Mechanical behavior of aluminum-lithium alloys at cryogenic temperatures

Abstract

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...

Aluminum alloys for ALS cryogenic tanks :

“…These values are larger (about 50 percent) than those reported by Glazer, et al [9] from data on an earlier vintage 2090-T81 at equivalent test temperatures and orientation (L) . Our strain rate (9xl0~-^s"^) was about an order of magnitude lower than that used by Glazer, et al However, except at 4 K, this effect on strainhardening rates is expected to be insignificant.…”

“…The increase of E at low temperatures is approximately 10 percent, of the same order as the increase in E for pure Al [6]. ASTM E 1304 S-L 10,9 (9,8) S-T 10,10 (9.9) Ledbetter, 1990, dynamic data (x)®; MIL-HDBK recommended value for 2219 (n)^.…”

“…Recently, Glazer et al [9] suggested that, for Al-Li alloys, specimen necking occurs when the true stress in the specimens reaches the strain-hardening rate. They plot strain-hardening rate and true stress for 2090-T81, L orientation, versus true strain to illustrate the coincidence of the two values at tensile failure .…”

Cryogenic mechanical property testing system directly cooled by G-M cryocooler

“…Mechanical properties are mainly dependent on the composition, phase structure and defects, but not constant and often changes as a function of temperature. Materials often have some undesirable or unusual properties in cryogenic temperatures that make them interesting or unsafe to use in cryogenic engineering [2,[4][5][6][7]. For example, the body-centred-cubic metals show good fracture toughness at room temperature but brittleness at cryogenic temperatures [8].…”

Microstructural Influence on the Work Hardening of Aluminum-Lithium Alloy 2090 at Cryogenic Temperatures

“…Previous studies have revealed an excellent strength-toughness combination for 2090 at cryogenic temperatures.£6, [8][9][10][11][12][13] Subsequently, much of the work on 2090 has concentrated on understanding the source of the excellent mechanical properties observed at cryogenic temperatures.£1- 3,6,[8][9][10][11][12][13][14] Although the determining mechanism remains an issue of debate, it is generally agreed upon that the deformation behavior, the ability of a material to work harden, plays an important part in determining the subsequent fracture toughness.£6, 8,1O,12] The majority of work on 2090 and other like alloys associates increases in workhardening with greater slip homogeneityJI5-17] Jata and Starke [16] relate this greater homogeneity with a decrease in slip planarity. The planarity of slip is, in turn, controlled by the ability of dislocations to cross-slip.…”