High Temperature Gradient Directional Solidification Research Articles

Unveiling the dynamic instability mechanism of microstructure transformation in faceted oxide eutectic composite ceramics

Microstructure control is a great challenge in the high-temperature gradient directional solidification of eutectic composite ceramics due to the complex solidification behavior. Herein, the microstructure transformation of faceted Al2O3/Er3Al5O12 thermal emission eutectic composite ceramics is explored over wide ranges of compositions (13.5 mol%–22.5 mol% Er2O3) and solidification rates (2–200 μm/s). Entirely coupled eutectics with primary phases suppressed are fabricated and the coupled zone is broadened in a wide range of 15.5 mol%–22.5 mol% Er2O3 at low solidification rates. The competitive growth between eutectic and dendrite is evaluated on the basis of the maximum interface temperature criterion. In addition, the mechanisms of irregular eutectic spacing selection and adjustment under different solidification rates are revealed based on Magnin–Kurz model. A successful prediction of lamellar to rod-like eutectics is achieved associated with the dynamic instability of lamellar eutectic and the corresponding enlarged coexistence region is mapped based on the interface undercooling. According to the well microstructure tailoring, the flexural strength of Al2O3/Er3Al5O12 eutectic composite ceramics has improved from 508 MPa up to 1800 MPa due to the refined eutectic spacing with low fluctuation. The eutectic composite ceramics show strong selective optical absorption and the intensity increases with the refining microstructure. The as-designed Al2O3/Er3Al5O12 composites with microstructural tailoring have great potential as integrations of structural and functional materials.

Probing the intrinsic phonon transport properties of the SnTe thermoelectric material

Based on the Boltzmann transport theory combined with first-principles calculations, we analyze the lattice anharmonicity of SnTe and the respective contributions of the lattice thermal conductivity (κL) from six different phonon modes. The Grüneisen parameter (γ), three-phonon scattering phase space (SPS), phonon lifetime (τ), and phonon velocity (v) were obtained quantitatively. These calculations indicate that large τ with maximum value of 104 ps and high v of 4800 m/s for acoustic branches make the acoustic modes a major contributor to the high κL of SnTe (～78% for acoustic branches and ～22% for Optical branches). Further, the dependence of κL on phonon mean free path shows that designing nanostructures with characteristic length of less than 14.5 nm could be an effective way to decrease the κL. In addition, we employed high-temperature gradient directional solidification technique to grow the SnTe single crystal and the measured κL vaules of the single crystal match reasonably with the theoretical calculation results.

Investigation of Microstructure Evolution and Phase Selection of Peritectic Cuce Alloy During High-Temperature Gradient Directional Solidification.

In this work, a CuCe alloy was prepared using a directional solidification method at a series of withdrawal rates of 100, 25, 10, 8, and 5 μm/s. We found that the primary phase microstructure transforms from cellular crystals to cellular peritectic coupled growth and eventually, changes into dendrites as the withdrawal rate increases. The phase constituents in the directionally solidified samples were confirmed to be Cu2Ce, CuCe, and CuCe + Ce eutectics. The primary dendrite spacing was significantly refined with an increasing withdrawal rate, resulting in higher compressive strength and strain. Moreover, the cellular peritectic coupled growth at 10 μm/s further strengthened the alloy, with its compressive property reaching the maximum value of 266 MPa. Directional solidification was proven to be an impactful method to enhance the mechanical properties and produce well-aligned in situ composites in peritectic systems.

Directional solidification of Fe-Al-Ta eutectic by electron beam floating zone melting

Fe-Al-Ta eutectic composites have been paid more attentions for high temperature application. Nevertheless, still there is a lack of research on microstructural characteristics and crystal growth mechanism of this eutectic obtained by high temperature gradient directional solidification technique. As such, Fe-Al-Ta eutectic composites were prepared by high temperature gradient electron beam floating zone melting technique in the present paper. Microstructures, solid/liquid interface morphologies and preferential orientations of the Fe-Al-Ta eutectic composites at different solidification rates were studied. Moreover, effect of interface perturbation wavelength on stability of the solid/liquid interface was calculated according to M-S theory and Kurz-Fisher model, respectively. These are consistent with the experimental results well.

Microstructure and thermoelectric properties of un-doped Mg2Si1−xSnx single crystals prepared by high temperature gradient directional solidification

Un-doped single crystals of Mg2Si1−xSnx (x = 0.55, 0.65 and 0.75) were successfully prepared by high temperature gradient directional solidification (HGDS). In the Mg2Si0.45Sn0.55 crystal, Mg2Si precipitates were observed in the solidified microstructure, and no precipitates in the single crystals of Mg2Si0.35Sn0.65 and Mg2Si0.25Sn0.75. By measuring the electronic transport properties of these three single crystals, the Mg2Si0.35Sn0.65 has a largest PF value, about 2.5 times more than that of the nanocrystalline prepared by solid-state reaction methods. The corresponding ZT values of Mg2Si0.35Sn0.65 single crystal are greatly improved. It indicates that, the Mg2Si1−xSnx crystals prepared by HGDS can not only have a uniform microstructure, but also optimize the TE performance of the crystal. In addition, the first-principles calculation has been conducted to examine the intrinsic properties of Mg2Si1−xSnx single crystals, and the calculated data agree well with the experimental results.

Probing anisotropy of Seebeck coefficient and enhanced thermoelectric performance of Mg2Si0.35Sn0.65 single crystal

A single crystal of Sb-doped Mg2Si0.35Sn0.65 has been successfully prepared using high temperature-gradient directional solidification. The composition distribution of the crystal is uniform, and the preferred growth direction of the crystal is [111] orientation. The Seebeck coefficient (S) of the crystal is anisotropic in different orientations, and the electrical and thermal conductivity are isotropic properties. The S value in the [111] orientation can reach −265 μVK−1, which is much larger than the reported results. Also, the power factor in the [111] orientation has a value of 5.56 mWm−1K−2, which leads to a maximum ZT of 1.3 at 730 K. Results of Hall testing and first principles calculations agree well with the experiment results.

Orientation Characteristics of Single Crystal Superalloys with Different Preparation Methods

Single crystal superalloys have been prepared separately by a grain selector and a seeding technique in a high temperature gradient directional solidification furnace. The orientation characteristics were measured by XRD. Results indicate that for the grain selector method, the crystal solidification begins at the cooling plate with random nucleation, and through mutual competitive growth at the starter block. The grains enter the spiral selector, and finally the single crystal superalloys which are close to <001> direction are obtained. By the seeding technique, by epitaxial growth of partially melted seed crystals, single crystal superalloys which have the same orientation as the seed crystal are obtained.

Orientation evolution of single-crystal superalloys under different solidification interface

The single-crystal superalloys with different deviation angle are prepared by using the liquid metal cooling high-temperature gradient directional solidification furnace, and the orientation evolution of single-crystal superalloys under different solidification interface is studied. The results indicate that the increase of the deviation angle will make the planar interface unstable. With the increase of the solidification rate, the growth direction of the crystal will deviate from the direction of heat flow to the preferred orientation direction. Under the developed dendrite crystal growth, the branches of the dendrite will follow the preferred orientation, nearly free from the effect of the heat flow.

A Study on Shape Memory Performance of Cu-Al-Ni-Be Alloy Single Crystal

In this paper, single crystal of CuAlNiBe quaternary shape memory alloy was prepared in a high temperature gradient directional solidification furnace with a selective growing crystallizer. And its shape memory performance characters were systematically compared with other series copper base shape memory alloys. The results show that the single crystal of CuAlNiBe quaternary shape memory alloy has better shape memory properties.

Preparation of single crystal CuAlNiBe SMA and its performances

A novel quaternary CuAlNiBe shape memory alloy (SMA) Cu–11.5Al–3.8Ni–0.4Be (wt%) was designed and synthesized in this study. The single crystal of this alloy was fabricated by means of a high temperature gradient directional solidification furnace with a selective growing crystallizer. After solid solution treatment, the phase transformation points (including M s, M f, A s and A f) were determined by DSC; the bending shape memory performance, the maximal recoverable deformations, the tensile strength and the elongation percentage were measured. Its maximum recoverable strain reached 10% which approximately equals to or exceeds the reported maximum recoverable strain for polycrystalline TiNi alloy. The number of bend-relaxing times arrived at 750. Both results implied that this single crystal alloy may become a promising candidate to challenge TiNi SMA.

Behavior and Preparation of Single Crystal of Cu-Al-Ni-Be SMA

In this paper, single crystal of CuAlNiBe shape memory alloy (SMA) was prepared in a high temperature gradient directional solidification furnace with a selective growing crystallizer. And its performance characters were systematically studied. The results show that the mechanical and shape memory properties of the single crystal of CuAlNiBe SMA alloy has much better than that of the polycrystalline material. This may be explained that the stress concentration at grain boundary caused by the difference of bilateral strains is the dominant reason of deterioration of fatigue properties; therefore single crystal alloy possesses the excellent properties duo to its elimination of grain boundary.

Solidification characteristics of Tb 0.27Dy 0.73Fe x ( x = 1.8, 1.95) alloys during the directional solidification

Solidification characteristic of intermetallic compound is complex and dependent on its composition and the solidification conditions. Tb 0.27Dy 0.73Fe x ( x = 1.8, 1.95) alloy, as an intermetallic compound, exhibits different solidification behavior during its directional solidification. Experimental investigations of the behavior were carried in a super high temperature gradient directional solidification equipment developed by authors. The relationships between the solidification behavior and the solidification condition of Tb 0.27Dy 0.73Fe x ( x = 1.8, 1.95) alloys were determined. For Tb 0.27Dy 0.73Fe 1.8 alloy, solidification characteristics with short cylindrical, plate-like and mixed structure were exhibited under the different solidification conditions and zone with a plate-like structure is enlarged with the increase of the temperature gradient and the crystal growth velocity. For Tb 0.27Dy 0.73Fe 1.95 alloy, however, Terfenol-D crystal always grows with a plate-like structure in the experimental ranges. Plate-like structure zone is also enlarged with the increase of the crystal growth velocity and the temperature gradient. The experiment results are helpful for preparing an appropriate Terfenol-D crystal.

Preparation of Terfenol-D with precise 〈110〉 orientation and observation of the oriented growth crystal morphology

In a super high temperature gradient directional solidification equipment developed by the authors, φ7 mm rare earth giant magnetostrictive rods with precise 〈110〉 orientations were manufactured successfully without any seed crystal. A dispersion of only 3° exists between the 〈110〉 oriented growth direction and the axis of the rod. During the directional solidification process, there exists a certain relationship between the crystal growth morphology and the crystalline orientation degree. The crystallites’ orientation degree can be determined conveniently through the observation of the crystal morphology. Under the condition of the temperature gradient of 800 K/cm and the withdrawal velocity of 6 μm/s∼10 μm/s, Tb 0.27Dy 0.73Fe 1.9 rod with 〈110〉 oriented crystals can be obtained when the solidified part of the rod reaches 20 mm. When the length of the solidified part is at least 30 mm, the 〈110〉 oriented crystallines can grow in a stable way.

Technique of melting and casting of (TbDy)Fe 2 matrix alloy bars

The technique of melting and casting rare earth giant magnetostrictive bars was investigated with super high temperature gradient directional solidification equipment developed by the authors. A number of φ7×120-mm bars with a smooth appearance were manufactured successfully; bars with different dimensions can be obtained by changing the molds. The results reveal that the fabrication process is available and simple. Under normal melting, the losses of Tb and Dy can be controlled to within 1 and 3%, respectively. The mean composition errors of the rare earth elements Tb and Dy are less than 1.5%. Metallographic examinations reveal that the microstructure is notably dependent on the mold temperature. In addition, the full process from melting and casting to directional solidification can be performed in the same equipment, and investment in facilities for melting and casting is not required.