Columnar Growth Research Articles

Impact of Substrate upon Morphology, Luminescence, and Wettability of ZnMgO Layers Deposited by Spray Pyrolysis

In this work, we report on a comparative study of the topology, luminescence, and wettability properties of ZnMgO films prepared by a cost-effective spray pyrolysis technology on GaAs substrates with (100), (001), and (111) crystallographic orientations, as well as on Si(100) substrates. Deposition on nanostructured GaAs substrates was also considered. It was found that film growth is not epitaxial or conformal, but rather, it is granular, depending on the nucleating sites for the crystallite growth. The distribution of nucleation sites ensured the preparation of nanostructured films with good uniformity of their topology. The observed difference in columnar growth on Si substrates and pyramidal growth on GaAs ones was explained in terms of the impact of chemical bonding in substrates. The films grown on GaAs substrates with a (001) orientation were found to be made of larger crystallites compared to those deposited on substrates with a (111) orientation. These effects resulted in a difference in roughness of a factor of 1.5, which correlates with the wetting properties of films, with the most hydrophobic surface being found on films deposited on GaAs substrates with a (111) orientation. The prospects for photocatalytic and gas sensor applications of films produced on flat substrates, as well as for plasmonic and other applications of films deposited on nanostructured substrates, are discussed, taking into account the results of the analysis of their photoluminescence properties.

Probing rapid solidification pathways in refractory complex concentrated alloys via multimodal synchrotron X-ray imaging and melt pool-scale simulation

AbstractRefractory complex concentrated alloys (RCCAs) show potential as the next-generation structural materials due to their superior strength in extreme environments. However, RCCAs processed by metal additive manufacturing (AM) typically suffer from process-related challenges surrounding laser material interaction defects and microstructure control. Multimodal in situ techniques (synchrotron X-ray imaging and diffraction and infrared imaging) and melt pool-level simulations were employed to understand rapid solidification pathways in two representative RCCAs: (i) multi-phase BCC + HCP Ti0.4Zr0.4Nb0.1Ta0.1 and (ii) single-phase BCC Ti0.486V0.375Cr0.111Ta0.028. As expected, laser material interaction defects followed similar systematic trends in process parameter space for both alloys. Additionally, both alloys formed a single-phase (BCC) microstructure after rapid solidification processing. However, significant differences in microstructure selection between these alloys were discovered, where Ti0.4Zr0.4Nb0.1Ta0.1 showed a mixture of equiaxed and columnar grains, while Ti0.486V0.375Cr0.111Ta0.028 was dominated by columnar growth. These behaviors were well described by the influence of undercooling effects on columnar-to-equiaxed transition (CET). Distinct microstructure formation in each alloy was verified through CET predictions via analytical melt pool simulations, which showed a ~ 5 × increase degrees in undercooling for Ti0.4Zr0.4Nb0.1Ta0.1 compared to Ti0.486V0.375Cr0.111Ta0.028. Overall, these results show that microstructure control based on modulating the freezing range must be balanced with process considerations which resist defect formation, such as solidification crack formation in RCCAs. Graphical abstract

Thermodynamic behavior and microstructure evolution of Inconel 718 alloy by laser metal deposition

The temperature evolution, stress evolution, and dendrite growth during the laser metal deposition (LMD) process of Inconel 718 alloy were simulated using a multi-scale model that combines the finite element method and the Cellular Automata (CA) method. This model established the relationship between the thermal behavior of the molten pool and the solidification characteristics, and predicted the morphological size of the solidified structure. Research indicates that under conditions of higher cooling rate and lower shape control factor, equiaxed grains are prone to form, with a lower degree of elemental segregation in this region, which is conducive to the precipitation of granular Laves phase. Conversely, under conditions of lower cooling rate and higher shape control factor, columnar grains are more likely to form, with a higher degree of elemental segregation in this region, leading to the precipitation of chain-like Laves phase. The results of thermo-mechanical coupled simulation suggest that the maximum transient thermal stress is located at the overlap position, and when the component cools down to room temperature, the maximum normal stress along the scanning direction is observed. Dendritic growth results demonstrate that during the equiaxed grain growth process in the top region of the melt pool, the released solute is constrained by time and space, leading to the accumulation of solute and the formation of interdendritic segregation. In areas with a higher grain density, the solute accumulation zones overlap, resulting in an increase in solute concentration, which inhibits dendrite growth. In the middle region of the melt pool, during the columnar grain growth process, the solute concentration in the gaps between adjacent grains continuously increases, resulting in micro-segregation phenomena. Experimental measurements have shown that the temperature field's melt pool morphology, stress values along the scanning path, and dendritic morphology are in good agreement with the experimental outcomes.

Evolutionary Mechanism of Solidification Behavior in the Melt Pool during Disk Laser Cladding with 316L Alloy

Laser cladding is an emerging environmentally friendly surface-strengthening technology. During the cladding process, the changes in molten pool temperature and velocity directly affect the solidification process and element distribution. The quantitative revelation of the directional solidification mechanism in the molten pool during the cladding process is crucial for enhancing the quality of the cladding layer. In this study, a multi-field coupling numerical model was developed to simulate the coating process of 316L powder on 45 steel matrices using a disk laser. The instantaneous evolution law of the temperature and flow fields was derived, providing input conditions for simulating microstructure evolution in the molten pool’s paste zone. The behavior characteristics of the molten pool were predicted through numerical simulation, and the microstructure evolution was simulated using the phase field method. The phase field model reveals that dendrite formation in the molten pool follows a sequence of plane crystal growth, cell crystal growth, and columnar crystal growth. The dendrites can undergo splitting to form algal structures under conditions of higher cooling rates and lower temperature gradients. The scanning speed of laser cladding (6 mm/s) has minimal impact on dendrite growth; instead, convection within the molten pool primarily influences dendrite growth and tilt and solute distribution.

Epitaxial Au/Fe4N/MgO thin films on GaAs (001) substrates

We investigate the growth of Au/FexNy/MgO trilayers on GaAs(001) substrates by plasma-assisted molecular beam epitaxy. The optimization of the growth conditions made it possible to obtain the compound of stoichiometric Fe4N. Microstructural studies show that Fe4N forms 3D islands at the initial stages of growth. As the Fe4N thickness increases, a columnar growth sets in leading to a strong texturing and to the formation of grains having the same crystallographic orientation. The growth is epitaxial with the relationship GaAs (001) [110]//MgO (001) [110]//Fe4N (001) [110]//Au (012) [0–32]. A chemical analysis at the nanoscale reveals that the interfaces are rather sharp with a limited interdiffusion. Magnetic characterizations show that a trilayer containing a 1-nm-thick Fe4N layer is already ferromagnetic. The easy magnetization axis is in-plane independent of the Fe4N layer thickness (from 1 to 6 nm). This study shows the potential to use Fe4N as a spin injector for spin-optoelectronic applications.

The MdCo gene encodes a putative 2OG-Fe (II) oxygenase that positively regulates salt tolerance in transgenic tomato and apple

Salinity stress is a significant environmental factor that impacts the growth, development, quality, and yield of crops. The 2OG-Fe (II) oxygenase family of enzyme proteins plays crucial roles in plant growth and stress responses. Previously, we identified and characterized MdCo, which encodes a putative 2OG-Fe (II) oxygenase, a key gene for controlling the columnar growth habit of apples. In this study, we explored the role of MdCo in salt stress tolerance. Expression analysis suggested that MdCo exhibits high expression in roots and is significantly induced by NaCl stress. Ectopic expression of MdCo exhibited enhanced salt stress tolerance in transgenic tomatoes, and these plants were characterized by better growth performance, and higher chlorophyll content, but lower electrolyte leakage and malondialdehyde (MDA), and less hydrogen peroxide (H2O2) and superoxide radicals (O2-) under salt stress. Overexpression of MdCo can effectively scavenge reactive oxygen species (ROS) by enhancing the activities of antioxidant enzymes and up-regulating the expression of stress-associated genes under salt stress, thereby enhancing salt tolerance in apple calli. Collectively, these findings provide new insights into the function of MdCo in salt stress tolerance as well as future potential application for apple breeding aimed at improving salt stress tolerance.

Study of phase evolution, mechanical, and tribological behaviour of a novel β-phase strengthened Ti-alloy with Fe and Co alloying via a laser material deposition route

A new Ti64-Fe-Co alloy was studied via a laser-material-deposition method. Associated with high growth restriction factors, β-eutectoid (Fe, Co) alloying minimizes anisotropy (due to the prior columnar β-grain growth) in the laseradditive -manufacturing (AM) built Ti-parts along with enhanced properties. Fe alloying for Ti64 via laser-AM results in superior strengthening with reduced ductility, where a good combination of strength and elongation properties can be achieved for a Ti-Fe-Co system. However, studies on Ti-Fe-Co-based systems via laser processing are limited. Co possesses a similar electronic property as Fe and the Ti-Fe system accompanies further solute addition. The effect of Co alloying the Ti64-Fe system was investigated in this study via a pre-placed laser-material-deposition method. A Ti64–5Fe-5Co alloy was formulated referring to CALPHAD simulations, along with a Ti64–10Fe composition. The new Ti-alloys were fabricated via both the laser-material-deposition and conventional vacuum arc melting routes, and the characteristics of the fabricated samples were examined. XRD analysis shows the evolution of the β-Ti phase with Fe and Co alloying, which agrees with the thermodynamic phase simulation results. The corresponding SEM micrographs depicted a β-phase dominant matrix with α-dendrites, and the solidification path was discussed. Considerable grain refinement was obtained for the laser-deposited alloys compared to the arc-melted ones, with an improved hardness value twice that of the commercial Ti64. A reduced stiffness for the laser-deposited Ti64–5Fe-5Co was observed from the load-vs-displacement characteristics, indicating matrix softening with Co alloying, which might benefit the elongation properties. The studied Ti-alloy shows a 30 % improvement in the dry-sliding wear characteristic with reduced fretting and abrasion compared to the commercial Ti64. The studied Ti-alloys were located in the theoretical d-electron space and new (Bo) ̅-(Md) ̅ vectors were introduced. Further studies on this would help in extending the applicability of laser-AM-built Ti-components.

Shape Anisotropy of Grains Formed by Laser Melting of (CoCuFeZr)17Sm2

For permanent magnetic materials, anisotropic microstructures are crucial for maximizing remanence Jr and maximum energy product (BH)max. This also applies to additive manufacturing processes such as laser powder bed fusion (PBF-LB). In PBF-LB processing, the solidification behavior is determined by the crystal structure of the material, the substrate, and the melt-pool morphology, resulting from the laser power PL and scanning speed vs. To study the impact of these parameters on the textured growth of grains in the melt-pool, experiments were conducted using single laser tracks on (CoCuFeZr)17Sm2 sintered magnets. A method was developed to quantify this grain shape anisotropy from electron backscatter diffraction (EBSD) analysis. For all grains in the melt-pool, the grain shape aspect ratio (GSAR) is calculated to distinguish columnar (GSAR < 0.5) and equiaxed (GSAR > 0.5) grains. For columnar grains, the grain shape orientation (GSO) is determined. The GSO represents the preferred growth direction of each grain. This method can also be used to reconstruct the temperature gradients present during solidification in the melt-pool. A dependence of the melt-pool aspect ratio (depth/width) on energy input was observed, where increasing energy input (increasing PL, decreasing vs) led to higher aspect ratios. For aspect ratios around 0.3, an optimum for directional columnar growth (93% area fraction) with predominantly vertical growth direction (mean angular deviation of 23.1° from vertical) was observed. The resulting crystallographic orientation is beyond the scope of this publication and will be investigated in future work.

Twin experiments and detailed investigation of data assimilation system for columnar dendrite growth in thin film

Accurately predicting dendrite growth during alloy solidification is crucial for enhancing the quality of metallic products. Recently, data assimilation has emerged as a promising tool for integrating two cutting-edge methods for studying dendritic growth, viz. in situ X-ray observation experiments and phase-field (PF) simulations, to elucidate important parameter(s) of the simulation and produce a “digital twin” of a growing dendrite. This study was conducted to evaluate the performance of a data assimilation system, employing an ensemble Kalman filter, through systematic twin experiments focused on columnar dendrite growth in a thin film of a binary alloy. The results show that the data assimilation system effectively estimates multiple parameters and dendrite morphology simultaneously. Furthermore, two approaches—domain decomposition method and parameter estimation method from a part of the domain—to reduce computational costs are investigated. Both methods can effectively reduce the computational cost of data assimilation. Additionally, data assimilation using voxel observation data of varying resolutions, mimicking actual X-ray images, is performed. The study findings discourage the direct use of voxel data owing to incompatible PF simulations. PF relaxation computation is explored as a solution for generating observation data with smooth PF profiles. The results show that while relaxation computation enhances the estimation accuracy for high-resolution voxel data, its efficacy diminishes for low-resolution data. These detailed investigations of data assimilation provide valuable insights for utilizing actual in situ X-ray observation data in dendrite growth studies.

A comparative study of TiO2, Ta2O5 and Nb2O5 coated Ti6Al4V titanium alloy for biomedical applications

Refractory metal oxides such as titanium dioxide (TiO2), tantalum oxide (Ta2O5), and niobium oxide (Nb2O5) coated-titanium and its alloys for medical applications have recently gained significant attention due to their remarkable properties, including high hardness, strong chemical inertness, and excellent biocompatibility. However, comparative studies among them remain scarce. In this study, thin films of Ta2O5, Nb2O5, and TiO2 were deposited on Ti6Al4V titanium alloy using radio frequency sputtering technology. Their properties were comparably evaluated through a series of tests, including scratching, nanoindentation, friction and wear, potentiodynamic polarization, and cell cultivation experiments. The results revealed that all three coatings, with a thickness of approximately 2.6–2.8 μm, exhibited an amorphous columnar growth pattern with dense surfaces resembling cauliflower. The bonding force of the TiO2 coating exceeds 50 N, which is 26.2 times and 14.5 times greater than that of Ta2O5 and Nb2O5, respectively. Furthermore, all coated samples demonstrate superior wear resistance, corrosion resistance, and biocompatibility compared to bare Ti6Al4V. Among the coatings, TiO2 showed the best performance, followed by Ta2O5, while Nb2O5 exhibited the least favorable performance. These findings revealed the performance differences among the three ceramic coatings on Ti6Al4V samples and offer valuable insights for further research and the selection of coatings to enhance the properties of Ti6Al4V titanium alloy for biomedical applications.

Influence of deposition pressure on microstructure, mechanical and electrical properties of niobium thin films

Niobium (Nb) thin films were deposited on a Si substrate using magnetron sputtering system by varying the deposition pressure. The effect of deposition pressure on the microstructure, residual stresses, and electrical properties was investigated by systematically varying the deposition pressure from 0.15 to 0.60 Pa. The Nb thin films were characterized using hard x-ray reflectivity, grazing incidence x-ray diffraction, four point probe method, and atomic force microscopy. The films grown at lower deposition pressures had smooth surfaces and more compact structures, which are advantageous for lowering electrical resistance but had higher compressive stress. On the other hand, the films grown at higher deposition pressures had columnar type growth with less dense structures, which leads to increase in electrical resistance. However the nature of stresses transform from compressive to tensile. Hard X-ray reflectivity performed on Nb films provides direct insight into the growth and the microstructure which were correlated with the mechanical and electrical properties.

Controlled Crystallization and Enhanced Performance of γ-CsPbI3 Perovskite Through Methylammonium Iodide-Assisted Coevaporation.

Cesium lead triiodide (CsPbI3) perovskites have garnered significant attention owing to their suitable bandgap for tandem silicon substrates and excellent chemical stability. However, γ-CsPbI3 prepared via low-temperature co-evaporation is limited by a narrow black phase processing window and random crystal orientation, hindering its optoelectronic performance and industrial applications. This study introduced trace amounts of methylammonium iodide (MAI) into the co-evaporation system, enhancing the crystallization process, promoting columnar grain growth, and stabilizing the γ-phase perovskite, resulting in films with improved structural integrity and reduced defect density. The optimal Pb/Cs ratio for achieving the best photoelectric performance shifted from 1:1 to 1.1:1 in the presence of MAI. Additionally, the incorporation of MAI allowed for more efficient longitudinal carrier transport, as evidenced by the enhanced photoluminescence (PL) intensity. The bandgap of CsPbI3 remained approximately at 1.7eV before the δ-phase transition, ensuring suitability for photovoltaic applications. Ultimately, a photovoltaic device with 12% efficiency is achieved in the p-i-n structure without additional post-annealing of the CsPbI3 perovskite films, demonstrating the practical benefits of MAI incorporation.

Recent Developments in Fe-Based Electrodeposited Thin Films

Electrodeposition is a commonly employed technique for synthesizing magnetic thin films, serving both fundamental research and practical applications. Of particular interest are Fe-based alloys like NixFe1-x and Fe1-xGax, as fundamental properties of ferromagnets as saturation magnetization (Msat), Curie temperature (TC), magnetic anisotropy (K) and magnetostriction constant (λ) strongly depend on the composition [1-2]. This work presents our latest findings on Fe-based thin films and multilayers, exploring diverse phenomena ranging from controlling magnetization switching in magnetoelastic structures to fine-tuning the out-of-plane (OOP) component of magnetization [3-4].Materials exhibiting moderate perpendicular magnetic anisotropy can develop stripe domains above a critical thickness (tcr). In addition to the interest in the basic understanding of this magnetic texture, nowadays, there is an increasing attention in this type of domains since they can be used for example for spin waves generation. The Ni80Fe20 alloy known as permalloy has been widely investigated because of its low coercivity and null magnetostriction. Sputtered Ni80Fe20 layers has been reported to have tcr around 300 nm [5], and its formation is generally believed to be due to columnar growth. However, for applications like magnetoimpedance, these stripes can be undesirable, hindering the growth of sufficiently thick layers to fabricate reliable sensors. Therefore, it is of interest the control of stripe domains in NixFe1-x films and whether the electrodeposition technique can also be used to tailor the appearance of this magnetic texture. Initially, we have examined the formation of stripe domains in the Ni90Fe10 alloy that is also of great interest because of its low coercivity and negative magnetrostriction (λ ~ -20 ppm). Subsequently, we have analyzed the magnetic behavior of electrodeposited NixFe1-x films in which the Fe content has been increased up to 30 at. %. Experimental results show that for Ni90Fe10 films magnetic stirring during growth suppresses the presence of stripes even for a thickness exceeding 1 µm. On the other hand, when growing with unstirred electrolyte, tcr can be reduced if a perpendicular magnetic field of 100 Oe is applied during samples growth. Eventually, it has been observed that tcr depends on the Fe content in the NixFe1-x and no stripes are observed for a content above 13 at.% when no perpendicular magnetic field is applied during growth.We have also investigated a bilayer structure comprised of two magnetic layers with opposite signs for the magnetostriction constant, Ni90Fe10 (λ ~ -20 ppm) and Fe70Ga30 (λ~ 70 ppm) to analyze the effect of magnetoelasticity on the magnetization reversal process. The exchange correlation length is a key parameter that determines the transition region between two opposite directions of the magnetization that depends on characteristics such as the magnetic anisotropy and the exchange stiffness constant. It is also used to quantify the domain wall thickness (δ) that can be considered as the distance over which the magnetic moments are correlated by exchange interactions, and that plays a fundamental role on magnetic systems relying on interfacial interactions as spring magnets or exchange-biased systems. Since the thickness of the layers cannot be higher than δ to avoid the magnetic switching of uncoupled regions, this implies that the thickness of layers is intrinsically limited in systems with interfacial coupling. We have studied Ni90Fe10/Fe70Ga30 bilayers with thicknesses exceeding δ to analyze the magnetic switching in these new magnetoelastic bilayer structures. 500 nm-thick Ni90Fe10 and Fe70Ga30 single layers have coercive fields of 22 Oe and 42 Oe, respectively, which guarantees that independent magnetic switchings from each layer can be experimentally observed.This has been confirmed by micromagnetic simulations performed by OOMF in which magnetoelasticity has not been included. However, when measuring the hysteresis loops of different NiFe/FeGa samples with thicknesses exceeding δ it is observed an in-unison magnetization reversal in all cases. Since magnetoelasticity is not an interfacial effect, it enables to control the magnetization reversal process of the whole bilayer regardless of the thickness being possible to promote this simultaneous reversal process regardless of the layer thickness (Fig.1).In conclusion, these results highlight the capability of electrodeposition to achieve new magnetic systems with enhanced functionalities.[1] J. M. D. Coey. Magnetism and Magnetic Materials. Cambridge University Press (2010)[2] Q. Xing, Y. Du, R. J. McQueeney, T. A. Lograsso. Acta Mater. 56 (2008) 4536−4546.[3] N. Coton, J. P. Andres, A. Cabrera, M. Maicas, R. Ranchal. J. Appl. Phys. 134 (2023) 103904.[4] N. Coton, J.P. Andres, E. Molina, M. Jaafar, R. Ranchal. J. Magn. Magn. Mater. 565 (2023) 170246.[5] M. Romera, R. Ranchal, D. Ciudad, M. Maicas, C. Aroca, J. Appl. Phys. 110 (2011) 083910 Figure 1

Microstructures and mechanical properties of Co-based Elgiloy fabricated by wire arc additive manufacturing

Wire arc additive manufacturing (WAAM) has emerged as a promising method for additive manufacturing due to its high efficiency and cost-effectiveness. This study aimed to fabricate Co-based Elgiloy by WAAM based on cold metal transfer technology. An 18-layers thin alloy wall was prepared. The microstructure, texture and mechanical properties of the as-deposited samples were investigated. Results indicated that the prepared alloy had good formability and was composed of a single face center cubic structure. The tensile properties of the alloy exhibited anisotropy due to the large number of columnar grains growing along the building direction in the alloy. The tensile samples along the building direction exhibited lower yield strength but better elongation due to the columnar grain growth direction and the softening effect of the remelted zone.

Preparation and tribological properties of WS2 solid lubricating coating with dense structure using HiPIMS

In this study, a dense WS2 coating was deposited via high-power impulse magnetron sputtering (HiPIMS) after solving the glow discharge problem, and it was compared with WS2 coating deposited via radio frequency (RF) sputtering. The latter showed a columnar growth, with a loose and porous microstructure. The columnar crystal growth of the former was inhibited, resulting in a more compact coating structure and a more uniform, flatter and smoother surface. Therefore, the structure and tribological properties of WS2 coating prepared via HiPIMS were investigated under different loads and frictional frequencies. Under low loads (up to 15 N), the coefficient of friction (COF) decreased with polishing wear mechanism. Under a high load of 20 N, the COF rapidly increased, the wear rate was the highest, and the wear mechanism changed from polishing to abrasive wear. At low frequencies (5 and 10 Hz), the coating had a low and stable COF, good wear resistance and the wear mechanism was abrasive wear. At high frequencies (≥15 Hz), the COF and wear rates rapidly increased and the wear mechanism changed from abrasive to adhesive wear and plastic fracture. However, the friction and wear rates were similar across different wear mechanisms.

Influence of contouring on the microstructure and micromechanical properties of Laser Powder Bed Fusion (LPBF) processed Al6061 enhanced with reactive particles

Age-hardenable aluminum alloys are of interest for powder bed fusion (PBF) additive manufacturing to produce lightweight, high-performance custom components. The successful processing of these alloys depends on specific alloying and print techniques to overcome defects that significantly degrade mechanical properties of PBF aluminum. The Reactive Additive Manufacturing (RAM) process modifieds alloy feedstock with reactive precursors which inoculate equiaxed microstructures during solidification, resulting in a reduction of columnar grain growth and shrinkage cracks. Additionally, surface quality is addressed by contouring, which retraces the perimeters of printed layers to produce smooth, fully fused surfaces. However, the combination of RAM and contouring methods has not been previously described in the literature, raising questions about the extent of refinement which is achieved when RAM particles are remelted by contouring laser sweeps. In this research, X-ray microcomputed tomography (XCT) was used to provide a complete characterization of RAM particles and defects in an AA6061-RAM alloy in 3D. The non-destructive XCT technique was used to quantify pore and particle distributions in the contoured alloy volumes and compared directly to hatched regions. The resulting three-dimensional analysis was correlated to traditional 2D microstructure quantification and nanoindentation experiments, providing a multimodal quantitative description of sub-micron grains, reaction products, and micromechanical properties. The study reveals that RAM particles inhibit cracking while simultaneously reducing the variance in mechanical properties between hatched and contoured regions of the build volume.

Microstructure evolution and grain growth characteristics of IN625 in laser surface melting: Effects of laser power and scanning speed

This study investigated the effects of laser power and scanning speed on the microstructure evolution and grain growth characteristics of IN625 in laser surface melting. Laser powers of 400 W, 600 W and 800 W at scanning speeds from 200 mm/min to 800 mm/min were employed to melt the surface of as-cast IN625 using a continuous Yb-doped fibre laser. The microstructure from the surface to the melt pool boundary was characterised by scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). It was shown that a cellular structure was developed at low energy densities (≤ 240 J/mm2), since low energy densities resulted in more rapid solidification. A coarse grain region was found near the surface after melting and a columnar grain growth region was formed close to the melt pool boundary at increasing laser power. Large angle grain boundaries were eliminated and medium angle grain boundaries exhibited an area fraction of 90 % after laser surface melting. This was due to the fact that the twinned grains were fully melted in the melt pool and no twins were formed after solidification. An analytical approach was proposed to the estimate the melt pool depth and good agreement between experimental and calculated melt pool depth was obtained at laser power of 400 W and 600 W.

Structures in grain-refined directionally solidified hypoeutectic Al-Cu alloys: Benchmark experiments under microgravity on-board the International Space Station

Benchmark solidification experiments were successfully performed under microgravity conditions on-board the International Space Station (ISS) within the ESAprogramme CETSOL (Columnar-to-Equiaxed Transition in SOLidification Processing). Cylindrical samples of grain-refined Al-4wt.%Cu, Al-10wt.%Cu and Al-20wt.%Cu alloys were directionally solidified in a gradient furnace to investigate columnar and equiaxed dendritic growth structures as well as the columnar to equiaxed transition under diffusive conditions. The determination of temperature gradients; interface velocities; and cooling rates at liquidus, solidus, and eutectic front positions provides well-defined thermal experimental characterization. The evaluation of the flight samples demonstrates that no significant macrosegregation along the sample axis occurred and no radial effects were observed. Therefore, purely diffusive solidification behaviour without any residual melt convection can be assumed for these microgravity experiments. The analyses of the microstructure in longitudinal cross-sections show dendritic structures without any pore formation and the averaged eutectic fraction is largely constant along the sample. The samples of refined Al-4wt.%Cu alloy show a sharp CET from columnar dendrites to a fine equiaxed steady-state grain structure whereas in the samples of refined Al-10wt.%Cu and Al-20wt.%Cu alloy, only equiaxed dendritic grain growth is observed. A quantitative analysis of the equiaxed grain morphology shows, that the shapes of the equiaxed dendrites depend on the applied temperature gradient, but the grain sizes in radial and longitudinal directions are identical. Therefore, a fully equiaxed dendritic growth structure without dendrite elongation was obtained. Compared to experiments in microgravity with non-refined Al-Cu alloys the average equiaxed grain size is about three times smaller.

Microstructure, mechanical properties, and wettability of CrN and CrSiCN coatings with graded architectures

Protective CrN-based coatings are of interest for applications requiring a combination of corrosion resistance and toughness. Here, the processing-microstructure-properties relationships for metallic Cr, binary CrN, and quaternary CrSiCN coatings are reported. Hexamethyldisilazane (HMDS) was used as a single-source organosilicon precursor for Si and C during plasma-enhanced magnetron sputtering (PEMS). By varying the availability of reactive species during coating growth, graded coating architectures were produced. Scanning and transmission electron microscopy revealed that as Si and C content increased, the coating microstructure evolved from a coarse, columnar growth structure (0 at.% Si), to a nanocolumnar structure (3 at.% Si), and, finally, to a dense, nanoscale structure (6 at.% Si). Coating surface topography was characterized using optical microscopy and atomic force microscopy. While the CrSiCN coating deposited at the highest HMDS flow rate exhibited the lowest nanoscale surface roughness (Rq = 6.74 nm), it also had the greatest area fraction of microscopic surface defects. Evidence of the formation of secondary Cr2N phases within the graded interlayers of the coatings was acquired from selected area electron diffraction analysis. However, the practical adhesion of the coatings, as evaluated by the Rockwell C indentation test, was not adversely affected. Nanoindentation testing revealed that greater incorporation of Si and C into the CrN-based coatings increased coating hardness from 25.4 ± 1.6 GPa to 30.7 ± 1.8 GPa and decreased apparent elastic modulus from 295 ± 14 GPa to 284 ± 12 GPa. Contact angle goniometry was conducted to determine the wettability of the coatings. It was found that sessile water contact angles decreased from 86.5° ± 1.6° to 77.3° ± 2.1° as coating Si and C content increased.

Femtosecond laser ultrafast photothermal exsolution

Exsolution, as an effective approach to constructing particle-decorated interfaces, is still challenging to yield interfacial films rather than isolated particles. Inspired by in vivo near-infrared laser photothermal therapy, using 3 mol% Y2O3 stabilized tetragonal zirconia polycrystals (3Y-TZP) as host oxide matrix and iron-oxide (Fe3O4/γ-Fe2O3/α-Fe2O3) materials as photothermal modulator and exsolution resource, femtosecond laser ultrafast exsolution approach is presented enabling to conquer this challenge. The key is to trigger photothermal annealing behavior via femtosecond laser ablation to initialize phase transition from monoclinic zirconia (m-ZrO2) to tetragonal zirconia (t-ZrO2) and induce t-ZrO2 columnar crystal growth. Fe-ions rapidly segregate along grain boundaries and diffuse towards the outmost surface, and become ‘frozen’, highlighting the potential to use photothermal materials and ultrafast heating/quenching behaviors of femtosecond laser ablation for interfacial exsolution. Triggering interfacial iron-oxide coloring exsolution is composition and concentration dependent. Photothermal materials themselves and corresponding photothermal transition capacity play a crucial role, initializing at 2 wt%, 3 wt%, and 5 wt% for Fe3O4/γ-Fe2O3/α-Fe2O3 doped 3Y-TZP samples. Due to different photothermal effects, exsolution states of ablated 5 wt% Fe3O4/γ-Fe2O3/α-Fe2O3-doped 3Y-TZP samples are totally different, with whole coverage, exhaustion (ablated away) and partial exsolution (rich in the grain boundaries in subsurface), respectively. Femtosecond laser ultrafast photothermal exsolution is uniquely featured by up to now the deepest microscale (10 μm from 5 wt%-Fe3O4-3Y-TZP sample) Fe-elemental deficient layer for exsolution and the whole coverage of exsolved materials rather than the formation of isolated exsolved particles by other methods. It is believed that this novel exsolution method may pave a good way to modulate interfacial properties for extensive applications in the fields of biology, optics/photonics, energy, catalysis, environment, etc.