Eutectic Composites Research Articles

High thermoelectric performance of GeTe–Ag8GeTe6 eutectic composites

The (GeTe)1−x(Ag8GeTe6)x composites with x=0.02, 0.05, 0.08 and 0.11 were prepared by melting, ball milling and spark plasma sintering techniques. The thermoelectric properties of the composites were investigated. The experimental results show that all samples are composites consisting of the primary solidification phase GeTe and eutectic microstructures with GeTe and Ag8GeTe6 phases. The composites are p-type semiconductors. As compared to the pure compound GeTe, the existence of Ag8GeTe6 compound in the GeTe phase enhances its electrical resistivity at low temperatures while reduces its electrical resistivity at high temperatures. Also, the Seebeck coefficient of the composite is slightly decreased due to the Ag8GeTe6 compound, but its thermal conductivities are reduced significantly. As a result, the figure of merit ZT for the materials can be enhanced. The maximum figure of merit ZT was obtained as 1.15 in the sample (GeTe)0.89(Ag8GeTe6)0.11 at 673K. This value is much higher than that for the pure GeTe, leading to a substantial improvement in the thermoelectric performance of the material.

Wear Behaviour of Eutectic Al-Si Alloy-Graphite Composites Fabricated by Combined Modified Two-Stage Stir Casting and Squeeze Casting Methods

Dry sliding wear behaviour of eutectic Al-Si alloy-graphite composites was investigated employing a pin-on-disc wear test rig. Results revealed that the wear and friction coefficients decreased linearly with increasing weight percentage of graphite particles. Wear resistance of the composite increased considerably with increasing sliding velocity at constant load. In contrast, the friction coefficient of Al-7.5 wt.% Gr composite increased when the sliding velocity was increased from 1 m/s to 2 m/s at 49 N. Worn-out surfaces of wear specimens after the test were examined by scanning electron microscopy to study the morphology of worn surfaces. EDS analysis was carried out to investigate the influence of mechanically mixed layer (MML) which comprises oxides and iron, and this acted as an effective tribolayer in enhancing the wear resistance at higher sliding velocity.

Effects of Ternary Additions on the Microstructure of Directionally-Solidified MoSi2/Mo5Si3 Eutectic Composites

ABSTRACTMicrostructural variations of directionally-solidified (DS) MoSi2/Mo5Si3 eutectic composites caused by ternary additions of V, Cr, Nb, Ta and W were investigated. Ternary addition of relatively large amount of V resulted in the apparent modification of the morphology as well as the macroscopic inclination angle of one type of the interfaces extending nearly along the growth direction in the DS ingots. Ternary addition of Cr was found to change the microstructures of the DS ingots from a cellular-type with coarse boundary regions to a plate-like cellular-type with increasing the amount of the ternary addition. No significant change of scriptlamellar morphology was observed for the DS ingots containing Nb, Ta and W irrespective of the amount of the ternary addition.

Plastic Deformation of Directionally-Solidified MoSi2/Mo5Si3 Eutectic Composites

ABSTRACTDeformation behavior of the directionally-solidified MoSi2/Mo5Si3 eutectic composites has been investigated as a function of the average thickness of MoSi2 phase over a temperature range from 900 to 1500°C. The average thickness of both MoSi2 and Mo5Si3 phases in the directionally-solidified ingots with script-lamellar morphologies grown by optical floating zone method decreases with increasing the growth rate. Plastic deformation was observed above 1000°C for all the DS ingots grown at different growth rates when the loading axis is parallel to [1¯10]MoSi2 close to the growth direction. Yield stress decreases monotonically with increasing temperature. Yield stress at 1400°C increases drastically with decreasing the average thickness of MoSi2 phase.

Microstructure and mechanical properties of Al2O3/ZrO2 eutectic ceramic composites prepared by explosion synthesis

Al2O3/ZrO2 eutectic ceramics were prepared by explosion synthesis using Al and Zr(NO3)4 as raw materials. The Al2O3/ZrO2 eutectic microstructure showed that the rod-like ZrO2 phases with a diameter 200nm were embedded orderly in Al2O3 matrix. With the increasing of the reaction temperature, the volume content of the Al2O3/ZrO2 eutectic increased accordingly, and the diameter and phase spacing of the rod-like ZrO2 decreased, which was mainly attributed to the sufficient diffusion at higher temperature and the large degree of supercooling. Due to the fine microstructure of the Al2O3/ZrO2 eutectic, the Vickers hardness and fracture toughness can reach 17.7GPa and 10.3MPam1/2, respectively.

A Few Aspects on the Processing and Deformation Behavior of Advanced Eutectic Alloys

The present work is aimed to develop homogeneous ultrafine eutectic composites without any micrometer size second phase dispersions. Ti70.5Fe29.5 is chosen as a model eutectic system and ingots were prepared by arc melting upon addition of different amount of Sn. The effect of Sn addition to Ti70.5Fe29.5 on the interlamellar spacing, microstructure, and elastic properties has been studied. The load dependence microhardness or indentation size effect (ISE) in ultrafine lamellar eutectic has been investigated to understand the mechanism of ISE in terms of strain gradient on the evolution of geometrically necessary dislocations. Furthermore, high-resolution transmission microscopy of deformed composites has been performed in order to reveal the role of different eutectic phases during the transfer of slip across their interface.

Effect of Mg addition on microstructural, mechanical and environmental properties of Nb–Si eutectic composite

The paper reports the effect of addition of small amount of Mg on the mechanical and oxidation properties of Nb–Nb3Si eutectic composites in Nb–Si system under the condition of suction casting. Mg addition increases the volume fraction of primary dendrites of Nb solid solution. This phase contains significant amount of strengthening precipitates. Two different precipitates are identified. The large plate shaped precipitates are that of hcp phase, while fine coherent precipitates have the structure similar to recently identified δ-Nb11Si2 phase. The Mg addition improves both the strength and ductility of the composite at room temperature (∼1.4GPa and ∼5% engineering strain) as well as at 700°C(∼1.2GPa and ∼7% engineering strain). The presence of Mg results in a complex barrier layer which significantly increases the oxidation resistance up to a temperature of at least 1000°C.

Interfacially Driven Deformation Twinning in Bulk Ag-Cu Composites

Interfaces and interface/defect interactions increasingly dominate the mechanical response of materials as the dimensions of the grains decrease to the nanoscale. Recently, we reported unusually profuse deformation twinning in Ag-Cu layered eutectic composites with bilayer thicknesses in the submicron regime (~200 nm–400 nm) at room temperature and low strain rates. Using atomistic simulations and dislocation theory, we propose that the Ag-Cu interface facilitated deformation twinning in Cu by permitting the transmission of twinning partials from Ag to Cu. In this way, twins in Ag can provide an ample supply of twinning partials to Cu to support and sustain twin growth in Cu during deformation. Interface-driven twinning as revealed by this study suggests the exciting possibility of altering the roles of dislocation slip and twinning through the design of heterophase interface structure and properties.

Ab-initio calculation of the coefficients of thermal expansion for MeB2 (Me–Ti, Zr) and LaB6 borides and LaB6–MeB2 eutectic composites

The coefficients of thermal expansion of transition-metal borides, lanthanum hexaboride, and eutectic composites thereof are calculated using the a-priori pseudopotential method. To determine the energy of the electron–ion system as a function of the lattice parameters at nonzero temperatures, it is proposed that the energy scales of the system be compared at absolute zero and nonzero temperatures. The coefficient of thermal expansion of the composites shows an abrupt increase in the temperature range from 750 to 1000 K.

Synthesis and Investigation of Al<sub>2</sub>O<sub>3</sub>/SiO<sub>2</sub> /ZrO<sub>2</sub> Powders via Selective Laser Sintering

This research focused on the synthesis and investigation of the thermal properties and microstructure of the Al2O3/SiO2 /ZrO2 system applied to dental field. The composite ceramic was studied by scanning electron microcopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Detailed investigations of the different proportions of materials on the preparation and microstructural phases of ternary eutectic were presented. Furthermore, the crystallization process was investigated by using DSC and XRD. The results indicate that sintering microstructure of the ternary eutectic composite is greatly influenced by the materials proportions. The synthetically thermal analysis shows that the eutectic temperature of ternary Al2O3/SiO2 /ZrO2 composite is 1040°C, is well matching the phase diagram of Al2O3/SiO2 /ZrO2.

Enhanced hydrogen desorption from the Co-catalyzed LiBH4–Mg(BH4)2 eutectic composite

Co-based catalyst can significantly improve the dehydrogenation kinetics of the eutectic composite of LiBH4–Mg(BH4)2 (1/1 M ratio). The onset hydrogen desorption temperature of the composite is at about 155 °C, which is ca. 245, 110 or 27 °C lower than that of LiBH4, Mg(BH4)2 or pristine LiBH4–Mg(BH4)2, respectively. Upon holding the samples at 270 °C, the Co catalyzed composite can release hydrogen at a rate 1.6 times faster than that of the pristine one. Electron Paramagnetic Resonance (EPR) characterization evidenced that Co was in a reduced state of Co+ which may serve as the functional species in catalyzing the dehydrogenation of the composite.

Microstructure of eutectic composites Ln 1−x Ln′ x MnO3 (Ln = Eu or Tb; Ln′ = Y or Ho) near the transition between the orthorhombic structure and hexagonal structures

Multiferroic crystals Ln1 − x MnO3 (Ln = Eu or Tb; Ln′ = Y or Ho) have been grown by floating-zone melting; the compositions of these crystals are close to the point of concentration transition from the orthorhombic structure to the hexagonal phase. Scanning microscopy has revealed that the crystal boules have a regular lamellar structure in the form of alternating layers of the orthorhombic and hexagonal phases; this structure is typical of the materials obtained by the directional solidification of eutectic composites. The local chemical and phase compositions and the unit-cell parameters of the phases have been determined. The microstructure and morphology of the lamellar structures are described for different growth conditions. The mutual orientation of the crystallographic axes of the orthorhombic and hexagonal phases in the neighboring layers has been determined by electron backscatter diffraction. The orientational features are considered from the point of view of the crystal structure of both phases.

Microstructure and properties of Ni–Ni3Si composites by directional solidification

Ni–Ni3Si composites are prepared by the Bridgman directional solidification technology under different growth conditions, aiming to improve the ductility of the Ni3Si compound and investigate the relationship between solidification microstructure and the properties. Microstructure of the Ni–Ni3Si hypoeutectic in situ composites transforms from regular lamellar eutectic to cellular structure then to dendritic crystal with the increase of the solidification rate. Ni–Ni3Si eutectic composites display regular lamellar eutectic structure at the solidification rate R=6.0–40.0 μm/s and the lamellar spacing is decreased with the increase of the solidification rate. Moreover, the Ni–Ni3Si hypoeutectic composites present lower micro-hardness than pure Ni3Si, which indicate Ni–Ni3Si hypoeutectic composites have higher ductility, whereas the ductility of the Ni–Ni3Si eutectic composites has scarcely been improved. This is caused by the formation of the metastable Ni31Si12 phase in the Ni–Ni3Si eutectic composites.

Improving the Mechanical Properties of Fe-Nb-(Ni-Mn) Dendrite-Ultrafine Eutectic Composites via Controlling the Primary Phase Features

Tuning of microstructure by addition of austenite stabilizers effectively enhances the mechanical properties in Fe-Nb-(Ni-Mn) dendrite-ultrafine eutectic composites. The Fe93Nb7 alloy displays the improved plasticity up to 10 pct due to the introduction of a ductile α-Fe dendrite into the ultrafine eutectic matrix. Meanwhile, the Fe78Nb7Ni10Mn5 alloy, which forms an in-situ martensitic α′-Fe dendritic phase reinforced ultrafine eutectic composite exhibits excellent combination of a high fracture strength of 1.6 GPa and a large plastic strain of 11 pct. The investigations reveal that the characteristics of the modulated primary dendrites in the dendrite-ultrafine eutectic composites play an important role in manipulating the generation and propagation of shear bands, thus resulting in the improved mechanical properties and plastic deformation behavior.

Ruby micro-piezospectroscopy in GdAlO3/Al2O3(/ZrO2), Er3Al5O12/Al2O3(/ZrO2) and Y3Al5O12/Al2O3(/ZrO2) binary and ternary directionally solidified eutectics

Abstract The fluorescence from (naturally present) Cr 3+ impurities was used to measure the residual stress in the alumina phase of six melt-grown ceramic eutectic composites associating gadolinum aluminum perovskite (GAP), erbium aluminum garnet (EAG) or yttrium aluminum garnet (YAG) with α-alumina and cubic zirconia. Such measurements are reported for the first time in the GAP containing eutectics. In the usual hydrostatic assumption, we conclude to a residual compression in the range of ∼70–400 MPa depending on the sample composition. The validity of the hydrostatic assumption is questioned when a microscope is used for the measurements.

Solidification of Al2O3–YAG eutectic composites with off-metastable eutectic composition from undercooled melt produced by melting Al2O3–YAP eutectics

Abstract It has been reported that solidification of the Al2O3–YAG equilibrium eutectic structure follows melting of the Al2O3–YAP metastable eutectic structure. Since the exothermic heat due to solidification was consumed by the endothermic heat due to melting, a fine and uniform eutectic structure was obtained. However, the composition of the Al2O3–YAG eutectic structure is restricted to the metastable eutectic composition. In this paper, Al2O3–YAG eutectic compacts with an off-metastable eutectic composition were prepared by the addition of Al2O3 particles to Al2O3–YAP eutectic particles with diameters less than 20 μm. In compositions ranging from 18.5 mol%Y2O3 to 13.5 mol%Y2O3, dense Al2O3–YAG eutectic compacts were formed without any Al2O3 segregation. The flexural strength and the fracture toughness remained almost unchanged with the increase in the Al2O3 phase. The addition of Al2O3 particles to the Al2O3–YAP eutectic particles enabled the matrix phase to change from the YAG phase to the Al2O3 phase.

Residual stresses in Al2O3–ZrO2 (3 mol.% Y2O3) directionally solidified eutectic ceramics as a function of temperature

Abstract Directionally solidified eutectics are in situ composites grown from the melt. Due to the differences in the thermoelastic properties of the different phases present in the material, these composites often exhibit residual stresses that can affect their mechanical properties. In this work we use neutron diffraction to investigate residual stresses in Al 2 O 3 –ZrO 2 eutectic composites as a function of temperature, for samples processed at two different growth rates, 10 mm/h and 750 mm/h. Our results show that the stress-free temperature is in the range of 1200 ± 200 °C. We explain the experimental observations based on the thermoelastic properties of the phases in the material and confirm our measurements using a simple, self-consistent model.

Microstructure Characteristics of Fe-xCr-C Hardfacing Alloys

A series of Fe-xCr-C hardfacing alloys were successfully fabricated on a substrate of 20g steel by gas tungsten arc welding (GTAW) process. These claddings were disigned to observe hypoeutectic, near-eutectic, and hypereutectic structures with various Cr3C2 and M7C3 carbides at room temperature. According to X-ray diffraction(XRD) and optical microscopy (OM), the hypoeutectic and eutectic composites include the Fe-Cr solid solution(α+A) and Cr3C2 carbides. Hypereutectic structures consist of α+A+M+M7C3 respectively. The cladding of hypereutectic microstructure with great amounts of primary M7C3 carbides had the highest hardness of the all conditions.

Microstructure and high-temperature mechanical behavior of melt-growth Al2O3/Er3Al5O12/ZrO2 ternary eutectic composites

The microstructural and high-temperature mechanical characteristics of directionally solidified rods of Al2O3–Er3Al5O12–ZrO2 ternary eutectic oxides processed by the laser-heated floating zone method at different growth rates have been investigated. The eutectic microstructure displayed an entangled three-dimensional network of Al2O3 and Er3Al5O12 phases of similar sizes, elongated along the growth direction; the minority zirconia phase formed small fibers into the alumina phase. The interphase spacing is reduced with increasing solidification rate, changing about 2μm down to 200nm. These microstructural features are essentially the same exhibited by Al2O3–Y3Al5O12–ZrO2 composites processed by the same technique. Compressive deformation tests performed at 1400°C at constant strain rate showed that the creep resistance decreased when increasing the growth rate due to the refinement of the microstructure.

High Temperature Characteristics of Unidirectionally Solidified Al<sub>2</sub>O<sub>3</sub>/GAP Eutectic Composites with a Novel Microstructure

Much attention has been paid to unidirectionally solidified ceramic composites as candidates for high-temperature structural materials. We have recently developed potential ceramic eutectics, which are named Melt Growth Composites (MGC). The Al2O3/GAP(GdAlO3) binary MGC has a novel microstructure, in which continuous networks of single-crystal Al2O3 phases and single-crystal GAP phases interpenetrate without grain boundaries. Chain structure in the Al2O3/GAP binary system is formed due to the frequent branching of both phases resulted in the entangled structure. Therefore, the Al2O3/GAP binary MGC has excellent high-temperature characteristics in the air atmosphere at very high temperatures. In the paper, high temperature strength, thermal stability of microstructure and strength, and fracture toughness of the Al2O3/GAP binary MGC are reported.