Mechanical behavior of aluminum-lithium alloys at cryogenic temperatures
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INTRODUCTION
THE 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 A1-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 longitudinal (L) and long transverse (LT) directions. Lz'3 The most extensively studied alloy is alloy 2090-T81, of nominal composition AI-2.7Cu-2.2Li-0.12Zr in weight percent. The cryogenic mechanical properties of alloy 2090-T81 have been characterized z'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, and elongation in both L and LT directions and the fracture toughness in L-T and T-L orientations 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 J. GLAZER, R.R. SAWTELL, and S.L. VERZASCONI, Graduate Student Research Assistants, and J. W. MORRIS, Jr., Professorof Metallurgy, are with LawrenceBerkeleyLaboratory,Berkeley,CA 947209 Manuscript submittedJanuary 5, 1987. METALLURGICAL TRANSACTIONS A
Figure 1. Similar data for 2219-T87 alloy are included for comparison. 5'6'7 At 4 K, 2090-T81 plate has a strengthtoughness combination that is superior to that obtained to date for any other aluminum alloy. In addition, the tensile elongation in the longitudinal direction increases approximately 60 pct between room temperature and 4 K. The mechanisms responsible for the variation in mechanical properties with temperature are not currently understood and are the subject of this investigation. Because dramatic changes in fracture toughness between room and cryogenic temperatures in other systems are usually associated with significant changes in the fracture mode, this possibility was examined first. However, as illustrated by Figure 2, decreasing the temperature did not alter th
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