Early Stages Of Solidification Research Articles

In-situ formation behavior of NbC-reinforced Fe-based laser cladding coatings

An NbC-reinforced Fe-based composite coating was fabricated through laser cladding. NbC phases existed as particles, polyhedrons, dendrites, and networks in the coating. The liquid–solid interface engulfed the NbC particles in the early stages of solidification, thereby decreasing their sizes. Polyhedral NbC phases evolved from NbC particles that were not engulfed by the interface through a long growth process. Dendritic and network NbC phases presented as eutectic substances that formed in the terminal stages of solidification. The Nb content in the coating decreased because of metal matrix dilution. The dilution rate of Nb was similar to that of the coating. Moreover, not all of the Nb in the coating can react with carbon to form NbC. Approximately one-third of the Nb existed in the coating as solid solution atoms.

Mechanical activation of natural acidic igneous rocks for use in cement

Mechanical activation of three natural acidic igneous rocks, including granite, pumice, and perlite, was studied using a planetary mill. Positive effects of milling were demonstrated with respect to the compressive strength of mortars with the partial substitution of cement by mechanically activated acidic powders. A set of mortar specimens was made with 0%, 10%, 20%, and 30% activated powder replacement.X-ray diffraction and mineralogy studies revealed that the initial granite consists of fully crystalline phases, while pumice and perlite contain not only amorphous but also crypto-crystalline phases. It was found that pumice and perlite rapidly lost their crystalline nature through milling, remaining in solely amorphous phases after 15min of milling, whereas granite maintained its crystalline nature even after 45min. Milling generates a larger specific surface area, much more remarkable for pumice than for granite and perlite, but due to reactive agglomeration, there was a limitation on the specific surface area of pumice. All three mechanically activated acidic powders had significant effects on strength development especially at the early ages of solidification. But in the long term their effects depended on material type and grinding condition. The greatest strength improvement was achieved by mechanically activating perlite for 30min, at 10% substitution in cement mix. Cement consumption can be reduced by up to 30% in weight using mechanically activated powders in cement mix while keeping the 28day compressive strength at a satisfactory level. In addition, ultrasonic pulsation of mechanically-activated perlite indicated that the specific surface area was mostly influential and available in the early stages of solidification, while structural changes appeared to be so at later stages of hardening.

Modeling diffusion-governed solidification of ternary alloys – Part 2: Macroscopic transport phenomena and macrosegregation

Part 1 of this two-part investigation presented a multiphase solidification model incorporating the finite diffusion kinetics and ternary phase diagram with the macroscopic transport phenomena (Wu et al., 2013). In Part 2, the importance of proper treatment of the finite diffusion kinetics in the calculation of macrosegregation is addressed. Calculations for a two-dimensional (2D) square casting (50×50mm2) of Fe–0.45wt.%C–1.06wt.%Mn considering thermo-solutal convection and crystal sedimentation are performed. The modeling result indicates that the infinite liquid mixing kinetics as assumed by classical models (e.g., the Gulliver–Scheil or lever rule), which cannot properly consider the solute enrichment of the interdendritic or inter-granular melt at the early stage of solidification, might lead to an erroneous estimation of the macrosegregation. To confirm this statement, further theoretical and experimental evaluations are desired. The pattern and intensity of the flow and crystal sedimentation are dependent on the crystal morphologies (columnar or equiaxed); hence, the potential error of the calculated macrosegregation caused by the assumed growth kinetics depends on the crystal morphology. Finally, an illustrative simulation of an engineering 2.45-ton steel ingot is performed, and the results are compared with experimental results. This example demonstrates the model applicability for engineering castings regarding both the calculation efficiency and functionality.

Iron contamination in cast quasi-single crystalline silicon ingots

Iron is an important metallic impurity in crystalline silicon for solar cells and it can significantly influence the minority carrier lifetime. We numerically investigated the transport and distribution characteristics of iron impurity in the directional solidification process for quasi-single crystalline (QSC) silicon ingots, and attempted to reveal the relationship between the distributions of iron concentration and the minority carrier lifetime map. Additionally, the seed preservation time was varied to investigate its influence on the diffusion and distribution of iron impurity. The results show that iron diffusion from the silicon melt to the seed crystal during the preservation process can lead to a peak of iron concentration near the seed-crystal interface at the early stage of solidification. This peak gradually disappears during crystal growth, and iron diffusion results in a large region enriched with iron at the ingot bottom after complete solidification. Therefore, we conclude that the iron impurity is related to the large low lifetime region at the ingot bottom. We also discussed the effect of iron contamination on the high-low-high lifetime distribution pattern in the axial direction near the seed-crystal interface, which is a phenomena sometimes reported in QSC silicon ingots. The effect of seed preservation time on iron contamination in the QSC silicon ingot was also investigated. We found that the single-crystalline seed is highly contaminated even after a short time of seed preservation.

Time-resolved X-ray diffraction studies of solidification microstructure evolution in welding

High-brilliance and high-energy polychromatic X-ray diffraction (XRD) has been used for time-resolved studies of rapid solidification microstructure evolution in situ during simulated spot welding in austenitic steels. Weld pools of 5.0mm diameter and 0.6–1.5mm depth were formed at the steel plate surfaces by radiation heating from halogen lamps. Solidification was initiated by powering off the lamps, and completed within 1.5s while measuring diffraction from the solidifying grains of the pool at a 1kHz frame rate. The data contain time-resolved information on individual grain growth and overall solid fraction evolution, and furthermore reveal prominent individual and collective motion of grains during early stages of solidification, presumably caused by convective currents in the pool. Ultrafast polychromatic high-energy XRD is novel in studies of microstructure evolution during welding. The experimental technique could quite readily be used for similar studies in real welding.

Solidification of a colloidal hard sphere like model system approaching and crossing the glass transition.

We investigated the process of vitrification and crystallization in a model system of colloidal hard spheres. The kinetics of the solidification process was measured using time resolved static light scattering, while the time evolution of the dynamic properties was determined using time resolved dynamic light scattering. By performing further analysis we confirm that solidification of hard sphere colloids is mediated by precursors. Analyzing the dynamic properties we can show that the long time dynamics and thus the shear rigidity of the metastable melt is highly correlated with the number density of solid clusters (precursors) nucleated. In crystallization these objects convert into highly ordered crystals whereas in the case of vitrification this conversion is blocked and the system is (temporarily) locked in the metastable precursor state. From the early stages of solidification one cannot clearly conclude whether the melt will crystallize or vitrify. Furthermore our data suggests that colloidal hard sphere glasses can crystallize via homogeneous nucleation.

Transient growth of solid nuclei in the liquid – A numerical study on early stages of solidification

A sharp interface model describing transient non-equilibrium solidification processes for planar interfaces is extended to describe early stages of nucleus growth in an undercooled melt. Interface velocity and interface concentrations are calculated until full equilibrium is reached. Curvature pressure and latent heat release are considered. Realistic Gibbs free energies and concentration gradients in both phases are included in the simulation calculations. The calculations bridge the gap between the length scales of Molecular Dynamics simulations and conventional continuum models that assume local equilibrium at the solid/liquid interface.

Advanced Solute Conservation Equations for Dendritic Solidification Processes Part I: Experiments and Theory

AbstractThe macrosegregation formed in dendritic equiaxed structure during early stages of solidification of Al‐4.5%Cu alloy has been studied by experimental work and by metallurgical study of cast samples taken from the experimental work. An experimental work was conducted to study the coupled effect of natural convection streams, interdendritic strain and mushy permeability of Al‐4.5%Cu aluminum alloy solidified in horizontal rectangular parallelepiped cavity at different superheats. The metallurgical study includes macro‐microstructure evaluation, measurements of grain size of equiaxed crystals and macrosegregation analysis. This study shows that the level of surface segregation exhibiting as positive segregation varies with superheat whereas the rest of inner ingot areas show the light fluctuation in segregation values. In addition to experimental work, there is a mathematical study which contains a complete derivation of local solute redistribution equations based on Fleming's approach under different solute diffusion mechanisms in the dendritic solid. This derivation includes also the effects of interdendritic strain and mushy permeability on the local solute redistribution distribution. Owing to the length of the study, it is presented in two parts. The first part describes the experimental work and its results as well as a detail derivation of solute conservation equations. This part also involves comparison and discussion between existing and proposed solute conservation equations. The second part contains the mathematical analyses of a two dimensional mathematical model of fluid flow, heat flow, solidification, interdendritic strain and macrosegregation. Also, this part also contains the numerical simulations by using finite difference technique “FDT” to create convection patterns, heat transfer, interdendritic strain, and macrosegregation distributions. This part also includes comparisons between the available measurements and model predications as well as full discussion of different model simulations. The mechanism of interdendritic strain generation and macrosegregation formation during solidification of dendritic equiaxed structure under different diffusion mechanisms in dendritic solid has also been explained and discussed.

Growth and interaction of dendritic equiaxed grains: In situ characterization by synchrotron X-ray radiography

This paper addresses dendritic equiaxed growth during grain structure formation in alloy solidification, and focuses on the growth and interaction of primary dendrite arms, which drive and limit grain development, respectively. Dedicated solidification experiments on Al–10wt.% Cu alloy were carried out at the European Synchrotron Radiation Facility, enabling live observation by in situ and real-time synchrotron X-ray radiography. Equiaxed solidification was achieved by cooling nearly isothermal samples, and the growth of the grains was continuously monitored from the very early stages of solidification up to a state where changes in grain structure were no longer visible to the observer’s naked eye. The time evolution of individual equiaxed grains is analysed through measurements of the lengths and tip velocities of the primary dendrite arms driving grain growth. It is found that equiaxed dendrite propagation is characterized by two growth regimes. The first regime, in which a gradual increase in the tip growth rate is observed, corresponds to free dendritic growth, when the relative distance between grains is large enough to consider a grain as isolated. During this regime, the far-field liquid concentration ahead of the advancing dendrite tip remains constant, equal to the initial alloy composition, and the driving force for growth is the undercooling applied by cooling of the sample, in agreement with three-dimensional models of free dendritic growth. The second regime shows a decrease in the tip velocity towards zero and is attributed to grain interaction when the solute diffusion field in front of the advancing dendrite tip under consideration overlaps with the solute field associated with the growth of the grain(s) facing it. Solute concentration profiles are measured in the extra dendritic liquid ahead of dendrite tips interacting with neighbour grains, from which the far-field solute concentration and neighbour–interaction distance are deduced. These measurements confirm that impingement of the solute boundary layers is the main cause of dendrite growth being slowed down and stopped.

3D In Situ Imaging of Aluminium Alloys During Solidification

X-ray tomography has become a widely used 3D characterisation technique in materials science either using laboratory tomographs or large X-rays facilities. The advantage of large X-rays facilities is the high photon flux which allows fast tomography to be performed. With fast acquisition CCD cameras, it is possible to obtain a 3D image of the material in less than 30 s with a spatial resolution of the order of 1 or 2 μm. We will show that this allows to study the influence of copper addition on the formation of intermetallics formation in Al–Si–Fe–Cu alloys: we will show that copper seems to have beneficial effects. With CMOS cameras it is possible to reduce acquisition times down to values of about 0.15 s. We will show that this allows to obtain in situ 3D information on the early stage of solidification in aluminium alloys, only accessible up to now using radiography : the solidification front rate was measured on several individual dendrites an Al–20wt%Cu alloy and estimated to be around 10 μm/s.

Grain refining mechanism of carbon inoculation on Mg-Al alloy

The processing parameters and refining mechanism of Mg-Al alloy treated with a newly developed carbon inoculant under different conditions were investigated experimentally in this work. Results show that the finest α-Mg grain in AZ91D alloy can be obtained after processing at about 740°C, and the average grain size of α-Mg grain decreases from about 180.4 to 85.6 μm by adding mass fraction win = 1% inoculant into melt. However, no evident refinement was achieved with excessive inoculant for Mn-free Mg-9Al alloy. Scanning electron microscope (SEM) photo, energy dispersive spectroscopy (EDS) analysis on the Mn-contained intermediate phase and differential scanning calorimetry (DSC) results indicate that Mn element plays an important role in the heterogeneous nucleation of α-Mg grain. In the early stage of solidification, Al-Mn-C compound particles formed on the surface of Al4C3 nano-particles should be the potential nuclei for primary α-Mg and probably responsible for the grain refinement achieved in the carbon inoculation process.

The evolution of AlTiSi intermetallic phases in Ti-added A356 Al–Si alloy

Although there are reports in open literature on the formation and morphologies of AlTiSi particles in aluminum alloys, there is not much information on its growth mechanism or orientation relationship with the aluminum matrix. The flakes and blocks morphologies of AlTiSi intermetallics in foundry alloys is similar to that of Al3Ti. However, there is not any report to date to specify the favorite conditions under which either of these morphologies would prevail. This has technological bearing since the plate like morphology could grow to such extends to disturb or even hinder the flow of molten alloy within feeding channels.The application of optical, SEM and TEM in analysis of as-cast Ti-added A356 Al–Si alloy shown the formation of AlTiSi intermetallics with different morphologies and sizes. In addition, the two main morphologies of block and flake AlTiSi particles are originated from the fine needles and spherical particles form during the early stages of solidification. The orientation relationships between the intermetallic phases and Al matrix are also examined.

Morphological Instability in Early Stages of Solidification of Pure Metals Part II: Experimental Analysis

A simple experimental setup has been used to investigate the morphological instability observed in many solidification processes. It is shown that the mold with smooth surface exhibits prominent thickness nonuniformities. Periodic grooves in the mold surface results in more uniform contact along the mold-shell interface, and therefore leads more uniform growth of the shell. Experimental results for the moving solidification front have also been compared to the results of a simple theoretical model introduced in Part I, and a good agreement has been obtained.

Morphological Instability in Early Stages of Solidification of Pure Metals Part I: A Theoretical Model

A simple one dimensional model has been introduced to investigate the morphological instability observed in many solidification processes. It is shown that the solidified shell material with higher thermal conductivity might result in planar shell growth, whereas the mold material with higher thermal conductivity may cause irregular growth of the shell which, generally, causes cracking near the surface, and the thicker mold causes faster growth of the shell, and the higher thermal contact resistance leads to faster growth of the shell.

In situ and real-time analysis of the growth and interaction of equiaxed grains by synchrotron X- ray radiography

The phenomena involved during equiaxed growth are dynamic, so that in situ and real-time investigation by X-ray imaging is compulsory to fully analyse the microstructure formation. The experiments on Al - 10 wt% Cu alloy of this paper are carried out at the European Synchrotron Radiation Facility (ESRF) in Grenoble (France). Equiaxed growth was achieved in nearly isothermal conditions and continuously monitored from the very early stages of solidification to an asymptotic state. First, measurements of dendrite arms velocity for a same grain showed slight differences in the early stages of the growth. This effect is attributed to a gravity-related "self - poisoning" of the grain. Then, the propagation of primary dendrite arms was analysed and two successive growth regimes were observed. First, due to the relative distance with neighbour grains, each grain could be considered as isolated (i.e. growing freely) and tip growth rate gradually increased. In a subsequent phase, tip growth rate slowly decreased towards zero, due to the proximity of neighbouring grains. Using an image analysis technique, we were able to measure the solute profiles in the liquid phase between interacting arms. These measurements confirmed that solutal impingement is responsible for stopping the grain growth.

Ultra Fast <i>In Situ</i> X-Ray Micro-Tomography: Application to Solidification of Aluminium Alloys

X-ray micro-tomography has been applied recently in a wide range of research fields (damage in materials, solidification …). Thanks to the high flux of synchrotrons and specific cameras the total time to acquire a scan was considerably reduced. The use of a specific camera based on CMOS technology allows dividing the acquisition time for a complete scan by a factor of 100. Therefore we have been able to perform in situ solidification of aluminium-copper alloys at high cooling rates (between 1 and 10°C/s) and we will show results concerning the evolution of the microstructure in 3D in the early stage of solidification, in particular the morphology of the solid phase and the kinetics of growth.

Double-Diffusive Convection on the Solid-Liquid Interface in the Early Stage of Solidification

In the present study, we report on the effect of double diffusive convection associated with solute discharge in the early stage of unidirectional solidification from above. Two-dimensional numerical simulations have been conducted to clarify the concentration, temperature and heat flux on the solid-liquid interface under a constant solid layer growth rate. The moving boundary due to solid layer growth was formulated according to the boundary tracking method. The simulations were conducted by varying the initial conditions; concentration, solid layer growth rate and heating temperature at the bottom. The effects of thermal and solutal Rayleigh number on the concentration and the convective heat flux at the solid-liquid boundary were discussed, and the associated correlations to predict those values as functions of the initial thermal and solutal Rayleigh numbers as well as the speeds of solidification were derived.

非脱ガス処理鋼の連続鋳造における水素起因ブレークアウトの解析

Hydrogen-induced sticker breakouts in aluminum-killed steel production without a degassing route have often been reported elsewhere, but so far, yet the mechanism has not been thoroughly explained quantitatively. In order to obtain a better understanding of this phenomenon, hydrogen gas evolution from the solidified shell at the early stage of solidification in continuous casting was analyzed, taking into account the δ→γ transformation. It is concluded that lower hydrogen contents, lower casting speeds and higher consumption rates of mold powder are preferable to prevent the hydrogen-induced breakouts of aluminum-killed non-degassed steel.

Morphological Instability in Early Stages of Solidification of Pure Metals Part II: Experimental Analysis

A simple experimental setup has been used to investigate the morphological instability observed in many solidification processes. It is shown that the mold with smooth surface exhibits prominent thickness nonuniformities. Periodic grooves in the mold surface results in more uniform contact along the mold-shell interface, and therefore leads more uniform growth of the shell. Experimental results for the moving solidification front have also been compared to the results of a simple theoretical model introduced in Part I, and a good agreement has been obtained.

Morphological Instability in Early Stages of Solidification of Pure Metals Part I: A Theoretical Model

A simple one dimensional model has been introduced to investigate the morphological instability observed in many solidification processes. It is shown that the solidified shell material with higher thermal conductivity might result in planar shell growth, whereas the mold material with higher thermal conductivity may cause irregular growth of the shell which, generally, causes cracking near the surface, and the thicker mold causes faster growth of the shell, and the higher thermal contact resistance leads to faster growth of the shell.