Cryogenic toughness of commercial aluminum-lithium alloys: Role of delamination toughening
- PDF / 4,146,283 Bytes
- 13 Pages / 590.28 x 785 pts Page_size
- 96 Downloads / 161 Views
I. I N T R O D U C T I O N T H E rapid development of advanced aluminum-lithium alloys in recent years has been driven largely by numerous potential structural applications in the aerospace industry requiring high-strength, high-modulus, and lowdensity metallic materials. More recently, however, there has been increasing interest in the cryogenic properties of these alloys, following reports of a marked increase in ductility, fatigue resistance, and especially fracture toughness with d e c r e a s e in temperature from ambient to 4 K. l~-5]Thus, although developed primarily as low-density high-strength airframe materials, AI-Li alloys have additionally become attractive candidate materials for liquid-hydrogen, -oxygen, and -natural gas fuel tanks, in particular for existing and future transatmospheric and hypersonic aircraft applications. The mechanistic origin of the significantly improved cryogenic properties of aluminum-lithium alloys currently is uncertain, although behavior is not unlike other high-strength aluminum alloys ( e . g . , 2219) which display somewhat higher Kit values at lower temperatures, t61 Explanations based on higher strain-hardening rates, associated with more homogeneous plastic deformation, t7,81 and solidification of Na, K, H-rich liquid phases at grain boundaries I2'3'91 have been proposed for A1-Li alloys, although such suggestions seemingly are inconsistent with results 14'~~ which show i n c r e a s e d lowtemperature fracture toughness for the longitudinal (L-T,T-L) orientations but d e c r e a s e d toughness for the K.T. V E N K A T E S W A R A R A O is a Postdoctoral Research Engineer, and R.O. RITCHIE is a Professor and the Director of the Center for Advanced Materials, Materials and Chemical Sciences Division, Lawrence Berkeley Laboratory, and Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720. W E I K A N G Y U , formerly with the Department of Materials Science and Mineral Engineering, University of California, Berkeley, is with Raychen Corporation, Menlo Park, CA. Manuscript submitted April 1, 1988. METALLURGICAL TRANSACTIONS A
short-transverse (S-L,S-T) orientations. The latter observations led to proposals t4,1~ that the effect was associated more with a greater tendency for short-transverse cracking along high angle grain boundaries at low temperatures, similar to the early work of Carman et al. llll on the toughness of 7075-T6. Such increased shorttransverse cracking leads to enhanced crack deflection II~ and, more importantly, to delamination perpendicular to the crack plane t4,'~ for fracture in the L-T and T-S orientations, as illustrated for the crack-divider and crackarrester orientations, respectively, in Figure 1. Since extensive low-temperature fracture-toughness data on commercial aluminum-lithium alloys are not readily available in the literature, the prime objective of the present study is to examine whether the excellent cryogenic deformation and toughness properties reported for 2090-T8E41 ~l.7.~~ are commo
Data Loading...
