Cast Metal-Matrix Composites

Metal-Matrix Composites (MMCs) are engineered combinations of two or more materials (one of which is a metal) in which tailored properties are achieved by systematic combinations of different constituents. Conventional monolithic materials have limitations in terms of achievable combinations of strength, stiffness, coefficient of expansion, and density.

Engineered MMCs consisting of continuous or discontinuous fibers (designated by an f), whiskers (w), or particles in a metal result in combinations of very high specific strength and specific modulus:

Furthermore, systematic design and synthesis procedures can be developed to achieve unique combinations of engineering properties, such as high elevated-temperature strengths, fatigue strength, damping properties, electrical conductivity, thermal conductivity, and coefficient of thermal expansion. In a broader sense, several cast materials with two-phase microstructures in which the volume and shape of the phases are governed by phase diagrams have long been produced in foundries. Modern cast MMCs differ from these older materials in that any selected volume, shape, and size of reinforcement can be introduced into the matrix; these tailored materials represent a new product opportunity for foundry men.

A variety of methods for producing MMCs, including foundry techniques, have recently become available. The potential advantage of preparing these composite materials by foundry techniques is near net shape fabrication in a simple and cost-effective manner. In addition, foundry processes lend themselves to the manufacture of large numbers of complexly shaped components at high production rates, which is required by automotive and other consumer-oriented industries.

Structurally, cast MMCs consist of continuous or discontinuous fibers, whiskers, or particles in an alloy matrix that solidifies in the restricted spaces between the reinforcing phases to form the bulk of the matrix. By carefully controlling the relative amounts and distributions of the ingredients constituting a composite and by controlling the solidification conditions, MMCs can be imparted a tailored set of useful engineering properties that cannot be realized with conventional monolithic materials.

In addition, the solidification microstructure of the matrix is refined and modified because of the fibers and particles, indicating the possibility of controlling micro segregation, macro segregation, and grain size in the matrix. This represents an opportunity to develop new matrix alloys. The creep rupture life of an Al/Al₂O₃ composite and its creep behavior were studied. The metal matrix composite was produced by using a squeeze casting technique.

High-temperature tensile tests and creep experiments were conducted on a 15 vol pct alumina fiber-reinforced AC2B Al alloy metal matrix composite (MMC). The high-temperature tensile strength of Al/Al₂O₃ composite is 14 pct higher than that of an AC2B Al alloy. The steady-state creep rate and the creep life were measured. The stress exponents of the AC2B and Al/Al₂O₃ composites were found to be 4 and 12.3, respectively. A new equation for predicting creep life was established, which was based on the conservation of the creep strain energy. The theoretical predictions were compared with those of the experiment results, and a good agreement was obtained.

As mentioned before, discontinuously reinforced metal matrix composites (MMCs) are attractive for many structural applications, because these materials exhibit unusual combinations of mechanical, physical, and thermal properties. These properties include high modulus and strength, good wear resistance, good heat resistance, and low thermal expansion. In many cases, MMCs were used in high-temperature conditions because of their good heat resistance. Since the life of many components operating at high temperatures is limited by creep deformation, it is important to predict creep rupture life.

The influence of fiber volume fraction on creep, the role of the interfaces between fibers and the matrix, and the reasons for the very high values of creep stress exponents and activation energies have recently been studied. Nardone and Strife have studied the creep behavior of Al 2124/20 pct SiC at temperatures between 150°C and 300°C. They found that the minimum strain rate of the composite obeys Eq., with values of n and Q equal to 8.4 and 277 kJ/mol, respectively, when considered in the lower temperature range. However, in the higher temperature range, the values of n and Q are equal to 21 and 431 kJ/mol, respectively.

Fiber-Metal Wettability and Its Effect on Casting Quality

Cast MCs are made by introducing fibers or particles into molten or partially solidified metals, followed by casting of these slurries in molds. Alternatively, a preform of fibers or a prepacked bed of particles is made and infiltrated by molten alloys, which then freeze in the interfiber spaces to form the composite.

In both these processes, mixing and wetting between molten alloys and dispersoids are desirable in terms of ease of fabrication and ultimate distribution of particulates and fibers. Graphite/magnesium, graphite/aluminum, and several other fiber-reinforced metals are valuable structural materials because they combine high specific strength and stiffness with a near-zero coefficient of thermal expansion and high electrical and thermal conductivities.

The primary difficulty in fabricating these fiber-reinforced metals is poor wetting and bonding between fibers and metals. However, compatibility and bonding between the fiber and the metal in these systems are induced by the chemical vapor deposition of a thin layer of titanium and boron onto the fibers to achieve wetting. These coated fibers are air-unstable because the titanium-boron coating is rapidly oxidized when exposed to air and because molten metal does not wet the fiber. To circumvent this difficulty, air stable coatings of silicon dioxide that are wetted by magnesium have been used for graphite fibers.

These coatings are deposited on the fiber surfaces using silicon-base organometallic compounds. The fibers are simply passed through the organometallic solution, which is then chemically converted by either hydrolysis or pyrolysis to form the silicon dioxide coating. The flexible coated fibers can then be wound or laid up and held in place with a removable binder for selective reinforcement. They are then incorporated into magnesium near-net shape structures by the pressure infiltration of magnesium.

Alumina ceramics (Al₂O₃) are widely used in contact with metals in applications such as joining, sealing, and metal-matrix composites. High metal/alumina bond strength is usually required in such applications; this may be achieved by promoting the metal/alumina wettability. Much work has been done on the wettability in metal/Al₂O₃ and other systems. In particular, the wetting properties of Al/Al₂O₃ are extremely sensitive to the ubiquitous atmospheric impurities, mainly oxygen, that are present in the furnace atmosphere even under controlled test conditions contact.

The addition of alloying elements to the melt and modification of the substrate through deposition of surface coatings can affect the wettability. For example, in Al/ Al₂O₃ decreases upon addition of Mg to Al even at low temperatures due possibly to oxide disruption, Mg adsorption at the solid-liquid interface and lowering of the liquid surface tension. Similarly Ce, Li, and other additives to Al improve the wettability with Al₂O₃. Likewise, coatings of carbon, Ni, Ti, Au, Cu, TiN, C, and TiB₂ on Al₂O₃ have been found to lower the contact angle of alumina ceramics with Al.

The interface strength of bi-material couples is measured using the push-off test, push-out test, pull-out test, fragmentation test, laser spallation, or similar micromechanical techniques. In early studies on A1₂O₃/Me couples (Me = Al, Ni, Ag, or Cu), a push-off test was employed to monitor the influence of alloying additions on the interface strength directly on solidified sessile-drop/substrate couples. The push-off test consists of applying a stress parallel to the substrate to shear the solidified sessile drops. It was found that in the Ni/Al₂O₃ system, the additions of 1 to 2 pct Cr and 0.02 to 0.08 pct Y to Ni increased the push-off stress, but higher additions caused interfacial weakening even though the contact angle continued to decrease (as in Ni(Cr)/ AI₂O₃), or remained sensibly constant (as in Ni(Y)/Al₂O₃).

Contact angle measurements were made at different temperatures and for different lengths of time, with or without alloying additions to Al and coatings on the substrate. Metallic coatings are unstable in contact with molten metals at high temperatures. This raises the question: is there an effect of thin substrate coatings on bonding when such films might not (or, at best, only marginally) affect the contact angle?

Titanium and tin films were used in the study; Ti is an active wetting agent in many ceramic/metal couples (Cu/C, Sn/C, Cu/Al₂O₃, Cu nitrides, and Cu/carbides), and unlike Ti, Sn is a nonreactive additive. Thus, selection of Ti and Sn coatings was meant to provide comparative data on the effects of reactive and nonreactive thin coatings on wettability and interface strength of Al to A1₂O₃.

The relationship between contact angle, or work of adhesion (Wad), and the interfacial shear strength as obtained from a modified push-off shear test was studied in the Al/Al₂O₃ system as a function of temperature, alloying Al with Ti or Sn, and substrate coatings of Ti and Sn. A high Wad corresponds to high shear strength in the pure-Al/Al₂O₃ system.

The Ti coatings effectively lower the contact angle and improve the droplet/substrate bond strength; however, Ti alloying of Al does not lower the contact angle or improve the bond strength. The shear strength of Al/Al₂O₃ and Al/Ti- coated Al₂O₃ increases with increasing temperature, with the reactive Al/Ti/Al₂O₃ couple exhibiting the highest strength at all temperatures presumably due to a beneficial interface structure. The room-temperature shear tests conducted on thermally cycled sessile-drop test specimens revealed that the greatest interface weakening occurred in slow contact-heated samples with short contact times and fast contact- heated samples with long contact times.