Showing papers on "Grain size published in 2002"
TL;DR: A thermomechanical treatment of Cu is described that results in a bimodal grain size distribution, with micrometre-sized grains embedded inside a matrix of nanocrystalline and ultrafine (<300 nm) grains, which impart high strength, as expected from an extrapolation of the Hall–Petch relationship.
Abstract: Nanocrystalline metals--with grain sizes of less than 100 nm--have strengths exceeding those of coarse-grained and even alloyed metals, and are thus expected to have many applications. For example, pure nanocrystalline Cu (refs 1-7) has a yield strength in excess of 400 MPa, which is six times higher than that of coarse-grained Cu. But nanocrystalline materials often exhibit low tensile ductility at room temperature, which limits their practical utility. The elongation to failure is typically less than a few per cent; the regime of uniform deformation is even smaller. Here we describe a thermomechanical treatment of Cu that results in a bimodal grain size distribution, with micrometre-sized grains embedded inside a matrix of nanocrystalline and ultrafine (<300 nm) grains. The matrix grains impart high strength, as expected from an extrapolation of the Hall-Petch relationship. Meanwhile, the inhomogeneous microstructure induces strain hardening mechanisms that stabilize the tensile deformation, leading to a high tensile ductility--65% elongation to failure, and 30% uniform elongation. We expect that these results will have implications in the development of tough nanostructured metals for forming operations and high-performance structural applications including microelectromechanical and biomedical systems.
2,531 citations
TL;DR: A review of the literature on grain refinement by heterogeneous nucleation and alloying can be found in this paper, where the types of grain refiner, Al-Ti-B master alloys in particular, and their methods of manufacture are discussed.
Abstract: Grain refinement of aluminium and its alloys is common industrial practice. The field has been extensively investigated by many workers over the past 50 years, not only to develop efficient grain refiners for different aluminium alloys, but also to achieve an understanding of the mechanism of grain refinement. The present review confines itself to the literature on grain refinement by heterogeneous nucleation and alloying. Initially, the fundamentals of grain refinement by inoculants are outlined. The types of grain refiner, Al-Ti-B master alloys in particular, and their methods of manufacture are next discussed. The grain refining tests to assess the efficiency of the grain refiners and the grain refining behaviour of aluminium alloys are also discussed in brief. The performance of a grain refiner, as well as the response of an aluminium alloy to grain refinement, is influenced by the microstructure of the grain refiner as controlled by the process parameters involved in its preparation and the alloying elements present in the aluminium alloy. The roles of these factors, and particularly the roles of poisoning elements such as Si, Cr, Zr, Li, are reviewed. The paper also reviews the mechanisms of grain refinement, the fading and poisoning phenomena, and the trends in the development of new grain refiners for aluminium alloys containing poisoning elements.
700 citations
TL;DR: The hardness of coarse-grained materials is inversely proportional to the square root of the grain size as mentioned in this paper, but as Van Swygenhoven explains in her Perspective, at nanometer scale grain sizes this relation no longer holds.
Abstract: The hardness of coarse-grained materials is inversely proportional to the square root of the grain size. But as
Van Swygenhoven
explains in her Perspective, at nanometer scale grain sizes this relation no longer holds. Atomistic simulations are providing key insights into the structural and mechanical properties of nanocrystalline metals, shedding light on the distinct mechanism by which these materials deform.
586 citations
TL;DR: In this article, underdoped Bi2Sr2CaCu2O8+δ superconductors are shown to be a mixture of two different short-range electronic orders with the long-range characteristics of a granular superconductor.
Abstract: Granular superconductivity occurs when microscopic superconducting grains are separated by non-superconducting regions; Josephson tunnelling between the grains establishes the macroscopic superconducting state1. Although crystals of the copper oxide high-transition-temperature (high-Tc) superconductors are not granular in a structural sense, theory suggests that at low levels of hole doping the holes can become concentrated at certain locations resulting in hole-rich superconducting domains2,3,4,5. Granular superconductivity arising from tunnelling between such domains would represent a new view of the underdoped copper oxide superconductors. Here we report scanning tunnelling microscope studies of underdoped Bi2Sr2CaCu2O8+δ that reveal an apparent segregation of the electronic structure into superconducting domains that are ∼3 nm in size (and local energy gap 50 ± 2.5 meV. These observations suggest that underdoped Bi2Sr2CaCu2O8+δ is a mixture of two different short-range electronic orders with the long-range characteristics of a granular superconductor.
581 citations
TL;DR: In this article, it was shown that grain boundaries are in a metastable thermodynamic equilibrium in the presence of solute atoms and, therefore, grain coarsening is stopped as there is no driving force.
511 citations
TL;DR: In this paper, the authors investigated the UFG microstructures in the surface layer of an aluminum alloy 7075 heavily worked by ultrasonic shot peening and found that the grain subdivision into the subgrains was the main mechanism responsible for grain refinement.
418 citations
TL;DR: In this paper, the aluminum alloys 6063-T5 and T4 were friction-stir welded at different tool rotation speeds (R), and then distributions of the microstructure and hardness were examined in these welds.
Abstract: The aluminum (Al) alloys 6063-T5 and T4 were friction-stir welded at different tool rotation speeds (R), and then distributions of the microstructure and hardness were examined in these welds. The maximum temperature of the welding thermal cycle rose with increasing R values. The recrystallized grain size of the weld increased exponentially with increasing maximum temperature. The relationship between the grain size and the maximum temperature satisfied the static grain-growth equation. In the as-welded condition, 6063-T5 Al was softened around the weld center, whereas 6063-T4 Al showed homogeneous hardness profiles. Different R values did not result in significant differences in the hardness profile in these welds, except for the width of the softened region in the weld of 6063-T5 Al. Postweld aging raised the hardness in most parts of the welds, but the increase in hardness was small in the stir zone produced at the lower R values. Transmission electron microscope (TEM) observations detected a similar distribution of the strengthening precipitates in the grain interiors and the presence of a precipitation-free zone (PFZ) adjacent to the grain boundaries in all the welds. Microstructural analyses suggested that the small increase in hardness in the stir zone produced at the lower R values was caused by an increase in the volume fraction of PFZs.
411 citations
TL;DR: In this article, two fine-grained 7075Al alloys with a grain size of 3.8 and 7.5 μm were subjected to friction stir processing (FSP) with different processing parameters.
380 citations
TL;DR: In this paper, the grain-size scaling of the Coble creep is found to decrease from d−3 to d−2 when the grain diameter becomes of the order of the GB width.
370 citations
TL;DR: In this article, four olivine polycrystals ranging in mean grain size d from 3 to 23 μm have been fabricated, characterized, and mechanically tested in torsion at high temperatures and seismic frequencies, and both the shear modulus and the dissipation of shear strain energy Q−1 have been measured as functions of oscillation period To, temperature T, and grain size.
Abstract: [1] In order to investigate the processes responsible for the attenuation of seismic shear waves in the Earth's upper mantle, four olivine polycrystals ranging in mean grain size d from 3 to 23 μm have been fabricated, characterized, and mechanically tested in torsion at high temperatures and seismic frequencies. Both the shear modulus, which governs the shear wave speed VS, and the dissipation of shear strain energy Q−1 have been measured as functions of oscillation period To, temperature T, and, for the first time, grain size. At sufficiently high T all four specimens display similar absorption band viscoelastic behavior, adequately represented for 1000 < T < 1200 or 1300°C and 1 < To < 100 s, by the expression Q−1 = A [Tod−1 exp (−E/RT)]α with A = 7.5 × 102 s−α μmα, α = 0.26 and E = 424 kJ mol−1. This mildly grain-size-sensitive viscoelastic behavior of melt-free polycrystalline olivine is attributed to a combination of elastically and diffusionally accommodated grain boundary sliding, the latter becoming progressively more important with increasing T and/or To. Extrapolation to the larger (mm-cm) grain sizes expected in the Earth's upper mantle yields levels of dissipation comparable with those observed seismologically, implying that the same grain-size-sensitive processes might be responsible for much of the observed seismic wave attenuation. The temperature sensitivity of VS is increased substantially by the viscoelastic relaxation allowing the lateral variability of wave speeds to be associated with relatively small temperature perturbations.
343 citations
TL;DR: In this paper, the activation energy for the grain and grain boundary conduction and its variation with grain size have been reported and the conduction mechanism is found to be due to the hopping of both electrons and holes.
Abstract: Electrical conductivity and dielectric measurements have been performed for nanocrystalline NiFe2O4 spinel for four different average grain sizes, ranging from 8 to 97 nm. The activation energy for the grain and grain boundary conduction and its variation with grain size have been reported in this paper. The conduction mechanism is found to be due to the hopping of both electrons and holes. The high-temperature conductivity shows a change of slope at about 500 K for grain sizes of 8 and 12 nm and this is attributed to the hole hopping in tetrahedral sites of NiFe2O4. Since the activation energy for the dielectric relaxation is found to be almost equal to that of the dc conductivity, the mechanism of electrical conduction must be the same as that of the dielectric polarization. The real part e' of the dielectric constant and the dielectric loss tanδ for the 8 and 12 nm grain size samples are about two orders of magnitude smaller than those of the bulk NiFe2O4. The anomalous frequency dependence of e' has been explained on the basis of hopping of both electrons and holes. The electrical modulus analysis shows the non-Debye nature of the nanocrystalline nickel ferrite.
TL;DR: In this article, the first stage of the electrochemical reduction of crystallized spinel in lithium cells was investigated by means of in situ X-ray diffraction, and it was shown that the formation of the intermediate phase was highly dependent on the discharge rate, the texture of the active material (i.e., crystallite size, specific surface area), and the cycling temperature.
Abstract: The first stage of the electrochemical reduction of crystallized spinel in lithium cells was investigated by means of in situ X-ray diffraction. Through the use of tailor-made materials prepared from Co-alkoxide precursors, we observed that the formation of the intermediate phase previously evidenced by several authors was highly dependent on the discharge rate, the texture of the active material (i.e., crystallite size, specific surface area), and the cycling temperature. When starting from a highly divided oxide and/or using a low current, we found that this plateau was actually associated with the formation of α-CoO, subsequently leading to metallic cobalt upon further reduction. Alternatively, was formed when using materials with a large crystallite size or/and applying a high discharge rate, later on similarly decomposing into divided metal. These findings and the related competition between different reaction paths represent an explanation for numerous electrochemical observations, and for the need of fast insertion in such host materials to stabilize intermediate lithiated compounds. This work illustrates the major influence of the initial texture as well as the temperature on the reactivity of the 3d-metal based oxides recently reinvestigated for their electrochemical performances as negative electrode materials. Also, it emphasizes the implications of the reactive grain size evolution upon cycling. © 2002 The Electrochemical Society. All rights reserved.
TL;DR: In this article, a study of nanocrystalline oxide thin film processing and influence of microstructure on the electrical properties of Gd3+-doped CeO2 thin films was reported.
TL;DR: In this paper, the formation of α″ martensite and its influence on Young's modulus and mechanical properties of forged Ti-29Nb-13Ta-4.6Zr (wt pct) alloy is reported.
Abstract: An investigation on the formation of α″ martensite and its influence on Young’s modulus and mechanical properties of forged Ti-29Nb-13Ta-4.6Zr (wt pct) alloy is reported in this article. For ice-water-quenched specimens after solution treatment at 1023, 1123, and 1223 K in the single β-phase field for 1.8, 3.6, 14.4, and 28.8 ks, X-ray diffraction and internal friction measurements showed that the volume fraction of the α″ martensite changes with both solution temperature and time. This effect has been attributed mainly to the influence of grain size of the β-parent phase on the stability of the β phase and, consequently, on the martensitic start (Ms) temperature. A critical grain size of 40 µm was identified for the β phase, below which the martensitic transformation is largely suppressed because of low MS temperature. With the β grain size increasing above this critical value, the volume fraction of the α″ martensite increases significantly at first and then decreases gradually with further grain growth. The α″ martensite was shown to possess good ductility and, compared to the β phase, lower strength and hardness but nearly identical Young’s modulus in the studied alloy.
TL;DR: In this article, the authors used complex conductivity measurements to predict the hydraulic conductivity of unconsolidated materials, including natural sediments and artificial sand/clay mixtures.
Abstract: [1] We use complex conductivity measurements to predict the hydraulic conductivity (K) of unconsolidated materials. The samples include natural sediments and artificial sand/clay mixtures. We apply the Borner et al. [1996] model, which is based on the Kozeny-Carman equation and incorporates electrical estimates of formation factor (F) and specific surface area per unit volume-to-porosity ratio (Spor), from the real (σ′) and imaginary (σ″) conductivity components respectively. We find that K correlates with σ″ but shows no correlation with F, which we attribute to the wide range in grain size for these materials. The Borner model appears primarily dependent on the K - σ″ relation. The relationship between σ″ and Spor is nonlinear and appears to depend upon material type. Further examination shows that σ″ is well correlated with effective grain size (d10) and is relatively independent of the material type. We propose a simple Hazen-type equation in which the effective grain size is estimated from σ″. This simple model provides order of magnitude estimates of K for a range of unconsolidated sediments.
01 May 2002-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the tensile and yield strengths of the nanocomposite material with SiC content less than 2 vol.% were higher than those for pure nanocrystalline Ni of comparable grain size.
Abstract: Nanocomposite materials consisting of a nanocrystalline Ni matrix (grain size 10–15 nm) reinforced with sub-micron size SiC particulates (average particle size: 0.4 μm) up to 10.5 vol.% have been produced by pulse electrodeposition. Substantial improvements in mechanical properties including hardness, yield and tensile stress were obtained for the nanocomposite material, as compared with conventional Ni–SiC composites with a matrix grain size in the micrometer range. Tensile strengths up to four times that for conventional polycrystalline Ni and two times that for conventional polycrystalline Ni–SiC of comparable SiC content was measured. The tensile and yield strengths of the nanocomposite material with SiC content less than 2 vol.% were higher than those for pure nanocrystalline Ni of comparable grain size. For these nanocomposites an unexpected increase in tensile ductility was also observed when compared to pure nanocrystalline nickel. At higher SiC content (>2 vol.%) the strength and ductility were found to decrease to the detriment of the nanocomposite. Particle clustering was considered the main cause of this decrease.
TL;DR: In this paper, a strain gradient dependent crystal plasticity approach is used to model the constitutive behavior of polycrystal FCC metals under large plastic deformation, where material points are considered as aggregates of grains, subdivided into several fictitious grain fractions: a single crystal volume element stands for the grain interior whereas grain boundaries are represented by bi-crystal volume elements.
Abstract: A strain gradient dependent crystal plasticity approach is used to model the constitutive behaviour of polycrystal FCC metals under large plastic deformation. Material points are considered as aggregates of grains, subdivided into several fictitious grain fractions: a single crystal volume element stands for the grain interior whereas grain boundaries are represented by bi-crystal volume elements, each having the crystallographic lattice orientations of its adjacent crystals. A relaxed Taylor-like interaction law is used for the transition from the local to the global scale. It is relaxed with respect to the bi-crystals, providing compatibility and stress equilibrium at their internal interface. During loading, the bi-crystal boundaries deform dissimilar to the associated grain interior. Arising from this heterogeneity, a geometrically necessary dislocation (GND) density can be computed, which is required to restore compatibility of the crystallographic lattice. This effect provides a physically based method to account for the additional hardening as introduced by the GNDs, the magnitude of which is related to the grain size. Hence, a scale-dependent response is obtained, for which the numerical simulations predict a mechanical behaviour corresponding to the Hall–Petch effect. Compared to a full-scale finite element model reported in the literature, the present polycrystalline crystal plasticity model is of equal quality yet much more efficient from a computational point of view for simulating uniaxial tension experiments with various grain sizes.
TL;DR: In this paper, the microstructural evolution and thermal properties of nanocrystalline (nc) Cu during mechanical attrition were investigated by using quantitative x-ray-diffraction and thermal analysis techniques.
Abstract: The microstructural evolution and thermal properties of nanocrystalline (nc) Cu during mechanical attrition were investigated by using quantitative x-ray-diffraction and thermal analysis techniques. Upon milling of the Cu powders with coarse grains, the grain sizes are found to decrease gradually with the milling time, and remain unchanged at a steady-state value (about 11 nm) with continued milling. The microstrain and the stored enthalpy increase to maximum values during the grain refinement, and decrease then increase to the second maxima and decrease again within the milling stage of steady-state grain size, while the lattice parameter remains unchanged during the entire milling process. The grain boundary (GB) enthalpy of the nc Cu was estimated, showing a GB softening-hardening-softening cyclic variation within the steady-state milling. The present work indicated with clear experimental evidence that even within the milling stage of steady-state grain size, the microstructure (both the GB's and the crystallites) of nc materials is still changing, which may result from the GB sliding.
TL;DR: In this article, the compaction curves are related directly to observations made from thin sections prepared at different stress levels, and the image analyses of the thin sections show that the degree of grain fracturing increases continuously as a function of stress level.
TL;DR: In this article, the performance of sand cast magnesium alloy MEZ was investigated for unrefined (MEZU) and Zr-grain-refined(MEZR) microstructures in 5% NaCl solution using salt spray, immersion, in situ examination of the corrosion morphology, ESEM, electron probe microanalysis, hydrogen evolution and polarisation curves.
TL;DR: In this paper, molecular dynamics simulation is used to study the tensile mechanical properties of face-centred cubic Ni nanocrystalline materials with mean grain size of 12 nm, and three samples are considered: one as-prepared, another annealed at 800 K, and one in which additional structural disorder has been introduced to the grain boundary region.
TL;DR: In this article, the effects of hot pressing parameters on the TiB 2 ceramic microstructure and mechanical properties were studied by measuring three point bending testing and single edge notched bending tests, respectively.
Abstract: In this paper, a titanium diboride ceramic was produced by the hot pressing sintering method. The effects of hot pressing parameters on the TiB 2 ceramic microstructure and mechanical properties were studied. The bending strength and fracture toughness were measured by three point bending testing and single edge notched bending tests (SENB), respectively. The microstructure features of the TiB 2 sintered material were revealed by means of SEM and TEM. The results show that the TiB 2 grain size increases quickly with the increasing temperature and time during hot pressing sintering. The density and the TiB 2 grain size have a great influence on the mechanical properties. The bending strength decreases with increasing TiB 2 grain size, whilst the fracture toughness increases.
TL;DR: In this article, a power law relationship between grain size and toughness increment was found for nanocrystalline tetragonal zirconia ceramics, and a linear relationship between yttria content and inverse critical grain size was found.
TL;DR: In this article, the Seebeck coefficient of sintered degenerate n-type PbTe samples with small grain sizes ranging from 07 to 4 μm was investigated and the effects of grain size on their thermoelectric properties were then investigated.
Abstract: Sintered degenerate n-type PbTe samples with small grain sizes ranging from 07 to 4 μm were prepared and the effects of grain size on their thermoelectric properties were then investigated The Seebeck coefficient of the sintered samples increased almost double when the grain size decreased from 4 to 07 μm On the other hand, their electrical and thermal conductivity decreased with decreasing grain size Accordingly, decreasing their grain size increased their thermoelectric figure-of-merit A maximum value of the figure-of-merit of the obtained small grain-size samples was significantly higher than that of large grain-size samples with the same carrier concentration reported This favorable result was caused mainly by the increase in the Seebeck coefficient The influences of grain boundaries on the increase in the Seebeck coefficient were discussed It is concluded that the Seebeck coefficient was increased by potential barrier scattering, which occurred at the grain boundaries in the sintered samples
TL;DR: In this paper, the electrical conductivity of ZnO films has been studied in order to elucidate the structural and electrical properties at low temperature, showing that zinc monoacetate is an intermediate product prior to the formation of zinc hydroxide.
TL;DR: In this paper, single ferroelectric lead titanate (PTO) grains down to 15 nm were fabricated by chemical solution deposition and the grain-size-dependent domain configuration was studied using three-dimensional piezoresponse force microscopy (PFM).
Abstract: Single ferroelectric lead titanate (PTO) grains down to 15 nm were fabricated by chemical solution deposition. Varying the dilution of the precursor solution leads to different grain sizes between 15 and 200 nm. The grain-size-dependent domain configuration was studied using three-dimensional piezoresponse force microscopy (PFM). It is found that the PTO grains in a dense film contain laminar 90° domain walls, whereas separated PTO grains show more complicated structures of mainly 180° domain walls. For grains smaller than 20 nm, no piezoresponse was observed and we suppose this could be due to the transition from the ferroelectric to the superparaelectric phase which has no spontaneous polarization. Recent calculations showed that the ferroelectricity of fine ferroelectric particles decrease with decreasing particle size. From these experiments the extrapolated critical size of PTO particles was found to be around 4–14 nm.
TL;DR: In this article, a tensile testing technique utilizing MEMS force sensors for in situ mechanical characterization of sub-micron scale freestanding thin films in SEM and TEM is presented.
Abstract: We present a novel tensile testing technique utilizing MEMS force sensors for in situ mechanical characterization of sub-micron scale freestanding thin films in SEM and TEM. Microfabrication techniques are used to cofabricate the thin film specimens with force sensors to produce the following unique features: (1) small setup size to fit in SEM and TEM for in situ experiments, (2) ability to measure tensile pre-stress in specimen, (3) alignment between specimen and applied loading axes with lithographic precision, (4) no extra gripping mechanism required, and (5) ability to measure creep strain in the material. The technique allows single or multilayers of materials that can be deposited/grown on silicon substrate to be tested. We demonstrate the technique by testing a 100 nm thick, 8.8 μm wide and 275 μm long freestanding aluminum specimen (average grain size about 50 nm) in situ inside an environmental SEM chamber, and present another setup for similar experiment in TEM. Experimental results strongly suggest that at this size scale: (1) elastic modulus does not change, (2) size effects on yield strength are pronounced (63 times the bulk pure aluminum yield stress), and (3) permanent strain hardening effects are absent.
TL;DR: In this article, a fine-grained ZK60 magnesium alloy with the grain size of 14 μm was processed by equal-channel angular extrusion and the material exhibited low temperature superplasticity.
TL;DR: Grain growth in systems of anisotropic grain boundary energy and mobility is investigated by computer simulations in a two-dimensional textured polycrystalline system in this article, where the energy and the mobility are allowed to depend on both grain boundary inclination and misorientation.
TL;DR: In this article, the authors investigated the photoluminescence and time-resolved PL spectra of zinc oxide (ZnO) films as a function of the grain size of the microcrystals.
Abstract: Photoluminescence (PL) and time-resolved PL spectra of zinc oxide (ZnO) films were investigated as a function of the grain size of the microcrystals. Correlation was found between the grain size and the optical properties—in the bound exciton states, both the PL intensity and PL decay time increased with increasing the grain size. This correlation can be well explained by the existence of nonradiative-surface and/or -interface states in the grain boundaries of ZnO microcrystals.