Beneficial influence of copper and manganese alloying elements on the mechanical properties of metallic composite materials based on the eutectic Al-5.7% Ni alloy

We have developed a new type of metallic composite material based on the binary eutectic Al-5.7 wt% Ni alloy. The structure of the initial alloys prepared by simple casting consists of strong, fine fibres of the Al₃Ni phase and a ductile aluminium matrix. These alloys are then transformed into dispersion-hardened materials by isostatic extrusion in which the Al₃Ni fibres are broken and dispersed in the ductile matrix. The materials thus prepared present a mechanical strength of the same order as the unidirectionally solidifed eutectic alloy: the tensile strength is nearly 300 MPa at room temperature. We observed in this work that the mechanical strength is remarkably increased by the addition of a small amount of copper or manganese: it attains about 400 MPa by the addition of 2 to 3wt% Cu, and more than 500 MPa by the addition of 3wt% Mn. These alloying elements also produce beneficial effects on the mechanical properties at high temperature.     

Density measurements after tensile and creep tests on pure and slightly oxidised aluminium

In order to investigate the influence of dispersed oxide particles on crack-formation in Al-Al₂O₃ alloys, series of tensile tests (20 to 550† C) and creep tests (450† C) were carried out on pure Al and on Al-0.7wt% Al₂O₃ specimens. Density measurements performed on broken samples showed no changes in pure Al but considerable decreases in the slightly oxidised Al. The observed influence of oxide is strong for the small percentage if compared with the behaviour of Al-Al₂O₃ alloys with much higher oxide contents.     

The solidification and mechanical properties of chill-cast AI-AI3IMi and AI-AI2Cu eutectic alloys

This paper describes the effect of chill-casting on the solidification behaviour and mechanical properties of the AI-AI₃Ni and AI-AI₂Cu eutectic alloys. Cellular microstructures were obtained by casting the eutectic alloys into preheated split-steel moulds mounted on either a water-cooled or plain copper chill, to promote growth along the length of the ingot and not radially from the mould wall. This produced the required cellular microstructure with good alignment of AI₃Ni fibres or AI₂Cu lamellae within the cells, with an interfibre/interlamellar spacing of 1 m. The experimental solidification results showed an increase in solidification rate with increasing distance from the chill associated with a decrease in interfibre/interlamellar spacing along the length of the solidifying ingot. There were no significant variations in the room-temperature tensile properties of the two chill-cast aluminium based eutectic alloys for the various casting conditions. Variations in solidification rate along the ingots for the different chill-casting conditions were not sufficient to affect the stress-strain behaviour of the chill-cast alloys. The room-temperature tensile behaviour of the chill-cast AI-AI₃Ni eutectic alloy was very similar to, and that of the AI-AI₂Cu eutectic alloy significantly different from, those obtained by Lawson and Kerr. The ultimate tensile strengths of the chill-cast eutectic alloys were not as high as those of the corresponding unidirectionally-solidified eutectic alloys prepared at a slow and constant solidification rate although the reasons for this were different for the two alloys. The ultimate tensile strength of the chill-cast AI-AI₃Ni eutectic alloy was found to be in reasonable agreement with that expected from the rule of mixtures for discontinuous fibre reinforcement.     

The tensile fracture behaviour of the aligned Al-Al3Ni and Al-CuAl2 eutectics at various temperatures

The tensile behaviour of single crystals of the unidirectionally-solidifled Al-Al₃Ni fibrous and Al-CuAl₂ lamellar eutectics has been investigated at ambient and elevated temperatures. Fractured specimens have been studied directly by scanning electron microscopy, and longitudinal sections through each fracture surface by optical microscopy. At low temperatures, fracture of the Al-Al₃Ni eutectic is caused by the brittle failure of Al₃Ni fibres. At intermediate temperatures broken fibres may be present without causing composite fracture, whilst at high temperatures fibre pull-out occurs. The properties of the Al-CuAl₂ eutectic are controlled by the brittle CuAl₂ phase, except when the temperature is such that both phases are plastic. During fracture at high temperature the Al-CuAl₂ eutectic undergoes micromorphological degradation in the vicinity of the fracture surface.     

Solidification behaviors of the emulsified Al-4wt%Fe alloy powders

The emulsified Al-4wt%Fe alloy powders showed several different microstructures depending on the amount of undercooling such as Al₁₃Fe₄, Al _x Fe primary intermetallic, Al-Al₃Fe or Al-Al₆Fe eutectics and -Al cellular structure. The presence of these phases depends on a competitive growth mechanism, which was determined by the undercooling prior to solidification. The amount of undercooling of the powders was monitored by differential thermal analysis and was matched with the microstructures. The difference of the amount of undercooling which corresponds to the microstructures between the experiment and the previous value converted by the LKT theory was rationalized through the solidification behaviors. A microstructure selection map of Al-4Fe alloy powders for tailored solidification was also suggested.     

Tensile yielding of Al-Al3Ni eutectic crystals

Directionally-solidified single crystals of Al-Al₃Ni eutectic have been grown at differing rates to produce fibre spacings of the Al₃Ni-phase of between 1.26 and 2.26m. After heat-treatment to produce a low matrix dislocation density, the tensile yield stress of the various crystals was measured. Comparison of the yield stresses of crystals of differing fibre spacing indicate that the resolved shear stress is lower at smaller fibre spacings, suggesting yield is not by the Orowan mechanism. It is concluded that on yield, dislocations propagate from sources near the fibre-matrix interfaces, and that due to their higher elastic modulus, the fibres tend to repel the matrix dislocations.     

Metallurgical structure and alkali leaching of the Al/Al3Ni eutectic

Raney-Nickel catalysts were prepared by aluminium leaching out of aluminium-rich binary Al-Ni alloys. In order to understand the behaviour of NiAl₃ during alkali leaching, different metallurgical structures of the Al/Al₃Ni eutectic were prepared as precursor alloys. This eutectic showed a fibrous morphology with Al₃Ni fibres embedded in an aluminium matrix. After alkali leaching, transmission electron microscopy observations showed that the fibrous microstructure was retained. The fibres were formed with small nickel crystallites. During the early stages of leaching, a reaction front was observed which remained parallel to the Al/Al₃Ni interface.     

Precipitation in an Al-1.78 wt % Hf alloy after rapid solidification

A metallographic study has been made of an Al-1.78 wt % Hf alloy after rapid solidification, and after high temperature annealing of the as-cast alloy. After solidification, Hf stays in supersaturated solid solution with a rather inhomogeneous distribution. On annealing, spheres and dendrites of an intermediate Al₃Hf phase withLI ₂ structure, and perfectly coherent with the matrix, are formed. The spheres form by a continuous, the dendrite by a discontinuous precipitation reaction. After continued annealing theLI ₂ structure is replaced by a laminated structure ofLI ₂ andDO₂₂, whereDO₂₂ is the equilibrium structure of Al₃Hf. The precipitation reactions in the systems Al-Hf and Al-Zr are compared.     

Effect of Cu addition on microstructures and tensile properties of high-pressure die-casting Al-5.5Mg-0.7Mn alloy

In this study, Cu was added into the high-pressure die-casting Al-5.5Mg-0.7Mn (wt%) alloy to improve the tensile properties. The effects of Cu addition on the microstructures, mechanical properties of the Al-5.5Mg-0.7Mn alloys under both as-cast and T5 treatment conditions have been investigated. Additions of 0.5 wt%, 0.8 wt% and 1.5 wt% Cu can lead to the formation of irregular-shaped Al₂CuMg particles distributed along the grain boundaries in the as-cast alloys. Furthermore, the rest of Cu can dissolve into the matrixes. The lath-shaped Al₂CuMg precipitates with a size of 15–20 nm × 2–4 nm were generated in the T5-treated Al-5.5Mg-0.7Mn-xCu (x = 0.5, 0.8, 1.5 wt%) alloys. The room temperature tensile and yield strengths of alloys increase with increasing the content of Cu. Increasing Cu content results in more Al₂CuMg phase formation along the grain boundaries, which causes more cracks during tensile deformation and lower ductility. Al-5.5Mg-0.7Mn-0.8Cu alloy exhibits excellent comprehensive tensile properties under both as-cast and T5-treated conditions. The yield strength of 179 MPa, the ultimate tensile strength of 303 MPa and the elongation of 8.7% were achieved in the as-cast Al-5.5Mg-0.7Mn-0.8Cu alloy, while the yield strength significantly was improved to 198 MPa after T5 treatment.     

Interfacial reactions in the liquid diffusion couples of Mg/Ni,Al/Ni and Al/(Ni)-Al2O3 systems

The interfacial reactions between various molten metals and solid plates were investigated in this diffusion couple study. The molten metals were pure magnesium, pure aluminium, aluminium-rich Al-Mg alloy, and aluminium-rich Al-Cu alloys, and the solid plates were pure nickel plate, alumina plate, and nickel-plated alumina plate. The interfacial reactions in the diffusion couples were determined by using optical microscopy, scanning electron microscopy and electron probe microanalysis in regard to the formation of intermetallic phases, the dissolution rates of the nickel plates, and the morphology of the interfaces. Mg₂Ni phase was found in the pure Mg/Ni plate diffusion couples, and the Al₃Ni and Al₃Ni₂ phases were observed in the pure Al/Ni plate and Al-alloys/Ni plate diffusion couples. In the Al-Cu alloy/Ni-plated alumina plate diffusion couple, Al₂O₃ formed at the interface, while spinel particles were found in the diffusion couples of Al-7.4wt% Mg alloy/Ni-plated alumina plate. Experimental difficulty was encountered in preparing the diffusion couples with alumina plate, and a gap existing at the interface prohibited reactions between the molten metal with alumina plate.     

Coupled eutectic growth in Al-Fe alloys

The response to isothermal soaking at 773 to 913 K (0.83 to 0.99T _m) for holding times upto 1000 h is reported for Al-3 wt% Fe/metastable Al-Al₆Fe eutectic (10vol%Al₆Fe) directionally grown at 1.24 mm sec^–1. Breakdown is initiated by pinching-off and spherodization of Al₆Fe eutectic rods within eutectic cells and by growth of equilibrium Al₃Fe at grain boundaries and cell boundaries. Compared with equivalent Al-Al₃Ni, results indicate enhanced thermal stability of Al-Al₆Fe eutectic pending consumption by growing Al₃Fe. Hardness decreased with increased soaking time according to an Orowan relationship with Al₆Fe particle spacing.     

Morphological characteristic of the conventional and melt-spun Al-10Ni-5.6Cu (in wt.%) alloy

The Al-10Ni-5.6Cu alloy was prepared by conventional casting and further processed melt-spinning technique. The resulting conventional cast and melt-spun ribbons were characterized using X-ray diffraction, optical microscopy, scanning electron microscopy together with energy dispersive spectroscopy, differential scanning calorimetry and microhardness techniques. The X-ray diffraction analysis indicated that ingot samples were α-Al, intermetallic Al₃Ni and Al₂Cu phases. The optical microscopy and scanning electron microscopy results show that the microstructures of rapidly solidified ribbons are clearly different from their ingot alloy. Al-10Ni-5.6Cu ribbons reveal a very fine cellular structure with intermetallic Al₃Ni particles. Moreover, at high solidification rates the melt-spun ribbons have a polygonal structure dispersed in a supersaturated aluminum matrix. The differential scanning calorimetry measurements revealed that exothermic reaction was between 290 °C and 440 °C which are more pronounced in the ternary Al-10Ni-5.6Cu alloy.     

Filamentary preforms of Al-13wt % Si alloy reinforced with TiC-coated and pre-treated carbon fibres

Filamentary preforms of aluminium-13 wt % Si alloy reinforced with TiC-coated carbon fibres were obtained by liquid metal infiltration using K₂ZrF₆ as wetting agent. The interfacial structure was investigated using analytical electron microscopy (TEM, SEM) and the fibre strength measured following each step of processing. The reaction between the K₂ZrF₆ deposit and the molten alloy is discussed on the basis of the reaction products formed at the fibre-matrix interface, which mainly consisted of fluoride compounds (K₃AlF₆), different zirconium-rich suicides (Si₂Zr, SiZr (Al), (Al, Si)₂Zr) and crystalline -alumina. Large amounts and/or inhomogeneous distributions of the K₂ZrF₆ deposit should be avoided, because the massive precipitation of colonies of intermetallics around fibres is conducive for the formation of harmful aluminium carbides and favours brittle fracture of the composite. The utilization of TiC-coatings formed by the reactive chemical vapour deposition process gave satisfactory results, both in terms of composite elaboration and fibre protection during liquid infiltration.     

Coupled eutectic growth in Al-Fe alloys

The conditions for fully eutectic growth in Al-Fe alloys at a temperature gradient of 20 K mm^–1 are reported for ranges of composition from 2.2 to 6.1 wt % Fe and of growth velocity from 0.03 to 10 mm sec^–1. All six main classes of growth structure (i.e. Al-Al₃Fe or Al-Al₆Fe eutectics either alone or together with primaryAl or Al₃Fe) were obtained, some of them reported for the first time for steady-state conditions. Observed concentration-dependences both of the limiting growth velocity for primary Al₃Fe and of the interphase spacing for the fully eutectic Al-Al₆Fe displacing it are in good agreement with theory. Hardness levels for the Al-Al₆Fe eutectic as a function of concentration are similar to those forAl dendritic structures grown in much thinner sections under splat-cooling conditions. The significance of some observed transitions in growth morphology for eutectic cells, Al₆Fe eutectic rods andAl dendrites is discussed.     

Microstructure development in eutectic Al–Fe alloy during directional solidification under high magnetic fields at different growth velocities

In this study, the Al-1.9 wt.% Fe alloy was directionally solidified at different growth velocities under various high magnetic fields. The effect of high magnetic fields on microstructural evolution of the alloys during directional solidification and their dependence on the growth velocity were investigated. The microstructures near the transition growth regions of the alloys between two different growth velocities were observed. With increasing growth velocity, the microstructure exhibited a eutectic to cellular eutectic to hypoeutectic structure transformation. Applying high magnetic fields increased the spacing of the eutectic Al₃Fe phase and decreased the alignment degree of the eutectic Al₃Fe phase at 1 µm/s, decreased the size of the eutectic cells at 10 µm/s, and promoted development and branching of the primary Al dendrites at 100 µm/s. Near the transition growth region, applying high magnetic fields stopped the growth of existing eutectic Al₃Fe and promoted nucleation of the new eutectic Al₃Fe phase for 1–10 μm/s, and accelerated transformation of the growth behavior from cellular eutectic to hypoeutectic for 10–100 μm/s. The evolution of the eutectic growth behavior caused by the high magnetic fields can be attributed to suppression of convection and the corresponding decrease in solute migration owing to the Lorentz force.

     

Some aspects on the microstructure and crystallization of rapidly solidified Al-27% Mn alloy

In a previous paper we reported observations of the icosahedral quasistructure (m \(\bar 3 \bar 5\) symmetry) in Al-14 wt % Mn ribbons. In this paper, the microstructure of rapidly solidified Al-27 wt % Mn ribbons is studied where, in addition to the icosahedral phase, the decagonal quasistructure (10/m or 10/m m m symmetry) is also found to occur along with the equilibrium Al₆Mn and Al₄Mn phases, as well as Al. The effect of annealing (in the range 400 to 600° C for 1 to 100 h) on the stability of these structures was studied, using X-ray diffraction and transmission electron microscopy. While the icosahedral phase becomes unstable at 400° C and crystallizes into Al₆Mn, the decagonal phase is seen to persist even after 500° C/100h. The existence of the decagonal phase at 600° C together with the Al₄Mn phase suggests that the decagonal phase arises from a commensurate → incommensurate transformation due to atomic displacement within the Al₄Mn phase.     

Microstructural investigation of a rapidly solidified Ti-6Al-4V-1B-0.5Y alloy

A Ti-6Al-4V-1B-0.5Y (wt%) alloy has been prepared by consolidation of the melt-spun alloy fibres. The microstructures of the melt-spun fibre and the consolidated alloy were examined by different techniques. It was found that in the consolidated alloy, titanium boride and yttrium oxide particles have a refined particle size and a uniform distribution in the (+) matrix compared with the microstructure of the same alloy obtained by conventional ingot metallurgy. The boride phase in the consolidated alloy mainly has a needle-shaped morphology and has been identified by electron diffraction to be orthorhombic TiB with a B27 structure, while the yttrium oxide has a cuboidal morphology and has been identified as bcc Y₂O₃. Detailed TEM examination also revealed that yttrium addition has a strong influence on the TiB morphology by comparing the microstructures of Ti-6Al-4V-1B alloys with and without yttrium addition. Under similar processing conditions, the TiB phase in the consolidated alloys without yttrium addition mainly has a nearly equiaxed morphology with a finer particle size, while the TiB phase in the consolidated alloy with yttrium addition will mainly have a needle-shaped morphology. This effect of yttrium addition on the TiB morphology has been discussed in terms of heterogeneous nucleation and the reduced undercooling.     

Structural study on Al-26 mass% Si-8 mass% Ni powder

The structure of Al-26 mass% Si-8 mass% Ni alloy, a material used as an input product for manufacturing light weight products via powder metallurgy, was investigated by in situ powder diffraction techniques up to 700°C. Thermal expansion of the dominant phases indicates substitutional alloying of silicon by nickel atoms, forming a solid solution phase stable in the temperature region from 330 to 550°C. The presence of Al, Si, Al₃Ni and Si₃Ni was determined by phase analysis from a sample annealed at 700°C (re-melted) and cooled down to room temperature. EXAFS analysis of as prepared and re-melted samples documented similar local atomic structure around the Ni atoms in both stages.     

Off-eutectic composite solidification and properties in Al-Ni and Al-Co alloys

To investigate the range of coupled eutectic growth in Al-Co alloys from 1 to 4 wt% Co and Al-Ni alloys from 5 to 10 wt% Ni directional solidification, using rates from 0.8×10^–2 cm/sec to 10.6×10^–2 cm/sec, was employed. Both alloy systems exhibited coupled zones skewed towards the hypereutectic compositions. Fully eutectic structures were obtained in the ranges Al-1 wt% Co to Al-3 wt% Co and Al-5.7 wt% Ni to Al-9.2 wt% Ni.The off-eutectic alloys which exhibit a fully eutectic structure behave as reinforcing composite materials, with the tensile strength and microhardness increasing as the volume fraction of the strengthening phase increases.     

Fabrication of Al2O3 continuous fibre reinforced Al–Cu alloys by axial infiltration process

Abstract

This paper describes the fabrication of Al₂O₃ continuous fibre reinforced Al-Cu alloys by an axial infiltration process which is expected to be used in the production of stick, bar, or platelike composites. A discussion on the infiltrating process gave equations for the critical infiltration pressure and the size of composite defects. Microscopic observations and microprobe analyses on Al-4.43Cu, Al-6.48Cu, Al-10.11Cu, and Al-4.45Cu-1.54Mg (wt-%) matrix composites identified the solidification process of matrix alloys in the presence of Al₂O₃ fibres. The approximate relationships between microstructure, interspace size, and the matrix composition are described schematically. Microsegregation of Cu and Mg in the composites are also analysed quantitatively.