The mechanical properties of aluminum-lithium alloy 2090-T8E41 were evaluated at 298 K, 77 K, and 4 K. Previously reported tensile and fracture toughness properties at room tem.,lerature were confirmed. This alloy exhibits substantially improved properties at cryogenic temperatures; the strength, elongation, fracture toughness and fatigue crack growth resistance all improve simultaneously as the testing temperature decreases. This alloy has cryogenic properties superior to those of aluminum alloys currently used for cryogenic applications. This report was done with support from the Department of Energy. Any conclusions or opinions expressed in this report represent solely those of the author(s) and not necessarily those of The Regents of the University of California, the Lawrence Berkeley Laboratory or the Department of Energy. Reference to a company or product name does not imply approval or recommendation of the product by the University of California or the U.S. Department of Energy to the exclusion of others that may be suitable.

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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