Slow Solidification Research Articles

Stabilization of metastable γ-co: Combustion synthesis and rapid processing

This study reports a novel method for stabilizing the metastable γ-Co phase, which has a face-centered cubic structure. This phase is known for its challenging synthesis due to its tendency to transform into the stable hexagonal-closed packed (ε-Co) allotrope. Combustion synthesis and a rapid pressure-less sintering approach overcome the traditional barriers to γ-Co preparation. The combustion process involves exothermic reactions in a solution of cobalt nitrate hexahydrate and hexamethylenetetramine, generating enough heat to increase the synthesis temperature above the γ↔ε phase transition temperature (∼690 K). Rapid cooling of the solid product by gases is critical to preventing the reverse transition to ε-Co, thus stabilizing the γ-Co phase with a minimal amount of the stable phase. Thermal analysis has demonstrated that the decomposition of cobalt nitrates hexahydrate leads to the formation of cobalt oxides. These oxides are then reduced to Co by methane (CH4) and hydrazine (N2H4), released during hexamethylenetetramine decomposition. Kinetic data analysis shows that the rate-limiting step in forming Co is the CH4 (or N2H4)-mediated reduction of CoO. When subjected to rapid pressure-less sintering at 1275 K, the combustion synthesized materials consolidate into compact samples with a relative density of ∼90 % and micrometer-size grains. High-resolution electron microscopy investigation shows increased lattice dislocation due to the ε→γ transition at high-temperature processing. Prepared γ-Co exhibits a higher nanohardness and Young's modulus than ε-Co prepared by slow melt solidification and rolling methods.

In-situ formation of uniform Mg2Ni catalyst and hydrogen storage properties of Mg[sbnd]41Ni alloys with varied grain sizes and morphologies

In order to clarify the cyclic microstructure evolution process and the influence of grain size on the hydrogen de−/absorption properties of magnesium-based alloys. Mg-41 wt%Ni (Mg41Ni) alloys with varying grain sizes were prepared using different solidification rates. By analyzing the cross sections of ball-milled particles, it is found that the amorphous layer generated during ball milling is not conducive to the activation stage. Additionally, the coarse primary Mg2Ni and the lamellar Mg2Ni in the eutectic break into smaller Mg2Ni particles during the hydrogen absorption and release cycles. Thus, the fast solidified alloys can absorb 2.1 wt% of hydrogen gas at 125 °C, and complete decomposition can be achieved in 2.4 min at 300 °C. The activation energy of dehydrogenation decreases from 93.2 kJ/mol for slow solidification to 85.9 kJ/mol for fast solidification. Different kinetic models are used to fit and investigate the rate-controlling process of hydrogen desorption. The activated sample consists of the Mg matrix and uniformly distributed fine Mg2Ni particles due to the self-refinement, which significantly shortens the diffusion distance of H atoms. Besides, the multitude of pores generated inside the particles also facilitate the ingress and outflow of hydrogen.

Efficient red light polymerization facilitated by low energy excitation based on a single-atom substituted naphthalimide

Photoinduced polymerization with low-energy, longer wavelength light has attracted enormous attention in material/biological applications owing to its unique advantages of greener and deep curing compared to conventional harsh UV-triggered polymerization. However, the low-energy polymerization methods often rely on heavy atom-containing sensitizers or suffer from the slow solidification process, which restricts their applicability in several areas. Herein we construct a novel efficient photoinitiating system based on a single-atom substituted pure organic triplet photosensitizer, prepared from a common naphthalimide chromophore through a simple one-pot process, and exemplified its potential to enable the efficient and rapid polymerization (just a few tens of seconds) of acrylate upon low-intensity (0.36 W/cm2) red-light illumination. We show that the single atom replacement (oxygen with sulfur) can redshift the absorption profile of naphthalimide chromophore over 150 nm and also effectively promotes its intersystem crossing efficiency (from 0 to 86.4%). For the first time, the thionaphthalimide (NIS) was introduced in triplet-triplet annihilation upconversion, showing a high upconversion quantum yield of (4.4–12.9%) with a large anti-Stokes shift (0.65–0.80 eV) in various solvents. We successfully employed the upconverted annihilator's singlet to trigger the photoinitiator via Förster resonance energy transfer (FRET) for polymerization. Furthermore, this photoinitiating system is found to be effective enough to be operable under sunlight illumination and useful for the fabrication of high-quality flexible products via photolithography approach. Thus, our results are beneficial for the future design of eco-friendly, low-energy activated chemical conversion systems.

Processing and microstructure–property relations of Al-Mg-Si-Fe crossover alloys

This study introduces new alloys, which combine the age-hardening capability of Al-Mg-Si alloys with the microstructure-controlling effect on processing of primary Fe-rich intermetallic phases used in foil stock. In detail, the processing and microstructure–property relations in new crossover aluminum alloys derived from 6xxx and 8xxx foil stock alloys, is shown. A highly Fe-rich intermetallic phase content was deployed to conceptually mimic high scrap content. Fast and slow solidification rates were applied to represent thin strip and direct chill casting, respectively. The effects of adding Fe and Mn to alloy 6016 were examined, while the Si consumed in primary phases was partly adjusted to maintain age-hardening potential. It was shown that upon thermomechanical processing, primary intermetallic phases in the new alloys are finely fragmented and well dispersed, resulting in strong grain refinement and a uniform texture. Attractive combinations of strength and ductility were revealed, also in material processed under direct chill casting conditions. The new alloys’ high elongation values of up to 30%, and their age-hardening response, were similar to those seen in commercial alloy 6016, while their strain hardening capacity was significantly greater. This can be attributed mainly to the formation of geometrically necessary dislocations near primary Fe-rich intermetallic phases. The study discusses microstructure refinement on the basis of particle stimulated nucleation. It uses a simple model to describe the individual contributions to yield strength, including the effect of primary phases. It also models the effect of these particles on increased strain hardening and ductility.

Atomistic-informed kinetic phase-field modeling of non-equilibrium crystal growth during rapid solidification

A novel method based on molecular dynamics (MD) is developed to make the kinetic phase-field (PF) model quantitative in predicting non-equilibrium crystal growth during rapid solidification. MD-calculated variations of the diffuse solid-liquid (SL) interface width versus interface velocity are used to parameterize the kinetic PF model. Two approaches are adopted to study temperature independent and temperature dependent interfacial properties on the accuracy of predictions. MD simulations of slow and rapid solidification regimes for an fcc metal (Ni) show that the SL interface width decreases by increasing the solidification velocity. Fitting the dynamic response of the interface width to the traveling wave solution of hyperbolic PF equation determines the target SL interfacial properties, namely propagation velocity and diffusion coefficient. Independently, the MD calculations of nonlinearity in velocity versus undercooling is used to validate the atomistic-informed kinetic PF model. Both parabolic and kinetic PF models parameterized by temperature-independent material properties can accurately simulate the linear portion of near-equilibrium crystal growth during solidification. However, they both fail to predict the crystal growth kinetics during rapid solidification. The kinetic PF model parameterized with the temperature-dependent SL interfacial properties can accurately predict both the equilibrium and non-equilibrium crystal growth during slow and rapid solidification. MD simulation results on Ni along with some analytical analysis on the variation of interface width versus interface velocity show that for fcc metals, in general, {110} interface has a smaller propagation velocity in comparison to {100} interface, resulting in a larger non-linear behavior at smaller undercooling.

Magnetic Field-Dependent Microstructure Evolution of Solidified Co39.2Ni39.2Al21.6 Eutectic Medium-Entropy Alloy

A (Fe, Cr)-free Co39.2Ni39.2Al21.6 eutectic medium-entropy alloy (EMEA) was designed and fabricated to study the microstructure and its evolution during slow solidification under different intensities of high static magnetic field (0 T, 5 T and 10 T). It was found that the original microstructure was characterized by FCC/BCC mixed herringbone eutectics consisting of two types of lamellar structures: a curved and wormy anomalous eutectic in the fringe, and a straight and long regular eutectic in the center. Nano-sized L10-type martensite layers are also distributed on the BCC lamellar as the martensitic transformation product. The FCC and BCC phases were enriched in Co and Al elements, respectively, while Ni element was distributed homogenously in both phases. With increasing magnetic field intensity, the herringbone eutectic structures remained stable, without the formation of a primary phase, while the phase constitution and the orientation relationships in the eutectic structures remained unchanged, with no obvious magnetically induced alignments. However, the lamellar spacing of the regular lamellar eutectic decreased significantly from 3.3 μm (0 T) to 1.93 μm (10 T); by contrast, the volume fraction of the anomalous eutectics increased considerably from 28.35% (0 T) to 55.14% (10 T), and the assumption that the imposed convection and destabilization of lamellar eutectics is controlled by the magnetic field is discussed in depth. Our results show a great potential for tailoring microstructures and properties by applying a strong magnetic field during the solidification process of EMEAs.

Role of structural hierarchy on mechanics and electrochemistry of wire arc additive manufactured (WAAM) single phase titanium

The absence of understanding of correlations between structure, mechanics, and electrochemical behavior in single-phase additive-manufactured titanium components impedes the advancement of large-scale processing. The present study addresses this scientific knowledge gap by investigating the evolution of structural hierarchy and its impact on the hardness and corrosion resistance of wire arc additive manufactured (WAAM) commercially pure titanium (cp-Ti) along three mutually orthogonal deposition directions. Integrated X-ray diffraction and optical microscopy revealed that a single-phase α-Ti monolithic structure is devoid of observable deposition tracks and interfaces with traces of titanium oxide. Homogeneous nucleation in the single phase at low undercooling and slow solidification rates during WAAM resulted in coarse grains of size 1 mm. The grains are randomly oriented with slight preferential texture along the hexagonal close-packed basal {0001} planes due to their parallel orientation with the close-packed body-centered cubic {011} plane during the β – α phase transition. The single-phase structure results in a uniform distribution of microhardness along layer-by-layer buildup direction and the bulk with an average of 1.6 GPa, which is about 80 % of conventionally cast cp-Ti. An increased dislocation density at the surface and from thermal strain during WAAM enhances nanohardness to 3.0 GPa. The absence of galvanic potential difference between two adjacent grains and the coarse size imparts high corrosion resistance with a rate of 80 × 10−5 mm per year, less than 50 % of other additive-manufactured titanium alloys. Overall, this study advances the state-of-the-art in homogenous, single-phase WAAM processing as a potential large-sale additive manufacturing alternative to conventionally processed Ti components.

Energy storage performance improvement of phase change materials-based triplex-tube heat exchanger (TTHX) using liquid–solid interface-informed fin configurations

The future of power generation is predicted to be greener with greater uptake in renewable energy which will be more intermittent and make matching demand harder. Latent thermal energy storage using phase change materials (PCMs) could provide a solution to that problem. PCMs can store large amounts of energy in small volumes, however, the main issue is the low conductivity of PCMs, which limits the rate that energy can be stored due to the slow melting and solidification processes. In the present study, we design and optimise a flow structure-informed fin configuration for triplex-tube heat exchangers (TTHXs) that accelerates the PCM melting process to allow for faster energy storage rates. The computational fluid dynamics modelling is developed to evaluate different TTHX geometries and find the most efficient fin geometries. The results show that an optimal fin geometry can provide a 57.4 % reduction in melting time, which demonstrates that fins offer a very effective way of reducing the melting times of PCMs without taking up much volume. We also found that fins in the lower half of the PCM are more effective in reducing total melt time than fins in the upper half. It is revealed that curved fins offer better performance when compared to similar straight fins. Lastly, it is observed that horizontal or angled surfaces allow for more natural convection in TTHX. The findings can be used to inform future studies on energy storage performance improvement of PCMs.

Preparation of ultra-large intermetallic particles in Al alloys for accurate determination of their mechanical properties

The mechanical properties of intermetallic phases are important parameters for designing aluminum alloys. However, accurate measurement of these properties was obstructed by the subsidence of small intermetallic particles in the soft matrix. In this work, the alloy compositions are designed through CALculation of PHAse Diagram (CALPHAD) so that each target phase is primarily formed in a wide temperature range during slow solidification, thus intermetallic particles as large as 0.6–3 mm were prepared by the solution growth method. The hardness and elastic moduli of 6 important intermetallic phases in aluminum alloys, including Al6(Mn, Fe), Al2Cu, α-Al8Fe2Si, β-Al9Fe2Si2, Al3FeSi2 and Mg2Si were obtained. The elastic moduli and hardness values of Al6(Mn, Fe) and Al3FeSi2 were measured for the first time by nanoindentation and Vickers hardness tests as EAl6(Mn, Fe) = 171 ± 10 GPa, HAl6(Mn, Fe) = 501 ± 5 HV and EAl3FeSi2 = 199 ± 10 GPa, HAl3FeSi2 = 811 ± 12 HV. The properties measured in this work are overall higher than other experimental data and closer to theoretically predicted values. For this sake, we believe these data are most accurate for these 6 phases. In addition, this work provides a simple method to prepare ultra-large single crystal particles in Al alloys.

Wear characteristics of laser-deposited AlCoCrCuFeNi high entropy alloy with finite element analysis

BackgroundWear is a destructive phenomenon and one of the principal causes of material failure in moving components during surface interaction while in service. AlCoCrCuFeNi high-entropy alloy with its many properties is a potential material for aero-engine applications attributed to its outstanding relatively lightweight, high strength, good thermal, oxidation, and corrosion resistance properties. Hence, the investigation into the tribological behaviour of AlCoCrCuFeNi high-entropy alloys is essential to reduce maintenance costs and prolong the service life of this advanced material for aerospace applications. Most AlCoCrCuFeNi high-entropy alloy compositions were fabricated via arc melting, which has been reported to have defects attributed to slow solidification, consequently reducing the mechanical properties of the alloy with limited reports on other fabrication methods. Therefore, there is a need for the use of advanced manufacturing techniques for fabricating these alloys to improve the tribological properties. In this study, AlCoCrCuFeNi high-entropy alloy was fabricated via laser metal deposition. The influence of the laser processing parameters, rapid solidification, and the applied load on the tribological properties of the as-built alloys under dry conditions has been studied for aerospace applications. The counter ball rolling friction analysis was also investigated using COMSOL Multiphysics.ResultsThe results showed that at a high laser power of 1600 W and a scan speed of 12 mm/s, the lowest wear rates and highest hardness values were observed. The average coefficient of friction at room temperature was 0.1 and 0.3 at a speed of 21 m/s. The dominant wear mechanism at room temperature was abrasive wear as the wear rate increased linearly with an increase in load from 10 to 20 N. The scan speed had the most significant influence on the wear behaviour of the as-built high-entropy alloy attributed to the rapid rate of solidification which occurs at higher scan speeds.ConclusionsThe study examines the wear characteristics of high-entropy alloys fabricated via laser deposition technique in comparison with those fabricated via conventional routes. Although there were similarities in the phase structures of both techniques, the results showed that the wear resistance of the laser-deposited high-entropy alloy was comparatively higher than the same alloy prepared via conventional methods. Laser additive manufacturing was concluded to be a more successful method in fabricating high-entropy alloys.

Effect of Slow Solidification of Ultra-thick Continuous Casting Slab on Solidification Structure and Macrosegregation

Effect of Slow Solidification of Ultra-thick Continuous Casting Slab on Solidification Structure and Macrosegregation

On the role of Sc in powders and spray deposits of hypoeutectic Al-Cu

ABSTRACT Scandium (Sc) is an expensive alloying element which has an important effect on the properties of aluminum alloys. It is of interest to determine for a variety of solidification conditions the effect of minor addition, in the hypoeutectic levels, of Sc on Al alloys. This study builds on past work on rapid and slow solidification of Al-4.5Cu-0.4Sc (wt%) by comparing previous microstructures to those obtained using spray forming of strip. The effect of Sc (0.4 wt%) on the scale of the microstructure of as-atomised powders and as-deposited strips (SD) is found to be negligible in the investigated alloys. The eutectic undercooling of the Al-4.5Cu-0.4Sc (wt%) alloy is the same for both powders and SD. This is attributed to eutectic nucleation and growth following the same path in both solidification processes, characterised by an extended solubility of Sc in the primary α-Al matrix, as evidenced by the resulting Sc-rich precipitates and inherent age hardening upon annealing of the as-solidified samples. This work contributes to building an understanding of rapidly cooled structures and their subsequent self-aging in advanced manufacturing processes.

Fluid flow and vortex-shaped structure formation mechanisms ofAl-Ag-Ge alloy

<p indent="0mm">Fluid flow and solidification mechanisms of Al₇₀Ag₂₀Ge₁₀ alloy were systematically investigated using electromagnetic levitation, induction, arc, and resistance melting techniques coupled with finite element simulation method. A unique type of vortex-shaped microstructure was formed in these four experiments. This was different from the result under slow solidification conditions. The formation of the vortex-shaped structure was affected by fluid flow and cooling rate of alloy melt. The Lorentz force caused turbulent flow in alloy melt, while the turbulent flow during arc melting was driven by arc force under electromagnetic levitation or induction melting conditions. The surface tension and buoyancy force caused laminar flow in alloy melt during the resistance melting process. Due to rapid flow, the Ag₂Al phase nucleated ahead of the Al phase, primarily under near-equilibrium conditions. The metastable Ag₂Al dendrites grew as vortex-shaped morphology and preserved the melt flow pattern during solidification. The increased cooling rate caused a rapid growth of metastable Ag₂Al dendrites with increased vortex density. The curvature radius of metastable Ag₂Al dendrites decreased with enhanced melt flow; meanwhile, the more drastic nucleation and growing competition among phases occurred. Finally, irregular Al, Ag₂Al, and Ge ternary eutectics fully formed inside the vortex-shaped structure.

Strongly out-of-equilibrium growth morphologies in fast solidifying eutectics

In metal printing processes such as laser-based additive manufacturing, typical solidification velocities yield a strongly out-of-equilibrium (SOE) growth regime that is intermediate between the slow solidification in conventional metal casting and rapid solidification regimes characterized by severe solute trapping. Using phase-field simulations supported by experimental observations, we provide evidence for an inherent growth mode of the SOE regime in slightly hypereutectic Al-Ni, leading to the destabilization of the planar eutectic coupled growth front and the development of a protruded state. We interpret this scenario theoretically in terms of a concomitant growth of different solidification modes, here the coupled eutectic together with $\ensuremath{\alpha}$ primary phase. As another example of concomitant growth under SOE conditions, we present simulation results for single-phase solidification of a Ni fcc-phase exhibiting dendrites and doublons.

The Roles of Ni and Co in Dendritic Growth and Tensile Properties of Fe‐Containing Al–Si–Cu–Zn Scraps under Slow and Fast Solidification Cooling

Elementary correction of Al scraps contaminated with Fe has emerged as an alternative to existing recycling processes. The appropriate management of solidification parameters may regulate both the microstructure and the properties in a highly effective way. As shown in this article by José E. Spinelli and co-workers, number 2100822, this may allow for use in new applications with greater performance requirements.

NbB refining capability: Effects of slow and rapid solidification on dendritic spacings and grain sizes of a 6201 alloy

NbB inoculants are refiners that might be of interest to the Al industry. Grain size (GS) and secondary dendrite arm spacing (SDAS) are two important microstructural variables to consider when assessing a refiner's performance. Both of these attributes may have an impact on the alloy’s properties. The interplay of grain refiner and cooling rate on grain size, SDAS, and grain shape in the 6201 alloy is investigated in this article. It is demonstrated that GS associated with rapid solidification of the 6201 + NbB alloy was roughly 7 times smaller than that associated with slow solidification. Furthermore, regardless of the processing method (either slow or fast solidification), α-Al cells are generated for GS/SDAS ratio below and equal to 6.

NMR Spectral Characteristics of Ultrahigh Pressure High Temperature Impact Glasses of the Giant Kara Crater (Pay-Khoy, Russia)

In this study, we carried out the analysis of the impact melt vein glasses from the Kara impact crater (Russia) in comparison to low-pressure impact melt glasses (tektites) of the Zhamanshin crater (Kazakhstan). 27Al, 23Na, and 29Si MAS NMR spectra of the samples of these glasses were analyzed. The samples of the natural glass contained inclusions of crystalline phases, paramagnetic elements that greatly complicate and distort the NMR signals from the glass phase itself. Taking into account the Mossbauer distribution of Fe in these glasses, the analysis of the spectra of MAS NMR of glass network-former (Si, Al) and potential network-modifiers (Na) of nuclei leads to the conclusion that the Kara impact melt vein glasses are characterized by complete polymerization of (Si,Al)O4 tetrahedral structural units. The NMR features of the glasses are consistent with the vein hypothesis of their formation under conditions of high pressures and temperatures resulting in their fluidity, relatively slow solidification with partial melt differentiation, polymerization, and precipitation of mineral phases as the impact melt cools. The 70 Ma stability of the Kara impact vein glass can be explained by the stabilization of the glass network with primary fine-dispersed pyroxene and coesite precipitates and by the high polymerization level of the impact glass.

Microstructural and mechanical characterization of quasicrystalline Al-Cu-Fe foams

Quasi-crystalline metal foams are considered a new class of materials, as they differ fundamentally from conventional materials in their atomic structure and mechanical properties. Their manufacturing methods are under constant development. In this article, the microstructural characterization and the mechanical behavior of quasicrystalline foams obtained via slow solidification and heat treatment with the presence of molten metal is presented. The studied Al63Cu28Fe9 alloy was thermally treated at 800 °C for short times from 5 to 15 min and for longer times of 180 and 360 min in order to promote the formation of a metal structure constituted by icosahedral phase with a high percentage of porosity. The microstructural characterization was performed by using the X-ray diffraction (XRD) technique, scanning electron microscopy (SEM) and optical microscopy (OM). The thermal properties of the as-cast alloy and the changes in the density of the thermally treated samples were characterized by a differential thermal analysis (DTA) and Archimedes method, respectively. The highest percentage of macroporosity (40%) was obtained in the heat-treated sample at 800 °C for 360 min. Under these heat treatment conditions, a completely porous structure (foam) mainly constituted by quasicrystalline icosahedral phase was formed. The maximum compressive stress value and average plateau stress reached for the alloy heat-treated during 360 min were 80 MPa and 30 MPa, respectively.

Microstructures and mechanical properties of wire arc additive manufactured 5183-Al: Influences of deposition dimensions

Wire arc additive manufacturing (WAAM) is a promising technology to fabricate near-net large-scale structures with reduced cost and improved flexibility. Due to the cathode cleaning phenomenon of electric arc and relatively slow solidification rates, WAAM is suitable for depositing various series of aluminum alloys. The WAAM of complex aluminum parts usually requires deposition layers with diverse dimensions. Understanding the relationship between microstructures, mechanical properties, and deposition dimensions is thus necessary. In this work, WAAM based on gas tungsten arc welding is used to produce 5183-Al components. Three single path multi-layered parts with different layer widths were deposited using ER5183 wires. The influences of layer widths on grain morphology, defects, and mechanical properties were investigated experimentally. And the intrinsic influencing mechanism is analyzed with results from the numerical simulation on the thermal process. Results show that different layer widths lead to changes in solidification rates and temperature gradients, significantly influencing grain morphologies. However, the mechanical properties are found insensitive to the layer width, especially for the microhardness and strength. Larger layer width results in better ductility due to pores reduction. This research provides new findings on the relationship between microstructures, mechanical properties, and deposition dimensions, which is helpful for layer and path geometric optimization in WAAM of complex structures.

The Roles of Ni and Co in Dendritic Growth and Tensile Properties of Fe‐Containing Al–Si–Cu–Zn Scraps under Slow and Fast Solidification Cooling

Studies of Fe‐contaminated Al‐based scraps have drawn attention for years. However, little has been researched about the effects of cobalt (Co) and nickel (Ni) additions to contaminated scraps both under slow solidification (directional solidification, DS) and fast solidification regime (copper mold centrifugal casting, CC). As such, the current study examines a representative low‐alloy Al–Si chemistry from industrial scraps, i.e., the Al–7%Si–0.6%Fe–0.35%Cu–0.25%Zn (% by weight). This alloy is changed independently using Ni and Co. It is observed that a mixture of uneven distributed α and β Fe‐bearing particles prevails for all tested alloys and solidification conditions. The coexistence of both phases agrees with the charts proposed by Gorny's work for the range of secondary dendritic spacing, λ 2, observed. Although the Fe‐containing particles are smaller for fast‐solidified samples, they still maintain the acute morphology and, in some cases, the Chinese script shape. This results in a reduction in ductility of the order of 65% for the nonmodified and Ni‐modified samples considering mean λ 2 varying from 20 μm (DS) to 6.5 μm (CC), despite improvements in mechanical strength. Furthermore, addition of Co results in a higher number of Fe‐based particles within the microstructure. In this case, the largest particle/α‐Al interfacial area results in slightly poor ductility (~6–8%) and higher strength (175–205 MPa), with less ductility loss when DS and CC samples are compared.