Showing papers on "Grain size published in 2014"
TL;DR: Gradient microstructures, in which the grain size increases from nanoscale at the surface to coarse-grained in the core, were recently discovered to be an effective approach to improving ductility.
Abstract: Steels can be made stronger, tougher, or more resistant to corrosion either by changing composition (adding in more carbon or other elements) or by modifying their microstructures. An extreme microstructural route for strengthening materials is to reduce the crystallite size from the micrometer scale (“coarse-grained”) to the nanoscale. Nanograined aluminum or copper (Cu) may become even harder than high-strength steels, but these materials can be very brittle and crack when pulled (deformed in tension), apparently because strain becomes localized and resists deformation. However, nanograined metals can be plastically deformed under compression or rolling at ambient temperature, implying that moderate deformation can occur if the cracking process is suppressed. Tremendous efforts have been made to explore how to suppress strain localization in tensioned nanomaterials and make them ductile. Gradient microstructures, in which the grain size increases from nanoscale at the surface to coarse-grained in the core, were recently discovered to be an effective approach to improving ductility ( 1 – 4 ).
755 citations
TL;DR: In this article, the authors investigated various binary, ternary and quaternary alloys made from the equiatomic high-entropy alloy FeNiCoCrMn.
659 citations
TL;DR: A promising future for zirconia restorations, with combined translucency and mechanical properties, can be realized by reducing its grain size.
411 citations
TL;DR: In this paper, the influence of changes in crystallographic texture on the Hall-Petch (H-P) relationship for an Mg alloy was investigated, and the texture variations were facilitated by changing the uniaxial tensile loading orientation with respect to the normal direction of the rolled Mg plate.
294 citations
TL;DR: This work provides a route toward scaled-up synthesis of high-quality monolayer MoS2 for electronic and optoelectronic devices and shows a field mobility comparable to those achieved from exfoliatedMoS2.
Abstract: We report a scalable growth of monolayer MoS2 films on SiO2 substrates by chemical vapor deposition. As-grown polycrystalline MoS2 films are continuous over the entire substrate surface with a tunable grain size from ∼20 nm up to ∼1 μm. An obvious blue-shift (up to 80 meV) of photoluminescence peaks was observed from a series samples with different grain sizes. Back-gated field effect transistors based on a polycrystalline MoS2 film with a typical grain size of ∼600 nm shows a field mobility of ∼7 cm2/(V s) and on/off ratio of ∼106, comparable to those achieved from exfoliated MoS2. Our work provides a route toward scaled-up synthesis of high-quality monolayer MoS2 for electronic and optoelectronic devices.
264 citations
TL;DR: In this article, the authors investigated the formation of annealing twins in the FCC-structured equiatomic CoCrFeMnNi high-entropy alloy by arc melting and drop casting.
239 citations
TL;DR: Grain refinement through severe plastic deformation enables synthesis of ultrahigh-strength nanostructured materials that impedes dislocation motion and enables an extreme tensile strength of 7 GPa, making this alloy the strongest ductile bulk material known.
Abstract: Grain refinement through severe plastic deformation enables synthesis of ultrahigh-strength nanostructured materials. Two challenges exist in that context: First, deformation-driven grain refinement is limited by dynamic dislocation recovery and crystal coarsening due to capillary driving forces; second, grain boundary sliding and hence softening occur when the grain size approaches several nanometers. Here, both challenges have been overcome by severe drawing of a pearlitic steel wire (pearlite: lamellar structure of alternating iron and iron carbide layers). First, at large strains the carbide phase dissolves via mechanical alloying, rendering the initially two-phase pearlite structure into a carbon-supersaturated iron phase. This carbon-rich iron phase evolves into a columnar nanoscaled subgrain structure which topologically prevents grain boundary sliding. Second, Gibbs segregation of the supersaturated carbon to the iron subgrain boundaries reduces their interface energy, hence reducing the driving force for dynamic recovery and crystal coarsening. Thus, a stable cross-sectional subgrain size $l10\text{ }\text{ }\mathrm{nm}$ is achieved. These two effects lead to a stable columnar nanosized grain structure that impedes dislocation motion and enables an extreme tensile strength of 7 GPa, making this alloy the strongest ductile bulk material known.
216 citations
TL;DR: In this article, the effect of grain size on the energy storage properties of TiO 3 (BST) ceramics with various grain sizes (0.5-5.6μm) was investigated.
Abstract: (Ba 0.4 Sr 0.6 )TiO 3 (BST) ceramics with various grain sizes (0.5–5.6 μm) were prepared by conventional solid state reaction methods. The effect of grain size on the energy storage properties of BST ceramics ( T c ≈ −65 °C) was investigated. With decreasing grain sizes, a clear tendency toward the diffuse phase transition was observed and the dielectric nonlinearity was reduced gradually, which can be explained by the Devonshire's phenomenological theory (from the viewpoint of intrinsic polarization). Based on the multi-polarization mechanism model, the relationship between the polarization behavior of polar nano-regions (the extrinsic nonlinear polarization mechanisms) and grain size was studied. The variation of the grain boundary density was thought to play an important role on the improvement of dielectric breakdown strength, account for the enhanced energy density, which was confirmed by the complex impedance spectroscopy analysis based on a double-layered dielectric model.
209 citations
TL;DR: In this paper, the effects of grain size on the rate-dependent thermomechanical responses of polycrystalline superelastic NiTi (with an average grain size from 10 to 90nm) under both monotonic and cyclic tensile loading-unloading were investigated.
183 citations
TL;DR: In this paper, the degradation of ferroelectric properties of atomic layer deposited Hf0.5Zr 0.5O2 films with increasing thickness was mitigated by inserting 1'nm-thick Al2O3 interlayer at middle position of the thickness of the FE film.
Abstract: The degradation of ferroelectric (FE) properties of atomic layer deposited Hf0.5Zr0.5O2 films with increasing thickness was mitigated by inserting 1 nm-thick Al2O3 interlayer at middle position of the thickness of the FE film. The large Pr of 10 μC/cm2, which is 11 times larger than that of single layer Hf0.5Zr0.5O2 film with equivalent thickness, was achieved from the films as thick as 40 nm. The Al2O3 interlayer could interrupt the continual growth of Hf0.5Zr0.5O2 films, and the resulting decrease of grain size prevented the formation of non-ferroelectric monoclinic phase. The Al2O3 interlayer also largely decreased the leakage current of the Hf0.5Zr0.5O2 films.
180 citations
TL;DR: In this paper, the effect of various well-studied alloying elements on the microstructure and soft magnetic properties of the Fe-based amorphous/nanocrystalline system is compared.
Abstract: The intention of the present study is to review and compare the effect of various well-studied alloying elements on the microstructure and soft magnetic properties of the Fe-based amorphous/nanocrystalline system. The state-of-the-art Fe-based amorphous/nanocrystalline alloys have been developed because of their unique soft magnetic properties such as low core loss, high permeability(104–105 at 1 kHz) and low magnetostriction (< 10 ppm) as compared to conventional silicon steels which are also called electrical steels. In Fe-based amorphous/nanocrystalline system, the chemical composition and microstructural features particularly grain size play an indispensable role on the saturated magnetization (Bs) and coercivity (Hc) values. An ideal Fe-based soft magnetic material is defined as a material possessing higher Bs and lower Hc. The problem of the new material is its low Bs value (for commercial material is 1.4 T) than silicon steels (≈ 2 T). In addition to Bs content of new material, many attempts have been made to reduce the Hc value which could be achieved via a decrease of grain size (< 50 nm). To reach this goal (Bs↑ and Hc↓), the effect of a variety of elements on the microstructure, crystallization process and soft magnetic properties of the Fe-based amorphous/nanocrystalline alloys has been investigated so far. The aim of all these studies is to find an appropriate replacement for conventional silicon steels because of their high core loss and low permeability. Effect of alloying elements including Si, B, Cu, Nb, Zr, N-doping, P, Ni, Co, H-doping, Ge and W on the microstructure and magnetic properties is the main subject of this study in order to shed light on the dependence of magnetic properties with composition and grain size.
TL;DR: In this article, the impact of laser power, scanning speed, and laser path on the microstructure of NiTi cylinders was studied, where the laser power was varied from 56 to 100 W and the scanning speed from about 100 to 300 mm/s.
TL;DR: In this paper, a solidification microstructure process map was developed using finite element analysis for deposition of single beads of Ti-6Al-4V via electron beam wire feed AM processes.
Abstract: The ability to deposit a consistent and predictable solidification microstructure can greatly accelerate additive manufacturing (AM) process qualification. Process mapping is an approach that represents process outcomes in terms of process variables. In this work, a solidification microstructure process map was developed using finite element analysis for deposition of single beads of Ti-6Al-4V via electron beam wire feed AM processes. Process variable combinations yielding constant beta grain size and morphology were identified. Comparison with a previously developed process map for melt pool geometry shows that maintaining a constant melt pool cross sectional area will also yield a constant grain size. Additionally, the grain morphology boundaries are similar to curves of constant melt pool aspect ratio. Experimental results support the numerical predictions and identify a proportional size scaling between beta grain widths and melt pool widths. Results further demonstrate that in situ indirect control of solidification microstructure is possible through direct melt pool dimension control.
TL;DR: In this paper, Li7La3Zr2O12 (LLZO) pellets with a grain size of 100-200 μm and a relative density of 94% were prepared by conventional solid-state processing at a sintering temperature of 1100 °C, 130 °C lower than previously reported.
Abstract: Al-substituted Li7La3Zr2O12 (LLZO) pellets with a grain size of 100–200 μm and a relative density of 94% were prepared by conventional solid-state processing at a sintering temperature of 1100 °C, 130 °C lower than previously reported. Morphological features and the presence of impurities were evaluated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). Femtosecond Laser Induced Breakdown Spectroscopy (LIBS) was used to visualize the distribution of impurities. The results suggest that chemical composition of the powder cover strongly affects morphology and impurity formation, and that particle size control is critical to densification. These properties, in turn, strongly affect total ionic conductivity and interfacial resistance of the sintered pellets.
TL;DR: In this article, the authors demonstrate that the local demagnetization factor decreases as the grain size is reduced, which explains the higher coercivity and lower temperature dependence of magnetization in magnetization reversals with smaller grain sizes.
TL;DR: In this article, it was shown that the dominant factor for extra grain refinement by alloying was due to the effect of solute-matrix atomic-size mismatch and modulus interaction on the mobility of edge dislocations.
TL;DR: In this paper, the authors found that either grain size or porosity can individually affect the stress at which new cracks are initiated at different cooling stages, and that slowly cooled rock specimens have longer stable crack propagation regions than rapidly cooled rocks.
13 Jan 2014-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, a rule of mixtures based approach for determining the characteristic length of the microstructure for heterogeneous weld metal was introduced, and the proposed grain size parameter, the volume-weighted average grain size, was measured experimentally for nine structural steel weld metals and two base materials.
Abstract: The strength of polycrystalline metals increases with a decrease in grain size according to the Hall–Petch relationship. However, heterogeneous microstructures deviate from this relationship depending on the distribution of grain sizes. This paper introduces a rule of mixtures based approach for determining the characteristic length of the microstructure for heterogeneous weld metal. The proposed grain size parameter, the volume-weighted average grain size, is measured experimentally for nine structural steel weld metals and two base materials. The weld metals are found to have a large variety of grain size distributions that are noticeably broader than those of the base material due to differences in phase contents. The results show that the volume-weighted average grain size is able to capture the influence of grain size distribution on the strength of welded structural steel. Based on the experimental results, a modified Hall–Petch relationship is formulated for the strength prediction of heterogeneous microstructures. The modified relationship is also found to be applicable to data from the literature.
TL;DR: In this article, it was shown that 90° domain wall motion during both strong (above coercive) and weak (below coercive) electric fields is greatest at these intermediate grain sizes, correlating with the enhanced permittivity and piezoelectric properties observed in BaTiO3.
Abstract: The dielectric and piezoelectric properties of ferroelectric polycrystalline materials have long been known to be strong functions of grain size and extrinsic effects such as domain wall motion. In BaTiO3, for example, it has been observed for several decades that the piezoelectric and dielectric properties are maximized at intermediate grain sizes (≈1 μm) and different theoretical models have been introduced to describe the physical origin of this effect. Here, using in situ, high-energy X-ray diffraction during application of electric fields, it is shown that 90° domain wall motion during both strong (above coercive) and weak (below coercive) electric fields is greatest at these intermediate grain sizes, correlating with the enhanced permittivity and piezoelectric properties observed in BaTiO3. This result validates the long-standing theory in attributing the size effects in polycrystalline BaTiO3 to domain wall displacement. It is now empirically established that a doubling or more in the piezoelectric and dielectric properties of polycrystalline ferroelectric materials can be achieved through domain wall displacement effects; such mechanisms are suggested for use in the design of new ferroelectric materials with enhanced properties.
TL;DR: In this paper, the empirical Hall-Petch relationship mathematically describes grain boundary strengthening and provides guidance for a straightforward way to produce stronger materials by increasing the average crystallite grain size.
TL;DR: In this paper, a finite element (FE) model for orthogonal machining of Inconel 718 alloy and prediction of the microstructure changes during the process is presented.
20 Oct 2014-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, the effect of prior austenite grain size on martensite-austenite (M-A) constituent during the second pass reheating and its consequent influence on impact toughness was investigated.
Abstract: Structure–mechanical property relationship studies were carried out on Gleeble simulated intercritically reheated coarse-grained heat affected zone (ICCGHAZ) of 700 MPa linepipe steel microalloyed with Nb. The design of experiments was aimed at varying reheat temperature in the first pass to obtain different coarse grain size in the HAZ. This enabled the study of the effect of prior austenite grain size on martensite–austenite (M–A) constituent during the second pass reheating and its consequent influence on impact toughness. We elucidate here the role of phase transformation and the fraction, size, shape, distribution, and carbon content of M–A constituent on impact toughness. The data suggests that the fraction of M–A constituent is not influenced by grain size, but the size of M–A constituent is influenced by the prior austenite grain size, which consequently governs toughness. Coarse austenite grain size increases the size of M–A constituent and lowers the HAZ toughness. Coarse austenite grain associated with coarse M–A constituent along grain boundary is the dominant factor in promoting brittle fracture. The combination of fine prior austenite grain size and smaller M–A constituent is favorable in obtaining high toughness. Good toughness is obtained on refining the prior austenite grain size in the CGHAZ during first pass and hence ICCGHAZ in the second pass.
TL;DR: In this article, the effect of nanograins on the reduction in lattice thermal conductivity can surpass that of nanoprecipitates at the intermediate temperature range, while simultaneously suppressing bipolar effect.
Abstract: We present nanocrystalline PbS, which was prepared using a solvothermal method followed by spark plasma sintering, as a promising thermoelectric material The effects of grains with different length scales on phonon scattering of PbS samples, and therefore on the thermal conductivity of these samples, were studied using transmission electron microscopy and theoretical calculations We found that a high density of nanoscale grain boundaries dramatically lowered the thermal conductivity by effectively scattering long-wavelength phonons The thermal conductivity at room temperature was reduced from 25 W m−1 K−1 for ingot-PbS (grain size >200 μm) to 23 W m−1 K−1 for micro-PbS (grain size >04 μm); remarkably, thermal conductivity was reduced to 085 W m−1 K−1 for nano-PbS (grain size ∼30 nm) Considering the full phonon spectrum of the material, a theoretical model based on a combination of first-principles calculations and semiempirical phonon scattering rates was proposed to explain this effective enhancement The results show that the high density of nanoscale grains could cause effective phonon scattering of almost 61% These findings shed light on developing high-performance thermoelectrics via nanograins at the intermediate temperature range Thermoelectric materials that transform waste heat generated by equipment or buildings into electricity are emerging as an important green energy technology Currently, researchers are trying to improve thermoelectric substances by embedding within them nanoscale precipitates that allow these materials to capture more heat An international team led by Jiaqing He from the South University of Science and Technology of China has now discovered a way to improve this process by systematically introducing nanoscale crystal structure defects, or ‘nanograins’, into lead sulfide (PbS) particles Their approach tripled the thermoelectric performance of this low-cost mineral from its bulk state without introducing charge-disrupting centers commonly associated with nanoscale precipitates Detailed analysis revealed that the densely packed nanograins trap heat by scattering solid-state vibrations, or phonons, while simultaneously suppressing ‘bipolar’ interactions between charge carriers that can diminish thermoelectric power We report here on the effects of grains of PbS with different length scales on thermal conductivity reduction and bipolar effect ‘suppression’ through macro-properties/microstructure analysis We found that nanograins can achieve the above goals simultaneously Combining experimental results and theoretical calculations, we found that the effect of nanograins on the reduction in lattice thermal conductivity can surpass that of nanoprecipitates Improved properties corresponding to the lowest lattice thermal conductivity in a PbQ (Q=Te, Se, S) system (05 W m K−1 at 923 K) and the highest ZT value in PbQ nanocrystalline materials were achieved by the nanograin method
TL;DR: In this article, the effect of grain size distribution (sorting coefficient ranging from 1.5 to 1.03), grain size (average grain size ranging from 0.75 to 2.25mm), and the heterogeneities of different mineral grains (quartz, K-feldspar, plagioclase) on the onset of cracking were examined.
Abstract: Crack initiation in uniaxial compressive loading of rocks occurs well before the peak strength is reached. The factors that may influence the onset of cracking and possible initiating mechanisms were explored using a discrete element numerical approach. The numerical approach was based on grain-based model that utilized the Voronoi tessellation scheme to represent low porosity crystalline rocks such as granite. The effect of grain size distribution (sorting coefficient ranging from 1.5 to 1.03), grain size (average grain size ranging from 0.75 to 2.25 mm), and the heterogeneities of different mineral grains (quartz, K-feldspar, plagioclase) on the onset of cracking were examined. The modelling revealed that crack initiation appears to be a tensile mechanism in low porosity rocks, and that shear cracking along grain boundaries is only a prominent mechanism near the peak strength. It was also shown that the heterogeneity introduced by the grain size distribution had the most significant effect on peak strength and crack initiation stress. The peak strength ranges from 140 to 208 MPa as the grain size distribution varies from heterogeneous to uniform, respectively. However, the ratio of crack initiation to peak stress showed only minor variation, as the heterogeneity decreases. The other factors investigated had only minor effects on crack initiation and peak strength, and crack initiation ratio.
TL;DR: In this paper, the effect of Y-doping on the microstructure and electrical properties of PBLZST anti-ferroelectric (AFE) ceramics has been investigated.
TL;DR: In this paper, the dependence of ionic conductivity on the grain size of a lamellar block copolymer electrolyte was studied, using a freeze-dried mixture of poly(styrene)-block-poly(ethylene oxi...
Abstract: A systematic study of the dependence of ionic conductivity on the grain size of a lamellar block copolymer electrolyte was performed. A freeze-dried mixture of poly(styrene)-block-poly(ethylene oxi...
TL;DR: In this article, the grain boundary energy is derived and systematically studied in terms of temperature, grain size, concentration and solute segregation for binary systems of 44 solvents and 52 solutes, using readily available elemental data, such as moduli and liquid enthalpy of mixing.
TL;DR: In this paper, the effect of sintering conditions and concomitant microstructure of dental zirconia (ZrO 2 ) ceramics on their low-temperature degradation (LTD) behavior remains unclear.
TL;DR: In this article, the area ratios between the first, second, and third order peaks of ZnS identified as the T2(LO) mode decrease with increasing grain size, attributed to changes in the bandgap energy from quantum confinement due to the varying grain size between the films/particles.
Abstract: Near-resonant Raman scattering measurements of zinc sulfide nanoparticles and thin films have been made and correlated to grain and particle size, respectively, using a 325 nm wavelength excitation source. The area ratios between the first, second, and third order peaks of ZnS identified as the T2(LO) mode decrease with increasing ZnS grain size. This is an effect attributed to changes in the bandgap energy from quantum confinement due to the varying grain size between the films/particles, as noted by a shift in the room temperature photoluminescence emission corresponding to the free exciton emission energy. While Raman scattering spectroscopy is typically limited to identification of phases and their crystalline properties, it is possible to attain more than such straightforward information by calibrating the spectral features to variations between sets of samples. These results open the possibility of making a quantitative grain size estimation in ZnS thin films and nanostructures, as well as in other material systems where ZnS may be expected as a secondary phase, such as Cu2ZnSnS4. Additionally, more commonly used excitation wavelengths for Raman scattering, such as 514 and 532 nm, are shown to be of limited use in characterizing ZnS thin films due to the extremely low Raman scattering efficiency of ZnS in films with sub-micron thicknesses.
TL;DR: In this paper, the influence of grain boundary segregation on the strength of a nanostructured austenitic stainless steel was investigated, and it was shown that grain boundary segregations can lead to significant enhancement of the yield stress.