Grain Size Evolution Research Articles

Thermal stability and grain growth behavior of nanocrystalline Mg 2Si

Thermal stability and grain growth behavior of nanocrystalline Mg 2Si (nano-Mg 2Si), prepared by using mechanically activated solid-state reaction plus hot-pressing in vacuum, were investigated by the method of in situ high-temperature X-ray diffraction. The result indicates that the evolution of grain size d with isothermal-annealing time t for nano-Mg 2Si can be well described by the formula d − d 0 = ct 1/ n with grain growth exponent n = 6, 5 and 4 at 700, 800 and 900 °C, respectively, indicating that nano-Mg 2Si has a good thermal stability. Simultaneously, an Arrhenius plot of rate constant c against the reciprocal of T yields a straight line, from which an activation energy of 112 ± 1 kJ/mol is derived for the grain growth of nano-Mg 2Si.

Model for the evolution of Nb–Si–N thin films as a function of Si content relating the nanostructure to electrical and mechanical properties

DC reactive magnetron sputtering was used for the deposition of Nb–Si–N thin films at 250 °C substrate temperature. Si content was varied by changing the current applied on the Si target, whereas that on Nb target was kept constant. The evolution of morphology, grain size and lattice constant has been analyzed by Transmission Electron Microscopy and X-rays diffraction investigations. Hardness, stress and resistivity measurements provide complementary information concerning the macroscopic properties of the films. In the Nb–Si–N films, 3 distinct concentration regions have been observed depending on the Si content. For low Si content (CSi≤4 at.%) the Si atoms substitute the Nb atoms in the NbN lattice. Exceeding the solubility limit, a nanocomposite film grows; it is made of NbN:Si nanocrystallites surrounded by an amorphous SiNy layer. Further increase of Si content (CSi≥7 at.%) implies a reduction of the crystallite size, while the thickness of the SiNy layer at the crystallite surface remains constant (∼1.3 monolayer). The amount of the SiNy amorphous phase in the films increases through the increase of the surface to volume ratio of the crystallites. The structure of the films is responsible for the increase of hardness from 25 to 34 GPa through a 2-step mechanism, i.e., by forming a solid solution of Si atoms in NbN lattice and by forming a nanocomposite material. The large variation of the temperature coefficient of resistivity is well correlated with the thickness of SiNy layer on the NbN:Si crystallite surface. Based on the three concentration regions, a three-step model is proposed for the film formation of the Nb–Si–N thin films. This model correlates nanoscale structures with macroscopic properties of the films.

Time-dependent strain localization in viscous media with state-dependent viscosity

The temporal evolution of viscous rheologies with dependence on an additional state variable is examined. A localization measure is introduced that quantifies the change in the degree of localization in time. Three sample rheologies are analyzed in detail, each representative of a larger class of rheologies: (1) a grain-size dependent viscosity with grain growth and diminution, (2) shear heating with temperature-dependence according to the Arrhenius law, and (3) shear heating with temperature dependence in the Frank-Kamenetzky approximation. All three rheologies display stages of temporal increase and decrease of localization, depending on the initial conditions. This localization behavior is not discernible in plots of strain rate versus strain at constant driving stress which are the typical output of creep experiments. The grain-size dependence of olivine leads to effectively non-Newtonian behavior with a stress exponent of about 5. If the laws describing the grain-size evolution are applicable to the mantle, then the lower mantle and those parts of the upper mantle that are dominated by diffusion creep thus have a stronger stress-dependence than the depth range governed by dislocation creep, provided that the timescale of deformation is greater than 1 0 5 – 1 0 6 years.

Effect of impurities on grain growth in cold ice sheets

On the basis of a detailed study of the ice microstructure of the European Project for Ice Coring in Antarctica (EPICA) ice core at Dome Concordia, Antarctica, we analyze the effect of impurities (solubles, and insolubles, that is, dust particles) on the grain growth process in cold ice sheets. As a general trend, the average grain size increases with depth. This global increase, induced by the normal grain growth process, is punctuated by several sharp decreases that can be associated with glacial‐interglacial climatic transitions. To explain the modifications of the microstructure with climatic changes, we discuss the role of soluble and insoluble impurities on the grain growth process, coupled with an analysis of the pinning of grain boundaries by microparticles. Our data indicate that high soluble impurity content does not necessarily imply a slowdown of grain growth kinetics, whereas the pinning of grain boundaries by dust explains all the observed modifications of the microstructure. We propose a numerical model of the evolution of the average grain size in deep ice cores that takes into account recrystallization processes such as normal grain growth and rotation recrystallization as well as the pinning effect induced by dust particles, bubbles, and clathrates on the grain boundaries. Applied to the first 2135 m of the Dome Concordia core, the model reproduces accurately the measured mean grain radius. This indicates a major role of dust in the modification of polar ice microstructure and shows that the average grain size is not a true paleothermometer, as it is correlated with climatic transitions through the dust content of the ice.

Surface morphologies and properties of pure and antimony-doped tin oxide films derived by sol–gel dip-coating processing

A simple laboratory technique for the routine preparation of antimony-doped tin oxide (ATO) on float glass substrates (25 mm × 76 mm × 1 mm) was described. As-prepared thin films were dried at temperature of 100 ± 5 °C and annealed at temperatures of 400–550 °C. Microstructural and morphological analyses of as-prepared films were performed at different conditions. The evolution of grain size and the morphologies of ATO films were analyzed by means of atom force microscopy (AFM) and digital microscope. The studies suggested that higher Sb-doped level and higher annealing temperature led to a decrease in the surface roughness of the deposited films. The XRD patterns revealed that as-prepared ATO films were in the crystallization of a tetragonal rutile structure of SnO 2 with highly (1 1 0) preferred orientation. Their optical properties were analyzed by U-3310 spectrophotometer. The transmission of the ATO thin films was obtained as high as 80–90% in visible region, but decreased substantially in IR region. The sheet resistance of the investigated thin films was determined by four-probe method, showing that it was about 85–100 Ω □ −1which decreased with the increase of antimony-doped concentration.

Nucleation and growth of SnO2 nanocrystallites prepared by pulsed laser deposition

We report in detail the synthesis and microstructural characterization of SnO2 nanocrystallites produced using a pulsed laser deposition (PLD) method. A detailed observation of the microstructural evolution of the as-prepared SnO2 nanocrystallites has been carried out, and their grain sizes, shapes, crystallography characteristics, and morphologies have been also determined. The nucleation and growth processes of the SnO2 nanocrystallites are discussed in more detail in order to examine how the PLD techniques and conditions affect the evolution of grain sizes, shapes, crystallography characteristics and morphologies.

An experimental study of Ostwald ripening of olivine and plagioclase in silicate melts: implications for the growth and size of crystals in magmas

We carried out an experimental study to characterize the kinetics of Ostwald ripening in the forsterite-basalt system and in the plagioclase (An65)-andesite system. Eight experiments were done in each system to monitor the evolution of mean grain size and crystal size distribution (CSD) with time t; the experiments were performed in a 1-atmosphere quench furnace, at 1,250°C for plagioclase and 1,300°C for olivine. Very contrasted coarsening kinetics were observed in the two series. In the plagioclase series, the mean grain size increased as log(t), from ≈3 μm to only 8.7 μm in 336 h. The kinetic law in log(t) means that Ostwald ripening was rate-limited by surface nucleation at plagioclase-liquid interfaces. In the olivine series, the mean grain size increased as t 1/3, from ≈3 μm to 23.2 μm in 496 h. A kinetic law in t 1/3 is expected when Ostwald ripening is rate-limited either by diffusion in the liquid or by grain growth/dissolution controlled by a screw dislocation mechanism. The shape of olivine CSDs, in particular their positive skewness, indicates that grain coarsening in the olivine experiments was controlled by a screw dislocation mechanism, not by diffusion. As the degrees of undercooling ΔT (or supersaturation) involved in Ostwald ripening are essentially <1°C, the mechanisms of crystal growth identified in our experiments are expected to be those prevailing during the slow crystallisation of large magma chambers. We extrapolated our experimental data to geological time scales to estimate the effect of Ostwald ripening on the size of crystals in magmas. In the case of plagioclase, Ostwald ripening is only efficient for mean grain sizes of a few microns to 20 μm, even for a time scale of 105 years. It can, however, result in a significant decrease of the number of small crystals per unit volume, and contribute to the development of convex upwards CSDs. For olivine, the mean grain size increases from 2–3 μm to ≈70 μm in 1 year and 700 μm in 103 years; a mean grain size of 3 mm is reached in 105 years. Accordingly, the rate of grain size-dependent processes, such as compaction of olivine-rich cumulates or melt extraction from partially molten peridotites, may significantly be enhanced by textural coarsening.

Development of dislocation-based unified material model for simulating microstructure evolution in multipass hot rolling

Dynamic recrystallization and recovery are two competing processes. Both may continue after hot deformation, such as during passes in multipass hot rolling processes, reducing dislocation density of materials and allowing larger plastic deformation to be achieved. The main objective of this research is to develop a set of mechanism-based unified viscoplastic constitutive equations which model the evolution of dislocation density, recrystallization and grain size during and after hot plastic deformation. This set of constitutive equations are determined for a C-Mn steel using an evolutionary programming (EP) optimization technique and implemented into the commercial finite element (FE) solver ABAQUS for process simulations. Numerical procedures to simulate multipass rolling are developed. FE analysis is carried out to simulate the evolution of grain size, dynamic/static recrystallization and recovery, and to rationalize their effects on the viscoplastic flow of the material in a two-pass hot rolling process.

The potential for adaptive evolution of pollen grain size in Mimulus guttatus

We tested whether pollen grain size (PGS) shows heritable variation in three independent populations of Mimulus guttatus by imposing artificial selection for this character. In addition, we looked for correlated responses to selection in a range of 15 other floral characters. Heritable variation in PGS was found in all three populations, with heritabilities of between 19 and 40% (average 30%). After three generations, upward and downward lines differed on average by 30% in pollen volume. No consistent patterns of correlated response were found in other characters, indicating that PGS can respond to selective forces acting on PGS alone. Possible selection mechanisms on PGS in this species could include intermale selection, if large pollen grains produce more competitive gametophytes; or optimization of patterns of resource allocation, if local mate competition varies.

An analysis of the grain growth kinetics in Mn2O3 nanocrystals

The isothermal grain growth of Mn2O3 nanocrystals, prepared by the chemical liquid-phase homogeneous precipitation route, was investigated at various temperatures between 200 and 500 °C for different annealing times. X-ray diffraction patterns and transmission electron micrographs show that the average grain sizes are in the range of 4–50 nm. The grain growth data were analyzed using two different models. The first model, assuming normal grain growth as that in conventional polycrystalline materials, yields large grain growth exponent (n) and extremely low activation energy (Q). Although it can describe the evolution of grain sizes, it fails to give satisfactory physical interpretation of n and Q, both beyond the theoretical predictions. The second model is based on the structural relaxation of the interface component in nanocrystalline materials. In this case, the ordering of distorted interfaces by structural relaxation proceeds with grain growth. This structure relaxation model not only describes the evolutions of grain growth well, but also makes reasonable attribution of the low activation energy to the short-range rearrangement of atoms in the interface region as well.

On the influence of in-situ reactions on grain size during reactive atomization and deposition

A fundamental study of the factors that govern grain size of 5083 Al processed via reactive atomization and deposition (RAD) is reported. Microstructural observation shows that the average grain size in RAD 5083 Al is slightly smaller than that in the material processed via N2 spray deposition (SDN). A numerical approach, together with measurements of the temperature histories inside the deposited materials, is implemented to analyze the influence of in-situ reactions during RAD process on the evolution of grain size. The numerical results show that RAD 5083 Al possesses a slightly higher density of nuclei relative to that present in SDN 5083 Al on a per unit volume of deposited material basis at the beginning of the slow solidification of remaining liquid phase. Furthermore, the RAD material exhibits a slightly lower coarsening extent during the slow solidification. Grain growth is negligible during the solid-phase cooling. Accordingly, the calculated grain size in RAD 5083 Al is slightly smaller than that in SDN 5083 Al, consistent with the observed results.

Kinetics of Austenite Grain Growth during a Continuous Heating of a Niobium Microalloyed Steel

Grain growth is a thermally activated process in which the average grain size increases as temperature and time increases. The driving force for grain growth results from the decrease in the free energy associated with the reduction in total grain boundary energy. There are several known factors that influence the migration of grain boundaries such as second phase particles precipitated in the matrix and the solute elements segregated at grain boundaries. The austenite grain boundaries are revealed using the thermal etching method. Carbon extraction replicas were prepared to determine the composition and size of precipitates present in the matrix. In this work, the evolution of the average prior austenite grain size (PAGS) of a low carbon steel microalloyed with niobium is studied as a function of temperature and heating rate. Austenite grains show a two-stage growth. It has been found that as heating rate increases, the grain coarsening temperature (TGC) increases and the grain size at that temperature decreases. TGC temperature lies around 40-60°C below the temperature for complete dissolution of carbonitrides (TDISS).

In situ investigation of MnAs/GaAs(001) growth and interface structure using synchrotron x-ray diffraction

Using grazing-incidence synchrotron x-ray diffraction, we have quantitatively studied the strain evolution of epitaxial MnAs films in situ during growth by molecular-beam epitaxy. We find that the MnAs layer grows in four distinct stages, being compressively strained at the beginning of growth and relaxing as the thickness increases. The evolution of the in-plane grain size is determined as a function of layer thickness. We find a one-dimensional ordered array of periodic misfit dislocations at the interface, having a periodicity of 4.9±0.05 nm along GaAs[110]. An annealing of the film increases the grain size as well as improves the ordering of dislocation array.

Coarsening Behavior and Coercivity in L10-Ordered Intermetallic Fe-Pd Ferromagnets with Equiaxed Grain Morphology

ABSTRACTThermo mechanical processing of equiatomic FePd has been used to establish a nearly equiaxed microstructure that consists of mono-variant L10 ordered grains. The technically important hard magnetic properties, e.g. coercivity, of the uniaxial ferromagnetic L10-FePd with equiaxed microstructure depend on the grain size. Thus, here, the evolution of grain size during the later stages of annealing has been studied using SEM and computer assisted image analysis with the goal to determine the activation parameters for the relevant coarsening processes. Using a classical grain growth analysis the activation parameter for coarsening of the 5% largest grains in the populations of L10-ordered grains in FePd has been determined as approximately 30kcal/mol.

Modelling of microstructural evolution in multipass hot rolling

Mechanism-based unified viscoplastic constitutive equations are developed to model the evolution of grain size, recovery, dynamic and static recrystallisation and their effects on viscoplastic flow of materials. This set of constitutive equations are determined for a C–Mn steel using a GA-based optimisation technique and implemented into the commercial FE solver ABAQUS for process simulations. Numerical procedures to simulate multipass rolling are developed. FE analysis is carried out to rationalise the evolution of grain size, dynamic/static recrystallisation and recovery, viscoplastic flow and stress relaxation in a multipass hot rolling process.

Simulation of microstructures for Alloy 718 blade forging using 3D FEM simulator

Mechanical properties of Ni–Cr–Fe-based Alloy 718 depend very much on the grain size, as well as the presence of strengthening phases, γ′ and γ″. The grain size of superalloy parts can be controlled by the thermo-mechanical processes, including heating, forging and cooling sequences. In order to predict the evolution of grain size in Alloy 718 blade forging, a series of constitutive equations for dynamic recrystallization, meta-dynamic recrystallization and grain growth were developed and implemented into a 3D FE simulator. The evolution of grain structure in the process of two-step blade forging of Alloy 718 was simulated using the 3D FE simulator combined with a microstructure module. The microstructure prediction tool was validated by comparing the simulated grain structure with that of the experimental blade-type forging.

A set of unified constitutive equations for modelling microstructure evolution in hot deformation

Recrystallisation and recovery are two competing processes. Metals showing high degrees of dynamic recovery are unlikely to recrystallise dynamically, since it is difficult to accumulate the dislocation density to a certain level to initiate recrystallisation. Recrystallisation and recovery may continue after hot deformation, such as during passes in multipass hot rolling processes. Both processes reduce dislocation density of materials and allow large plastic deformation to be achieved. The main aim of this research is to develop a set of mechanism-based unified viscoplastic constitutive equations, which models the evolution of dislocation density, recrystallisation and grain size during and after hot plastic deformation. Deformation mechanisms of metals at high temperatures and techniques of modelling the physical effects are analysed. The effects of dislocation density and recrystallisation on viscoplastic flow and grain size evolution of a micro-alloyed steel are rationalised. Optimisation techniques and procedures are developed to determine the unified viscoplastic constitutive equations from experimental data.

Metadynamic and Static Recrystallization of Hypereutectoid Steel

The metadynamic and static recrystallization behavior in hypereutectoid steel containing 1 % carbon was determined by hot compression testing. Compression tests were performed using double hit schedules at temperatures between 900 to 1 050°C, strain rates of 0.01 to is -1 and recrystallization times of 0.1 to 500s. The characteristics of static and metadynamic recrystallization are distinctly different. Results show that the metadynamic kinetics was twice as fast as the static kinetics. These data were used to generate equations to predict the kinetics of static and metadynamic recrystallization, as well as the evolution of grain size, after recrystallization.

Abnormal grain growth in {111} textured Cu thin films

An analysis is presented that addresses the grain growth in 0.5 μm thick Cu films on Si substrates. Samples were thermally cycled to several maximum temperatures up to 500 °C in order to identify the evolution of grain size with temperature. Grain-morphological studies by focussed ion beam microscopy, as well as transmission and scanning electron microscopy were performed before and after thermal cycling. Additionally, in-situ measurements of stress and resistivity were carried out. During the first heating up to 500 °C the grains grow from 60 nm to approximately the tenfold size. The enlargement occurs by the growth of a few grains which consume the surrounding (stagnant) fine-grained matrix until the large grains meet and the fine-grained matrix is completely consumed. No other texture components than {111} were found before and after thermal cycling. An estimation yields the grain-boundary energy as the decisive driving force for the abnormal grain growth observed.

Analytical model of pass-by-pass strain in rod (or bar) rolling and its applications to prediction of austenite grain size

Rod rolling is a process where the deformation state of the workpiece between the work rolls is quite different from the strip rolling process. However, in most microstructure evolution models, the simple area strains (natural logarithm of the area reduction ratio) multiplied by a constant have been used to compute pass-by-pass evolution of austenite grain size (AGS) in rod (or bar) rolling, without any verification. The strains at a given pass play a crucial role in determining the recrystallization behavior (static or dynamic). In this study, an analytical model that calculates the pass-by-pass strain and strain rate in rod rolling has been developed and verified by conducting four-pass (oval–round) bar and plate rolling experiments. Numerical simulations have then been carried out for the four-pass rolling sequence using the area strain model and the new analytical model, focusing on the effect of the method for calculating the strain on the recrystallization behavior and evolution of AGS. The AGS predicted was compared with those obtained from hot torsion tests. It is shown that the analytical model developed in this study is more appropriate in the analysis of bar (or rod) rolling. It was found that the recrystallization behavior and evolution of AGS during this process were influenced significantly by the calculation method for the deformation parameters (strain and strain rate). The pass-by-pass strain obtained from the simple area strain model is inadequate to be used as an input to the equations for recrystallization and AGS evolution under these rolling conditions.