Substrate For Heterogeneous Nucleation Research Articles

Effect of La addition on microstructure evolution of hypoeutectic Al–6Si alloys

Solidification experiments were carried out for Al–6Si alloys with different La addition levels. The results demonstrate that the average sizes of the α-Al grains and the eutectic Si have a tendency of first decreasing and then almost keeping constant with the La addition. About (0.04–0.06) wt% La addition is enough to achieve a satisfactory grain refinement and modification. An analytical model based on the Wilson equation, the Miedema model and the thermodynamics was proposed to investigate the mechanisms of the α-Al grain refinement and the eutectic Si modification in the hypoeutectic Al–6Si alloys with the trace La addition. The La effect on the grain refinement of α-Al and the modification of the eutectic Si are mainly attributed to the increase in the efficiency of the heterogeneous nucleation substrates to nucleate α-Al and the amount of La atoms in the eutectic Si. The grain refinement and the modification results achieve the maximum when the La concentration is near its maximum solid solubility in the primary α-Al phase.

Chemically reactive membrane crystallisation reactor for CO2–NH3 absorption and ammonium bicarbonate crystallisation: Kinetics of heterogeneous crystal growth

The feasibility of gas-liquid hollow fibre membrane contactors for the chemical absorption of carbon dioxide (CO2) into ammonia (NH3), coupled with the crystallisation of ammonium bicarbonate has been demonstrated. In this study, the mechanism of chemically facilitated heterogeneous membrane crystallisation is described, and the solution chemistry required to initiate nucleation elucidated. Induction time for nucleation was dependent on the rate of CO2 absorption, as this governed solution bicarbonate concentration. However, for low NH3 solution concentrations, a reduction in pH was observed with progressive CO2 absorption which shifted equilibria toward ammonium and carbonic acid, inhibiting both absorption and nucleation. An excess of free NH3 buffered pH suitably to balance equilibria to the onset of supersaturation, which ensured sufficient bicarbonate availability to initiate nucleation. Following induction at a supersaturation level of 1.7 (3.3 M NH3), an increase in crystal population density and crystal size was observed at progressive levels of supersaturation which contradicts the trend ordinarily observed for homogeneous nucleation in classical crystallisation technology, and demonstrates the role of the membrane as a physical substrate for heterogeneous nucleation during chemically reactive crystallisation. Both nucleation rate and crystal growth rate increased with increasing levels of supersaturation. This can be ascribed to the relatively low chemical driving force imposed by the shift in equilibrium toward ammonium which suppressed solution reactivity, together with the role of the membrane in promoting counter-current diffusion of CO2 and NH3 into the concentration boundary layer developed at the membrane wall, which permitted replenishment of reactants at the site of nucleation, and is a unique facet specific to this method of membrane facilitated crystallisation. Free ammonia concentration was shown to govern nucleation rate where a limiting NH3 concentration was identified above which crystallisation induced membrane scaling was observed. Provided the chemically reactive membrane crystallisation reactor was operated below this threshold, a consistent (size and number) and reproducible crystallised reaction product was collected downstream of the membrane, which evidenced that sustained membrane operation should be achievable with minimum reactive maintenance intervention.

PRODUCT CHARACTERISTICS IN SIMULTANEOUS CRYSTALLIZATION OF NaCl AND CaSO4 FROM AQUEOUS SOLUTION WITH SEEDING

Zero liquid discharge strategies for industrial wastewater treatment have become prominent in recent years. When evaporative simultaneous crystallization is applied, knowledge about the particle crystallization mechanisms and morphology are important to ease solids downstream handling and for recovery of their valuable components. In this work, batch simultaneous crystallization of sodium chloride (NaCl) and calcium sulphate hemihydrate (CaSO4.0.5H2O) from aqueous solution was studied. It was found that CaSO4.0.5H2O is not an effective substrate for NaCl heterogeneous nucleation, but agglomerates with NaCl particles instead. CaSO4.0.5H2O particles on the surface of NaCl crystals sterically hamper agglomeration of NaCl particles with each other and, for a sufficiently high CaSO4.0.5H2O seed load, cover the NaCl crystals so that NaCl supersaturation rises, inducing NaCl primary nucleation. Simultaneous crystallization of CaSO4.0.5H2O and NaCl yields a satisfactory product for downstream handling even for high CaSO4.0.5H2O content in the crystallizer. However, seeding with CaSO4.0.5H2O is not recommended as it reduces the mean size and increases product size dispersion. NaCl seeding may be considered if it is desirable to separate CaSO4.0.5H2O from NaCl downstream the crystallizer by size classification, as it favors CaSO4.0.5H2O to build up in product sizes of 200 μm and below.

A first principles investigation on the mechanism of TiC act as heterogeneous nucleation substrate of Mg phase to refine grains in AZ91

The lattice misfit of small Miller indices crystal faces between Mg and TiC is calculated based on Bramfitt two-dimensional lattice misfit theory. The electronic structure, adhesion energy and interfacial energy of Mg(001)/TiC(111) interface are investigated by first-principle calculation. The bond characteristics between interfacial atoms are analyzed by charge density difference and electron localization function. The calculation shows that the two-dimensional lattice misfit of Mg(001)/TiC(111) interface is 4.9%, which indicates TiC is suitable to act as an effective heterogenous nucleation substrate for Mg. The band structure and density of states calculations prove that the chemical bonds in Mg bulk structure are mainly metallic bonds, while that in TiC bulk structure are mixed bonds composed of polar covalent bonds and metallic ones. At the Mg-C interface, the electrons between Mg atoms and C atoms are shared to form covalent bonds, and the charge accumulation is obvious. The bonds in the Mg-Ti interface are metallic bonds, and the charge density between Mg atom and Ti atom is uniformly distributed. Compared with the Mg-Ti interface, the Mg-C interface has a larger adhesion energy (Wad = 6.96 J/m2) and a smaller interfacial energy (γ = 1.41 J/m2). Therefore, the crystallographic orientation most likely to produce a heterogeneous nucleation is Mg(001)//TiC(111), and the interface structure is optimally formed as the Mg-C type model.

Segregation of Ca at the Mg/MgO interface and its effect on grain refinement of Mg alloys

It has been reported that native MgO particles in Mg alloy melts can act as heterogeneous nucleation substrates such that grain refinement of Mg alloys is achieved. A recent study showed the addition of Ca, combined with the native MgO particles, significantly improves grain refinement of Mg and its alloys. However, the mechanism underlying the grain refining phenomenon is not well understood due to the lack of direct experimental evidence. In this work, we investigated the segregation of Ca atoms at the Mg/MgO interface and its effect on grain refinement in Mg-0.5Ca alloys by utilizing advanced analytical electron microscopy. The experimental results focus on the chemical and structural information at the interface between MgO and the Ca solute. Adsorption layers rich in Al, N and Ca have been detected on {1 1 1} facets of MgO particles, with the lattice structure resembling the structure of MgO. It is suggested that the significant grain refinement improvement can be attributed not only to the growth restriction due to the presence of Ca addition but also to the specific chemistry and structure of the adsorption layers.

Investigation on heterogeneous nucleation substrate of Y2O3 as NbC in hypereutectic Fe–Cr–C hardfacing coating by experiment and first-principles calculation

Three kinds of hypereutectic Fe–Cr–C hardfacing coatings were designed to research the refinement effect and heterogeneous nucleation mechanism of NbC on nano-Y2O3 in the coatings. The microstructure and elemental maps of Fe–Cr–C–Nb–Y2O3 coating were investigated by field emission scanning electron microscope. The phase structures of the coating and heterogeneous nucleation were researched by transmission electron microscope (TEM). The lattice mismatch between Y2O3 and NbC was computed by Bramfitt’s two-dimensional lattice mismatch theory. The adhesive strength, interface energy and electronic properties of Y2O3(111)/NbC(110) interfaces were calculated by a first-principles method. The bonding characteristics of heterogeneous nucleation interfaces were analyzed by charge density difference, electron localization function and Bader charge population. The experimental results show that the primary M7C3 can be refined by NbC, when Nb additive was added into the Fe–Cr–C coating. When nano-Y2O3 and Nb additive were added into the coating simultaneously, the primary M7C3 can be further refined. The SEM results show that NbC can be effective heterogeneous nucleation substrate of the primary M7C3 and there is a compound containing rare earth (RE) element Y in NbC particle. The TEM results verified the RE compound in NbC particle is Y2O3, and they combine together tightly. The calculation results indicate that the two-dimensional lattice mismatch of Y2O3(111)/NbC(110) interface is 6.8%, which reveals that Y2O3 meets the geometric condition to act as a medium-effective heterogeneous nucleation substrate of NbC. In all interface models, the adhesive strength of O-Bridge model is the largest (Wad = 9.75 J/m2) and its stability is the best (γ = − 0.87 J/m2). The major bond type of O-terminated interfaces is covalent one. Actually, the interfacial energy of O-terminated interface is negative, which reveals that their formation process is a process of energy reduction and structure stabilization. It means Y2O3 and NbC can form stable interface structure. It can be predicted that the heterogeneous nucleation occurs preferably on the O-terminated interfaces. Therefore, Y2O3 can serve as an effective heterogeneous nucleation substrate of NbC to refine it.

Semisolid Casting of Short Freezing Range Alloys

Semisolid casting and non-dendritic solidification of commercially pure tin (about 1 °C freezing range) and Zamak 3 alloy (about 10 °C freezing range) by a modified serpentine channel method were studied. It was shown that semisolid casting of very small freezing range metals with a non-dendritic structure was possible using this method. The results showed that the wall of the copper serpentine channel mold acted as a substrate for heterogeneous copious nucleation of primary solid particles and the channel provided sufficient self-steering action to disperse the nuclei in the melt. The average diameter and shape factor of the primary particles in the semisolid cast CP-Sn sample was measured to be about 107 μm and 0.75, respectively. The average diameter and shape factor of the primary particles in the semisolid cast Zamak 3 alloy was measured to be about 16 μm and 0.8, respectively. Hardness of semisolid samples was slightly higher than those of conventional gravity cast samples.

Novel hierarchical TiO2@ZIF-8 for photodecolorization of semi-real sample bromothymol blue aqueous solution

A TiO2@ZIF-8 hierarchical structure built up of a well-ordered array was achieved by an easy handling of Zn(NO3)2·6H2O, 2-methylimidazole and TiO2 mixture in DMF with solvothermal behavior. It is believed that solvothermal cause to the driving force for the accomplished TiO2 rod to self-assemble into ZIF-8, which performed as the substrate for secondary heterogeneous nucleation, and afterwards the crystal growth happened into development of a TiO2@ZIF-8 rod arrangement. The TiO2@ZIF-8 hierarchical rod array structure was characterized by FTIR, XRD, SEM, EDX, ICP, DRS, TG, NH3–TPD and BET and employed as photocatalyst for photodecolorization of bromothymol blue (BTB) aqueous solution under Hg lamp irradiation. The optimum values of the experimental parameters which affect the decolorization efficiency were attained as: 0.1 g L−1 of the 7% TiO2@ZIF-8.30 ppm dye concentration and pH 6.5. The decolorization kinetics was fitted well to the Langmuir–Hinshelwood first-order rate law. It was found that the catalyst displayed significantly high catalytic stability, and the activity loss is appearing after four BTB decolorization cycles.

Grain refinement of Al-Si9.8-Cu3.4 alloy by novel Al-3.5FeNb-1.5C master alloy and its effect on mechanical properties

A novel Al-3.5FeNb-1.5C master alloy with uniform microstructure was prepared using a melt reaction process for this study. In the master alloy, basic intermetallic particles such as NbAl3, NbC act as heterogeneous nucleation substrates during the solidification of aluminium. The grain refining performance of the novel master alloy on Al-Si9.8-Cu3.4 alloy has also been investigated. It is observed that the addition of 0.1 wt.% of Al-3.5FeNb-1.5C master alloy can induce very effective grain refinement of the Al-Si9.8-Cu3.4 alloy. The average grain size of α-Al is reduced to 22.90 μm from about 61.22 μm and most importantly, the inoculation of Al-Si9.8-Cu3.4 alloy with FeNb-C is not characterised by any visible poisoning effect, which is the drawback of using commercial Al-Ti-B master alloys on aluminium cast alloys. Therefore, the mechanical properties of the Al-Si9.8-Cu3.4 alloy have been improved obviously by the addition of the 0.1 wt.% of Al-3.5FeNb-1.5C master alloy, including the yield strength and elongation.

Effect of gadolinium addition on microstructural evolution and solidification characteristics of Al-15%Mg2Si in-situ composite

The current study investigated the effects of gadolinium (Gd) addition (0, 0.3, 0.5, 1.0, and 2.0wt%) on microstructural development and solidification characteristics of hypereutectic Al-15%Mg2Si in-situ composite. Microstructural examinations were carried out via optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and electron backscattered diffraction (EBSD). The results showed that an addition of 1.0wt% Gd, changed the morphology of primary Mg2Si from coarse dendritic to fine truncated octahedral shape. Furthermore, the average size of primary Mg2Si particles decreased from 40μm to 20μm and density increased from 495 to 1167particles/mm2. Besides that, cooling curve thermal analysis (CCTA) technique revealed that Gd addition increased nucleation and growth temperatures of primary Mg2Si phase. It is proposed that modification and refinement mechanism of Gd addition are mainly attributed to formation of tiny Al3Gd particles as the heterogeneous nucleation substrates for primary Mg2Si, which results in a refined distribution of these particles and poisoning effect by adsorbing Gd atoms, preferentially on {100} facets of Mg2Si crystals. Moreover, it is believed that formation of Gd intermetallic compounds is responsible for refinement of primary Mg2Si particles.

Influence of Fe on Al4C3 as a heterogeneous nucleation substrate: a first-principles study

To elucidate the influence of Fe on the crystal structure of Al4C3 and its carbonaceous refining effect on primary Mg grains, the adsorption energy and electronic structure of adsorption were investigated via first-principles calculations. The results show that H1 and are the most stable adsorption sites for the Al-terminated and C-terminated Fe/Al4C3(0 0 0 1) adsorption systems. Based on their formation energies, the adsorption system is more stable than the H1 adsorption system, indicating that Fe atoms should adsorb on the C-terminated surface rather than on the Al-terminated surface. Analyzing the atomic structure of the adsorption system revealed that the surface structure of Al4C3(0 0 0 1) is reconstructed when the adsorption coverage exceeds 0.25 ML of Fe atoms. Thus, Fe adsorption may weaken the heterogeneous nucleation potency of Al4C3 particles. This explains the observed experimental phenomenon that excessively high Fe contents negatively influence the refinement of Mg alloys with added carbon.

Role of In-Situ Generated CO2 Bubbles in Heterogeneous Nucleation of Solid Solutes in the Precipitation by Pressure Reduction of Gas-Expanded Liquid (PPRGEL) Process

Precipitation by pressure reduction of a gas-expanded liquid (PPRGEL) entails dissolution of subcritical CO2 in the solution of an organic solvent and a solid solute resulting in a gas-expanded liquid (GEL). Depressurization of a GEL leads to evolution of CO2 bubbles along with a large decrease in temperature facilitating solid precipitation. An illustrative system comprising CO2 (1), acetone (2), and cholesterol (3) is considered. It is proposed that CO2 bubbles generated from the depressurized GEL provide a substrate for heterogeneous nucleation of cholesterol. The size of nucleated CO2 bubbles and their rate of generation with time are predicted. The effective size of the CO2 bubbles ranges over 200–500 μm and their generation rate is ∼106–107 #/s. Comparing these with the corresponding solute nuclei size and nucleation rates, and considering the GEL–solid interfacial energetics, we conclude that the CO2 bubbles provide the required surface area for solid nucleation.

Enhanced aging kinetic of Al3BC/6061 Al composites and its micro-mechanism

In this work, the aging behavior of Al3BC/6061 Al composites was investigated and the impacts of Al3BC on the aging kinetic as well as its micro-mechanism were unveiled. It is found that the aging kinetic of the composites is considerably accelerated and the peak aging time at 175 °C comes down from 14 to 6 h. Both positive and negative impacts due to the incorporation of Al3BC on the aging kinetic of the composites were discussed. The high density dislocations can either act as the heterogeneous nucleation substrate or play as the diffusion path to expedite precipitation process, while the low vacancy concentration and solute segregation on the interface suppress the formation of GP zone and reduce the chemical driving force for precipitation, respectively. Obviously, the positive effects caused by the dislocation contribute more in the current work. Microstructure characterizations verify that needle-like β″ phase and lath Q′ phase precipitate in the matrix and are impacted by the Al3BC differently. In the area free of Al3BC particle, they distributed uniformly in the matrix. However, in the vicinity of Al3BC particles, the precipitation of β″ is inhibited for the low vacancy concentration and segregation of Mg, Si elements on the Al3BC/Al interface, while the precipitation of Q′ phase is accelerated for the preferred nucleation of Q′ phase on the dislocation.

Analysis of C-S-H growth rates in supersaturated conditions

The growth rate of calcium-silicate-hydrate (C-S-H) was analyzed by following the evolution of calcium and silicon concentrations in supersaturated solutions. In these experiments, the supersaturated solution was produced by mixing a saturated calcium hydroxide solution and a solution obtained from the hydration of tricalcium silicate. A continuous decrease of the silicon concentration over time was observed during the experiments and the C-S-H formation rate was calculated from the amount of silicon that was precipitated between two consecutive analyses.The data obtained in this study demonstrate that the interfacial growth rate of C-S-H depends mainly on the supersaturation with respect to this phase, the availability of calcite as a substrate for heterogeneous nucleation and the calcium concentration in solution. A mean value of approximately 10nmol of C-S-H per m2 per second was obtained for the interfacial growth rate of C-S-H in conditions that are relevant for the hydration of tricalcium silicate.

First-principles study on Al4Sr as the heterogeneous nucleus of Mg2Si

The interfacial structure, electronic structure, work of adhesion and interfacial energy of the Al4Sr(100)/Mg2Si(100) interface have been studied with first-principles calculations to clarify the heterogeneous nucleation potential of the Al4Sr particle for a primary Mg2Si phase. Eight models of the Al4Sr(100)/Mg2Si(100) interface with OT and HCP stacking were adopted for the interfacial model geometries. The results show that the Al-Mg terminations of HCP and Al-Si terminations of OT stacking, with lower interfacial spacing and higher interfacial adhesion, are the most favorable structures after relaxation. Al-Mg- and Al-Si-terminated interfaces, with a lower interfacial distance, form chemical bonds more easily. Metallic bonds are formed near the Al-Mg-terminated interface, while the Al-Si-terminated interface exhibits predominantly covalent bond characteristics. Moreover, the calculated interfacial energies of both terminations are negative in conditions involving excess Mg atoms. The interfacial energies of Al-Si are lower than those of Al-Mg termination, indicating that the Al-Si-terminated interface is more stable. From thermodynamic analysis, we discover that the Al4Sr particle can be an effective heterogeneous nucleation substrate for Mg2Si in a Mg-Al-Si alloy melt.

Engineering the heterogeneous nuclei in Al-Si alloys for solidification control

Aluminium alloys are particularly advantageous for transportation industries to reduce the weight of the vehicles and thus pollution. The optimisation of the performances of Al alloys can be achieved by engineering and enhancing the heterogeneous nucleation of the melt. In this work, we demonstrate how the inoculation of commercial Al-Si alloys by Nb-based phases permits to improve and control the solidification process. We show that NbB2/Al3Nb are potent heterogeneous nucleation substrates for Al-Si alloys. The improved nucleation makes the material structure less dependent on the processing conditions such as solidification time and chemistry of the alloy (i.e. Si content). This permits to design and fabricate lighter structural components with improved and more isotropic performances.

The Role of Lattice Misfit on Heterogeneous Nucleation of Pure Aluminum

α-Alumina (Al2O3) single crystals with different termination planes were used as heterogeneous nucleation substrates for liquid aluminum to varying lattice misfits at the interface between substrate and newly nucleated aluminum grain. Undercooling during the nucleation process was measured for interface configurations with varied lattice misfit, while the solidified Al/Al2O3 interfaces were directly observed by high-resolution transmission electron microscopy (HRTEM). Based on experimental results, the effect of lattice misfit on nucleation behavior was systematically investigated following previous misfit-interfacial energy models, with clarification being made by the undercooling measurement and HRTEM observations of the interfaces in the Al/Al2O3 system. When the misfit is smaller than 13 pct, both experimental results and theoretical analysis show that the currently existing models through modification and incorporating energy calculation can be used to fit the detected undercooling of investigated system. Beyond 13 pct, a new hypothesis was developed to accommodate lattice misfit with stacking faults such as microtwins according to the HRTEM analysis. The interfacial energy is then replaced by the stacking fault energy accumulated in the strained area. It is shown that the lattice misfit plays an important role in determining the heterogeneous nucleation of liquid aluminum. The nucleation undercooling is then able to be predicted by the theoretically calculated interfacial energy using the integrated models developed in the work. The prediction results were also verified by the HRTEM analysis on the nucleation interface of the Al/Al2O3 systems and detected undercooling on corresponding systems.

Modification effect of calcium-magnesia phosphate fertilizer on microstructure and mechanical properties of Mg2Si/Mg-4Si composite

In order to modify in-situ synthesized Mg2Si particles in Mg2Si/Mg-4Si composite, the modification effect of calcium-magnesia phosphate fertilizer on primary Mg2Si phase in Mg2Si/Mg-4Si composite was investigated by means of X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analysis. The results indicate that the morphology of the primary Mg2Si phase apparently changes from coarse dendrites to fine dispersive polygonal particles, and the mean size is decreased from 277 μm to 17 μm. With the addition of 4.0wt.% calcium-magnesia phosphate fertilizer as a modifier, the ultimate tensile strength and elongation of the Mg2Si/Mg-4Si composite are increased from 78.7 MPa and 2.1% to 105.2 MPa and 2.6%, as compared to those of the base composite, which is probably attributed to the formation of the phosphorous compound and the cluster of Ca compounds that acted as the heterogeneous nucleation substrates of the primary Mg2Si particles, resulting in a refined distribution of these precipitates.

Heterogeneous Nb-Based Nuclei for the Grain Refinement of Al-Si Alloys

Nb-based intermetallics are, generally, low-density high-temperature materials used for structural applications or cryogenic superconductors. In this work, we report the development of an Al(96)-Nb(2)-B(2) master alloy where in situ-formed micrometric Nb-based intermetallics (i.e. NbB2 and Al3Nb) are used for a completely different purpose: to promote the refinement of Al-Si alloys by taking advantage of enhanced heterogeneous nucleation. Nb-based intermetallics have the right characteristics, like low density, stability at high temperature and good lattice match, to be used as heterogeneous nucleation substrates. It was found that the addition of these Nb-based intermetallics permits the significant refinement of the microstructural features of the Al-Si alloy studied. The enhanced heterogeneous nucleation makes the grain size of the material far less dependent on the cooling rate, which is one of the critical parameters influencing the variation of the properties of the alloy.

Microstructures, Wear Behavior and Mechanical Properties of the TiC Ceramic Particulate Locally Reinforced Ni-Hard4 White Cast Iron Matrix

Titanium carbide ceramic particles could act as inoculants to generate fine microstructure and helps to increase the wear resistance for steel. In this article, the effect of titanium addition on the microstructure, wear and mechanical properties of Ni-Hard4 white cast iron was studied. The study was undertaken with five laboratory pieces of cast iron made with different titanium amounts. The microstructures were examined by optical microscopy, scanning electron microscopy equipped by EDS. The impact energy, hardness and wear resistance of the samples were determined. Thermodynamic calculation showed that solute titanium atoms can react with carbon and form TiC in modified samples. The results indicated that the morphology of chromium carbides can be improved by adding a suitable amount of titanium. This improvement was correlated with the emergence of TiC ceramic particles. These particles can act as the substrates for heterogeneous nucleation of primary chromium carbides, which result in significant refinement of the final average carbide diameter. In addition, the hardness and wear resistance were improved without significant variations in fracture toughness. The wear resistance was increased with titanium addition owing to change microstructure and the precipitation hardening of fine ceramic particles in the martensitic matrix.