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In the intensive materials development activities for electronic paclazging and thermal management applications, the subclass of materials in which SiC particles reinforce aluminum alloy matrices has emerged as one with an especially attractive combination of physical properties, manufacturing flexibility, and cost. One benefit of these materials is the ability to tailor the physical properties through the selection of both reinforcement and alloy variables to match the thermal expansion coefficient of other electronic materials. In addition, the manufacturing flexibility of the various processes allows for shape complexity as well as selective reinforcement placement in the component to optimize system producibility. Finally, because raw materials are inherently inexpensive and low-cost production routes have been identified, aluminum composites may offer a range of cost-effective solutions to emerging problems in electronic paclazging and thermal-management applications.

Fatigue crack growth (FCG) experiments were conducted on controlled variations of Type 7075 and 7050 aluminum alloys. Alloy FCG resistance was ranked under constant amplitude and simple variable amplitude load spectra. Fracture mechanics and fractographic approaches were used to interpret causes for variation in ranking of 7XXX aluminum alloy FCG resistance with loading conditions. The interpretation is built around clarification of a controlling FCG mechanism that is dependent upon interaction of microstructure and load history. This clarification represents a necessary first step toward knowing which microstructure or which design (test) procedure is optimum for a particular class of application, for example, fighter as opposed to bomber or transport aircraft.

The cryogenic mechanical properties of aluminum-lithium alloys are of interest because these alloys are attractive candidate materials for cryogenic tankage. Previous work indicates that the strength-toughness relationship for alloy 2090-T81 (Al-2.7Cu-2.2Li-0.12Zr by weight) improves significantly as temperature decreases. The subject of this investigation is the mechanism of this improvement. Deformation behavior was studied since the fracture mode did not change with temperature. Tensile failures in 2090-T81 and -T4 occur at plastic instability. In contrast, in the binary aluminum-lithium alloy studied here they occur well before plastic instability. For all three materials, the strain hardening rate in the longitudinal direction increases as temperature decreases. This increase is associated with an improvement in tensile elongation at low temperatures. In alloy 2090-T4, these x:esults correlate with a decrease in planar slip at low temperatures. The improved toughness at low temperatures is believed to be due to increased stable deformation prior to fracture. IntroductionThe cryogenic properties of low-density structural materials have acquired considerable importance because of their current and potential uses in space vehicles. For example, the proposed hypersonic and transatmospheric vehicles are expected to use cryogenic fuels such as liquid hydrogen. The large quantity of fuel required to power these vehicles suggests that the cryogenic fuel tank will be structural. Applications of this sort have spurred interest in the cryogenic properties of high-strength aluminum alloys. Although mechanical property data exist for a number of aluminum alloys, to date there has been little work aimed at understanding the mechanisms that control mechanical behavior at cryogenic temperatures.Aluminum-lithium alloys are attractive for cryogenic tankage because they have both lower densities and higher elastic moduli than the aluminum alloys currently used in these applications, such as the Al-Cu alloy 2219-T87, from which the cryogenic external tank of the space shuttle is constructed. In addition, previous work has indicated that at low temperatures aluminum-lithium alloys display improved toughness and an improved strength-toughness relationship in the Land LT directions [1][2][3]. The most extensively studied alloy is alloy 2090-TBl, of nominal composition Al-2.7Cu-2.2Li-0.12Zr in weight 2 percent. The cryogenic mechanical properties of alloy 2090-T81 have been characterized [1,2] and are summarized below.The most striking feature of the low temperature behavior of alloy 2090-T81 'is that the yield strength, ultimate tensile strength, elongation and fracture toughness in both the L and LT directions increase as temperature decreases. Although some other aluminum alloys, including 2219-T87, display this behavior [4], 2090-T81 alloy shows a significantly greater improvement in mechanical properties. The improvement in the strength-toughness relationship with decreasing temperature is illustrated in Figure 1. Si...

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