Microstructure Of Single Crystals Research Articles

0.98(K0.5Na0.5)NbO3–0.02(Bi0.5Na0.5)(Zr0.85Sn0.15)O3 Single Crystals Grown by the Seed-Free Solid-State Crystal Growth Method and Their Characterization

(K0.5Na0.5)NbO3-based single crystals are of interest as high-performance lead-free piezoelectric materials, but conventional crystal growth methods have some disadvantages such as the requirement for expensive Pt crucibles and difficulty in controlling the composition of the crystals. Recently, (K0.5Na0.5)NbO3-based single crystals have been grown by the seed-free solid-state crystal growth method, which can avoid these problems. In the present work, 0.98(K0.5Na0.5)NbO3–0.02(Bi0.5Na0.5)(Zr0.85Sn0.15)O3 single crystals were grown by the seed-free solid-state crystal growth method. Sintering aids of 0.15 mol% Li2CO3 and 0.15 mol% Bi2O3 were added to promote single crystal growth. Pellets were sintered at 1150 °C for 15–50 h. Single crystals started to appear from 20 h. The single crystals grown for 50 h were studied in detail. Single crystal microstructure was studied by scanning electron microscopy of the as-grown surface and cross-section of the sample and revealed porosity in the crystals. Electron probe microanalysis indicated a slight reduction in K and Na content of a single crystal as compared to the nominal composition. X-ray diffraction shows that the single crystals contain mixed orthorhombic and tetragonal phases at room temperature. Raman scattering and impedance spectroscopy at different temperatures observed rhombohedral–orthorhombic, orthorhombic–tetragonal and tetragonal–cubic phase transitions. Polarization–electric field (P–E) hysteresis loops show that the single crystal is a normal ferroelectric material with a remanent polarization (Pr) of 18.5 μC/cm2 and a coercive electrical field (Ec) of 10.7 kV/cm. A single crystal presents d33 = 362 pC/N as measured by a d33 meter. Such a single crystal with a large d33 and high Curie temperature (~370 °C) can be a promising candidate for piezoelectric devices.

On-Substrate Fabrication of CsPbBr3 Single-Crystal Microstructures via Nanoparticle Self-Assembly-Assisted Low-Temperature Sintering.

The growth of all-inorganic perovskite single-crystal microstructures on substrates is a promising approach for constructing photonic and electronic microdevices. However, current preparation methods typically involve direct control of ions or atoms, which often depends on specific lattice-matched substrates for epitaxial growth and other stringent conditions that limit the mild preparation and flexibility of device integration. Herein, we present the on-substrate fabrication of CsPbBr3 single-crystal microstructures obtained via a nanoparticle self-assembly assisted low-temperature sintering (NSALS) method. Sintering guided by self-assembled atomically oriented superlattice embryos facilitated the formation of single-crystal microstructures under mild conditions without substrate dependence. The as-prepared on-substrate microstructures exhibited a consistent out-of-plane orientation with a carrier lifetime of up to 82.7 ns. Photodetectors fabricated by using these microstructures exhibited an excellent photoresponse of 9.15 A/W, and the dynamic optical response had a relative standard deviation as low as 0.1831%. The discrete photosensor microarray chip with 174000 pixels in a 100 mm2 area showed a response difference of less than 6%. This method of nanoscale particle-controlled single crystal growth on a substrate offers a perspective for mild-condition preparation and in situ repair of crystals of various types. This advancement can propel the flexible integration and widespread application of perovskite devices.

Thermally stable strong texture in additively manufactured cobalt-based superalloys

Cobalt-based γ'-strengthened superalloy exhibits tremendous potential in numerous fields, such as power generation equipment and the aerospace industry. The characteristics of laser powder bed fusion technology have aligned well with the demands for superalloy's strong texture or single crystal microstructure. However, the post-heat treatment processes often disrupt the initial microstructure of the as-printed parts due to unavoidable recrystallization. In this study, we obtained a strong <101> texture in an as-printed Co-5Al-14V (at.%) ternary cobalt-based superalloy, which is difficult to achieve in nickel-based superalloys. Intriguingly, this <101> texture remains thermally stable even after aging treatment at a near γ' solvus temperature. Hence, a <101> textured microstructure with an evenly distributed γ'(L12) precipitates was attained, which benefits the improvement of high-temperature creep resistance. The unique microstructure characteristics revealed in this work can be beneficial for the development of cobalt-based superalloys and shed light on the microstructural engineering of other LPBF-manufactured superalloys.

Investigation of columnar to equiaxial transition criterion and solidification conditions for Ni-based superalloy in laser powder bed fusion

Laser powder bed fusion (L-PBF) is a pioneering additive manufacturing (AM) technology for superalloys. The study of the columnar to equiaxial transition (CET) of a superalloy in the L-PBF process is important for controlling the grain morphology of the superalloy and achieving the production of a superalloy with a single-crystal microstructure. This study aims to reveal the fundamental law of CET of superalloys in the L-PBF process. An algorithm was developed to successfully extend the application of the CET model to cover the solidification rate interval of the L-PBF process, and the accuracy of the model was experimentally verified. The extended CET model indicated that the critical temperature gradient from columnar epitaxial growth to equiaxial growth does not increase monotonically with increasing solidification rate but exhibits a peak. The temperature gradient at the solidification interface of the L-PBF molten pool was greater than this peak. In addition, the solidification mode of the alloy remained epitaxial after adjusting the laser power and scanning rate, because adjusting these two parameters exerted a weak effect on the temperature gradient at the solidification interface of the L-PBF molten pool. The excessively high temperature gradient of L-PBF also makes it difficult to change the solidification mode from epitaxial to equiaxial at preheating temperatures below 1000 K. Changes in the morphology of the molten pool due to the increased temperature of the deposited layer are of more concern in terms of microstructure formation. The results of this study are expected to provide guidance for designing process parameters for optimizing the microstructures of superalloys fabricated using L-PBF.

Ion Aggregation-Induced Crystallization Strategy for Printing Morphology-Controllable Perovskite Single-Crystal Arrays and Integrated Devices

Perovskite single crystals have emerged as attractive structures for integrated devices. However, due to uncontrolled crystallization, the versatile and scalable fabrication of arrays of perovskite single crystals with controllable morphology and location remains a great challenge. Here, we report a facile printing strategy to controllably fabricate perovskite single-crystal arrays with well-defined morphology and location. Through modulating perovskite precursor ion aggregation in microdroplets by controlling the temperature of substrates, perovskite single-crystal arrays with controlled morphologies are fabricated, which can be selectively integrated on silicon for high-performance-type tailored laser arrays. More importantly, integrated perovskite single-crystal microstructures are printed, showing an efficiently coupling property with well-maintained intrinsic characters of the original signals. This strategy enables fully inkjet-printed morphology-controllable perovskite single-crystal arrays and functional coupling-structure arrays, which offers new opportunities for integrated photonic and optoelectronic devices.

Complete mineralization of phenolic compounds in visible-light-driven photocatalytic ozonation with single-crystal WO3 nanosheets: Performance and mechanism investigation

Complete mineralization of phenolic compounds into CO2 and H2O is desirable for removing them in wastewater, but it is challenging due to the generated recalcitrant intermediates, which requires highly effective advanced oxidation process with proper catalysts. Herein, we found that single-crystal WO3 nanosheets (NSs)-based photocatalytic ozonation (PCO) can realize complete mineralization of phenols (phenol and 2-chlorophenol) under visible light irradiation. Almost 100% mineralization ratio of phenols was achieved through WO3 NSs-based PCO system within short time. By comparing their performances with those of polycrystalline WO3 nanoparticles, detecting and analyzing the intermediates, identifying the dominant radicals and conducting some electrochemical characterizations, the origin of superior catalytic activity of WO3 NSs was uncovered, the mineralization pathways and the overall mechanism were proposed. The excellent PCO performance of WO3 NSs was contributed to their nanosheet morphology with single-crystal microstructure and good dispersion, which can provide continuous interior channels for the photogenerated charge transport from the bulk to surface of WO3 NSs and enough active sites for the surface reactions triggered by these charges. This work puts forwards new ideas to design highly active photocatalysts for PCO and helps deepen understanding of the catalytic mechanism of PCO.

Machine learning reinforced microstructure-sensitive prediction of material property closures

This study addresses a machine learning (ML)-reinforced strategy to build both linear and non-linear property closures for metallic materials. A property closure is a closed space of material properties that contains all possible values of the closure variables. The material properties of metals are significantly dependent on the underlying microstructure texture. Here, the polycrystalline material is expressed by the Orientation Distribution Function (ODF) that relates to the volume densities of the crystallographic orientations. Theoretically, the property closures of volume-averaged material properties can be derived using single-crystal microstructure solutions; however, this theory is not valid for non-linear properties. Therefore, we use an ML-reinforced strategy to generate both linear and non-linear material property closures using the Linear Regression (LR) and Artificial Neural Network (ANN) method with Bayesian Regularization. The closures for material properties, such as the elastic stiffness parameters and critical buckling load, are generated for Titanium, Magnesium, and Aluminum. The outcomes of the ML surrogate models for these properties are compared to each other. The results demonstrate that the ANN model with Bayesian regularization is capable of predicting both linear and non-linear material properties with almost 100% accuracy. However, the linear regression algorithm is found to be not as accurate as of the Bayesian inference for the non-linear property even though it provides similar accuracy as ANN for the linear property. Therefore, ANN with Bayesian regularization is utilized for predicting the property closure of critical buckling load, which is a non-linear property.

Role of pre-existing dislocations on the shock compression and spall behavior in single-crystal copper at atomic scales

Large-scale molecular dynamics simulations are carried out to investigate the role of pre-existing dislocation loops on the shock-induced deformation and spall behavior of single-crystal Cu microstructures. This study investigates the role of loading orientation and initial density of pre-existing dislocations on the decay behavior of the Hugoniot elastic limit (HEL) as well as the damage nucleation and growth behavior during spall failure of single-crystal Cu systems. The results suggest that the presence of pre-existing dislocation loops results in a decrease of the shock wave velocity and a substantial decay of the HEL values. The increased decay behavior is attributed to the decrease in the density of Shockley partials at the shock front as the shock wave travels through the metal as compared to defect-free initial single-crystal microstructures. Similarly, the presence of pre-existing dislocations is observed to result in increased values for the spall strength as compared to defect-free initial single-crystal microstructures wherein a higher density of dislocations results in the nucleation of a larger number of smaller voids. The decay behavior of the HEL values is observed to have a power–law dependence on the shock propagation distance with the initial dislocation density as a parameter. Similarly, a power–law dependence is also proposed for the number of voids nucleated at the spall plane with a dependence on the size of the voids as well as the initial density of dislocations. The evolution of microstructure (dislocation densities and voids) for the various loading orientations and initial densities of dislocations is discussed.

DenMap single crystal solidification structure feature extraction: Automation and application

A fully automatic, standardised, and validated dendritic core detection algorithm, DenMap, is applied to analyse CMSX-4® and CMSX-10® single crystal microstructure. Through the utilisation of novel imaging techniques such as Gaussian deconvolution and minimisation-based thresholding, the newly enhanced version of DenMap demonstrates a significant improvement in detection accuracy and range of applications. The results indicate dendrites preferentially form channels, with array disorder increasing with height from the chill plate. A new twisting phenomenon has been observed that may have significant impact on the development of low and high angle grain boundaries and porosity formation within the microstructure. DenMap provides the foundation for 3D reconstruction methodologies and advanced pattern recognition techniques. This new standardised approach will facilitate the development of machine learning algorithms and eventually AI for single crystal solidification studies, particularly for the purpose of process optimisation.

Microstructural Investigations of Ni-Based Superalloys by Directional Solidification Quenching Technique.

The improvement of the mechanical properties of Ni-based superalloys is achieved in most cases by modifying the chemical composition. Besides that, the processing can be modified to optimize the as-cast microstructure with regard to the mechanical properties. In this context, the present study highlights the solidification mechanism of several Ni-based superalloys by conducting experiments using a modified, laboratory-scale Bridgman-Stockbarger furnace. In that context, the single-crystal rods are partially melted, directionally solidified and quenched sequentially. Several characterization methods are applied to further analyze the influence of the alloying elements and the variation of the withdrawal rate on the as-cast microstructure. Four stages of solidification are distinguished whereby the morphology observed in the different stages mainly depends on the cooling rate and the local concentration of the carbide forming elements. The effect of carbide precipitation and the effect on the as-cast microstructure is investigated by employing energy dispersive X-ray spectrometry (EDX) and electron backscatter diffraction (EBSD) analysis techniques. A local polycrystalline structure is observed in the single-crystal system as consequence of the influence of the carbon content and the cooling rate. The present work aims to develop strategies to suppress the formation of the polycrystalline structure to maintain the single-crystal microstructure.

Dendritic Packing and Porosity Formation within Single Crystal CMSX-4® Ni-Base Superalloy

Specialised imaging techniques are instrumental for extracting the wealth of information locked within as-cast microstructures. Recently, a novel characterisation tool (Shape-Limited Primary Spacing) was developed for determining primary dendrite arm spacing (PDAS), dendritic packing pattern and porosity formation within single crystal microstructures. In this work, the algorithm is applied to a single crystal CMSX-4® Ni-base superalloy to investigate the relationship between dendritic packing pattern and condensation porosity. It is concluded that porosity formation is reduced when regular organised dendritic packing patterns form. This work elucidates new unknown relationships between packing and porosity and may help to develop improved process-property models for defect prediction.

Characterization of Microstructure, Precipitations and Microsegregation in Laser Additive Manufactured Nickel-Based Single-Crystal Superalloy.

In this study, the microstructure, precipitations, and microsegregation in the laser additive manufactured thin-wall structure of a single-crystal superalloy are synthetically characterized. The influence of a subsequent heat treatment on the microstructure and precipitations is discussed. The results show that under the given processing conditions, the single-crystal microstructure is regenerated perfectly with small misorientation angles in the thin-wall structure. The crystal morphology shows obvious diversity and instability with the incremental height of thin-wall structure. With the increase of manufacturing height, both the primary and secondary dendritic arm spacings of epitaxial columnar dendrites first increase rapidly and then reach a dynamic balanced state. The distribution of precipitations and pores keeps symbiosis in the interdendritic region and shows periodic band characteristic with high density in the band region and low density in the inner region of plate layers. The microsegregation of element atoms in the microstructure shows a three-dimensional network distribution. The concentration of element atoms keeps good consistency with high value in the three-dimensional network and nearly standard value in the outside region. The subsequent heat treatment process contributes to the occupation of as-processed pores by the expanded mature precipitations with good blocky shape. Further optimization of the heat treatment process for improving the lattice coherency of precipitated γ’ phase and γ matrix in the laser additive manufactured single-crystal superalloy is needed and valuable.

Applications of pattern recognition for dendritic microstructures

The Primary Dendrite Arm Spacing (PDAS) is the most important length scale in directionally solidified single crystal alloys. It determines the propensity for defect formation, solution heat treatment times and mechanical properties of the material. In this work a CMSX4 single crystal sample was imaged under a Scanning Electron Microscope (SEM). An automatic dendritic mapping (DenMap) algorithm using Normalised Cross-Correlation (NCC) is combined with Shape-Limited Primary Spacing (SLPS) to determine the local nearest neighbour dendrites and the corresponding dendritic packing. The algorithm located the dendritic centres, calculated the local PDAS, packing pattern, and relationship between PDAS and packing pattern for 256 dendrites in 1 minute 10 seconds. This is the first fully automatic method to produce a clear Gaussian distribution of local PDAS and packing pattern; thus, enabling rapid data gathering potential for single-crystal microstructures.

Out-of-plane transport in ZrSiS and ZrSiSe microstructures

A recent class of topological nodal-line semimetals with the general formula MSiX (M = Zr, Hf and X = S, Se, Te) has attracted much experimental and theoretical interest due to their properties, particularly their large magnetoresistances and high carrier mobilities. The plateletlike nature of the MSiX crystals and their extremely low residual resistivities make measurements of the resistivity along the [001] direction extremely challenging. To accomplish such measurements, microstructures of single crystals were prepared using focused ion beam techniques. Microstructures prepared in this manner have very well-defined geometries and maintain their high crystal quality, verified by the observations of quantum oscillations. We present magnetoresistance and quantum oscillation data for currents applied along both [001] and [100] in ZrSiS and ZrSiSe, which are consistent with the nontrivial topology of the Dirac line-node, as determined by a measured π Berry phase. Surprisingly, we find that, despite the three dimensional nature of both the Fermi surfaces of ZrSiS and ZrSiSe, both the resistivity anisotropy under applied magnetic fields and the in-plane angular dependent magnetoresistance differ considerably between the two compounds. Finally, we discuss the role microstructuring can play in the study of these materials and our ability to make these microstructures free-standing.

Oxygen-18 Tracer Measurements of Anion Diffusion in Uranium Dioxide Thin Films

Oxygen 18 was used as a tracer to quantify anion diffusion in thin-film UO2 using secondary ion mass spectroscopy to measure one-dimensional depth profiles. Both thermal and heavy ion bombardment (1.8 MeV Kr+) treatments were employed over a temperature range from 295 K to 623 K. Textured and single-crystal thin-film samples were grown using reactive-gas magnetron sputtering at ambient temperature. Both microstructures resulted in similar thermal activation energies, Ea = 0.2 eV. This activation energy is significantly lower than the known value for intrinsic anion vacancy-self diffusion in stoichiometric UO2.00 (Ea = 2.5 eV). We attribute this to an interstitialcy mechanism in our hyper-stoichiometric films. The activation energy for irradiated textured films was approximately half that of thermal diffusion, consistent with the chemical rate theory. The opposite was true for the single-crystal microstructure (irradiated Ea = 0.36 eV). This may be due to radiation-induced changes in the microstructure. The mixing parameter was quantified on the anion sublattice as well, ξ = 2.1 ± 0.2 A5eV−1.

Shape memory effect of dual-phase NiMnGaTb ferromagnetic shape memory alloys

The evolution of microstructure, reverse martensitic transformation and the correlated influence on shape memory effect was investigated in as-cast and directionally solidified dual-phase NiMnGaTb alloys. The directionally solidified alloys exhibit single-crystal microstructure, preferred dendrite microstructure, and mussy dendrite microstructure in the specimens grown at a withdrawal rate (v) of 10, 50 and 200, and 1000 μm/s, respectively. The precipitates dispersively distribute in the martensite matrix for the directionally solidified alloys. With the refined grains and particle precipitates, the reverse martensitic transformation gradually shifts to lower temperatures and the temperature span is significantly broadened. The directional solidification technology can effectively enhance the strains recovered due to shape memory effect (esme) and decrease the compressive stress required to trigger the reorientation of twins (σ) via the realization of preferred orientation, while the maximal esme and minimum σ can reach 4.96% and 14 MPa in v = 10 μm/s specimens, respectively. The formation of dendrite morphology degrades the shape memory strain, and esme decreases with the growth of secondary dendritic arms.

Microstructure stability of γ′ + β Ni–Al coated single-crystal superalloy N5 annealed at 1100 °C

γ′ + β Binary-phase Ni–Al coatings were prepared on second-generation single-crystal superalloy Rene N5 (N5) substrates by electron beam physical vapor deposition. Inter-diffusion behavior between coatings and substrates at 1100 °C and its effects on microstructure stability of the substrates were investigated. A 3-μm-thick substrate diffusion zone (SDZ) layer forms beneath the coating/substrate interface after 5-h heat treatment. The SDZ layer is composed of γ′-Ni3Al phases and needle-like precipitates growing along (100) $$\left\langle {110} \right\rangle$$ or (100) $$\left\langle { 1 {\bar{\text{1}}\text{0}}} \right\rangle$$ direction. After 100-h annealing, the thickness of the SDZ layer increases to ~17 μm, much lower than that of the single β-NiAl-coated substrate, while keeping single-crystal microstructure.

Erosion mechanisms during abrasive waterjet machining: Model microstructures and single particle experiments

The erosion mechanisms during abrasive waterjet (AWJ) machining have been examined for a variety of materials. However, no systematic study has considered the effect of the microstructure–property relationship on the erosion mechanisms in metals. In this work, the influence of microstructure and mechanical properties on the erosion mechanisms is investigated using AWJ controlled-depth milling and single particle impact experiments performed on nanocrystalline, microcrystalline and single crystal nickel samples. The resulting footprints and subsurface microstructure evolution were analysed using advanced characterization techniques. The erosion rate of the target metal is found to correlate positively with grain size and negatively with hardness but this correlation is nonlinear. The subsurface microstructure of the single crystal and microcrystalline are altered, while only the texture of the nanocrystalline nickel is modified. The grain refinement mechanism observed in microcrystalline and single crystal microstructure is elucidated by electron backscatter diffraction. It proceeds by the generation of dislocations under severe plastic deformation, which transforms into subgrains before forming new grains under further strain. Therefore, severe plastic deformation induced by AWJ machining leads to surface nanocrystallization and induces substantial subsurface work-hardening, as revealed by nanoindentation tests and confirmed by single particle impacts, with the consequence that the erosion rate decreases with decreasing grain size. This work clarifies the erosion mechanisms in pure metals and highlights the dynamic nature of AWJ machining as a result of the complex interplay between microstructure, mechanical properties and material removal mechanisms, providing new insights into AWJ controlled-depth milling technique.

Features of the structural states of KNbO3 single crystals before and after fast-neutron irradiation

Neutron irradiation is a unique tool for forming new structural states of ferroelectrics, which cannot be obtained by conventional methods. The inf luence of the irradiation by two doses of fast neutrons (F = 1 × 1017 and 3 × 1017 cm–2) on the structure and properties of KNbO3 single crystals has been considered for the first time. The developed method for taking into account the experimental correction to the diffuse scattering has been used to analyze the structural changes occurring in KNbO3 samples at T = 295 K and their correlations with the behavior of dielectric and nonlinear optical characteristics. The irradiation to the aforementioned doses retains the KNbO3 polar structure, shifting Т С to lower temperatures and significantly affecting only the thermal parameters and microstructure of single crystals. Neutron irradiation with small atomic displacements provides a structure similar to the high-temperature modification of an unirradiated KNbO3 crystal.

Laws of Formation of Nanostructured Condition in Single Crystals of Highly Refractory Alloy VZhM4-VI Microalloyed with Lanthanum

Results of metallographic studies of the microstructure of single crystals of alloy VZhM4-VI with residual content of lanthanum varied from 0.0002 to 0.026% after a complete heat treatment and after testing for long-term strength are presented. The mechanism of the action of lanthanum on the formation of nanostructure in single crystals of VZhM4-VI is studied. It is shown that creation of nanostructure in the alloy raises its high-temperature properties.