Directionally solidified Soviet superalloy,ZHS6-K

Directional solidification of the Soviet superalloy, ZHS6-K, has been carried out in an argon atmosphere. Tensile and stress-rupture properties have been measured for the investment cast and directionally solidified (DS) alloy. The DS alloy shows a several fold increase in rupture life and ductility compared with the investment cast alloy It also shows improved tensile properties. Stress-rupture and tensile fracture behaviour has been examined.     

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.     

Deformation and fracture in directionally solidified Co-CoAl eutectic

The effect of growth defects known as lamellar terminations on the yielding and fracture behaviour of Co-CoAl eutectic single crystals was studied using tensile tests and finite-element modelling. The yield strength and strain to fracture were found to decrease with increasing termination density. Observations of deformed surfaces and serial sectioning experiments on fractured tensile specimens revealed that crack initiation during the fracture process was enhanced by the presence of lamellar terminations. The fracture surfaces were found to have a staircase-type appearance, which indicated that the final fracture process was discontinuous with a step-wise propagation from one CoAl lamella to adjacent CoAl lamellae. A computer simulation was conducted to determine the stress distributions about lamellar terminations in model microstructures, since the experimental results suggested that the lamellar terminations behaved as stress concentrations in the microstructure. The finite-element calculation confirmed that lamellar terminations can influence the yielding process; the stress at which the first slip system was activated was found to decrease with increasing termination density.     

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.     

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.     

Structure and magnetic properties of directionally solidified Bi-MnBi eutectic alloys

An alloy containing about 2 at% Mn and 98 at% Bi should solidify at 230°C to give a eutectic consisting of about 3 vol % ferromagnetic MnBi in a matrix of diamagnetic Bi. Directional solidification of this alloy produces a structure of elongated parallel MnBi rods with diameters from 0.1 to 1.5 μm, decreasing with increasing growth rate. Samples solidified slowly (growth rate less than 5 cm/h) show magnetic properties at room temperature and down to 4.2 K that are consistent with the known magnetic properties of bulk equilibrium MnBi. Samples solidified more rapidly (20-80 cm/h) show considerably more complicated magnetic behavior. At room temperature they have less than half the moment expected for the equilibrium MnBi composition, and a second magnetic phase appears when the temperature is lowered below about 240 K. The coercive field of this phase rises rapidly with decreasing temperature, reaching the limit of our experimental apparatus (about 120 kOe) near 100 K. Magnetic measurements with the field applied perpendicular to the growth axis show the effects of anisotropy, due to particle alignment, although samples prepared at very rapid freezing rates (300 cm/h) appear to be poorly aligned. Samples prepared by arc melting on a water-cooled hearth are similar to high-growth-rate samples. Annealing at 200°C was necessary in order to obtain reproducible magnetic data. The results are generally consistent with the presence in the samples of a mixture of the normal equilibrium MnBi phase and a variant of the known high-temperature phase (HTP) with a substantially depressed Curie temperature. Structural studies using electron diffraction from thinned sections indicate the presence of both normal (LTP) and high-temperature (HTP) MnBi.     

Microstructures and fracture toughness of directionally solidified Mo-ZrC eutectic composites

Microstructures and fracture toughness of arc-melted and directionally solidified Mo–ZrC eutectic composites were investigated in this study. Two kinds of directionally solidified composites were prepared by spot-melting and floating zone-melting. Microstructure of the arc-melted composite (AMC) consists of equiaxed eutectic colonies, in which ZrC particles are dispersed. The spot-melted composite (SMC) exhibits spheroidal colony structure, which is rather inhomogeneous in size and morphology. ZrC fibers in the eutectic colonies are aligned almost parallel to the growth direction. Well aligned, homogeneous columnar structure with thin ZrC fibers evolves in the floating zone-melted composite (FZC). Texture measurement by X-ray diffractometry revealed that the growth direction of Mo solid solution (Mo_SS) in FZC is preferentially 〈100〉, while that of SMC is scattered. Fracture toughness K_q evaluated by three point bending test using the single edge notched beam method is >13 MPa m^1/2 for AMC, 20 MPa m^1/2 for SMC and 9 MPa m^1/2 for FZC. Intergranular fracture along colony boundaries is often observed in AMC. In contrast, transgranular fracture is dominant in SMC and FZC, although significant gaps caused by intergranular fracture are occasionally observed in SEM micrographs of SMC. Fracture surface in FZC is wholly flat. Pull-out of ZrC occurs owing to Mo/ZrC interfacial debonding in intergranularly fractured regions of AMC and SMC.

Coarse elongated colonies in SMC and FZC induce transgranular fracture instead of intergranular fracture. Intergranular fracture and interfacial debonding in AMC and SMC causes frequent crack deflection accompanied by ligament formation and crack branching, which is responsible for the high fracture toughness of the composites. Preferred 〈100〉 growth of Mo_SS phase in FZC leads to brittle 〈100〉 cleavage fracture associated with low fracture toughness.     

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.     

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}     

Transmission electron microscope study of a directionally solidified Cu-MgCu2 eutectic

The crystallography and the interface structure of a unidirectionally solidified Cu-MgCu₂ eutectic alloy have been examined by transmission electron microscopy. The microstructure of the eutectic was found to be lamellar and regularly interrupted by faults. The preference of the particular orientation relationship could not be explained by relative atomic densities of the planes comprising the interface. Based on the defect contrast observed and extinction distance calculations, it is suggested that the fine array of defects observed at the interface may be characterized as steps with step vectors parallel to or Dislocations were also observed at the interface but they were rarely regular.     

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.     

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.     

A directionally solidified Ni-base eutectic alloy reinforced by a mixed hafnium-zirconium monocarbide

The nickel-base eutectic alloy (Ni-10Cr-5Al-5Cr-8. 3Hf-1. 1Zr-0.7C, NASA HAFNIC-10) was directionally solidified to produce aligned refractory monocarbides in a nickel-rich matrix. The aligned carbides, (Hf, Zr) C, were present as rod- and ribbontypes, with a volume loading of about 8%. A blocky type of Ni₃Al () precipitate, about 45% volume loading, was produced in the nickel-base matrix. The growth direction of both aligned carbide and matrix was parallel to 1 0 0. The effect of thermal cycling between 425 and 1100° C using a 2.5 min cycle time was examined. No microstructural degradation of the aligned carbide fibres occurred after 2000 cycles.Formerly with National Aeronautics and Space Administration as a N.R.C. Research Associate, Lewis Research Center, Cleveland, USA.     

快速凝固/粉末冶金(RS/PM)高硅铝合金材料的研究

采用冷压 +热挤压工艺制备快速凝固高硅铝合金材料 ,并探讨了挤压温度、保温时间、挤压比等工艺参数对材料组织及性能的影响 .研究结果表明 ,与高硅铝合金铸造材料相比 ,本研究所制的材料硅相尺寸得到了显著细化 ,且分布更为均匀 ,材料的抗拉强度和延伸率有明显的提高 .采用光学金相显微镜、SEM、XRD等手段对快速凝固高硅铝合金粉末及大块材料的微观结构及相组织进行了深入研究 .