Competitive Grain Research Articles

Single crystal growth and characterization of Ba2ScNbO6 – A novel substrate for BaSnO3 films

Large single crystals of the double-perovskite Ba2ScNbO6 were grown from the melt for the first time. With a lattice parameter at room temperature of 4.11672(1) Å, this cubic double-perovskite has an excellent lattice match to BaSnO3, PbZr0.9Ti0.1O3, LaInO3, BiScO3, and other perovskites of contemporary interest. Differential thermal analysis showed that Ba2ScNbO6 melts at 2165 ± 30 °C in an inert atmosphere. Competitive grain growth was visualized by energy dispersive Laue mapping. X-ray diffraction rocking curve measurements revealed full width at half maximum values between 21 and 33 arcsec for 002 and 004 reflections. The crystals were sufficiently large to yield (1 0 0)-oriented single-crystal substrates with surface areas as large as 10 × 10 mm2.

Revealing competitive columnar grain growth behavior and periodic microstructural banding in additively manufactured Ti-6Al-4 V parts by selective electron beam melting

Powder-bed fusion additive manufacturing of Ti-6Al-4 V has been of tremendous interest in both academia and industry. As the two ubiquitous microstructural features, columnar grain and microstructural banding in selective electron beam melting of Ti-6Al-4 V are systematically studied by experimental and simulation methods. Three basic build geometries (i.e. V-, I- and A-shaped parts) are employed to study the columnar grain growth behavior. We find that columnar prior β-Ti grains grow epitaxially and competitively by following the classic competitive grain growth model in additive manufacturing of Ti-6Al-4 V. It results in increasingly stronger crystallographic texture with the rising build height. We also observe the consistently occurring microstructural banding (∼100 µm in period) normal to build direction due to the overlapped heat affected zones that are formed in the layerwise thermal cyclic process. In addition, a large quantity of sub-columnar grains exist within the microstructure for selective electron beam melted Ti-6Al-4 V. It is demonstrated that the periodic microstructural banding determines the appearance of sub-columnar grains which may affect the competition behavior for the growth of columnar grains.

Competitive grain growth and dendrite morphology evolution in selective laser melting of Inconel 718 superalloy

Microstructure control is a great challenge for selective laser melting (SLM) mainly because of nonuniform and ultra-rapid heating and cooling in the process. It is related to the control of dendrite morphology evolution and competitive grain growth by adjusting the process parameters of SLM. In this study, a multiscale simulation approach, which combines the finite element method (FEM) and the phase field (PF) method, is performed to investigate the dendrite morphology and stray grain growth under various SLM parameters for Inconel 718 superalloy. The cooling characteristics of melt pool, including the cooling rate, thermal gradient/solidification ratio (G/R), and the slope of the solidification front are investigated. Meanwhile, experiment verification adopting the manufacturing parameters identical to the simulation is performed. It is shown that simultaneously increasing or decreasing the laser power and scan speed leads to a drastic change of melt pool aspect ratio (MPAR), and this in turn affects the competitive growth of stray grains. Overall, a larger MPAR is found to suppress the growth of stray grains more effectively. Also, the experimental results are found to agree well with the simulation results. Three mechanisms of stray grain growth are revealed from the simulation results, and their roles are analzyed. This study provides important insights into the microstructure control in the laser additive manufacturing processes.

Patterned Wettability Surface for Competition‐Driving Large‐Grained Perovskite Solar Cells

AbstractNovel photovoltaic perovskite solar cells (PSCs) with high‐efficient photovoltaic property are largely in thrall to the uncertain perovskite grain size and inevitable defects. Here, inspired by the competitive growth between tree and grass in the forest system, a competitive perovskite grain growth approach via micro‐contact print (MicroCP) method (CD disk as templates) for printing wettability‐patterned substrate is proposed, aiming to achieve large‐grained perovskite and avoid discontinuous perovskite films caused by the low wettability of substrates. A MicroCP process is employed to construct a patterned wettability surface for the perovskite competitive growth mechanism on the electrode surface. This approach modifies the substrates quickly, ensures the uniform coverage of perovskite due to the function of ‐NH2 and Pb2+ bonds, and converts the perovskite films composed of small grains and pinholes into high‐quality perovskite films, free from pinholes and made up of large grains, resulting in efficiencies over 20% for the MicroCP PSCs.

Comprehensive study on mechanisms for grain morphology evolution and texture development in powder bed fusion with electron beam of Co–Cr–Mo alloy

Grain morphology, size, and growth direction are crucial in determining the performances of metallic implant components. Understanding the effects of the manufacturing characteristics of powder bed fusion with electron beam (PBF-EB), an additive manufacturing process, on microstructure formation and anisotropy development during solidification is essential to achieve flexible microstructure control. In this study, PBF-EB was employed to fabricate a Co–Cr–Mo alloy and the grain morphology and texture formation with different process parameters were analyzed by experimental characterization with the aid of computational thermal–fluid dynamics simulations. It was found that the epitaxial growth with resulting columnar grain and near-cubic texture tended to be dominant in the solidification process, due to competitive grain growth and the heat flow characteristics in the snake-scanning strategy. However, the molten pool connection between adjacent melt tracks resulted in the random orientation of <001> in the horizontal plane, producing a fiber-like texture. In addition, nucleation and new grain growth rather than extensive epitaxial growth could be achieved by manipulating the molten pool geometry and overlap between adjacent melt tracks. Increasing the slope of the solid/liquid interface of the molten pool and decreasing the remelting fraction of adjacent melt tracks favored the formation of new grains with random orientations to restrict the extensive epitaxial growth of the columnar grains and attenuate the microstructural anisotropy.

On microstructural homogenization and mechanical properties optimization of biomedical Co-Cr-Mo alloy additively manufactured by using electron beam melting

The electron beam melting (EBM), a layer-by-layer additive manufacturing (AM) technique, has been recently utilized for fabricating metallic components with complex shape and geometry. However, the inhomogeneity in microstructures and mechanical properties are the main drawbacks constraining the serviceability of the EBM-built parts. In the present study, we found remarkable microstructural inhomogeneity along build direction in the EBM-built Co-based alloy, owing to the competitive grain growth and subsequent isothermal γ-fcc → ε-hcp phase transformation, which affects the corresponding tensile properties significantly. Then, we succeeded in eliminating the inhomogeneities, modifying the phase structures and refining grain sizes via comprehensive post-production heat treatment regimes, which provides a valuable implication for improving the reliabilities of AM-built metals and alloys. The Co-based alloy can be selectively transformed into predominant ε or predominant γ phase by the regime, and the grains were refined to 1/10 of the initial sizes by repeated heat treatment. Finally, we investigated the tensile properties and fracture behaviors of the alloy before and after each heat treatment. The γ → ε strain-induced martensitic transformation is the major deformation mode of the γ phase, meanwhile the formation of stripped ε phase at {111}γ habit planes contributed to a good combination of strength and ductility. Nevertheless, the ε phase was deformed mainly by (0001)ε <11 2¯0 >ε basal and {1 1¯00}ε <11 2¯0 >ε prismatic slip systems, exhibiting very limited ductility and strength. In addition, the ε grains act as secondary hardening factor in the samples consisting of dual γ/ε phase, leading to a non-uniform deformation behavior.

Quantifying competitive grain overgrowth in polycrystalline ZnO thin films

The grain size evolution of polycrystalline thin films, which form by competitive grain overgrowth as commonly interpreted using the van der Drift model, is understood to follow a power-law scaling of the average grain size d with film thickness h, i.e. d ∝ hα. While simulations have identified a growth exponent α = 0.4 for three-dimensional growth, previous experimental studies did not confirm this value, instead finding α values in the range of ∼0.5–0.7. Here we study competitive grain overgrowth using a system of ZnO thin films grown by low-pressure metal–organic chemical vapor deposition. We present quantitative data on the evolution of grain size and orientation across the thickness of thin films, obtained by automated crystal orientation mapping of “double-wedge” transmission electron microscopy samples. The data from a-textured ZnO films, grown under three different conditions, are compared against van der Drift model predictions of self-similarity of the grain size distribution and the power-law scaling. The results are further interpreted by comparing to simulations of faceted polycrystalline film growth, which we adapt to the ZnO system by including idiomorphic growth shapes with a six-fold symmetry and random or biased nuclei orientations. As well as showing the predicted self-similarity of grain size distributions during growth, for the first time our experimental data confirm a power-law growth exponent of α = 0.4, as also predicted by the simulations using randomly oriented nuclei. Nevertheless, interpretation of this result is contingent on the absence of factors such as textured nucleation and renucleation during film growth. Indeed, only one film, grown at a higher ratio of H2O/DEZ precursor gases, displaying random initial nucleation, and minimal grain renucleation during growth, shows a proper conformance to the model nature and predictions.

Quantifying Competitive Grain Overgrowth in Polycrystalline ZnO Thin Films

The grain size evolution of polycrystalline thin films, which form by competitive grain overgrowth as commonly interpreted using the van der Drift model, is understood to follow a power-law scaling of the average grain size d with film thickness h, i.e. d ∝ hα. While simulations have identified a growth exponent α = 0.4 for three-dimensional growth, previous experimental studies did not confirm this value, instead finding α values in the range of ~0.5-0.7. Here we study competitive grain overgrowth using a system of ZnO thin films grown by low-pressure metal-organic chemical vapor deposition. We present quantitative grain data on the evolution of grain size and orientation across the thickness of thin films, obtained by automated crystal orientation mapping of double-wedge transmission electron microscopy samples. The data from a-textured ZnO films, grown under three different conditions, are compared against van der Drift model predictions of self-similarity of the grain size distribution and the power-law scaling. The results are further interpreted by comparing to simulations of facetted polycrystalline film growth, which we adapt to the ZnO system by including idiomorphic growth shapes with a six-fold symmetry and random or biased nuclei orientations. As well as showing the predicted self-similarity of grain size distributions during growth, for the first time our experimental data confirm a power-law growth exponent of α = 0.4, as also predicted by the simulations using randomly oriented nuclei. Nevertheless, interpretation of this result is contingent on the absence of factors such as textured nucleation and renucleation during film growth. Indeed, only one film, grown at a higher ratio of H2O/DEZ precursor gases, displaying random initial nucleation, and minimal grain renucleation during growth, shows a proper conformance to the model nature and predictions.

Gradients of microstructure, stresses and mechanical properties in a multi-layered diamond thin film revealed by correlative cross-sectional nano-analytics

Thin diamond films deposited by chemical vapour deposition (CVD) usually feature cross-sectional gradients of microstructure, residual stress and mechanical properties, which decisively influence their functional properties. This work introduces a novel correlative cross-sectional nano-analytics approach, which is applied to a multi-layered CVD diamond film grown using microwave plasma-enhanced CVD and consisting of a ∼8 μm thick nanocrystalline (NCD) base and a ∼14.5 μm thick polycrystalline (PCD) top diamond sublayers. Complementary cross-sectional 30 nm beam synchrotron X-ray diffraction, depth-resolved micro-cantilever and hardness testing and electron microscopy analyses reveal correlations between microstructure, residual stress and mechanical properties. The NCD sublayer exhibits a 1.5 μm thick isotropic nucleation region with the highest stresses of ∼1.3 GPa and defect-rich nanocrystallites. With increasing sublayer thickness, a 110 fibre texture evolves gradually, accompanied by an increase in crystallite size and a decrease in stress. At the NCD/PCD sublayer interface, texture, stresses and crystallite size change abruptly and the PCD sublayer exhibits the presence of Zone T competitive grain growth microstructure. NCD and PCD sublayers differ in fracture stresses of ∼14 and ∼31 GPa, respectively, as well as in elastic moduli and hardness, which are correlated with their particular microstructures. In summary, the introduced nano-analytics approach provides complex correlations between microstructure, stresses, functional properties and deposition conditions.

Directional solidification casting technology of heavy-duty gas turbine blade with liquid metal cooling (LMC) process

In this work, some important factors such as ceramic shell strength, heat preservation temperature, standing time and withdrawal rate, which influence the formability of directionally solidified large-size blades of heavy-duty gas turbine with the liquid metal cooling (LMC) process, were studied through the method of microstructure analysis combining. The results show that the ceramic shell with medium strength (the high temperature flexural strength is 8 MPa, the flexural strength after thermal shock resistance is 12 MPa and the residual flexural strength is 20 MPa) can prevent the rupture and runout of the blade. The appropriate temperature (1,520 °C for upper region and 1,500 °C for lower region) of the heating furnace can eliminate the wide-angle grain boundary, the deviation of grain and the run-out caused by the shell crack. The holding time after pouring (3-5 min) can promote the growth of competitive grains and avoid a great deviation of columnar grains along the crystal orientation , resulting in a straight and uniform grain structure. In addition, to avoid the formation of wrinkles and to ensure a smooth blade surface, the withdrawal rate should be no greater than the growth rate of grain. It is also found that the dendritic space of the blade decreases with the rise of solidification rate, and increases with the enlarging distance between the solidification position and the chill plate.

Grain Morphology and Texture Development in a Co-Cr-Mo Alloy Fabricated by Powder Bed Fusion with an Electron Beam

Grain morphology, size, and growth direction are crucial in determining the performances of metallic implant components. Understanding the effects of the manufacturing characteristics of powder bed fusion with an electron beam (PBF-EB), an additive manufacturing process, on microstructure formation and anisotropy development during solidification is essential to achieve flexible microstructure control. In this study, PBF-EB was employed to fabricate a Co-Cr-Mo alloy and the grain morphology and texture formation with different process parameters were analyzed by experimental characterization with the aid of computational thermal-fluid dynamics simulations. It was found that the epitaxial growth with resulting columnar grain and near-cubic texture tended to be dominant in the solidification process, due to competitive grain growth and the heat flow characteristics in the snake-scanning strategy. However, the molten pool connection between adjacent melt tracks resulted in the random orientation of in the horizontal plane, producing a fiber-like texture. In addition, nucleation and new grain growth rather than extensive epitaxial growth could be achieved by manipulating the molten pool geometry and overlap between adjacent melt tracks. Increasing the slope of the solid/liquid interface of the molten pool and decreasing the remelting fraction of adjacent melt tracks favored the formation of new grains with random orientations to restrict the extensive epitaxial growth of the columnar grains and attenuate the microstructural anisotropy.

Grain Optimization: Optimizing Grain Selection Design in the Single-Crystal Solidification of Ni-Based Superalloys (Crystal Research and Technology 12/2018)

Crystal Research and TechnologyVolume 53, Issue 12 1870022 Cover PictureFree Access Grain Optimization: Optimizing Grain Selection Design in the Single-Crystal Solidification of Ni-Based Superalloys (Crystal Research and Technology 12/2018) First published: 13 December 2018 https://doi.org/10.1002/crat.201870022AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Graphical Abstract This figure represents selection phenomenon of single crystal during directional solidification. The process begins by arbitrarily nucleation of equiaxed grains at the copper chill contact area. Further progress of solidification brings about a columnar structure which is accompanied by competitive grain growth. A single grain finally encompass the upper part after solidification of spiral grain selector has come to an end. More details can be found in article number 1800108 by Fazlollah Sadeghi, Ahmad Kermanpur, and Ebrahim Norouzi. Volume53, Issue12December 20181870022 RelatedInformation

Three-dimensional grain growth during multi-layer printing of a nickel-based alloy Inconel 718

Heterogeneous grain structure is a source of the inhomogeneity in structure and properties of the metallic components made by multi-layer additive manufacturing (AM). During AM, repeated heating and cooling during multi-layer deposition, local temperature gradient and solidification growth rate, deposit geometry, and molten pool shape and size govern the evolution of the grain structure. Here the effects of these causative factors on the heterogeneous grain growth during multi-layer laser deposition of Inconel 718 are examined by a Monte Carlo method based grain growth model. It is found that epitaxial columnar grain growth occurs from the substrate or previously deposited layer to the curved top surface of the deposit. The growth direction of these columnar grains is controlled by the molten pool shape and size. The grains in the previously deposited layers continue to grow because of the repeated heating and cooling during the deposition of the successive layers. Average longitudinal grain area decreases by approximately 80% when moving from the center to the edge of the deposit due to variable growth directions dependent on the local curvatures of the moving molten pool. The average horizontal grain area increases with the distance from the substrate, with a 20% increase in the horizontal grain area in a short distance from the third to the eighth layer, due to competitive solid-state grain growth causes increased grain size in previous layers.

Optimizing Grain Selection Design in the Single‐Crystal Solidification of Ni‐Based Superalloys

AbstractAn attempt is made to model the design of grain selection during single‐crystal solidification of an Ni‐based superalloy by the Bridgman method. Various geometrical designs of the starter block and spiral grain selector are chosen and their effects on crystal orientation of the single‐crystal part are studied. The competitive grain selection is simulated utilizing the cellular automaton finite element module of the ProCAST software. The simulated results are validated against the experimental observation of grain structure along with the crystal orientation measured by the electron backscatter diffraction (EBSD) and rotational orientation X‐ray diffraction (RO‐XRD) techniques. The results show that in order to obtain the single crystal with a minimum deviation from longitudinal axis, the height and diameter of the starter block as well as the height and the take‐off angle of the spiral should be optimized. It is found that the diameter/height ratio of the starter block should be less than 0.6 to obtain a well‐oriented single crystal prior to entrance of solidification front into the spiral. The height of starter block is found to be more influential than its diameter in selecting the suitable single grain.

Evaluation of carbohydrate properties and end-use quality of hexaploid triticale and its relationship to solvent retention capacity

Triticale is a competitive grain in foraging, but it has only limited usage in the food industry due to its relatively unfavorable end-use quality. Improvement of carbohydrate characteristics could be an option to partially compensate for its weaker protein quality and to have positive influences on technological properties. In the present study, carbohydrate composition and its relation to baked bread properties of triticale whole meal flour were studied, and utilization of the solvent retention capacity (SRC) method to predict bread-making quality was evaluated. Carbohydrate composition of the analysed triticale genotypes exhibited notable genetic variance with advantageous carbohydrate composition. Many carbohydrate parameters, including damaged starch, total starch, total dietary fiber and sugar concentrations and arabinoxylans levels, influenced the baked bread characteristics and SRC profiles of the triticale. Values for triticale whole meal bread properties were between those of wheat and rye, and triticale genotypes, with bread properties most similar to wheat, showed the same SRC profile as wheat. Therefore, SRC values could be used to predict bread-making quality of triticale genotypes; in particular, based on results of correlation analyses, lactic acid SRC % is a more specific test, for assessing bread-making quality.

Root competition resulting from spatial variation in nutrient distribution elicits decreasing maize yield at high planting density

Nutrient spatial heterogeneity occurs ubiquitously in soil, but its impact on plant growth and competition throughout the whole growth cycle remains unclear. We assessed the effects of different nutrient supply patterns and planting density on maize (Zea mays L.) growth rate, root competitive intensity and grain yield. Maize was grown with homogeneous or heterogeneous nutrient (ammonium and phosphorus) supplies under low and high planting densities in field conditions. Nutrient patches promoted root proliferation and plant growth rate in the short term; however, with nutrient depletion and roots still proliferating in the patch zone, the plant growth rate was surpassed by that of plants in the homogeneous ammonium and phosphorus treatments with time. Consequently, final shoot biomass showed no difference between nutrient supply patterns at a given planting density. Furthermore, the temporal decoupling between root growth and soil nutrient availability induced greater competitive intensity in the heterogeneous than homogeneous treatments between the vegetative stage and anthesis. Under these conditions, heterogeneous ammonium and phosphorus supply markedly decreased grain yield at high planting density. Our results demonstrate that heterogeneous nutrient supply-induced root competition at high planting density modulates the dynamics of plant development and decreases final productivity.

Competitive grain boundary segregation of phosphorus and carbon governs delamination crack in a ferritic steel

The present study aims at unveiling the influence of trace amounts of phosphorus on the macro-scale delamination in a ferritic steel. Two different steels with trace amounts of phosphorus were examined to reveal the cause: One was made by long-time high temperature holding to maximize the phosphorus segregation, and the other one was made by short-time high temperature holding to minimize the phosphorus segregation. The atom probe results at the grain boundaries ahead of the delamination-related cracks provide a strong evidence that phosphorus-enrichment and carbon-depletion for the long-time high temperature holding induces a larger degree of delamination as compared to the short-time high temperature holding. Reasons for the different segregation tendency were discussed by correlating the isothermal holding time at high temperature and the competitive segregation between phosphorus and carbon.

Competitive grain growth during directional solidification of a polycrystalline binary alloy: Three-dimensional large-scale phase-field study

Competitive grain growth during the directional solidification of a polycrystalline binary alloy is investigated by performing systematic three-dimensional large-scale phase-field simulations with the GPU supercomputer TSUBAME2.5 at the Tokyo Institute of Technology. Contrary to two-dimensional investigations, in which an unusual growth of unfavorably oriented (UO) grains has been observed frequently in preference to favorably oriented (FO) ones, the grain selection in the present three-dimensional simulations follows essentially the Walton and Chalmers model, in which FO grains are predominant. The UO dendritic grains persist for longer times than the UO cellular grains, and the FO dendritic grains remain smaller than the FO cellular ones. The change in the number of grains follows a power law, with an exponent that is much lower than that of the Kolmogorov's model describing a purely geometrical growth selection.

Printability and microstructure of the CoCrFeMnNi high-entropy alloy fabricated by laser powder bed fusion

The CoCrFeMnNi high-entropy alloy is a promising candidate for metal additive manufacturing. In this study, 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. CoCrFeMnNi showed good printability with high consolidation and uniform high hardness. It is shown that microstructure in the printed alloy is governed by epitaxial growth and competitive grain growth. As a consequence, a bi-directional scanning pattern without rotation in subsequent layers generates a dominant alternating sequence of two crystal orientations.

Producing Ni-base superalloys single crystal by selective electron beam melting

The possibility to produce Ni-base superalloy single crystals by selective electron beam melting (S-EBM) is demonstrated. The production of single crystals specimens was achieved by a tight control of the processing conditions without requiring a grain selector or a crystal seed. The melting parameters are controlled so as to promote columnar grains and intensify the competitive grain growth.