Unidirectional Solidification Experiments Research Articles

Effect of Solid/Liquid and Eutectic Front Velocities on Microstructure Evolution in Al-20%Cu Alloys

During the solidification process, microstructures are affected by the experimental conditions, the thermophysical characteristics of the alloy, and the type of grain-refining particles. Unidirectional solidification experiments were performed in a vertical Bridgman-type furnace to investigate the effect of the solidification front velocity on the solidified microstructure of a non-refined and refined Al-20%Cu alloy. The samples were solidified by rapidly increasing the sample velocity (v) range from 0.02 mm/s to 0.2 mm/s while maintaining an almost constant temperature gradient (~5 K/mm). As a result, despite changes in the solid/liquid front velocity along the sample, the microstructure of the non-refined alloys remained columnar. In the refined alloy, the columnar structure changed into an equiaxed structure at two different front velocities.

Comparison of Dynamical and Empirical Simulation Methods of Secondary Dendrite Arm Coarsening

The physical and mechanical properties of an entirely (wrought alloys) or partly (cast alloys) dendritically solidified alloy strongly depend on the secondary dendrite arm spacing (SDAS). The casting practice and the simulation of solidification need a usable but simple method to calculate the SDAS during and at the end of solidification as a function of the cooling rate. Based on many solidification experiments, a simple equation to calculate the SDAS (empirical method) is known to use the local solidification time, which can be obtained from the measured cooling curves (equiaxed solidification), or can be calculated from the temperature gradient and front velocity (directional solidification). This equation is not usable for calculating the SDAS during solidification. Kirkwood developed a semi-empirical method based on the liquid phase’s diffusion, which contains only one geometric factor that seems constant for different alloys. This equation contains some physical parameters that depend on the temperature, so the equation cannot be integral in closed form. In the present work, first, we show the effect of the curvature of the solid/liquid interface on the equilibrium concentrations and then the different processes of SDA coarsening. In our earlier paper, we demonstrated that using the empirical method, the final SDAS can be calculated with acceptable correctness in the case of four unidirectional solidification experiments of Al-7wt%Si alloy. The present work shows that numerically integrated Kirkwood’s equations used the known cooling curve; the SDAS can be calculated at the end and during solidification in good agreement with these experimental results. Compared to the two calculation methods, we stated that the correctness of the methods is similar. Still, the results of the solidification simulation (the microsegregation) will be more correct using the dynamical method. It is also shown that with the dynamical method, the SDAS can be calculated from any type of cooling curve, and using the dynamical method, it is proved that some different SDASs could belong to the same local solidification time.

Temperature Field Evolution of Seeding during Directional Solidification of Single-Crystal Ni-Based Superalloy Castings

By combined numerical simulations and unidirectional solidification experiments, the temperature field evolution of seeding for nickel-based single-crystal superalloys was studied. At the steady state, the position of the mushy zone in the seed moves down as the melt is poured into the mold. The holding time required for the temperature field of the seed segment to reach the steady state is less for seeds with casting than for those without casting. The holding time required to completely eliminate randomly oriented broken dendrites in the melt-back zone is much longer than that required for the temperature field to achieve a steady state. A short incubation stage is required before the temperature field evolution process; then, the migration rate of the isotherm gradually increases until it is the vicinity of the withdrawal rate. Finally, the effect of temperature field evolution on the formation of stray grains in seed segments is discussed.

Solidification Simulation of Al-Si Alloys with Dendrite Tip Undercooling

A novel solution approach is proposed for the numerical simulation of the solidification process of binary Al-Si hypoeutectic alloys during upward and downward solidification modes. Undercooling is always observed during solidification, but the phenomenon could not be considered in the present-day numerical solution. In this approach, the temperature distribution in the mushy zone was used to define the fraction of solid, which enabled the evaluation of the effect of dendrite tip undercooling on the characteristics of the binary alloy solidification. The present numerical algorithm was found to significantly reduce the computation time. Transient temperature distribution and solidification time from the numerical analysis, with consideration of natural convection due to temperature and concentration gradients, have been successfully simulated and validated with experiment results. Numerical results with consideration of dendrite tip undercooling have better agreement with experimental results. The effect of dendrite tip undercooling on the fluid flow (velocity profile), G, R and λ1 for both upward and downward solidification modes of Al-Si alloys have been investigated and discussed. Consideration of undercooling was found to increase G and reduce R in both solidification modes. During downward solidification, considering undercooling significantly increased flow velocity and decreased λ1. The primary dendrite arm spacing could be validated with results from uni-directional solidification experiments only when dendrite tip undercooling was considered.

The effect of melt flow induced by RMF on the meso- and micro-structure of unidirectionally solidified Al–7wt.% Si alloy Benchmark experiment under magnetic stirring

This work deals with the effect of melt flow on the solidified structure by RMF. The microstructure of the first non-stirred part of the Al-7Si alloy is columnar. After switching on stirring first a transient zone was formed with eutectic “cupola”. After the first transient zone, the “Christmas Tree Like” microstructure was formed. During the solidification of stirred part, a “tulip-like” interface evolves. During the last two decades, many algorithms were developed to simulate the solidification process for different casting methods like ingots, continuous casting of steel and the direct chill cast of aluminum. Experiments performed under exactly known conditions and with the detailed knowledge of meso- and micro-structures are required for validating these simulations. The aim of this paper is to give a data set to validate these simulations. Unidirectional solidification experiments were performed by using a rotating magnetic field (RMF) to study the effect of melt flow on the solidified micro- and meso-structure of the Al–7wt.% Si binary alloy. The first and the third 1/3 parts of samples were solidified without magnetic stirring, and the second (middle) 1/3 part was solidified by using magnetic stirring. The magnetic induction was 10 mT, the temperature gradient was ∼7 K/mm, and the sample movement velocity was 0.1 mm/s. On the longitudinal section of the sample, the columnar/equiaxed transition (CET), the equiaxed/columnar transition (ECT), the secondary dendrite arm spacing (SDAS), and the macrosegregation (concentration distribution and the amount of eutectic) were investigated. The primary dendrite arm spacing (PDAS) and the grain structure were studied on the cross-section after color etching. The effect of melt flow induced by RMF on the meso- and microstructure of unidirectionally solidified Al–7wt.% Si alloy Benchmark experiment under magnetic stirring .

Determination of the conditions of laminar/turbulent flow transition using pressure compensation method in the case of Ga75In25 alloy stirred by RMF

The effect of melt-flow on the microstructure has been investigated by performing several unidirectional solidification experiments where the melt was flown in a rotating magnetic field. It is a well-known fact that the angular frequency of the melt cylinder always differs from the angular frequency of the magnetic field. However, it proves to be very difficult to determine during the experiments. In our present study, the magnetic Taylor number and the Reynolds number were determined as a function of the radius of melt cylinder, the magnetic induction, and the angular frequency of magnetic field using the real angular frequency defined by the so-called pressure-compensation method developed earlier in the case of Ga75In25 alloy. The simulations developed for performing similar experiments can be checked as well as the different experiments can be compared correctly by using the obtained measurement results. By knowing the critical Reynolds number belonging to the laminar/turbulent flow transition, the value of critical magnetic induction belonging to the transition was determined as a function of the radius of the melt cylinder.

Upward Unsteady-State Solidification of Dilute Al–Nb Alloys: Microstructure Characterization, Microhardness, Dynamic Modulus of Elasticity, Damping, and XRD Analyses

Aluminium alloys form many important structural components, and the addition of alloying elements contributes to the improvement of properties and characteristics. The objective of this work is to study the influence of thermal variables on the microstructure, present phases, microhardness, dynamic modulus of elasticity, and damping frequency in unidirectional solidification experiments, which were performed in situ during the manufacturing of Al–0.8 Nb and Al–1.2 Nb (wt.%) alloys. Experimental laws for the primary (λ1) and secondary (λ2) dendritic spacings for each alloy were given as a function of thermal variables. For Al–0.8%wt Nb, λ1 = 600.1( T ˙ )−1.85 and λ2 = 186.1(VL)−3.62; and for Al–1.2%wt Nb, λ1 = 133.6( T ˙ )−1.85 and λ2 = 55.6(VL)−3.62. Moreover, experimental growth laws that correlate the dendritic spacings are proposed. An increase in dendritic spacing influences the solidification kinetics observed, indicating that metal/mold interface distance or an increase in Nb content lowers the liquidus isotherm velocity (VL) and the cooling rate (Ṫ). There is also a small increase in the microhardness, dynamic modulus of elasticity, and damping frequency in relation to the composition of the alloy and the microstructure.

Macrosegregation Formation in an Al\u2013Si Casting Sample with Cross-sectional Change During Directional Solidification

A unidirectional solidification experiment of hypoeutectic Al-7.0 wt% Si alloy against gravity direction in a cylindrical mold with cross-sectional change was made, and the macrosegregation in different parts of the as-solidified sample was investigated (Ghods et al. in J Cryst Growth 441:107–116, 2016; J Cryst Growth 449:134–147, 2016). The current study is to use a two-phase columnar solidification model to analyze the segregation mechanisms as used in this experiment. Following flow phenomena and their contributions to the formation of macrosegregation are simulated and compared: (1) solidification shrinkage-induced feeding flow; (2) thermo-solutal convection; and (3) combined thermo-solutal convection and shrinkage-induced feeding flow. The shrinkage-induced feeding flow leads to an inverse (positive) segregation in the bottom part, and a severe negative segregation in the part below cross-sectional change. Thermo-solutal buoyancy leads to a so-called steepling convection in the main part of the sample (away from the bottom and cross-sectional change), and this kind of flow leads to a positive macrosegregation near the sample surface. The calculations have successfully explained the experimental result of macrosegregation.

The correlation of microstructure features, dry sliding wear behavior, hardness and tensile properties of Al-2wt%Mg-Zn alloys

The effect of Zn addition on the as-cast microstructures, mechanical properties and dry sliding wear behavior of Al-2wt%Mg-Zn alloys is investigated in this study. Two Al-Zn-Mg ternary alloys, with 5% and 8% Zn contents and 2 wt% Mg, were developed: 752 alloy (Al-5wt%Zn - 2 wt%Mg) and 782 alloy (Al-8wt%Zn-2wt%Mg). Unidirectional vertical upward solidification experiments were performed using a metallic water-cooled mold instrumented by thermocouples. Samples extracted along the length of the resultant ingots were characterized by optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD), and mechanical properties were analyzed by hardness, microhardness, tensile and dry sliding wear tests. The results showed that the microstructures for both alloys are composed of an α-Al dendritic matrix and MgZn2 and Al2Mg3Zn3 interdendritic precipitates. An experimental equation is derived relating the secondary dendrite arm spacing (λ2) to the solidification cooling rate (Ṫ). Experimental relationships show that both Brinell Hardness (HB) and ultimate tensile strength (UTS) increased with the decrease in λ2. The wear resistance is shown to increase with the increase in the alloy Zn content and with the decrease in λ2. A correlation between wear, hardness and ultimate tensile strength was proposed, in which wear decreases with the increase in both HB and UTS. The observed wear mechanism is shown to be abrasive for all examined alloys and experimental conditions.

Dynamics of spacing adjustment and recovery mechanisms of ABAC-type growth pattern in ternary eutectic systems

In directionally solidified 2D samples at ternary eutectic compositions, the stable three-phase pattern is established to be lamellar structure with ABAC stacking, where A, B, and C are crystalline phases. Beyond the stability limits of the ABAC pattern, the system uses various spacing adjustment mechanisms to revert to the stable regime.In this study, the dynamics of spacing adjustment and recovery mechanisms of isotropic ABAC patterns were investigated using three-phase In–Bi–Sn alloy. Unidirectional solidification experiments were performed on 23.0 and 62.7µm-thick samples, where solidification front was monitored in real-time from both sides of the sample using a particular microscopy system. At these thicknesses, the pattern was found to be 2D during steady-state growth, i.e. both top and bottom microstructures were the same. However, during spacing adjustment and recovery mechanisms, 3D features were observed. Dynamics of two major instabilities, lamellae branching and elimination, were quantified. After these instabilities, two key ABAC pattern recovery mechanisms, namely, phase invasion and phase exchange processes, were identified and analyzed. After elimination, ABAC pattern is recovered by either continuous eliminations of all phases or by phase exchange. After branching, the recovery mechanisms are established to be phase invasion and phase exchange.

Evaluation of dendrite morphology using fractal dimension and dimensionless perimeter in unidirectionally solidified Al-Si Alloys

The dendrite morphology of unidirectionally solidified Al-Si alloys was evaluated by measuring the fractal dimension and dimensionless perimeter of dendrites. In an unidirectional solidification experiment, columnar crystals grew from a bottom chill and columnar to equiaxed transition (CET) occurred at the upper part of an ingot. Then, equiaxed crystals were formed at the top of the ingot. Different dendrite morphology was observed in longitudinal, transverse and oblique sections, however, the fractal dimension or dimensionless perimiter of the dendrites in the sections with same local solidification time showed same values, and continuously decreased with increase in the local solidification time through columnar, CET and equiaxed regions. It can be considered that the fractal dimension and dimensionless perimiter of dendrites are controlled by local solidification time and irrespective of dendrite morphology. This result demonstrated the potential of the fractal dimension and dimensionless perimiter as a parameter for estimating local solidification time of an ingot in which the measurement of SDAS is difficult.

Unidirectional Solidification of Pb-Sn Alloys in a Rotating Magnetic Field

Cylindrical Pb-Sn alloy samples (diameter: 8 mm, length: 120 mm) of different compositions (30, 40 and 50 wt.% of Sn) were prepared from high pure (4N) components. The unidirectional solidification experiments have been performed according to the upward vertical Bridgman-method by using a rotating magnetic field (RMF) with a magnetic induction of 150 mT and with a frequency of 50 Hz. The sample-movement velocity was constant (0.05 mm/s) and the temperature gradient changed from 7 to 3 K/mm during the solidification process. The first half of samples was solidified without using the magnetic field and the second half was solidified by using the magnetic field. Under the influence of this strong flow induced by the magnetic field, the columnar microstructure of the first part decomposed and a characteristic "Christmas tree"- like macrosegregated structure with equiaxed Pb-dendrites was developed. The secondary dendrite arm spacing (SDAS) and the volume percent of primary Pb-phase (dendrite) were measured by an automatic image analyser on the longitudinal polished sections along the whole length of the samples. The effect of the forced melt flow on the micro-and macrostructure was studied in case of the different sample compositions.

Cellular growth of single-phase Zn–Ag alloys unidirectionally solidified

Transient unidirectional solidification experiments have been carried out with a single-phase Zn–Ag alloy under cooling rates in the range of 0.1–40 K s−1. The resulting macrostructure is shown to be typified by columnar grains aligned along the heat flow direction and the microstructure is characterized by a cellular morphology along the entire casting length with no evidence of cellular/dendritic transition. A regular to plate-like cells transition is shown to occur for cooling rates higher than 10K s−1. Experimental results from the literature on the cellular growth of single-phase Zn–Ag alloys in steady-state solidification conditions are compared with the results of the present investigation. An experimental growth law relating the cell spacing with the cooling rate is proposed, which is shown to be able to represent both the steady-state and transient growth regimes of single-phase Zn–Ag alloys. The Bouchard–Kirkaldy model is shown to be the only theoretical growth model that matches the cellular experimental scatters in both solidification regimes.

Top-view approach for in-situ observation of growth morphology in bulk transparent organic alloys

A new experimental method for in-situ observation of microstructure formation in top view during unidirectional solidification of bulk, transparent, organic alloys is presented. This method allows observing growth patterns over an extended interface area with high resolution and minimal optical aberrations. With (D)camphor–neopentylglycol–succinonitrile (DC–NPG–SCN) alloys a series of unidirectional solidification experiments were performed in order to validate the set-up. By means of multi-focus exposition eutectic cells were observed over a depth of several millimeters, followed by 3D reconstruction of their shape. The method also allows capturing the integral interface dynamics and measuring its relative undercooling.

Formation of ordered nano-wire microstructures in thermoelectric Pb–Ag–Sb–Te

Regular arrays of nano-wire microstructures containing Ag2Te rods with diameter <400nm and inter-rod distance <1μm are formed in a matrix of AgSbTe2+δ-Sb2Te, which makes an ordered nano-wire composite thermoelectric material. This microstructure is formed during eutectic solidification processing. To understand and guide production of uniform bulk samples of this composite, the liquidus projection of quaternary Pb–Ag–Sb–Te system at 36.0at.% Te isoplethal section is constructed experimentally, using quenched samples. A three-phase univariant reaction of the quaternary Pb–Ag–Sb–Te system, L→AgSbTe2+Ag2Te+δ-Sb2Te, is investigated, between a ternary-eutectic melt (Ag–40.0at.% Sb–36.0at.% Te) and a quaternary melt (Pb–22.0at.% Ag–40.0at.% Sb–36.0at.% Te), at 496.1°C and 503.9°C, respectively. High-resolution transmission electron microscopy confirms that these three phases are simultaneously present at the nanometer scale. Furthermore, unidirectional solidification experiments of the ternary eutectic alloy using the Bridgman method are carried out to examine the alloy’s solidification behavior.

Correlation between dendrite arm spacing and microhardness during unsteady-state directional solidification of Al–Ni alloys

Hypoeutectic Al–Ni alloys are characterised by the formation of as-cast structures composed by a dendritic aluminium-rich matrix surrounded by a eutectic mixture (α + β), where α is the Al-rich phase and β is the Al3Ni intermetallics. The morphology, size and distribution of the intermetallic particles decisively affect the mechanical properties of these alloys. This study aims to determine experimental laws relating the primary dendritic arm spacing (λ1) and Vickers microhardness (HV) of hypoeutectic Al–Ni alloys. Upward unidirectional solidification experiments were performed in order to obtain castings with significant differences in the scale of microstructural parameters along the casting length. It was found that the microhardness values were directly influenced by both the amount of solute and λ1 and Hall–Petch-type equations relating the microindentation and λ1 are proposed.

Solubility and formation of ternary Widmanstätten precipitates in PbTe in the pseudo-binary PbTe–Bi2Te3 system

A unidirectional solidification experiment by Bridgman method has been performed for the Pb_(14)Bi_(28.8)Te_(57.2) composition, which lies on the pseudo-binary PbTe–Bi_2Te_3 system, resulting in the formation of Widmanstatten precipitates of a ternary compound, most likely with the structure of PbBi_2Te_4 in the PbTe matrix. The formation of the precipitates is caused by the decrease of bismuth solubility in the PbTe phase with decreasing temperature. The PbTe-rich part of the PbTe–Bi_2Te_3 phase diagram was investigated from the compositional variations in the unidirectionally solidified sample and the diffusion couples. This proved that the solubility decreases with decreasing temperature: 15.6 ± 0.9 (583 °C) to 62^(+21)_(−17) (450 °C) at.% Bi. The orientation relationship between the matrix and precipitates has been examined by electron backscatter diffraction technique; precipitation occurs on {111} habit planes in PbTe with orientation relationship (0001)_(precipitate)//{111}_(PbTe) and _(precipitate)// _(PbTe). The thermoelectric properties in PbTe with Widmanstatten precipitates as examined by the scanning Seebeck probe method is –46 ± 2 μVK^(−1).

Microstructure evolution and room temperature deformation of a unidirectionally solidified Nb-22Ti-16Si-3Ta-2Hf-7Cr-3Al-0.2Ho (at.%) alloy

Unidirectional solidification (DS) experiments were performed on a Nb-22Ti-16Si-3Ta-2Hf-7Cr-3Al-0.2Ho (at.%) alloy and the growth rates were selected as 6, 10 and 16 mm/h, respectively. The effect of solidification rates on the microstructure development and room temperature deformation was investigated. The DS alloys were mainly composed of Nb solid solution (Nbss) and Nb 5Si 3 phases. All the Nbss/Nb 5Si 3 eutectic colonies aligned well along the growth direction of the DS specimens. A small amount of hex-Nb 5Si 3 phase was identified as intergranular phase when observed from the transverse section of the DS specimens. There was an orientation relationship between Nbss and hex-Nb 5Si 3, which was determined as [ 111 ] Nb / / [ 22 4 ¯ 3 ] h and the two almost parallel planes of ( 1 1 ¯ 0 ) Nb / / ( 1 1 ¯ 00 ) h with an include angle about 2.2°. The DS alloys showed improved room temperature compressive strength and fracture toughness as compared with the casting alloy.

Crystallography of solid–liquid interface and evolution of texture during directional solidification of Tb 0.3Dy 0.7Fe 1.95 alloy

The effect of growth texture on the magnetostriction of ternary Tb 0.3Dy 0.7Fe 1.95 was studied by conducting unidirectional solidification experiments using a zoning set-up. Detailed texture evolutions were studied using X-ray diffraction on samples obtained by varying growth rates from 18 to 72 cm/h, under a temperature gradient of 100 °C/cm. The estimated texture co-efficient and pole figures of the samples indicate that during the onset of the solidification, <110> and <331>/‘rotated <110>’ texture components nucleate and grow in all the samples. However, as the solidification progresses, <112> texture component becomes dominant at higher growth rate. This results in an improvement of magnetostriction from 1000 to 1300 microstrains for samples grown at growth rates of 18 and 72 cm/h respectively. The transition of preferred growth direction occurs through intermediate orientations <123>. An attempt has been made in this paper to explain the occurrence of different growth texture by considering the stability of growing interface, its planar packing fraction and atomic stacking sequence of several low index planes.

A suggested effective method for unidirectional solidification under rotating magnetic field in the space experiments

In order to save costs and time during the unidirectional solidification experiments, it is necessary to obtain more and more information by solidifying one sample in a rotating magnetic field (RMF) to be performed on the space station. The macro- and microstructure developing during solidification is significantly influenced by the intensity and frequency of magnetic field, the solid/liquid interface velocity as well as the temperature gradient. A method by which one of the aforementioned four parameters can repeatedly be changed during the solidification is described. By this method, the transient phenomena occurring during parameters modification can also be observed. The results of earth experiments carried out by this method with Al7Si1Fe alloy are presented.