Both aircraft and aerospace vehicles have increased rapidly in size and capacity in recent years. This increase has accelerated the demand for materials with high strength-weight and modulus-weight ratios. Limits are being approached and sharp increases can no longer be anticipated by alloy development. Very few structural metals or alloys have ultimate strength-to-density ratios above 1,000,000 in. Modulus-to-density ratios of the common structural metals, magnesium, aluminum, titanium, and steel, are all approximately 100,000,000 to 1.
Materials such as boron, silicon carbide, beryllium, aluminum oxide, boron nitride, and even fine steel wire far exceed these values in one or both of the above ratios. These high values are attained by single crystal whiskers or fine continuous fibers of these materials. To make use of such properties, the materials must be incorporated in some type of matrix so as to achieve the desired structural shapes.
In employing metal matrices, several methods have been used to achieve distribution of the reinforcement in the matrix. These include powder metallurgy, electrodeposition, plasma flame spraying, molten metal impregnation, and solid state diffusion bonding. After careful consideration of advantages and limitations, work at Harvey Engineering Laboratories has been concentrated on two methods, diffusion bonding for building up the matrix around continuous fibers and molten metal encapsulation with whiskers.
Most of the work has been done in the development of an aluminum matrix composite with continuous fine steel wire as the reinforcing material. A number of compositions have been employed, but Type 355 stainless steel wire in 2024 aluminum alloy has had most use.
Heat, pressure, plastic deformation, and coatings have been used in various combinations for effecting bonding of the steel wire in the aluminum matrix. Material from 0.040 in. thick up to 0.75 in. thick has been made. Plate as large as 1 ft. wide by 8 ft. long has been produced. Micro and macro structure and mechanical properties have been studied for various fabricating procedures and plate sizes.
Tests have been conducted to determine the feasibility of incorporating brittle reinforcing materials such as beryllium and boron in the aluminum matrix. Composites can be produced without breakage of the reinforcing fiber. Work to date shows that with continuous unidirectional fibers, the tensile strength and modulus of elasticity can be predicted on the basis of the properties of the components and the percentage of each in the composite.
Representative applications for composites are cryogenic tankage, airframe and missile structural components, helicopter blades, and turbine engine compressor blades. Many other uses will develop as the properties of this class of materials become more fully characterized.