Composites boron/aluminum

Let s address the issue of nonlinear material behavior, i.e., nonlinear stress-strain behavior. Where does this nonlinear material behavior come from Generally, any of the matrix-dominated properties will exhibit some degree of material nonlinearity because a matrix material is generally a plastic material, such as a resin or even a metal in a metal-matrix composite. For example, in a boron-aluminum composite material, recognize that the aluminum matrix is a metal with an inherently nonlinear stress-strain curve. Thus, the matrix-dominated properties, 3 and Gj2i generally have some level of nonlinear stress-strain curve. 

Boron has high neutron absorption and the boron-aluminum composites are being investigated for nuclear applications. Single-ply boron-epoxy composites have microwave polarization properties with potential applications in antenna and radome designs. 01... 

Low-Temperature Specific Heat. Although no liquid-helium-temperature data exist for boron, the specific heat has been measured between 13 and 305 K by Johnston et al [ ]. As a result, a Debye temperature of 1219 K has been assigned to the temperature range of 60 to 150 K [ ]. This value is assumed to be applicable at low temperatures and is used in synthesizing a value for the lattice specific heat coefficient ( 3 or jS) of the boron/aluminum composite, on the assumption that the mixture principle is applicable, viz. ... 

The loss of three-dimensionality in the higher temperature regime is not a consequence of internal sample geometry—the fibers are macroscopic. For example, the specific heat of the boron/aluminum composite contains the cubic term and conforms to the mixture principle for bulk metallic ingredients. Moreover, experiments performed previously and discussed elsewhere [ ] demonstrate the dominance of a quadratic term in the low-temperature specific heat of a fiberglass-cloth-reinforced resin. The tendency to lower-order dimensionality is presumably a property of the lattice dynamics of the polymeric chains and rings characteristic of the resin matrix. 

Polycrystalline-Alumina-Reinforced Aluminum Alloy Tensile Strength of Boron/Aluminum Composites Compressive Strength... 

Boron-reinfixced aluminum is a technologically mature continuous-fiber MMC (Fig. 2). Applications for this composite include tubular truss members in the mid-fiiselage structure of the space shuttle orbiter and cold plates in electronic microchip carrier multilayer boards. Fabrication processes for boron/aluminum composites are based on hot-press diffusion bonding of alternating layers of aluminum foil and boron fiber mats (foil-fiber-foil processing) or plasma-spraying methods. 

The corrosion properties of boron/aluminum composites are extensively reviewed in Ref 3. This section summarizes the significant findings. 

Electronic-Grade MMCs. Metal-matrix composites can be tailored to have optimal thermal and physical properties to meet requirements of electronic packaging systems, eg, cotes, substrates, carriers, and housings. A controUed thermal expansion space tmss, ie, one having a high precision dimensional tolerance in space environment, was developed from a carbon fiber (pitch-based)/Al composite. Continuous boron fiber-reinforced aluminum composites made by diffusion bonding have been used as heat sinks in chip carrier multilayer boards. 

Typical S-N (stress versus number of cycles) curves for various metals and composite materials are shown in Figure 6-4 [6-3]. The boron-epoxy composite material curve is much flatter than the aluminum curve as well as being flatter than the curves for any of the metals shown. The susceptibility of composite materials to effects of stress concentrations such as those caused by notches, holes, etc., is much less than for metals. Thus, the initial advantage of higher strength of boron-epoxy... 

Historically, polymer-matrix composite materials such as boron-epoxy and graphite-epoxy first found favor in applications, followed by metal-matrix materials such as boron-aluminum. Ceramic-matrix and carbon-matrix materials are still under development at this writing, but carbon-matrix materials have been applied in the relatively limited areas of reentry vehicle nosetips, rocket nozzles, and the Space Shuttle since the early 1970s. 

Many barium aluminosilicate-based compositions will eventually react with the chromium oxide or aluminum oxide scales on the metal interconnect or metal edge rails to form barium chromate or a celsian phase at the interface [6], This can cause a mechanical weakness that is easily delaminated. Also, compositions that contain boron can react over time with water (steam) to produce B2(OH)2 or B(OH)3 gas. This can decompose the glass and greatly limit the lifetime of the seal. Thus many of the new investigations have emphasized low or no boron glass compositions. 

On the supposition that the total number of unit cells keeps invariable and no aluminum atoms are lost during the boronation, the composition of unit cell and the population of vacancies can be estimated as listed in composition of unit cell (I) in Table 2. It can be seen that the vacancies occupy about 30-50% of total T sites after the boronation. However, it should be noted that the population of vacancies thus obtained by chemical analysis is only a bulk average result. The composition on the surface of crystallites is actually different from that in the bulk because the dissolution of silicon starts first from the outer surface, so that the vacancies on the surface are much more than those in the interior of crystallites. Such a large number of vacancies on the surface will result in corrosion and dissolution of the surface parts of crystal particles. Therefore, the number of unit cells in the sample after the boronation is actually less than that before the boronation, whereas boron atoms in each unit cell should be more than those shown in composition of unit cell (1) in Table 2. On the other hand, if all the 64 T sites are occupied by silicon and trivalent atoms, we can give another set of compositions as shown in composition of unit cell (II) in Table 2. The real composition of a unit cell should be between these two sets of compositions, that is, the 64 T sites are neither occupied completely nor vacated so severely that the collapse of the framework occurs. It can also be seen that the introduction of boron atoms is so limited that there are no more than 1.5 atoms per unit cell even though the repeated boronation is performed. 

In another paper [ the results of specific heat measurements on 10 metallic alloy samples were considered. This paper discusses specific heat measurements on four composite (i.e., fiber-reinforced) materials, one of which (boron/aluminum) is essentially metallic, and the other three are resin-based. The resin-based composites are more difficult to measure than metallic samples, and in analyzing the resulting data, the assembling of an appropriate fitting function is more complicated. As with the Fe-Ni base alloys [ ], specific heats were measured in the low-temperature range (3 to 20 K) and at the intermediate temperatures 80 K and 300 K. Because of difficulties associated with long thermal-relaxation times at these temperatures, considerable experimental scatter is associated with the results for the resin-based specimens. 

Although absent in the fitting functions for metallic alloys [ ] and metallic composites such as boron/aluminum, the presence of a quadratic term in T is essential for a proper description of the specific heat-temperature dependences of the resin-based materials in the temperature range above 6 K. Below that temperature, good fits are obtained with what are essentially cubic functions. 

Compositions of highest heat output using boron, aluminum, and titanium or zirconium, with oxidizers such as potassium nitrate or perchlorate, are not normally needed in strictly pyrotechnic lire transfer, but they are important in the initiation of solid propellants, be it for small items such as gas cartridges or for larger grains. Formulas 182, 183, and 184 are some of the better-known examples. 

The principal reinforcements for metal matrices include continuous fibers of carbon, boron, aluminum oxide, silica, aluminosilicate compositions and... 

A.J. Pyzik, R.A. Newman, A. Wetzel and E. Dubensky, Composition Control in Aluminum Boron Carbide Composites , Proceedings of SO" International Conference on Advanced Ceramics And Composites at Cocoa Beach, FI, The American Ceramic Society, 2006. 

Morris, J. R. and Tanzilli, R. A. (1987) Aluminum Nitride-Boron Nitride Composite Article and Method of Making Same, U.S. Pat. 4666,873. 

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