Study of 6061-Al2O3p composites produced by reciprocating extrusion

A reciprocating extrusion process was developed to consolidate 6061-Al₂O_3p composites from mixed powders. The 6061 alloy powder was first dehydrated in a vacuum chamber at 450 °C and then mixed with 12.5 μm Al₂O₃ powder in various volume fractions: 0, 5, 10, 20, and 30 pct. The mixed powders were hot pressed at 300 °C under a pressure of 300 MPa and finally extruded reciprocatingly 14 times at 460 °C. The results show that the composites were fully densified, with no sign of pores or oxide layers observable in the optical microscope. The Al₂O₃ particles were distributed uniformly in the matrix. As compared with 6061 alloys, the composites demonstrated a smaller precipitation hardening and elongation, but exhibited a higher Young’s modulus and a larger work hardening capacity. The degradation of precipitation hardening was due to the loss of Mg, which reacts with Al₂O₃ to form MgAl₂O₄. The large work-hardening capacity is attributable to the incompatibility between Al₂O₃ and the matrix, which possibly generates more dislocations to harden the matrix. The composites had much higher friction coefficients and greater wear resistances than the 6061 alloy against steel disc surface. The friction coefficient of the 6061-30 vol pct Al₂O_3p composite was double that of the 6061 alloy and the wear resistance was 100-fold. As compared with similar composites reported previously, these composites possessed much higher elongation at the same strength level. A 30 vol pct Al₂O_3p still displayed an elongation of 9.8 pct in the T6 condition. All of these improvements are attributed to the merits, including full densification of the bulk, uniform dispersion of the Al₂O₃ particles in the matrix, and strong binding between the Al₂O₃ particles and the matrix resulting from reciprocating extrusion.     

Interfacial Reactions in Al-Mg (5083)/SiCp Composites during Fabrication and Remelting

The interfaces of aluminum alloy composites (5083) reinforced by SiC particles (as-received, oxidized 3.04 wt pct and 14.06 wt pct) were studied. The composites were fabricated by compocasting and certain samples were also remelted at 800 °C for 30 minutes. The reaction mechanisms between SiC _p and liquid Al and between the SiO₂ layer and Al(Mg) are discussed. The crystal boundaries of the MgO (or MgAl₂O₄) reaction products are believed to be the diffusion paths (or channels) during the interfacial reactions. A SiO₂ layer, formed by oxidation of the SiC particles prior to their incorporation into the melt, plays an important role in preventing the SiC _p from being attacked by the matrix. The interfacial reaction products are affected by both the alloy composition and the thickness of the initial SiO₂ layer.     

Modification of the interface in SiC/Al composites

Methodologies both to avoid the formation of Al₄C₃ and to tailor the interfacial structures in a SiC/2014 Al composite were demonstrated. Modification of the interfacial structures in the SiC/2014 Al composite was made by forming SiO₂ layers on the surfaces of SiC via passive oxidation at elevated temperatures. In the 2014 Al composite reinforced with the oxidized SiC, MgAl₂O₄ and Si crystals were observed to be present at the interfacial region as a result of the reaction between the SiO₂ layer and the matrix. On the other hand, in the case of the 2014 Al composite reinforced with unoxidized SiC, SiC was found to react with the Al matrix to form both Al₄C₃ and Si. Qualitative measurements of the interfacial bonding strength were carried out on composites having various types of interfaces and thicknesses. Detailed interfacial structures and phase identifications, which were examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), were presented.     

Interfacial reactions in aluminosilicate short fiber-reinforced aluminum matrix composites

Aluminosilicate short fibers are one of the less expensive reinforcements used for the fabrication of metal matrix composites (MMCs). The present investigation evaluates the interfacial characteristics of Al-7Si-0.4Mg (356) alloy reinforced with 10 wt pct aluminosilicate short fibers using optical microscopy, electron microscopy, and X-ray analysis. The fibers used are standard- and zirconiagrade aluminosilicate short fibers. The interfacial analysis has shown the formation of MgAl₂O₄ and Si in both grades of fibers. In addition, ZrAl₃ formation is observed in the zirconia-grade fiber because of the interaction between the matrix and the dispersoid. The zirconia-grade fiber is more susceptible to interfacial reaction than the standard-grade fiber because of the presence of the highly reactive ZrO₂ phase and a lower amount of the Al₂O₃ phase, which provides resistance to the reaction.     

Interfacial characteristics for brazing of aluminum matrix composites with Al-12Si filler metals

Discussions concerning the interfacial reactions and characterizations in brazing aluminum matrix composites are documented in this study. Joints of alumina particulate reinforced 6061 aluminum matrix composites were made using an Al-12 (wt pct) Si filler metal by vacuum brazing. The resulted maximum bonding strengths were 75.4, 81.5, and 71.8 MPa for 10, 15, and 20 vol pct alumina reinforcement, respectively. The microstructural examinations revealed that the bonding strength was strictly related to the reinforced alumina particles and the reaction products presented at the joint interfaces. During brazing, Mg segregated at the joining interface and alumina/6061 Al interface. Further, reactions between alumina and 6061 Al matrix resulted in the formation of Mg-rich phases, such as MgAl₂O₄ and MgO, near the joining interface and the alumina reinforcement. The Si in the filler material penetrated into the metal matrix composites (MMCs) matrix and segregated at the alumina/6061 Al interfaces. This phenomenon can be confirmed by a joint between two alumina bulk specimens.     

Porosity nucleation in metal-matrix composites

The mechanism of porosity nucleation in pressure infiltration casting of metal-matrix composites (MMCs) is investigated. Five interfacial configurations are investigated for a variety of matrix/reinforcement systems. Interfaces with negative curvature such as cavity are found to be potential sites for porosity formation. The Al/Al₂O₃ system is most susceptible to porosity nucleation for the systems considered. Appropriate matrix alloying such as Mg in the Al/Al₂O₃ system and Mg and Cu in the Al/SiC system and reinforcement coatings such as Cu coating on SiC significantly reduce the contact angle, enhance wettability at the interface, and could be effective for suppressing porosity formation. Other effective methods include careful control of the cooling condition as well as the applied pressure.     

Aluminum matrix composites: Fabrication and properties

Aluminum alloy matrix composites containing 1 to 30 wt pct of fibrous and particulate nonmetals varying in size from 0.06 μm to 840 μm were fabricated. The composites were cast into cylindrical molds for friction and wear tests, hot extrusion and tensile tests. The distribution of the nonmetals in the cast ingots was homogeneous. Friction and wear tests were done on a pin (52100 bearing steel) and dish type machine without lubrication. It was found that composites containing ∼10 wt pct or more of SiC, TiC, Si₃N₄, Al₂O₃, glass, solid waste slag, and silica sand wear less than the pure matrix alloy, but have slightly higher average coefficients of friction. Wear in composites containing soft particles, especially MgO and boron nitride was higher than the pure matrix alloy. The average coefficient of friction of all the composites was in the range of 0.35 to 0.58. Increasing the sliding velocity reduced this range to ∼ 0.4 to 0.45. The longitudinal tensile properties of the extruded composites (with the exception of loss of ductility in some cases) are comparable to that of the matrix alloys. Improvements in strength or ductility were noted. For example, addition of 15 wt pct of 3 μm size Al₂O₃ particles raised the yield and ultimate strength of the Al-4 pct Cu-0.75 pct Mg alloy matrix from 227 to 302 MPa, and 356 to 403 MPa, respectively. The corresponding percent elongation decreased from 25.8 to 12.5. The fact that the various composites can be readily cast and hot formed suggests a variety of engineering applications. AKIRA SATO, formerly Visiting Scientist at Massachusetts Institute of Technology, Cambridge.     

Effect of Combined Addition of Cu and Aluminum Oxide Nanoparticles on Mechanical Properties and Microstructure of Al-7Si-0.3Mg Alloy

In this study, an ultrasonic cavitation based dispersion technique was used to fabricate Al-7Si-0.3Mg alloyed with Cu and reinforced with 1 wt pct Al₂O₃ nanoparticles, in order to investigate their influence on the mechanical properties and microstructures of Al-7Si-0.3Mg alloy. The combined addition of 0.5 pct Cu with 1 pct Al₂O₃ nanoparticles increased the yield strength, tensile strength, and ductility of the as-cast Al-7Si-0.3Mg alloy, mostly due to grain refinement and modification of the eutectic Si and θ-CuAl₂ phases. Moreover, Al-7Si-0.3Mg-0.5Cu-1 pct Al₂O₃ nanocomposites after T6 heat treatment showed a significant enhancement of ductility (increased by 512 pct) and tensile strength (by 22 pct). The significant enhancement of properties is attributed to the suppression of pore formation and modification of eutectic Si phases due to the addition of Al₂O₃ nanoparticles. However, the yield strength of the T6 heat-treated nanocomposites was limited in enhancement due to a reaction between Mg and Al₂O₃ nanoparticles.     

Interface interactions during fabrication of aluminum alloy-alumina fiber composites

The feasibility of fabricating fiber-reinforced aluminum alloys by addition of discontinuous fibers to vigorously agitated, partially solid metal slurries was investigated. In the first phase of the program, reported herein, emphasis was placed on the study of interface interactions between polycrystalline A1₂O₃ fibers and Al-2 to 8 pct Mg, Al-4.5 pct Cu and Al-4.5 pct Cu-1 to 2 pct Mg alloys. In general, it was observed that the incorporation of fibers could be readily achieved by this technique, and that fibers appeared wetted after a few minutes of contact with the melt. The composites produced exhibited an intimate, void free bond between the constituents. In addition, a region of significantly altered microstructure resulted from accumulation of oxide and/or aluminate particles which either formed within the melt and were attached to the moving fibers, or used the fiber surface as a substrate to grow on. Microscopic examination of this interaction zone and thermodynamic considerations indicate that it consists of fine α-Al₂O₃, aluminates, oxides of the alloying elements, and probably some intermetallic compounds. For example, it is shown that a stable MgAl₂O₄ spinel forms at the interface of A1₂O₃ fibers and Al-Mg alloys. Examination of composite specimens fractured under tension indicated that the interfaces produced were strong enough to permit the transfer of loads at strengths in the order of 250 to 350 MPa.     

Internal friction in AlCu-Al2O3 metal-matrix composites

In metal-matrix composites (MMCs), the metal matrix is exposed to plastic deformation and damage accumulation in the region close to the reinforcements, following mechanical or thermal stress. In this connection, Al-4 wt pct Cu-based MMCs reinforced with 20 vol pct Al₂O₃ fibers were characterized by internal friction (IF) measurements. The IF measurements as a function of the vibration amplitude present a solid friction peak connected with the loosening of metal-fiber interfaces, while plastic deformation was associated with a high amplitude IF background. On this basis, IF measurements allowed us to identify the distribution of internal stresses and damage accumulation at matrix-fiber interfaces or plastic flow in the matrix in different thermomechanical conditions. Furthermore, IF measurements allowed damage accumulation consequent to mechanical fatigue to be followed.     

Combustion synthesis of Ni3Al and Ni3Al-matrix composites

The self-propagating mode of combustion synthesis (SHS) of Ni₃Al starting from compacts of stoichiometrically mixed Ni and Al powders readily forms fully reacted structures with about 3 to 5 pct porosity, if green density of the compacts is greater than about 75 pct of theoretical. SHS-produced Ni₃Al matrix composites with up to 2 wt pct A1₂O₃ whiskers also have relatively low porosity levels. Porosity increases rapidly with lower green densities, higher Al₂O₃, or SiC whisker contents, and the degree of reaction completeness diminishes. The SiC whiskers undergo reaction with the matrix, while Al₂O₃ whiskers are nonreactive. All of these observations correlate well with temperature measurements made during the course of the reaction. The SHS mode can be achieved with agglomerated particle size ratioD _Al/D _Ni ≥ 1, larger than the limit established from studies of the thermal explosion mode of combustion synthesisD _Al/D _Ni ≃ 0.3. This paper is based on a presentation made in the symposium “Reaction Synthesis of Materials” presented during the TMS Annual Meeting, New Orleans, LA, February 17–21, 1991, under the auspices of the TMS Powder Metallurgy Committee.     

Characterization of a diffusion-bonded Al-Mg alloy/SiC interface by high resolution and analytical electron microscopy

The interfacial structure of a diffusion-bonded Al-4.55 at. pct Mg/SiC interface was examined by conventional and high-resolution transmission electron microscopy. Formation of Mg₂Si, MgO, and Al₂MgO₄ was observed. The monoclinic Mg₂Si phase formed at the Al/SiC interface, while the oxides MgO and Al₂MgO₄ formed at the monoclinic Mg₂Si/Al interface. It is shown that the formation of these phases can be predicted using simple thermodynamic criteria such as the relative bond strengths between Al, Si, C, O, and Mg. In addition, precipitation of some equilibrium Al₈Mg₅ precipitate was also observed at the interface. The interfacial structure observed in the Al-Mg/SiC system is contrasted with that observed in the pure Al/SiC system.     

Intermetallic-reinforced light-metal matrix in-situ composites

Transition-metal trialuminide intermetallics such as Al₃Zr and Al₃Ti, having low densities and high elastic moduli, are good candidates for the in-situ reinforcement of light-metal matrices based on Al and Mg alloys. In this work, in-situ composites based on Al and Al-Mg matrices reinforced with an Al₃Zr intermetallic were successfully processed by conventional ingot metallurgy. The microstructural studies showed that “needle” or “feathery”-like particles of Al₃Zr phase, whose volume fraction increased with increasing concentration of Zr, were formed in the Al matrix in the investigated range of Zr contents from 0.9 to 11.6 at. pct. Properties of Al-Zr alloys were investigated as a function of volume fraction of Al₃Zr. It is shown that the density, hardness, and yield strength of the in-situ Al/Al₃Zr composites can be quite adequately described by the composite rule-of-mixtures (ROM) behavior. Alloying of a binary Al-2.4 at. pct Zr alloy with Mg up to ∼25 at. pct reduces profoundly its density and, additionally, strengthens the matrix by a Mg solid-solution strengthening mechanism.     

Foamability of MgAl2O4 (Spinel)-Reinforced Aluminum Alloy Composites

A novel foamable aluminum alloy has been developed. It contains sub-micron-sized MgAl₂O₄ (spinel) particles that are generated in situ by a reaction of SiO₂ with a molten Al-Mg alloy. The study involves an optimization of parameters such as Mg concentration, SiO₂ particles size, and reaction time and shows that a composite containing MgAl₂O₄ particles as chief reinforcement in the matrix leads to effective foaming. Composites containing large sized transition phases and particle agglomerates in the matrix yield poor foam structure. The best foamable composite obtained contained 3.4 vol. pct of ultrafine (80 nm to 1 μm) MgAl₂O₄ particles uniformly distributed in an Al-Si alloy matrix. The corresponding metal foam contained 75 pct porosity and exhibited a uniform distribution of cells.     

Fabrication of Al-3.7?Pct Si-0.18?Pct Mg Foam Strengthened by AlN Particle Dispersion and its Compressive Properties

Al-3.7 pct Si-0.18 pct Mg foams strengthened by AlN particle dispersion were prepared by a melt foaming method, and the effect of foaming temperature on the foaming behavior was investigated. Al-3.7 pct Si-0.18 pct Mg alloy containing AlN particles was prepared by noncompressive infiltration of Al powder compacts with molten Al alloy in nitrogen atmosphere, and it was foamed at different foaming temperatures ranging from 1023 to 1173 K. The porosity of prepared foam decreases and the pore structure becomes homogeneous with increasing foaming temperature. When the foaming temperature is higher than 1123 K, homogeneous pores are formed in the prepared ingot without using oxide particles and metallic calcium granules, which are usually used for stabilizing a foaming process. This stabilization of the foaming at high temperatures is possibly caused by Al₃Ti intermetallic compounds formed at high temperature and AlN particles. Compression tests for the prepared foams revealed that the absorbed energy per unit mass of prepared Al-3.7 pct Si-0.18 pct Mg foam is higher than those of aluminum foams strengthened by alloying or dispersion of reinforcements. It is remarkable that the oscillation in stress, which usually appears in strengthened aluminum foams, does not appear in the plateau stress region of the present Al-3.7 pct Si-0.18 pct Mg foam. The homogeneity in cell walls and pore morphology due to the stabilization of pore formation and growth by AlN and Al₃Ti particles is a possible cause of this smooth plateau stress region.     

Microstructural observations of pressure cast Ni3Al/Al2O3 and Ni/Al2O3 composites

Pressure castings of Ni₃Al(IC218)/Al₂O₃ and Ni/Al₂O₃ composites, made with continuous DuPont FP α-Al₂O₃ and DuPont PRD166 α-Al₂O₃+20 wt pct partially stabilized ZrO₂ 20 μm diameter fibers, were examined by optical, scanning electron microscope (SEM), and transmission electron microscope (TEM) techniques. According to optical magnifications, excellent infiltration took place. However, in SEM and TEM magnifications, small gaps were found adjacent to regions where bonding had taken place between fibers. On the basis of available evidence, the gap formation was attributed to trapped gases and microshrinkage. Titanium was added to the metal to promote infiltration. Diffusion of Ti into the fibers of the Ni/Al₂O₃ composites occurred, but similar diffusion into the fibers of the IC218/Al₂O₃ composites did not take place. The qualitatively higher bond strength of the interfaces of the Ni/Al₂O₃ composites was ascribed to the diffusion of Ti into Al₂O₃. No interface reaction layer was found in any of the composites. Very little grain growth was found to take place in either the FP or PRD 166 fibers after casting and after a subsequent ten day anneal at 1150 °C.     

Co-deformation processing and modeling of <Emphasis Type="Italic">In-Situ</Emphasis> multiphase composites

A series of in-situ, deformation-processed metal matrix composites were produced by direct powder extrusion of blended constituents. The resulting composites are comprised of a metallic Ti-6Al-4V matrix containing dispersed and co-deformed discontinuously reinforced-intermetallic matrix composite (DR-IMC) reinforcements. The DR-IMCs are comprised of discontinuous TiB₂ particulate within a titanium trialuminide or near-γ Ti-47Al matrix. Thus, an example of a resulting composite would be Ti-6Al-4V+40 vol pct (Al₃Ti+30 vol pct TiB₂) or Ti-6Al-4V+40 vol pct (Ti-47Al+40 vol pct TiB₂), with the DR-IMCs having an aligned, high aspect ratio morphology as a consequence of deformation processing. The degree to which both constituents deform during extrusion has been examined using systematic variations in the percentage of TiB₂ within the DR-IMC, and by varying the percentage of DR-IMC within the metal matrix. In the former instance, variation of the TiB₂ percentage effects variations in relative flow behavior; while in the latter, varying the percentage of DR-IMC within the metallic matrix effects changes in strain distribution among components. The results indicate that successful co-deformation processing can occur within certain ranges of relative flow stress; however, the extent of commensurate flow will be limited by the constituents’ inherent capacity to plastically deform.     

Effect of Processing Parameters on Microstructure and Mechanical Properties of an Al-Al11Ce3-Al2O3In-Situ Composite Produced by Friction Stir Processing

Friction stir processing (FSP) was applied to produce aluminum-based in-situ composites from powder mixtures of Al-5 mol pct CeO₂. A billet of powder mixtures was prepared using the conventional pressing and sintering route. The sintered billet was then subjected to multiple passages of FSP. This technique has combined the hot-working nature of FSP and the exothermic reaction between Al and CeO₂. The reinforcing phases were identified as Al₁₁Ce₃ and δ ^* -Al₂O₃. The Al₂O₃ particles with an average size of ~10 nm are uniformly distributed in the aluminum matrix, which has an average grain size of approximately 390 to 500 nm. Both the sintering temperature and the tool traversing speed used in FSP have significant influence on the microstructure and mechanical properties of the composite. The composite produced by sintering at 833 K followed by FSP with a tool traversing speed of 30 mm/min possesses an enhanced modulus (E = 109 GPa) and strength (ultimate tensile strength (UTS) = 488 MPa) as well as a tensile ductility of ~3 pct.     

Oxidation behavior of single-crystal Al2O3-fiber-reinforced Ni3Al-based composites

A series of single-crystal Al₂O₃-fiber-reinforced Ni3Al-based intermetallic matrix composites were fabricated by pressure casting. The matrices employed were binary Ni₃Al, Ni₃Al-0.5 at. pct Cr, and Ni₃Al-0.34 at. pct Zr. The development of microstructure upon oxidation in air at either 1100 °C or 1200 °C was investigated by optical, scanning, and transmission electron microscopy. In air-oxidized binary Ni₃Al, some of the fibers were fully or partially covered with a layer of oxide. A weak fiber/matrix bond in this system, which led to fiber debonding during composite processing, is believed to be responsible for the ingress of O into the composite and oxidation of the matrix in the debonded regions at the fiber/matrix interface. Addition of Cr to Ni₃Al resulted in an almost threefold increase in fiber/matrix bond strength. No oxidation of the interface was observed. A thick layer of oxide was formed around all the fibers when the composite was thermally cycled prior to isothermal annealing. Addition of Zr to Ni₃Al resulted in the formation of a layer of ZrO₂ on the surface of the fibers during composite processing. The ZrO₂ layer provided a fast path for the diffusion of O, which led to the formation of a rootlike oxide structure around the fibers. The rootlike structure consisted of a network of Al₂O₃-covered ZrO₂.