Nucleation Undercooling Research Articles

The Nucleation Potency of In Situ-Formed Oxides in Liquid Iron

The nucleation potency of iron oxides was verified experimentally through nucleation undercooling of liquid iron using aerodynamic levitation technology for minimized container contaminations. Steady undercooling values were subsequently obtained from multiple melting and freezing thermal cycles, with the average undercooling values of 223 K ± 3 K and 75 K ± 6 K (223 °C ± 3 °C and 75 °C ± 6 °C) for FeO-contained liquid and Fe3O4-contained liquid, respectively. The statistical results showed a negligible difference in the sizes and numbers of particles between FeO and Fe3O4 particles, indicating that the nucleation potency difference is attributed to the nature of nucleants rather than particle size or numbers. Furthermore, high-resolution transmission electron microscopy analysis showed that the potential nucleation interfaces can be assumed as $$ \{ 1 1 0\}_{{\delta {\text{{-}Fe}}}} //( 0 0\bar{2})_{\text{FeO}} $$ and $$ \{ 1 1 2\}_{{\delta {\text{{-}Fe}}}} //(\bar{2} 0 2 )_{{{\text{Fe}}_{3} {\text{O}}_{4} }} $$ , based on the detected exposed crystal planes of the oxide particles. Both the interfaces have relatively large values of lattice misfit, consistent with the experimentally measured undercooling based on Turnbull’s lattice matching theory.

A Nucleation Progenitor Function approach to polycrystalline equiaxed solidification modelling with application to a microgravity transparent alloy experiment observed in-situ

A Nucleation Progenitor Function (NPF) approach that accounts for the interdependence between nucleation and growth during equiaxed solidification is proposed. An athermal nucleation density distribution, based on undercooling, is identified as a progenitor function. A Kolmogorov statistical approach is applied assuming continuous nucleation and growth conditions. The derived progeny functions describe the (supressed) distribution of actual nucleation events. The approach offers the significant advantage of generating progeny functions for volumetric (3D) data and projected image (2D) data. The main difference between 3D and 2D data in transparent alloy experiments is due to a stereological correction for over-projection. Progeny functions can be analysed to obtain statistical output information, e.g., nucleation counts, average nucleation undercooling and standard deviation. The statistical output data may be calculated in a formative (running) or a summative (final) mode. The NPF kinetics have been incorporated into a transient thermal model of equiaxed solidification. The model has been applied to characterise a microgravity solidification experiment with the transparent alloy system Neopentylgycol-30 wt%(d)Camphor. The model predicted thermal and observed nucleation and growth data with a good level of agreement.

Effect of cooling rates of production process of Al-3%B-3%Sr master alloy on grain refinement and eutectic modification efficiency in cast A356 alloy

In this paper, size and morphology of the grain refiner and modifier particles in the Al-3%B-3%Sr master alloy production by using different cooling rates were investigated. Two Al-3%B-3%Sr master alloys were produced with 0.2 and 10°C/s, respectively. The grain refinement and eutectic modification efficiency of the Al-3%B-3%Sr master alloy were tested in casting process of A356 alloy by addition of 4wt.% and holding times for 10-120 min. The experimental result showed that microstructure of the M1 alloy (Slow cooling) consisted of larger solidified particles of AlB2, SrB6and Al4Sr in the matrix of α-Al compared to the M2 alloy (Rapid cooling). The addition of the M1 alloy in cast A356 alloy, it was found that small grain size and fully modify eutectic silicon were obtained from the holding time in a range of 10-60 min. While the addition of M2 alloy, a small grain size was achieved in shorter holding time in a range of 10-30 min but the eutectic silicon was partly modify. From the thermal analyzed result, solidification of un-modified A356 alloy was changed after addition of Al-3%B-3%Sr master alloy. It was clearly observed that both the undercooling of nucleation and eutectic reaction was reduced and the solidification time was shifted to longer.

Modelling of nucleation in Impulse atomized undercooled droplets

Numerous previous researchers have used nucleation models as part of their simulation of droplet solidification. None of these have verified the nucleation portion of these models with experimental results. In this work, a heterogeneous nucleation model based on the classical nucleation theory was coupled with a thermal model of a falling droplet through a stagnant gas. The pre-exponential factor, which is usually unknown, was determined to obtain an undercooling similar to experimental results for a 196 μm droplet solidified in helium. A stochastic element was added by using a cumulative density probability function, leading to a range of possible nucleation undercoolings and nucleation points. The primary and eutectic undercoolings as well as the number of nucleation points were then compared with additional experimental results of Al-Cu alloy droplets. Good agreement was found between the calculated and the experimental undercoolings. Modeling results also suggest the presence of one nucleation point in the majority of cases and are in agreement with experimental observations.

Effects of scandium addition on microstructure, mechanical and thermal properties of cast Be-Al alloy

The microstructure modification, mechanical property reinforcement, and thermal behaviors of arc-melted casting beryllium-aluminum (Be-Al) alloys with the addition of various contents of scandium (Sc) were studied in this work. The addition of 0.2wt% Sc shortened the secondary dendritic arm spacing (SDAS) of Be dendrites; the addition of 0.4wt% Sc further decreased the SDAS, weakened the dendritic structure of Be, and afforded equiaxed/cellular alloy microstructure with the finest Be grain size; however, higher additions of Sc till 3.0wt% enlarged the SDAS of Be and coarsened the Be grains. An explanation for the microstructure variation was proposed. The addition of Sc resulted in the formation of intermetallic compounds Be13Sc and Al3Sc distributed in the Be and Al matrices and along the Be/Al grain boundaries. It was observed that Sc preferentially formed Be13Sc and rendered the Be phase less plastic. The addition of Sc increased the hardness and elastic modulus of Be-Al alloys while strengthening the Be phase to a higher extent than Al. Thermal analysis showed that the Al phase had the highest latent heat of 167.6 J/g during melting and a low nucleation undercooling of 16.4 °C during solidification with the addition of 0.4wt% Sc. Heat treatment and water quenching changed the relative contents of Be13Sc and Al3Sc and further strengthened the alloys.

Modeling of Dendritic Evolution of Continuously Cast Steel Billet with Cellular Automaton

In order to predict the dendritic evolution during the continuous steel casting process, a simple mechanism to connect the heat transfer at the macroscopic scale and the dendritic growth at the microscopic scale was proposed in the present work. As the core of the across-scale simulation, a two-dimensional cell automaton (CA) model with a decentered square algorithm was developed and parallelized. Apart from nucleation undercooling and probability, a temperature gradient was introduced to deal with the columnar-to-equiaxed transition (CET) by considering its variation during continuous casting. Based on the thermal history, the dendritic evolution in a 4 mm × 40 mm region near the centerline of a SWRH82B steel billet was predicted. The influences of the secondary cooling intensity, superheat, and casting speed on the dendritic structure of the billet were investigated in detail. The results show that the predicted equiaxed dendritic solidification of Fe-5.3Si alloy and columnar dendritic solidification of Fe-0.45C alloy are consistent with in situ experimental results [Yasuda et al. Int J Cast Metals Res 22:15–21 (2009); Yasuda et al. ISIJ Int 51:402–408 (2011)]. Moreover, the predicted dendritic arm spacing and CET location agree well with the actual results in the billet. The primary dendrite arm spacing of columnar dendrites decreases with increasing secondary cooling intensity, or decreasing superheat and casting speed. Meanwhile, the CET is promoted as the secondary cooling intensity and superheat decrease. However, the CET is not influenced by the casting speed, owing to the adjusting of the flow rate of secondary spray water. Compared with the superheat and casting speed, the secondary cooling intensity can influence the cooling rate and temperature gradient in deeper locations, and accordingly exerts a more significant influence on the equiaxed dendritic structure.

In-situ studies of multicrystalline silicon nucleation and growth on α- and β-Si3N4 coated substrates

The growth of silicon on various nitride coated quartz substrates were studied using in-situ observation of the solidification process. Three different coating types were used: One consisting entirely of α-Si3N4 particles, one of only β-Si3N4 particles, and a third of a 50/50 mixture of the above mentioned coating powders. The mean particle size of the α- and β-particles was about 0.3 μm and 5.7 μm, respectively. Three different cooling rates were used for each coating type: 2, 5 and 10 K/min. It was observed that the samples were similar at the lowest cooling rate, but at 5 K/min and higher the samples differed significantly. The biggest difference was seen in the α-particle coating, which showed significant dendritic growth, compared to the more faceted growth observed from the other coatings. All coatings containing the β-particles showed similar growth characteristics. These samples were also analyzed by electron-backscattered diffraction (EBSD) on both the vertical and horizontal planes. No clear trend in preferred crystal orientations was observed; however, it was seen that the density of Σ3 boundaries, especially parallel twins, increased with cooling rate. A gradual increase in the Σ3 grain boundary density was seen for the coatings containing β-Si3N4 particles, while the α-particle coating showed a faster increase. The grain sizes were also analyzed from the horizontal EBSD maps. Again there was a clear difference between the samples containing β-particles and the sample with α-particle coating. The ones containing β-particles showed a decrease in grain size with increasing cooling rate, while the opposite was true for the other α-particle coated samples. This was attributed to the rapid dendritic growth, which caused the grain structure to be dominated by one single grain. The difference in growth for the three coating types was explained using the athermal nucleation theory, which states that there is a correlation between particle sizes and nucleation undercooling.

Ultrasound Assisted Casting of an AM60 Based Metal Matrix Nanocomposite, Its Properties, and Recyclability

An AM60 magnesium alloy nanocomposite reinforced with 1 wt % of AlN nanoparticles was prepared using an ultrasound (US) assisted permanent-mould indirect-chill casting process. Ultrasonically generated cavitation and acoustic streaming promoted de-agglomeration of particle clusters and distributed the particles throughout the melt. Significant grain refinement due to nucleation on the AlN nanoparticles was accompanied by an exceptional improvement in properties: yield strength increased by 103%, ultimate tensile strength by 115%, and ductility by 140%. Although good grain refinement was observed, the large nucleation undercooling of 14 K limits further refinement because nucleation is prevented by the formation of a nucleation-free zone around each grain. To assess the industrial applicability and recyclability of the nanocomposite material in various casting processes, tests were performed to determine the effect of remelting on the microstructure. With each remelting, a small percentage of effective AlN nanoparticles was lost, and some grain growth was observed. However, even after the third remelting, excellent strength and ductility was retained. According to strengthening models, enhanced yield strength is mainly attributed to Hall-Petch strengthening caused by the refined grain size. A small additional contribution to strengthening is attributed to Orowan strengthening.

Heterogeneous nucleation and grain growth of inoculated aluminium alloys: An integrated study by in-situ X-radiography and numerical modelling

A comprehensive study on the heterogeneous nucleation and grain growth of Al-10 wt.%Cu alloys inoculated with Al-5Ti-1B was carried out. To further reveal the solute segregation stifling mechanism, in-situ near-isothermal melt solidification experiments with constant cooling rates and greatly suppressed melt convection were realized by in-situ microfocus X-radiography study. The kinetics of heterogeneous nucleation and grain growth under the isolated influence of cooling rate and addition level of inoculant particles has been quantitatively studied. Moreover, novel image processing and analysis approaches have been proposed, to determine the maximum nucleation undercooling and solid volume fraction at nucleation ceasing. To better understand the heterogeneous nucleation and grain growth behaviors under the in-situ experiment conditions, a new grain size prediction model in which both pure globular growth kinetics and dendritic growth kinetics including spherical/globular to dendritic transition (GDT) has been developed. The quantitative agreements between the simulation results and experimental results in terms of grain size, maximum nucleation undercooling and solid fraction at nucleation ceasing, have confirmed the validity of the solute segregation stifling mechanism for castings without recalescence. Furthermore, it is demonstrated that globular growth kinetics is an acceptable approximation for grain size prediction purposes of well grain-refined aluminum alloys. However, for poorly inoculated aluminum alloys with well-developed coarse dendritic grains, an application of dendritic growth kinetics significantly improves the grain size prediction power of the model.

Microstructural Quantification of Rapidly Solidified Undercooled D2 Tool Steel

Rapid solidification of D2 tool steel is investigated experimentally using electromagnetic levitation (EML) under terrestrial and reduced gravity conditions and impulse atomization (IA), a drop tube type of apparatus. IA produces powders 300 to 1400 μm in size. This allows the investigation of a large range of cooling rates (~100 to 10,000 K/s) with a single experiment. On the other hand, EML allows direct measurements of the thermal history, including primary and eutectic nucleation undercoolings, for samples ~6 to 7 mm in diameter. The final microstructures at room temperature consist of retained supersaturated austenite surrounded by eutectic of austenite and M7C3 carbides. Rapid solidification effectively suppresses the formation of ferrite in IA, while a small amount of ferrite is detected in EML samples. High primary phase undercoolings and high cooling rates tend to refine the microstructure, which results in a better dispersion of the eutectic carbides. Evaluation of the cell spacing in EML and IA samples shows that the scale of the final microstructure is mainly governed by coarsening. Electron backscattered diffraction (EBSD) analysis of IA samples reveals that IA powders are polycrystalline, regardless of the solidification conditions. EBSD on EML samples reveals strong differences between the microstructure of droplets solidified on the ground and in microgravity conditions. While the former ones are polycrystalline with many different grains, the EML sample solidified in microgravity shows a strong texture with few much larger grains having twinning relationships. This indicates that fluid flow has a strong influence on grain refinement in this system.

Effect of the melt superheat on equiaxed solidification of Al-20 wt% Cu alloy investigated by in situ synchrotron radiography

Effect of the melt superheat on equiaxed solidification of Al-20wt% Cu alloy was investigated by in-situ synchrotron radiography at Beijing Synchrotron Radiation Facility. For comparison, the corresponding DSC analysis was also conducted. It was found that the grain size decreased with increasing the melt superheat. The relationship between the final mean grain size and the melt superheat can be expressed as: d=4919.3×ΔT-0.33. During solidification, the mean grain size increased sharply in the first 70s, then reached the final grain size gradually. Furthermore, with increasing the melt superheat, the mean nucleation rate increased, which can be attributed to the fact that increasing the melt superheat led to an increase in nucleation undercooling, and the growth rate and the duration of free growth stage decreased. As the melt superheat increased from 100°C to 160°C, the mean nucleation rate increased by 78.2% while the mean growth rate only decreased by 19.3%, which indicated that the high mean nucleation rate and the consequent low mean growth rate may be the real reasons for grain refinement. The increased nucleation density caused earlier growth deceleration due to solutal impingement effects.

On the microstructural refinement in commercial purity Al and Al-10 wt% Cu alloy under ultrasonication during solidification

Physical grain refinement is examined under high-intensity ultrasonication during solidification in commercial purity Al (CP-Al) and binary Al-10wt% Cu alloy melts cooled naturally in air and compared against chemical inoculation using Al-5Ti-1B grain refiner. The coarse dendritic unrefined base microstructure was completely replaced with a fine equiaxed grain structure in the case of either inoculation or ultrasonication. However, ultrasonication produced more effective refinement over chemical inoculation with a two-fold and eight-fold increase in the grain density in CP-Al and Al-10%Cu alloy, respectively. While combining chemical inoculation with ultrasonication produced the finest grain structure in CP-Al, no further improvement over ultrasonication was noted for the Al-10%Cu alloy. Noticeable reduction in nucleation undercooling, of similar magnitude to chemical inoculation, was observed under ultrasonication. Cooling curve observations indicate strongly enhanced heterogeneous nucleation under ultrasonication. It appears that although chemical inoculation utilises higher potency nucleants, more nucleation events are favoured under cavitation.

Dependence of Solidification for Bi2Te3\u2212xSex Alloys on Their Liquid States

The resistivity versus temperature (ρ-T) behaviours of liquid n-type Bi2Te3−xSex (x = 0.3, 0.45 and 0.6) alloys are explored up to 1050 °C. A clear hump is observed on all ρ-T curves of the three studied Bi2Te3−xSex melts during the heating process, which suggests that a temperature-induced liquid-liquid structural transition takes place in the melts. Based on this information, the solidification behaviours and microstructures of the alloys with different liquid states are investigated. The samples that experienced liquid structural transition show that the nucleation and growth undercooling degrees are conspicuously enlarged and the solidification time is shortened. As a result, the solidified lamellae are refined and homogenized, the prevalence of low-angle grain boundaries between these lamellae is increased, and the Vicker Hardness is enhanced. Atom probe tomography analyses prove that there is no segregation or nanoprecipitation within the grains, but the Te-rich eutectic structure and the evolution of composition near the Te-matrix phase boundary are investigated in a sample that experienced liquid structural transition. Our work implies that the solidification behaviours of Bi2Te3−xSex alloys are strongly related to their parent liquid states, providing an alternative approach to tailor the thermoelectric and mechanical properties even when only a simple solidification process is performed.

Effect of Construction Manner of Mould Cluster on Stray Grain Formation in Dummy Blade of DD6 Superalloy

AbstractA dummy blade of Ni-based single crystal (SX) superalloy was modelled to investigate the stray grain (SG) defect by both experiment and simulation. Three construction manners of mould cluster (CMMC) with 0°, 45° and 90° assembling angle were considered. The experimental results reveal a strong dependence of SG sensitivity upon CMMC. Profuse SGs took place in the case of 0°, but almost no one shown in those of 45° and 90°. The FEM simulation results indicate that SG occurrence is not only determined by critical nucleation undercooling, but also the geometrical characteristic, shape of liquidus isotherm and other three important effects (shadow effect, channel effect and dimension effect). The tendency of SG formation increases with the increase of number of these effects. By means of the combination effect consisting of the above three effects, SG occurrence can be qualitatively predicted and process window is more accurately modified.

Effect of cooling rate and Al content on solidification characteristics of AZ magnesium alloys using cooling curve thermal analysis

The solidification behavior of AZ Magnesium alloys in various cooling conditions was investigated using a computer-aided cooling curve thermal analysis method. In each case, the cooling curve and its first and second derivative curves have been plotted using accurate thermal analysis equipment and solidification characteristics were recognized from these curves. The cooling rates used in the present study range from 0.22 to 8.13 °C s−1. The results of thermal analysis show that the solidification parameters of AZ alloys such as nucleation temperature (T N,α), nucleation undercooling (∆T N,α), recalescence undercooling (∆T R,α), range of solidification temperature (∆T S) and total solidification time (t f) are influenced by variation of cooling rate. Also, the effect of Al content on these parameters was studied. Microstructural evaluation was carried out to determine the correlation between the cooling rate and secondary dendrite arm spacing.

Nucleation, grain orientations, and microstructure of Sn-3Ag-0.5Cu soldered on cobalt substrates

Abstract The potential of cobalt substrates to control the microstructure of 550 μm Sn-3Ag-0.5Cu ball grid array (BGA) joints is explored. It is shown that cobalt substrates give a small and reproducible nucleation undercooling for tin and prevent the formation of large primary Ag 3 Sn blades and Cu 6 Sn 5 rods. βSn dendrites grew from the CoSn 3 reaction layer and the βSn grains are shown to inherit their orientation from the CoSn 3 layer due to heterogeneous nucleation with the following orientation relationship: (100) Sn ||(100) CoSn3 with [001] Sn ||[001] CoSn3 . Changes in the dendrite and eutectic microstructure are shown to be caused by the reduced nucleation undercooling for βSn and the altered solidification path due to cobalt substrate dissolution.

Refining Mechanism of 7075 Al Alloy by In-Situ TiB₂ Particles.

The nucleation undercooling of TiB2/7075 Al matrix composites, the microstructure observed after solidification at different cooling rate, and the size and distribution of TiB2 particles were investigated. The experimental results have shown that the grain sizes of TiB2/7075 Al matrix composites firstly decreased, then increased, and finally decreased again with the increase of TiB2 content. The nucleation undercooling of TiB2/7075 Al matrix composites first increased, then decreased, and finally increased again with the increase of TiB2 content when the cooling rates was 5 and 10 °C/min respectively, but kept decreasing with the increase of TiB2 content at a cooling rate of 20 °C/min. The melting and solidification process showed no significant change with the decrease of cooling rate in 9.0% TiB2/7075 Al matrix composites. Most small particles can act as heterogeneous nucleus, which induced grain growth and were captured into the grain by the solid/liquid interface. At the same time, most of the larger particles and a minority of the small TiB2 particles are pushed into the grain boundary; locating in the grain boundary can hinder the Al atoms from diffusing during the solidification process and restrain α-Al phase growth. The influence of particles shifted from dominating by locating to dominating by nucleation as the quantity of TiB2 particles increased.

Key aspects of carbide precipitation during solidification in the Ni-base superalloy, MAR M002

Key differences have been shown to exist between the predicted solidification path using thermodynamic calculations compared with that determined experimentally using thermal analysis coupled with microscopy in the Ni-base superalloy, MAR M002. When nucleation undercooling is negligible, the measured liquidus temperature is significantly greater than that obtained from thermodynamic calculations. Primary solidification corresponds to freezing of γ-phase, and a near-constant freezing rate occurs during primary growth prior to nucleation and initial growth of MC carbides. This nucleation and growth occurs via a eutectic reaction, L → γ + MC during the latter stages of solidification, and freezing terminates through the four-phase eutectic reaction, L → γ + MC + γ′. There is a two-fold decrease in freezing rate during the terminal stages of solidification. The freezing sequence of the carbides is markedly different to that predicted by thermodynamic calculations, which instead predict growth of MC directly from the liquid before nucleation of γ-phase and also to the formation of borides, which were not experimentally observed.

Sn-Ag合金の固液界面エネルギーに及ぼすZnの微量添加の影響

In order to assess the effect of Zn addition in Sn-2Ag alloy on the undercooling of the primary (β-Sn) phase, tip radii of the columnar dendrites of primary (β-Sn) have been systematically investigated. Sn-2Ag-x Zn alloys were prepared for the experiments, where x=0, 0.05 and 0.6 mass%. These alloys were unidirectionally solidified at growth velocities of 4, 10, 20, 30 and 40 μm/s. The tip radii of columnar dendrites of primary (β-Sn) on the longitudinal cross section of quenched samples were characterized. It was found that the tip radius decreased with increasing growth velocity and with increasing amounts of Zn. The effect of Zn addition on the solid/liquid interfacial energy was analyzed by assuming that the addition of Zn to Sn-2Ag alloy will not change the physical parameters such as the diffusion coefficient of the solute, solidification temperature range, and entropy of fusion. The addition of small amounts of Zn may considerably decrease the solid/liquid interfacial energy of primary (β-Sn). This may possibly decrease the undercooling for nucleation of the primary (β-Sn) phase in Sn-2Ag-x Zn alloy.

Numerical Simulation of Porosity for Al Based Alloys

The final integrity of a casting is greatly influenced by the presence of porosity. Progressive way to predict presence of porosity is the use of modern computer simulation programs. The main aim of performed experiments is to verify possibilities of this promising method of porosity prediction. A calculation of advanced porosity prediction was performed for an aluminium alloys by advanced porosity module included in ProCAST software. This calculation takes into account all basic phenomena, which are at the origin of micro and macro porosity. For experiment purposes was used mold with specific shape – Sanduhrprobe. Materials used in experiments were not loaded from software database, because results could be distorted by deviations from particular material we used. To achieve precise results, we used thermal analysis to get accurate data about used alloys. Important solidification events, which affects porosity formation, such as recoalescence and nucleation undercooling temperature, coherence point and rigidity point have been determined from cooling curves and its first derivate. These data were then included to the database of simulation software and used in simulation process.