of the starting reactants, Dol reaction rims and polycrystalline Mg-Cal layers were formed. The widths of the layers were in the ranges 4–117 and 30–147 μm, respectively, depending on the microstructure of starting materials and experimental conditions. Annealing experiments with polycrystalline reactants in contact with each other resulted in a ~22-fold increase in Dol rim thickness compared to a contact between two single crystals and a larger Mg-Cal layer width by a factor of 5 (cf. Helpa et al. 2014). This suggests that the microstructure of magnesite controls migration of the reaction front. For polycrystalline starting materials, axial stress accelerated Mg-Cal growth rates but not Dol growth rates. Highly strained torsion samples showed Dol formation along grain boundaries in Mg-Cal as well as in the polycrystalline calcite reactant. A reduction of Dol rim thickness between polycrystalline reactants deformed in torsion is possibly caused by concurrent grain coarsening of polycrystalline magnesite. Dol and Mg-Cal growth kinetics between single crystals were unaffected by the addition of ~0.3 wt% water. The experiments demonstrate that Dol reaction kinetics strongly correlate with magnesite reactant grain sizes, while Mg-Cal growth depends on the calcite reactant grain sizes. The dolomite-forming mineral reaction kinetics are not significantly affected by concurrent deformation. In contrast, deformation enhances Mg-Cal formation, especially at small calcite grain sizes that promote efficient grain boundary diffusion. Therefore, the fastest reactions forming Dol and Mg-Cal in nature are expected to occur in very fine-grained reactants. Concurrent deformation may drastically enhance reaction kinetics if grain size reduction of the reactants occurs by, for example, cataclasis or dynamic recrystallization.

Temperature-Dependent Flow Behavior and Microstructural Evolution During Compression of As-Cast Mg-7.7Al-0.4Zn

The microstructure and mechanical properties improve substantially by hot working. This aspect in as-cast Mg-7.7Al-0.4Zn (AZ80) alloy is investigated by compression tests over temperature range of 30-439°C and at strain rates of 5 × 10−2, 10−2, 5 × 10−4 and 10−4 s−1. The stress exponent (n) and activation energy (Q) were evaluated and analyzed for high-temperature deformation along with the microstructures. Upon deformation to a true strain of 0.80, which corresponds to the pseudo-steady-state condition, n and Q were found to be 5 and 151 kJ/mol, respectively. This suggests the dislocation climb-controlled mechanism for deformation. Prior to attaining the pseudo-steady-state condition, the stress-strain curves of AZ80 Mg alloy exhibit flow hardening followed by flow softening depending on the test temperature and strain rate. The microstructures obtained upon deformation revealed dissolution of Mg17Al12 particles with concurrent grain growth of α-matrix. The parameters like strain rate sensitivity and activation energy were analyzed for describing the microstructure evolution also as a function of strain rate and temperature. This exhibited similar trend as seen for deformation per se. Thus, the mechanisms for deformation and microstructure evolution are suggested to be interdependent.

Role of Microstructure and Surface Defects on the Dissolution Kinetics of CeO2, a UO2 Fuel Analogue.

The release of radionuclides from spent fuel in a geological disposal facility is controlled by the surface mediated dissolution of UO2 in groundwater. In this study we investigate the influence of reactive surface sites on the dissolution of a synthesized CeO2 analogue for UO2 fuel. Dissolution was performed on the following: CeO2 annealed at high temperature, which eliminated intrinsic surface defects (point defects and dislocations); CeO2-x annealed in inert and reducing atmospheres to induce oxygen vacancy defects and on crushed CeO2 particles of different size fractions. BET surface area measurements were used as an indicator of reactive surface site concentration. Cerium stoichiometry, determined using X-ray Photoelectron Spectroscopy (XPS) and supported by X-ray Diffraction (XRD) analysis, was used to determine oxygen vacancy concentration. Upon dissolution in nitric acid medium at 90 °C, a quantifiable relationship was established between the concentration of high energy surface sites and CeO2 dissolution rate; the greater the proportion of intrinsic defects and oxygen vacancies, the higher the dissolution rate. Dissolution of oxygen vacancy-containing CeO2-x gave rise to rates that were an order of magnitude greater than for CeO2 with fewer oxygen vacancies. While enhanced solubility of Ce(3+) influenced the dissolution, it was shown that replacement of vacancy sites by oxygen significantly affected the dissolution mechanism due to changes in the lattice volume and strain upon dissolution and concurrent grain boundary decohesion. These results highlight the significant influence of defect sites and grain boundaries on the dissolution kinetics of UO2 fuel analogues and reduce uncertainty in the long term performance of spent fuel in geological disposal.

Microstructural and constitutive analysis in process modeling of hot working: The case of a Mg-Zn-Mn alloy

This study presents an analysis of the microstructural evolution and flow stress variation with strain of a Mg-2%Zn-1%Mn alloy during hot deformation, carried out through a combination of torsion experiments, electron backscatter diffraction (EBSD) studies and constitutive analysis. Straining was observed to produce dynamic recrystallization (DRX). It emerged that such DRX was sluggish at 250°C, while almost complete grain refinement occurred at 350°C. A further increase in temperature led to concurrent grain growth and dynamic recrystallization, resulting in coarse grain size. A modified strain-dependent form of the Garofalo equation was used in the constitutive analysis, obtaining a more than satisfactory description of the experimental results. The extrapolation of the model in the low strain rate/high strain regions led to an equally satisfactory description of the experimental data. The analysis of the variation of the activation energy for hot working, which was equivalent to the activation energy for self-diffusion, suggested that deformation was recovery-controlled in most of the range of the experimental conditions investigated.