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.     

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.     

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.     

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.     

Structure and mechanical properties of unidirectionally solidified low alloy steels

Abstract

Two low alloy steels have been unidirectionally solidified in a liquid metal cooling Bridgman crystal grower. The dendrite morphology and dendrite arm spacing have been determined as a function of distance along the bars for solidification at various rates and under different temperature gradients. Microsegregation in the as solidified material was studied by electron probe microanalysis. The tensile properties of heat treated unidirectionally solidified material were determined and their values of elongation to failure found to be considerably greater than for the conventionally cast material. Similarly, the impact energy values of heat treated unidirectionally solidified material are higher than those of the conventionally cast alloy. The tensile and impact properties are discussed in terms of the strain incompatibility present during deformation both at the dendritic grain boundary and at the individual dendrite. Incompatibility of strain leads to a propensity for secondary cracking at these boundaries, the amount of which depends upon the detailed morphology of the dendrite which is determined by the solidification parameters.

MST/880     

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.     

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-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}     

The microstructure and mechanical properties of unidirectionally solidified Al−Si alloys

Hypereutectic Al−Si alloys with minor additions of Sr were directionally solidified with a temperature gradient of 125°C cm⁻¹ in the liquid. Silicon in the range 14–17 wt%, Sr in the range 0.0–0.5 wt% and specimen traction velocities between 1 and 1500 μm sec⁻¹ were used. The relationship between hardness and traction velocity and spacing in eutectic silicon morphologies is defined and shown to be of the same form as that for yield stress. The possibility of using hardness measurements to be of the same form as that for yield stress. The possibility of using hardness measurements to indicate mechanical properties is discussed. The complex regular silicon structure makes a significant contribution to the hardness of hypereutectic alloys. This makes the relationship between hardness and traction velocity more complex adding difficulties to the use of hardness to measure mechanical properties.     

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.     

Crystallographic characterization and indentation mechanical properties of LaB6-ZrB2 directionally solidified eutectics

LaB₆-ZrB₂ eutectics were directionally solidified by a zone melting process. The microstructure and crystallography were characterized by X-ray diffraction, optical microscopy and transmission electron microscopy. The anisotropies of hardness and indentation fracture toughness were assessed by making measurements along different crystallographic directions on both transverse and longitudinal sections. While the hardness did not vary appreciably between the two orientations, the fracture toughness was observed to be highly anisotropic. Toughening behavior was observed on longitudinal sections with crack deflection and bridging mechanisms apparent. Cracks were difficult to initiate on transverse sections along directions perpendicular to the growth direction, but for 20 N loads an indentation fracture toughness of 11 MPa could be measured.     

Investigation on Ni3Al-Ni7Hf2 unidirectionally solidified eutectic composite

The formation of a Ni₃Al()+Ni₇Hf₂ unidirectionally solidified lamellar eutectic composite has been investigated in this paper. The results show that Ni-5.8Al-32Hf alloy, which has the Ni₃Al+Ni₇Hf₂ eutectic structure, is a suitable composition of D.S eutectic material. The melting range of this composition is 41 °C as determined by DTA. The critical ratio of G/R for Ni₃Al+Ni₇Hf₂ eutectic is found to be 5× 10⁵ °C · s · cm^–2, and the lamellar Ni₃Al+Ni₇Hf₂ eutectic aligned parallel to the direction of solidification was made with R = 5 m/s and G = 250 °C/cm. The investigation shows that the lamellar eutectic has a preferred crystallographic orientation between the Ni₃ Al and Ni₇Hf₂ lamellae, i.e., (111)_NiAl//(100)_NiHf and [110]_NiAl//[010]_NiHf. The lamellar Ni₇Hf₂ did not degrade or coarsen obviously, and no harmful phase formed in the interface of Ni₃Al/Ni₇Hf₂ after long time soaking of 1100 °C/110 h. This demonstrates that the Ni₃Al+Ni₇Hf₂ lamellar eutectic has high interface thermal stability.     

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.