Columnar Structure Research Articles

A Donor-Acceptor Molecular Octopus and the CLICK Procedure in Columnar Liquid Crystal Phases.

Two star-shaped mesogens with a (meso-tetraphenylporphinato) zinc (II) core and bithiophene conjugated arms with 3,4,5-trisdodecyloxyphenyl periphery were synthesized. One of these molecules was decorated with four fullerenes via an aliphatic spacer. This is the sterically overcrowded compound with an octapodal morphology. The other star lacks the fullerenes and provides free space between the conjugated arms. This mesogen does not aggregate in solution, but in solid state it forms a hexagonal columnar and a highly ordered oblique helical columnar phase, while the octopus molecule assembles in an amorphous solid. Photophysical studies of the octapodal compound in solution and the solid thin film reveal the formation of J-type aggregates, in which the interaction between donors (porphyrin) and acceptors (fullerene) dominates leading to absorption bands in the NIR region of the spectra. The mixture of both compounds results in a self-assembly which is called the Click procedure. Fullerenes of the octopus nanosegregate in the pockets of the star mesogens generating hexagonal columnar structures with a regular stacking along the columnar axis. Thus providing free space is a tool to control the competition between supramolecular interactions and nanosegregation. Such liquid-crystalline donor-acceptor structures may play a role in future LC photovoltaic applications.

Formation conditions of the tungsten porous thin film with pulsed laser deposition under various gas atmosphere

Abstract Pulsed laser deposition (PLD) under gas atmospheres has been used to fabricate thin films for various applications. In this study, PLD was performed under various gas atmospheres (helium, oxygen, and argon) using tungsten (W) to investigate the morphology of thin films. Various types of structures were formed, including uniform, nanoparticles, and columnar structures. In particular, the substrate fabricated at an argon pressure of 100 Pa had a high porosity and a low light reflectance in the 200–1400 nm wavelength range. In addition, it was shown that the growth of the thin film thickness was non-linear with respect to time, and the formation of a fuzz-like structure may be influenced by particle diffusion in the gas phase and on the substrate.

Dual-Wave ZnO Film Ultrasonic Transducers for Temperature and Stress Measurements.

ZnO film ultrasonic transducers for temperature and stress measurements with dual-mode wave excitation (longitudinal and shear) were deposited using the reactive RF magnetron sputtering technique on Si and stainless steel substrates and construction steel bolts. It was found that the position in the substrate plane had a significant effect on the structure and ultrasonic performance of the transducers. The transducers deposited at the center of the deposition zone demonstrated a straight columnar structure with a c-axis parallel to the substrate normal and the generation of longitudinal waves. The transducers deposited at the edge of the deposition zone demonstrated inclined columnar structures and the generation of dominant shear or longitudinal shear waves. Transducers deposited on the bolts with dual-wave excitation were used to study the effects of high temperatures in the range from 25 to 525 °C and tensile stress in the range from 0 to 268 MPa on ultrasonic response. Dependencies between changes in the relative time of flight and temperature or axial stress were obtained. The dependencies can be described by second-order functions of temperature and stress. An analysis of the contributions of thermal expansion, strain, and the speed of sound to changes in the time of flight was performed. At high temperatures, a decrease in the signal amplitude was observed due to the decreasing resistivity of the transducer. The ZnO ultrasonic transducers can be used up to temperatures of ~500 °C.

High-rate Deposition of Crystalline TiHfN Ultra-thin Films by Closed-field Dual-cathode DC Unbalanced Reactive Magnetron Sputtering without External Substrate Heating

High deposition rate titanium hafnium nitride (TiHfN) ultra-thin film deposition was successfully prepared by closed-field dual-cathode DC unbalanced reactive magnetron sputtering. All prepared films were polycrystalline. The morphology and atomic composition of the TiHfN ultra-thin film were characterized by field-emission scanning electron microscopy (FE-SEM) and energy dispersive spectroscopy (EDS). The columnar structure could be promoted by increasing the deposition time. Lastly, the surface-enhanced Raman scattering (SERS) activity was investigated by Rhodamine 6G (R6G) drop-dried TiHfN ultra-thin film surface. The TiHfN ultra-thin films deposited at 20 s were found to have a high SERS activity, whose detection of R6G molecule at 10-5 M. The result could open preliminary studies on ternary transition metal nitride (TTMN) thin films for the alternative plasmonic sensors as SERS chips.

A flexible strategy to fabricate trumpet-shaped porous PDMS membranes for organ-on-chip application.

Porous polydimethylsiloxane (PDMS) membrane is a crucial element in organs-on-chips fabrication, supplying a unique substrate that can be used for the generation of tissue-tissue interfaces, separate co-culture, biomimetic stretch application, etc. However, the existing methods of through-hole PDMS membrane production are largely limited by labor-consuming processes and/or expensive equipment. Here, we propose an accessible and low-cost strategy to fabricate through-hole PDMS membranes with good controllability, which is performed via combining wet-etching and spin-coating processes. The porous membrane is obtained by spin-coating OS-20 diluted PDMS on an etched glass template with a columnar array structure. The pore size and thickness of the PDMS membrane can be adjusted flexibly via optimizing the template structure and spinning speed. In particular, compared to the traditional vertical through-hole structure of porous membranes, the membranes prepared by this method feature a trumpet-shaped structure, which allows for the generation of some unique bionic structures on organs-on-chips. When the trumpet-shape faces upward, the endothelium spreads at the bottom of the porous membrane, and intestinal cells form a villous structure, achieving the same effect as traditional methods. Conversely, when the trumpet-shape faces downward, intestinal cells spontaneously form a crypt-like structure, which is challenging to achieve with other methods. The proposed approach is simple, flexible with good reproducibility, and low-cost, which provides a new way to facilitate the building of multifunctional organ-on-chip systems and accelerate their translational applications.

Morphological and Microbial Diversity of Hydromagnesite Microbialites in Lake Salda: A Mars Analog Alkaline Lake

ABSTRACTLake Salda, a terrestrial analog for the paleolake in Jezero Crater on Mars, hosts active, subfossil, and fossil hydromagnesite microbialites, making it an ideal location to study microbialite formation and subsequent processes. Our understanding of this record is still limited by an incomplete knowledge of the macro‐ and mesoscale morphotypes of microbialites, along with their spatial distribution and correlation with microbial and geochemical processes that influence microbialite formation. In this study, we investigated the spatial distribution, morphotypes, mineralogy, geochemistry, and microbial diversity of the microbialites and identified six distinct zones (Zone I to Zone VI) with major microbialite build‐ups in Lake Salda. Newly identified microbialites were classified based on the macro‐ and mesostructures. Our work shows that the lake contains stromatolites, thrombolites, stromatolitic thrombolites, dendrolites, and microbially induced sedimentary structures. At macroscale, Lake Salda microbialites exhibit hemispheres, stacked domes, and laterally linked columnar structures while minicolumns, knobs, mesoclots, laminae, and botryoidal structures are common at mesoscale. The macro‐ and mesoscale distribution of different microbialite types spatially correlates with microbial community composition and water depth. Deep‐growing microbialites with a low abundance of Cyanobacteria (∼1%–4%) and high abundance of Firmicutes (28%–93%) exhibit steeply convex lamination, producing finger‐like minicolumnar mesostructures. In contrast, shallow‐growing microbialites with a low abundance of Firmicutes (0%–5%) and high abundance of Cyanobacteria (11%–37%) have well‐preserved gently convex millimeter‐scale lamination, resulting in cauliflower mesostructures. Palygorskite ((Mg, Al)2Si4O10(OH)) is identified in the diatom‐rich microbial layer of the deep‐growing microbialites. Regardless of the microbialite types, hydromagnesite and aragonite are present in the extracellular polymeric substance (EPS)‐rich zone of the shallow and deep‐growing microbialites. Overall, environmental changes (e.g., water depth and, accommodation space) play a major role in the formation and spatial distribution of different microbialite morphologies at the macro‐ and mesoscale. Differences in the relative abundance of dominant microorganisms between mesostructured types suggest that mesomorphology may be influenced by changes in microbial diversity. Spatial variations in the microbialite morphotypes, along with the abundant presence of entombed biomass (e.g., mineralized filaments), may indicate areas that have a high potential for the preservation of biosignatures.

Evaluation of friction and wear depth during the cyclic loading on partly melted LPBF particles of LPBF Cu alloy

In this work, the resistance against nanoindentation of the Laser Powder Bed Fusion (LPBF) CuCrZr alloy specimen was studied at the ramp load of 2000–3000 μN on 100 points. It was found that the triangular shape indentation cavity and the pile-up material increased along the diagonal direction. The LPBF CuCrZr alloy contained few partly melted LPBF particles which have three different structures namely columnar, cellular and equiaxed. The nanowear test was conducted on the above three structures. The tribological property of partly melted LPBF particles was analyzed at 2000 μN cyclic load. The wear depth in the partly melted LPBF particles at the columnar structure for cycle-1, cycle-2 and cycle-3 were 836 nm, 918 nm and 980 nm, respectively. The wear depth in the partly melted LPBF particles at the cellular structure for cycle-1, cycle-2 and cycle-3 were 650 nm, 780 nm and 810 nm, respectively. The wear depth in the partly melted LPBF particles at the equiaxed structure for cycle-1, cycle-2 and cycle-3 were 650 nm, 720 nm and 780 nm, respectively. The obtained outcomes were correlated with the tribological behavior of non-defective part. The non-defective part exhibited higher wear resistance than the partly melted LPBF particles. The percentage increase in wear resistance at the columnar non-defective part over partly melted LPBF particles for cycle-1, cycle-2 and cycle-3 were 22.24 %, 20.5 % and 20.4 %, respectively. Similarly, the cellular and equiaxed non-defective parts have higher wear resistance when compared to the partly melted LPBF particles. Moreover, the decrease in Coefficient of friction was observed from one cycle to the next cycle both in the partly melted LPBF particles and non-defective part in all three structures.

Fracture strain improving via structural austenitic stainless steel-coated 22MnB5 plates

A total of 316 austenitic stainless steel (ASS) coatings were deposited on 22MnB5 press-hardening steel plates by varying the substrate bias voltages using a magnetron sputtering technique and quenched in a flat mold with cooling water. The substrate bias current and ion-bombardment energy densities increased up to 0.5 mA/mm2 and 50 J/mm2 at a high voltage of −100 V, and, therefore, the variations in the coatings’ morphologies were owing to the increases in the ion bombardment. The porous bulk pattern appeared in the quenched ASS coating at −100 V, clearly different from the porous columnar structure of the others at −50 and −75 V. The γ-Fe and α-Fe phases were observed from the diffraction peaks, presented the ASS coating and 22MnB5 substrate, respectively. There was no intermetallic compound (IMC) peak detected. In addition, the diffusion layer of the quenched ASS-coated plate was observed, in which its morphology was different from the quenched martensite (M) plate, and proved to be the α-Fe microstructure by the very low α-Fe peak at the standard (110) position. The bright image of TEM and the select area electron diffraction pattern indicated the nanostructure of the quenched ASS coating. The nanohardness of the ASS coating, diffusion layer, and M plate (7.36, 3.60, and 6.25 GPa, respectively) was detected, indirectly proving an α-Fe structure of the diffusion layer. The fracture angles of the 316-coated plates significantly improved from 51.82° at −50 V, 53.77° at −75 V to 57.38° at −100 V, as the ASS structure changed to be porous bulk pattern. The clearance of IMC, coating’s porous bulk pattern, and the low hardness α-Fe diffusion layer benefited for the fracture strain by lengthening the pathway of the crack’s development and blocking the further crack’s propagation, respectively.

Comparative analysis of structural characteristics and thermal insulation properties of ZrO2 thin films deposited via chemical and physical vapor phase processes

ZrO2 is a flagship material for the development of efficient thermal barrier coatings not only for aerospace applications but also on steel molds for plastic injection, especially to produce high quality miniaturized parts of low environmental impacts. In the present study, ZrO2 thin films of thicknesses ranging between 4 and 40 μm were deposited on steel substrates by two key gas phase technologies, magnetron sputtering (PVD) and direct liquid injection metal organic CVD (DLI-MOCVD). The film morphology, structure and chemical composition were comparatively investigated and correlated to their heat transfer properties. All films were nearly stoichiometric and presented a columnar structure mainly formed of the monoclinic crystalline phase. The magnetron sputtering coatings were denser and smoother than the CVD ones, for which highly porous tree-like microstructures were observed without degradation of their mechanical properties. All films exhibited efficient thermal insulation properties, the thickest 40 µm magnetron sputtering coating displaying the most significant reduction in heat transfer. The CVD films provided the highest thermal gradient decrease across their thickness, probably thanks to their higher porosity associated to a multi-angle tree-like columnar structure, both acting as scattering zones of phonons involved in heat flow diffusion. By controlling the local deposition conditions influencing the nucleation and growth mechanisms, the film porous microstructure can thus be tuned to optimize the coating thermal properties.

Influence of Defects and Microstructure on the Thermal Expansion Behavior and the Mechanical Properties of Additively Manufactured Fe-36Ni.

Laser-based powder bed fusion of metals (PBF-LB/M) is a widely used additive manufacturing process characterized by a high degree of design freedom. As a result, near fully dense complex components can be produced in near-net shape by PBF-LB/M. Recently, the PBF-LB/M process was found to be a promising candidate to overcome challenges related to conventional machining of the Fe64Ni36 Invar alloy being well known for a low coefficient of thermal expansion (CTE). In this context, a correlation between process-induced porosity and the CTE was presumed in several studies. Therefore, the present study investigates whether the unique thermal properties of the PBF-LB/M-processed Fe64Ni36 Invar alloy can be tailored by the selective integration of defects. For this purpose, a full-factorial experimental design, representing by far the largest processing window in the literature, was considered, correlating the thermal expansion properties with porosity and hardness. Furthermore, the microstructure and mechanical properties were investigated by scanning electron microscopy and quasi-static tensile tests. Results by means of statistical analysis reveal that a systematic correlation between porosity and CTE properties could not be determined. However, by using specific process parameter combinations, the microstructure changed from a fine-grained fan-like structure to a coarse columnar structure.

Optimizing Selective Laser Melting of Inconel 625 Superalloy Through Statistical Analysis of Surface and Volumetric Defects

This article delves into optimizing and modeling the input parameters for the selective laser melting (SLM) process on Inconel 625. The primary aim is to investigate the microstructure within the interlayer regions post-process optimization. For this study, 100 layers with a thickness of 40 µm each were produced. Utilizing the design of experiments (DOE) methodology and employing the Response Surface Method (RSM), the SLM process was optimized. Input parameters such as laser power (LP) and hatch distance (HD) were considered, while changes in microhardness and roughness, Ra, were taken as the responses. Sample microstructure and surface alterations were assessed via scanning electron microscopy (SEM) analysis to ascertain how many defects and properties of Inconel 625 can be controlled using DOE. Porosity and lack of fusion, which were due to rapid post-powder melting solidification, prompted detailed analysis of the flaws both on the surfaces of and in terms of the internal aspects of the samples. An understanding of the formation of these imperfections can help refine the process for enhanced integrity and performance of Inconel 625 printed material. Even slight directional changes in the columnar dendrite structures are discernible within the layers. The microstructural characteristics observed in these samples are directly related to the parameters of the SLM process. In this study, the bulk samples achieved a microhardness of 452 HV, with the minimum surface roughness recorded at 9.9 µm. The objective of this research was to use the Response Surface Method (RSM) to optimize the parameters to result in the minimum surface roughness and maximum microhardness of the samples.

Effect of sputtering power and thickness ratios on the materials properties of Cu–W and Cu–Cr bilayer thin films using high power impulse magnetron and DC magnetron sputtering

This study investigates the influence of Cu thickness ratios on the structural, morphological, and mechanical properties of sputtered Cu–W and Cu–Cr bilayer thin films. Employing high power impulse magnetron sputtering (HiPIMS), five distinct thickness ratios of 1:3, 3:5, 1:1, 5:3, and 3:1 were analyzed and compared to bilayer films developed using direct current magnetron sputtering (DCMS). The microstructural and surface characteristics of these films were evaluated using x-ray diffraction (XRD), atomic force microscopy, and scanning electron microscopy. Electrical properties were measured using a four-point probe, while mechanical properties were assessed through nanoindentation. Results reveal that increasing Cu thickness in Cu–W and Cu–Cr bilayers inversely affects hardness, grain size, and roughness, highlighting the influence of thickness ratios on film properties. Films with a higher Cu thickness ratio in both Cu–W and Cu–Cr bilayer systems deposited by HiPIMS exhibited lower hardness, smaller grain size, and reduced average roughness. Cross-sectional analysis and XRD confirmed the impact of thickness ratio on crystal phase and microstructure, indicating smoother columnar structures. Specifically, the HiPIMS-deposited Cu–Cr 3-1 film exhibited the lowest resistivity, at 4.77 μΩ cm, and hardness, measuring 8.26 GPa. Moreover, the 1:1 ratio films of Cu–W and Cu–Cr demonstrated hardness values of 13.81 and 11.37 GPa, respectively, which were 1.39 times higher than the films grown by DCMS. Additionally, variations in the bilayer thickness ratio significantly affected the electrical properties of the films. The enhanced properties of HiPIMS films are attributed to the higher peak power density of the target, leading to increased ion energy and deposition of dense grain structures.

Augmenting microstructure and tribological performance of wire arc additive manufactured PH13-8Mo stainless steel via TiC/TiB2 nano-particles incorporation

This study aimed to investigate the effect of TiC and TiB2 nano-inoculants addition on the microstructure and tribological behavior of PH13-8Mo stainless steel processed through wire arc additive manufacturing (WAAM). The incorporation of TiC/TiB2 inoculants demonstrated notable efficiency in mitigating the anisotropic wear and scratch response and enhancing wear resistance observed in the as-printed state. This improvement was ascribed to the refinement of the grain structure, disruption of the columnar structure, and increased content of the retained austenite. Notably, TiB2 inoculation exhibited superior grain refinement and achieved the highest hardness. However, the TiC-inoculated condition demonstrated the best wear resistance, attributed to its excellent combination of hardness and fracture resistance, and a higher contribution of strain-induced martensite transformation during wear testing. Additionally, the implementation of post-printing solutionizing and aging treatment was found to improve the scratch and wear resistance of the alloy, attributed to the formation of nano-sized β-NiAl precipitates. The main wear mechanism observed involved oxidation wear, adhesive wear, and three-body abrasive wear. The findings of this study highlight the significant potential of incorporating ceramic-based nano-particles for improving the wear resistance of WAAM PH13-8Mo components, particularly in demanding applications like injection molding dies where superior abrasion resistance is paramount.

Ultrasonic vibration assisted directed energy deposition of titanium alloy: Microstructure control, strengthening mechanisms and fatigue crack behavior

Large-scale titanium alloy structural components fabricated using direct energy deposition (DED) technology have broad application prospects. However, the columnar grain structures and inhomogeneous microstructure due to the inherent characteristics of additive manufacturing seriously affects the mechanical properties of components. In this study, high-intensity ultrasound vibration was introduced into the wire-arc DED deposition of TC4-DT titanium alloy. The results showed that ultrasonic vibration assistance (UVA) significantly improves the grain structure (refine prior-β grain size and weaken α texture) of the as-deposited component, without introducing additional porosity. Compared to conventional wire-arc DED deposited components, the yield strength and tensile strength of UVA alloys increased by 22.23 % and 15.06 %, respectively. Furthermore, the difference in α grain structure caused by UVA results in different fatigue crack growth behaviors. The disorder orientation of the α laths enhanced the fatigue crack initiation resistance of the wire-arc DED component with UVA. This study shows the excellent mechanical properties of UVA DED fabricated TC4-DT alloy, paving the way for the design, fabrication, and application of large structural components of titanium alloys.

Insight into elongation and strength enhancement of heat-treated LPBF Ni-based superalloy 718 using in-situ SEM-EBSD

This study presents a detailed investigation into the deformation behaviour and the factors affecting the strength and ductility of laser powder bed fusion (LPBF) Ni-based superalloy 718 (Inconel 718) in both as-printed and heat-treated conditions using in-situ SEM + EBSD. The results show that following post-deposition high homogenization temperatures, the columnar grain structure of the as-printed Inconel 718 alloy successfully transformed to a nearly uniform grain size and the subsequent aging processes facilitated the precipitation of strengthening phases (γ′, γ''). The specimen subjected to heat treatment (HT1) exhibited higher strength but lower ductility, while the as-printed specimen shows higher tensile elongation due to high initial dislocation density. In contrast, the heat-treated (HT2) specimen presented a more optimal combination of strength and ductility. The underlying mechanism for the enhanced tensile properties in the HT2 specimen was nearly homogeneous deformation trend across the grains and ultrafine precipitation of hardening phases. In addition, the twin boundaries were stable in the early deformation stages and maintained their orientation. Multiple slip systems were activated at the high-deformation stage, resulting in a high proportion of geometric necessary dislocations. Further, the interaction of deformation-induced dislocation with twin boundaries transformed them into general boundaries such as high and low-angle grain boundaries, ultimately increasing ductility. On contrary, in the HT1 specimen, the deformation inhomogeneity in grains enabled the strain localization at grain boundaries, which ultimately caused the early formation of cracks.

Molten CMAS resistance strategy for PS-PVD TBCs based on laser textured and Al-modified bionic structure

Plasma spray-physical vapor deposition (PS-PVD) is a promising third-generation thermal barrier coatings (TBCs) technique. Feather-like columnar TBCs with excellent strain tolerance and low thermal conductivity can be achieved using PS-PVD. However, molten CMAS (CaO–MgO–Al2O3–SiO2) can penetrate coatings and accelerate PS-PVD TBCs failure due to the feather-like columnar structure. Hence, a strategy is proposed to alleviate molten CMAS corrosion. The super-hydrophobicity structure is fabricated via laser texturing on the surface of PS-PVD TBCs to repel molten CMAS wetting and spreading. Then, a thin layer of the Al-film is deposited on the laser-textured surface. Next, the Al-modified layer is in situ synthesized after vacuum heat treatment, preventing the infiltration of molten CMAS into the TBCs and reducing the coating damage. The results show that the contact angle of laser textured and Al-modified PS-PVD TBCs (LT-Al) at room temperature increased from 12.3° to 168.8°. The wetting and spreading behavior of molten CMAS of as-sprayed (AS), laser textured (LT), and LT-Al coatings is observed in situ at 1230 °C for 1800 s. The LT-Al coatings exhibited excellent CMAS corrosion resistance, attributed to the laser-textured micro-nano structures and Al-modified layer protection. The findings may be an effective approach for solving the disadvantage of PS-PVD feather-like columnar structure TBCs.

Treelike PS-PVD coating: Hierarchical branching by shading and sintering

A structure zone model (SZM) with columnar structures for conventional physical vapor deposition (PVD) has been assigned to plasma spray-physical vapor deposition (PS-PVD) coatings. However, many different comprehensive properties between PS-PVD and conventional PVD coatings reveal the essential different microstructures. In this work, an impact-diffusion model was established based on the impact and diffusion behavior of depositing units. The microstructure formation of PS-PVD coatings was simulated. Results clearly display that the PS-PVD coating shows a novel treelike structure with hierarchical branches rather than a well-known columnar structure. The hierarchical branching is determined by the shadowing effect. Besides, temperature-promoting diffusion of vapor units can generate the in-situ sintering of the coatings. In-situ sintering happens during the deposition process and can govern the branching behavior. A novel treelike SZM is established as an enlargement of the conventional one, marked by successive transformations in branching and densifying. The treelike SZM is experimentally proved by the electron microscopy images and mechanical properties of typical PS-PVD YSZ coatings.

Fatigue analysis of selective laser melting Inconel 718 and the mechanism of ductile–brittle continuous alternating steps during fatigue process

In this paper, Inconel 718 powber fabricated by selective laser melting were studied to explore the mechanical characteristics and microstructures. The results of microstructure show that the material exhibits the columnar, honeycomb, and mixed crystal structures. The microhardness is very uniform, with a average value of 317 HV. Tensile test shows the ultimate tensile strength of 785Mpa, elongation of 4.93%, which is high stress brittle fracture. The fatigue strength of 180 MPa was obtained from the fatigue test. The fatigue life was sensitive to the effect of stress. In the scanning electron microscope observation, small crack propagation was found in the fracture, and a large number of step-like fracture characteristics of repeated transformation of toughness and brittleness were found. The mechanism analysis of the step-like ductile–brittle fracture is carried out, which is divided into four stages. Mathematically analyzing the tough-brittle fracture of the step type, where the crack is subjected to both the stress field around the pore and the stress field at the crack tip on the way to extension, a stress field model for crack propagation around the pore and a fracture toughness formula for transverse and longitudinal propagation of tough-brittle steps applicable to this paper are proposed.

Rossby wave instability and substructure formation in 3D non-ideal MHD wind-launching discs

ABSTRACT Rings and gaps are routinely observed in the dust continuum emission of protoplanetary discs (PPDs). How they form and evolve remains debated. Previous studies have demonstrated the possibility of spontaneous gas rings and gaps formation in wind-launching discs. Here, we show that such gas substructures are unstable to the Rossby wave instability (RWI) through numerical simulations. Specifically, shorter wavelength azimuthal modes develop earlier, and longer wavelength ones dominate later, forming elongated (arc-like) anticyclonic vortices in the rings and (strongly magnetized) cyclonic vortices in the gaps that persist until the end of the simulation. Highly elongated vortices with aspect ratios of 10 or more are found to decay with time in our non-ideal magnetohydrodynamic (MHD) simulation, in contrast with the hydro case. This difference could be caused by magnetically induced motions, particularly strong meridional circulations with large values of the azimuthal component of the vorticity, which may be incompatible with the columnar structure preferred by vortices. The cyclonic and anticyclonic RWI vortices saturate at moderate levels, modifying but not destroying the rings and gaps in the radial gas distribution of the disc. In particular, they do not shut-off the poloidal magnetic flux accumulation in low-density regions and the characteristic meridional flow patterns that are crucial to the ring and gap formation in wind-launching discs. Nevertheless, the RWI and their associated vortices open up the possibility of producing non-axisymmetric dust features observed in a small fraction of PPDs through non-ideal MHD, although detailed dust treatment is needed to explore this possibility.

Modelling grain refinement under additive manufacturing solidification conditions using high performance cellular automata

Despite increasing applications of additively manufactured parts, they still suffer from anisotropic mechanical properties and can experience cracking due to coarse columnar grain structures induced by metal 3D printing. Microstructural control is promising avenue to overcome these challenges, but requires deeper understanding of factors controlling solidification, particularly regarding grain refinement. Addressing this gap, this study explores grain refinement in Al-Cu alloys with a process-microstructure linking cellular automata-finite difference (CAFD) approach supported by single-track laser surface remelting (LSR) experiments. To enhance the understanding of nucleation behaviour in alloys under additive manufacturing solidification conditions, this research analyses the influence of nucleation parameters on the post-LSR microstructures. Additionally, this study explores the computational dimensions of microstructure modelling, testing CA mesh sensitivity effects and benchmarking our CAFD models on two high-performance computing platforms. The CAFD model captures well the key microstructural features observed in LSR Al-Cu alloys with and without grain refiner addition. It is shown that the maximum nucleation density has a significant effect on the final microstructure, resulting in different proportions of coarse columnar grains resulting from epitaxial solidification, newly nucleated thin columnar grains, and equiaxed grains.