Non-faceted Phase Research Articles

Rapid Eutectic Growth Kinetics of Undercooled Nb-Si Alloys at Electrostatic Levitation State

The rapid eutectic growth kinetics of undercooled Nb-Si alloys was investigated by using electrostatic levitation (ESL) technique combined with triggering method. For eutectic Nb-17.3 at.% Si alloy, the rapidly solidified microstructure was characterized by (Nb) dendrites and (Nb)+Nb3Si eutectics, which was different from the master alloy composed of complete eutectic structures. The transition of primary phase from the faceted Nb3Si phase to non-faceted (Nb) phase and the formation of complete eutectic microstructure were observed within hypereutectic Nb-18.7 at.% Si alloy with the increase of undercooling. Three distinct solidification paths leading to different microstructures and constituent phases were revealed by controlling the melt undercooling. Furthermore, the phase selection mechanism was clarified from the perspective of nucleation kinetics. Based on the experimental and analytical results, the coupled zone of (Nb)+Nb3Si eutectic was qualitatively predicted. The final microstructures solidified at different undercoolings were all composed of the (Nb) phase and Nb3Si phase, whose crystaline structures were both confirmed by transmission electron microscopy analysis. Besides, it was found that the volume fractions of (Nb) and Nb3Si phases could be undercooling control adjusted by. According to the nano-indentation and Vickers micro-indentation tests, the mechanical properties varied significantly with solidification paths. Compared with the master alloy, the hardness of hypereutectic Nb-18.7 at.% Si alloy at 157 and 473 K undercoolings was decreased by 42.7 % and 9.9 %, while the fracture toughness was enhanced remarkably by 593.1 % and 144.8 %, respectively. This may provide a desirable approach for designing Nb-Si alloys with improved performances.

Investigation of Peritectic Solidification Morphologies by Using the Binary Organic Model System TRIS-NPG.

Under pure diffusive growth conditions, layered peritectic solidification is possible. In reality, the competitive growth of the primary α-phase and the peritectic β-phase revealed some complex peritectic solidification morphologies due to thermo-solutal convection. The binary organic components Tris-(hydroxylmenthyl) aminomethane-(Neopentylglycol) were used as a model system for metal-like solidification. The transparency of the high-temperature non-faceted phases allows for the studying of the dynamic of the solid/liquid interface that lead to peritectic solidification morphologies. Investigations were carried out by using the Bridgman technic for process conditions where one or both phases solidify in a non-planar manner. Different growth conditions were observed, leeding to competitive peritectic growth morphologies. Additionally, the competitive growth was solved numerically to interpret the observed transparent solidification patterns.

Studying on the morphology of primary phase by 3D-CT technology and controlling eutectic growth by tailoring the primary phase

Morphology and alignment of the structure determine material properties. In the present work, the morphologies of the typical primary Al2Cu and Al3Ni phases in hypereutectic alloys are studied by 3D-CT technology, and their growth characteristics have been clarified further. The experimental result reveals that the Al2Cu and Al3Ni phases grow with a faceted characteristic at a lower growth rate, and the transition from the faceted phase to the non-faceted phase occurs with the increase of growth rate. It is also found that the application of the temperature gradient or/and the magnetic field is capable of controlling the growth of the primary Al2Cu and Al3Ni phases, and the alignments of the primary Al2Cu and Al3Ni phases remarkably affect that of the Al–Al2Cu and Al–Al3Ni eutectic. This implies that the eutectic phase can be controlled by tailoring the primary phases during imposing the magnetic field or/and temperature gradient. Finally, a simple model of controlling the growth of the eutectic phase by tailoring the primary phase is proposed. The present work can deepen the understanding on the growth characteristics of the Al2Cu and Al3Ni phases and offer a new method to control the eutectic phase by tailoring the primary phase.

The mechanism of eutectic modification by trace impurities

In the quest toward rational design of materials, establishing direct links between the attributes of microscopic building blocks and the macroscopic performance limits of the bulk structures they comprise is essential. Building blocks of concern to the field of crystallization are the impurities, foreign ingredients that are either deliberately added to or naturally present in the growth medium. While the role of impurities has been studied extensively in various materials systems, the inherent complexity of eutectic crystallization in the presence of trace, often metallic impurities (‘eutectic modification’) remains poorly understood. In particular, the origins behind the drastic microstructural changes observed upon modification are elusive. Herein, we employ an integrated imaging approach to shed light on the influence of trace metal impurities during the growth of an irregular (faceted–non-faceted) eutectic. Our dynamic and 3D synchrotron-based X-ray imaging results reveal the markedly different microstructural and, for the first time, topological properties of the eutectic constituents that arise upon modification, not fully predicted by the existing theories. Together with ex situ crystallographic characterization of the fully-solidified specimen, our multi-modal study provides a unified picture of eutectic modification: The impurities selectively alter the stacking sequence of the faceted phase, thereby inhibiting its steady-state growth. Consequently, the non-faceted phase advances deeper into the melt, eventually engulfing the faceted phase in its wake. We present a quantitative topological framework to rationalize these experimental observations.

Compression properties enhancement of Al-Cu alloy solidified under a 29 T high static magnetic field

Bulk solidification of Al-40 wt% Cu hypereutectic alloy has been investigated using a high static magnetic field (HSMF). The results show that, by imposing a 29 T HSMF, the melt convection has been completely suppressed in the mushy zone and the whole scale. The primary Al2Cu faceted phase changed to non-faceted phase because of the growth enhancement of Al2Cu precipitates. Al-Al2Cu eutectic was destabilized as a coexisting of lamellar and rod-like α-Al phase under 0 T whereas it was stabilized as a lamellae under the 29 T HSMF. The dimension of primary Al2Cu precipitates has been refined. The interlamellar spacing of eutectic Al-Al2Cu structure has been reduced from 6.3 µm to 2–3 µm. The primary Al2Cu precipitates were almost oriented to its < 001 > crystal direction which was parallel to the direction of the vertical HSMF. The morphologies of shrinkages displays as approximately regular spheres. Compressive strength of Al-40 wt% Cu alloy has been enhanced from 248 MPa to 431 MPa and the maximum stain has been improved from 3.6% to 5%. This paper reveals the effect of HSMF on Al2Cu precipitates, provides a new sight about refinement theory and new pathway to fabricate brittle materials with excellent mechanical properties.

Fluctuation of Solid-Liquid Interface of Faceted Phase and Nonfaceted Phase by Periodic Temperature Variation

In order to examine how the solid-liquid interface responds to temperature variation depending on the materials characteristics, i.e. faceted phase or nonfaceted phase, the moving solid-liquid interface of transparent organic material, as a model substance for metallic materials (pivalic acid, camphene, salol, and camphor-50wt% naphthalene) was observed in-situ. Plots of the interface movement distance against time were obtained. The solid-liquid interface of the nonfaceted phase is atomically rough; it migrates in continuous mode, giving smooth curves of the distance-time plot. This is the case for pivalic acid and camphene. It was expected that the faceted phases would show different types of curves of the distance-time plot because of the atomically smooth solid-liquid interface. However, salol (faceted phase) shows a curve of the distance-time plot as smooth as that of the nonfaceted phases. This indicates that the solid-liquid interface of salol migrates as continuously as that of the nonfaceted phases. This is in contrast with the case of naphthalene, one of the faceted phases, for which the solidliquid interface migrates in “stop and go” mode, giving a stepwise curve of the distance-time plot.

Microstructure of the directionally solidified ternary eutectic ceramic Al2O3/MgAl2O4/ZrO2

The directionally solidified Al2O3/MgAl2O4/ZrO2 ternary eutectic ceramic was prepared via induction heating zone melting. Smooth Al2O3/MgAl2O4/ZrO2 eutectic ceramic rods with diameters of 10mm were successfully obtained. The results demonstrate that the eutectic rods consist of Al2O3, MgAl2O4 and ZrO2 phases. In the eutectic microstructure, the MgAl2O4 and Al2O3 phases form the matrix, the ZrO2 phase with a fibre or shuttle shape is embedded in the matrix, and a quasi-regular eutectic microstructure formed, presenting a typical in situ composite pattern. During the eutectic growth, the ZrO2 phase grew on non-faceted phases ahead of the matrix growing on the faceted phase. The hardness and fracture toughness of the eutectic ceramics reached 12GPa and 6.1MPa·m1/2, respectively, i.e., two times and 1.7 times the values of the pre-sintered ceramic, respectively. In addition, the ZrO2 phase in the matrix reinforced the matrix, acting as crystal whiskers to reinforce the sintered ceramic.

Thermal Conduction Performance of Fiber-Enhanced Polyalcohol Binary Systems

Polyalcohol has poor heat conduction performance. A fiber-enhanced polyalcohol binary system combines polyalcohol with a copper fiber net to improve its heat conduction performance. In a binary system without fibers, more heat input increases the wall temperature near the container than when the fiber is present. Compared to a polyalcohol binary system, pentaerythritol/Tris hydroxymethyl aminomethane (PE/TRIS) without fibers, fiber-enhanced polyalcohol system PE/TRIS shows quicker response to energy input from the exterior region. The phase change temperature in a fiber-enhanced polyalcohol binary system is much lower than that in a polyalcohol system without fibers. This is because of the metastate plastic state that presents a nonfaceted phase interface with a larger radius of curvature in a polyalcohol system without fibers. The porous structure of the fiber is smaller than the size of the phase interface in a polyalcohol system without fibers and can increase the equilibrium pressure and make the phase change easier.

Halo formation in arc-melted Cr–Nb alloys

Haloes of Laves phase Cr2Nb around the primary dendrites Cr were observed in the arc-melted Cr–12% Nb hypoeutectic alloy, while no halo of non-faceted Cr phase around the Cr2Nb Laves phase occurred in the Cr–20% Nb hypereutectic alloy. This observation differs from the haloes formed in metal–intermetallic alloy systems. An explanation on the formation of Cr2Nb halo was presented by considering the solidification behavior of the leading phase and non-reciprocal nucleation characteristics of the Cr2Nb/Cr eutectic. In the Cr–12% Nb alloy, primary dendrites Cr worked as a good nucleant available for the occurrence of the halo Cr2Nb that subsequently solidified as the leading phase in the eutectic. For the Cr–20% Nb alloy, primary dendrites Cr2Nb first precipitated as the leading phase inducing the eutectic formation, resulting in no halo growth. In addition, the coupled zone of the Cr–Nb alloy was theoretically predicted in agreement with the experimental results.

Abnormal growth relationship between Si and other phases in undercooled Al–Cu–Si alloy

The dependence of the growth relationship between Si and other two phases on undercooling was investigated in terms of microstructural characteristics in Al77.2Cu13.6Si9.2 ternary alloy. As undercooling increases, the independent growth of faceted Si phase and the cooperative growth of Al and θ phases are replaced by the attaching growth of Al phase on faceted Si phase in the ternary eutectic. At the same time, θ phase and a small amount of non-faceted Si phase tend to grow cooperatively in both pseudobinary and ternary eutectics. Once undercooling exceeds a critical value, the independent nucleation and growth of the three non-faceted phases result in anomalous ternary eutectic.

Formation mechanism of the primary faceted phase and complex eutectic structure within an undercooled Ag–Cu–Ge alloy

The solidified microstructure of bulk undercooled Ag40Cu30Ge30 alloy consists of three parts: primary (Ge) phase, the complex structure of (Ag + Ge) and (Ag + e 2) pseudobinary eutectics, and (Ag + Ge + e 2) ternary eutectic. In comparison, the pseudobinary eutectic no longer appears in an alloy droplet solidified in a drop tube. Once the undercooling exceeds 225 K and the cooling rate is greater than 2×103 K s−1, the microstructure of the solidified droplet is totally composed of anomalous ternary eutectic. In both cases, the primary (Ge) phase exhibits various faceted growth morphologies at different undercoolings, such as columnar block, long dendrite, equiaxed dendrite and rod-like crystal. Some refined side branches grow from the equiaxed (Ge) dendritic branches composed of {111} twins, which is ascribed to the rapid epitaxial growth of (Ag + Ge) pseudobinary eutectic from the (Ge) dendritic branches. Moreover, both the primary (Ge) phase and the (Ge) phase in the (Ag + Ge) pseudobinary eutectic are effective heterogeneous nuclei for the (Ag+e 2) pseudobinary eutectic. As undercooling increases, the (Ge) phase in the (Ag + Ge+e 2) ternary eutectic transforms from faceted to non-faceted phase, while the independent nucleation and growth of the (Ag) and e 2 phases in the ternary eutectic displaces their previous cooperative growth. These growth kinetics transitions result in the formation of anomalous ternary eutectic.

Morphology and microstructure of M2C carbide formed at different cooling rates in AISI M2 high speed steel

In as-cast structure of AISI M2 high speed steel, M2C carbide prevails, the morphology of which has crucial influence on distribution and dimension of carbides in final products. In this study, the morphology and microstructure of M2C formed at different cooling rates have been investigated by scanning electron microscope, X-ray diffraction, transmission electron microscope, and electron back-scatter diffraction. The results show that the morphology of M2C carbide changes from the plate-like type to the fibrous one with increasing cooling rates. Surprisingly, the microstructure between plate-like and fibrous M2C is significantly different. Twining and stacking faults are observed in the plate-like M2C, which is supported by great misorientations between adjacent carbides. However, no planar faults are identified in fibrous M2C and the carbides in one colony have almost identical orientation. It is expected that plate-like M2C grows as a faceted phase, while fibrous M2C formed at high cooling rates is likely a non-faceted phase. The difference of liquid/solid interface structure is supposed to result in the morphology change of M2C.

Oscillatory growth of nonfaceted dendrite and faceted plate during unidirectional solidification

Nonfaceted dendrite and faceted plate in succinonitrile-0.7 wt.% salol and camphor-47.4 wt.% and −35 wt.% naphthalene mixtures were in situ observed during unidirectional solidification. Nonfaceted dendrite oscillates in the growth direction during unidirectional solidification, alternatively repeating fast and slow growth. The faceted phase, whose growth is operated by a two dimensional nucleation mode, also shows oscillation of growth velocity. The oscillation in the faceted phase is due to the intrinsic growth nature, while in the nonfaceted phase it is due to experimental artifacts, that is, thermal fluctuations in the cold and hot zones. The implications of the observed dendrite tip fluctuation in relation with the initiation of dendrite sidebranching have been discussed.

Rapid solidification of undercooled Al-Cu-Si eutectic alloys

Under the conventional solidification condition, a liquid aluminium alloy can be hardly undercooled because of oxidation. In this work, rapid solidification of an undercooled liquid Al80.4Cu13.6Si6 ternary eutectic alloy was realized by the glass fluxing method combined with recycled superheating. The relationship between superheating and undercooling was investigated at a certain cooling rate of the alloy melt. The maximum undercooling is 147 K (0.18T E). The undercooled ternary eutectic is composed of α(Al) solid solution, (Si) semiconductor and θ(CuAl2) intermetallic compound. In the (Al+Si+θ) ternary eutectic, (Si) faceted phase grows independently, while (Al) and θ non-faceted phases grow cooperatively in the lamellar mode. When undercooling is small, only (Al) solid solution forms as the leading phase. Once undercooling exceeds 73 K, (Si) phase nucleates firstly and grows as the primary phase. The alloy microstructure consists of primary (Al) dendrite, (Al+θ) pseudobinary eutectic and (Al+Si+θ) ternary eutectic at small undercooling, while at large undercooling primary (Si) block, (Al+θ) pseudobinary eutectic and (Al+Si+θ) ternary eutectic coexist. As undercooling increases, the volume fraction of primary (Al) dendrite decreases and that of primary (Si) block increases.

Modeling structure parameters of irregular eutectic growth: Modification of Magnin-Kurz theory

A new approach to irregular eutectic growth (faceted/nonfaceted crystallization) in Fe-C and Al-Si alloys has been presented in this article. The results of unidirectional crystallization of the irregular eutectic in the Fe-C, Al-Si, and Al-Fe systems were used for the experimental verification of the resulting model. For the oriented graphite, α(Al)-Si and α(Al)-Al3Fe eutectics, a decrease of the interlamellar spacing λ and in protrusion δβ of the nonfaceted phase (austenite, α(Al)) by the leading faceted phase (graphite, silicon, and Al3Fe), the increase of growth rate v was observed. The Magnin-Kurz theory of irregular eutectic growth has been modified in order to better understand the physical mechanisms driving the crystallization process. A comparison of the measured and calculated average λ values has revealed good agreement for the (γ)Fe-graphite, α(Al)-Si, and α(Al)-Al3Fe eutectics. The developed model also considered the influence of the material constants of the examined alloys on the interlamellar spacing and protrusion of the leading phase—graphite, silicon, and Al3Fe. It has been found that material constants such as the wetting angle, diffusion coefficient, and Gibbs-Thomson coefficient are of great importance in this eutectic growth.

MICROSTRUCTURAL EVOLUTION OF HIGHLY UNDERCOOLED EUTECTIC Ni 78.6Si 21.4 ALLOY

A large undercooling level up to 550K (0.386T E ) was achieved in eutectic Ni 78.6 Si 21.4 melt by the combination of molten-glass and cyclic superheating. A microcrystalline structure is obtained at large undercooling. Surprisingly, the morphology of α(Ni) phase transits from the non-faceted phase to faceted phase at large undercooling of 390K. Based on the classical nucleation theory and transient nucleation theory, the process of microstructure evolution and competitive nucleation was analyzed, and the refinement of crystal structure is determined by the high nucleation rate under large undercooling.

Organic alloy systems suitable for the investigation of regular binary and ternary eutectic growth

Transparent organic alloys showing a plastic crystal phase were investigated experimentally using differential scanning calorimetry and directional solidification with respect to find a suitable model system for regular ternary eutectic growth. The temperature, enthalpy and entropy of phase transitions have been determined for a number of pure substances. A distinction of substances with and without plastic crystal phases was made from their entropy of melting. Binary phase diagrams were determined for selected plastic crystal alloys with the aim to identify eutectic reactions. Examples for lamellar and rod-like eutectic solidification microstructures in binary systems are given. The system (d)Camphor–Neopentylglycol–Succinonitrile is identified as a system that exhibits, among others, univariant and a nonvariant eutectic reaction. The ternary eutectic alloy close to the nonvariant eutectic composition solidifies with a partially faceted solid–liquid interface. However, by adding a small amount of Amino-Methyl-Propanediol (AMPD), the temperature of the nonvariant eutectic reaction and of the solid state transformation from plastic to crystalline state are shifted such, that regular eutectic growth with three distinct nonfaceted phases is observed in univariant eutectic reaction for the first time. The ternary phase diagram and examples for eutectic microstructures in the ternary and the quaternary eutectic alloy are given.

一方向凝固させたFe-C共晶合金の固液界面形状に及ぼすSの影響

Groove depth of the non-faceted phase at the solid-liquid interface and spacing of the faceted phase were measured for vertically solidified austenite-graphite irregular eutectics to investigate the effect of the addition of interfacial active element sulphur.Groove depth, δ (mm), and inter-graphite spacing, λ (mm), were expressed as functions of solidification rate R(mm/s):(This article is not displayable. Please see full text pdf.) Groove depth was strongly influenced by the growth condition in the groove, for example, the temperature distribution in the groove, and was not so much influenced by the local growth condition at the eutectic tips which protrude into liquid; and it increased with a small addition of sulphur.Inter-graphite spacing decreased with decreasing sulphur, and approached the theoretical value derived by Jackson and Hunt if the eutectic tips were aligned on and the growth directions of the tips were perpendicular to the macroscopic solid-liquid interface plane; and it increased with a small addition of sulphur.

Faceting and morphological regularity of eutectic structures: The AuSn 4-Sn system

The AuSn 4-Sn eutectic formed with a faceted AuSn 4 intermetallic phase and a non-faceted phase can present a perfect lamellar structure over regions several square millimeters in area and 1 cm long. In this case, directional solidification must be carried out under a marked thermal gradient (450 K cm ′-1): a Bridgman furnace with a heat pipe was needed to obtain this gradient. The stability region of the growth coupling of the lamellar structure is given as a function of the ratio gradient over growth rate and of the gold content. No equation was found correlating the interlamellar spacing and the growth rate. Due to the very privileged crystallographic nature of the solid/solid interfaces new lamellae must be germinated to enable the interlamellar spacing to adapt to the growth rate. This is a very slow mechanism and a steady growth cannot be obtained during solidification of a specimen several centimeters long. The interpretation of the growth modes observed throws doubt on the traditional analysis of eutectic growth. The role of the solid/solid and solid/liquid interfaces during growth and that of the anisotropy of the interface properties are emphasized.

Solidification of undercooled Sn-Bi and Pb-Sb alloys

In hypoeutectic and eutectic alloys of Sn-Bi and Pb-Sb, the first phase to solidify from the melt is the nonfaceted phase, tin and load, respectively. Since primary tin and lead are poor nucleants for the second phase of the eutectic, the faceted phase, substantial undercooling below the equilibrium eutectic temperature occurs before the eutectic is nucleated. Once nucleated the faceted phase does not grow preferentially to return the melt composition to the equilibrium eutectic composition. Instead growth of a complex regular structure takes place from a melt whose composition is displaced from the equilibrium eutectic composition towards the faceted component. Reheating experiments have confirmed that the complex regular structure is rich in the faceted component. This structure grows at a temperature depressed several degrees below the equilibrium eutectic temperature. An hypothesis is presented to explain the growth of a structure rich in the faceted component at a depressed temperature.