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DOE OSTI2021

Three-dimensional Hydrodynamic Simulations of Convective Nuclear Burning in Massive Stars Near Iron Core Collapse

Nonspherical structure in massive stars at the point of iron core collapse can have a qualitative impact on the properties of the ensuing core-collapse supernova explosions and the multimessenger signals they produce. Strong perturbations can aid successful explosions by strengthening turbulence in the postshock region. Here we report on a set of 4π 3D hydrodynamic simulations of O- and Si-shell burning in massive star models of varied initial masses using MESA and the FLASH simulation framework. We evolve four separate 3D models for roughly the final 10 minutes prior to and including iron core collapse. We consider initial 1D MESA models with masses of 14, 20, and 25 M ⊙ to survey a range of O/Si-shell density and compositional configurations. Further, we characterize the convective shells in our 3D models and compare them to the corresponding 1D models. In general, we find that the angle-average convective speeds in our 3D simulations near collapse are three to four times larger than the convective speeds predicted by MESA at the same epoch for our chosen mixing length parameter of α MLT = 1.5. In three of our simulations, we observe significant power in the spherical harmonic decomposition of the radial velocity field at harmonic indices of ℓ = 1–3 near collapse. Our results suggest that large-scale modes are common in massive stars near collapse and should be considered a key aspect of presupernova progenitor models.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2016

Simulated high-latitude soil thermal dynamics during the past 4 decades

Soil temperature ( T s ) change is a key indicator of the dynamics of permafrost. On seasonal and interannual timescales, the variability of T s determines the active-layer depth, which regulates hydrological soil properties and biogeochemical processes. On the multi-decadal scale, increasing T s not only drives permafrost thaw/retreat but can also trigger and accelerate the decomposition of soil organic carbon. The magnitude of permafrost carbon feedbacks is thus closely linked to the rate of change of soil thermal regimes. In this study, we used nine process-based ecosystem models with permafrost processes, all forced by different observation-based climate forcing during the period 1960–2000, to characterize the warming rate of T s in permafrost regions. There is a large spread of T s trends at 20 cm depth across the models, with trend values ranging from 0.010 ± 0.003 to 0.031 ± 0.005 °C yr -1 . Most models show smaller increase in T s with increasing depth. Air temperature ( T sub>a ) and longwave downward radiation (LWDR) are the main drivers of T s trends, but their relative contributions differ amongst the models. Different trends of LWDR used in the forcing of models can explain 61 % of their differences in T s trends, while trends of T a only explain 5 % of the differences in T s trends. Uncertain climate forcing contributes a larger uncertainty in T s trends (0.021 ± 0.008 °C yr -1 , mean ± standard deviation) than the uncertainty of model structure (0.012 ± 0.001 °C yr -1 ), diagnosed from the range of response between different models, normalized to the same forcing. In addition, the loss rate of near-surface permafrost area, defined as total area where the maximum seasonal active-layer thickness (ALT) is less than 3 m loss rate, is found to be significantly correlated with the magnitude of the trends of T s at 1 m depth across the models ( R = -0.85, P = 0.003), but not with the initial total near-surface permafrost area ( R = -0.30, P = 0.438). The sensitivity of the total boreal near-surface permafrost area to T s at 1 m is estimated to be of -2.80 ± 0.67 million km 2 °C -1 . Finally, by using two long-term LWDR data sets and relationships between trends of LWDR and T s across models, we infer an observation-constrained total boreal near-surface permafrost area decrease comprising between 39 ± 14 × 10 3 and 75 ± 14 × 10 3 km 2 yr -1 from 1960 to 2000. This corresponds to 9–18 % degradation of the current permafrost area.

58 GEOSCIENCES↗

NASA NTRS1982

Modelling of photodegradation in solar cell modules of substrate and superstrate design made with ethylene-vinyl acetate as pottant material

A computer model which simulates, in principle, the chemical changes in the photooxidation of hydrocarbons using as input data a set of elementary reactions, corresponding kinetic rate data and appropriate initial conditions was developed. The Model was refined and exploited to examine more closely the photooxidation and photostabilization of a hydrocarbon polymer. The results lead to the following observations. (1) The time to failure, tau sub f (chosen as the level of 5% C-H bond oxidation which is within the range anticipated for marked change in mechanical properties) varies as the inverse square root of the light intensity. However, tau sub f is almost unaffected by both the photoinitiator type and concentration. (2) The time to failure decreases with the rate of abstraction of C-H by peroxy radicals but increases with the rate of bimolecular radical termination controlled by diffusion. (3) Of the various stabilization mechanisms considered, the trapping of peroxy radicals is distinctly the most effective, although the concommitant decomposition of hydroperoxide is also desirable.

Somersall, A. C.↗

NASA NTRS1984

Separation processes during binary monotectic alloy production

Observation of microgravity solidification processes indicates that outside of sedimentation, at least two other important effects can separate the phases: critical-point wetting and spreading; and thermal migration of second-phase droplets due to interfacial tension gradients. It is difficult to study these surface tension effects while in a unit gravity field. In order to investigate the processes occurring over a temperature range, i.e., between a consolute point and the monotectic temperature, it is necessary to use a low-gravity environment. The MSFC drop tube (and tower), the ballistic trajectory KC-135 airplane, and the Space Shuttle are ideal facilities to aid formation and testing of hypotheses. Much of the early work in this area focuses on transparent materials so that process dynamics may be studied by optical techniques such as photography for viewing macro-processes; holography for studying diffusional growth; spinodal decomposition and coalescence; ellipsometry for surface wetting and spreading effects; and interferometry and spectroscopy for small-scale spatial resolution of concentration profiles.

Frazier, D. O.↗

NASA NTRS1998

Long Term Effects of Poultry Litter on Soil Physical and Chemical Properties in Cotton Plots

Poultry litter and compost can alter the moisture holding capacity of a soil. These organic materials can also increase the nutrient status of a soil during the decomposition process by microbial actions. The objective of this study was to evaluate the effect of poultry litter and compost on the dielectric constant and moisture holding capacity of soil. The Delta-T theta-probe was used to measure volumetric soil water content and the apparent dielectric constant of the upper 6-cm of the soil profile. Soil texture, pH, and organic matter were also determined for each plot. Results of these analyses indicated that the pH of the soil ranged from 6.4 to 7.7 and the volumetric soil moisture content ranged from 0.06 to 0.18 cu m/cu m for the upper 6-cm of the soil profile. The effect of poultry litter and compost on soil properties resulted in an increase in the volumetric moisture content and dielectric constant of the soil due to the improvement of the soil structure.

Surrency, J.↗

DOE OSTI2015

Simulation of injector dynamics during steady inductive helicity injection current drive in the HIT-SI experiment

We present simulations of inductive helicity injection in the Helicity Injected Torus with Steady Inductive helicity injection (HIT-SI) device that treats the entire plasma volume in a single dynamic MHD model. A new fully 3D numerical tool, the PSI-center TETrahedral mesh code, was developed that provides the geometric flexibility required for this investigation. Implementation of a zero-β Hall MHD model using PSI-TET will be presented including formulation of a new self-consistent magnetic boundary condition for the wall of the HIT-SI device. Results from simulations of HIT-SI are presented focusing on injector dynamics that are investigated numerically for the first time. Asymmetries in the plasma loading between the two helicity injectors and progression of field reversal in each injector are observed. Analysis indicates cross-coupling between injectors through confinement volume structures. Injector impedance is found to scale with toroidal current at fixed density, consistent with experimental observation. Comparison to experimental data with an injector drive frequency of 14.5 kHz shows good agreement with magnetic diagnostics. Global mode structures from Bi-Orthogonal decomposition agree well with experimental data for the first four modes.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2022

HiPACE++: A portable, 3D quasi-static particle-in-cell code

Modeling plasma accelerators is a computationally challenging task and the quasi-static particle-in-cell algorithm is a method of choice in a wide range of situations. In this work, we present the first performance-portable, quasi-static, three-dimensional particle-in-cell code HiPACE++. By decomposing all the computation of a 3D domain in successive 2D transverse operations and choosing appropriate memory management, HiPACE++ demonstrates orders-of-magnitude speedups on modern scientific GPUs over CPU-only implementations. The 2D transverse operations are performed on a single GPU, avoiding time-consuming communications. The longitudinal parallelization is done through temporal domain decomposition, enabling near-optimal strong scaling from 1 to 512 GPUs. HiPACE++ is a modular, open-source code enabling efficient modeling of plasma accelerators from laptops to state-of-the-art supercomputers.

97 MATHEMATICS AND COMPUTING↗

DOE OSTI2012

Partial substitution of Mo{sup 6+} by S{sup 6+} in the fast oxide ion conductor La{sub 2}Mo{sub 2}O{sub 9}: Synthesis, structure and sulfur depletion

Powder-solid state reaction route using La{sub 2}(SO{sub 4}){sub 3} as sulfur source was used to prepare compositions of the solid solution La{sub 2}Mo{sub 2-y}S{sub y}O{sub 9}. Single phases were only obtained in the substitution range extending up to y=0.8 (40 mol% S) at the annealing temperature of 850 Degree-Sign C with regard to the limit of stability of the lanthanum sulphate reactant. Within the synthesis conditions, a stabilization of the high temperature {beta}-form is observed from and above y=0.1 (5 mol% S). Temperature-controlled X-ray diffraction and thermogravimetric analyses have shown that La{sub 2}Mo{sub 2-y}S{sub y}O{sub 9} raw powders undergo thermal decompositions in two steps. Heating above 900 Degree-Sign C, a sulfur depletion to the benefit of molybdenum in La{sub 2}Mo{sub 2-y}S{sub y}O{sub 9} raw powders leads to the formation of La{sub 2}SO{sub 6}. At higher temperature, the exsolved La{sub 2}SO{sub 6} phase then decomposes into La{sub 2}O{sub 3}, which in turn reacts with the sulfur-depleted La{sub 2}Mo{sub 2}O{sub 9} phase to form La{sub 2}MoO{sub 6}. The present study also reveals that depending on the substitution rate y, the sulfur depletion can be induced by ball-milling of raw powders. Along the La{sub 2}Mo{sub 2-y}S{sub y}O{sub 9} series, the isovalent substitution of molybdenum by sulfur tends to restrict in magnitude, or even to suppress above 400 Degree-Sign C, the distortive thermal expansion of the cubic {beta}-type structure, thus strongly decreasing the conductance at high temperature. - Graphical abstract: La{sub 2}O{sub 3}-MoO{sub 3}-'SO{sub 3}' ternary phase diagram showing the exsolution path at low temperature (white arrows) and the total decomposition path at high temperature (black arrows) of {beta}-La{sub 2}Mo{sub 2-y}S{sub y}O{sub 9} raw powders. Highlights: Black-Right-Pointing-Pointer Isovalent substitution of molybdenum by sulfur in La{sub 2}Mo{sub 2}O{sub 9} up to 40 mol%. Black-Right-Pointing-Pointer Stabilization of the {beta}-form for a sulfur content greater than or equal to 5 mol%. Black-Right-Pointing-Pointer Decomposition by sulfur exsolution induced by thermal treatment or ball-milling. Black-Right-Pointing-Pointer Reduction or even cancellation of the distortive thermal expansion above 400 Degree-Sign C. Black-Right-Pointing-Pointer Decrease of conductance at high T involved by the low thermal expansion above 400 Degree-Sign C.

36 MATERIALS SCIENCE↗

DOE OSTI2026

High Pressure X-ray Diffraction and Equation of State of Hydrazine

Synchrotron X-ray diffraction has been used to investigate the structure and equation of state (EOS) of hydrazine (N 2 H 4 ) up to 54.3 GPa at 298 K. The diffraction patterns could be fit to a monoclinic unit-cell structure and put strong constraints on previously reported phase transitions documented by vibrational spectroscopy over this pressure range. Pressure–volume ( P–V ) data were fit using a Vinet EOS, yielding parameters: V 0 = 45.2 Å 3 /molecule (fixed), K 0 = 11.8(7) GPa, and K 0 ′ = 6.5(2). Previously measured high-pressure vibrational frequency shifts were used to estimate the vibrational free energy and model P–V–T isotherms from 0 to 1200 K. The results of the P–V–T isotherms are compared to existing shock Hugoniot data on hydrazine and 298 K isotherms for assemblages of possible decomposition products. This comparison suggests dissociation at high density under shock loading. Good correspondence was found between the static lattice EOS as calculated by the model and the previously reported EOS as calculated by density functional theory. Finally, these results resolve existing uncertainties about the EOS and crystal symmetry of hydrazine at high pressure and provide valuable baseline information on this important energetic material.

diffraction↗

DOE OSTI2013

Kinetics of Methane Hydrate Decomposition Studied via in Situ Low Temperature X-ray Powder Diffraction

Gas hydrates are known to have a slowed decomposition rate at ambient pressure and temperatures below the melting point of ice termed self-preservation or anomalous preservation. As hydrate exothermically decomposes, gas is released and water of the clathrate cages transforms into ice. Two regions of slowed decomposition for methane hydrate, 180 200 K and 230 260 K, were observed, and the kinetics were studied by in situ low temperature x-ray powder diffraction. The kinetic constants for ice formation from methane hydrate were determined by the Avrami model within each region and activation energies, Ea, were determined by the Arrhenius plot. Ea determined from the data for 180 200 K was 42 kJ/mol and for 230 260 K was 22 kJ/mol. The higher Ea in the colder temperature range was attributed to a difference in the microstructure of ice between the two regions.

anomalous preservation↗

DOE OSTI2019

Projected Changes to Extreme Precipitation Along North American West Coast From the CESM Large Ensemble

Precipitation events along the North American (NA) west coast are strongly modulated by atmospheric rivers, yet the mechanisms of their influences on the probability distributions of precipitation events are not well studied. Simulations from the Community Earth System Model (CESM) large ensemble under global warming are investigated using a moisture budget conditioned onto precipitation events for the recurrence intervals ranging from 0.1 to 50 years. In the midlatitudes, the increases in precipitation intensity and accumulation for all events over the NA west coast are predominantly controlled by moisture increases. In contrast, changes in the subtropical precipitation distributions in southwestern NA are associated with moisture increases and duration decreases for all events, with additional dynamical amplification for the heaviest precipitation events. These interpretations from the conditional moisture budget are more consistent with future projection of atmospheric rivers than the conventional mean and transient decomposition of the moisture budget.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2019

Conversion of Coal Wastes and Coal Plus Other Opportunity Fuel Mixtures, Such as Coal Plus Municipal Solids, to Produce Electricity and/or Liquid Fuels

For the foreseeable future, the Nation’s energy demand will continue to come largely from indigenous fossil energy resources and hydrocarbon fuels. Specifically waste coal plus other opportunity fuels need to be utilized as a feedstock to eliminate a waste stream and optimized to maximize energy yield and cleanly minimize pollutant emissions. Modular equipment is needed for installation in coal preparation locations, military installations, and research park facilities. Thermal decomposition of coal, biomass, and mixed solid waste (MSW) have been shown to reduce a waste stream while providing liquid and gaseous fuel products with low pollutant emissions. However, thermal decomposition processes have several challenges associated with preprocessing biomass and MSW, feeding and conveying material into the reactor, and post processing of syngas to remove undesirable products while upgrading to increase yield of high value products. Increasing total energy output from power plants by utilizing biomass and mixed wastes ultimately lowers energy costs, harmful environmental impacts of emissions, and has significant impacts on the energy security and sustainability of the U.S. economy. Entrained flow gasifiers (EFGs) produce very little CH4 and a wide range of H2/CO ratios (0.5–2.0) but are limited in their ability to effectively feed large biomass or MSW particles into the reactor. A solution to the feeding problems is to use pyrolysis torrefaction (PT) for conversion of waste materials to create a feedstock similar to coal and capable of grinding down and mixing with the coal feed. Mainstream experimentally explored reaction pathways for the conversion of coal waste plus municipal solids (CWPMS) and demonstrated a bench-scale reactor used for torrefaction of the municipal solid waste (MSW) to create a coal-like feed for blending with the coal waste. The material was processed in a pilot-scale EFG to determine product yield and emissions with CWPMS fuels, as well as the ash content and slagging properties of these feedstocks. The PT-EFG system was experimentally validated, where emissions and yield data were used to develop a high-fidelity process model and economic analysis. The following was accomplished in the Phase I: Determined optimal torrefaction process conditions for MSW and biomass through TGA; maximized heating value of fuel while maximizing final mass of torrefied product Demonstrated torrefaction of MSW and biomass feedstocks at 334°C, pulverized to uniform 74 µm particle size, and processed into coal-like feedstocks Demonstrated 8 lb/h pilot-scale EFG using MSW and biomass blends with waste coal; demonstrated 100% biomass in EFG Determined major effects of MSW torrefaction and co-gasifying high-ash coal waste-MSW blends have on pilot-scale EFG Demonstrated blending low-ash, low-sulfur torrefied biomass with high-ash waste bituminous and anthracite refuse coals generates a higher quality syngas compared to syngas from high-ash waste coals Demonstrated a high carbon conversion (> 99.5%) and syngas with 29.47% H2 content, and 0% CH4 content; greater water–gas shift occurs in EFG than predicted by equilibrium ASPEN Plus model Blended lower-ash, high-alkali biomass MSW to lower slag viscosities of the high-ash coal refuse sufficiently for better slag flow compared to feeding the high-ash coal Used low-sulfur biomass to greatly reduce the EFG syngas H2S concentrations and produce non-leachable slag and quench water with very little organic and trace metal contamination Developed a process model and TEA showing the modular PT EFG is cost competitive to existing power plants with the advantage of using low- or negative-value feedstocks such as waste coal stockpiles and non-landfilled MSW; providing a large incentive to implement an economical PT-EFG process

30 DIRECT ENERGY CONVERSION↗

DOE OSTI2023

Assessing the Intrinsic Roles of Key Dopant Elements in High–Nickel Layered Oxide Cathodes in Lithium–Based Batteries

A rational compositional design is critical for utilizing LiNiO 2 -based cathodes with Ni contents > 90% as promising next-generation cathode materials. Unfortunately, the lack of a fundamental understanding of the intrinsic roles of key elements, such as cobalt, manganese, and aluminum, makes the rational compositional design of high-Ni cathodes with a limited range of dopants (<10%) particularly challenging. Here, with 5% single-element doped cathodes, viz., LiNi 0.95 Co 0.05 O 2 , LiNi 0.95 Mn 0.05 O 2 , and LiNi 0.95 Al 0.05 O 2 , along with undoped LiNiO 2 (LNO), the influences of the dopants are systematically examined through a control of cutoff charge energy density and a common practice of cutoff charge voltage. Comprehensive investigations into the electrochemical properties, combined with in-depth analyses of the structural and interfasial stabilities and electrolyte decomposition pathways through advanced characterizations, unveil the following: i) the intrinsic role of dopants regulates the cathode energy density or state-of-charge and, more critically, the occurrence of H2–H3 phase transition, which essentially dictates cyclability; ii) undoped LNO can be stabilized well with the avoidance of H2–H3 phase transition; and iii) Co provides more merits overall with an optimized electrochemical operating condition. Finally, this work provides guidance for the compositional design of high-energy-density high-Ni cathodes and sheds light on the challenges of removing Co.

25 ENERGY STORAGE↗

DOE OSTI2016

Deep eutectic solvent approach towards nickel/nickel nitride nanocomposites

Nickel nitride is an attractive material for a broad range of applications including catalysis. However preparations and especially those targeting nanoscale particles remain a major challenge. Herein, we report a wet-chemical approach to produce nickel/nickel nitride nanocomposites using deep eutectic solvents. A choline chloride/urea deep eutectic solvent was used as a reaction medium to form gels containing nickel acetate tetrahydrate. Heat treatment of the gel in inert atmosphere forms nanoparticles embedded within a nitrogen-doped carbon matrix. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were used to understand the decomposition profile of the precursors and to select pyrolysis temperatures located in regions of thermal stability. X-ray diffraction (XRD) confirmed the presence of metallic nickel, whereas X-ray photoelectron spectroscopy (XPS) suggested the existence of a nickel nitride surface layer. According to transmission electron microscopy (TEM) analysis these mixed phase, possibly core-shell type nanoparticles, have very defined facets. Furthermore, these materials represent a unique opportunity to tune catalytic properties of nickel-based catalysts through control of their composition, surface structure, and morphology; in addition to employing potential benefits of a nitrogen-doped carbon support.

36 MATERIALS SCIENCE↗

DOE OSTI2016

Enhanced photocatalytic and photoelectrochemical activities of reduced TiO 2-x /BiOCl heterojunctions

A key issue to design highly efficient photoelectrodes for hydrogen production is how to prohibit the rapid carrier recombination. In order to use the visible light and reduce the recombination of electrons and holes, reduced TiO 2-x /BiOCl heterojunctions are successfully synthesized and the photoelectrodes are assembled in this work. The effects of various Bi/Ti molar ratios on the structural, morphological, optical, photoelectrochemical and photocatalytic activities of the resultant samples are investigated systematically. The TiO 2-x nanoparticles contain Ti 3+ , Ti 2+ , and oxygen vacancies (Ov), while the BiOCl nanosheets exposed {001} facet. Ultraviolet–visible diffuse reflectance spectroscopy (UV–vis DRS) results indicate that the existence of Ti 3+ , Ti 2+ and Ov expand the light-response range. Linear scan voltammetry and electrochemical impedance spectroscopy results indicate that more efficient electron transportation is presented in the heterojunctions with the appropriate Bi/Ti molar ratio. Consequently, the reduced TiO 2-x /BiOCl heterojunction with the most appropriate Bi/Ti molar ratio exhibits a high photocurrent density of 0.755 mA cm -2 with photoconversion efficiency up to 0.634%, 10.5 and 22.6 times larger than that of pure TiO 2 and BiOCl. Furthermore, this heterojunction exhibit 48.38 and 12.54 times enhancement for the visible-light decomposition of rhodamine B compared with pure TiO 2 and BiOCl.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2013

The effect of emissive biased limiter on the magnetohydrodynamic modes in the IR-T1 tokamak

A moveable emissive biased limiter (EBL) for the investigation of spatial and temporal structure of MHD modes in IR-T1 tokamak, based on mirnov oscillations, was designed and constructed. The biasing has been considered to improve the global confinement by setting up an electric field at the plasma edge. Radial electric field (E{sub r}) modifies edge plasma turbulence, plasma rotation, and transport. Mirnov oscillations using singular value decomposition (SVD) and wavelet techniques were analyzed. SVD algorithm has been employed to analyze the frequency and wavenumber harmonics of the MHD fluctuations. The time-resolved frequency component analysis has been performed using wavelets. The EBL was applied to plasma at 10 ms with negative polarity. The results show that after applying EBL, the m = 2 mode is grown, m = 3 mode is suppressed, and H{sub {alpha}} radiation is decreased. Furthermore, results of the wavelet analysis of mirnov coil in the time range of 8-12 ms indicate that 1.5 ms after applying EBL, the MHD frequency is reduced from 45 kHz to 25 kHz.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

NASA NTRS1986

Isotopic characterisation of prebiotic synthesis of organic material

Many primitive meteorites contain an insoluble organic material, much like terrestrial kerogen, whose mode of origin is currently unknown. When sujbected to stepwise decomposition, this material, unlike its terrestrial counterpart, reveals characteristic release patterns for the stable isotopes of carbon, hydrogen and nitrogen as a function of fractional release of each element. The purpose of this study is to try to match those release patterns using organic matter synthesised in the laboratory under controlled conditions. If successful, such a study would shed light on the origin of kerogen-like organic matter in the early solar system and, by extension, on prebiotic organic synthesis in general. The range of possible syntheses, starting materials and reaction conditions to be investigated is considerable. Samples analysed to date include: a heavy oil produced by Fischer-Tropsch-type catalysis of CO + H2; a solid residue generated by a plasma discharge in CO + H2 + N2; a solid deposited on the electrodes of a Miller-Urey synthesis operating on CH4 + H2O + N2; and a solid residue formed by polymerization of light hydrocarbons procured by a Miller-Urey discharge acting on CH4. Significant structure is observed in the release patterns for the carbon and hydrogen isotopes from the synthetic samples, though there is little evidence for isotopic fractionation during the analysis itself.

Kerridge, J. F.↗

DOE OSTI2018

A unified framework for polycrystal plasticity with grain boundary evolution

Plastic deformation in polycrystals is governed by the interplay between intra-granular slip and grain boundary-mediated plasticity. However, while the role played by bulk dislocations is relatively well-understood, the contribution of grain boundaries (GBs) has only recently begun to be studied. GB plasticity is known to play a key role along with bulk plasticity under a wide range of conditions, such as dynamic recovery, superplasticity, severe plastic deformation, etc., and developing models capable of simultaneously capturing GB and bulk plasticity has become a topic of high relevance. In this paper we develop a thermodynamically-consistent polycrystal plasticity model capable of simulating a variety of grain boundary-mediated plastic processes in conjunction with bulk dislocation slip. The model starts from the description of a single crystal and creates lattice strain-free polycrystalline configurations by using a specially-designed multiplicative decomposition developed by the authors. This leads to the introduction of a particular class of geometrically necessary dislocations (GND) that define fundamental GB features such as misorientation and inclination. The evolution of the system is based on an energy functional that uses a non-standard function of the GND tensor to account for the grain boundary energy, as well as for the standard elastic energy. Our implementation builds on smooth descriptions of GBs inspired on diffuse-interface models of grain evolution for numerical convenience. Here we demonstrate the generality and potential of the methodology by simulating a wide variety of phenomena such as shear-induced GB sliding, coupled GB motion, curvature-induced grain rotation and shrinkage, and polygonization via dislocation sub-grain formation.

36 MATERIALS SCIENCE↗