Aluminum Matrix Composites with Discontinuous Silicon Carbide Reinforcement

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 oriented.

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 composites.

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 data.

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 lower ductilities.

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 strength.

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 strengths observed.

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 fabrication.

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 applications.

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:

1. 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.
2. 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 of reinforcement.
3. 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 mechanical working.
4. 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 fracture.
5. 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.