Large Undercooling Research Articles

Liquid phase separation and subsequent dendritic solidification of ternary Fe35Cu35Si30 alloy

Liquid Fe35Cu35Si30 alloy has achieved the maximum undercooling of 328 K (0.24TL) with glass fluxing method, and it displayed triple solidification mechanisms. A critical undercooling of 24 K was determined for metastable liquid phase separation. At lower undercoolings, α-Fe phase was the primary phase and the solidification microstructure appeared as homogeneous well-defined dendrites. When the undercooling exceeded 24 K, the sample segregated into Fe-rich and Cu-rich zones. In the Fe-rich zone, FeSi intermetallic compound was the primary phase within the undercooling regime below 230 K, while Fe5Si3 intermetallic compound replaced FeSi phase as the primary phase at larger undercoolings. The growth velocity of FeSi phase increased whereas that of Fe5Si3 phase decreased with increasing undercooling. For the Cu-rich zone, FeSi intermetallic compound was always the primary phase. Energy-dispersive spectrometry analyses showed that the average compositions of separated zones have deviated substantially from the original alloy composition.

Rapid eutectic growth and applied performances of Fe[sbnd]Al[sbnd]Nb alloy solidified under electromagnetic levitation condition

Electromagnetic levitation provides a containerless condition for materials processing, which is a desired technique to produce optimized microstructures and properties. In this paper, the large undercooling and rapid solidification of high-temperature Fe67.5Al22.8Nb9.7 alloy were realized by means of electromagnetic levitation method. The undercooling range from 13 to 150K was achieved, and the microstructure consisted of Nb(Fe, Al)2+(αFe) pseudoeutectic grains and a small amount of primary Nb(Fe, Al)2 phase. The eutectic growth velocity was measured and found to increase by nearly 4 times with enhanced undercooling. Both eutectic grain size and interlamellar spacing decreased more than 50% with the increase of undercooling. Owing to the structural refinement, the micromechanical property of eutectics in terms of Vickers microhardness was improved, whereas that of primary phase remained unchanged. The soft magnetism of the undercooled alloy was reduced according to the variation of hysteresis loops. Besides, the thermal expansion coefficient of this alloy increased by two times within the temperature range from 300 to 1573K, and the alloy density variation was determined accordingly.

Effect of Nb on the Growth Behavior of Co3Sn2 Phase in Undercooled Co-Sn Melts

The growth behavior of the primary β-Co3Sn2 phase in (Co67Sn33)100−x Nb x (x = 0, 0.5, 0.8, 1.0) hypereutectic alloys at different melt undercoolings was investigated systematically. The growth pattern of the β-Co3Sn2 phase at low undercooling changes with the Nb content from fractal seaweed (x = 0, 0.5) into dendrite (x = 0.8) and then returns to fractal seaweed (x = 1.0) as a response to the changes in interface energy anisotropy and interface kinetic anisotropy. As undercooling increases, the dendritic growth of the β-Co3Sn2 phase in (Co67Sn33)99.2Nb0.8 alloy gives way to fractal seaweed growth at an undercooling of 32 K (−241 °C). At larger undercooling, the fractal seaweed growth is further replaced by compact seaweed growth, which occurred in the other three alloys investigated. The growth velocity of the β-Co3Sn2 phase slightly increases at low and intermediate undercooling but clearly decreases at larger undercooling due to the Nb addition. The growth velocity sharply increases as the growth pattern of the Co3Sn2 phase transits from fractal seaweed into compact seaweed.

Theoretical prediction and experimental observation for microstructural evolution of undercooled nickel–titanium eutectic type alloys

The high undercooling and rapid solidification of hypoeutectic Ni–13%Ti, eutectic Ni–14%Ti and hypereutectic Ni–16%Ti alloys were realized by electromagnetic levitation and drop tube techniques. The competitive growth of primary (Ni) dendrite and Ni3Ti compound with (Ni+Ni3Ti) eutectic caused significant microstructural variations beyond some critical undercoolings. For the hypoeutectic Ni–13%Ti alloy, the solidification microstructure was characterized by primary (Ni) dendrites plus interdendritic (Ni+Ni3Ti) eutectics in the moderate undercooling range below 196 K. When the alloy melt was undercooled beyond 196 K, complete solute trapping occurred and thus induced the formation of metastable single-phase microstructure. The microstructural morphology of eutectic Ni–14%Ti alloy appeared as (Ni+Ni3Ti) lamellar structures in the small undercooling regime below 40 K, whereas it displayed a hypoeutectic microstructure dominated by the primary (Ni) dendrites at larger undercoolings. In the case of hypereutectic Ni–16%Ti alloy, an irregular eutectic structure was formed at the undercoolings higher than 41 K and it was significantly refined under substantial undercooling conditions. These microstructural transitions were further analyzed by calculating the coupled zone of (Ni+Ni3Ti) lamellar eutectic, which leaned to the Ni3Ti phase side and covered a composition range from 13.1 to 19.0 wt%Ti.

Change aspects of microstructure and mechanical behavior of Bi and Zn-doped Sn–0.5Cu solders for microelectronic applications

Sn–Cu alloys were recommended as a promising substitute for traditional Sn–Pb alloy in wave soldering applications. In the present study, the change of microstructure, thermal and mechanical behavior associated with alloying of Bi and Zn into Sn–0.5Cu solder have been investigated. DSC analysis reveals that large kinetic undercooling arising from the rapid solidification condition is the vital mechanism to realize the eutectic lamellar structures for hypoeutectic Sn–Cu solder. Unlike Bi addition, Zn doping is very effective in reducing undercooling and the onset temperature, although the pasty range is slightly increased. Correlations between thermal analysis, microstructure, and mechanical behavior were established. One conspicuous feature of Zn addition is that Zn promotes the nucleation of fine β-Sn grains, which in turn increases the ductility, but the yield strength, ultimate tensile strength and Young’s modulus are decreased. These properties emerge the plastic energy dissipation ability of solder joints at room temperature. In contrast, the higher mechanical strength of Bi-containing solder is contributed by the solid solution effects and precipitations hardening mechanism of Bi.

The TTT Curves of the Heterogeneous and Homogeneous Crystallization of Lithium Disilicate – A Stochastic Approach to Crystal Nucleation

The present study explores the temperature and time dependence of heterogeneous crystal nucleation in a lithium disilicate glass using the stochastic approach. In particular a single lithium disilicate sample was repeatedly (284 runs) undercooled to 1173 K in a PtRh-crucible and the crystallization onset time during an isothermal hold was detected in each run. The statistical distribution of the times elapsed before crystallization is described by a first order reaction with a heterogeneous crystal nucleation rate of (9.19 ± 0.04) × 10-4 s-1 while individually shaped crystallization exotherms of each run were recorded which indicate growth of a single or only few crystals during crystallization of the entire volume. The data were used together with results of previous stochastic experiments and those of double-stage heat treatments to calculate the crystallization time of a fraction of 10-4 percent for all temperatures between glass transition and melting. The derived TTT diagram shows a double nose of crystallization in the volume at large undercoolings (0.53–0.61 Tm) and crystallization at the surface at small undercoolings (0.62–0.92 Tm) initiated by homogeneous and heterogeneous crystal nucleation, respectively. The critical cooling rate at the heterogeneous nose is approximately 73 K s-1.

Crystallization of glass-forming liquids: Specific surface energy

A generalization of the Stefan-Skapski-Turnbull relation for the melt-crystal specific interfacial energy is developed in terms of the generalized Gibbs approach extending its standard formulation to thermodynamic non-equilibrium states. With respect to crystal nucleation, this relation is required in order to determine the parameters of the critical crystal clusters being a prerequisite for the computation of the work of critical cluster formation. As one of its consequences, a relation for the dependence of the specific surface energy of critical clusters on temperature and pressure is derived applicable for small and moderate deviations from liquid-crystal macroscopic equilibrium states. Employing the Stefan-Skapski-Turnbull relation, general expressions for the size and the work of formation of critical crystal clusters are formulated. The resulting expressions are much more complex as compared to the respective relations obtained via the classical Gibbs theory. Latter relations are retained as limiting cases of these more general expressions for moderate undercoolings. By this reason, the formulated, here, general relations for the specification of the critical cluster size and the work of critical cluster formation give a key for an appropriate interpretation of a variety of crystallization phenomena occurring at large undercoolings which cannot be understood in terms of the Gibbs’ classical treatment.

Anomalous Halo Formation in an Arc-Melted ScNi-Sc2Ni Off-Eutectic Binary Alloy

Diverse non-equilibrium eutectic structures have attracted numerous experimental and theoretical studies. One special type is the formation of a halo of one phase around a primary dendrite of another phase. In our experiments, it was occasionally observed that ScNi halos grow as dendritic morphology around the primary ScNi dendrites in an arc-melted ScNi-ScNi off-eutectic binary alloy. The formation of this anomalous halo structure was then well reproduced by employing quantitative phase-field simulations. Based on the phase-field simulation, It was found that (i) the large undercooling and growth velocity of the ScNi phase during solidification causes the formation of halos; and (ii) the released latent heat induces the recalescence phenomenon, and changes the solidification sequence largely, resulting in the anomalous halo structure in the Sc-34 at % Ni alloy.

Crystallization of Polyethylene at Large Undercooling.

Extremely fast crystallization of high-density polyethylene and random copolymers of ethylene with up to 16 mol % 1-octene was observed for the first time by ultrafast scanning calorimetry. In order to account for the inherently high crystallization rate of polyethylenes, in nonisothermal and isothermal crystallization experiments cooling rates up to 1 000 000 K/s and crystallization times as short as 10 μs, respectively, were employed. It was possible to supercool the melt of high-density polyethylene down to 57 °C and the melt of a random ethylene/1-octene copolymer with 16 mol % 1-octene down to -33 °C, without prior crystallization. At these temperatures, the characteristic time of the primary crystallization process is of the order of magnitude of 100 μs. Complete vitrification of the liquid would require cooling even faster than 1 000 000 K/s. Compared to the homopolymer, the cooling-rate dependence of the crystallization temperatures and the temperature dependence of the characteristic time of primary crystallization of random ethylene/1-octene copolymers both are nearly parallel shifted to lower temperatures. Fast crystallization under conditions of reduced linear crystal growth rate is possibly caused by boosting homogeneous nuclei density up to 1027 m-3 and urgently requires further investigation.

Enhanced mechanical properties of a CoCrFeNi high entropy alloy by supercooling method

A CoCrFeNi high entropy alloy has been solidified with a high undercooling up to 300K adopting glass fluxing method. Results show that the compressive yield strength of the alloy is enhanced about 3 times: from 137MPa for a traditional casting condition to 455MPa for the samples processed at large undercooling. The enhanced mechanical properties are attributed to both the refined grain size and complex phases obtained through the supercooling method, which appears to be an efficient way to modify the microstructure and improve the properties of high entropy alloys.

In situ observations of the rapid solidification for undercooled Al30Si70 alloy melt

Abstract

Strong kinetic effect of polyethylene glycol 6000 under directional solidification condition

Interface characterizes describes how the atoms/molecules attach themselves to the solid/liquid interface from the liquid when the crystallization takes place, which plays a key role in revealing the kinetic mechanism during the crystal growth. For common non-facet/non-facet metallic systems, the kinetic undercooling is usually small and it becomes only significant when the growth velocity is high. However, high growth velocity can be usually realized under large undercooling condition. In this case, the interface temperature cannot be measured, thus the kinetic undercooling cannot be determined quantitatively either. Compared with the atom and small molecule materials, the polymer has its distinctive characteristic of different long chains, which are entangled together in a liquid state. Thus the crystallization of the polymer system usually proceeds in the two-dimensional manner, which provides an ideal way to obtain large kinetic undercooling under the small growth velocity condition. The directional crystallization technique has been widely adopted to study the scaling law of undercooling and growth velocity due to its accurate controlling of growth velocity and temperature gradient. Therefore, it offers an appropriate way to make a quantitative investigation. In this paper, the in-situ observations of the solidification of polyethylene glycol 6000 at different pulling velocities are performed and the interface temperature is examined as well by using the directional crystallization technique. The effect of the pulling rate on the growth kinetics is examined. The results reveal that the interface temperature decreases and the undercooling increases gradually with the pulling velocity increasing. A change in the growth regime is observed at T=13.5 K, where regime Ⅱ-regime Ⅲ transition occurs according to Hoffman's kinetic theory of polymer crystallization. The comparison of undercooling between the present work and DSC isothermal crystallization is made, and it shows that the data obtained in the directional growth and the isothermal growth follow the same trends but the undercooling in isothermal growth is larger than in directional growth under the same growth velocity. This indicates that the undercooling in the latter case is over-estimated since it contains the thermal undercooling. Undercooling is the driving force for crystallization, which usually includes solute undercooling, curvature undercooling, thermal undercooling, and kinetic undercooling. Because of the flat interface and the pure material, there is no solute undercooling nor curvature cooling in the present case. The thermal undercooling is also zero in the unidirectional crystallization process. Thus the total undercooling in the present work is the kinetic undercooling. The maximum kinetic undercooling reaches 20 K, indicating that the interface kinetic controlling growth takes place due to the two-dimensional nucleation in polymer.

Multi-transformations in rapid solidification of highly undercooled hypoeutectic Ni–Ni3B alloy melt

Abstract

Dendrite Growth Kinetics in Undercooled Melts of Intermetallic Compounds

Solidification needs an undercooling to drive the solidification front. If large undercoolings are achieved, metastable solid materials are solidified from the undercooled melt. Containerless processing provides the conditions to achieve large undercoolings since heterogeneous nucleation on container walls is completely avoided. In the present contribution both electromagnetic and electrostatic levitation are applied. The velocity of rapidly advancing dendrites is measured as a function of undercooling by a High-Speed-Camera. The dendrite growth dynamics is investigated in undercooled melts of intermetallic compounds. The Al50Ni50 alloy is studied with respect to disorder trapping that leads to a disordered superlattice structure if the melt is undercooled beyond a critical undercooling. Disorder trapping is evidenced by in situ energy dispersive diffraction using synchrotron radiation of high intensity to record full diffraction pattern on levitated samples within a short time interval. Experiments on Ni2B using different processing techniques of varying the level of convection reveal convection-induced faceting of rapidly growing dendrites. Eventually, the growth velocity is measured in an undercooled melt of glass forming Cu50Zr50 alloy. A maximum in the growth velocity–undercooling relation is proved. This is understood by the fact that the temperature dependent diffusion coefficient counteracts the thermodynamic driving force for rapid growth if the temperature of the undercooled melt is approaching the temperature regime above the glass transition temperature. The analysis of this result allows for determining the activation energy of atomic attachment kinetics at the solid–liquid interface that is comparable to the activation energy of atomic diffusion as determined by independent measurements of the atomic diffusion in undercooled Cu50Zr50 alloy melt.

Remelting-induced anomalous eutectic formation during solidification of deeply undercooled eutectic alloy melts

Anomalous eutectics are argued to form due to the remelting of the primary solid during solidification of deeply undercooled eutectic alloy melts. As an indicator of the tendency for anomalous eutectic formation, the remelted fraction of the primary eutectic was analyzed systematically based on the eutectic dendrite growth theory. For eutectic alloys with either larger equilibrium solute distribution coefficients or gentle liquidus slopes, the primary eutectic is highly supersaturated with solute and more prone to remelting. When the eutectic composition is set to different values, (e.g. the eutectic point is closer to one phase of the eutectic), the two eutectic phases under rapid growth change their compositions simultaneously, but their remelted fractions during temperature recalescence do not vary significantly. Three representative binary eutectic alloys Ag–39.9at.%Cu, Ni–19.6at.%P and Pd–16.0at.%P – their eutectic products are solid solution–solid solution, solid solution–stoichiometric intermetallic compound and stoichiometric intermetallic compound–stoichiometric intermetallic compound, respectively, were solidified at large undercooling. Anomalous eutectics were observed in the first two eutectic alloys whose eutectic structure consists of at least one solid solution phase, whereby the stoichiometric intermetallic compound phase remained highly oriented whereas the solid solution phase had a near random distribution of orientations. For the Pd–16.0at.%P alloy, however, the primary eutectic retained its original morphology as the final solidification structure regardless of the degree of undercooling. All these experimental results validate the argument that anomalous eutectics result from the remelting of the primary solid.

Experimental platform for solidification and in-situ magnetization measurement of undercooled melt under strong magnetic field.

An experimental platform for solidification of undercooled melt and synchronous measurement of magnetization under strong magnetic field is reported. The facility can be used for in-situ measurement of the magnetization of the undercooled melts and to studying the non-equilibrium solidification from deeply undercooled metals and alloys in magnetic field. The key advantages of this apparatus are first, we can obtain large undercooling in high fields by glass fluxing technique in combination with cyclical superheating and supercooling method, which is comparable to the maximum undercooling obtained by traditional method without magnetic field. Second, we can have a controlled thermal history with linear heating and cooling speed with high accuracy. Third, the temperature and magnetization measurement system are fast and on-line, which is very suitable for the investigation of both the structure transitions in the liquid in a wide temperature range (overheated and undercooled state) and the solid state phase transformations.

Experimental Test of Tammann’s Nuclei Development Approach in Crystallization of Macromolecules

A first attempt to probe the size distribution of homogeneously formed nuclei in polymers was realized employing Tammann’s two-stage crystal nuclei development method and fast scanning calorimetry. A transfer heating rate of 500 000 K·s–1 prevents nuclei growth on heating in poly(e-caprolactone) (PCL). Data collected in a wide range of crystallization temperatures allow, in combination with a theoretical model based on classical nucleation theory, for an estimate of the nuclei size distribution and the growth rate. The employed temperature profile was adapted from Tammann’s two-stage crystal nuclei development method implying formation of nuclei at large undercooling (low temperatures) and following their isothermal growth at higher temperatures. Fast scanning calorimetry allowed us to reach the deep supercooling of the melt at 100 000 K·s–1 avoiding homogeneous nuclei formation and heterogeneous nuclei growth. Then crystal nuclei were allowed to form isothermally at the temperature corresponding to the m...

Effect of strontium addition on silicon phase and mechanical properties of Zn–27Al–3Si alloy

The evolution of silicon phase and mechanical properties of Zn–27Al–3Si alloy with Sr addition were investigated. The results showed that primary Si crystals were gradually modified from polygonal, plate-like and star-like to larger nodular with increasing the addition contents of Sr. A bimodal distribution of eutectic Si crystals was found. One was the coarse eutectic (CE) Si nucleating and growing from the primary Si crystals, the other was the fine eutectic (FE) Si nucleating at other sites (not primary Si). The average size and aspect ratio of FE Si crystals significantly decreased and whose morphology was transformed from a coarser plate-like structure to a fine fibrous structure with more branches with increasing Sr content. Through electron probe microanalysis mapping (EPMA) and cooling curves, it was concluded that the nodular primary Si crystals nucleated and grew by producing a large undercooling (about 40°C) and preferential attachment of Sr to silicon phase. The mechanical properties were investigated by tensile test with various concentration of Sr. It was found that the ultimate tensile strength (UTS) and elongation (EI) increased by 62.6% and 162%, respectively. The improvements in UTS and EI were attributed to the morphology change of silicon phase as well as the improved feeding solidification shrinkage.

Competitive nucleation and growth of (Ge) and CuAl2phases subjected to different solidification conditions

We adapted DSC and drop tube processing techniques to investigate the solidification behavior of (Ge) and CuAl2 phases in the Al–Cu–Ge ternary alloy comparatively. For the DSC sample only undercooled to 32 K with a low cooling rate, (Ge) phase is the leading nucleating phase and then CuAl2 phase nucleates. Primary (Ge) block, (Ge+CuAl2) and (Ge+Al) pseudobinary eutectics coexist in the microstructure. In the drop tube, the more competitive nucleation and growth of (Ge) and CuAl2 phases are caused primarily by high cooling rate and then secondly by large undercooling. On condition that undercooling and cooling rate exceed critical values, simultaneous nucleation of (Ge) and CuAl2 phases from the alloy melt and subsequently their independent growth contribute to the formation of anomalous (Ge+CuAl2) pseudobinary eutectic grain. Under such condition, a small amount of ternary eutectic grain forms due to the late exclusion of Al solute.

Effects of copper content and liquid separation on the microstructure formation of Co–Cu immiscible alloys

Abstract Applying a melt-fluxing method combined with cyclical superheating, rapid solidification of undercooled Co70Cu30, Co50Cu50 and Co30Cu70 melts was performed to investigate the effects of alloy composition and immiscibility on the resultant non-equilibrium solidification microstructure. As for the samples without liquid separation, grain refinement of primary α-Co dendrite occurs clearly with increases in both copper content and initial undercooling. In contrast, the influence of alloy composition on microstructure evolution is weakened due to the occurrence of liquid separation at larger undercooling, where slight variations in the solute concentration and micro-hardness for Co-rich phase are obtained. After a quantitative thermodynamic calculation, the process of liquid separation is described, which gives an excellent explanation for the observed experimental results.