Role of magnesium in cast aluminium alloy matrix composites

Wetting between the dispersoid and the matrix alloy is the foremost requirement during the preparation of metal matrix composites (MMC) especially with the casting/liquid metal processing technique. The basic principles involved in improving wetting fall under three categories: (i) increasing the surface energies of the solids, (ii) decreasing the surface tension of the liquid matrix alloy, and (iii) decreasing the solid/liquid interfacial energy at the dispersoid matrix interface. The presence of magnesium, a powerful surfactant as well as a reactive element, in the aluminium alloy matrix seems to fulfil all the above three requirements. The role played by magnesium during the synthesis of aluminium alloy matrix composites with dispersoids such as zircon (ZrSiO₄), zirconia (ZrO₂), titania (TiO₂), silica (SiO₂), graphite, aluminium oxide (Al₂O₃) and silicon carbide (SiC), has been analysed. The important role played by the magnesium during the composite synthesis is the scavenging of the oxygen from the dispersoid surface, thus thinning the gas layer and improving wetting and reaction-aided wetting with the surface of the dispersoid. The combinations of magnesium and aluminium seem to have some synergistic effect on wetting.     

Wetting behaviour of SiC ceramics: Part II—Y2O3/Al2O3 and Sm2O3/Al2O3

Wettability is the most significant phenomenon in SiC liquid phase sintering. The wetting of Y₂O₃/Al₂O₃ and Sm₂O₃/Al₂O₃ on SiC was analysed by the “Sessil drop” method. The wetting of liquid on solid during liquid phase sintering is very important. The behaviour of the additive on the SiC plate was observed using an imaging system with a CCD camera, and the contact angle measurements were analysed by Qwin Leica software. The samples were cut transversally and characterized by scanning electron microscopy and X-ray spectrometry (SEM/EDS). The wetting was found to be strongly influenced by the temperature; the SiC/additive contact angle decreased with increasing temperature. The YA and SA additives presented low contact angle values, indicating their good wetting on SiC in the argon atmosphere. The contact angle could not be measured when the test was performed in the nitrogen atmosphere because bubbles formed in the liquid during the test. The best atmosphere for this sintering was found to be argon, which allows uniform spreading.     

Benetzungswinkel und Grenzflächenenergien der geschmolzenen Metalle Bi,Pb, Cu und Ni in Kontakt mit festem Al2O3

Wetting Angles and Interfacial Energies of the Liquid Metals Bi, Pb, Cu and Ni in Contact with Solid Al₂O₃ Sessile drop experiments were used to determine the wetting angle (θ) between polycrystalline Al₂O₃ and the liquid metals Bi, Pb, Cu and Ni in argon-atmosphere. It was found that at their melting points the liquid metals do not wet the Al₂O₃ (θ > 90°). Using available literature data the work of adhesion and the interfacial energies in the investigated systems were calculated. Both show a linear dependence with temperature. The temperature coefficients were calculated. In the case of Cu and Ni the wetting behaviour is improved (θ < 90°) at higher temperature below the melting point of Al₂O₃.     

Synthesis of an Al2O3/Al co-continuous composite by reactive melt infiltration

The route for the fabrication of an Al₂O₃/Al co-continuous composite by reactive melt infiltration was investigated using scanning electron microscopy, energy dispersive X-ray microanalysis and X-ray diffraction analysis. It was found that in the process of molten aluminium infiltration into the SiO₂ preform, the chemical reaction of 3SiO₂ + 4Al → 2Al₂O₃ + 3Si occurred at the infiltration front, and generated a transition zone containing a new type of continuous porosity about 100 μm in width. The reaction continued with further infiltration of molten aluminium alloy into this porosity which reacted with the residual SiO₂ until all the SiO₂ was transformed into Al₂O₃. A comparison was made between this route and that by direct infiltration of molten aluminium alloy into the open porosity of an Al₂O₃ preform. As a result of the increased wetting ability of the molten aluminium alloy by the chemical reaction, reactive melt infiltration took place at a higher rate for the SiO₂ preform than that for the direct infiltration of the Al₂O₃ preform. A fracture surface examination demonstrated a toughening effect provided by the continuous aluminium alloy in the composite.     

A novel approach to determine high temperature wettability and interfacial reactions in liquid metal/solid interface

In many technical processes, high temperature wetting of a liquid metal phase on a solid substrate occurs via an extensive chemical reaction and the formation of a new solid compound at the interface. For instance, good adhesion of the zinc coating to the steel surface is one of the most important requirements that the hot-dip galvanizing process has to fulfill. Good adhesion directly depends on the formation of a defect-free Fe₂Al₅ inhibition layer at the interface. The complex surface chemistry of oxides on the steel surface which is a result of segregation and selective oxidation upon recrystallization annealing significantly influences the kinetics of the correlated reactive wetting. This article presents the development of a novel advanced technique for the investigation of high temperature wetting process up to a temperature of 1100 K and provides first new insights in the mechanisms of the reactive wetting process in presence of oxides on the surface. The method is based on the sessile drop method with an additional spinning technique to get rid off the liquid metal phase at any chosen wetting time, thusly opening the way to access the interfacial reaction layer directly. The presented work focuses on model alloys of interest which are mainly relevant to the industrial steel grades. Emphasis is put both on the wettability of liquid Zn and on the interfacial reactions during reactive wetting process. Insights into such reactive phenomena are fundamental demand to improve the hot-dip galvanizability of advanced high strength steel grades.     

Wetting and adhesion of Ni-Al alloys on α-Al2O3 single crystals

The effect of Al additions on the wetting and adhesion of Ni on an -Al₂O₃ single crystal was studied. Contact angles were measured by the sessile drop technique under vacuum or in He atmosphere. The morphological and chemical features of metal-vapour and metal-oxide interfaces were determined by scanning electron microscope (SEM), microprobe analysis and profilometry. The work of adhesion of Ni-Al alloys on Al₂O₃ substrates was significantly higher than for pure Ni and Al components. This result was explained by co-operative adsorption of aluminium and oxygen atoms at the Ni-Al₂O₃ interface. The influence of oxidation of the alloy on wetting and bonding is also discussed.     

Contribution to the study of reactive wetting in the CuTi/Al2O3 system

The wetting (kinetics of spreading and stationary contact angles) of CuTi alloys on monocrystalline alumina under high vacuum, at a temperature of 1373 K, by the sessile drop technique was investigated. The morphological and chemical characteristics of the metal-ceramic interface were determined by scanning electron microscopy and microprobe analysis. When the results are analysed, three distinct effects of the Ti solute on wetting can be identified and evaluated semi-quantitatively: (a) a reduction in the solid-liquid interfacial tension by adsorption into the liquid side of the interface; (b) a reduction in this tension by formation of a TiO metallic-like oxide layer in the solid side of the interface; (c) a contribution to the wetting driving force due to the free energy released at the interface by the reaction between Ti and Al₂O₃.     

Wettability and spreading kinetics of molten aluminum on copper-coated ceramics

The purpose of this study was to investigate the influence of Cu-coating on the spreading kinetics and equilibrium contact angles of aluminum on ceramics using a sessile drop technique. Al₂O₃ and SiC plates were coated by electroless plating. The copper film overcomes the low wetting of the uncoated samples by dissolution in the drop at 800 °C in argon, showing an intrinsically favorable effect on the adhesion energy. Just after 2 min, the contact angle decreased to 12.6° and 26°for Al/Cu–Al₂O₃ and Al/Cu–SiC, respectively. However, a de-wetting behavior was observed, reaching equilibrium contact angles of 58.3° and 45.5° for the couples. The dissolution reaction rate at the triple junction was so high that the spreading process was controlled by local diffusion rather than chemical reaction kinetics.     

Facetting and optical perfection in Czochralski grown garnets and ruby

Studies of ruby (Al₂O₃/Cr³⁺) and rare-earth aluminium garnet single crystals, in particular the mixed garnets formed between Y₃Al₅O₁₂ (YAG) and Dy₃Al₅O₁₂ (DyAG), have shown that the formation of facets on the solid/liquid interface, which give rise to a strained central core within the crystals, is dependent upon the shape of the solid/liquid interface. Development of the strained and facetted core can be prevented by modifying the growth conditions to produce a flat solid/liquid interface and as a result the optical perfection of the crystals is greatly improved. Certain crystals, e.g. DyAG, grow naturally with aflat interface, and in the present work this has been shown to be due to the optical absorption characteristics of this material. In other materials, e.g. YAG and ruby, the interface shape can be controlled by the rate at which the growing crystal is rotated. The changes in temperature gradient produced in a YAG melt by changes of crystal rotation rate have been measured, and their effect upon crystal perfection is described.     

Migration of liquid phase in low temperature sintering of AlN

In the process of low-temperature sintering of AlN ceramics, the reaction of the sintering aids YF₃ and CaF₂ with superficial Al₂O₃, inherently contained in AlN lattice, results in formation of liquid phase. Nevertheless, the uniformly dispersed liquid phase is prone to migrate from the bulk to the surface of the samples, opposing densification. The analysis of the experimental results indicates that fresh liquid phase can continuously arrive from the bulk to the surface due to chemical reactions and crystallization which occur at the surface as well as wettalibilty and capillarity phenomena. The surface is depleted of liquid phase since the latter is consumed due crystallization and carbothermal reduction reactions with the elements of the atmosphere of the furnace N₂ and C, resulting in formation of a dense layer of crystals of Al₂Y₄O₉, CaYAl₃O₇ and Y₂O₃, grown perpendicularly to the surface. The chemical and structural features of this newly formed crystalline surface layer generate a significant difference of the wetting regimes and the capillary forces between the surface and the bulk, favouring pumping of the liquid from the bulk to the surface.     

Wetting improvement of carbon or silicon carbide by aluminium alloys based on a K2ZrF6 surface treatment: application to composite material casting

A surface treatment with aqueous solutions of K₂ZrF₆ has been carried out to improve, in dramatic manner, the wetting of carbon (or SiC)-base ceramics by liquid light alloys at low temperatures (i.e. within the 700 to 900°C range). The mechanism which is thought to be responsible for the wetting improvement involves two steps: (i) K₂ZrF₆ reacts with aluminium with the formation of K₃AlF₆, other complex fluoride species and intermetallics, (ii) K₃AlF₆ dissolves the alumina thin layer, coating the liquid light alloy and enables the wetting of the ceramics. The mechanism has been worked out from sessile drop experiments, solid state chemistry experiments and composite casting. The K₂ZrF₆ surface treatment appears to be particularly suitable for processing composite materials made of carbon (or SiC) fibrous preforms and aluminium-base matrices according to techniques directly derived from the light alloy foundry.     

Effect of electric fields on solid-state reactions between oxides

Interdiffusion in sintered polycrystalline pellets of MgO and Al₂O₃ at temperatures in the range 1300 to 1450° C was shown by scanning electron microscope energy dispersive X-ray probe (EDAX) and X-ray diffraction techniques to be enhanced by the application of direct current electric fields, particularly when the alumina pellet is of negative polarity, under which conditions the diffusion activation energy of Mg in Al₂O₃ is significantly increased. The diffusion process involved, principally, the transport of Mg by grain-boundary diffusion and possibly vapour-phase transport into the interstices of the comparatively porous alumina pellets; only minimal counter-migration of aluminium was observed in these experiments. The Mg-content of the discrete spinel grains within the reaction interface region is increased by electrolysis above the values calculated from solid solubility data, particularly where the alumina pellet is of positive polarity. The excess Mg is most apparent at the side of the spinel grains nearest the reaction interface, and precipitates as a surface MgO layer on the grains. The effective electrical mobility of the migrating species was calculated from the down-field shift of the diffusion profile and compared with values calculated from the electrical conductivity of the system.     

Compatibility between alumina fibres and aluminium

An investigation is carried out on the interfacial wetting behaviour and reactions between aluminium and alumina fibres (85mass% Al₂O₃ and 15mass% SiO₂). Aluminium is coated onto alumina fibres by a vacuum evaporation technique and the surface of the fully coated fibres and the edge of the partially coated fibres are examined by scanning electron microscope after heat treatments at various temperatures. Within a temperature regime between 943 and 1273 K, occurrence of such interfacial reactions as 4Al(I) + Al₂O₃(s) 3Al₂O₃(g) and 4Al(I) + 3SiO₂(s) 2Al₂O₃(s) + 3Si(s) are detected. It is found that molten aluminium can cover the alumina fibre surface but it peels off near the edge of the coating film on a partially coated fibre, showing the very weak interface cohesion. This is ascribed to the lack of a stable compound formation at the interface. Results of tensile test show that the strength of the coated fibres is degraded after heat-treating at above the melting point of aluminium. The culprits for the tensile failure of alumina fibres are evaluated by the Weibull distribution theory.     

Formation of alkoxy-derived yttrium aluminium oxides

Monoclinic Y₄Al₂O₉ and hexagonal YAlO₃ crystallize at low temperatures from amorphous materials prepared by the hydrolysis of yttrium and aluminium double alkoxides. Hexagonal YAlO₃ transforms to the cubic phase with a garnet structure as an intermediate product at elevated temperatures. The formation process of YAlO₃ is described. Solid solutions of hexagonal YAlO₃ crystallize between 50 and 62.5 mol % Al₂O₃. Yttrium aluminium garnet Y₃Al₅O₁₂(YAG) is formed by transformation of the solid solution.     

Surface patterning nanoparticle-based arrays

In this study, focused ion beam lithography is used to pattern different size and shape island arrays on silicon wafers. Cavity arrays of inverse shapes are then made on silicone mold surfaces by polymerization. After that, Al₂O₃ nanoparticle-based island arrays are created by a surface feature transfer and freeze casting process using an Al₂O₃ colloidal suspension. The effects of silicone mold surface wettability and freezing rate on the Al₂O₃ nanoparticle pattern quality are investigated. The results show that coating the silicone mold surface with a 10 nm thick Au–Pt layer makes the Al₂O₃ nanoparticle suspension more wetting on the mold surface and also likely reduces the dry Al₂O₃ nanoparticle adhesion to the mold surface. Freezing rate should be lower than 1 °C/min to avoid cracks or loose Al₂O₃ nanoparticle packing in the freeze cast features. When these factors are properly controlled, the reported patterning process allows reproduction of micron-size feature arrays from Al₂O₃ nanoparticle suspensions. The studied approach should be applicable to most nanoparticle-based materials and open numerous opportunities for direct-device fabrication.     

Aluminization of nickel-formation of intermetallic phases and Ni2Al3 coatings

The occurrence and growth mechanisms of the various intermetallic phases of the Al-Ni system formed during pack aluminization of unalloyed nickel have been investigated with respect to the aluminium activity in the pack. Several types of coatings were obtained: (1) a Ni₂Al₃ coating formed by inward aluminium diffusion in a high activity cement of pure aluminium; (2) a Ni-rich NiAl coating formed by outward nickel diffusion in a low activity pack constituted by an Al-Ni alloy; (3) a mixed type of coating exhibiting the phases Ni₂Al₃, Al-rich NiAl, Ni-rich NiAl and Ni₃Al in four superposed layers, formed in a pack containing an Al-Cr alloy; (4) a high temperature, high activity type of coating formed above 950° C with an outer layer exhibiting a hypereutectic structure of NiAl₃ grains in a eutectic matrix due to precipitation from the liquid state. The optimum cementation conditions, for the production of maximum thickness and quality Ni₂Al₃ coatings were determined. The influence of surface reactivity and pack activity on the coating quality parameters was investigated.     

Study of the Ti/Al2O3 interface

The Ti/Al₂O₃ (1 ¯1 0 2) interface formation has been investigated by X-ray photoelectron spectroscopy and Auger electron spectroscopy (AES). The results showed that when an active metal titanium was evaporated on to a room-temperature Al₂O₃ (1 ¯ 1 0 2) surface in ultrahigh vaccum, a Ti/Al₂O₃ interface region of about 200 nm was formed, and in the first several monolayers of titanium, the titanium was oxidized due to the active oxygen anions on the surface. Therefore, the pure Ti/Al₂O₃ interface was replaced gradually by a titanium oxides/Al₂O₃ interface, which has a stronger interaction than the former. The change of shape of the photoemission lines and the shift of binding energy of aluminium, oxygen and titanium with increasing coverage of titanium showed that the formation of the Ti-O bond at the interface is due to titanium transferring its electrons to Al³⁺ via O^2– anions in the Al-O bond, whereby the Al³⁺ was reduced to metallic aluminium, Al⁰. The AES intensity profile also proved the existence of the reduced species Al⁰. This suggests that the reaction layer consists of a multiphasic mixture: the Ti-O type phase, the (Ti, Al)₂O₃ phase and metallic aluminium phase.     

Formation and hot isostatic pressing of ZrO2 solid solution in the system ZrO2-Al2O3

In the system of ZrO₂-Al₂O₃, cubic ZrO₂ solid solutions containing up to 40 mol% Al₂O₃ crystallize at low temperatures from amorphous materials prepared by the simultaneous hydrolysis of zirconium and aluminium alkoxides. At higher temperatures, they transform into tetragonal solid solutions. Metastable ZrO₂ solid solution powders containing 25 mol% Al₂O₃ have been sintered at 1000–1150 °C under 196 M Pausing the hot isostatic pressing technique. The solid solution ceramics consisting of homogeneous microstructure with an average grain size of 50 nm exhibited a very high fracture toughness of 23 MN m ^–1.5. They have been characterized by X-ray diffraction and electron probe surface analyses.     

The effect of substrate surface roughness on the wettability of Sn-Bi solders

The effect of substrate surface roughness on the wettability of Sn-Bi solders is investigated by the eutectic Sn-Bi alloy on Cu/Al₂O₃ substrates at 190 °C. To engineer the surface with different roughnesses, the Cu-side of the substrates is polished with sandpaper with abrasive number 100, 240, 400, 600, 800, 1200, and 1 m alumina powder, respectively. Both dynamic and static contact angles of the solder drops are studied by the real-time image in a dynamic contact angle analyzer system (FTA200). During dynamic wetting, the wetting velocity of the solder drop decreases for the rougher surface. However, the time to reach the static contact angle seems to be identical with different substrate surface roughness. The wetting tip of the solder cap exhibits a waveform on the rough surface, indicating that the liquid drop tends to flow along the valley. As the solder drops reach a static state, the static contact angle increases with the substrate surface roughness. This demonstrates that the wettability of solders degrades as the substrates become rough.     

Zone-melted unidirectional solidification of particulate dispersed composites

In order to solve the problem of particles settling and agglomeration in front of solidifying interface in unidirectional solidification (UDS) experiments, a zone-melted process has been utilized in this study. The experimental results show that, the melting zone could be kept in 30–40 mm width and the zone melted UDS experiments are realized with Al₂O₃ particle reinforced aluminum-matrix composites. But particle settling still occurs in the liquid, and becomes severe as the particle volume fraction decreases. However, when the volume fraction of the particles is more than 20–22 vol.%, no further settling occurs under a solidification rate of 8–16 mm/h. Investigation on the interaction of particles and solid/liquid interface reveals that the Al₂O₃ particles are rejected into liquid and pushed by the growing solid phase in Al₂O_3(P)/Al and Al₂O_3(P)/Al-0.23wt.%Ce composites. Some particles are mechanically entrapped between cells, and distributed along the crystal grain boundaries.