Substrate For Heterogeneous Nucleation Research Articles

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.

Detection and Analysis of Fine Particles Acting as the Nucleation Site of Aluminum Grains Using FIB-TEM

The paper describes a technique development using focused ion beam and transmission electron microscopy to find and analyze fine particles which acted as the heterogeneous nucleation substrate for aluminum grains. An aluminum alloy was melted, sheared intensively by a melt conditioning unit, and cast by a laboratory scale horizontal twin roll caster. The intensively sheared alloy showed low segregation of solute elements on the surface as well as at the centerline, probably by enhanced nucleation of aluminum grains from fine oxide particles. In order to verify the possibility of the heterogeneous nucleation of aluminum grains, several techniques using focused ion beam and transmission electron microscopy were tried to find any oxide particles in the grains. Finally, a technique using the lifting-out of a lamella combined with the serial milling by focused ion beam detected a fine particle at the center of one aluminum grain and transmission electron microscopy confirmed the particle as an oxide. The intense nucleation of aluminum grains on fine dispersed oxides in the intensively sheared aluminum alloy can induce uniform distribution of solute elements and consequently minimize the severe centerline and surface segregations.

Refinement of the grain size of the LM25 alloy (A356) by 96Al–2Nb–2B master alloy

Nb-based intermetallics contained in the master alloy act as potent heterogeneous nucleation substrate for the nucleation of primary Al dendrites. The addition of the 96Al–2Nb–2B master alloy to the LM25 alloy permits to significantly refine its microstructural features and the refinement is achieved in a great range of cooling rates. The formation of columnar grains at slow cooling rates is prevented.

Formation of endogenous MgO and MgAl2O4particles and their possibility of acting as substrate for heterogeneous nucleation of aluminum grains

Aluminum containing 4 wt.% magnesium was oxidized at a temperature for different oxidation times and analyzed by high-resolution electron microscopy. A thin oxidized layer of about 5 µm, which is composed of MgO, forms at short oxidation time and gradually increases. High-resolution microstructures reveal that the oxidized layers are porous regardless of oxidation time. After extended oxidation time, discrete MgAl2O4 particles formed as a result of the reaction of initially formed MgO, liquid aluminum, and oxygen introduced from air through the porous MgO. Furthermore, it is clear by high-resolution lattice images that MgAl2O4 particles are covered with thin Al2O3, whereas MgO is bonded intimately to aluminum. Therefore, MgAl2O4 particles that form naturally during oxidation are difficult to act as a direct substrate for nucleation of aluminum grains because of the coverage of Al2O3. In contrast, MgO shows the possibility of acting as a substrate for the aluminum nucleation. The formation mechanism of MgO and MgAl2O4 and their possibility of acting as substrates for nucleation of aluminum grains suggest that atomic level bonding and mismatches of nucleant/nucleus metal should be considered for correct evaluation of the possibility of heterogeneous nucleation of metallic matrix on a potent nucleant. © 2015 The Authors. Surface and Interface Analysis Published by John Wiley & Sons, Ltd.

Modification of aluminium and aluminium alloys by AL-B master alloy

Chemical compositions and microstructures affect on mechanical – physical and working properties of aluminium and aluminum alloys. Transition elements, such as Ti, V, Cr, Zr in solid solution greatly reduce the electrical conductivity of aluminium and its alloys. For reduction of detrimental effects of transition elements, Al-B master alloys are added into molten aluminium to occur reactions of boron and transition elements to form diborides of titanium, vanadium, chromium and zirconium, which are markedly insoluble in molten aluminium, then these transition elements have an insignificant effects on conductivity. In addition, Al-B master alloys is also used as a grain refiner of aluminium and aluminium alloys. Aluminium borides particles in Al-B master alloys act as substrates for heterogeneous nucleation of aluminium and its alloys. Al-B master alloys are prepared from low cost materials, such as boric acid H3BO3 and cryolite Na3AlF6, by simple melting method, easily realize in electrical wire and cable factories.

Microstructure and mechanical properties of Al–1Mn and Al–10Si alloy circular clad ingot prepared by direct chill casting

An innovative direct chill casting process to prepare Al–10wt%Si and Al–1wt%Mn alloy circular clad ingots has been developed in the present study. The experimental casting parameters were determined by theoretical analysis, numerical simulation and experimental processes. The interface of clad ingots was investigated by methods of metallographic examination, electron probe microanalysis (EPMA) and transmission electron microscopy (TEM). The results showed that excellent metallurgical bonding of two different aluminum alloys could be achieved by direct chill casting. The Al–1Mn alloy which was poured into the mold earlier served as the substrate for heterogeneous nucleation of Al–10Si alloy. Because of diffusion of Si and Mn elements, a diffusion layer with a thickness of about 40μm on average between the Al–10Si and Al–1Mn alloys could be obtained. The tensile strength of the clad ingot was 106.8MPa and the fractured position was located in the Al–1Mn alloy side, indicating the strength of the interfacial region is higher than that of Al–1Mn alloy.

Viscoelastic Phase Separation and Interface Assisted Crystallization in a Highly Immiscible iPP/PMMA Blend.

Polymer blends with dynamic asymmetry have attracted much interest recently. In this study, we report a more typical case where the dynamically asymmetric system is highly immiscible. We find that there is a transient network growth and phase inversion for the slow minor component. The network structure shows a hierarchical growth behavior, which is the result of competition between a slow relaxation-controlled concentration growth on local scale and a fast hydrodynamic growth on large scale. When phase separation couples with a subsequent crystallization, the interfacial boundary may assist lateral crystallization and irregular spherulites would grow epitaxially around the amorphous component-rich domains. The interface may play the role of substrates for heterogeneous nucleation. These phenomena may help us with morphological control in material processing.

Nucleation and Crystallization of Lysozyme: Role of Substrate Surface Chemistry and Topography

In an effort to better understand, direct and control the crystallization of molecular and macromolecular compounds, an approach using colloidal templates as substrates for heterogeneous nucleation was investigated. These templates combine both tuneable chemical functionalities and geometrical features, altering the crystal–substrate interactions. Colloidal templates were prepared from silica nanoparticles, where the surface chemistry was modified by silanization. Particle size varied from 30 to 700 nm, with silanols, NH2, CF3, phenyl, or dodecyl as surface functional groups. Here, we report on the template assisted crystallization of chicken egg white lysozyme (CEWL). Nucleation was dramatically affected by the surface chemistry and topography of the templates. Using 220 nm particles, hydrophobic templates generally produced fewer, larger crystals, while a larger number of small crystals were obtained on hydrophilic templates. The use of different particle sizes also affected the crystal size, the optimal for nucleation being 432 nm. Classical Nucleation Theory (CNT) can interpret surface chemistry effects but does not support the effect of particle size. This paper reports that the combined use of both geometrical and chemical interactions results in an increased ability to control the nucleation and growth of protein crystals.

Nucleating role of an effective in situ Mg 3P 2 on Mg 2Si in Mg–Al–Si alloys

Refinement and modification effects of the Al–3.5P master alloy on Mg 2Si phases in Mg–4Al–2Si alloys were studied in this work. Mg 2Si phases in Mg–4Al–2Si alloys can be modified well at 740 °C with the addition of 1.5% Al–3.5P master alloy. The mean size of primary Mg 2Si decreases from about 70 μm to less than 15 μm, and the morphology changes from coarse dendritic or equiaxed to polygonal shape. In addition, the amount of eutectic Mg 2Si obviously decreases and the morphology changes from coarse Chinese script to fine fiber shape. XRD and EPMA results show that a reaction between AlP and Mg occurs in the melt forming a new Mg 3P 2 phase which serves as the heterogeneous nucleation substrates of Mg 2Si.

Grain refinement of AM60B magnesium alloy by SiC particles

AM60B alloy has been refined by SiC particles and the corresponding refining mechanism has been mainly discussed. The results indicate that the addition of 0.2 wt% SiC particles in form of mixture with Mg powder decreases the grain size from 317 μm of the not refined alloy to 46 μm. The decrease of β phase and formation of Mg2Si and Al4C3 phases well demonstrate that the reactions of 3SiC + 4Al = Al4C3 + 3Si and 2Si + Mg = Mg2Si occur during refining treatment. In addition, the crystal nuclei are composed of two kinds of elements, Al and C. All of these imply that the formed Al4C3 particles are the actual heterogeneous nucleation substrates.

First-principle study of the AlP/Si interfacial adhesion

AlP is heterogeneous nucleation substrate of primary Si in hypereutectic Al–Si alloys, while studies on the nucleation mechanism at atomic level are absent. The pseudopotential-based DFT calculations have been carried out to investigate the atomic and electronic structure, bonding and adhesion of the AlP/Si interface. In total, eight geometries have been investigated, in which the interfacial stacking sequence is different. The favorable interfaces can be deduced for the reason that adhesive interface energies ( W ad ) are different, which cannot be obtained from the traditional mismatch theory. The interfacial density of states and Mulliken population are also investigated. It is found that the main bond between AlP and Si is covalent Al–Si or P–Si bond, accompanying some ionic characteristic.

Sunlight-assisted fabrication of a hierarchical ZnO nanorod array structure

A ZnO hierarchical structure built up of a well ordered nanorod array was obtained by a simple treatment of zincate ion aqueous solution and a zinc foil mixture with sunlight irradiation. The as-prepared ZnO nanorods are all single crystalline with a uniform [0001] preferential growth direction and have an average diameter of approximately 40 nm and a length of approximately 300 nm. It is believed that sunlight gave rise to the driving force for the preformed ZnO nanorod to self-assemble into plates, which served as the substrate for secondary heterogeneous nucleation, and subsequently the crystal growth resulted into formation of a ZnO nanorod array. The formation mechanism of the ZnO hierarchical nanorod array structure was discussed, based on controlled experiment results. Raman spectroscopy, cathodoluminescence and UV-Vis absorbance spectra of the as-prepared ZnO product have been performed.

Influence of foreign metal ions on crystal growth and morphology of brushite (CaHPO 4, 2H 2O) and its transformation to octacalcium phosphate and apatite

Brushite, forming tabular crystals, has been precipitated at 25 °C in the presence of each of 14 different di- and trivalent metal ions. The influence of these ions at micromolar concentrations on the solvent-mediated phase transformation of brushite to more basic calcium phosphates has been studied as well. The effect of additives on brushite crystallization was pH-dependent, which could be related to the presence or absence of amorphous precipitate. In the latter case the course of the crystallization process could be followed by recording pH as function of time. For half of the additives kinetic analysis was possible and showed that the crystal growth mechanism is surface nucleation. Edge free energy is lowered in the presence of an additive. Zn favoured aggregates, and the transition metals with the exceptions of Mn(II), Co(II) and Cu(II) favoured irregular growth. Zn inhibited lateral growth, as did Cd and Cr(III) at low and Cu(II) at high pH. Most of the ions have a marked effect on the transformation to octacalcium phosphate (OCP) and hydroxyapatite (HAP) as well. In both cases Cu(II) and Zn are strong inhibitors, whereas Pb(II) is a moderate promotor. Fe in both oxidation states, Co(II), Mn(II) and Sr are intermediate in effect on phase transformation. Inhibition may be caused by adsorbed foreign ions impeding growth of nuclei or by poisoning of the substrate for heterogeneous nucleation, i.e. brushite crystals. Promotion is explained by the formation of nuclei with suitable crystal structure, e.g. apatite/pyromorphite (Ca,Pb) 5OH(PO 4) 3 in the case of Pb.

Nucleation kinetics during the solidification of aluminum alloys

The nucleation kinetics of individual grains during solidification of an aluminium alloy are experimentally investigated by X-ray diffraction at a synchrotron source. The investigated aluminium alloy contained solute titanium along with micrometer size TiB2 particles, which act as substrates for heterogeneous nucleation of aluminium grains during solidification. The nucleation rate of aluminium grains during solidification is measured by different cooling rates ranging from 1 to 40K/min. The measurements indicate that for the continuous cooling, the nucleation rate strongly decreases with time. The nucleation process is complete for solid fractions below 30% for all cooling rates. The maximum nucleation rate significantly increases with increasing cooling rates.

Controlling Polymorphism with Membrane-Based Crystallizers: Application to Form I and II of Paracetamol

The selective crystallization of either stable or metastable polymorph of paracetamol was obtained by membrane crystallization. The membrane acts as a selective medium for solvent evaporation and as active substrate for heterogeneous nucleation. This allows switching between a thermodynamically and a kinetically controlled nucleation stage.

Effect of titanium on the morphology of primary M 7C 3 carbides in hypereutectic high chromium white iron

This paper studies the effect of titanium concentration on the morphology of primary M 7C 3 carbides in hypereutectic high chromium white cast iron by means of the optical microscopy (OM), the scanning electron microscopy (SEM), the X-ray diffraction (XRD), the energy dispersive X-ray spectrometry (EDS) and Leica image analyzer. Theoretical calculation shows that solute Ti atoms can react with carbon and form TiC particles under certain conditions in the melt of hypereutectic high chromium cast iron. The experimental results show that the morphology of primary M 7C 3 carbides can be improved by adding a suitable amount of Ti. This improvement correlated with the emergence of TiC particles. These particles can act as the substrates for heterogeneous nucleation of primary M 7C 3 carbides, which results in significant refinement of the final grain size.

Magma-mixing in the genesis of Hercynian calc-alkaline granitoids: an integrated petrographic and geochemical study of the Sázava intrusion, Central Bohemian Pluton, Czech Republic

The Devonian–early Carboniferous (354.1±3.5 Ma: conventional zircon U–Pb age) Sázava intrusion (biotite-amphibole quartz diorite, tonalite and granodiorite) of the Central Bohemian Pluton (CBP) associated with bodies of (olivine, pyroxene-) amphibole gabbro, gabbrodiorite, (quartz) diorite and rare hornblendite, gives an opportunity for a comparative study of a rather shallow, calc-alkaline magma-mixing zone at two levels, separated by a vertical difference of approximately 1 km. The deeper section (Příbram) displays the direct evidence for the existence of a long-lived, periodically tapped and replenished, floored magma chamber (MASLI). The contacts between the subhorizontal sheet-like basic bodies and the surrounding, commonly cumulus-rich, Sázava granitoid, are arcuate, and cut by a series of veins and ascending pipes. Shallow-dipping swarms of strongly elongated and flow-aligned mafic microgranular enclaves (MME), concordant with the contacts of the basic bodies, are commonplace. The higher level (Teletín) section shows relatively independent basic intrusions, some of them distinctly hybrid in character and mainly of quartz dioritic composition, surrounded by relatively homogeneous, nearly cumulus-free Sázava tonalite rich in texturally variable MME. Larger quartz microdiorite bodies and the MME, both interpreted as hybrids, contain varying proportions of highly heterogeneous plagioclase megacrysts with complex zoning, particularly well shown by cathodoluminescence (CL). Most often the megacrysts have cores of labradorite-anorthite, partly resorbed and overgrown by andesine rims but some are strongly brecciated and fragments have been annealed by rim growth. Also characteristic are long prisms of apatite, oikocrysts of quartz and K-feldspar and zoned amphibole. The latter has brown pargasite and magnesiohastingsite cores, resorbed and overgrown by magnesiohornblende, compositionally similar to the amphibole in the Sázava tonalite. The brown cores are probably relics of a higher P–T stage, with temperature exceeding 900 °C. Plots of major-element analyses of the Teletín quartz microdiorite and the MME fall close to the line joining the compositions of the evolved Sázava granodiorite-tonalite and the associated gabbros. The estimated minimal proportions of the acidic end-member range from ca. 15% to 20% for the quartz microdiorite and ca. 40% to 60% for the enclaves. Field relationships, macroscopic and mineralogical features and whole-rock geochemistry are consistent with a hybrid origin of the quartz microdiorite masses and enclaves. Unzoned calcic plagioclase cores are interpreted as having crystallized from a basic magma prior to its injection into a high-level acidic magma chamber. The sudden decrease in Ca activity and introduction of water, due to magma-mixing with tonalitic melt, led to their resorption and brecciation. Quartz ocelli that are abundant locally in some basic bodies are most likely xenocrysts introduced from the felsic magma. In the basic melt, they were resorbed and became a substrate for heterogeneous nucleation of the amphibole or pyroxene coronas. Strong undercooling of the basic magma resulted in the growth of long prismatic apatite together with dendritic, boxy cellular plagioclase. Both types of plagioclase cores were subsequently overgrown by more sodic mantles. At the same time, small unzoned plagioclase crystals nucleated in the groundmass. Following thermal re-adjustment, crystallization of the hybrid magma ended with considerably slower growth of quartz and K-feldspar oikocrysts.

Effect of the citrate sol–gel synthesis on the formation of MgAl2O4 ultrafine powder

Abstract Nanocrystalline MgAl 2 O 4 with average crystal sizes of about 100 nm were prepared by citrate sol–gel process at lower temperatures. The presence of small amounts of MgAl 2 O 4 as crystal seed could obviously accelerate the crystallization of MgAl 2 O 4 . The effects of the amount and the size of crystal seeds on the formation of MgAl 2 O 4 were investigated. The inceptive formation temperature of the MgAl 2 O 4 not containing any crystal seeds was 600 °C, while the specimen containing 1 wt.% MgAl 2 O 4 used as a crystal seed was 550 °C. The decrease in the inceptive formation temperature was due to MgAl 2 O 4 serving as a substrate for heterogeneous nucleation which accelerated nucleation. The amount and the size of crystal seeds showed little influence on the formation of MgAl 2 O 4 spinel. The prepared ultrafine MgAl 2 O 4 spinel were characterized by X-ray diffraction (XRD), differential thermal analysis and thermogravimetric (TG–DTA) and scanning electron microscopy (SEM).

Crystallization behaviour of PP and carbon nanofibre blends

Crystallization behaviour of blends of different MFI isotactic polypropylenes (PP), and blends of PP with carbon nanofibre have been investigated by DSC and polarizing optical microscope. Both higher MFI PP component and the carbon nanofibre in the blend influence the nucleation activity of the melt during non-isothermal crystallization. In presence of carbon nanofibre, the sherulitic growth rate is highly disturbed. The calculation of nucleation activity indicates that carbon nanofibres act as active substrate for heterogeneous nucleation.

Interface investigations of alumina and aluminosilicate short-fiber-reinforced aluminum-alloy composites

The interfacial microstructures of alumina (Al 2O 3) and aluminosilicate (Al 2O 3·SiO 2) short-fiber-reinforced aluminum-alloy composites were studied by means of transmission electron microscopy (TEM). The experimental results showed that the SiO 2 content in the fiber has a marked effect on the interface microstructures of the composites even though a silica binder is not used. The reaction product MgAl 2O 4 spinel oxide was formed at the interface of the aluminosilicate-fiber-reinforce aluminum-alloy composite, however, no spinel oxide was observed at the interface of alumina-fiber-reinforced composite. The alumina fiber could act as a heterogeneous nucleation substrate for the primary silicon phase as a result of the low disregistry (5.62%) between the (0001) of δ-Al 2O 3 and the (110) of silicon.