High-temperature strength and thermal stability of a unidirectionally solidified Al2O3/YAG eutectic composite

A unidirectional solidification method was investigated to manufacture Al2O3/YAG eutectic composites with high-temperature resistance that would make them usable at very high temperatures. We were successful in manufacturing a single-crystal Al2O3/single-crystal YAG eutectic composite with a dimension of 40 mm in diameter and 70 mm in length containing no colonies or pores. This composite also displayed excellent high-temperature strength characteristics. The flexural strength was in the range 350400 MPa from room temperature up to 2073 K (just below its melting point of about 2100 K) with no apparent temperature dependence. During tensile tests above 1923 K, the eutectic composite showed evidence of plastic deformation occurring by dislocation motion, and a yield phenomenon similar to many metals was observed. In addition, the microstructure of the composite was extremely stable: after 1000 h of heat treatment at 1973 K in an air atmosphere there was no growth. The above superior high-temperature characteristics are caused by such factors as the eutectic composite having a single-crystal Al2O3/single-crystal YAG structure, the formation of a compatible interface with no amorphous phase and thermal stability, and the combined effect of a YAG phase with superior high-temperature characteristics. © 1998 Chapman & Hall     

The structure of the unidirectionally solidified Al-Al2Au eutectic

The structure of a number of unidirectionally solidified Al-Al₂Au alloys of eutectic and off-eutectic compositions has been investigated over a wide range of growth rates (1.6×10^–4 to 1.66×10^–2cm sec^–1) using a thermal gradient of approximately 80 to 100 lamellar interface || (001)_{Al 2 Au} || (01 1) _Al [ 1 1 0 ]_{Al 2 Au} || [ 1 0 0 ] _Al growth direction of lamellae and rods || [ 1 1 0 ]_{Al2 Au} || [ 1 0 0 ]_Al \begin{gathered} lamellar interface \left\| {(001)_{Al_{ 2} Au} } \right.\left\| {(01 1)} \right._{Al} \hfill \\ \left[ {1 1 0} \right]_{Al_{ 2} Au} \left\| {\left[ {1 0 0} \right]} \right._{Al} \hfill \\ growth direction of \hfill \\ lamellae and rods \left\| {\left[ {1 1 0} \right]_{Al_2 Au} \left\| {\left[ {1 0 0} \right]_{Al} } \right.} \right. \hfill \\ \end{gathered}     

Compressive properties of the unidirectionally solidified Sb-Ge eutectic

The structure and compressive properties of the faceted/faceted Sb-Ge eutectic, unidirectionally solidified over a wide range of growth rates, have been examined and compared with those of the faceted/non-faceted Al-Si and Zn-Ge eutectics. The UCS of the Sb-Ge eutectic was found to be independent of the scale of the microstructure. It is considered that this behaviour is the result of the presence of a brittle matrix and poor matrix/second phase coherence.     

The microstructure of unidirectionally solidified Ag-Ge eutectic alloys

Ag-Ge alloys containing 15 to 22 wt% Ge were unidirectionally solidified to investigate the growth conditions for fully eutectic growth (the coupled region) in the range of growth rate 1.4 x 10^–4 to 1.1 cm sec^–1 and at a temperature gradient of 200° C cm^–1. Primary silver was not formed in the hypereutectic Ag-Ge alloys, implying that the coupled region of Ag-Ge alloys may be different from that of the other nf-f alloy systems such as Al-Si, Fe-C, Al-Ge and Al-Fe, whose coupled regions are usually skewed towards the faceted component. It was also observed that the morphologies of primary silver, primary germanium and eutectic structure were changed with increasing growth rate. Lamellar colonies were formed prominently in the fast-grown hypereutectic alloys. As the growth rate increased the tendency for branching in massive primary germanium was so pronounced that a lamellar colony was finally formed.     

Electrical anisotropic property of some unidirectionally solidified eutectic alloys

The eutectic systems of Cd-Pb, Bi-Cd and Bi-Zn were alloyed using chemically pure raw materials in an electric furnace. The electrical resistivities were determined at room temperature in terms of the microstructures of the eutectic alloys. Electrical resistivities measured parallel and perpendicular to the unidirectional microstructure at different solidification rates were quite distinctive. The electrical resistivities measured were consistent with calculated values. The present study indicates qualitatively that unidirectional solidification of ideal conductor-insulator eutectic systems could produce electrical anisotropic materials.     

The structure of the unidirectionally solidified Al-AlSb binary eutectic

The structures of four unidirectionally solidified A-Sb alloys of near eutectic composition were determined over a wide range of growth rates (0.6 to 49 cm/h). No cellular macrostructure was observed. At the lower solidification rates a broken lamella structure was formed; isolated grains with randomly arranged rods were also present. At higher solidification rates all grains were of the broken lamella type, more perforated and finer. With a metallographic technique it was possible to arrive at the conclusion that broken lamellae are mainly of two types: perforated lamella and branched ribbons. Crystallographic orientations were also determined; the interlamella spacing varied according to the relation =AR ^–n, where R is the growth rate, with n=0.40.     

The tensile deformation of unidirectionally solidified Al-Al3Ni and Al-Al2Cu eutectics

Transmission electron microscopy has been used to investigate the dislocation structures produced in single crystals of Al-Al₃Ni and Al-Al₂Cu eutectics deformed in tension. Matrix dislocation densities in the as-grown eutectics are very high, owing to differential thermal contraction effects. The subsequent deformation behaviour of the eutectic crystals is shown to be controlled by the initial high dislocation density, and a constraint effect due to the presence of closely spaced fibres or lamellae.     

Structure and mechanical properties of unidirectionally solidified Zn-Ge eutectic alloys

The Al-Si eutectic alloys are known to undergo various structural transitions when unidirectionally solidified. This paper describes another metal/non-metal combination, Zn-Ge, which undergoes closely similar morphological changes. The tensile and compressive properties of the unidirectionally solidifed Zn-Ge eutectic have been examined and compared with those of the Al-Si eutectic. It is shown that the marked compressional stiffness of Al-Si alloys containing 〈100〉 type branched silicon dendrites only arises because of the lateral constraints of the aluminium matrix and does not occur in the Zn-Ge system.     

Al-CuAl2 eutectic structure in unidirectionally solidified rods by the Ohno continuous casting process

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

The temperature-dependence of the mechanical properties of unidirectionally solidified silver-germanium eutectic

Work-hardening curves of the unidirectionally solidified silver-germanium eutectic have been determined by tensile deformation between room temperature and 615° C. The transition stress, ₀ between elastic and plastic deformation of the germanium platelets, exhibits the same temperature dependence ( ₀ exp Q/KT) over the whole temperature range. ₀ is determined, however, by two effects: the temperature-dependent critical shear stress of germanium and the lowering of the shear strength, , of the phase interfaces. Estimates of yield 14 kg mm^–2 at room temperature and <0.12 kg mm^–2 at 615° C. At low temperatures (<400° C), the composite fracture is initiated by the fracture of fibres (platelets), whereas at higher temperatures, the matrix fails first. As long as the interface shear strength is sufficiently large, the composite fracture is retarded resulting in a pronounced maximum of the fracture strain at 550° C.     

强脉冲磁场中Al-Cu共晶定向凝固组织的演变

脉冲磁场作用于Al Cu共晶凝固的界面,研究了定向凝固组织的演变.随着脉冲磁场强度的提高,Al-Cu共晶定向凝固组织经历了由规则柱晶到破碎枝晶、粗化枝晶到重新规则化柱晶三个演化阶段;在重新规则化柱晶试样中,共晶片层间距减小,晶团间富铜相析出明显.将感生电势场与溶质扩散场相耦合,分析了脉冲磁场对凝固界面稳定性的影响,发现强脉冲磁场在金属熔体引起感生电势场效应,在凝固界面前沿诱发具有振荡特征的电致迁移,从而促进晶间扩散和减小成分过冷区域.     

Barothermal analysis and structure of the eutectic Al-Ni (2.7 at % Ni) alloy

Phase transformations of the 2.7Ni + 97.3Al eutectic alloy have been characterized by differential barothermal analysis (DBA) at temperatures of up to 750°C and pressures of up to ∼100 MPa. The results demonstrate that the Al matrix of the as-prepared alloy was saturated with micropores. After melting and crystallization in compressed argon, the micropore density increased. As a result of melting and solidification, the fine-grained structure of the as-prepared alloy transformed to a macrocrystalline, dendritic structure. Electron microscopy was used to determine the volume fraction of the intermetallic phase NiAl₃ in the Al matrix. Supersaturated solid solutions of nickel in aluminum decompose at 626°C to give a mixture of Al〈Ni〉 and NiAl₃. At 100 MPa, the nickel-aluminum solid-solution range extends to 2.7–2.8 at % Ni, and the nickel content at the eutectic point may reach 3.1–3.3 at %. The Al and NiAl₃ in the alloy solidified at 100 MPa had reduced lattice parameters. We determined the microhardness of the Al matrix after DBA.     

Constitution of the Ni3Cr-Ni3Al-Ni3W system

Isothermal sections of the Ni-Cr-Al-W system have been investigated at 75 at % Ni and temperatures of 1523 and 1273 K, by means of phase compositional analysis, X-ray diffraction and microscopical examination. The alloys studied lay in the range 2.5 to 10 at % Cr, 12.5 to 20 at% Al, 2.5 to 6.25 at % W, The phases formed were, and the bcc solid solution based on tungsten (designated ₂). The maximum extent of the region was found to be 3 at % each of chromium and tungsten. Preferential partitioning of tungsten to occurred. Study of an Ni-10Cr-12.5Al-2.5W alloy aged at 1273 and 1073 K, after quenching from 1573 K, showed that changes in and compositions and lattice parameters occur as a function of ageing time.