Thin Melt Layer Research Articles

Karwowskiite, Ca9(Fe2+0.5□0.5)Mg(PO4)7—A New Merrillite Group Mineral from Paralava of the Hatrurim Complex, Daba-Siwaqa, Jordan

Crystals of karwowskiite, Ca9Mg(Fe2+0.5□0.5)(PO4)7, a new mineral of the merrillite group, were found on an amygdule wall in the central part of an anorthite–tridymite–diopside paralava of the Hatrurim Complex, Daba-Siwaqa, Jordan. The amygdule was filled with a sulfide melt, which after crystallization gave a differentiated nodule, consisting of troilite and pentlandite parts and containing tetrataenite and nickelphosphide inclusions. Karwowskiite crystals are colorless, although sometimes a greenish tint is observed. The mineral has a vitreous luster. The microhardness VHN25 is 365 (12), corresponding to 4 on the Mohs hardness scale. Cleavage is not observed, and fracture is conchoidal. The calculated density is 3.085 g/cm3. Karwowskiite is uniaxial (−): ω = 1.638 (3), ε = 1.622 (3) (λ = 589 nm), and pleochroism is not observed. The composition of karwowskiite is described by the empirical formula: Ca9.00(□0.54Fe2+0.23Mg0.12Na0.04 Sr0.03 Ni0.03K0.01) Σ1.00Mg1.00(PO4)7.02. Karwowskiite is distinct from the known minerals of the merrillite subgroup with the general formula A9XM[TO3(Ø)]7, where A = Ca, Na, Sr, and Y; X = Na, Ca, and □; M = Mg, Fe2+, Fe3+, and Mn; T = P; and Ø = O, in that the X site in it is occupied by Fe2+0.5□0.5. Karwowskiite is trigonal, space group R-3c with a = 10.3375 (2) Å, c = 37.1443 (9) Å, and V = 3437.60 (17) Å3. Karwowskiite crystallizes at temperatures lower than 1100 °C in a thin layer of secondary melt forming on the walls of amygdules and gaseous channels in paralava as a result of contact with heated gases which are by-products of the combustion process.

Seismic visibility of melt at the core-mantle boundary from PKKP diffracted waves

The core-mantle boundary (CMB) is a complex and poorly understood region of the Earth. The existence of melt or partial melt at the CMB is strongly debated with evidence coming from a range of sources. While partial melt has been inferred in specific locations using seismology, often as an explanation for ultra-low velocity zones (ULVZs), no global layer of melt has been observed despite evidence that such melt should accumulate at the CMB.Using high frequency synthetic seismograms we have systematically examined the effect of a one-dimensional melt layer at the CMB on the global seismic wavefield. We find that PKKP diffracted waves are an extremely sensitive, robust and previously underutilised seismic phase for studying the CMB and, in synthetic testing, could resolve very thin melt layers. We have constructed a global data set of PKKP diffracted observations to attempt to observe or exclude melt at the CMB. We find that within the bounds of our data, we are unable to robustly exclude or observe a melt layer at the CMB on the order of single kilometres thick. The presence of a thin layer of unobservable melt at the CMB would have profound impacts for the internal dynamics of the Earth.

Elastic amplification of the Rayleigh–Taylor instability in solidifying melts

The concomitant mechanical deformation and solidification of melts are relevant to a broad range of phenomena. Examples include the preparation of cotton candy, the atomization of metals, the manufacture of glass fibers, and the formation of elongated structures in volcanic eruptions known as Pele's hair. Usually, solid-like deformations during solidification are neglected as the melt is much more malleable in its initial liquid-like form. Here we demonstrate how elastic deformations in the midst of solidification, i.e., while the melt responds as a very soft solid ([Formula: see text] Pa), can lead to the formation of previously unknown periodic structures. Namely, we generate an array of droplets on a thin layer of liquid elastomer melt coated on the outside of a rotating cylinder through the Rayleigh-Taylor instability. Then, as the melt cures and goes through its gelation point, the rotation speed is increased and the drops stretch into hairs. The ongoing solidification eventually hardens the material, permanently "freezing" these elastic deformations into a patterned solid. Using experiments, simulation, and theory, we demonstrate that the formation of our two-step patterns can be rationalized when combining the tools from fluid mechanics, elasticity, and statistics. Our study therefore provides a framework to analyze multistep pattern formation processes and harness them to assemble complex materials.

The Nature of Anomalous Particles (Granules) in Rapidly Quenched PREP Powders: I. A Multiscale Study of the Heating Zone of the Rotating Electrode Used to Obtain the Rapidly Quenched PREP Ni-Based Superalloy Powder

The uniformity of composition of rapidly quenched PREP particles of the powder of high temperature Ni-based superalloys and stainless steels is characterized by an important feature—the occurrence of anomalous particles (granules) with a significantly different content, mainly of microalloying interstitial elements, carbon and boron, as well as active carbide- and boride-forming alloying elements. A detailed multi-scale experimental study of the heating zone of the crater of the Ni-based superalloy electrode after its use to obtain rapidly quenched PREP powder was carried out in order to find the nature and mechanisms of the formation of anomalous granules. Direct nuclear physics methods of activation autoradiography on carbon, track autoradiography on boron, metallography, SEM, EDX, OIM were used. In the electrode crater, the heat-affected zone (HAZ) and the partially melted zone (PMZ) were detected. Intense migration of boron to the electrode surface due to the formation of thermal macrocracks was also revealed. The behavior of carbon is determined by the formation of a thin layer of melt on the surface of the crater. The features of the evolution of the terminal solidification region TSR and incipiently melted regions IMR, the main type of heterogeneity of the composition of the dendritic structure of Ni-based superalloys and stainless steels, are revealed. The interrelation of the evolution of these areas is established, which is a consequence of the thermodynamic principle of the reversibility of the processes of solidification and melting, respectively, in the smelting of an ingot electrode and in the process of subsequent atomization. The analysis of the influence of the behavior of boron, carbon, and the characteristics of the crater structure on the nature and mechanism of the formation of anomalous granules using the PREP method for producing rapidly quenched powder of the Ni-based superalloy is performed.

Simulation of the Glass Transition of a Thin Aluminum Melt Layer at Ultrafast Cooling under Isobaric Conditions

It is shown that a sharp increase in viscosity, a change in the decay rate of correlations, and the appearance of nonequilibrium and transverse sound in a thin film of aluminum melt during rapid cooling occur in the same temperature range. The analysis is performed using the shear stress autocorrelation functions calculated by the molecular dynamics method. The cooling rate dependences indicate the transition of the metastable state into the amorphous one rather than a phase transition.

Evidence for Tyndall melt-figure production during spallation-induced pressure spikes associated with ice crushing

In discussing ice crushing in the brittle regime Gagnon (2016) had speculated that small Tyndall melt figures, functioning in conjunction with a previously observed thin squeeze-film slurry layer of melt and ice particles at the ice/structure interface, could play a role in the removal of ice from the interface. Here we present supporting evidence for the existence of the melt figures. Using high-speed imaging, small features were observed to rapidly appear and grow in size at the ice/surface interface during pressure spikes caused by spallation of ice in the contact zone during crushing experiments conducted at −10 °C. The observations correlate with data from a previous study of inwardly-propagating Tyndall melt figures that nucleate at the surface of ice, situated in an oil-filled pressure vessel, when subjected to rapid adiabatic pressure increments. That is, during the ice-crushing experiments the mean growth length of the features was 1.5 ± 0.6 mm, where associated ice/surface interfacial pressure spikes (from similar tests) were about 36 MPa. For comparison, the earlier melt-figure study yielded a growth length of ~ 1.7 mm, which is within reasonable agreement with the ice-crushing value for the same rapid pressure increment. Furthermore, the features had orientations that generally were aligned with the two diagonal axes that connected opposing corners of the four-sided pyramid-shaped samples, where confinement was greatest. Hence the internal stress, and associated superheating, in hard-zone ice at the crushing interface during pressure spikes was greatest along the two axes. The features tended to grow in these directions, where the most heat was available, so this suggests they were melt figures. An incidence where a twin pair of features exhibited an angular separation close to a crystal symmetry angle also suggests the features were melt figures. The orientation data generally indicate that orientations of melt figures in non-uniformly rapidly-stressed ice may differ from orientations in uniformly stressed ice. We conjecture that in the present study small melt figures form a tenuous liquid matrix of shallow depth (roughly 0.2 mm) in the hard-zone ice surface during pressure spikes. A unique erosive effect may occur as the material in the top portion of this weakened layer is sheared off by, and entrained in, the ambient viscous flow of slurry.

Axial segregation in crystal growth from a thin melt layer

During a crystal growth from a thin melt layer, the transport of dopant occurs mostly by diffusion. One can describe the dopant distribution by simplified analytical equations, as, for example, those previously obtained by Tiller.We performed numerical 2D axi-symmetric computations of heat and mass transfer in a thin melt layer (0.05<h/d<0.2), in order to find the limits of the validity of such approaches. Sb doped Ge, in a cylindrical volume with a diameter of 200 mm, was used as a model material. Compared with the heat transfer, the mass transfer was found to be much more sensitive to residual convection. Some restrictions in the application of the analytical equations for the description of the experimental data are revealed and necessary recommendations to improve the results are made. In order to provide the uniform axial dopant distribution in the very last portion of the ingot, too, it is necessary to vary the rate of solidification by changing the cooling rate with time.

Frictional melt homogenisation during fault slip: Geochemical, textural and rheological fingerprints

Abstract Volcanic environments often represent structurally active settings where strain localisation can promote faulting, frictional deformation, and subsequent melting along fault planes. Such frictional melting is thermodynamically a disequilibrium process initiated by selective melting of individual mineral phases and softening of volcanic glass at its glass transition as a response to rapid frictional heating. The formation of a thin melt layer on a fault plane surface can drastically accelerate or terminate slip during fault motion. A comprehensive understanding of the physical and chemical properties of the frictional melt is required for a full assessment of slip mechanisms, as frictional rheology depends on the contributions from selectively melted mineral and glass phases as well as the physical effects of restite fragments suspended in the frictional melt. Here, we experimentally investigate the impact of host-rock mineralogy on the compositional and textural evolution of a frictional melt during slip. High-velocity rotary shear (HVR) experiments were performed under controlled, volcanically relevant, coseismic conditions (1 m s−1 slip rate and 1 MPa normal stress) using three intermediate dome lavas with contrasting mineral assemblages, sampled from volcanic systems where fault friction is evident: (1) an amphibole-bearing andesite (Soufriere Hills Volcano, Montserrat); (2) an amphibole-poor dacite (Santiaguito dome complex, Guatemala); and (3) an amphibole-free andesite (Volcan de Colima, Mexico). For each sample, five HVR experiments were terminated at different stages of frictional melt evolution, namely: (1) at the onset of melting, (2) upon formation of a steady-state melt layer, and (3) after 5 m, (4) 10 m, and (5) 15 m of slip at steady-state conditions. Progressive mixing and homogenisation of selective, single-phase melts within the frictional melt layer through double-diffusion convection demonstrates the control of melt composition on slip behaviour. Amphiboles melted preferentially, leading to consistently lower shear stress (∼1 MPa less than amphibole-poor samples) and pronounced shear-weakening during the frictional melting of amphibole-bearing lavas. The results highlight the implications of mineral assemblages on fault slip, including conduit flow processes, which may influence the style of eruptions, and run-out distances of rapid granular flows.

Joining Performance and Microstructure of the 2024/7075 Aluminium Alloys Welded Joints by Vaporizing Foil Actuator Welding

Vaporizing foil actuator welding (VFAW) was used for joining 2024-T3 and 7075-T6 aluminum alloy sheets, and the resulting joint microstructure was analyzed. 2024/7075 aluminum alloy pairs with suitable processing parameters can be prepared by using VFAW. Dynamic preform addresses the poor formability problem of target material and advantage of VFAW on dissimilar materials in some conditions. But with standoff sheet inserting in the flyer and target, 2024/7075 welded pairs gets the better weld strength, compared with flyer preformed method. The microstructure of the circular weld area of the welded joint showed a wave interface, in which a thin melt layer formed at the center and edge parts. The crystal grains near the bonding interface were remarkably elongated and refined. Therefore, the joining of the 2024/7075 pairs was facilitated through plastic forming and melting.

Mineralogical and compositional features of rock fulgurites: A record of lightning effects on granite

Fulgurites are a naturally occurring glass formed when sand, rock, or soil is struck by atmospheric electrical discharges (lightning). The aim of this paper is to provide insights into the conditions occurring in rocks during the lightning strike. Rock fulgurites collected from Mt. Mottarone, Baveno (Piedmont, Italy) have been investigated to assess the mineralogical and compositional changes occurring in granite due to a lightning strike. X-ray powder diffraction showed that the samples represent the dominant granitic rock type of the Baveno massif, the so-called “Pink Baveno.” Fulgurite coats the surface of the granite as a brown-black, glassy to very fine-grained porous layer. Powder diffraction data for the fulgurite reveal the presence of cristobalite and quartz crystals in a glass matrix, suggesting that temperature exceeded ~1700 °C at near atmospheric conditions, assuming thermodynamic equilibrium. Electron probe microanalysis of the glass revealed that it is mainly composed of SiO 2 and Al 2 O 3 and that it has a porosity of 5–7 area% in the studied zones. The presence of the amorphous phase indicates that the abrupt electrical (Joule) heating of the rock surface yielded high temperatures, producing a thin melt layer on the surface, which then cooled adiabatically. Idealized physical model was developed to simulate the effects of Joule heating and subsequent thermal conduction close to the rock surface during and after a lightning strike. The quantity of organic matter in the glass, obtained via Elemental Analyzer, suggests that rapid quenching of the melt trapped NO x and CO x gases produced during heating. Raman spectroscopy revealed the presence of polyaromatic hydrocarbon molecules, which, combined with the Elemental Analyzer data, suggest that organic matter was pyrolyzed at around 300–350 °C and then trapped in the glass matrix of the studied rock fulgurites.

Zero CO2 steelmaking in a future low carbon economy. 1. Energy conservation in smelting hematite ore directly to refined iron slab

ABSTRACTThe emphasis is on energy conservation. Efficient charge preheating by heat recovery from not only the molten reduction slag but even more important is to eliminate water cooling in the continuous casting of the molten iron, which overflows automatically from the primary melt circulation loop. It is proposed that a liquid metal coolant is used to capture the latent heat of solidification and also the sensible heat associated with cooling the near net shape cast refined iron slab from the melting point down to say 1000°C or so by counter-current radiative heat transfer to secure 800°C charge preheating. The preheated crushed hematite ore plus flux is distributed along the full length of the electrical conductively heated charge incorporation zone of the principal melt circulation loop at a multiple number of locations. The resulting wustite containing thin melt layer floating on the circulating molten iron is counter-currently reduced by hydrogen at atmospheric pressure.

Bulk Crystallization of Supercooled Metal Films

In current work nonequilibrium theory of bulk crystallization of melts rapidly quenched to deeply supercooled state is developed. The problem of solidification of a thin layer of metal melt brought into contact with a massive cold substrate is considered.

Ge distribution in Si0.9Ge0.1 alloy ingot grown from thin melt layer

We have studied experimentally and theoretically the possibility to obtain a uniform single crystal of SiGe alloy enriched at the Si side. The content of the second component in a crystal 15mm in diameter and 40mm in length grown by the modified floating zone technique from the charge of 79.8 at.% Si and 20 at.% Ge composition with 0.2% B admixture has been investigated using selected area X-ray analysis in different points and in line scanning mode along and across the crystal axis. The longitudinal changes in the germanium concentration proved to be well described by the analytical equation previously derived for conditions of Sb (Ga) doped Ge growth from a thin melt layer in the presence of a heater submerged into the melt. For a more accurate description of the experimental data we have made allowance for the change in the melt layer thickness between the growing crystal and the bottom of the submerged heater. The lateral distribution of the second component not exceeding 5% over the crystal diameter can be significantly improved by reducing the curvature of the phase interface during the growth.

Bulk Crystallization of Supercooled Metal Films

The overwhelming majority of modern materials are knowingly produced by crystallization from melts. Leaving apart powder metallurgy and some other technologies, the mainstream is melting and homogenizing a charge followed by its transition to a solid state. In this case, technologies based on a method of ultrafast hardening from a liquid state, the key point of which is putting a substance into a deep metastable state with a further intense phase transition, are of particular importance. This allows producing amorphous, nanocrystalline, bioactive, and other promising materials widely used in modern industry and technologies, microelectronics, medicine, etc. In particular, they can be used as durable anti-corrosion coatings. Since the quality and physicochemical properties of these materials depend directly on their morphology, the skill in controlling this morphology is an effective way of producing materials with specified functional characteristics. This requires the ability to describe in detail the kinetics of phase transitions at all stages of the process. Like any other first-order phase transitions, crystallization occurs via fluctuation-induced nucleation and growth of centers of a new phase. Hence, the overall kinetics of such a process is determined by nucleation and the growth rate of nuclei. These quantities are related to a set of various thermodynamic and kinetic parameters that are important under specific conditions and primarily to a degree of metastability of the parent phase. The main problem here is finding the time dependence of the fraction of the crystalline phase and the size distribution of the nuclei. The kinetic theory of fluctuation-induced nucleation and crystal growth is elaborated pretty well, whereas the thermophysical aspects of this problem are poorly investigated. In particular, the effect of heat release at the phase transition on the rate of the process is insufficiently studied. Theoretical analysis of phase transition shows that for the most cases the process can be divided into two particular stages. The first one is the nucleation stage. The majority of new phase nuclei form during this particular stage. Nonlinear nature of nucleation expressed in strong dependence of nucleation rate from the degree of metastability. At the beginning of this stage the process is conditioned by the factors producing metastability, though the stage ends due to reduction in the degree of metastability. This reduction results from the transition of a portion of material from metastable phase to stable phase accompanied by release of latent heat of phase transition. Typically the portion of the new phase is relatively small at the moment of the end of nucleation stage. This is due to the fact that formation of even small portion of a new phase is enough for significant reduction in nucleation rate. The main part of initial phase transits to stable state during the subsequent growth of crystals already formed during nucleation stage. In studying the phase transition the nucleation stage attracts great interest, but it is a problem of great complexity. It involves thermodynamics of small systems, nucleation kinetics, knowing the growth mechanism in a wide range of parameters, correct accounting for effect that assembly of crystals has on metastability, etc. Combined with account for the factors producing metastability it all leads to challenging, nonlinear mathematical problem described by the system of integro-differential equations. At this time the complete solution of this problem is yet to be found. In current work nonequilibrium theory of bulk crystallization of melts rapidly quenched to deeply supercooled state is developed. This theory correctly takes into account the heat release due to phase transition. It is achieved by using the real temperature fields formed around crystals in kinetic equations. Limits of validity of theories using heat release obtained from global heat balance are estimated. Proposed theory allows to describe accurately the kinetics of crystal nucleation and growth in metastable liquid and to determine the microstructure of solidified material. Particular attention was paid to the mechanism of crystal growth in highly nonequilibrium conditions and with account taken of the shrinkage during solidification. The problem of solidification of a thin layer of metal melt brought into contact with a massive cold substrate is developed. Based on the amorphization conditions formulated, the thickness of the amorphous sublayer formed near substrate is determined. The suggested model of bulk crystallization of melt makes it possible to find the morphology of the solidified layer. The work is supported by RFBR (grant #15-08-04474).

Experimental Investigation and Modeling of a New High Speed Coating Process

Abstract Health regulations are becoming increasingly restrictive which leads to develop solvent free laminating processes. Hot melt adhesives represent an interesting solution to bond two pre-printed substrates of polymer for packaging applications for example, but their high viscosities require extruding the polymer formulation through important die gaps (between 0.5 mm and 1 mm) in order to limit the extrusion pressure. Thus, delivering a final very thin layer of hot melt (a few μm) between the polymer substrates requires applying very high draw ratios between the die and the contact point on the substrate. These high draw ratios would result in marked thickness and width instabilities (called draw resonance) when using usual stretching distances as in cast film (a few cm) or in coating thermoplastic layers on a metal or paper substrate (more than 10 cm). Surprisingly, reducing drastically the stretching distance (around 1 mm) and applying a differential pressure between the two sides of the melt allow postponing the instability occurrence. Nevertheless the processing window is narrow and the objective of the present paper is to describe the cotformatating process developed at Bostik Company, to characterize the drawing instabilities and to investigate the influence of different processing parameters. In addition, a numerical model explaining these surprising stabilizing conditions is proposed. This model results in the definition of a high speed coating processing window.

THERMOPHYSICAL ASPECTS OF THE AMORPHOUS METALS PRODUCTION

An algorithm of solving a thermal problem of a thin melt layer quenching was developed. The amorphous structure forming technology is realized through the rapid heat removal and ensuring the rapid melt quenching. The quenching rate should not be lower than 104...106 K/s for alloys and 106 K/s for pure metals. As a result of the experimental studies, the amorphous structure of the deposited metal has been obtained and the technology implementation conditions have been defined.

Conceptual zero CO2 mine site continuous smelting of goethitic high phosphorus iron ore to refined steel with enhanced safety

A projected low energy continuous route for viably accessing vast reserves of high phosphorus iron ores at existing mine sites has been identified. Rather than upgrading the iron ore in preparation for export, goethitic ore, for example, should be autonomously smelted with zero CO2 emission to refined steel product at the mine site of the future. Enhanced safety in comparison with alternative options currently under consideration is mandatory. The proposed response to the challenge adapts established Ruhrstahl Hereaus (RH) steel vacuum degassing or other appropriate technology to closed loop circulate a carbon free molten iron carrier medium to transport input thermal energy to sites associated with hydrogen reduction of wüstite above its melting point at a moving thin oxidic melt layer. Carrier medium deoxidation, in advance of RH based melt circulation and electrical conductive heating, is implemented safely to satisfy all thermal energy demands in the melt circulation loop, operating at essentially atmospheric pressure.

Three‐dimensional flow in a thin annular layer of silicon melt with bidirectional temperature gradients

Bidirectional temperature gradients coexist virtually in surface tension driven flows. However, the simulations have been performed to the flow with only one temperature gradient. A series of 3 D numerical simulations are conducted to investigate the Marangoni‐thermocapillary flow of silicon melt in a thin annular layer with bidirectional temperature gradients. The temperature gradients are produced by the temperature difference ΔT between walls and the constant heat flux q on the bottom, respectively. When changing q, the melt presents different state evolutions at different ΔT. Furthermore, two critical q are found, one makes the minimum melt temperature higher than the crystallization temperature and the other makes the flow unsteady. Both of the critical heat fluxes decrease with increasing ΔT. q contributes more to the elevation of the melt temperature, while ΔT contributes more to the enhancement of the melt instability. In addition, the melt on the free surface flows mainly along the radial direction.

Scale bridging between atomistic and mesoscale modelling: applications of amplitude equation descriptions

Amplitude equations are discussed as an extension of phase field models, which contain atomic resolution and allow one to describe polycrystalline structures, lattice deformations and defects. The interaction of adjacent grains, which are separated by a thin melt layer, leads to structural interactions if the grains are slightly misplaced, similar to the concept of γ-surfaces. We are able to predict these interactions essentially analytically, leading to a superposition of short-ranged interaction terms related to the individual density waves. Deviations from the analytical predictions are found only at short distances between the grains and are most pronounced in situations with different ranges of the contributions. Furthermore, we demonstrate the ability of the amplitude equation model to predict dislocation pairing transitions at high temperatures, which supports earlier findings using molecular dynamics and phase field crystal simulations. To effectively perform the numerical simulations, we present a way to implement the model on graphics cards. An enormous acceleration of the code in comparison to a single CPU code by up to two orders of magnitude is reached.

Ge Distribution in Si0.9Ge0.1 Alloy Ingot Grown from Thin Melt Layer

We studied experimentally and theoretically the possibility to obtain a uniform single crystal of SiGe alloy enriched at the Si side. The content of the second component in a crystal 15 mm in diameter and 40 mm in length grown by the modified floating zone technique from the charge of 79.8 at.% Si and 20 at.% Ge composition with 0.2% B admixture has been investigated using selected area X − ray analysis in different points and in line scanning mode along and across the crystal axis. The longitudinal changes in the germanium concentration of proved to be well described by the analytical equation previously derived for conditions of Sb (Ga) doped Ge growth from a thin melt layer in the presence of a heater submerged into the melt. For a more accurate description of the experimental data we made allowance for the change in the melt layer thickness between the growing crystal and the bottom of the submerged heater. The lateral distribution of the second component, not exceeding 5% over a diameter of the crystal, can be significantly improved by reducing the curvature of the phase interface during the growth.