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Papers by Carlos Tome

International Journal of Plasticity, 2018

We propose a multi-scale modeling approach that can simulate the microstructural and mechanical b... more We propose a multi-scale modeling approach that can simulate the microstructural and mechanical behavior of metal/alloy parts with complex geometries subjected to multi-axial load path changes. The model is used to understand the biaxial load path change behavior of 316L stainless steel cruciform samples. At the macroscale, a finite element approach is used to simulate the cruciform geometry and numerically predict the gauge stresses, which are difficult to obtain analytically. At each material point in the finite element mesh, the anisotropic viscoplastic selfconsistent model is used to simulate the role of texture evolution on the mechanical response. At the single crystal level, a dislocation density based hardening law that appropriately captures the role of multi-axial load path changes on slip activity is used. The combined approach is experimentally validated using cruciform samples subjected to uniaxial load and unload followed by different biaxial reloads in the angular range°°[27 , 90 ]. Polycrystalline yield surfaces before and after load path changes are generated using the full-field elasto-viscoplastic fast Fourier transform model to study the influence of the deformation history and reloading direction on the mechanical response, including the Bauschinger effect, of these cruciform samples. Results reveal that the Bauschinger effect is strongly dependent on the first loading direction and strain, intergranular and macroscopic residual stresses after first load, and the reloading angle. The microstructural origins of the mechanical response are discussed.

Acta Materialia, 2018

On the first direct observation of de-twinning in a twinning-induced plasticity steel On the firs... more On the first direct observation of de-twinning in a twinning-induced plasticity steel On the first direct observation of de-twinning in a twinning-induced plasticity steel

In this work, a crystallographic thermal creep model is proposed for Zr alloys that accounts for ... more In this work, a crystallographic thermal creep model is proposed for Zr alloys that accounts for the hardening contribution of solutes via their time-dependent pinning effect on dislocations. The core-diffusion model proposed by Soare and Curtin (2008a) is coupled with a recently proposed constitutive modeling framework (Wang et al., 2017, 2016) accounting for the heterogeneous distribution of internal stresses within grains. The Coble creep mechanism is also included. This model is, in turn, embedded in the effective medium crystallographic VPSC framework and used to predict creep strain evolution of polycrystals under different temperature and stress conditions. The simulation results reproduce the experimental creep data for Zircaloy-4 and the transition between the low (n~1), intermediate (n~4) and high (n~9) power law creep regimes. This is achieved through the dependence on local aging time of the solute-dislocation binding energy. The anomalies in strain rate sensitivity (SRS) are discussed in terms of core-diffusion effects on dislocation junction strength. The mechanism-based model captures the primary and secondary creep regimes results reported by Kombaiah and Murty (2015a, 2015b) for a comprehensive set of testing conditions covering the 500 to 600 o C interval, stresses spanning 14 to 156 MPa, and steady state creep rates varying between 1.5• 10-9 s-1 to 2• 10-3 s-1. There are two major advantages to this model with respect to more empirical ones used as constitutive laws for describing thermal creep of cladding: 1) specific dependences on the nature of solutes and their concentrations are explicitly accounted for; 2) accident conditions in reactors, such as RIA and LOCA, usually take place in short times, and deformation takes place in the primary, not the steady-state creep stage. As a consequence, a model that accounts for the evolution with time of microstructure is more reliable for this kind of simulation.

Acta Materialia, 2017

Basal slip and {0112} twinning are two major plastic deformation mechanisms in hexagonal closedpa... more Basal slip and {0112} twinning are two major plastic deformation mechanisms in hexagonal closedpacked magnesium. Here we quantify the critical stresses associated with basal slip and twinning in single-crystal and bi-crystal magnesium samples by performing in situ compression of micropillars with different diameters in a scanning electron microscope. The micropillars are designed to favor either slip or twinning under uniaxial compression. Compression tests imply a negligible size effect related to basal slip and twinning as pillar diameter is greater than 10 mm. The critical resolved shear stresses are deduced to be 29 MPa for twinning and 6 MPa for basal slip from a series of micropillar compression tests. Employing full-field elasto-visco-plastic simulations, we further interpret the experimental observations in terms of the local stress distribution associated with multiple twinning, twin nucleation, and twin growth. Our simulation results suggest that the twinning features being studied should not be close to the top surface of the micropillar because of local stress perturbations induced by the hard indenter. Published by Elsevier Ltd on behalf of Acta Materialia Inc.

Acta Materialia, 2016

The mechanical response of a low carbon steel under complex strain path changes is analyzed here ... more The mechanical response of a low carbon steel under complex strain path changes is analyzed here in terms of dislocation storage and annihilation. The mechanical tests performed are cyclic shear and tensile loading followed by shear at different angles with respect to the tensile axis. The material behavior is captured by a dislocation-based hardening model, which is embedded in the Visco-Plastic Self-Consistent (VPSC) polycrystal framework taking into account the accumulation and annihilation of dislocations, as well as back-stress effects. A new and more sophisticated formulation of dislocation reversibility is proposed. The simulated flow stress responses are in good agreement with the experimental data. The effects of the dislocation-related mechanisms on the hardening response during strain path changes are discussed.

Textures and Microstructures, 1996

Polycrystalline models are used for the calculation of the yield loci of hcp aggregates with diff... more Polycrystalline models are used for the calculation of the yield loci of hcp aggregates with different crystallographic textures. Predictions are made using both a classical Taylor Full Constraints (FC) approach and a viscoplastic selfconsistent (VPSC) formulation. Typical deformation modes for low and high temperature are assumed to be active. The vertex configuration of the Single Crystal Yield Surface (SCYS) and the relative activity of different deformation modes as a function of the applied stress are also presented and discussed. For low temperature, when twinning is active, a non centro-symmetric behavior is observed in textured polycrystals along the direction of the -axis texture component. Such effect, associated with the directionality of twinning, is well reproduced by the VPSC model. For high temperature, the predicted yield loci are centro-symmetrical but also strongly dependent on the texture of the aggregate.

In this work, a physics-based thermal creep model is developed based on the understanding of the ... more In this work, a physics-based thermal creep model is developed based on the understanding of the microstructure in Fe-Cr alloys. This model is associated with a transition state theory-based framework that considers the distribution of internal stresses at sub-material point level. The thermally activated dislocation glide and climb mechanisms are coupled in the obstacle-bypass processes for both dislocation and precipitate-type barriers. A kinetic law is proposed to track the dislocation densities evolution in the subgrain interior and in the cell wall. The predicted results show that this model, embedded in the visco-plastic self-consistent framework, captures well the creep behaviors for primary and steady-state stages under various loading conditions. The roles of the mechanisms involved are also discussed.

Journal of Computational Physics, 2017

Accurate prediction of cladding mechanical behavior is a key aspect of modeling nuclear fuel beha... more Accurate prediction of cladding mechanical behavior is a key aspect of modeling nuclear fuel behavior, especially for conditions of pellet-cladding interaction (PCI), reactivity-initiated accidents (RIA), and loss of coolant accidents (LOCA). Current approaches to fuel performance modeling rely on empirical constitutive models for cladding creep, growth and plastic deformation, which are limited to the materials and conditions for which the models were developed. To improve upon this approach, a microstructurally-based zirconium alloy mechanical deformation analysis capability is being developed within the United States Department of Energy Consortium for Advanced Simulation of Light Water Reactors (CASL). Specifically, the viscoplastic self-consistent (VPSC) polycrystal plasticity modeling approach, developed by Lebensohn and Tomé [1], has been coupled with the BISON engineering scale fuel performance code to represent the mechanistic material processes controlling the deformation behavior of light water reactor (LWR) cladding. A critical component of VPSC is the representation of the crystallographic nature (defect and dislocation movement) and orientation of the grains within the matrix material and the ability to account for the role of texture on deformation. A future goal is for VPSC to obtain information on reaction rate kinetics from atomistic calculations to inform the defect and dislocation behavior models described in VPSC. The multiscale modeling of cladding deformation mechanisms allowed by VPSC far exceed the functionality of typical semi-empirical constitutive models employed in nuclear fuel behavior codes to model irradiation growth and creep, thermal creep, or plasticity. This paper describes the implementation of an interface between VPSC and BISON and provides initial results utilizing the coupled functionality.

Acta Materialia, 2016

We study the effect of nearest neighboring grains on the propensity for {1012} twin growth in Mg ... more We study the effect of nearest neighboring grains on the propensity for {1012} twin growth in Mg and Zr. Twin lamellae lying within one grain flanked by two neighboring grains with several orientations are considered. The fields of resolved shear stress on the twin system are calculated in the multicrystal using a three-dimensional full-field crystal plasticity Fast Fourier Transform approach. The calculations were carried out for Mg and Zr using slip threshold stresses corresponding to 300K and 76K, respectively, where twin activity is important. We show that the neighboring grain constraint tends to oppose further growth and that the critical applied stress needed to overcome this resistance depends on neighboring grain orientation, more strongly in Zr than in Mg. We also present results for a pair of adjacent and parallel twins at various spacings. It is found that their paired interaction increases the resistive forces for twin growth above that for an isolated twin. The critical spacing above which this enhanced resistance is removed is smaller for Zr than Mg. Our analysis reveals that these two disparate responses of Zr and Mg are both a consequence of the fact that Zr is elastically and plastically more anisotropic than Mg. Additional calculations carried out on Ti support this conclusion. These findings can help explain why, for the same grain size, more twins per grain form in Zr than in Mg, twins in Zr tend to be thinner than those in Mg, and the relationship between the thickness of the twin and its Schmid factor in Zr is not as strong as in Mg.

Materials Science and Engineering: A, 2015

In this work, we develop a polycrystal mean-field constitutive model based on an elastic-plastic ... more In this work, we develop a polycrystal mean-field constitutive model based on an elastic-plastic self-consistent (EPSC) framework. In this model, we incorporate recently developed subgrain models for dislocation density evolution with thermally activated slip, twin activation via statistical stress fluctuations, reoriented twin domains within the grain and associated stress relaxation, twin boundary hardening, and de-twinning. The model is applied to a systematic set of strain path change tests on pure beryllium (Be). Under the applied deformation conditions, Be deforms by multiple slip modes and deformation twinning and thereby provides a challenging test for model validation. With a single set of material parameters, determined using the flowstress vs. strain responses during monotonic testing, the model predicts well the evolution of texture, lattice strains, and twinning. With further analysis, we demonstrate the significant influence of internal residual stresses on: 1) the flow stress drop when reloading from one path to another, 2) deformation twin activation, 3) de-twinning during a reversal strain path change, and 4) the formation of additional twin variants during a cross-loading sequence. The model presented here can, in principle, be applied to other metals, deforming by multiple slip and twinning modes under a wide range of temperature, strain rate, and strain path conditions.

Materials Science Forum, 2011

Texture evolution in plastically deformed HCP metals is strongly influenced by the nucleation and... more Texture evolution in plastically deformed HCP metals is strongly influenced by the nucleation and growth of deformation twins and twin variant selection. Statistically based EBSD analyses of deformed microstructures in HCP metals indicate that the nucleation of deformation twins depends on, among other factors, the local stress fields arising from neighboring grain interactions at grain boundaries [1]. Inspired by these findings a probability model for twin nucleation was developed [2,3], based on the activation of defect sources statistically occurring in grain boundaries. This nucleation model was implemented in a Visco-Plastic Self-Consistent (VPSC) code. Because the latter is based on an Effective Medium assumption and the inclusion formalism, it only provides average stress values in the grains, and the nature of local stress fields at grain boundaries had to be considered in a heuristic manner. In order to have better insight on the effect of local textures on twin nucleation,...

International Journal of Plasticity, 2014

In this work, we present a multiscale physically based constitutive law for predicting the mechan... more In this work, we present a multiscale physically based constitutive law for predicting the mechanical response and texture evolution of body-centered cubic (BCC) metals as a function of strain-rate and temperature. In the model, deformation of individual single crystals results not only from the resolved shear stress along the direction of slip (Schmid law) but also from shear stresses resolved along directions orthogonal to the slip direction as well as the three normal stress components (non-Schmid effects). We account for coupled Schmid and non-Schmid effects through the modification of the resolved shear stress for both 1/2〈11 ത 1〉ሼ110ሽ and 1/2〈111 ത 〉ሼ112ሽ slip systems and the modification of the slip resistance for 1/2〈111 ത 〉ሼ112ሽ slip systems. The single crystal model is implemented into a self-consistent homogenization scheme containing a hardening law for crystallographic slip. The hardening law is based on the evolution of dislocation densities that incorporates strain-rate and temperature effects through the Peierls stress, thermally activated recovery, dislocation substructure formation and dislocation interactions. The polycrystal model is calibrated and validated using a set of mechanical and texture data collected on a tantalum-tungsten alloy, Ta-10W, at temperatures ranging from 298 K to 673 K and strain-rates from 10-3 to 2400 s-1. We show the model effectively captures the anisotropic hardening rate and texture evolution for all data using a single set of single-crystal hardening parameters. Comparisons between predictions and measured data allow us to discuss the role of slip on ሼ110ሽ and ሼ112ሽ in determining plasticity and texture evolution in Ta-10W.

International Journal of Solids and Structures, 2010

Various self-consistent polycrystal plasticity models for hexagonal close packed (HCP) polycrysta... more Various self-consistent polycrystal plasticity models for hexagonal close packed (HCP) polycrystals are evaluated by studying the deformation behavior of magnesium alloy AZ31B sheet under different uniaxial strain paths. In all employed polycrystal plasticity models both slip and twinning contribute to plastic deformation. The material parameters for the various models are fitted to experimental uniaxial tension and compression along the rolling direction (RD) and then used to predict uniaxial tension and compression along the traverse direction (TD) and uniaxial compression in the normal direction (ND). An assessment of the predictive capability of the polycrystal plasticity models is made based on comparisons of the predicted and experimental stress responses and R values. It is found that, among the models examined, the self-consistent models with grain interaction stiffness halfway between those of the limiting Secant (stiff) and Tangent (compliant) approximations give the best results. Among the available options, the Affine self-consistent scheme results in the best overall performance. Furthermore, it is demonstrated that the R values under uniaxial tension and compression within the sheet plane show a strong dependence on imposed strain. This suggests that developing anisotropic yield functions using measured R values must account for the strain dependence.

International Journal of Solids and Structures, 2012

The recently developed large strain elastic visco-plastic self-consistent (EVPSC) model, which in... more The recently developed large strain elastic visco-plastic self-consistent (EVPSC) model, which incorporates both slip and twinning deformation mechanisms, is used to study the lattice strain evolution in extruded magnesium alloy AZ31 under uniaxial tension and compression. The results are compared against in-situ neutron diffraction measurements done on the same alloy. For the first time, the effects of stress relaxation and strain creep on lattice strain measurements in respectively displacement controlled and load controlled in-situ tests are numerically assessed. It is found that the stress relaxation has a significant effect on the lattice strain measurements. It is also observed that although the creep does not significantly affect the trend of the lattice strain evolution, a better agreement with the experiments is found if creep is included in the simulations.

Physical Review B, 2009

Previously measured in situ x-ray diffraction is used to assess the development of internal elast... more Previously measured in situ x-ray diffraction is used to assess the development of internal elastic strains within grains of a sample of polycrystalline cobalt plastically deformed up to a pressure of 42.6 GPa. An elastoplastic self-consistent polycrystal model is used to simulate the macroscopic flow curves and internal strain development within the sample. Input parameters are single-crystal elastic moduli and their pressure dependence, critical resolved shear stresses, and hardening behavior of the slip and twinning mechanisms which are active in Co crystals. At 42 GPa, the differential stress in hcp-Co is 1.9Ϯ 0.1 GPa. The comparison between experimental and predicted data leads us to conclude that: ͑a͒ plastic relaxation plays a primary role in controlling the evolution and ordering of the lattice strains; ͑b͒ the plastic behavior of hcp-Co deforming under high pressure is controlled by basal and prismatic slip of ͗a͘ dislocations, and either pyramidal slip of ͗c + a͘ dislocations, or compressive twinning, or both. Basal slip is by far the easiest and most active deformation mechanism. Elastoplastic self-consistent models are shown to overcome the limitations of models based on continuum elasticity theory for the interpretation of x-ray diffraction data measured on stressed samples. They should be used for the interpretation of these experiments.

Philosophical Magazine, 2007

Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdf ... more Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdf This article maybe used for research, teaching and private study purposes. Any substantial or systematic reproduction, redistribution , reselling , loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

Materials Science and Engineering: A, 2003

In this work, we develop a modeling framework for predicting the visco-plastic deformation, micro... more In this work, we develop a modeling framework for predicting the visco-plastic deformation, microstructural evolution (distributions of grain shape and size) and texture evolution in polycrystalline materials during the equal channel angular extrusion (ECAE) process, a discontinuous process of severe shear straining. The foundation of this framework is a visco-plastic selfconsistent (VPSC) scheme. We consider a 908 die angle and simulate ECAE up to four passes for four processing routes, (A, C, B A and B C , as denoted in the literature) for an FCC polycrystalline material. We assume that the FCC single crystal has a constant critical resolved shear stress (CRSS), so that hardening by dislocation activity is suppressed, and the influence of grain shape distribution and texture as well as their interaction can be isolated. Many deformation microstructural features, such as grain size and shape distribution, texture, and geometric hardening Á/softening, were highly dependent on processing route. Using a grain subdivision criterion based on grain shape, route A was the most effective, then route B A and route B C and lastly route C, the least effective for grain size refinement, in agreement with redundant strain theory. For producing refined equiaxed grains, route B C was more effective than routes B A and A. We show that grain Á/grain interactions tend to weaken texture evolution and consequently geometric hardening and softening in all routes.

Materials Science and Engineering: A, 1994

The plastic properties of anisotropic polycrystalline aggregates and polyphase materials are in g... more The plastic properties of anisotropic polycrystalline aggregates and polyphase materials are in general non-homogeneous and, as a consequence, so is the local plastic deformation. We present in this work a model that describes the plastic behaviour of non-homogeneous materials composed of anisotropic regions (grains or phases). Our model is based on describing each region as a viscoplastic inclusion embedded in the effective medium represented by the other grains, and incorporates explicitly the grain interaction with its surroundings and the plastic anisotropy of grain and matrix. Within the model the grain response is coupled to the overall response of the polycrystal and the grain deformation may differ from the polycrystars. A characteristic of our approach is that those deformation systems with lower critical resolved shear stress tend to be more active, and less than five systems per grain are sufficient to accommodate the imposed overall deformation. In this work we explore the consequences and the limits of the model, and its dependence on the assumed rate sensitivity as well. We combine the self-consistent formulation with a volume fraction transfer scheme for treating the reorientation due to twinning, and simulate rolling textures of brass (f.c.c.), Zircaloy (h.c.p.), calcite (trigonal) and uranium (orthorhombic). We compare the results with experimental measurements and Taylor-type predictions, infer information concerning the microscopic deformation mechanisms and discuss the limits of applicability of the approach.

Journal of Nuclear Materials, 2008

Hardening and embrittlement are controlled by interactions between dislocations and irradiation i... more Hardening and embrittlement are controlled by interactions between dislocations and irradiation induced defect clusters. In this work we employ the visco plastic self consistent (VPSC) polycrystalline code in order to model the yield stress dependence in ferritic steels on the irradiation dose. We implement the dispersed barrier hardening model in the VPSC code by introducing a hardening law, function of the strain, to describe the threshold resolved shear stress required to activate dislocations. The size and number density of the defect clusters varies with the irradiation dose in the model. We find that VPSC calculations show excellent agreement with the experimental data set. Such modeling efforts can both reproduce experimental data and also guide future experiments of irradiation hardening.

International Journal of Plasticity, 2018

We propose a multi-scale modeling approach that can simulate the microstructural and mechanical b... more We propose a multi-scale modeling approach that can simulate the microstructural and mechanical behavior of metal/alloy parts with complex geometries subjected to multi-axial load path changes. The model is used to understand the biaxial load path change behavior of 316L stainless steel cruciform samples. At the macroscale, a finite element approach is used to simulate the cruciform geometry and numerically predict the gauge stresses, which are difficult to obtain analytically. At each material point in the finite element mesh, the anisotropic viscoplastic selfconsistent model is used to simulate the role of texture evolution on the mechanical response. At the single crystal level, a dislocation density based hardening law that appropriately captures the role of multi-axial load path changes on slip activity is used. The combined approach is experimentally validated using cruciform samples subjected to uniaxial load and unload followed by different biaxial reloads in the angular range°°[27 , 90 ]. Polycrystalline yield surfaces before and after load path changes are generated using the full-field elasto-viscoplastic fast Fourier transform model to study the influence of the deformation history and reloading direction on the mechanical response, including the Bauschinger effect, of these cruciform samples. Results reveal that the Bauschinger effect is strongly dependent on the first loading direction and strain, intergranular and macroscopic residual stresses after first load, and the reloading angle. The microstructural origins of the mechanical response are discussed.

Acta Materialia, 2018

On the first direct observation of de-twinning in a twinning-induced plasticity steel On the firs... more On the first direct observation of de-twinning in a twinning-induced plasticity steel On the first direct observation of de-twinning in a twinning-induced plasticity steel

In this work, a crystallographic thermal creep model is proposed for Zr alloys that accounts for ... more In this work, a crystallographic thermal creep model is proposed for Zr alloys that accounts for the hardening contribution of solutes via their time-dependent pinning effect on dislocations. The core-diffusion model proposed by Soare and Curtin (2008a) is coupled with a recently proposed constitutive modeling framework (Wang et al., 2017, 2016) accounting for the heterogeneous distribution of internal stresses within grains. The Coble creep mechanism is also included. This model is, in turn, embedded in the effective medium crystallographic VPSC framework and used to predict creep strain evolution of polycrystals under different temperature and stress conditions. The simulation results reproduce the experimental creep data for Zircaloy-4 and the transition between the low (n~1), intermediate (n~4) and high (n~9) power law creep regimes. This is achieved through the dependence on local aging time of the solute-dislocation binding energy. The anomalies in strain rate sensitivity (SRS) are discussed in terms of core-diffusion effects on dislocation junction strength. The mechanism-based model captures the primary and secondary creep regimes results reported by Kombaiah and Murty (2015a, 2015b) for a comprehensive set of testing conditions covering the 500 to 600 o C interval, stresses spanning 14 to 156 MPa, and steady state creep rates varying between 1.5• 10-9 s-1 to 2• 10-3 s-1. There are two major advantages to this model with respect to more empirical ones used as constitutive laws for describing thermal creep of cladding: 1) specific dependences on the nature of solutes and their concentrations are explicitly accounted for; 2) accident conditions in reactors, such as RIA and LOCA, usually take place in short times, and deformation takes place in the primary, not the steady-state creep stage. As a consequence, a model that accounts for the evolution with time of microstructure is more reliable for this kind of simulation.

Acta Materialia, 2017

Basal slip and {0112} twinning are two major plastic deformation mechanisms in hexagonal closedpa... more Basal slip and {0112} twinning are two major plastic deformation mechanisms in hexagonal closedpacked magnesium. Here we quantify the critical stresses associated with basal slip and twinning in single-crystal and bi-crystal magnesium samples by performing in situ compression of micropillars with different diameters in a scanning electron microscope. The micropillars are designed to favor either slip or twinning under uniaxial compression. Compression tests imply a negligible size effect related to basal slip and twinning as pillar diameter is greater than 10 mm. The critical resolved shear stresses are deduced to be 29 MPa for twinning and 6 MPa for basal slip from a series of micropillar compression tests. Employing full-field elasto-visco-plastic simulations, we further interpret the experimental observations in terms of the local stress distribution associated with multiple twinning, twin nucleation, and twin growth. Our simulation results suggest that the twinning features being studied should not be close to the top surface of the micropillar because of local stress perturbations induced by the hard indenter. Published by Elsevier Ltd on behalf of Acta Materialia Inc.

Acta Materialia, 2016

The mechanical response of a low carbon steel under complex strain path changes is analyzed here ... more The mechanical response of a low carbon steel under complex strain path changes is analyzed here in terms of dislocation storage and annihilation. The mechanical tests performed are cyclic shear and tensile loading followed by shear at different angles with respect to the tensile axis. The material behavior is captured by a dislocation-based hardening model, which is embedded in the Visco-Plastic Self-Consistent (VPSC) polycrystal framework taking into account the accumulation and annihilation of dislocations, as well as back-stress effects. A new and more sophisticated formulation of dislocation reversibility is proposed. The simulated flow stress responses are in good agreement with the experimental data. The effects of the dislocation-related mechanisms on the hardening response during strain path changes are discussed.

Textures and Microstructures, 1996

Polycrystalline models are used for the calculation of the yield loci of hcp aggregates with diff... more Polycrystalline models are used for the calculation of the yield loci of hcp aggregates with different crystallographic textures. Predictions are made using both a classical Taylor Full Constraints (FC) approach and a viscoplastic selfconsistent (VPSC) formulation. Typical deformation modes for low and high temperature are assumed to be active. The vertex configuration of the Single Crystal Yield Surface (SCYS) and the relative activity of different deformation modes as a function of the applied stress are also presented and discussed. For low temperature, when twinning is active, a non centro-symmetric behavior is observed in textured polycrystals along the direction of the -axis texture component. Such effect, associated with the directionality of twinning, is well reproduced by the VPSC model. For high temperature, the predicted yield loci are centro-symmetrical but also strongly dependent on the texture of the aggregate.

In this work, a physics-based thermal creep model is developed based on the understanding of the ... more In this work, a physics-based thermal creep model is developed based on the understanding of the microstructure in Fe-Cr alloys. This model is associated with a transition state theory-based framework that considers the distribution of internal stresses at sub-material point level. The thermally activated dislocation glide and climb mechanisms are coupled in the obstacle-bypass processes for both dislocation and precipitate-type barriers. A kinetic law is proposed to track the dislocation densities evolution in the subgrain interior and in the cell wall. The predicted results show that this model, embedded in the visco-plastic self-consistent framework, captures well the creep behaviors for primary and steady-state stages under various loading conditions. The roles of the mechanisms involved are also discussed.

Journal of Computational Physics, 2017

Accurate prediction of cladding mechanical behavior is a key aspect of modeling nuclear fuel beha... more Accurate prediction of cladding mechanical behavior is a key aspect of modeling nuclear fuel behavior, especially for conditions of pellet-cladding interaction (PCI), reactivity-initiated accidents (RIA), and loss of coolant accidents (LOCA). Current approaches to fuel performance modeling rely on empirical constitutive models for cladding creep, growth and plastic deformation, which are limited to the materials and conditions for which the models were developed. To improve upon this approach, a microstructurally-based zirconium alloy mechanical deformation analysis capability is being developed within the United States Department of Energy Consortium for Advanced Simulation of Light Water Reactors (CASL). Specifically, the viscoplastic self-consistent (VPSC) polycrystal plasticity modeling approach, developed by Lebensohn and Tomé [1], has been coupled with the BISON engineering scale fuel performance code to represent the mechanistic material processes controlling the deformation behavior of light water reactor (LWR) cladding. A critical component of VPSC is the representation of the crystallographic nature (defect and dislocation movement) and orientation of the grains within the matrix material and the ability to account for the role of texture on deformation. A future goal is for VPSC to obtain information on reaction rate kinetics from atomistic calculations to inform the defect and dislocation behavior models described in VPSC. The multiscale modeling of cladding deformation mechanisms allowed by VPSC far exceed the functionality of typical semi-empirical constitutive models employed in nuclear fuel behavior codes to model irradiation growth and creep, thermal creep, or plasticity. This paper describes the implementation of an interface between VPSC and BISON and provides initial results utilizing the coupled functionality.

Acta Materialia, 2016

We study the effect of nearest neighboring grains on the propensity for {1012} twin growth in Mg ... more We study the effect of nearest neighboring grains on the propensity for {1012} twin growth in Mg and Zr. Twin lamellae lying within one grain flanked by two neighboring grains with several orientations are considered. The fields of resolved shear stress on the twin system are calculated in the multicrystal using a three-dimensional full-field crystal plasticity Fast Fourier Transform approach. The calculations were carried out for Mg and Zr using slip threshold stresses corresponding to 300K and 76K, respectively, where twin activity is important. We show that the neighboring grain constraint tends to oppose further growth and that the critical applied stress needed to overcome this resistance depends on neighboring grain orientation, more strongly in Zr than in Mg. We also present results for a pair of adjacent and parallel twins at various spacings. It is found that their paired interaction increases the resistive forces for twin growth above that for an isolated twin. The critical spacing above which this enhanced resistance is removed is smaller for Zr than Mg. Our analysis reveals that these two disparate responses of Zr and Mg are both a consequence of the fact that Zr is elastically and plastically more anisotropic than Mg. Additional calculations carried out on Ti support this conclusion. These findings can help explain why, for the same grain size, more twins per grain form in Zr than in Mg, twins in Zr tend to be thinner than those in Mg, and the relationship between the thickness of the twin and its Schmid factor in Zr is not as strong as in Mg.

Materials Science and Engineering: A, 2015

In this work, we develop a polycrystal mean-field constitutive model based on an elastic-plastic ... more In this work, we develop a polycrystal mean-field constitutive model based on an elastic-plastic self-consistent (EPSC) framework. In this model, we incorporate recently developed subgrain models for dislocation density evolution with thermally activated slip, twin activation via statistical stress fluctuations, reoriented twin domains within the grain and associated stress relaxation, twin boundary hardening, and de-twinning. The model is applied to a systematic set of strain path change tests on pure beryllium (Be). Under the applied deformation conditions, Be deforms by multiple slip modes and deformation twinning and thereby provides a challenging test for model validation. With a single set of material parameters, determined using the flowstress vs. strain responses during monotonic testing, the model predicts well the evolution of texture, lattice strains, and twinning. With further analysis, we demonstrate the significant influence of internal residual stresses on: 1) the flow stress drop when reloading from one path to another, 2) deformation twin activation, 3) de-twinning during a reversal strain path change, and 4) the formation of additional twin variants during a cross-loading sequence. The model presented here can, in principle, be applied to other metals, deforming by multiple slip and twinning modes under a wide range of temperature, strain rate, and strain path conditions.

Materials Science Forum, 2011

Texture evolution in plastically deformed HCP metals is strongly influenced by the nucleation and... more Texture evolution in plastically deformed HCP metals is strongly influenced by the nucleation and growth of deformation twins and twin variant selection. Statistically based EBSD analyses of deformed microstructures in HCP metals indicate that the nucleation of deformation twins depends on, among other factors, the local stress fields arising from neighboring grain interactions at grain boundaries [1]. Inspired by these findings a probability model for twin nucleation was developed [2,3], based on the activation of defect sources statistically occurring in grain boundaries. This nucleation model was implemented in a Visco-Plastic Self-Consistent (VPSC) code. Because the latter is based on an Effective Medium assumption and the inclusion formalism, it only provides average stress values in the grains, and the nature of local stress fields at grain boundaries had to be considered in a heuristic manner. In order to have better insight on the effect of local textures on twin nucleation,...

International Journal of Plasticity, 2014

In this work, we present a multiscale physically based constitutive law for predicting the mechan... more In this work, we present a multiscale physically based constitutive law for predicting the mechanical response and texture evolution of body-centered cubic (BCC) metals as a function of strain-rate and temperature. In the model, deformation of individual single crystals results not only from the resolved shear stress along the direction of slip (Schmid law) but also from shear stresses resolved along directions orthogonal to the slip direction as well as the three normal stress components (non-Schmid effects). We account for coupled Schmid and non-Schmid effects through the modification of the resolved shear stress for both 1/2〈11 ത 1〉ሼ110ሽ and 1/2〈111 ത 〉ሼ112ሽ slip systems and the modification of the slip resistance for 1/2〈111 ത 〉ሼ112ሽ slip systems. The single crystal model is implemented into a self-consistent homogenization scheme containing a hardening law for crystallographic slip. The hardening law is based on the evolution of dislocation densities that incorporates strain-rate and temperature effects through the Peierls stress, thermally activated recovery, dislocation substructure formation and dislocation interactions. The polycrystal model is calibrated and validated using a set of mechanical and texture data collected on a tantalum-tungsten alloy, Ta-10W, at temperatures ranging from 298 K to 673 K and strain-rates from 10-3 to 2400 s-1. We show the model effectively captures the anisotropic hardening rate and texture evolution for all data using a single set of single-crystal hardening parameters. Comparisons between predictions and measured data allow us to discuss the role of slip on ሼ110ሽ and ሼ112ሽ in determining plasticity and texture evolution in Ta-10W.

International Journal of Solids and Structures, 2010

Various self-consistent polycrystal plasticity models for hexagonal close packed (HCP) polycrysta... more Various self-consistent polycrystal plasticity models for hexagonal close packed (HCP) polycrystals are evaluated by studying the deformation behavior of magnesium alloy AZ31B sheet under different uniaxial strain paths. In all employed polycrystal plasticity models both slip and twinning contribute to plastic deformation. The material parameters for the various models are fitted to experimental uniaxial tension and compression along the rolling direction (RD) and then used to predict uniaxial tension and compression along the traverse direction (TD) and uniaxial compression in the normal direction (ND). An assessment of the predictive capability of the polycrystal plasticity models is made based on comparisons of the predicted and experimental stress responses and R values. It is found that, among the models examined, the self-consistent models with grain interaction stiffness halfway between those of the limiting Secant (stiff) and Tangent (compliant) approximations give the best results. Among the available options, the Affine self-consistent scheme results in the best overall performance. Furthermore, it is demonstrated that the R values under uniaxial tension and compression within the sheet plane show a strong dependence on imposed strain. This suggests that developing anisotropic yield functions using measured R values must account for the strain dependence.

International Journal of Solids and Structures, 2012

The recently developed large strain elastic visco-plastic self-consistent (EVPSC) model, which in... more The recently developed large strain elastic visco-plastic self-consistent (EVPSC) model, which incorporates both slip and twinning deformation mechanisms, is used to study the lattice strain evolution in extruded magnesium alloy AZ31 under uniaxial tension and compression. The results are compared against in-situ neutron diffraction measurements done on the same alloy. For the first time, the effects of stress relaxation and strain creep on lattice strain measurements in respectively displacement controlled and load controlled in-situ tests are numerically assessed. It is found that the stress relaxation has a significant effect on the lattice strain measurements. It is also observed that although the creep does not significantly affect the trend of the lattice strain evolution, a better agreement with the experiments is found if creep is included in the simulations.

Physical Review B, 2009

Previously measured in situ x-ray diffraction is used to assess the development of internal elast... more Previously measured in situ x-ray diffraction is used to assess the development of internal elastic strains within grains of a sample of polycrystalline cobalt plastically deformed up to a pressure of 42.6 GPa. An elastoplastic self-consistent polycrystal model is used to simulate the macroscopic flow curves and internal strain development within the sample. Input parameters are single-crystal elastic moduli and their pressure dependence, critical resolved shear stresses, and hardening behavior of the slip and twinning mechanisms which are active in Co crystals. At 42 GPa, the differential stress in hcp-Co is 1.9Ϯ 0.1 GPa. The comparison between experimental and predicted data leads us to conclude that: ͑a͒ plastic relaxation plays a primary role in controlling the evolution and ordering of the lattice strains; ͑b͒ the plastic behavior of hcp-Co deforming under high pressure is controlled by basal and prismatic slip of ͗a͘ dislocations, and either pyramidal slip of ͗c + a͘ dislocations, or compressive twinning, or both. Basal slip is by far the easiest and most active deformation mechanism. Elastoplastic self-consistent models are shown to overcome the limitations of models based on continuum elasticity theory for the interpretation of x-ray diffraction data measured on stressed samples. They should be used for the interpretation of these experiments.

Philosophical Magazine, 2007

Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdf ... more Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdf This article maybe used for research, teaching and private study purposes. Any substantial or systematic reproduction, redistribution , reselling , loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

Materials Science and Engineering: A, 2003

In this work, we develop a modeling framework for predicting the visco-plastic deformation, micro... more In this work, we develop a modeling framework for predicting the visco-plastic deformation, microstructural evolution (distributions of grain shape and size) and texture evolution in polycrystalline materials during the equal channel angular extrusion (ECAE) process, a discontinuous process of severe shear straining. The foundation of this framework is a visco-plastic selfconsistent (VPSC) scheme. We consider a 908 die angle and simulate ECAE up to four passes for four processing routes, (A, C, B A and B C , as denoted in the literature) for an FCC polycrystalline material. We assume that the FCC single crystal has a constant critical resolved shear stress (CRSS), so that hardening by dislocation activity is suppressed, and the influence of grain shape distribution and texture as well as their interaction can be isolated. Many deformation microstructural features, such as grain size and shape distribution, texture, and geometric hardening Á/softening, were highly dependent on processing route. Using a grain subdivision criterion based on grain shape, route A was the most effective, then route B A and route B C and lastly route C, the least effective for grain size refinement, in agreement with redundant strain theory. For producing refined equiaxed grains, route B C was more effective than routes B A and A. We show that grain Á/grain interactions tend to weaken texture evolution and consequently geometric hardening and softening in all routes.

Materials Science and Engineering: A, 1994

The plastic properties of anisotropic polycrystalline aggregates and polyphase materials are in g... more The plastic properties of anisotropic polycrystalline aggregates and polyphase materials are in general non-homogeneous and, as a consequence, so is the local plastic deformation. We present in this work a model that describes the plastic behaviour of non-homogeneous materials composed of anisotropic regions (grains or phases). Our model is based on describing each region as a viscoplastic inclusion embedded in the effective medium represented by the other grains, and incorporates explicitly the grain interaction with its surroundings and the plastic anisotropy of grain and matrix. Within the model the grain response is coupled to the overall response of the polycrystal and the grain deformation may differ from the polycrystars. A characteristic of our approach is that those deformation systems with lower critical resolved shear stress tend to be more active, and less than five systems per grain are sufficient to accommodate the imposed overall deformation. In this work we explore the consequences and the limits of the model, and its dependence on the assumed rate sensitivity as well. We combine the self-consistent formulation with a volume fraction transfer scheme for treating the reorientation due to twinning, and simulate rolling textures of brass (f.c.c.), Zircaloy (h.c.p.), calcite (trigonal) and uranium (orthorhombic). We compare the results with experimental measurements and Taylor-type predictions, infer information concerning the microscopic deformation mechanisms and discuss the limits of applicability of the approach.

Journal of Nuclear Materials, 2008

Hardening and embrittlement are controlled by interactions between dislocations and irradiation i... more Hardening and embrittlement are controlled by interactions between dislocations and irradiation induced defect clusters. In this work we employ the visco plastic self consistent (VPSC) polycrystalline code in order to model the yield stress dependence in ferritic steels on the irradiation dose. We implement the dispersed barrier hardening model in the VPSC code by introducing a hardening law, function of the strain, to describe the threshold resolved shear stress required to activate dislocations. The size and number density of the defect clusters varies with the irradiation dose in the model. We find that VPSC calculations show excellent agreement with the experimental data set. Such modeling efforts can both reproduce experimental data and also guide future experiments of irradiation hardening.