Mechanical properties and stress-strain behavior were evaluated for several types of
commercially fabricated aluminum matrix composites, containing up to 40% vol
discontinuous SiC whisker, nodule, or particulate reinforcement. The elastic modulus
of the composites was found to be isotropic, to be independent of type of reinforcement,
and to be controlled solely by the volume percentage of SiC reinforcement present. The
yield/tensile strengths and ductility were controlled primarily by the matrix alloy and
temper condition. Type and orientation of reinforcement had some effect on the strengths
of composites, but only for those in which the whisker reinforcement was highly
Ductility decreased with increasing reinforcement content; however, the fracture strains
observed were higher than those reported in the literature for this type of composite.
This increase in fracture strain was probably attributable to cleaner matrix powder,
better mixing, and increased mechanical working during fabrication. Comparison of
properties with conventional aluminum and titanium structural alloys showed that the
properties of these low-cost, lightweight composites demonstrated very good potential
for application to aerospace structures.
The majority of effort in aluminum matrix composites has been directed toward
development of high performance composites, with very high strengths and module, for
use in specialized aerospace applications.
However, there are a number of other applications in aircraft engines and aerospace
structures where these very high properties may not be required, and where it could be
cost effective to use other metal matrix composites. For example, cost-, weight-, and
stiffness-critical components, such as engine static structures, do not require the
very high directional properties available with composites reinforced with aligned
continuous fibers. Replacement of such current aluminum, titanium, or steel structures
by low cost composites offers the potential of significant weight and cost savings.
For these reasons, efforts were initiated to assess the potential of applying low cost
aluminum matrix composites to these structures, using low-cost reinforcements and
low-cost composite fabrication processes, including powder metallurgy, direct casting,
and hot molding techniques.
Factors Influencing Modulus of Elasticity
The modulus of elasticity of 6061 Al
matrix composites increased with
increasing reinforcement content. This increase, however, is not linear, as in the
case of composites with continuous fibers aligned in the testing direction. The modulus
of the composites was below that expected from isostrain-type rule-of-mixtures behavior,
and tended to approach an isostress-type hyperbolic function with reinforcement content,
similar to that observed for transverse modulus behavior of continuous fiber
The reinforcement content was the dominant factor in the improvement of modulus of
elasticity in these SiC/Al composites. For a given reinforcement content, the modulus
tended to be isotropic with nearly equal values obtained from tests in both the
longitudinal and transverse directions. In addition, the modulus appeared to be
independent of type of reinforcement, with modulus values being within 5% of the
average value for all composites nested at any given reinforcement content, regardless
of type of reinforcement.
The modulus of the composites was also independent of the matrix alloy. Heat treatment
of the composites may have had a slight effect on modulus. The modulus of composite in
the T6-temper appeared to be slightly lower than the modulus measured on composites in
the as-fabricated F-temper. This reduction was slight (about 3 to 4%) and was not
consistent among all the matrix alloys tested, and may have been due to scatter in the
Factors Influencing Strength
The factors influencing the yield and tensile strengths of SiC/Al composites are
complex and interrelated, and the best way to evaluate this behavior is through
isolation of variables and analysis of stress-strain curves and fracture behavior.
Effect of Al matrix alloy. The Al matrix used for the SiC/Al composites
was the most important factor affecting yield strength and ultimate tensile strength of
these SiC/Al composites. Tests showed that SiC/Al composites with higher strength aluminum
matrix alloys, such as 2024/2124/7075 Al had higher strengths but
Composites with a 6061 Al matrix showed good strength and higher
ductility. Composites with a 5083 Al matrix failed in a brittle manner, with ultimate
strength related to failure strain. The 5083 Al alloy is not heat-treatable and has been
optimized to gain maximum properties by solid solution strengthening in the strain-hardened
H-temper. The addition of the SiC reinforcement probably overstrained the lattice, and
thus the alloy no longer had sufficient strain energy remaining to gain its potential
strength and ductility.
While heat treatment had little, if any effect of the modulus of elasticity of the
composites, it did affect the transition into plastic flow. Composites in the F-temper
strained elastically and then passed into a normal decreasing-slope plastic flow.
Composites tested in the T6-temper exhibited a slightly greater amount of elastic strain,
with the elastic proportional limit being increased from about 0.10 to 0.15% strain to
about 0.15 to 0.25% but the greater influence was a steepening of the slope of the
stress-strain curve at the inception of plastic flow, relative to that observed for
composites in the F-temper. The inception of plastic flow was marked by a continuation
of a slope that, while no longer elastic and starting to become plastic, approached
that of the elastic portion. This slope decreased with increasing strain, until
eventually reaching normal plastic flow leading to fracture at the ultimate tensile
This increase in elastic proportional strain limit and steepening of the stress-strain
curve were reflected by the higher yield and ultimate tensile strengths observed in
the heat-treated composites. The increase in flow stress of composites with each
heat-treatable matrix probably indicated the additive effects of dislocation interaction
with both the natural alloy precipitates and the synthetic SiC reinforcement. The
combination increased the lattice strain in the matrix, causing greater dislocation
tangling and requiring higher flow stresses for deformation, resulting in the higher
Experiments showed that the yield and ultimate tensile strengths of the SiC/Al composites,
with other parameters being constant, were primarily controlled by the intrinsic
yield/tensile strengths of the matrix alloys. Also, the yield and ultimate tensile
strengths of the composites, with 20% pct SiC reinforcement, were shown to be higher
than those of the same heat treated matrix alloys without reinforcement. The largest
increase in yield/tensile strengths over those of the unreinforced matrix alloy was
achieved by the SiC/6061 Al composites.
Factors Influencing Ductility
Ductility of SiC/Al composites, as measured by strain to failure, is again a complex
interaction of parameters. However, the prime factors affecting these properties are
reinforcement content, matrix alloy, and orientation.
With increasing reinforcement content, the failure strain of the composites is reduced,
and the stress-strain curves also reflect a change in the fracture mode. Preliminary
tensile tests, conducted on wrought aluminum specimens with no SiC reinforcement,
exhibited failure strains of about 15 pct, with a smooth 45 deg chisel-point shear
fracture across the thickness of the specimen. There was also a contraction in the
width of the specimen at the fracture plane.
Elevated Temperature Properties
Discontinuous SiC/Al composites continued to show an advantage over conventional aluminum
alloys at elevated temperatures.
Specimens tested at temperatures of 149° to 204°C (300° to 400°F) exhibiting
the same type of V-shaped, double shear lip transition fracture observed in tests at
room temperature. Specimens tested at 260°C (500°F) showed a slight increase in
plastic strain. While still transitional, the fracture showed more of a tendency for
the formation of a more ductile, single shear lip and was basically the same as that
observed at lower temperatures. Failure strain appeared to increase slightly at
315°C (600°F). The fracture showed a great deal of necking in both the width
and thickness direction of the specimen, and all four surfaces of the fracture area
necked in a ductile manner. This change in fracture behavior coincided with the
marked drop in ultimate tensile strength observed at 315°C (600°F).
Application of SiC/Al Composites to Aircraft Engine and Aerospace Structures
Studies show that these low cost SiC/Al matrix composites demonstrated a good potential
for application to aerospace structures and aircraft engine components. The composites
are formable with normal aluminum metal-working techniques and equipment at warm working
temperatures. They can also be made directly into structural shapes during
These composites merit additional work to determine fatigue, long-term stability, and
thermal cycle behavior to characterize more fully their properties and allow their
consideration for structural design for a variety of aircraft and spacecraft
The most significant aspect of these data was the increase in modulus over that of
competitive aluminum alloys. At 20 vol pct reinforcement, the modulus of SiC/Al composites
was about 50% above that of aluminum and approached that of titanium. This increase in
modulus was achieved with a material having a density one-third less than that of
titanium. Comparison of the properties of the various composites shows that the
modulus/density ratio of 20 vol pct SiC/Al composites was about 50% greater than that of
Al or Ti alloys, while at 30 vol pct SiC the advantage
was increased to about 70% and at 40 vol pct SiC the modulus was almost double that of
unreinforced Al or Ti structural alloys.
Studies were undertaken to evaluate the tensile behavior of low-cost discontinuous SiC/Al
composites, containing SiC-whisker, -nodule, or -particulate reinforcement. The effects
of reinforcement type, matrix alloy reinforcement content, and orientation were
determined by analysis of stress-strain curves and by SEM examination. These
investigations led to the following conclusions:
- Discontinuous SiC/Al composites offer a 50 to 100% increase over the modulus of
unreinforced aluminum and offer a modulus equivalent to that of titanium, but at a
third less density. The SiC/Al composites had modulus/density ratios of up to almost
twice those of titanium and aluminum structural alloys. The modulus of SiC/Al composites
tended to be isotropic and was controlled by the amount of SiC reinforcement.
- The yield and tensile strengths of SiC/Al composites demonstrated up to a 60%
increase over those of the unreinforced matrix alloys. Yield and ultimate tensile
strengths of the composites were controlled by the type and temper of the matrix alloy
and by reinforcement content. In general, these properties were independent of the type
- Ductility of SiC/Al composites, as measured by strain to failure, was dependent
upon reinforcement content and matrix alloy. Composites with ductile matrix alloys
and lower reinforcement contents exhibited a ductile shear fracture with a 5 to 12%
failure strain. As reinforcement content increased, the fracture progressed through a
transition and became brittle, reaching a <1 to 2% failure strain, at higher
reinforcement contents. The increase in ductility over that reported previously was
probably attributable to cleaner matrix alloy powders, better mixing, and increased
- A fine dimple network was observed in the fracture surfaces of composites with
higher strains. At lower fracture strains, a coarser dimple network was observed.
Composites failing in a brittle manner showed increasing amounts of cleavage
- The SiC-whisker reinforcement was generally oriented in the extrusion direction.
Composites with a higher degree of preferred orientation tended to have higher ultimate
tensile strength in the direction of whisker orientation. Composites with a more random
whisker orientation tended to be isotropic in strength.