A phase-field model for the diffusive melting of isolated dendritic fragments

A thermal phase-field model constructed in the "thin-interface" limit and incorporating a number of advanced numerical techniques such as adaptive mesh refinement, implicit time stepping, and a multigrid solver has been used to study the isolated diffusive melting of dendritic fragments. The results of the simulations are found to be fully consistent with the experimental observation of such melting in microgravity during the Isothermal Dendrite Growth Experiment. It is found that the rate at which the ratio of semi-major to semi-minor axes changes is a function of the melt Stefan number, which may help explain why both melting at (approximately) constant ratio and melting at slowly increasing ratio have been observed

Analysis of acoustic emissions from gas atomization

It is well known that during close-coupled gas atomization introduction of the melt into the gas stream affects the emitted noise. However, there has been virtually no study of this ‘acoustic signature’. In this paper we present a quantitative comparison of this acoustic signature for an atomizer under gas-only and gas + melt flow. We find that upon introduction of the melt there is strong absorption of frequencies in the 1-8 kHz range. These frequencies are characteristic of the resonance of droplets with 150-600 m diameters and may be indicative of the dynamics of the initial breakup of melt ligaments. Moreover, during atomisation we find that there are considerable low frequency (< 30 Hz) fluctuations in the intensity of the acoustic emissions. We show this may be related to atomizer pulsation, the quasi-periodic low frequency variation in the melt volume instantaneously at the atomizer tip

Spatially Resolved Velocity Mapping of the Melt Plume During High-Pressure Gas Atomization of Liquid Metals

We present details of an image analysis algorithm designed specifically to determine the velocity of material in the melt plume during high-pressure, close-coupled gas atomization. Following high-speed filming (16,000 fps) pairs of images are used to identify and track dominant features within the plume. Due to the complexity of the atomization plume, relatively few features are tracked between any given pair of images, but by averaging over the many thousands of frames obtained during high-speed filming a spatially resolved map of the average velocity of material in the plume can be built up. Velocities in the plume are typically very low compared to that of the supersonic gas, being around 30 m s−1 on the margins of the plume where the melt interacts strongly with the gas and dropping to < 10 m s−1 in the center of the melt plume. Consequently, the efficiency of the atomizer in transferring kinetic energy from the gas to the melt is correspondingly very low, with this being estimated as being no more than 0.1 pct

Mechanical properties of rapidly solidified Ni5Ge3 intermetallic

The congruently melting, single phase, intermetallic Ni5Ge3 has been subject to rapid solidification via drop-tube processing wherein powders with diameters between 850–150 μm are produced. At these cooling rates (850–150 μm diameter particles, 700–7800 K s−1) the dominant solidification morphology, revealed after etching, is that of isolated plate and lath microstructure in an otherwise featureless matrix. Selected area diffraction analysis in the TEM reveals the plate and lath are a disordered variant of ε-Ni5Ge3, whilst the featureless matrix is the ordered variant of the same compound. Microvicker hardness test result shows that mechanical properties improve with decreasing the particle size from 850 to 150 μm as a consequence of increasing the cooling rate

Phase-Field Modelling of Intermetallic Solidification

Many important intermetallic compounds display a faceted morphology during solidification close to equilibrium but adopt a more continuous, dendritic like morphology with increasing departure from equilibrium. We present a phase-field model of solidification that is able to both reproduce the Wulff shape at low driving force and to simulate a continuous transition from faceted to dendritic growth as the driving force is increased. A scaled ratio of the (perimeter)2 to the area is used to quantify the extent of departure from the equilibrium shape

Non-equilibrium processing of Ni-Si alloys at high undercooling and high cooling rates

Melt encasement (fluxing) and drop-tube techniques have been used to solidify a Ni-25 at.% Si alloy under conditions of high undercooling and high cooling rates respectively. During undercooling experiments a eutectic structure was observed, comprising alternating lamellae of single phase γ(NiSi) and Ni-rich lamellae containing of a fine (200-400 nm) dispersion of β- NiSi and α-Ni. This is contrary to the equilibrium phase diagram from which direct solidification to β-NiSi would be expected for undercoolings in excess of 53 K. Conversely, during drop-tube experiments a fine (50 nm) lamellar structure comprising alternating lamellae of the metastable phase NiSi and β-NiSi is observed. This is also thought to be the result of primary eutectic solidification. Both observations would be consistent with the formation of the high temperature form of the β-phase (β/β) being suppressed from the melt

Microstructural Evolution and Phase Formation in Rapidly Solidified Ni-25.3 At. Pct Si Alloy

The drop-tube technique was used to solidify droplets of the Ni-25.3 at. pct Si alloy at high cooling rates. XRD, SEM, and TEM analysis revealed that the metastable phase, Ni25Si9, formed as the dominant phase in all ranges of the droplets, with γ-Ni31Si12 and β 1-Ni3Si also being present. Three different microstructures were observed: the regular and anomalous eutectic structures and near single-phase structure containing small inclusions of a second phase, termed here as heteroclite structure. Both eutectic structures comprise alternating lamellae of Ni25Si9 and β 1-Ni3Si, which, we conjecture, is a consequence of an unobserved eutectic reaction between the Ni25Si9 and β 1-Ni3Si phases. The matrix of the heteroclite structure is also identified as the metastable phase Ni25Si9, in which twined growth is observed in the TEM. As the cooling rate is increased (particle size decreased), the proportion of droplets displaying the entire heteroclite structure tends to increase, with its fraction increasing from 13.91 pct (300 to 500 µm) to 40.10 pct (75 to 106 µm). The thermodynamic properties of the Ni25Si9 phase were also studied by in-situ heating during XRD analysis and by DTA. This showed the decomposition of Ni25Si9 to β 1 and γ-Ni31Si12 for temperatures in excess of 790 K (517 °C)

Morphology of order-disorder structures in rapidly solidified L12 intermetallics

Utilization of intermetallics in high temperature applications is limited due to their poor room temperature ductility. One route to overcoming this is disorder trapping (and subsequent anti-phase domain formation) during rapid solidification, motivating the study of disorder trapping in intermetallics. The single-phase, L1₂ intermetallic beta-Ni₃Ge has been rapidly solidified via drop-tube processing. At low cooling rates (850 – 500 um diameter particles, 700 – 2800 K/s) the dominant solidification morphology, revealed after etching, is that of isolated spherulites in an otherwise featureless matrix. Selected area diffraction analysis in the TEM reveals the spherulites to be partially disordered beta-Ni₃Ge, whilst the featureless matrix is the fully ordered variant of the same compound. Dark-field TEM imaging has confirmed that the spherulites grow as radially emanating fingers of the ordered phase, with disordered material in the space between the fingers

Determination of the origin of anomalous eutectic structures from in-situ observation of recalescence behaviour

A melt encasement (fluxing) method has been used to undercool Ag-Cu alloy at its eutectic composition. The recalescence of the undercooled alloy has been filmed at high frame rate. At low undercooling lamellar eutectic is observed to grow, giving way to a mixed anomalouslamellar structure at higher undercooling. In situ observation of the spot brightness reveals, as expected, that the lamellar eutectic grows via a planar front mechanism, while the anomalous eutectic grows via a more complex process characterised by a double recalescence. The first recalescence is non-space-filling (dendritic) in character and is followed shortly afterwards by a second recalescence which appears to be of the planar front type. Moreover, the first recalescence event appears to be to a temperature in excess of the equilibrium eutectic temperature. This is strongly suggestive that the anomalous eutectic morphology arises due to the growth and subsequent partial remelting of a dendritic morphology, probably a two-phase (eutectic) dendrite, followed by planar front growth of a lamellar eutectic into the residual liquid

Disorder-order morphologies in drop-tube processed Ni3Ge: Dendritic and seaweed growth

The single phase intermetallic β-Ni3Ge has been subject to rapid solidification via drop-tube processing. Droplets spanning the size range 212–38 μm, with corresponding cooling rates of 5800–54,500 K s−1, have been subject to microstructural investigation using SEM. Three dominant solidification morphologies have been identified with increasing cooling rate, namely; (i) well-defined dendrites with orthogonal side-branching, (ii) dendrites with non-orthogonal side-branching and (iii) dendritic seaweed. Selected area diffraction analysis in the TEM reveals that both types of dendrites are the disordered form of β-Ni3Ge in a matrix of the ordered, L12, form. However, the diffraction pattern from the dendritic seaweed cannot be mapped onto a cubic structure, indicating a change in the underlying crystallography coincident with the transition to the seaweed structure