Grain Size Evolution Research Articles

Velocity Gradients and the Evolution of Material Properties in a Narrow Layer near Surfaces of High Friction in Metal Forming Processes

Theoretical solutions for several rigid plastic models used to describe plastic flow in metal forming processes are singular in the vicinity of maximum friction surfaces. In particular, velocity gradients and the equivalent strain rate approach infinity near such surfaces. Such singular behavior can be excluded from consideration by choosing another friction law or material model. However, a different approach is proposed in the present paper. The starting point of this approach is that many experiments show that velocity gradients are very high in the vicinity of surfaces of high friction and that a narrow material layer is formed near such surfaces whose properties are very different from the properties in the bulk. Taking into account that the equivalent strain rate has a significant effect on the evolution of material properties, this experimental fact suggests that a theory based on the singular plastic solutions can be developed to describe the formation of the aforementioned material layer. In the present paper such a theory is proposed to describe the evolution of grain size. It is assumed that, in addition to the equivalent strain rate, the material spin has an effect of the evolution of grain size. It is then shown that the solutions for the material spin are singular as well. The interrelation between the present theory and strain gradient theories of plasticity is discussed. It is shown that it is necessary to account for the strain rate gradient to propose a more adequate theory to deal with the material flow near surfaces of high friction. Some experimental results on the formation of the narrow layer of ultra-fine grains in the vicinity of the fraction surface in extrusion are presented. An illustrative example to relate these experimental results and the new theory is given.

Grain-size evolution of polar firn: a new empirical grain growth parameterization based on X-ray microcomputer tomography measurements

AbstractFirn microstructure properties from six different sites in Greenland and Antarctica are investigated by means of X-ray microcomputer tomography. The optical effective radius is calculated from the specific surface area (SSA) and used as a measure of grain size. It is shown that the recently introduced spherical approximation of firn grains using the effective radius Reff is representative of grain size in the microwave frequency region. The measured profiles show the well-known increase of grain size with depth at all sites, where the increase is largest at near-surface depths. A large variability in grain size on the decimeter-to-centimeter scale as a result of different grain properties of single layers is superimposed on the overall trend at each site. A simple empirical parameterization of grain-size evolution is developed which allows the rapid grain growth in the uppermost layers of the firn to be predicted. The growth is driven by strong seasonal and diurnal temperature gradients. The model can be used to simulate grain-size profiles required by models of firn/microwave interaction (e.g. for retrieval of accumulation rates from satellite microwave sensors) in a more realistic fashion.

In Situ High-Energy XRD Study of the Hot-Deformation Behavior of a Novel γ-TiAl Alloy

ABSTRACTThe development of suitable hot-forming processes, e.g. forging, is an important step towards the serial production of TiAl parts. Several microstructure parameters change during hot-forming. However, the underlying mechanisms can normally only be inferred from post process metallographic studies.We used a deformation dilatometer modified for working in the HZG synchrotron beamlines at DESY for hot-deformation experiments. This setup enables the in situ monitoring of the interaction and evolution of microstructure parameters during processing. We observed the evolution of phase fractions, grain size and crystallographic texture during deformation while simultaneously recording the process parameters, like temperature, force and length change.Here we present the hot compressive deformation behaviour of a Ti-43Al-4Nb-1Mo-0.1B (in at.%) alloy. Several specimens were deformed at three temperatures each with two compression rates. During the experiments the Debye-Scherrer diffraction rings were continuously recorded.

Solution Combustion Synthesis of (Co,Fe)Cr2O4 pigments

Abstract The synthesis of mixed cobalt–iron chromites, Fe 1− Ψ Co Ψ Cr 2 O 4 (0 ≤ Ψ ≤ 1), has been studied by Solution Combustion Synthesis using urea as fuel. Characterization by XRD and SEM showed that crystallinity, cell parameters, crystallite size and grain size depend on Ψ . The higher is the cobalt content of the initial composition the greater are these four characteristics, but evolution of cell parameters, crystallite size and grain size is not regular, since the variations are more pronounced for the spinels with high iron content. Apparently, the method causes some deficiency in the oxygen content of the spinels and a slight enrichment in chromium with respect to the theoretical composition, but does not alter the proportions of iron and cobalt. All synthesized chromites showed a high tinting strength in ceramic glazes without any additional annealing step.

Layer-by-layer crystallization of Co2FeSi Heusler alloy thin films

Grain-size evolution with increasing annealing time has been investigated in polycrystalline Co2FeSi films. The samples were prepared by sputtering giving differing grain sizes. Large grains were formed after annealing at 500 °C, with grains over 200 nm forming in the L21 phase in a layer-by-layer mode. Further annealing causes a decrease in the average grain size, agreeing well with previously reported results for Co2MnSi. Magnetic measurements showed moments with values of up to 75% of those predicted from the Slater–Pauling curve providing further evidence for the formation of the L21 phase.

Process Modeling of Ti-6Al-4V Linear Friction Welding (LFW)

A fully coupled thermomechanical finite-element analysis of the linear friction welding (LFW) process is combined with the basic physical metallurgy of Ti-6Al-4V to predict microstructure and mechanical properties within the LFW joints (as a function of the LFW process parameters). A close examination of the experimental results reported in the open literature revealed that the weld region consists of a thermomechanically affected zone (TMAZ) and a heat-affected zone (HAZ) and that the material mechanical properties are somewhat more inferior in the HAZ. Taking this observation into account, a model for microstructure-evolution during LFW was developed and parameterized for the Ti-6Al-4V material residing in the HAZ. Specifically, this model addresses the problem of temporal evolution of the prior β-phase grain size (the dominant microstructural parameter in the HAZ) during the LFW process. This model is next combined with the well-established property versus microstructure correlations in Ti-6Al-4V to predict the overall structural performance of the LFW joint. The results obtained are found to be in reasonably good agreement with their experimental counterparts suggesting that the present computational approach may be used to guide the selection of the LFW process parameters to optimize the structural performance of the LFW joints.

Ball milling synthesis and stability under pressure in nanostructured HITPERM alloys

The main aim of the present work is to study the synthesis and stability of soft-magnetic nanostructured materials under high pressure–high temperature conditions. HITPERM alloy powders were prepared by high-energy ball-milling. The as-milled alloys consist of an iron-based nanocrystalline solid solution. The evolution of crystallinity and grain-size during the ball-milling process and investigations under external pressure up to 3GPa for temperatures up to 1200°C were evaluated using synchrotron radiation X-ray diffraction.

Predictive modeling of grain refinement during multi-pass cold rolling

Recently, grain refinement and grain misorientation have been experimentally studied for various materials with ultra-fine grained microstructures, which are achieved by the multi-pass cold rolling process. In this paper, a numerical framework is developed to model the evolution of grain size and grain misorientation based on a dislocation density-based material model. Novel finite element models embedded with the dislocation density-based material subroutine are developed to model the plastic deformation and microstructural evolution during the multi-pass cold rolling process. The multi-pass cold rolling processes of commercially pure titanium (CP Ti) and aluminum (AA 1200) are simulated in order to assess the validity of the numerical solution through comparison with experiments. The dislocation density-based material models are developed for CP Ti and AA 1200, which reproduce the observed material constitutive mechanical behavior under various strains, strain rates and temperatures occurring in the cold rolling process. It is shown that the developed model captures the essential features of the material mechanical behaviors and predicts a minimum grain size of below 100nm after five-pass cold rolling of CP Ti with equivalent strains up to 2.07 and the average incidental dislocation boundary (IDB) misorientation angle increased to 4.6° after six-pass cold rolling of AA 1200 with equivalent strains accumulated to 5.77.

Texture and Microstructure Evolution during Box Annealing of a Non-Oriented-Grain Electrical Steel

Non-oriented grain type electrical steels are used mainly in electrical rotating machines such as motors and compressors, in which the magnetization direction rotates 360 ° every cycle while remaining in the plane of the plate. The performance of these devices is affected by crystallographic texture of electrical steels due to strong anisotropy of magnetic properties. The electrical steel is supplied in the form of plates which are processed by cold rolling and subsequent annealing. Both, cold rolling and annealing directly influence the formation of crystallographic texture components. During annealing, recrystallization occurs, and this phenomenon gives rise to changes in texture that influences the quality of the final product and its application. Several works have been published in the study of the evolution of crystallographic texture and grain size in this type of electrical steel. In this work, samples have been taken in industrial conditions at various temperatures during the annealing in a coil box. Electrical steel samples cold rolled with reductions of 50% and 70% in thickness were removed during the process of annealing, and the evolution of texture with increasing temperature was studied. Aspects related to recrystallization, grain size and the evolution of texture and magnetic properties were discussed. Texture and recrystallization were studied by X-ray diffraction and electron backscatter diffraction (EBSD). The magnetic properties were measured in a vibrating sample magnetometer.

A hybrid simulation methodology for modeling dynamic recrystallization in UO2 LWR nuclear fuels

High burn-up, rim structures within uranium dioxide (UO2) light water reactor fuels exhibit marked differences in microstructure that are attributed to dynamic recrystallization. The recrystallization process has three distinct, interacting components: damage accumulation, nucleation and growth of damage-free regions, and subsequent evolution of recrystallized grains. In this paper, microstructural-scale simulation techniques for all three processes are presented and assembled into a hybrid tool for modeling the entire dynamic recrystallization process. The components of the model include a phenomenological model for damage accumulation and nucleation, a Cellular Automaton (CA) model for the growth and impingement of recrystallized grains, and a kinetic Monte Carlo (kMC) Potts model for subsequent grain growth. Preliminary results of the hybrid model demonstrate the evolution of a steady state grain size. Parametric simulations show the dependence of the steady state grain size on physical variables and on system size.

Grain-size distribution in the mantle wedge of subduction zones

[1] Mineral grain size plays an important role in controlling many processes in the mantle wedge of subduction zones, including mantle flow and fluid migration. To investigate the grain-size distribution in the mantle wedge, we coupled a two-dimensional (2-D) steady state finite element thermal and mantle-flow model with a laboratory-derived grain-size evolution model. In our coupled model, the mantle wedge has a composite olivine rheology that incorporates grain-size-dependent diffusion creep and grain-size-independent dislocation creep. Our results show that all subduction settings lead to a characteristic grain-size distribution, in which grain size increases from 10 to 100 μm at the most trenchward part of the creeping region to a few centimeters in the subarc mantle. Despite the large variation in grain size, its effect on the mantle rheology and flow is very small, as >90% of the deformation in the flowing part of the creeping region is accommodated by grain-size-independent dislocation creep. The predicted grain-size distribution leads to a downdip increase in permeability by ∼5 orders of magnitude. This increase is likely to promote greater upward migration of aqueous fluids and melts where the slab reaches ∼100 km depth compared with shallower depths, potentially providing an explanation for the relatively uniform subarc slab depth. Seismic attenuation derived from the predicted grain-size distribution and thermal field is consistent with the observed seismic structure in the mantle wedge at many subduction zones, without requiring a significant contribution by the presence of melt.

Evolution of sub-micron grain size and weak texture in magnesium alloy Mg–3Al–0.4Mn by a modified multi-axial forging process

This is the first successful attempt to produce simultaneously ultrafine grain size and weak texture in a single-phase magnesium alloy Mg–3Al–0.4Mn through an optimal choice of processing parameters in a modified multi-axial forging (MAF) process. An average grain size of ∼0.4 μm and a weak texture could be achieved. This has led to an increase in the strength as well as room-temperature ductility (55%). The plot of the yield loci shows a decrease in anisotropy after MAF.

Patterns of grain-size temporal variation of sediment transported by overland flow associated with moving storms: interpreting soil flume experiments

Abstract. This study describes and interprets the evolution of grain-size distribution of sediment yields generated in an experimental soil flume subjected to downstream and upstream moving rain storms. Results of laboratory experiments show that downstream moving storms cause more soil loss than do upstream moving storms. The pattern of sediment grain-size evolution in time during a runoff event exhibits a clear dependence on the direction of storm movement. A strong relationship between overland flow discharge and mean sediment size is found. Nevertheless, the mean grain-size of sediments transported during the rising limb of the hydrograph is coarser than during the recession limb of the hydrograph. This is more marked for downstream moving storms.

Numerical Simulation of Microstructure Evolution of TC6 Alloy Blade during Finish Forging

In order to predict the evolution of microstructure and grain size in TC6 alloy blade finish forging process and optimize the parameters, a series of constitutive equations for dynamic recrystallization and grain growth were developed and implemented into a 3D FE simulator. The grain size has been illustrated for the upsetting of TC6 alloy and the calculated grain size is in a good agreement with the experimental results, which shows that the microstructure prediction tool was validated and reliable. Then the distribution of stress, strain, temperature and microstructure, grain size and deformation has been predicted in TC6 alloy blade forging successfully using the 3D FEM at 930°C deformation temperature, 250°C die temperature and 50mm/s deformation velocity. The results show that the temperature of blade edge is higher than that of center because heat transfer, friction work and deformation work. The equivalent stress and strain have the same distribution as temperature after forging and the refined grain is obtained because of dynamic rescrystallization occurred with larger deformation. The uniform distribution of grain size are obtained in the blade and the shape after finish forging under the condition of technology parameters meet the user’s requirement comparing the calculated deformation with experimental results.

Fission-product behaviour in irradiated TRISO-coated particles: Results of the HFR-EU1bis experiment and their interpretation

It is important to understand fission-product (FP) and kernel micro-structure evolution in TRISO-coated fuel particles. FP behaviour, while central to severe-accident evaluation, impacts: evolution of the kernel oxygen potential governing in turn carbon oxidation (amoeba effect and pressurization); particle pressurization through fission-gas release from the kernel; and coating mechanical resistance via reaction with some FPs (Pd, Cs, Sr). The HFR-Eu1bis experiment irradiated five HTR fuel pebbles containing TRISO-coated UO 2 particles and went beyond current HTR specifications (e.g., central temperature of 1523 K). This study presents ceramographic and EPMA examinations of irradiated urania kernels and coatings. Significant evolutions of the kernel (grain structure, porosity, metallic-inclusion size, intergranular bubbles) as a function of temperature are shown. Results concerning FP migration are presented, e.g., significant xenon, caesium and palladium release from the kernel, molybdenum and ruthenium mainly present in metallic precipitates. The observed FP and micro-structural evolutions are interpreted and explanations proposed. The effect of high flux rate and high temperature on fission-gas behaviour, grain-size evolution and kernel swelling is discussed. Furthermore, Cs, Mo and Zr behaviour is interpreted in connection with oxygen-potential. This paper shows that combining state-of-the-art post-irradiation examination and state-of-the-art modelling fundamentally improves understanding of HTR fuel behaviour.

Comparison between Kinetic and Thermodynamic Effects on Grain Growth in Nano-Scale Materials

A detailed comparison between kinetic and thermodynamic effects on grain growth in nanocrystalline Gd doped ceria ceramics was given. From the thermodynamic standpoint, the evolution of grain size with annealing time can be described using an analytical model for grain boundary segregation upon isothermal grain growth. From the kinetic standpoint, the single isothermal grain growth can be understood in terms of a single, thermally-activated rate process with constant grain boundary energy, σb. The solute excess in grain boundaries increases with grain growth and tends to its saturated value, therefore, the inhibition of grain growth can be attributed to the reduction of grain boundary energy σb through solute segregation.

Multi-field coupled numerical simulation of microstructure evolution during the hot rolling process of GCr15 steel rod

In this paper, based on finite element (FE) software MSC.Marc, a multi-field coupled FE model is established to investigate the 18-pass hot rolling process of GCr15 steel rod. Thermal, mechanical and microstructural phenomena during the rolling process are coupled in the model. In this model, the microstructure evolution of GCr15 Steel is described by some empirical equations which are obtained using the Gleeble-3800 thermomechanical simulator, and a MSC.Marc subprogram is designed to predict the evolution of recrystallization behavior and austenite grain size. With the aid of the model, the paper analyzes detailedly the distribution and evolution of microscopic field-variables in rolled piece during the whole rolling process of GCr15 steel rod. Predicted values of grain size show good agreement with measured ones. The current model is beneficial to understand the effect of recrystallization behavior and control the microstructure evolution during hot rod rolling process.

Breaking Up: Comminution Mechanisms in Sheared Simulated Fault Gouge

The microstructural state and evolution of fault gouge has important implications for the mechanical behaviour, and hence the seismic slip potential of faults. We use 3D discrete element (DEM) simulations to investigate the fragmentation processes operating in fault gouge during shear. Our granular fault gouge models consist of aggregate grains, each composed of several thousand spherical particles stuck together with breakable elastic bonds. The aggregate grains are confined between two blocks of solid material and sheared under a given normal stress. During shear, the grains can fragment in a somewhat realistic way leading to an evolution of grain size, grain shape and overall texture. The ‘breaking up’ of the fault gouge is driven by two distinct comminution mechanisms: grain abrasion and grain splitting. The relative importance of the two mechanisms depends on applied normal stress, boundary wall roughness and accumulated shear strain. If normal stress is sufficiently high, grain splitting contributes significantly to comminution, particularly in the initial stages of the simulations. In contrast, grain abrasion is the dominant mechanism operating in simulations carried out at lower normal stress and is also the main fragmentation mechanism during the later stages of all simulations. Rough boundaries promote relatively more grain splitting whereas smooth boundaries favor grain abrasion. Grain splitting (plus accompanying abrasion) appears to be an efficient mechanism for reducing the mean grain size of the gouge debris and leads rapidly to a power law size distribution with an exponent that increases with strain. Grain abrasion (acting alone) is an effective way to generate excess fine grains and leads to a bimodal distribution of grain sizes. We suggest that these two distinct mechanisms would operate at different stages of a fault’s history. The resulting distributions in grain size and grain shape may significantly affect frictional strength and stability. Our results therefore have implications for the earthquake potential of seismically active faults with accumulations of gouge. They may also be relevant to the susceptibility of rockslides since non-cohesive basal shear zones will evolve in a similar way and potentially control the dynamics of the slide.

Grainsize evolution in ductile shear zones: Implications for strain localization and the strength of the lithosphere

At high stresses and low temperatures, grainsize reduction by dynamic recrystallization profoundly modifies rock rheology. Strain energy driven grain-boundary migration (ρGBM) is involved both in the nucleation of new grains by the grain-boundary bulging mechanism (BLG), and in the subsequent evolution of the microstructure. Above the D min line, which is a line in stress/grainsize space that defines the minimum size of nucleus that can form by BLG, ρGBM dominates the microstructure, and grain growth by surface energy driven grain-boundary migration (γGBM) is inhibited. The recrystallized grainsize is therefore dominated by the nucleation process, possibly controlled by the size of subgrains or dislocation cells within the old grains. This provides a first-order explanation for the experimentally observed grainsize-stress relationship. ρGBM is an important agent of recovery in rocks deformed by dislocation creep, sweeping out dislocations and counteracting work-hardening. We have derived a new flow law (DRX-assisted dislocation creep) based on this process, which exhibits grainsize sensitivity as a result of the role of ρGBM. If grainsize obeys the empirically-determined grainsize-stress relationship, DRX creep has an effective stress exponent of a little over 4, consistent with experimental observations and inferences from naturally deformed rocks. DRX creep may be an important agent in weakening quartz at low temperatures, whereas current flow law data suggest it may not be important in olivine. Rocks deformed and dynamically recrystallized above the D min line may switch from climb-assisted dislocation creep to grainsize-sensitive creep (Coble creep, DRX creep, or creep dominated by grain-boundary sliding), resulting in weakening. Lithospheric-scale shear zones are likely to evolve at approximately constant stress; under these conditions weakening results in an increase in strain rate, not a stress drop. The rate of dislocation motion, the dislocation density, and the dynamically recrystallized grainsize all remain the same, and grain growth will be inhibited by the activity of ρGBM. Hence the switches in deformation mechanism and the weakening they cause will be permanent, so long as the tectonic boundary conditions remain unchanged. Grainsize reduction caused by dynamic recrystallization may therefore play a fundamental role in lithospheric weakening, and may be a key process in the maintenance of plate tectonics.

Modeling the Flow Behavior of a High‐Manganese Steel Fe‐Mn23‐C0.6 in Consideration of Dynamic Recrystallization

AbstractIn this paper, the hot deformation behavior of a high manganese (23wt. % Mn and 0.6wt. % C) austenitic steel is analyzed. This steel is a ternary Fe‐Mn‐C alloy that shows twinning induced plasticity (TWIP). Since strip production involves hot rolling, the hot deformation behavior of this steel is analyzed and modeled by a phenomenological material model. The model takes into account strain hardening and softening by dynamic recrystallization (DRX), which is modeled using Avrami type kinetics. The evolution of grain size during DRX is also taken into account. Hot compression tests were performed in a temperature range of 900 to 1200 °C and various strain rates from 0.1 s−1 to 10 s−1 to identify the model parameters. The material model was coupled with a finite element code and validated using selected parameter combinations of the compression tests that were not used for parameter identification.