Eutectic Nucleation Research Articles

Nanoparticle-induced nucleation of eutectic silicon in hypoeutectic Al-Si alloy

In this work, significant refinement of eutectic grains in Sr-modified Al-10Si alloys was achieved upon addition of TiC0.5N0.5 nanoparticles (NPs). Results showed that the addition of NPs can lead to a marked reduction in the eutectic grain sizes from 1416μm for the matrix alloy to 328μm for the sample with 1.5vol.% additions. NPs within the eutectic were observed to exhibit two distribution types, i.e. intragranular distribution (within eutectic Si) and extragranular distribution (in the vicinity of eutectic Si). Based on TEM analysis, it is found that TiC0.5N0.5 possesses a specific crystallographic matching with Si, indicating that such NPs can act as heterogeneous nucleation sites for Si. By investigating NP effects on the eutectic nucleation, the NPs-induced nucleation mechanism is proposed to elucidate the refinement phenomenon.

The influence of Cu, Mg and Ni on the solidification and microstructure of Al-Si alloys

The influence of alloying elements (Cu, Mg, and Ni) on eutectic nucleation, eutectic grain morphology and the final microstructure of an Al-10Si commercial purity alloy in unmodified and Sr-modified conditions was investigated. It was found that the nucleation and eutectic grain growth morphologies of both the unmodified and Sr-modified Al-Si eutectic were significantly influenced by the addition of ternary alloying elements to a degree dependent on when the intermetallic phase formed during the solidification of the alloy with respect to the Al-Si eutectic. In cases where an intermetallic phase nucleated prior to the onset of the Al-Si eutectic reaction, the eutectic nucleation frequency was affected by changes to the available nuclei population. In cases where the intermetallic nucleated after the Al-Si eutectic, segregation of the ternary solutes in front of the Al-Si eutectic interface changed the nucleation and macroscopic growth dynamics. The changes in nucleation and growth dynamics of the Al-Si eutectic due to the presence of solute altered the morphology of the eutectic silicon considerably. This study has revealed a number of insights into the mechanisms of nucleation and growth of the Al-Si eutectic.

The influence of ternary alloying elements on the Al–Si eutectic microstructure and the Si morphology

The influence of the ternary alloying elements Cu, Mg and Fe on the Al–Si eutectic microstructure is investigated using a commercial purity Al–10wt%Si alloy in unmodified and Sr-modified conditions. A change in the Al–Si eutectic microstructure was associated with a change in the nucleation density of the eutectic grains caused by the addition of ternary alloying elements. When the ternary alloying element addition resulted in an increase in the eutectic nucleation frequency, a fibrous to flake-like transition was observed within the eutectic grain. When the ternary alloying element addition decreased the eutectic nucleation frequency significantly, a change in the eutectic morphology from flake-like to a mixture of flake-like and fibrous morphologies was observed. The mechanism of Al–Si eutectic modification is discussed. The growth velocity of the eutectic grain – liquid interface and the constitutional driving force available for growth are proposed as important parameters that influence the degree of eutectic modification in Al–Si alloys.

Optimization of the Homogenization Heat Treatment of Nickel-Based Superalloys Based on Phase-Field Simulations: Numerical Methods and Experimental Validation

A method for predicting the fastest possible homogenization treatment of the as-cast microstructure of nickel-based superalloys is presented and compared with experimental results for the single-crystal superalloy ERBO/1. The computational prediction method is based on phase-field simulations. Experimentally determined compositional fields of the as-cast microstructure from microprobe measurements are being used as input data. The software program MICRESS is employed to account for multicomponent diffusion, dissolution of the eutectic phases, nucleation, and growth of liquid phase (incipient melting). The optimization itself is performed using an iterative algorithm that increases the temperature in such a way that the microstructural state is always very close to the incipient melting limit. Maps are derived allowing describing the dissolution of primary γ/γ′-islands and the elimination of residual segregation with respect to temperature and time.

The Effect of Addition of Ce and Sr on the Solidification Path of Al–8Si–2Cu Alloy

Thermal analysis of Al–Si alloys is very useful for the non-destructive evaluation of the melt quality. The present work aimed at the study of the effect of addition of Ce and Sr on cooling curve parameters, microstructure and mechanical properties of Al–8Si–2Cu alloy. The thermal analysis parameters of melt treated alloys were determined. The addition of Ce influenced both primary and eutectic phase nucleation temperatures, whereas, Sr affected only the eutectic nucleation temperature. The depression in the eutectic arrest temperature was higher with Sr addition when compared to Ce. Consequently, a high degree of modification was achieved with Sr. The addition of Ce yielded fine equiaxed α-aluminium grains and Sr treatment resulted in columnar grains. The mechanical properties of the alloy significantly improved on simultaneous addition of Ce and Sr. The individual addition of Sr, Ce and combined addition of Ce + Sr to base alloy increased the tensile strength by about 40, 52 and 83 % respectively.

The influence of ternary Cu additions on the nucleation of eutectic grains in a hypoeutectic Al-10 wt.%Si alloy

The Influence of the ternary alloying element Cu on eutectic nucleation in an Al-10 wt.%Si alloy in unmodified and Sr-modified conditions was studied. Cu additions had a relatively minor effect on the unmodified eutectic nucleation frequency. In Sr-modified Al–Si alloys where the nucleation frequency of the eutectic grains is very low compared to the unmodified alloys, the addition of Cu significantly increased the nucleation frequency. Further increases in the Cu concentration resulted in a continuous increase in the number of eutectic grains and an associated decrease in their size. It is proposed that constitutional supercooling plays an important role in promoting the nucleation of eutectic grains ahead of the solidifying interface especially in the case of Sr-modified Al–Si alloys.

Three Dimensional Morphology of β-Al5FeSi Intermetallics in AlSi Alloys

Abstract Iron is the most common and detrimental impurity in casting alloys and has been associated with many defects. The main consequence of the presence or adding of iron to AlSi alloys is the formation Fe-rich intermetallics with especially deleterious β-Al5FeSi. β-Al5FeSi phases are most often called needles on 2D micro sections, whilst platelets in 3D geometry. The x-ray tomography results have demonstrated Ferich phases with shapes different from simple forms such as needles or platelets and presented bent and branched phases. β grown as complicated structure of bent and branched intermetallics can decrease feeding ability, strengthen pores nucleation and eutectic colonies nucleation leading to lower permeability of mushy zone and porosity in the castings.

Interplay Between Melt Flow and the 3D Distribution and Morphology of Fe-Rich Phases in AlSi Alloys

Abstract The presence of Fe aids in establishing the mechanical and physical properties of AlSi alloys and is also one of the main impurities leading to formation of β-Al5FeSi intermetallics. This study aims to understand the effect of fluid flow on the dendritic microstructure with intermetallics in Al-5/7/9 wt pct Si-0.2/0.5/1.0 wt pct Fe alloys that are directionally solidified under defined thermal and fluid flow conditions. We made extensive use of 3D X-ray tomography to obtain a better insight into the morphology and formation of the intermetallics. Three-dimensional (3-D) distribution of intermetallics presented here shows that the growth of large β-Al5FeSi due to forced flow occurs in the eutectic specimen center and together with an increase in the number density of β precipitates. The 3D reconstructions have verified the β shaped to be curved, bent with twining, branched, and to have imprints, holes, and propeller-shaped platelets. The 3D views showed that hole-shaped β arose from the lateral growth around α-Al dendrites. These views also confirmed the phenomenon of shortening of β as an effect of flow in the dendritic region, where β could be fragmented or completely remelted, and ultimately resulting in microstructures with shorter β-Al5FeSi and increases in number density. The analysis revealed an interaction between melt flow, 3D distribution, and the morphology of β-Al5FeSi. The growth of a large and complex group of β intermetallics can reduce the melt flow between dendrites and strengthen pore nucleation and eutectic colonies nucleation, leading to lower permeability of the mushy zone and increased porosity in the castings.

Effect of vanadium and chromium on the microstructural features of V-Cr-Mn-Ni spheroidal carbide cast irons

The objective of this investigation is to study the influence of vanadium (5.0wt%–10.0wt%) and chromium (0–9.0wt%) on the microstructure and hardness of Cr-V-Mn-Ni white cast irons with spheroidal vanadium carbides. The alloys’ microstructural features are presented and discussed with regard to the distribution of phase elements. The structural constituents of the alloys are spheroidal VC, proeutectoid cementite, ledeburite eutectic, rosette-shaped carbide eutectic (based on M7C3), pearlite, martensite, and austenite. Their combinations and area fraction (AF) ratios are reported to be influenced by the alloys’ chemical composition. Spheroidized VC particles are found to be sites for the nucleation of carbide eutectics. Cr and V are shown to substitute each other in the VC and M7C3 carbides, respectively. Chromium alloying leads to the formation of a eutectic (γ-Fe + M7C3), preventing the appearance of proeutectoid cementite in the structure. Vanadium and chromium are revealed to increase the total carbide fraction and the amount of austenite in the matrix. Cr is observed to play a key role in controlling the metallic matrix microstructure.

Computer Aided Cooling Curve Analysis and Microstructure of Cerium Added Hypereutectic Al–Si (LM29) Alloy

Thermal analysis of LM29 alloy and Ce added LM29 alloys was carried out. The effect of cerium addition on solidification parameters and microstructural features of hypereutectic Al-Si (LM29) alloy was studied using Newtonian analysis technique. Thermal analysis parameters such as primary and eutectic phase nucleation and solidus temperatures were determined. The addition of Ce to LM29 alloy decreased the nucleation temperature of primary silicon and eutectic silicon. The microstructural examination of Ce added LM29 alloys revealed the presence of a polyhedral shaped Al–Si–Ce compound that might have caused the refinement of primary and eutectic silicon. The dendrite coherency point temperature of LM29 alloy was found to be suppressed on addition of Ce.

Undercooling, Cooling Curves and Nodule Count for Near-eutectic Thin-walled Ductile Iron Castings

A solidification model for ductile iron, including Weibull formula for nodule count, has been presented. The principal assumptions of the kinetic nature of growth, depending on undercooling in respect of the eutectic equilibrium temperature and austenite liquidus line, have been adopted, disregarding the diffusion processes, which was justified by the rapid course of the crystallization process in a thin-walled casting. From this model, the following parameters can be determined: cooling curves, kinetics of austenite and eutectic nucleation, austenite and eutectic growth velocity, and volume fraction.The correctness of the mathematical model has been experimentally verified by comparison with literature data in the range of the most significant factors, which include temperature field, the value of maximum undercooling, the value of minimum and maximum temperature on a cooling curve, maximum undercooling, the recalescence and graphite nodule count interrelated with the casting cross-section.

Eutectic Condensation and Nucleation in Vapor–Liquid–Solid Grown ZnO Nanorod Arrays on Si Substrate

It is very important to understand the growth mechanism of ZnO nanostructures for control of the morphologies and further design of optoelectronic devices. In this paper, the eutectic condensation of Zn vapor and catalyst Au, nucleation and epitaxy of ZnO nanorods were intuitionally observed in chemical vapor deposition process by controlling the temperature and atmosphere. The vertically aligned ZnO nanorod arrays were fabricated on Au-coated Si substrate. A Zn-riched buffer layer was found on Si substrate. Under the atmosphere nearly without O2, it was clearly observed that the nano-sized sprouts appeared on the buffer layer and Au-Zn condensed bigger and bigger with increase of temperature. When the O2 was introduced, the as-grown nanorods almost kept the same size and density with those sprouts grown at the same temperature without O2. The morphologies and composition analysis provided direct evidence for a bottom-catalyzed vapor-liquid-solid mechanism of ZnO nanorod arrays on the lattice-mismatched substrate.

Solidification kinetics of a near eutectic Al-Si alloy, unmodified and modified with Sr

The purpose of this work was to explore the differences in solidification kinetics between unmodified and Sr modified eutectic Al-Si alloy as revealed by Fourier Thermal Analysis (FTA) and grain-growth kinetics characterization. Thermal analysis were performed in cylindrical stainless steel cups coated with a thin layer of boron nitride, using two type-K thermocouples connected to a data acquisition system. Grain growth kinetics characterization was carried out using solid fraction evolution and grain density data. FTA results for the non modified and modified alloys suggest that there are changes in the solidification rate during eutectic nucleation followed, during growth, by similar solidification rate evolutions, suggesting that this parameter is governed principally by the heat extraction conditions. On the other hand the change of the grain growth parameters estimated for the experimental probes suggest that the presence of Sr may modify the relationship between grain growth rate and undercooling in eutectic Al-Si.

Combined in situ X-ray radiographic observations and post-solidification metallographic characterisation of eutectic transformations in Al–Cu alloy systems

In this study thin Al–Cu hypoeutectic alloys were mounted in a Bridgman furnace and solidified near isothermally from above the liquidus to below the eutectic temperature. The eutectic transformation is easily identified from the X-ray absorption contrast difference between the semi-solid mush and the fully solid phase in the field of view (FOV). By virtue of real time in situ X-radiography, and subsequent image analysis techniques, it was possible to directly observe and accurately measure eutectic nucleation, as well as transformation rate across the FOV. Post-solidified samples were then subjected to further microstructural analysis, whereby the lamellar eutectic spacings were measured at random locations across the FOV region. The lamellar spacings were correlated with the eutectic transformation rate and showed reasonable agreement with theoretical predictions. Some interesting observations, such as, for example, in situ observations of solidification shrinkage and the effect of liquid feeding through the semi-solid mush, are also discussed.

The role of antimony in modifying of Al-Si-Cu cast alloy

The microstructure evolution and the tensile strength changes of hypoeutectic AlSi6Cu4 cast alloy modified by AlSb10 (0 - 10 000 ppm) was systematically investigated. The samples were study without and after T6 heat treatment. Several types of etching were used, which includes standard black and white etching (0.5 % HF), colour etching by Weck-Al (for documentation the eutectic cells) and deep etching by HCl (for eutectic Si morphology study). The results show that the addition of Sb into AlSi6Cu4 cast alloy should act as a modifier, so it supposes to change the eutectic Si morphology. However, its effect as a modifier is not as significant as we have expected. Its effect was more inoculation and caused refinement of microstructure what has led to mechanical properties increasing. Refinement of microstructure could obviously improve the size and distribution of eutectic cells. The eutectic cells are refined significantly in a fully modified eutectic microstructure (more than 1 000 ppm Sb). It can be speculated that the stick-fibrous transition of eutectic Si morphology involving in impurity modification may be independent of the frequency and mode of eutectic nucleation.

A solidification model for unmodified, Na-modified and Sr-modified Al-Si alloys

An addition of small amounts of Na and Sr is commonly used in the industry to modify the eutectic in Al-Si alloys. Both Na and Sr suppress nucleation of the eutectic forcing nucleation and growth to take place at higher undercooling than in the unmodified material. Thus the scale of the eutectic and the shape of the Si crystals are modified to a fine fibrous form so that the ductility of the material is increased. In the present work a one-dimensional numerical model is proposed that describes nucleation and growth of both primary dendrites and eutectic grains as a function of cooling conditions and modification. The model assumes that dendrites nucleate easily when the liquidus temperature is reached and that they grow as heat is extracted by the mould. Nucleation of the eutectic grains depends on local undercooling and growth is governed by a balance between growth of the eutectic grains and the rate at which heat is extracted by the mould. Experimental data is used to determine constants in the nucleation function. It is shown how cooling conditions and mode of modification influence nucleation and growth conditions.

Nucleation and growth of eutectic cell in hypoeutectic Al-Si alloy

The nucleation and growth of eutectic cell in hypoeutectic Al-Si alloy was investigated using optical microscopy and scanning electron microscopy equipped with electron backscattering diffraction (EBSD). By revealing the eutectic cells and analyzing the crystallographic orientation, it was found that both the eutectic Si and Al phases in an eutectic cell were not single crystal, representing an eutectic cell consisting of small ‘grains’. It is also suggested that the eutectic nucleation mode can not be determined based on the crystallographic orientation between eutectic Al phases and the neighboring primary dendrite Al phases. However, the evolution of primary dendrite Al phases affects remarkably the following nucleation and growth of eutectic cell. The coarse flake-fine fibrous transition of eutectic Si morphology involved in impurity elements modification may be independent of eutectic nucleation.

Solidification Analysis of an Al-19 Pct Si Alloy Using In-Situ Neutron Diffraction

In-situ neutron diffraction and thermal analysis techniques were used simultaneously to evaluate the kinetics of the nonequilibrium solidification process of an Al-19 pct Si binary alloy. Feasibility studies concerning the application of neutron diffraction for advanced solidification analysis were undertaken to explore its potential for high resolution phase analysis coupled with fraction solid/liquid analysis of phase constituents. Neutron diffraction patterns were collected in a stepwise mode during solidification between 983 K and 793 K (710 °C and 520 °C). The variation of intensity of the diffraction peaks was analyzed and compared to the results of conventional cooling curve analysis. Neutron diffraction was capable of detecting nucleation of the Si phase (primary and eutectic), as well as the Al phase during Al-Si eutectic nucleation. Moreover, neutron diffraction indicated the possibility of detecting the presence of Si peaks at near liquidus temperature and premature nucleation of α-Al prior to Al-Si eutectic temperature. The solid and liquid volume fractions were determined based on the change of intensity of neutron diffraction peaks over the solidification interval. Overall, the volume fraction determined was in good agreement with the results of the cooling curve thermal analysis, as well as calculations using the FactSage software. The potential of neutron diffraction for high resolution melt analysis required for advanced studies of grain refining, eutectic modification, etc. was illustrated. This study will help us better understand the solidification mechanism of Al-Si alloys used for various casting component applications.

Microstructure evolution and modification mechanism of the ytterbium modified Al–7.5%Si–0.45%Mg alloys

The effects of ytterbium (Yb) on the microstructure and eutectic solidification behavior of Al–7.5%Si–0.45%Mg alloys were investigated. Samples with different contents of Yb were quenched just after the commencement of eutectic arrest. Microstructures of the as-cast and quenched samples were examined by optical microscopy (OM) and scanning electron microscopy (SEM). The microstructural observations show that the addition of Yb causes a structural transformation of eutectic silicon from a coarse plate to a fine flake-like and some branched morphology. In addition, the formation of Yb-containing phases at higher content of Yb results in a deterioration of modification. Furthermore, the results of thermal and quenching analysis reveal that the addition of Yb reduces the temperatures of eutectic nucleation and growth. The growth rather than nucleation of eutectic is thought to be the primary cause of modification, and the corresponding undercooling caused by the presence of Yb plays an important role in the refinement of eutectic silicon.

Study of Heterogeneous Nucleation of Eutectic Si in High-Purity Al-Si Alloys with Sr Addition

Al-5 wt pct Si master-alloys with controlled Sr and/or P addition/s were produced using super purity Al 99.99 wt pct and Si 99.999 wt pct materials in an arc melter. The master-alloy was melt-spun resulting in the production of thin ribbons. The Al matrix of the ribbons contained entrained Al-Si eutectic droplets that were subsequently investigated. Differential scanning calorimetry, thermodynamic calculations, and transmission electron microscopy techniques were employed to examine the effect of the Sr and P additions on eutectic undercoolings and nucleation phenomenon. Results indicate that, unlike P, Sr does not promote nucleation. Increasing Sr additions depressed the eutectic nucleation temperature. This may be a result of the formation of a Sr phase that could consume or detrimentally affect potent AlP nucleation sites.