Directional Solidification Technology Research Articles

Removal of metal impurities from diamond wire saw silicon powder by vacuum electromagnetic directional solidification

The utilization of high-purity silicon (∼99.9999 %) is prevalent in the solar photovoltaic sector, and achieving a reduction in its cost has emerged as a pivotal engineering objective within the silicon solar industry. Recycling silicon from diamond wire saw silicon powder (DWSSP) is a promising solution to fulfill the continuously expanding of high purity silicon demand. Vacuum directional solidification technology using high-frequency electromagnetic induction heating was proposed to purify DWSSP for the first time efficiently. The silicon melt and gas interface reaction, as well as metal impurities’ microsegregation in silicon ingot were observed. The effects of vacuum level and electromagnetic force on the separation of silicon and metal impurities were investigated using thermodynamics and electromagnetic mechanics. Results indicate that the content of oxygen in the silicon reduced from 30.91 wt% to 5.08 wt%, and the actual sample composition including for uptake of residual gas contamination steadily declined as the vacuum level increased from 600 Pa to 10 Pa. Vacuum electromagnetic directional solidification reduced the content of metal impurities from 641 ppmw to 29.80 ppmw, increases the proportion of crystalline silicon area, and makes the silicon lattice arrangement more regular.

Review on Progress of Lamellar Orientation Control in Directionally Solidified TiAl Alloys.

TiAl alloys have excellent high-temperature performance and are potentially used in the aerospace industry. By controlling the lamellar orientation through directional solidification (DS) technology, the plasticity and strength of TiAl alloy at room temperature and high temperatures can be effectively improved. However, various difficulties lie in ensuring the lamellar orientation is parallel to the growth direction. This paper reviews two fundamental thoughts for lamellar orientation control: using seed crystals and controlling the solidification path. Multiple specific methods and their progress are introduced, including α seed crystal method, the self-seeding method, the double DS self-seeding method, the quasi-seeding method, the pure metal seeding method, and controlling solidification parameters. The advantages and disadvantages of different methods are analyzed. This paper also introduces novel ways of controlling the lamellar orientation and discusses future development.

Progress on improvement and application of magnetostrictive properties of the Fe-Ga alloy

Fe-Ga alloy is a novel material with high magnetostriction, good mechanical properties, and wide application prospects. At present, few reviews systematically summarize the improvement and application of magnetostrictive properties of Fe-Ga alloys. Combined with the latest research progress, this paper reviews Fe-Ga alloys and their magnetostrictive properties and expounds on the methods to improve the magnetostrictive properties of Fe-Ga alloys from three aspects: component design, preparation methods, and heat treatment. For example, doping Dy, Ce, Pr, and C elements, improving the heat treatment process such as introducing an H2S atmosphere and strong magnetic field, and improving the preparation methods such as directional solidification and rapid laser solidification technology. The magnetostrictive properties of Fe-Ga alloys have a wide application in the field sensors, transducers, flexible electronic devices, vibrators, actuators, and so on. This paper provides new ideas for improving the magnetostrictive properties of Fe-Ga alloy and promoting its wide application.

High temperature oxidation behavior of directionally solidified Fe(Al,Ta)/Fe2Ta(Al) eutectic composite

It is well known that Fe-Al intermetallic compounds have broad application prospects at high temperatures. The performance of Fe(Al,Ta)/Fe2Ta(Al) eutectic composite prepared by Bridgman directional solidification technology has been further improved as compared with that of Fe-Al intermetallic compounds. In this paper, high-temperature oxidation behaviors of directionally solidified Fe(Al,Ta)/Fe2Ta(Al) eutectic composite in static air were studied by isothermal oxidation test. The oxidized surface and cross section of directionally solidified Fe(Al,Ta)/Fe2Ta(Al) eutectic composite were characterized by scanning electron microscopy, X-ray diffraction and energy dispersive spectroscopy. The mechanism of high-temperature oxidation was analyzed from the point of view of kinetics and thermodynamics studies combined with the data of the Ellingham-Richardson diagram. The oxidation weight gain of the Fe(Al,Ta)/Fe2Ta(Al) eutectic composite at the same solidification rate is increased with temperature and Al2O3 is formed at the beginning of oxidation. However, a dense oxide film is not formed due to the small content of Aluminum, and the final oxide film for Fe(Al,Ta)/Fe2Ta(Al) eutectic composite after 100 hours′ oxidation is composed of Fe2O3 and Al3Fe5O12.

Research progress on microstructure and property regulation of ultra-high temperature oxide eutectic ceramics by high gradient directional solidification technology

With the rapid development of aerospace and other high-tech fields, a new generation of high perfor-mance, high efficiency ultrahigh temperature structural materials and their preparation technologies increasingly becomes the focus of the development of aerospace strategy in the world. Ultrahigh temperature alumina based eutectic ceramics have excellent intrinsic oxidation resistance, corrosion resistance, creep resistance, high strength and other excellent properties. Therefore, they have outstanding advantages and great development prospect in extreme environment such as ultrahigh temperature above 1 400℃, water and oxygen corrosion circumstances. In this paper, the current development status has been firstly introduced for the ultrahigh temperature oxide eutectic ceramics prepared by high gradient directional solidification technologies. Then, the formation mechanism and control method of the defects during the high gradient directional solidification process of the alumina based ultrahigh temperature eutectic ceramics have been summarized. Furthermore, the microstructure characteristics and homogenization control methods, crystal orientation and texture control methods, mechanical properties and strengthening-toughening control methods of oxide eutectic ceramics have been reviewed. Finally, the development trend and breakthrough point have been comprehensively prospected for the preparation of oxide eutectic ceramics with high performance and large size by high gradient directional solidification technologies.

The activation of compression twin pairs and plasticity improvement of directionally solidified Mg alloy

The <112¯0> orientated Mg-4.70Zn-0.38Y-0.14Zr alloy with high product of strength and elongation (PSE = 5346 MPa•%) was prepared using directional solidification technology. Electron Back Scatter Diffraction (EBSD) and High Resolution Transmission Electron Microscopy (HRTEM) were applied to explore the deformation mechanism and plasticity improvement of experimental alloys. Experiment results showed that the dominant deformation systems were non-basal slipping systems and compression twin pairs (CTPs). High critical resolved shear stress (CRSS) of non-basal slipping system increased the yield strength (180 MPa) and tensile strength (198 MPa). The CTP followed the geometrical relationship of TP1 (twinning plane of variant1)∥TP2 (twinning plane of variant2) and appeared in a wider misorientation range than tensile twin pair which was classified as twinning shear transmission and had high geometric coordination factor (m’). Calculation results showed that the stress concentration at the junction of CTP and grain boundaries was the same as inside the compression twin. 60% of x/<11 2¯ 0> (0°≤x < 180°) grain boundaries in experimental alloy satisfied this geometrical relationship, leading to the general activation of CTP and plasticity improvement (elongation is 27%).

Effect and mechanism of cold rolling and aging process on microstructure and properties of columnar grain C70250 copper alloy

The C70250 copper alloy billet produced by continuous directional solidification technology was cold rolled and aged. The influences of cold rolling deformation and aging process on the microstructure, mechanical and electrical properties of the alloy were investigated, and the mechanism was also revealed. Our results indicate that the peak aging tensile strength and electrical conductivity of the C70250 copper alloy gradually increase with the increase of cold rolling deformation amounts. When cumulative cold deformation amount reaches 95%, the aging treatment of 450 °C and 1 h was the optimal heat treatment process, the yield strength of the alloy is 720 MPa, the tensile strength is 758 MPa and the conductivity is 52.5% IACS under this condition. Compared with the traditional preparation process, the comprehensive properties are significantly improved. Large cold rolling reduction makes the columnar grain structure of C70250 copper alloy produce uniform shear deformation, promotes the uniform precipitation of Ni2Si phase in the aging process, leading to finer precipitates and more uniform distribution in the peak aging alloy. Dislocation slip is hindered, grain boundary migration capacity is reduced and the strength of the alloy is improved. The parallel shear band produced by large deformation cold rolling cuts the matrix evenly, which not only leads to the transformation of columnar grain structure into fiber strip structure, reduces the transverse grain boundary density, but also accelerates the precipitation of solute atoms, reduces the influence of solute atoms on electron scattering. Besides, the long fiber structure provides a channel for free electron transport, greatly reduces the barrier of electron transmission and improves the conductivity of the alloy.

Effects of Heating Power on Microstructure Evolution and Tensile Properties at Elevated Temperature by Directional Solidification for Ti2AlC/TiAl Composites

To improve the directionally solidified microstructure and tensile properties at elevated temperature, Ti46Al4Nb1C0.8Ta ingots are directional solidified by electromagnetic cold crucible with different heating powers. The microstructure is observed and characterized, tensile properties are tested, and related mechanisms are revealed. Results show that a relatively planar liquid–solid interface is formed under the low heating power (39 kW), which promoted formation of columnar crystals. When the heating power increases from 39 to 48 kW, degree of dent (H1) increases from 8.5 to 18.1 mm and influence length of the initial transition area (H2) increases from 4.3 to 28.6 mm. The formation of a concave liquid–solid interface is due to the high heating power increasing the temperature gradient. High heating power increases the electromagnetic stirring ability, which increases the time to reach stable growth. Tensile properties of the directionally solidified alloy are higher than those of the as‐cast alloy under the same testing temperature, e.g., the tensile strength is 342.5 MPa, an increase of 1.4 times, and the tensile strain is 4.4%, an increase of 1.3 times at 950 °C. This shows that the tensile properties and microstructure of TiAl‐based composites can be improved by the directional solidification technology.

Effect of MgO Content on Mechanical Properties of Directionally Solidified Pure Magnesium

The pure magnesium was fabricated by directional solidification, and the effect of the distribution characteristics of magnesium oxide (MgO) on the mechanical properties of pure magnesium was investigated. The statistical results showed that the area fraction, number and size of MgO were decreased gradually from the top region ingot to the bottom region ingot, and these reflected the advantages of directional solidification technology in the controllability of MgO distribution characteristics. The top parts of magnesium ingots have the highest tensile strength (44 MPa), which is mainly due to the presence of a large amount of the coarse MgO. Though the coarse MgO increases the strength obviously, it has harm for the ductility of magnesium. The top parts of magnesium ingots have higher ultimate tensile strength, but lower failure strain (13% and -21% respectively) than the ingot at the center. These results indicate that if the suitable size and amount of MgO existed in magnesium matrix, it could avoid the disadvantages of MgO and provide positive effect for both the strength and ductility of magnesium alloys.

High-temperature deformation resistance and creep resistance of a TiAl-based alloy fabricated by cold crucible directional solidification technology

In order to improve the high-temperature deformation resistance and creep resistance of TiAl-based alloys, cold crucible directional solidification (CCDS) technology was employed. A β-type TiAl-based alloy with the nominal composition of Ti44Al6Nb1Cr2V was prepared using the optimized CCDS parameters of 45 kW input power and 0.5 mm·min−1 solidification rate. Thermo-compression testing was utilized to evaluate the high-temperature deformation resistance and creep resistance of the CCDS Ti44Al6Nb1Cr2V alloy. Results show that the CCDS Ti44Al6Nb1Cr2V alloy billets contain aligned columnar grains and a high percentage of small-angle lamellae. Thermo-compression testing results in the radial direction of the CCDS alloy show a much higher peak stress than other reported results in similar conditions. The much higher hardening exponent and deformation activation energy are obtained, corresponding to the excellent high-temperature deformation resistance and creep resistance, which are because of the hard-oriented grains, weaker stress-strain coordination capability of lamella structure and relatively more hysteretic dynamic recrystallization. Thermo-compression testing results in the longitudinal direction of the CCDS Ti44Al6Nb1Cr2V alloy show the much higher peak stress than that in the radial direction, indicating the better high-temperature deformation resistance and creep resistance attributed to the hard-oriented lamellae in this condition.

Microstructure Evolution and Mechanical Properties of FeCoCrNiCuTi0.8 High-Entropy Alloy Prepared by Directional Solidification.

A CoCrCuFeNiTi0.8 high-entropy alloy was prepared using directional solidification techniques at different withdrawal rates (50 μm/s, 100 μm/s, 500 μm/s). The results showed that the microstructure was dendritic at all withdrawal rates. As the withdrawal rate increased, the dendrite orientation become uniform. Additionally, the accumulation of Cr and Ti elements at the solid/liquid interface caused the formation of dendrites. Through the measurement of the primary dendrite spacing (λ1) and the secondary dendrite spacing (λ2), it was concluded that the dendrite structure was obviously refined with the increase in the withdrawal rate to 500 μm/s. The maximum compressive strength reached 1449.8 MPa, and the maximum hardness was 520 HV. Moreover, the plastic strain of the alloy without directional solidification was 2.11%, while the plastic strain of directional solidification was 12.57% at 500 μm/s. It has been proved that directional solidification technology can effectively improve the mechanical properties of the CoCrCuFeNiTi0.8 high-entropy alloy.

Effect of Si Content on the Formation of {100} &lt;uvw&gt; Orientation in 0.27% Al Non-oriented Electrical Steel during Cell-to-Dendrite Transition Process

The effects of Si content (0.73%, 2.23%, 4.57%) and solidification rate (10 µm/s, 35 µm/s, 80 µm/s) on the as-cast microstructure and crystal orientation of 0.27% Al non-oriented electrical steel have been studied by directional solidification technology. Si atoms were inclined to segregate on the trunks and stems of cells and dendrites, and precipitate in the form of quadrilateral Fe-Si particles with size less than 4 μm. Increasing Si content would lead to a decrease in thermal conductivity of steel and directly cause an increase in temperature gradient of liquid steel, and as a result, make solid–liquid interface more stable and easier to grow as cell crystal. Moreover, decreasing solidification rate could further aggravate Si segregation at solid–liquid interface. Higher Si content (4.57%) together with lower solidification rate (10 µm/s) was more beneficial to the grain nucleation with {100} orientation, whereas higher solidification rate (80 µm/s) played a major role in the grain nucleation with {110} orientation.

Preparation of high-purity Ti–Si alloys by vacuum directional solidification

Abstract To prepare high-purity Ti–Si alloys, we newly used vacuum directional solidification technology to purify and separate Ti–Si alloys, which have different compositions (Ti-8.45 wt%Si, Ti-63.7 wt%Si, Ti-71.6 wt%Si, and Ti-84.1%wt%Si). The pulling-down rates in the directional solidification were to be 3 μm/s, 5 μm/s, and 7 μm/s, respectively, to investigate their effects on the purification. The results showed that vacuum directional solidification can purify Ti–Si alloys efficiently without discharging waste acid, slag and gas, and the slower pulling-down rate could result in better purification results. The impurities, Fe, Al, V, and Ni, could be removed due to their segregation behavior on the boundary of the solid and liquid phases. However, the impurities Ca and Mg could be removed using the vacuum technology because their vapor pressures were significantly higher than those of Si and Ti. TiSi2 could be precipitated and separated from the Ti-63.7 wt%Si and Ti-71.6 wt%Si alloys. The precipitated TiSi2 could be agglomerated at the bottom of the Ti–Si alloys, under the effect of gravity. The phases in the Ti–Si alloys, after vacuum directional solidification, were observed and determined using an electron probe micro-analyzer (EPMA). The results showed that the eutectic Ti-8.45 wt%Si alloy consisted of Ti5Si3 and Ti, the Ti-63.7 wt%Si and Ti-71.6 wt%Si alloys consisted of TiSi2 and eutectic Si–Ti alloy at the Si-rich side, and the Ti-84.1 wt%Si alloy consisted of Si particles and eutectic Si–Ti alloy at the Si-rich side. The study shows that vacuum directional solidification is an efficient and clean technology for the purification of Ti–Si alloys, and for the preparation of TiSi2 through the separation of Ti–Si alloys.

Microstructures and room temperature tensile properties of as-cast and directionally solidified AlCoCrFeNi2.1 eutectic high-entropy alloy

The microstructures and room temperature tensile properties of as-cast and directionally solidified AlCoCrFeNi2.1 eutectic high-entropy alloy were investigated using OM, SEM, EDS, TEM and tensile tests. The microstructure of as-cast alloy, including the well-aligned lamellar structures, the radial emanating lamellar structure and mesh-like structure, consists of NiAl-rich (B2) and CoCrFeNi-rich (L12) phases. From the longitudinal section of directionally solidified alloy, when the withdrawal rate is 6 μm/s, the solid-liquid interface morphology is planar, and the well-aligned lamellar structure is observed. The three-dimensional morphologies of well-aligned lamellar structures are well characterized by deep etching. With increasing the withdrawal rate to 15 μm/s, the solid-liquid interface morphology change from planar to cellular, and the microstructure changes from planar eutectic to cellular eutectic, even the dendritic eutectic at local region occurs at 60 and 120 μm/s. From the transverse section, the well-aligned lamellar structures occur in the cell interior, and the interlamellar spacing decreases gradually with increasing the withdrawal rates. The TEM result indicates that NiAl-rich phase has an orientation relationship with CoCrFeNi-rich phase of [1¯11] NiAl-rich//[1¯22]CoCrFeNi-rich and (211)NiAl-rich//(022¯)CoCrFeNi-rich. The nano-precipitates occur in NiAl-rich (B2) and CoCrFeNi-rich (L12) phases. The directionally solidified alloy possesses a good combination of strength and ductility at 60 μm/s. In particular, using the directional solidification technology is a good way to improve the ductility of the alloy (∼37%). The strengthening and plasticizing mechanisms are discussed by analyzing the fracture and side surfaces.

Effect of Zr Content on the Distribution Characteristic of the 14H and 18R LPSO Phases

Abstract Zirconium (Zr) is an essential element in Mg-Zn and Mg-Zn-Y system magnesium alloys. In this study, an interesting phenomenon that the content of Zr element could influence the size and the morphology of the long period stacking ordered (LPSO) phases, which has never been reported by previous works before. The Mg98.5-xZn0.5Y1Zrx (x =0, 0.1, 0.2 and 0.3 at. %) magnesium alloys were fabricated by directional solidification, and the effects of the Zr content on the distribution characteristics of the bulk LPSO phases (18R) and the lamellar LPSO phases (14H) were investigated. The directional solidification technology showed good controllability in LPSO phase’s distribution, and the morphology of LPSO phases in Mg98.5-xZn0.5Y1Zrx (x =0, 0.1, 0.2 and 0.3 at. %) alloys were observed clearly. The results showed that the amount and the morphology of the 14H and 18R LPSO phases within grains continuously decreased with the Zr content increasing. The continuous 14H lamellar structure changed to discontinuous. In addition, Zr element exhibited purification ability on the grain boundaries and refined effect on the 14H and 18R LPSO phases. This can be attributed to the influence of Zr atoms on stacking fault energy (SFE) and the attraction of Zr atoms to Mg atoms.

Effects of Al content and solidification rate on the crystal orientation of directionally solidified 1.3 wt% Si non-oriented electrical steel

1.3 wt%Si–0.14 ∼ 0.51 wt%Al non-oriented silicon steel as-cast rods with developed columnar structure and pronounced Goss texture were produced using directional solidification technology. The solute enrichments, microstructures and crystal orientations at solid-liquid interface of rods under different solidification rates were investigated. It is found that alloying element Al has little positive branch segregation at solid-liquid interface during equiaxed-to-columnar transformation process. Higher solidification rate is favorable to the homogeneous nucleation of long columnar Goss grains, while higher Al content is beneficial to the stable nucleation and further growth of columnar Goss grain.

Microstructure and enhanced thermoelectric performance of Te–SnTe eutectic composites with self-assembled rod and lamellar morphology

The eutectic Te–SnTe composites with self-organized rod and lamellar structure were synthesized using the directional solidification technology. The experiments show that the volume fraction and shape-tunable morphology of the SnTe phase vary with the alloy compositions. The thermoelectric properties of the composites are dependent on the volume fraction of SnTe phase and exhibit obvious anisotropy in different growth directions. The composites perpendicular to the growth direction possess much larger S and lower κL than that in the parallel direction. High resolution transmission electron microscopy (HRTEM) results illustrate that there are a given amount of dislocations and atomic mismatch stress field around the Te–SnTe eutectic interface areas, which effectively scatter the heat-carrying phonons, leading to an ultralow lattice thermal conductivity of 0.64 Wm−1K−1 for Sn-80at%Te eutectic alloy. The obtained maximum figure of merit is 0.40 for Sn-90at%Te alloy which is 1.5–1.9 times higher than that of pure SnTe and Te, respectively. It demonstrates that introducing large amounts of eutectic interfaces in Te–SnTe composites is an effective way to enhance the thermoelectric performance.

Shape memory effect of dual-phase NiMnGaTb ferromagnetic shape memory alloys

The evolution of microstructure, reverse martensitic transformation and the correlated influence on shape memory effect was investigated in as-cast and directionally solidified dual-phase NiMnGaTb alloys. The directionally solidified alloys exhibit single-crystal microstructure, preferred dendrite microstructure, and mussy dendrite microstructure in the specimens grown at a withdrawal rate (v) of 10, 50 and 200, and 1000 μm/s, respectively. The precipitates dispersively distribute in the martensite matrix for the directionally solidified alloys. With the refined grains and particle precipitates, the reverse martensitic transformation gradually shifts to lower temperatures and the temperature span is significantly broadened. The directional solidification technology can effectively enhance the strains recovered due to shape memory effect (esme) and decrease the compressive stress required to trigger the reorientation of twins (σ) via the realization of preferred orientation, while the maximal esme and minimum σ can reach 4.96% and 14 MPa in v = 10 μm/s specimens, respectively. The formation of dendrite morphology degrades the shape memory strain, and esme decreases with the growth of secondary dendritic arms.

The interaction and migration of deformation twin in an eutectic high-entropy alloy AlCoCrFeNi2.1

An eutectic high-entropy alloy consisting Al, Co, Cr, Fe and Ni elements was prepared by vacuum directional solidification technology. The alloy exhibits excellent comprehensive mechanical performance during tension at temperature range of 600–700 °C. The microstructure reveals the intersection of twin-twin is the prevailing deformation mechanism and the twins play a dual role in strengthening and toughening the alloy in the thermomechanical process. The deformation twin variants I and Π were formed by the edge dislocation 112¯ and the mixed dislocation 211¯ on the {111} crystal planes, respectively. Besides, the dislocation jogs and kinks caused by twin intersection on the slip planes can strengthen the alloy, which may contribute to the high strength (the tensile strengths at the 600° and 700° tensile tests are respectively780 MPa and 630 MPa.). Moreover, the coherent twin boundary migration has the function of coordinating deformation and contributes to the high ductility of the alloy.

Effect of Ag content and drawing strain on microstructure and properties of directionally solidified Cu-Ag alloy

Continuous columnar-grained Cu-Ag alloy bars containing different silver contents (6 wt%, 12 wt%, 24 wt%) with diameters of 10 mm were prepared by directional solidification technology firstly and drawn to 0.3 mm directly at room temperature without intermediate annealing in this study. With incremental Ag contents, eutectic microstructure distributed in the interdendritic region which parallel to the casting direction grew with a higher tensile strength but almost invariable elongation and electrical conductivity of the alloys. Meanwhile, the tensile strength of the alloy wires were enhanced greatly with increasing drawing strains owing to the remarkable fiber reinforcement effect, whereas the elongation and electrical conductivity decreased slowly. With the increase of drawing strains, columnar grains and eutectic phases were drawn into dense microcomposite structure, and the volume fraction of <111> texture in Cu and Ag phases increased in the three experimental alloys; while, the growth rate of <111> texture in Ag phases was lower than that in Cu phases, which is one of the main reasons for the excellent ductility of the alloys. These results may offer reference for the choice of Ag content and drawing strain, and then developing a short preparation process of Cu-Ag alloys with directional solidification + drawing (without intermediate annealing).