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Abstract: Co–24.0at.% Sn eutectic alloy melt was undercooled to different degrees below the equilibrium eutectic temperature. The dependence of solidification behavior on undercooling was established based on the experimental results of the solidification microstructure, crystal orientation and crystal growth velocity. In the entire undercooling range studied (0–203K), coupled eutectic growth of the α-Co and β-Co3Sn2 phases invariably takes place during the rapid solidification stage. The eutectic solidification interface advances in the alloy melt in seaweed morphologies rather than the well-known dendritic mode due to the weak interface energy anisotropy. But a critical undercooling of 175K exists, from which the eutectic solidification interface changes from a fractal into a compact seaweed pattern, accompanied with an abrupt increase of growth velocity. At high undercoolings the enhancement of the interface tip stability with the rise of crystal growth velocity should be responsible for the growth mode transition. [Copyright &y& Elsevier]

Abstract: A theoretical model for lamellar eutectic growth in undercooled eutectic alloy melts with a third element has been developed; it was assumed that no other phase forms. It is found that the tip radius of eutectic dendrite decreases with increasing content of the third element so as to diffuse away its enriched atoms ahead of the solidifying interface easily. A critical content exists below which the eutectic growth is accelerated within a certain undercooling range due to the third element addition, but slowed down otherwise. When more of the third element is contained, the growth velocity is always reduced. However, the eutectic lamellar spacing varies with the content in an opposite way. The influence of the third element addition is enhanced when the difference in distribution coefficients of the third element in two eutectic phases becomes greater. [Copyright &y& Elsevier]

Abstract: Two bulk Ni–Pd alloys Ni75Pd25 and Ni54.8Pd45.2 have been undercooled beyond their hypercooling limits. As undercooling increases, two grain refinements, occurring at low and high undercooling, respectively, were observed in the solidification of both alloys, even though the equilibrium crystallization temperature range of the latter is only 5K. Reaching of the hypercooling limit did not change the microstructural morphology abruptly, and dendritic substructure could still be found in the refined grains. Such experimental results cannot be interpreted satisfactorily by the theory that the break-up of dendrites during solidification is dominated by capillary force. It is proposed that the grain refinement at low undercooling results from the remelting associated with chemical superheating, but that at high undercooling is due to the post-solidification recrystallization. [Copyright &y& Elsevier]

Abstract: Ni54.6Pd45.4, a solid solution alloy without crystallization temperature range, has been undercooled by glass flux method up to the hypercooling limit. It was found that it solidified as pure metals. As undercooling increases, the secondary dendrite arm spacing decreases monotonously until a undercooling of 135K, above which the secondary branches considerably degenerate. The solidification with undercoolings higher than 160K leads to grain-refined structures. In this case, their average grain sizes continuously decrease with increasing undercooling even if the alloy has been hypercooled. The fact that grain refinement still takes place in the solidification of the hypercooled alloy indicates that the solidification time after recalescence is not a factor to dominate the grain refinement at large undercooling. Recrystallization of the solid in the post-solidification period should be responsible for the grain refinement. [Copyright &y& Elsevier]

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–39.9 at.%Cu, Ni–19.6 at.%P and Pd–16.0 at.%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–16.0 at.%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. [ABSTRACT FROM AUTHOR]

Abstract: Three Co–Sn alloys with compositions around the eutectic point were undercooled to different degrees below the equilibrium liquidus temperature and the solidification behaviors were investigated by monitoring the temperature recalescence and examing the solidification structure. It is revealed that the primary phase during rapid solidification changes complexly with the increasing undercooling in the off-eutectic alloys, while coupled eutectic growth takes place at all undercoolings in the eutectic alloy. Two types of anomalous eutectics form in the alloys: one evolving from coupled eutectics and the other from single phase dendrites or seaweeds. The crystallographic orientation of eutectic phases in the anomalous eutectic is dependent on which type their precursors belong to. [Copyright &y& Elsevier]

Abstract: Rapid solidification of undercooled Fe–Co–Cu alloys was investigated by means of fluxing purification and cyclic superheating technique. A transition in microstructure from dendrites to phase-separation occurred above a phase-separation undercooling ΔT sep. When ΔT<ΔT sep, dendrite was observed, the trunks were rich in Fe and Co, while Cu was rich at inter-dendrites. However, the phase-separated microstructure was obtained once ΔT>ΔT sep, with a large sphere of L1 phase located almost at the center of the sample and enwrapped by L2 phase. ΔT sep was 222, 88 and 45K for Fe50Co30Cu20, Fe25Co25Cu50 and Fe15Co15Cu70 alloys in this work, respectively. It was investigated that L1 phase solidified before L2 phase after liquid separation and followed different ways. [ABSTRACT FROM AUTHOR]

Abstract: How to explore an inherent linkage between non-equilibrium liqulid/solid transformation (LST) and subsequent solid-state transition (SST) is becoming more and more important for material preparation. Applying highly undercooled solidification and melting spinning, single-phase supersaturated solid solution (SSSS) was prepared for immiscible Fe–Cu and low-solid-solubility Fe–B alloys, respectively. By performing isothermal annealings at temperatures for the above SSSS, an inherent linkage between non-equilibrium LST and solid-state grain growth has been studied, in terms of an extended dendrite growth model and solute trapping model (for non-equilibrium solidification) and a numerical and an analytical thermo-kinetic model (for solid-state grain growth). For Fe–Cu SSSS, a relation between the metastable equilibrium grain size and the initial melt undercooling was derived, whereas, for Fe–B SSSS, a relation between the metastable equilibrium grain size and the ribbon thickness/rotational speed (in melt spinning) was deduced. The model prediction is consistent with the experimental result, indicating that the solid-state grain growth stops as a result of decrease in GB energy due to solute segregation to GBs. This further certifies that, for the alloys considered, the final microstructure is determined by both the non-equilibrium LST and the solid-state grain growth including GB segregation. [ABSTRACT FROM AUTHOR]

Abstract: Ag–Cu eutectic alloy was undercooled by the glass flux method and the solidification structure was investigated. It is revealed that when undercooling is not more than 70 K, the large difference in composition between two eutectic phases and very large thermal diffusion coefficient of the liquid result in cellular growth of the lamellar eutectics from the nucleation site. The variation in interface temperature during rapid solidification gives rise to a systematic change in microstructure within the sample. With the distance along the growth direction increasing, the finest lamellar spacing across the cellular eutectic rises, which indicates a gradually decreasing growth velocity of the primary eutectics. The primary lamellar eutectics near the nucleation site solidify under conditions far from equilibrium, and therefore are supersaturated with solute, and then partially remelted and ripened into anomalous eutectics. As undercooling increases, the area of the anomalous eutectics enlarges. [Copyright &y& Elsevier]

Abstract: Assuming thermal balance and solute conservation, a numerical model has been proposed to describe the recalescence behavior of bulk-undercooled Cu–Ni melts. Applying a finite-difference scheme, the transformed solid fraction upon recalescence is given as a function of the liquid temperature, while the average liquid concentration can be tracked by calculation of the liquid/solid (L/S) Gibbs energy difference, in combination with a dendrite growth model. Accordingly, a transition from non-equilibrium to equilibrium process has been described from the evolution of L/S Gibbs energy difference. Applying the present model, the experimentally observed maximum recalescence temperature can be well predicted. [Copyright &y& Elsevier]

Abstract: For undercooling less than a critical value (ΔT crit), peritectic solidification occurs upon solidification of Fe–Co alloy, as evidenced by a peritectic layer observed in the as-solidified microstructure. Combing the current experimental results with the dendrite model calculations, it has been concluded that the occurrence of peritectic reaction and transformation was concerned to both thermodynamic and kinetic preconditions: the temperature and the atomic diffusion. [Copyright &y& Elsevier]

Abstract: When a single-phase alloy solidifies in a low-undercooling range or above a critical undercooling, grain-refined structures are obtained. Taking Ni75Pd25 alloy as an example, the microscopic orientation of the refined grains was investigated by electron backscattered diffraction technology. It is revealed that the refined grains at low undercooling are completely randomly distributed. In the refined structure at high undercooling, certain grain boundaries with misorientation angles less than 5° can be observed, while most of the grain boundaries have large misorientation angles. The inverse pole figure indicates that the refined grains at high undercooling have a textured crystallographic orientation. The fact that twins exist in the refined structure at high undercooling and the dendritic substructure arms exhibit no misorientation supports such an argument that the grain refinement at high undercooling results from the recrystallization of the solidification dendrites. [Copyright &y& Elsevier]

Abstract: The grain refinement of Fe75Ni25 alloys at low undercooling was investigated by fluxing and cyclic superheating methods. Grain refinement was observed if the initial undercooling exceeds 56 K. Based on theoretical calculations and experimental observations, the grain refinement mechanism was achieved, and can be attributed to the breakup of dendrite–skeletons by remelting. [Copyright &y& Elsevier]

Peritectic Fe–Ni alloys were undercooled using glass fluxing. When subjected to an undercooling (ΔT) beyond a critical value (ΔT ∗ ≈130K), the peritectic reaction, L +δ→γ, normally occurring after primary recalescence, was found to be suppressed. This can be ascribed to a longer incubation time for the occurrence of the peritectic reaction than the time required for solidification of δ phase during post-recalescence, for ΔT >ΔT ∗. [Copyright &y& Elsevier]

Abstract: Ni80.3B19.7 alloy was undercooled using the glass-fluxing method. Applying high speed video (HSV) and metallographic methods, the recalescence behavior and as-solidified microstructure were observed. In combination with the analysis of the experimental results and the calculation on the basis of time dependent nucleation theory, the nucleation mechanism in undercooled Ni80.3B19.7 melt was investigated. Owing to the time required to form the critical nucleus is much smaller than the time required for solid/liquid (S/L) interface to touch the nucleus, the critical nucleuses ahead the S/L interface can form and develop into the nucleation events, which make the copious nucleation ahead the S/L interface be operative. The calculation is in quantitative agreement with the experimental results. [Copyright &y& Elsevier]

Abstract: In order to investigate forming mechanism of the metastable “dendrite core” in undercooled Fe–7.5 at.%Ni alloy melt, alloy samples subjected to undercoolings above the critical value for metastable phase formation were quenched and heat-treated. It was evidenced that solid-state transformation, i.e. from metastable δ phase to stable γ phase, cannot be completed during solidification, thus the remained δ phase forms the so-called metastable “dendrite core”. Meanwhile the solidification behavior during the second recalescence was described applying peritectic reaction theory. [Copyright &y& Elsevier]

The Fe alloy melts containing 7.5, 15, 22.5 and 30 at% Ni were bulk undercooled to investigate the structure evolution. When the undercooling of the four melts is lower than the critical value 110, 125, 175 and 325 K, respectively, only the stable face-centered cubic phase crystallizes. In this case a grain refinement caused by solid superheating is observed in all the alloys, but another grain refinement induced by recrystallization can merely occur in the Fe–30 at%Ni alloy undercooled by 190–325 K. Alternate crystallization of the metastable body-centered cubic phase occurs above the critical undercooling. It is indicated that the subsequent heterogeneous nucleation of the stable phase in the metastable solid and remaining liquid coexisting system is influenced not only by the morphology and surface area of the metastable solid, but also by the effective undercooling of the remaining liquid. On the basis of the experimental results and the theoretical analyses, a structure evolution map for bulk Fe–Ni system is constructed. [ABSTRACT FROM AUTHOR]

Abstract: Rapid solidification of undercooled peritectic Fe81.68Co18.32 alloy was investigated using fluxing method. With the increase of initial melt undercooling (ΔT), different microstructures were observed. For low ΔT, a typical peritectic microstructure, if ΔT exceeded a critical value, 177K, metastable phase was observed in the as-solidified microstructure. On this basis, different solidification modes for peritectic Fe81.68Co18.32 alloy were demonstrated. When ΔT <ΔT crit, the melt solidified with peritectic mode, peritectic reaction and peritectic transformation occurred, followed by extension growth of γ at the peritectic layer. When ΔT >ΔT crit, the melt solidified with competitive nucleation mode, competitive nucleation occurred between stable and metastable phase, and metastable phase nucleated primarily from the undercooled melt as a result. [Copyright &y& Elsevier]

Abstract: Applying melt-fluxing, how to predict the maximal recalescence temperature T R upon rapid solidification of bulk undercooled Cu70Ni30 alloy has been investigated. Reproducible experimental results show that T R decreases with the increase of the initial melting undercooling ΔT, and moreover, its value is much lower than that predicted by Level rule and Scheil equation, let alone the equilibrium liquidus temperature. Quantitative thermodynamic calculation, in combination with an analysis of solute distribution, demonstrates that, T R is affected by the difference of Gibbs free energy between the residual liquid and solid after recalescence. Subjected to sufficiently high ΔT, T R should be nearly equal to the thermodynamic T 0 point corresponding to the concentration of the residual melt after recalescence. [Copyright &y& Elsevier]

Abstract: In order to investigate phase selection between stable and metastable phases upon rapid solidification of Fe70Co30 single-phase alloy, different undercooling experiments were performed. The critical undercooling for forming metastable phase upon spontaneous crystallization was observed as ∼203K, whereas, it decreased to ∼169K if artificially triggering was adopted. This can be ascribed to a substantial reduction of the catalytic potency factor f(θ) of bcc phase in the case of triggered nucleation. [Copyright &y& Elsevier]