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Showing papers on "Grain growth published in 2011"


Journal ArticleDOI
TL;DR: In this paper, the authors presented the various types of microstructure of the Ti-6Al-4V alloy after post-fabrication heat treatments below or above the β transus.
Abstract: Selective laser melting (SLM) is a rapid manufacturing process that enables the buildup of very complex parts in short delays directly from powder beds. Due to the high laser beam energy during very short interaction times and the high solidification rates of the melting pool, the resulting microstructure is out-of-equilibrium and particularly textured. This type of as-fabricated microstructure may not satisfy the aeronautical criterion and requires post heat treatments. Optimized heat treatments are developed, in one side, to homogenize and form the stable phases α and β while preventing exaggerated grain growth. In the other side, heat treatment is investigated to relieve the thermal stresses appearing during cooling. This study is aimed at presenting the various types of microstructure of the Ti-6Al-4V alloy after postfabrication heat treatments below or above the β transus. Tensile tests are then carried out at room temperature in order to assess the effect of the microstructures on the mechanical properties. The fine as-fabricated microstructure presents high yield and ultimate strengths, whereas the ductility is well below the standard. A strong anisotropy of fracture between the two loading directions is noted, which is attributed to the manufacturing defects. Conventional and optimized heat treatments exhibit high yield and ultimate strengths while the ductility is significantly improved. This is due to a new optimization of the process parameters allowing drastic reduction of the number of defects. These two heat treatments enable now a choice of the morphology of the grains between columnar or equiaxial as a function of the type of loading.

921 citations


Journal ArticleDOI
TL;DR: The Interdependence Theory as mentioned in this paper links grain formation and nucleant selection to improve the ability to reveal the mechanisms of grain refinement, predict as-cast grain size and account for observations that only a small proportion of added inoculant particles nucleate grains.

437 citations


Journal ArticleDOI
TL;DR: In this paper, the influence of select rare earth elements; Gd, Nd, Ce, La and mischmetal (MM) on the sheet texture modification during warm rolling and annealing of a ZEK100 magnesium alloy, and the resulting formability and anisotropy during subsequent tensile testing at room temperature was examined.
Abstract: The current study examines the influence of select rare earth elements; Gd, Nd, Ce, La and mischmetal (MM) on the sheet texture modification during warm rolling and annealing of a ZEK100 magnesium alloy, and the resulting formability and anisotropy during subsequent tensile testing at room temperature. It was found that all the investigated RE elements led to weak sheet textures and hence promoted enhanced ductility and reduced anisotropy over conventional Mg sheet. Gd was of a particular interest because it gave rise to a desired Mg sheet texture despite its coarsest grain size resulting in promising mechanical properties. It is suggested that solute related effects on the grain boundary migration and the relative strengths of different deformation mechanisms are responsible for altering the common concepts of recrystallization and grain growth during annealing, and the activation scenarios of slip and twinning during deformation.

360 citations


Journal ArticleDOI
TL;DR: In this article, the influence of electrical and electromagnetic fields on grain boundary energetics and kinetics is unmistakable, and the fundamental mechanisms of these electrical interactions are discussed in the following ways: (i) dielectric loss and Joule heating in the crystal and at the grain boundary, (ii) coupling between mechanical stress and the electrochemical potential of charged species, (iii) interaction between applied electrical fields and the intrinsic fields that exist within the space charge layers, (iv) and the possibility of nucleating defect avalanches under electrical fields.
Abstract: Microwaves and spark plasma sintering (SPS) enhance sinterability. Simple electrical fields, applied by means of a pair of electrodes to bare specimens, have been shown to accelerate the rate of superplastic deformation, reduce the time and temperature for sintering, and to retard the rate of grain growth. By inference, the influence of electrical and electromagnetic fields on grain boundary energetics and kinetics is unmistakable. Often, in ceramics, grain boundaries are themselves endowed with space charge that can couple with externally applied fields. The frequency dependence of this coupling ranging from zero frequency to microwave frequencies is discussed. The classical approach for modeling grain growth, creep, and sintering, considers chemical diffusion (self-diffusion) under a thermodynamic driving force, underpinned by a physical mechanism that visualizes the flow of mass transport in a way that reproduces the phenomenological observations. In all instances, the final analytical result can be separated into a product of three functions: one of the grain size, the second related to the thermodynamic driving force, and the third to the kinetics of mass transport. The influence of an electrical field on each of these functions is addressed.The fundamental mechanisms of these electrical interactions are discussed in the following ways: (i) dielectric loss and Joule heating in the crystal and at the grain boundary, (ii) the coupling between mechanical stress and the electrochemical potential of charged species, (iii) the interaction between applied electrical fields and the intrinsic fields that exist within the space charge layers, (iv) and the possibility of nucleating defect avalanches under electrical fields. We limit ourselves to ceramics that have at least some degree of ionic character. In these experiments the electrical fields range from several volts to several hundred volts per centimeter, and the power dissipation from Joule heating is of the order of several watts per cubic centimeter of the specimen. Metals, where very high current densities are obtained at relatively low applied electric fields, leading to phenomenon such as electromigration, are not considered.

274 citations


Journal ArticleDOI
TL;DR: In this paper, the effects of compositional and environmental parameters on the kinetics of microstructural degradation are investigated for porous Ni/CGO anodes in solid oxide fuel cells (SOFC).

255 citations


Journal ArticleDOI
TL;DR: In this article, a physically motivated fitting formula for grain size distributions is derived using analytical predictions and numerical simulations, and the results for more complicated and realistic cases can be fitted with a black box recipe presented in this paper.
Abstract: Context. Grains in circumstellar disks are believed to grow by mutual collisions and subsequent sticking due to surface forces. Results of many fields of research involving circumstellar disks, such as radiative transfer calculations, disk chemistry, magneto-hydrodynamic simulations largely depend on the unknown grain size distribution.Aims. As detailed calculations of grain growth and fragmentation are both numerically challenging and computationally expensive, we aim to find simple recipes and analytical solutions for the grain size distribution in circumstellar disks for a scenario in which grain growth is limited by fragmentation and radial drift can be neglected.Methods. We generalize previous analytical work on self-similar steady-state grain distributions. Numerical simulations are carried out to identify under which conditions the grain size distributions can be understood in terms of a combination of power-law distributions. A physically motivated fitting formula for grain size distributions is derived using our analytical predictions and numerical simulations.Results. We find good agreement between analytical results and numerical solutions of the Smoluchowski equation for simple shapes of the kernel function. The results for more complicated and realistic cases can be fitted with a physically motivated “black box” recipe presented in this paper. Our results show that the shape of the dust distribution is mostly dominated by the gas surface density (not the dust-to-gas ratio), the turbulence strength and the temperature and does not obey an MRN type distribution.

245 citations


Journal ArticleDOI
TL;DR: In this article, the influence of nanoscale reinforcement on the mechanical behavior of ultrafine-grained composites was studied and it was shown that the presence of nanoparticles enhances strength by interacting with dislocations, while simultaneously retarding grain growth.

185 citations


Journal ArticleDOI
TL;DR: In this paper, the spark plasma sintering of an undoped commercial α-Al2O3 powder (0.14 μm) was investigated, and the SPS parameters such as the dwell temperature, applied external pressure, temperature of pressure application, dwell time and pulse pattern were varied.

175 citations


Journal ArticleDOI
TL;DR: In this paper, the processes of aggregation and subsequent grain growth of highly twinned, surfactant stabilized gold nanoparticles have been followed in real time using synchrotron X-ray diffraction (XRD) and small-angle Xray scattering (SAXS).
Abstract: The processes of aggregation and subsequent grain growth of highly twinned, surfactant stabilized gold nanoparticles have been followed in real time using synchrotron X-ray diffraction (XRD) and small-angle X-ray scattering (SAXS). This gives insight into the overall coalescence mechanism of metal nanocrystals. First, the capping ligands melt or desorb, which enables the nanocrystals to aggregate and join together. At longer times, grain growth is observed, and the stacking fault densities decrease. The time scale of the grain growth process is significantly longer than that of the particle aggregation. We contrast the behavior we observe to that of other nanoparticles and discuss the implications of our results on device fabrication.

173 citations


Journal ArticleDOI
TL;DR: In this article, the authors 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.

173 citations


Journal ArticleDOI
TL;DR: In this paper, the effect of production parameters on wear resistance of Al-Al2O3 composites was examined and it was found that increasing sintering temperature and time results in increasing density, hardness and wear resistance.

Journal ArticleDOI
TL;DR: In this article, the crystallite size, lattice strain, deformation stress, and strain energy density of AA 6061 matrix were determined precisely from the first five most intensive reflection of X-ray diffraction (XRD) and transmission electron microscopy (TEM).

Journal ArticleDOI
TL;DR: In this article, the authors employ non-equilibrium thermodynamics to propose a general equation for the mean grain size evolution in a deforming medium, under the assumption that the whole grain size distribution remains self-similar.
Abstract: P>We employ basic non-equilibrium thermodynamics to propose a general equation for the mean grain size evolution in a deforming medium, under the assumption that the whole grain size distribution remains self-similar. We show that the grain size reduction is controlled by the rate of mechanical dissipation in agreement with recent findings. Our formalism is self consistent with mass and energy conservation laws and allows a mixed rheology. As an example, we consider the case where the grain size distribution is lognormal, as is often experimentally observed. This distribution can be used to compute both the kinetics of diffusion between grains and of dynamic recrystallization. The experimentally deduced kinetics of grain size coarsening indicates that large grains grow faster than what is assumed in classical normal grain growth theory. We discuss the implications of this model for a mineral that can be deformed under both dislocation creep and grain size sensitive diffusion creep using experimental data of olivine. Our predictions of the piezometric equilibrium in the dislocation-creep regime are in very good agreement with the observations for this major mantle-forming mineral. We show that grain size reduction occurs even when the average grain size is in diffusion creep, because the largest grains of the grain size distribution can still undergo recrystallization. The resulting rheology that we predict for olivine is time-dependent and more non-linear than in dislocation creep. As the deformation rate remains an increasing function of the deviatoric stress, this rheology is not localizing.

Journal ArticleDOI
TL;DR: In this article, the influence of SiO2 doping on densification and microstructure evolution in Nd3xY3−3xAl5O12 (Nd:YAG) ceramics was examined.
Abstract: This paper examines the influence of SiO2 doping on densification and microstructure evolution in Nd3xY3−3xAl5O12 (Nd:YAG) ceramics. Nd:YAG powders were doped with 0.035–0.28 wt% SiO2 and vacuum sintered between 1484° and 1750°C. 29Si magic-angle spinning nuclear magnetic resonance showed that Si4+ substitutes onto tetrahedrally coordinated Al3+ sites. High-resolution transmission electron microscopy showed no grain boundary second phases for all silica levels in samples sintered at 1600°–1750°C. Coarsening was limited by a solute drag mechanism as suggested by cubic grain growth kinetics and transmission electron microscopy energy-dispersive X-ray spectroscopy observations of increased Nd3+ concentration near grain boundaries. Increasing SiO2 content increased both densification and grain growth rate and led to increasingly coarsening-dominated sintering trajectories. Fine-grained ( 82% real in-line transmission) ceramics were produced by sintering 0.035 wt% SiO2-doped ceramics at 1750°C for 8 h. Coarse-grained (18 μm), transparent samples were obtained with 0.28 wt% SiO2-doped Nd:YAG when sintered at 1600°C for 8 h.

Journal ArticleDOI
TL;DR: In this paper, the synthesis of kesterite Cu2ZnSnS4 (CZTS) polycrystalline thin films using cosputtering from binary sulfide targets was investigated.
Abstract: We investigate the synthesis of kesterite Cu2ZnSnS4 (CZTS) polycrystalline thin films using cosputtering from binary sulfide targets followed by annealing in sulfur vapor at 500°C to 650°C. The films are the kesterite CZTS phase as indicated by x-ray diffraction, Raman scattering, and optical absorption measurements. The films exhibit (112) fiber texture and preferred low-angle and Σ3 grain boundary populations which have been demonstrated to reduce recombination in Cu(In,Ga)Se2 and CdTe films. The grain growth kinetics are investigated as functions of temperature and the addition of Na. Significantly, lateral grain sizes above 1 μm are demonstrated for samples grown on Na-free glass, demonstrating the feasibility for CZTS growth on substrates other than soda lime glass.

Journal ArticleDOI
TL;DR: In this paper, the influence of second phases on olivine grain size was studied by quantitative microfabric analyses of samples of the Hilti massif mantle shear zone (Semail ophiolite, Oman).
Abstract: [1] The influence of second phases (e.g., pyroxenes) on olivine grain size was studied by quantitative microfabric analyses of samples of the Hilti massif mantle shear zone (Semail ophiolite, Oman). The microstructures range from porphyroclastic tectonites to ultramylonites, from outside to the center of the shear zone. Starting at conditions of ridge-related flow, they formed under continuous cooling leading to progressive strain localization. The dependence of the average olivine grain size on the second-phase content can be split into a second-phase controlled and a dynamic recrystallization–controlled field. In the former, the olivine grain size is related to the ratio between the second-phase grain size and volume fraction (Zener parameter). In the latter, dynamic recrystallization manifested by a balance between grain growth and grain size reduction processes yields a stable olivine grain size. In both fields the average olivine and second-phase grain size decreases with decreasing temperature. Combining the microstructural information with deformation mechanism maps suggests that the porphyroclastic tectonites (∼1100°C) and mylonites (∼800°C) formed under the predominance of dislocation creep. Since olivine-rich layers are intercalated with layer parallel, polymineralic bands in the mylonites, nearly equiviscous conditions can be assumed. In the ultramylonites, diffusion creep represents the major deformation mechanism in the polymineralic layers. It is this switch in deformation mechanism from dislocation creep to diffusion creep that forces strain to localize in the fine-grained polymineralic domains at low temperatures (<∼700°C), underlining the role of the second phases on strain localization in cooling mantle rocks.

Journal ArticleDOI
TL;DR: In this article, it has been observed that trace boron addition to Ti-6Al-4V alloy also ensures excellent microstructural homogeneity throughout the ingot.

Journal ArticleDOI
TL;DR: In this paper, multi-walled carbon nanotube (MWCNT)/nanostructured zirconia composites with a homogenous distribution of different MWCNT quantities (ranging within 0.5-5 wt%) were developed.

Journal ArticleDOI
TL;DR: In this article, a level set framework for the finite-element modeling of recrystallization in polycrystalline materials, including the nucleation, primary re-stallization and grain growth stages, is described.

Journal ArticleDOI
TL;DR: In this paper, the grain size of 300M steel was measured by using the linear intercept method on Olympus PMG3 metallographic microscope and the experimental results show that grain size increases with the increasing of heating temperature and holding time.
Abstract: The 300M steel was heated in an electric furnace at the temperatures of 850, 900, 950, 1000, 1050 °C, and holding time of 5, 10, 30, 60, 90, 120 min. The grain size of austenite was measured by using the linear intercept method on Olympus PMG3 metallographic microscope. The experimental results show that grain size of austenite increases with the increasing of heating temperature and holding time, and the grain growth model of austenite in 300M steel is in the following: d = 4.04 × 10 6 t 0.17 exp( − (1.32 × 10 5 / RT )).

Journal ArticleDOI
TL;DR: In this article, simultaneous shear coupling and grain rotation were observed experimentally during grain boundary migration in high-purity Al bicrystals subjected to an external mechanical stress at elevated temperatures.

Journal ArticleDOI
TL;DR: In this paper, grain boundary diffusivity on the cathode side has an apparent activation energy about 1 ǫ-eV lower than that of normal grain boundary diffusion.
Abstract: Grain growth in 8 mol% Y2O3-stabilized zirconia ceramics (8YSZ) under an electric current has been investigated. Enhanced grain growth on the cathode side starts at 1150°C, well below the conventional sintering temperature, while grain growth is dormant on the anode side until 1400°C. In fully dense samples, the grain size undergoes an abrupt transition, differing by a factor of more than 10 on the two sides. Porous samples also experience faster densification on the cathode side, but grain growth is postponed until full density is first reached. Estimated grain boundary diffusivity on the cathode side has an apparent activation energy about 1 eV lower than that of normal grain boundary diffusion. These results are attributed to supersaturated oxygen vacancies accumulated on the cathode side, causing cation reduction that lowers their migration barrier.

Journal ArticleDOI
TL;DR: In this paper, the influence of the pressure loading cycle during pulsed electrical current sintering (PECS) on the removal of oxide impurities was analyzed. And the critical temperature to evaporate the boron oxide impurity was determined to be 2000°C.
Abstract: Monolithic B 4 C, TiB 2 and B 4 C–TiB 2 particulate composites were consolidated without sintering additives by means of pulsed electric current sintering in vacuum. Sintering studies on B 4 C–TiB 2 composites were carried out to reveal the influence of the pressure loading cycle during pulsed electrical current sintering (PECS) on the removal of oxide impurities, i.e. boron oxide and titanium oxide, hereby influencing the densification behavior as well as microstructure evolvement. The critical temperature to evaporate the boron oxide impurities was determined to be 2000 °C. Fully dense B 4 C–TiB 2 composites were achieved by PECS for 4 min at 2000 °C when applying the maximum external pressure of 60 MPa after volatilization of the oxide impurities, whereas a relative density of 95–97% was obtained when applying the external pressure below 2000 °C. Microstructural analysis showed that B 4 C and TiB 2 grain growth was substantially suppressed due to the pinning effect of the secondary phase and the rapid sintering cycle, resulting in micrometer sized and homogeneous microstructures. Excellent properties were obtained for the 60 vol% TiB 2 composite, combining a Vickers hardness of 29 GPa, a fracture toughness of 4.5 MPa m 1/2 and a flexural strength of 867 MPa, as well as electrical conductivity of 3.39E+6 S/m.

Journal ArticleDOI
TL;DR: In this paper, a rigorous treatment for the evaluation of growth restriction factor Q in multicomponent alloys based on consistent thermodynamic descriptions of the alloy phase equilibria is presented.

Journal ArticleDOI
TL;DR: In this paper, the effects of rare earth element addition on the microstructure evolution, thermal stability and shear strength of AZ91 alloy were investigated in the as-cast and annealed conditions.

Journal ArticleDOI
TL;DR: In this paper, the interactive effect of grain and specimen sizes on the flow stress of sheet metal in microforming is investigated via the tensile test of pure copper and numerical modeling.
Abstract: In this research, the interactive effect of grain and specimen sizes on the flow stress of sheet metal in microforming is investigated via the tensile test of pure copper and numerical modeling. Models based on different assumptions are proposed to analyze the size effect phenomenon. It is found that the flow stress decreases linearly with the decrease of the ratio of specimen to grain sizes. The grain boundary thickness decreases and its volume fraction increases with the decrease of grain size. The variation of grain boundary thickness is not proportional to the variation of grain size. Furthermore, the fraction of grain boundary increases with the strain and the ratio of specimen to grain sizes. Based on the FE simulation, it is found that the simulated flow stress, which is modeled based on the identified grain boundary thicknesses using the proposed models, has a good agreement with the experimental result. In addition, the size effect on flow stress is also analyzed based on the surface layer model. The methodology to identify the surface and internal grain properties is proposed based on the experimental result. The identified properties are applicable in modeling of the interactive effect of specimen and grain sizes on flow stress. This research thus provides an in-depth understanding of the plastic deformation behavior in microforming process.

Journal ArticleDOI
Abstract: Grain growth by the accretion of metals in interstellar clou ds (called ‘grain growth’) could be one of the dominant processes that determine the dust content in galaxies. The importance of grain size distribution for the grain growth is demo nstrated in this paper. First, we derive an analytical formula that gives the grain size distr ibution after the grain growth in individual clouds for any initial grain size distribution. The time-scale of the grain growth is very sensitive to grain size distribution, since the grai n growth is mainly regulated by the surface to volume ratio of grains. Next, we implement the results of grain growth into dust enrichment models of entire galactic system along with the grain formation and destruction in the interstellar medium, finding that the grain growth in c louds governs the dust content in nearby galaxies unless the grain size is strongly biased to sizes larger than � 0.1 µm or the power index of the grain size distribution is shallower t han � 2.5. The grain growth in clouds contributes to the rapid increase of dust-to-gas r atio at a certain metallicity level (called critical metallicity in Asano et al. 2011 and Inoue 2 011), which we find to be sensitive to grain size distribution. Thus, the grain growth efficient ly increase the dust mass not only in nearby galaxies but also in high-redshift quasars, whose metallicities are larger than the critical value. Our recipe for the grain growth is applicabl e for any grain size distribution and easily implemented into any framework of dust enrichment in galaxies.

Journal ArticleDOI
TL;DR: In this article, the densification of hot-pressed ZrC ceramics with Zr and graphite additive was studied at 1800-2000°C and the results showed that Zr/C molar ratio at 1:2 was achieved by adding co-doped additive of Zr plus C.
Abstract: Densifications of hot-pressed ZrC ceramics with Zr and graphite additives were studied at 1800–2000 °C. ZrC with 8.94 wt% Zr additive (named ZC10) sintered at 1900–2000 °C achieved higher relative densities (>98.4%) than that of additive-free ZrC (<83%). The densification improvement was attributed to the formation of non-stoichiometric ZrC0.9, whereas there had rapid grain growth with grain size about 50–100 μm in ZC10. By adding co-doped additive of Zr plus C and adjusting the molar ratio of Zr/C, ZrC with co-doped additives with Zr/C molar ratio at 1:2 (named ZC12), ZrC ceramics with both high relative density (98.4%) and fine microstructures (grain size about 5–10 μm) were obtained at 1900–2000 °C. Effect of formation of non-stoichiometric ZrC1−x on densification of ZrC was discussed. The Vickers hardness and indentation toughness of ZC10 and ZC12 samples sintered at 1900 °C were 17.8 GPa and 3.0 MPa m1/2, 16.2 GPa and 4.7 MPa m1/2, respectively.

Journal ArticleDOI
TL;DR: In this article, the effect of trace Mn addition on the microstructure, texture and mechanical properties of the as-cast and as-extruded Mg-5.6.25.% Zn-0.3.% Ca (ZX51) alloys was investigated.
Abstract: The effect of trace Mn addition on the microstructure, texture and mechanical properties of the as-cast and as-extruded Mg–5.25 wt.% Zn–0.6 wt.% Ca (ZX51) alloys was investigated in this study. Mn addition had a negligible effect on the grain size of the as-cast ZX51 alloy. However, the addition of Mn led to the obvious decrease of grain size in the as-extruded Mg–5.25 wt.% Zn–0.6 wt.% Ca–0.3 wt.% Mn (ZXM510) alloy, because the Mn addition restricted the grain growth during the hot extrusion process. After the addition of Mn, the basal fiber texture with most of {0 0 0 2} planes parallel to the extrusion direction (ED) was significantly enhanced in the as-extruded ZXM510 alloy. Both tensile yield strength (TYS) and ultimate tensile strength (UTS) were increased in the as-extruded ZXM510 alloy, while the ductility was slightly decreased, which was ascribed to the grain refinement and texture strengthening.

Journal ArticleDOI
TL;DR: In this paper, the structural evolution of a nanocrystalline Ni-Fe alloy induced by high pressure torsion (HPT) was investigated, where grain growth occurred via grain rotation and coalescence, forming three-dimensional smallangle sub-grain boundaries.