Fabrication of laminated metal–intermetallic composites by interlayer in-situ reaction

A simple and cost-effective method, interlayer in-situ reaction process, has been developed to produce laminated metal-intermetallic materials. Layered NiAl₃ and Ni₂Al₃/Ni composites have been fabricated successfully by using the process. It is shown that volume friction of the intermetallic layers can be well controlled by the thickness of the metals. It is difficult to produce high strength composites if the original metals are directly exposed at high temperature. This is rectified by a pre-treating processing in which a prefect interface is formed to prevent the metals from oxidation at high temperature. The pre-treated composites have an improvement in tensile strength and thermal stability. SEM observations show that the composites exhibit a mixing fracture mode suggesting that the composites would have high toughness.     

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.     

Fabrication and properties of novel composites in the system Al–Zr–C

Composite bodies in the system Al–Zr–C, with about 95% relative density, were obtained by heating the compact body of powder mixture consisting of Al and ZrC (5 : 1 mol %) in Ar at 1100–1500°C for various lengths of time. Components of the material heated at more than 1200°C were Al, Al3Zr, ZrC and AlZrC2. The Al3Zr exhibited plate-like aggregation, and its size increased with increasing temperature. In the material heated at 1500°C for 1 h, the largest plate-like Al3Zr aggregation was 2000 m long and 133 m thick. Then the AlZrC2 was present as well-proportioned hexagonal platelet particles with a 8–9 m diameter and a 1–2 m thickness in the interior of the plate-like Al3Zr aggregation and Al matrix phase. The average three-point bending strength of the bodies was 140–190 MPa, and the maximum strength was 203 MPa in the body heated at 1300°C for 1 h. The body heated at 1500°C for 1 h showed high oxidation resistivity to air up to 1000°C.     

Microstructure evolution and elemental diffusion of SiCp/Al–Cu–Mg composites prepared from elemental powder during hot pressing

15vol%SiCp/Al–Cu–Mg composites were fabricated by hot pressing method using pure elemental powders. Microstructure evolution and elemental diffusion of Cu and Mg were studied. The microstructure of as-hot pressed composites and the elemental distribution of the composites before and after solution treatment were also investigated. The results showed that there were two types of eutectic liquid phases with different compositions after the compact was heated to 580 °C. After the compact was held at 580 °C for 60 min, the eutectic liquid was absorbed into the Al matrix and some equilibrium liquid phases formed in the boundaries of the initial Al particles. Meanwhile, Cu was homogeneously distributed in the Al particles while Mg tended to be distributed near the boundaries of the initial Al particles and in the SiC clusters. The presence of Al₂Cu, Mg₂Si, and some oxides of Mg was identified in the as-hot pressed composite. After solution treatment, Al₂Cu dissolved into the Al matrix, however, some Mg-rich compounds (silicide and oxide of Mg) did not dissolve into the matrix completely.     

Fabrication of bioglass infiltrated Al2O3–(m-ZrO2) composites

Using 80 vol.% of poly methyl methacrylate (PMMA) as a pore-forming agent to obtain interconnected porous bodies, porous Al₂O₃–(m-ZrO₂) bodies were successfully fabricated. The pores were about 200 μm in diameter and were homogeneously dispersed in the Al₂O₃–25 vol.% (m-ZrO₂) matrix. To obtain Al₂O₃–(m-ZrO₂)/bioglass composites, the molten bioglass was infiltrated into porous Al₂O₃–(m-ZrO₂) bodies at 1400°C. The material properties of the Al₂O₃–(m-ZrO₂)/bioglass composites, such as relative density, hardness, compressive strength, fracture toughness and elastic modulus were investigated.     

Fabrication of Al–Si coating on Ti–6Al–4V substrate by mechanical alloying

Al–Si coatings were synthesized on Ti–6Al–4V alloy substrate by mechanical alloying with Al–Si powder mixture. The as-prepared coatings had composite structures. The effects of Al–Si ratio, milling duration and rotational speed on the microstructure and oxidation behavior of coating were investigated. The results showed that the continuity and the anti-oxidation properties of the coating were enhanced with the increase of Al–Si weight ratio. The thickness of the coating largely increased in the initial 5-hour milling process and decreased with further milling. A rather long-time ball milling could result in the generation of microdefects in coating, which had an adverse effect on the oxidation resistance of coating. Both the thickness and the roughness of the coating increased with the raise of rotational speed. The low rotational speed would lead to the formation of discontinuous coating. The rotational speed had a limited effect on the coating oxidation behavior. Dense, continuous and high-temperature protective Al–Si coatings could be obtained by mechanical alloying with Al–33.3?wt.%Si powder at the rotational speed ranging from 250 to 350?rpm for 5?h.     

Assessment of diffusion barriers for Ti–SiC composites

Abstract

The application of chemical vapour deposition and physical vapour deposition coatings, either singly or in combination, onto SiC fibres is discussed in terms of their ability to enhance the high temperature stability of Ti–SiC composites. The thermal stability and success of potential barrier layers was assessed by studying the fibre-matrix interdiffusion and measurement of the mechanical properties of individual fibres following coating and thermal exposure. Measurements of the level of strength retention have proved to be a reliable method of assessing the effectiveness of potential diffusion barriers. Failures may result from one of three sources. For high strength fibres failures are SiC–core reaction zone initiated, for intermediate strength fibres failures are surface defect (SiC) initiated and for low strength fibres, failures are fibre–matrix reaction zone or coating initiated. To ensure high strength (i.e. core failures) it is essential that a carbon layer is retained at the SiC surface. The most successful barriers have been shown to be TiB₂ and PtAl₂ coatings preventing outward diffusion of carbon and minimising the interaction with the titanium matrix. From these results a life prediction model has been developed based on the fibre–coating interaction, which will predict fibre strength as a function of time at a given temperature.

MST/3001     

Effect of rolling temperature on the evolution of defects and properties of an Al–Cu alloy

The defects and properties of a precipitation hardening Al–Cu alloy 2017 were studied after rolling at room temperature (RT) and cryogenic (liquid N₂) temperature (CT). It is found that CT rolling produced practically the same hardness as RT rolling for a wide range of rolling strains. However, electrical resistivity measurement revealed a clear difference indicating different defect structures in the CT- and RT-rolled samples. This difference led to higher hardness, after subsequent ageing, for samples processed by CT rolling. It is deduced that precipitation occurred during RT rolling, which compensated for the effect of lower dislocation density (evaluated from X-ray diffraction) in RT-rolled sample, and consequently resulted in similar hardness in both RT- and CT-rolled samples. It is noted that after ageing, CT-rolled sample has higher strength (~35%) than the standard T4 treatment.     

Tensile behaviour of powder metallurgy processed (Al–Cu–Mg)/SiCp composites

Abstract

The tensile behaviour of Al–Cu–Mg alloy matrix composites produced by a powder metallurgy process was investigated as a function of particle size in the as extruded, homogenised, and peak aged conditions. The tensile behaviour of the corresponding matrix alloy which was produced in a similar manner, designated as Control, was also studied. There was a significant increase in the 0.2% yield strength of Control and all the metal matrix composites (MMCs) after homogenisation treatment (53–68%) and peak aging (93–109%), as compared to their values in the as extruded condition. The ultimate tensile strength (UTS) of Control as well as the MMCs also increases considerably after homogenisation treatment (39–70%), however, subsequent peak aging did not result in any further increase in UTS in case of any of the MMCs. It was found that the finer the reinforcement size, the higher the 0.2% yield strength and UTS in all the conditions. On the other hand, ductility decreased considerably after homogenisation treatment and subsequent peak aging. The results are discussed in the light of dislocation strengthening as well as reinforcement damage.     

Development of rolling textures in Cu–Ni alloys

Abstract

The development of texture during the cold rolling of Cu–12·5Ni and Cu–27Ni (wt-%) alloys has been studied using X-ray analysis and transmission electron microscopy (TEM). Pole figures and diffractometer intensity measurements from rolling sections confirm that the texture is of the ‘copper’ type, although the preferred orientation develops more slowly and is consequently less sharp than in the pure metal at equivalent strains. The microstructures were consistent with deformation by slip, no evidence of mechanical twinning being found despite the greater hardness of the alloys compared with copper. However, the presence of nickel in solid solution was found to alter the deformation sequence observed by TEM. Beyond 80% reduction (ε=2·0), the cell structure characteristic of deformed copper, both at low and high strains, was almost entirely replaced by an assembly of small, slightly elongated crystallites whose boundaries often lay at ~±35° to the rolling direction. Long microbands, associated with fine scale rippling in the optical microstructure, appeared after only ~90% reduction (ε=2·5), there being a much reduced tendency for such lamellae to group into transition bands than in copper. Compared with the pure metal, the macroscopic deformation of cupronickels thus proceeds more homogeneously, although larger orientation differences, e.g. of ~10;°, as measured by a precision convergent beam technique, existed between adjacent crystallites, adjacent microbands, and across crystallite/microband boundaries. Possible causes of these differences of behaviour in the alloys are discussed and related to the higher hardness and work hardening rates of Cu–Ni alloys.

MST/499     

Effects of Al–8.5Si–25Cu–xY filler metal foils on vacuum brazing of SiCp/Al composites

Rapidly solidified Al–8.5Si–25Cu–xY (wt-%, x?=?0, 0.05, 0.1, 0.2, 0.3, 0.4, and 0.5) foils were used as filler metal to braze Al matrix composites with high SiC particle content (SiCp/Al-MMCs), and the filler presented fine microstructure and good wettability on the composites. The joint shear strength first increased, then decreased and a sound joint with a maximum shear strength of 135.32?MPa was achieved using Al–8.5Si–25Cu–0.3Y as the filler metal. After Y exceeded 0.3%, a needle-like intermetallic compound, Al3Y, was found in the brazing seam, resulting in a dramatic decline in the shear strength of the brazed joints. In this research, the Al–8.5Si–25Cu–0.3Y filler metal foil was found to be suitable for the brazing of SiCp/Al-MMCs with high SiC particle content.     

Processing of Nb–Cu metal matrix composites

Abstract

During the development of new processing routes for Nb₃Sn superconductor, factors influencing the workability of two-phase metallic composites have been investigated. The ease with which such composites can be fabricated depends strongly on the relative hardnesses of the phases. Production of a regular, uniform filamentary structure is promoted by low hardness ratios in the initial composite.

MST/547     

Synthesis and microstructural characterization of Al–Ni3Al composites fabricated by press-sintering and shock-compaction

Aluminum matrix composites reinforced with nanocrystalline Ni3Al intermetallic particles, were synthesized using powder metallurgy techniques. Nanocrystalline Ni3Al was obtained by mechanical alloying of Ni75–Al25 stoichiometric mixture from elemental powders after 900 ks of milling with a 5 nm grain size average. Mixture powders of aluminum with 0.007, 0.02 and 0.04 volume fractions of Ni3Al intermetallic particles were compacted using two different compaction methods, the cold isostatic press and sintered at 873 K and the shock-compaction technique. Microstructure of shock-compacted composites showed fine particles of a few microns and also coarse particles less than 100 μm homogeneously distributed on the matrix, also the presence of micro-cracks and low porosity. However the nanoscale features of intermetallic was retained. On the other hand, the press and sintered composites showed good densification. The densities of the composites were about 90% and 94% of the theoretical density for the shock-compacted and press-sintered process, respectively. Finally, the results of hardness measurements showed that the nanocrystalline Ni3Al reinforcement improves the hardness of Al matrix for all conditions. The highest hardness was obtained for the Al–4 vol.%Ni3Al shock-compacted composite.     

Processing and properties of Al–Li–SiCp composites

Al–Li–SiC_p composites were fabricated by a modified version of the conventional stir casting technique. Composites containing 8, 12 and 18 vol% SiC particles (40 mm) were fabricated. Hardness, tensile and compressive strengths of the unreinforced alloy and composites were determined. Ageing kinetics and effect of ageing on properties were also investigated. Additions of SiC particles increase the hardness, 0.2% proof stress, ultimate tensile strength and elastic modulus of Al–Li–8%SiC and Al–Li–12%SiC composites. In case of the composite reinforced with 18% SiC particles, although the elastic modulus increases the 0.2% proof stress and compressive strength were only marginally higher than the unreinforced alloy and lower than those of Al–Li–8%SiC and Al–Li–12%SiC composites. Clustering of SiC particles appears to be responsible for reduced the strength of Al–Li–18%SiC composite. The fracture surface of unreinforced 8090 Al-Li alloy (8090Al) shows a dimpled structure, indicating ductile mode of failure. Fracture in composites occurs by a mixed mode, giving rise to a bimodal distribution of dimples in the fracture surface. Cleavage of SiC particles was also observed in the fracture surface of composites. Composites show higher peak hardness and lower peak ageing time compared with unreinforced 8090Al alloy. Macroand microhardness increase significantly after peak ageing. Ageing also results in considerable improvement in strength of the unreinforced 8090Al alloy and its composites. This is attributed to formation of δ^' (Al₃Li) and S^' (Al₂CuMg) precipitates during ageing. Per cent elongation, however, decreases due to age hardening. Al–Li–12%SiC, which shows marginally lower UTS and compressive strength than the Al–Li–8%SiC composite in extruded condition, exhibits higher strength than Al–Li–8%SiC in peak-aged condition.     

Microstructure evolution and mechanical properties of Nb-alloyed Cu–Zr–Al–Ni large-sized metallic glass composites

The effects of Nb on the microstructures and mechanical properties of large-sized (Cu_0.47Zr_0.47Al_0.06)_99???xNi₁Nb_x (x?=?0, 0.5, 1, 2?at.-%) bulk metallic glass composites were investigated. It is verified that the liquidus temperature (T_l) of the Nb-added alloys decreases to cause the increase of glass-forming ability (GFA). The addition of Nb adjusts the distribution and the volume fraction of B2-CuZr phase in the Cu–Zr–Al–Ni large-sized composites by changing the GFA of the alloys. The mechanical properties of the composites strongly depend on the volume fraction and distribution of B2-CuZr phase in the glassy matrix. The alloy with 0.5?at.-% Nb addition exhibits the high mechanical properties, which should be attributed to the uniform distribution and the proper volume fraction of B2-CuZr phase in the glassy matrix.     

Strategical parametric investigation on manufacturing of Al–Mg–Zn–Cu alloy surface composites using FSP

Friction stir processing (FSP) is a unique approach being presently researched for composite fabrication. In the present investigation, Al-B₄C surface composite was fabricated through FSP by incorporating B₄C powder particles into Al–Mg–Zn–Cu alloy (AA 7075) matrix. The influence of varying powder particle reinforcement strategies on the microstructure, powder distribution, microhardness, and wear resistance of the surface composite is reported. In addition, AA 6061/B₄C composites were prepared using the same parameter set and the powder distribution in the composite was compared to that in the AA 7075/B₄C composite. More homogeneous dispersion of B₄C powder was observed in AA 6061 as compared to AA 7075 substrate. Among the prepared AA 7075/B₄C composites, the best B₄C powder distribution was detected in samples processed using fine powder and incorporating the change in stirring direction between passes. The hardness and wear resistance of the prepared composites were almost doubled attributing to several strengthening mechanisms and B₄C powder distribution in the AA 7075 matrix.     

Nanocrystalline matrix TiC–Al3Ti and TiC–Al3Ti–Al composites produced by reactive hot-pressing of milled powders

Mechanically alloyed nanocrystalline TiC powder was short-term milled with 40 vol.% of Al powder. The powders mixture was consolidated at 1200 °C under the pressure of 4.8 GPa for 15 s and at 1000 °C under the pressure of 7.7 GPa for 180 s. The bulk materials were characterised by X-ray diffraction, light and scanning electron microscopy, energy dispersive spectroscopy, hardness, density and open porosity measurements. During the consolidation a reaction between TiC and Al occurred, yielding an Al₃Ti intermetallic. The microstructure of the produced composites consists of TiC areas surrounded by lamellae-like regions of Al₃Ti intermetallic (after consolidation at 1200 °C) or Al₃Ti and Al (after consolidation at 1000 °C). The mean crystallite size of TiC is 38 nm. The hardness of the TiC–Al₃Ti and TiC–Al₃Ti–Al composites is 13.28 GPa (1354 HV1) and 10.22 GPa (1041 HV1) respectively. The produced composites possess relatively high hardness and low density. The results obtained confirmed satisfactory quality of the consolidation with keeping a nanocrystalline structure of TiC.     

Synthesis of Cu–Zr–Al/Al2O3 amorphous nanocomposite by mechanical alloying

Two routes were used to produce Cu–Zr–Al/Al₂O₃ amorphous nanocomposite. First route included mechanical alloying of elemental powders mixture. In second route Cu₆₀Zr₄₀ alloy was synthesized by melting of Cu and Zr. Cu₆₀Zr₄₀ alloy was then ball milled with Al and CuO powder. It was not possible to obtain a fully amorphous structure via first route. The mechanical alloying of Cu₆₀Zr₄₀, Al and CuO powder mixture for 10 h led to the reaction of CuO with Al, forming Al₂O₃ particulate, and concurrent formation of Cu₆₂Zr₃₂Al₄ amorphous matrix. The thermodynamical investigations on the basis of extended Miedema’s model illustrated that there is a strong thermodynamic driving force for formation of amorphous phase in this system. Lack of amorphization in first route appeared to be related to the oxidation of free Zr during ball milling.     

Fabrication of ultrafine W – Cu powders by mechanical alloying

Abstract

Ultrafine composite powders of W – 15 wt-%Cu, W – 25 wt-%Cu, and W – 35 wt-%Cu have been fabricated by mechanical alloying. The effects of type of mill, process control agent, temperature of milling, and ball/powder ratio on the final products have been evaluated. The results show that the planetary ball mill possesses a higher impact energy intensity than that of the vibratory ball mill. The optimum milling time is confirmed by the formation of a nanocrystalline microstructure in the planetary ball mill after optimisation of the milling parameters. A steady state between cold welding and fracture is attained with a milling time of up to 25 h in the planetary ball mill under optimised conditions. Crystallites with sizes of 7 – 8 nm for W – Cu composite powders have been obtained after 25 h of ball milling. The powders obtained after mechanical alloying have been characterised in terms of their size, shape, phase constitution, and microstructural features using X-ray diffraction and scanning electron microscopy.     

Synthesis and characterisation of nanostructured Al–Al3V and Al–(Al3V–Al2O3) composites by powder metallurgy

In this study, the formation and characterisation of Aluminium (Al)-based composites by mechanical alloying and hot extrusion were investigated. Initially, the vanadium trialuminide (Al₃V) particles with nanosized structure were successfully produced by mechanical alloying and heat treatment. Al₃V–Al₂O₃ reinforcement was synthesised by mechanochemical reduction during milling of V₂O₅ and Al powder mixture. In order to produce composite powders, reinforcement powders were added to pure Al powders and milled for 5?h. The composite powders were consolidated in an extrusion process. The results showed that nanostructured Al-10?wt-% Al₃V and Al-10?wt-% (Al₃V–Al₂O₃) composites have tensile strengths of 209 and 226?MPa, respectively, at room temperature. In addition, mechanical properties did not drop drastically at temperatures of up to 300°C.