Fatigue crack propagation in aluminum- lithium alloy 2090: Part I. long crack behavior
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I.
INTRODUCTION
O V E R the past few years, there has been increasing interest in the development of ultra-light weight aluminumlithium alloys, specifically for aerospace structures 1'2'3 and, more recently, for cryogenic applications. 4 The motivation for using such materials is primarily their 7 to 10 pct lower density, although compared to traditional high strength aluminum alloys, A1-Li-X alloys additionally show 7 to 12 pct higher stiffness, generally superior fatigue crack propagation resistance, *-n and improved toughness at cryogenic temperatures. 4'13 On the negative side, however, they can suffer from poor short-transverse properties, low ductility, and fracture toughness, 13'14and have been shown to display significantly accelerated fatigue crack extension rates when cracks are microstructurally small.15 The low ductility and toughness of aluminum-lithium alloys can be traced, at least in part, to the inhomogeneous nature of their slip, resulting from coherent particle hardening of sperical 8' (A13Li) precipitates.16 In addition, the presence of equilibrium 8 (A1Li) precipitates at grain boundaries can cause precipitate free zones, which can induce further strain localization and promote intergranular failure. Consequently, for the development of commercial alloys, slip has been homogenized by introducing dispersoids (Mn, Zr) and semi-coherent/incoherent precipitates, such as T1(A12CuLi), 0'(A12Cu), or S(A12LiMg), through Cu or Mg additions.17 Concurrent developments in the thermomechanical processing have optimized aluminum-lithium microstructures for the best combinations of strength and K.T. VENKATESWARA RAO, Graduate Student, W. YU, Research Associate, and R.O. RITCHIE, Professor and Director, Center for Advanced Materials, Lawrence Berkeley Laboratory, are with the Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720. Manuscript submitted December 18, 1986.
METALLURGICAL TRANSACTIONS A
toughness, although the resulting material tends to be highly textured, particularly where small Zr additions are used to inhibit recrystallization. 18 Although most studies on aluminum-lithium alloys to date have reported generally improved fatigue resistance compared to traditional 2000-and 7000-series alloys, 4-12 due to their marked anisotropy, some concern has been raised over their crack propagation resistance as a function of plate orientation, and specifically with regard to shorttransverse properties (i.e., involving crack propagation in the rolling plane). Accordingly, the objective of the current stud3/is to investigate the mechanics and mechanisms of fatigue crack propagation behavior in an A1-Li-Cu-Zr alloy 2090-T8E41 as a function of orientation. In Part I, the behavior of through-thickness long (~>5 mm) cracks is examined in the L-T, T-L, T-S, S-L, S-T, and L + 45 orientations, as a function of load ratio, with specific emphasis on the role of crack tip shielding* in retarding crack growth. In * Crack tip shielding refers to a phenomenon where crack a
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