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:
- Al-Li / Al2O3
- Ti-6Al-4V / SiC
- Mg / Carbon
- Al / Carbon
- 6061 Al
- 2014 Al
- SiC (f)
- SiC (w)
- Al2O3 (f)
- B (f)
- C (f)
- Be (f)
- W (f)
- Al / boron
- Al / SiC
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
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
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
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.