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Showing papers on "Grain growth published in 2018"


Journal ArticleDOI
TL;DR: In this paper, a large-aspect-ratio grain-based thin film with low trap density was developed for high-performance inorganic perovskite CsPbI2Br solar cells.
Abstract: It is imperative to develop a large-aspect-ratio grain-based thin film with low trap density for high-performance inorganic perovskite CsPbI2Br solar cells. Herein, by using Mn2+ ion doping to modulate film growth, we achieved CsPbI2Br grains with aspect ratios as high as 8. It is found that Mn2+ ions insert into the interstices of the CsPbI2Br lattice during the growth process, leading to suppressed nucleation and a decreased growth rate. The combination aids in the achievement of larger CsPbI2Br crystalline grains for increased JSC values as high as 14.37 mA/cm2 and FFs as large as 80.0%. Moreover, excess Mn2+ ions passivate the grain boundary and surface defects, resulting in effectively decreased recombination loss with improved hole extraction efficiency, which enhances the built-in electric field and hence increases VOC to 1.172 V. As a result, the champion device achieves stabilized efficiency as high as 13.47%, improved by 13% compared with only 11.88% for the reference device.

336 citations


Journal ArticleDOI
TL;DR: In this article, nano-scale MnSiO3 Rhodonite particles were observed in 316L stainless steel fabricated by selective laser melting (SLM) and these oxide particles act as Zener-pinning particles providing dragging force to retard grain growth, and enhance strength through the comparison of Hall-Petch effects between SLM 316L and commercial wrought 316L under 1200°C isothermal heat treatment.

210 citations


Journal ArticleDOI
TL;DR: In this article, two scanning strategies were adopted to study the effect of scanning strategy on grain structure and crystallographic texture of selective laser melted (SLM) Inconel 718.

201 citations


Journal ArticleDOI
TL;DR: In this article, the influence of various process parameters on densification behavior, surface morphology, microstructure, and mechanical properties of selective laser melted (SLMed) maraging steel have been investigated.

194 citations


Journal ArticleDOI
TL;DR: The dimensionality of the as-fabricated perovskite film reveals an evolution from 2D, hybrid 2D/3D to 3D structure when the doping level of PEAI/PbI2 ratio varies from 2 to 0.05, which suggests the coexistence of low-dimensional perovSKite (PEA2 MAn-1 Pbn I3n+1 ) with various values of n after vapor reaction.
Abstract: The fabrication of multidimensional organometallic halide perovskite via a low-pressure vapor-assisted solution process is demonstrated for the first time. Phenyl ethyl-ammonium iodide (PEAI)-doped lead iodide (PbI2 ) is first spin-coated onto the substrate and subsequently reacts with methyl-ammonium iodide (MAI) vapor in a low-pressure heating oven. The doping ratio of PEAI in MAI-vapor-treated perovskite has significant impact on the crystalline structure, surface morphology, grain size, UV-vis absorption and photoluminescence spectra, and the resultant device performance. Multiple photoluminescence spectra are observed in the perovskite film starting with high PEAI/PbI2 ratio, which suggests the coexistence of low-dimensional perovskite (PEA2 MAn-1 Pbn I3n+1 ) with various values of n after vapor reaction. The dimensionality of the as-fabricated perovskite film reveals an evolution from 2D, hybrid 2D/3D to 3D structure when the doping level of PEAI/PbI2 ratio varies from 2 to 0. Scanning electron microscopy images and Kelvin probe force microscopy mapping show that the PEAI-containing perovskite grain is presumably formed around the MAPbI3 perovskite grain to benefit MAPbI3 grain growth. The device employing perovskite with PEAI/PbI2 = 0.05 achieves a champion power conversion efficiency of 19.10% with an open-circuit voltage of 1.08 V, a current density of 21.91 mA cm-2 , and a remarkable fill factor of 80.36%.

151 citations


Journal ArticleDOI
TL;DR: A357 alloy was manufactured by selective laser melting (SLM) and subjected to different heat treatments, by considering the stress relief step as critical, to establish the microstructure-mechanical property relationship and model the yield strength.

146 citations


Journal ArticleDOI
30 Nov 2018-Science
TL;DR: The colossal grain growth is achieved by minimizing contact stresses, resulting in a preferred in-plane and out-of-plane crystal orientation, and is driven by surface energy minimization during the rotation of the crystal lattice followed by “consumption” of neighboring grains.
Abstract: Single-crystal metals have distinctive properties owing to the absence of grain boundaries and strong anisotropy. Commercial single-crystal metals are usually synthesized by bulk crystal growth or by deposition of thin films onto substrates, and they are expensive and small. We prepared extremely large single-crystal metal foils by "contact-free annealing" from commercial polycrystalline foils. The colossal grain growth (up to 32 square centimeters) is achieved by minimizing contact stresses, resulting in a preferred in-plane and out-of-plane crystal orientation, and is driven by surface energy minimization during the rotation of the crystal lattice followed by "consumption" of neighboring grains. Industrial-scale production of single-crystal metal foils is possible as a result of this discovery.

145 citations


Journal ArticleDOI
TL;DR: In this paper, laser powder bed fusion of aluminum alloy (AA) 6061 used powder bed heating at 500°C in combination with other experimentally determined processing parameters to produce crack-free components.
Abstract: During solidification of many so-called high-performance engineering alloys, such as 6000 and 7000 series aluminum alloys, which are also unweldable autogenously, volumetric solidification shrinkage and thermal contraction produces voids and cracks. During additive manufacturing processing, these defects can span the length of columnar grains, as well as intergranular regions. In this research, laser powder bed fusion (LPBF) of aluminum alloy (AA) 6061 used powder bed heating at 500 °C in combination with other experimentally determined processing parameters to produce crack-free components. In addition, melt-pool banding, which is a normal solidification feature in LPBF, was eliminated, illustrating solidification process modification as a consequence of powder bed heating. Corresponding microindentation hardness and tensile testing of the as-fabricated AA6061 components indicated an average Vickers hardness of HV 54, and tensile yield, ultimate strength, and elongation values of 60 MPa, 130 MPa, and 15%, respectively. These mechanical properties and those of heat treated parts showed values comparable to annealed and T6 heat treated wrought products, respectively. X-ray diffraction and optical microscopy revealed columnar grain growth in the build direction with the as-fabricated, powder-bed heated product microstructure characterized by [100] textured, elongated grains (∼ 25 μm wide by 400 μm in length), and both intragranular and intergranular, noncoherent Al-Si-O precipitates which did not contribute significantly to the mechanical properties. The results of this study are indicative that powder bed heating may be used to assist with successful fabrication of AA6061 and other alloy systems susceptible to additive manufacturing solidification cracking.

136 citations


Journal ArticleDOI
TL;DR: In this paper, a three-dimensional numerical model is developed to evaluate fundamentals of grain structure evolution during metal additive manufacturing, and the results obtained show that specific solidification conditions in selective laser melting and grain selection associated with competitive nature of grain growth promote the development of coarse columnar grains with the most favorable growth direction misaligned with the build direction.

131 citations


Journal ArticleDOI
TL;DR: Ce-doped strontium bismuth titanate (SrBi4Ti4O15, SBT) powders were prepared by glycine-nitrate process at 400-500°C and the ceramics were sintered at 980°C as discussed by the authors.
Abstract: Ce-doped strontium bismuth titanate (SrBi4Ti4O15, SBT) powders were prepared by glycine-nitrate process at 400-500°C and the ceramics were sintered at 980°C. The phase composition, morphology and electric properties were investigated. It was found that the calcined powders consist of a single phase SBT and the calcination temperature is lower than that for the conventional solid state method. The morphology of Cedoped SBT is flake-like and the layer size decreases with Ce-addition from 12 to 2μm confirming that the addition of Ce3+ inhibits grain growth. The Curie temperature of Ce-doped SBT increased for about 20°C compared to the pure SBT. The tan δ was ~0.005 at 35°C and even below 0.05 up to 400°C. The temperature coefficient of dielectric constant was ~0.012 and the rate of frequency change was 0.01-0.04, which indicated the high stability of dielectric properties of the Ce-doped SBT. Impedance analysis revealed that the conduction mechanism of the Ce-doped SrBi4Ti4O15 ceramics is mainly grain conduction.

126 citations


Journal ArticleDOI
TL;DR: In this article, the effects of heat treatment and HIP on the microstructures of SLM processed specimens were analyzed by atom probe tomography, showing that the bi-modal grain size distribution observed in the as-processed condition can be maintained even after a heat treatment, due to a high density of intragranular Al3(ScxZr1-x) precipitates, and various other particles pinning the grain boundaries.
Abstract: Sc- Zr-modified Al-Mg alloy, processed by selective laser melting, offers excellent properties in the as processed condition, due to the formation of a desirable microstructure. As in conventional processing, such alloys are age hardenable, thereby precipitating a high fraction of finely dispersed coherent Al3(Scx Zr1-x) intermetallics, which serve for the improvement of the mechanical strength. Electron backscatter diffraction measurements and transmission electron microscopy were used to determine the effects of heat treatment and HIP on the microstructures of SLM processed specimens. In addition, the chemistry and number density of Al3Sc particles was analysed by atom probe tomography. The results show that the bi-modal grain size distribution observed in the as-processed condition can be maintained even after a heat treatment, due to a high density of intragranular Al3(ScxZr1-x) precipitates, and various other particles pinning the grain boundaries. A HIP post-processing can lead to grain growth in certain coarser grained areas, probably due to a local imbalance between driving and dragging forces, hence higher defect density and fewer pinning precipitates. Applying a heat treatment results in an increase of the density of ≤5 nm sized intragranular Al3(Scx Zr1-x) particles by a factor of 4–6, reaching 3·1023 m−3 to 5·1023 m−3.

Journal ArticleDOI
TL;DR: The microstructure and tensile properties of HfNbTaTiZr after cold working and annealing were investigated in this article, where cold work was introduced by axial compression followed by rolling resulting in a total thickness reduction of 89% without any evidence of cracking.
Abstract: The microstructure and tensile properties of HfNbTaTiZr after cold working and annealing were investigated. Cold work was introduced by axial compression followed by rolling resulting in a total thickness reduction of 89 pct without any evidence of cracking. The cold-worked material retained a single-phase microstructure and had a room temperature tensile yield stress σ0.2 = 1438 MPa, peak true stress σp = 1495 MPa, and true fracture strain ef = 5 pct. Annealing at 800 °C for up to 256 hours resulted in the precipitation of Nb and Ta rich particles with a BCC crystal structure inside a Hf-and-Zr-enriched BCC matrix. The second phase particles nucleated heterogeneously inside deformation bands and slip lines and coarsened during annealing. Analysis of the coarsening behavior suggested that kinetics were controlled by the diffusion of Nb and Ta. In the two-phase material, σ0.2 and σp decreased from 1159 to 1071 MPa and from 1174 to 1074 MPa, respectively, with an increase in particle diameter from 0.18 to 0.72 μm, while ef remained between 5 and 8 pct. Full recrystallization and normal grain growth, with the activation energy of 238 kJ/mol and activation volume of 5.3 to 9.6 m3/mol, occurred during annealing above 1000 °C. After heat treatment at this temperature, the alloy was characterized by a single-phase BCC structure with σ0.2 = 1110 to 1115 MPa, σp = 1160 to 1195 MPa, and ef = 12 to 19 pct with the maximum values attained after annealing for 1 hour.

Journal ArticleDOI
TL;DR: In this paper, single-layer and multi-layer builds were produced by laser powder bed fusion to study microstructure formation in rapid cooling and its evolution during repeated metal deposition.

Journal ArticleDOI
TL;DR: In this paper, a three-dimensional model for the prediction of dendritic growth for use with powder bed fusion (PBF) additive manufacturing is presented. But the model is validated by reproducing experimental grain structures of Inconel 718 test specimens manufactured by selective electron beam melting and the model represents the cutting edge of grain structure simulation in PBF and enables a reliable numerical prediction of appropriate beam parameters for arbitrary applications.

Journal ArticleDOI
TL;DR: In this article, the influence of grain growth inhibitors on the mechanical properties of WC-Co hardmetals is evaluated and graphically presented, in the absence of a binder phase and in alternative binder systems.
Abstract: The rapid coarsening of ultrafine and nanocrystalline WC powders during consolidation has challenged the production of bulk nano-structured hardmetals for decades. In response, it has become common practice to alloy WC-Co composites with carbides of transition metals in order to inhibit the WC grain growth during sintering. This review discusses various WC grain growth modes, details of grain growth inhibition and reports on the influences of grain growth inhibitors on the microstructure and properties of hardmetals found in current literature. The effectiveness of grain growth inhibitors to suppress WC grain growth during liquid-phase sintering is compared, and their limitations, with regards to sintering of nano-crystalline materials, are described. The effectiveness of grain growth inhibitors at under-eutectic temperatures, in the absence of a binder phase and in alternative binder systems, is analyzed. Currently recognized mechanisms of grain growth inhibition, on the basis of segregating thin films of carbide phases at WC/Co grain boundaries, are summarized and analyzed. The influence of grain growth inhibitors on the mechanical properties of WC-Co hardmetals are evaluated and graphically presented.

Journal ArticleDOI
TL;DR: In this paper, the possibility to produce Ni-base superalloy single crystals by selective electron beam melting (S-EBM) was demonstrated by a tight control of the processing conditions without requiring a grain selector or a crystal seed.

Journal ArticleDOI
TL;DR: In this paper, the enhanced and inhibited grain growth obtained using these field-assisted densification techniques were reported for different ceramic nanoparticle systems and related to their respective densification mechanisms.
Abstract: Spark plasma and flash sintering process characteristics together with their corresponding sintering and densification mechanisms and field effects were briefly reviewed. The enhanced and inhibited grain growth obtained using these field-assisted densification techniques were reported for different ceramic nanoparticle systems and related to their respective densification mechanisms. When the densification is aided by plastic deformation, the kinetics of grain growth depends on the particles’ rotation/sliding rate and is controlled by lattice and pipe diffusion. When the densification is aided by spark, plasma, and the particles’ surface softening, grain growth kinetics is controlled by viscous diffusion and interface reactions. Grain growth in both cases is hierarchical by grain rotation, grain cluster formation and sliding, as long as the plastic deformation proceeds or as long as plasma exists. Densification by diffusion in a solid state via defects leads to normal grain growth, which takes over at the final stage of sintering. Various field effects, as well as the effect of external pressure on the grain growth behaviour were also addressed.


Journal ArticleDOI
TL;DR: In this article, X-ray diffraction (XRD) analysis and microstructural investigations verified the formation of BN nano-platelets, TiN and SiO 2 in the Si 3 N 4 -doped TiB 2 sample via chemical reactions of Si3 N 4 with the oxide impurities (TiO 2 and B 2 O 3 ) present on the surface of TiB2 particles.

Journal ArticleDOI
TL;DR: In this paper, the influence of temperature gradient and cooling rate on the heterogeneous nucleation rate and growth kinetics of equiaxed grains have been studied quantitatively, and it is shown that under the same cooling rate, the nucleation rates of grains decreases with increasing temperature gradient.

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the processing-structure-property relations of a precipitation hardened aluminum alloy 2219 (AA2219) material with respect to deposition orientations and build layers.
Abstract: Issues with rapid grain growth, hot cracking and poor ductility have hindered the additive manufacturing and repair of aluminum alloys. Therefore, this is the first investigation to spatially correlate the processing-structure-property relations of a precipitation hardened aluminum alloy 2219 (AA2219) material with respect to deposition orientations and build layers. The AA2219 material was processed by a high deposition rate (1000 cm3/h) solid-state additive deposition process known as Additive Friction Stir Deposition or MELD. An equiaxed grain morphology was observed in the three orientations, where Electron Backscatter Diffraction (EBSD) identified a layer-dependent texture with a strong torsional fiber A texture in the top of the build transitioning to weaker textures in the middle and bottom layers. Interestingly, the tensile behavior reflected the texture layer-dependence with tensile strength increasing from the bottom to the top of the deposition. However, there were no statistically significant differences in hardness measured from the top to the bottom of the deposition. Furthermore, no orientation dependence on mechanical properties was observed for compression and tension specimens tested at quasi-static (0.001/s) and high (1500/s) strain rate. Transmission Electron Microscopy (TEM) determined a lack of θ′ precipitates in the as-deposited cross-section, therefore resulting in no precipitation strengthening.

Journal ArticleDOI
TL;DR: An ultrastrong lanthanum-doped nanocrystalline austenitic steel that is thermally stable and radiation-tolerant and usually candidates for nuclear reactors since they do not easily swell under irradiation is made.
Abstract: Nanocrystalline (NC) metals are stronger and more radiation-tolerant than their coarse-grained (CG) counterparts, but they often suffer from poor thermal stability as nanograins coarsen significantly when heated to 0.3 to 0.5 of their melting temperature (Tm). Here, we report an NC austenitic stainless steel (NC-SS) containing 1 at% lanthanum with an average grain size of 45 nm and an ultrahigh yield strength of ~2.5 GPa that exhibits exceptional thermal stability up to 1000 °C (0.75 Tm). In-situ irradiation to 40 dpa at 450 °C and ex-situ irradiation to 108 dpa at 600 °C produce neither significant grain growth nor void swelling, in contrast to significant void swelling of CG-SS at similar doses. This thermal stability is due to segregation of elemental lanthanum and (La, O, Si)-rich nanoprecipitates at grain boundaries. Microstructure dependent cluster dynamics show grain boundary sinks effectively reduce steady-state vacancy concentrations to suppress void swelling upon irradiation. Weaker ferritic/matensitic steels rather than stronger austenitic steels are usually candidates for nuclear reactors since they do not easily swell under irradiation. Here, the authors make an ultrastrong lanthanum-doped nanocrystalline austenitic steel that is thermally stable and radiation-tolerant.

Journal ArticleDOI
TL;DR: In this paper, a multiscale model is developed to investigate the evolution mechanisms of site-specific grain structures during additive manufacturing of metallic alloys, using the selective electron beam melting (SEBM) fabrication of Ti-6Al-4V as an example.

Journal ArticleDOI
TL;DR: In this paper, a comprehensive review on inherent anisotropic features of transition metal diboride (MB 2 ) and their implementation for tailoring the microstructure and properties of MB 2 -based Ultra-high temperature Ceramics (UHTCs) is presented.
Abstract: This is the first comprehensive review on inherent anisotropic features of transition metal diboride (MB 2 ) and their implementation for tailoring the microstructure and properties of MB 2 -based Ultra-high temperature Ceramics (UHTCs). The emphasis is on the processing approaches, microstructures, and properties of self-reinforced and/or textured MB 2 -based composites with elongated MB 2 grains. The crystal structure characteristics and grain growth behaviour of MB 2 are also critically reviewed. Benefiting from the tailored microstructure, the MB 2 -based ceramics exhibit some improved properties. Considering the success of Si 3 N 4 ceramics in the field of structural ceramics, it is expected that the potential MB 2 -based ceramic composites with abundant elongated MB 2 grains, textured structures, and controlled grain boundaries would possess improved fracture toughness, thermal shock resistance, and reliable high-temperature properties, which are desired for their practical applications. Accordingly, microstructure designing and tailoring provide an important perspective for the future development of UHTCs.

Journal ArticleDOI
TL;DR: In this paper, an age-hardening alloy, AXM100, which combines precipitation hardening and grain refinement by secondary-phase pinning, was designed via thermodynamic calculation.

Journal ArticleDOI
TL;DR: In this article, the second nearest neighbor modified embedded atom method (MEAM) potentials were used to simulate the spontaneous homogeneous nucleation from aluminum (Al) melt, and the crystal structure of nuclei, critical nucleus size, critical temperature, induction time and nucleation rate were determined.
Abstract: Homogeneous nucleation from aluminum (Al) melt was investigated by million-atom molecular dynamics (MD) simulations utilizing the second nearest neighbor modified embedded atom method (MEAM) potentials. The natural spontaneous homogenous nucleation from the Al melt was produced without any influence of pressure, free surface effects and impurities. Initially isothermal crystal nucleation from undercooled melt was studied at different constant temperatures, and later superheated Al melt was quenched with different cooling rates. The crystal structure of nuclei, critical nucleus size, critical temperature for homogenous nucleation, induction time, and nucleation rate were determined. The quenching simulations clearly revealed three temperature regimes: sub-critical nucleation, super-critical nucleation, and solid-state grain growth regimes. The main crystalline phase was identified as face-centered cubic (fcc), but a hexagonal close-packed (hcp) and an amorphous solid phase were also detected. The hcp phase was created due to the formation of stacking faults during solidification of Al melt. By slowing down the cooling rate, the volume fraction of hcp and amorphous phases decreased. After the box was completely solid, grain growth was simulated and the grain growth exponent was determined for different annealing temperatures.

Journal ArticleDOI
TL;DR: In this article, the character, morphology and distribution of inclusions have been studied in additively manufactured 17-4PH stainless steel parts produced from gas-atomized powder by selective laser melting, and a combination of advanced electron microscopy techniques has been used to show that such parts contain oxide inclusions ranging from a few nm to tens of μm across.


Journal ArticleDOI
TL;DR: Pt-Au, a proposed stable alloy consisting of two noble metals, is shown to exhibit extraordinary resistance to wear and is the first instance of an all-metallic material exhibiting a specific wear rate on the order of 10-9 mm3 N-1 m-1, comparable to diamond-like carbon and sapphire.
Abstract: Recent work suggests that thermally stable nanocrystallinity in metals is achievable in several binary alloys by modifying grain boundary energies via solute segregation. The remarkable thermal stability of these alloys has been demonstrated in recent reports, with many alloys exhibiting negligible grain growth during prolonged exposure to near-melting temperatures. Pt-Au, a proposed stable alloy consisting of two noble metals, is shown to exhibit extraordinary resistance to wear. Ultralow wear rates, less than a monolayer of material removed per sliding pass, are measured for Pt-Au thin films at a maximum Hertz contact stress of up to 1.1 GPa. This is the first instance of an all-metallic material exhibiting a specific wear rate on the order of 10-9 mm3 N-1 m-1 , comparable to diamond-like carbon (DLC) and sapphire. Remarkably, the wear rate of sapphire and silicon nitride probes used in wear experiments are either higher or comparable to that of the Pt-Au alloy, despite the substantially higher hardness of the ceramic probe materials. High-resolution microscopy shows negligible surface microstructural evolution in the wear tracks after 100k sliding passes. Mitigation of fatigue-driven delamination enables a transition to wear by atomic attrition, a regime previously limited to highly wear-resistant materials such as DLC.

Journal ArticleDOI
TL;DR: The experimental results showed that the originally recommended heat treatment process for cast and wrought materials was not effective for SLM-processed specimens and the HIP+ direct aging process was the most effective method among the post- processes to improve the creep behavior at 650 °C.
Abstract: The selective laser melting (SLM) process was used to fabricate an Alloy718 specimen. The microstructure and creep properties were characterized in both the as-built and post-processed SLM materials. Post-processing involved several heat treatments and a combination of hot isostatic pressing (HIP) and solution treatment and aging (STA) to homogenize the microstructure. The experimental results showed that the originally recommended heat treatment process, STA-980 °C, for cast and wrought materials was not effective for SLM-processed specimens. Obvious grain growth structures were obtained in the STA-1180 °C/1 h and STA-1180 °C/4 h specimens. However, the grain size was uneven since heavy distortion or high-density dislocation formed during the SLM process, which would be harmful for the mechanical properties of SLM-fabricated materials. The HIP+ direct aging process was the most effective method among the post-processes to improve the creep behavior at 650 °C. The creep rupture life of the HIP+ direct aging condition approached 800 h since the HIP process had the benefit of being free of pores, thus preventing microcrack nucleation and the formation of a serrated grain boundary.