Influence of precipitates on the mechanical response and substructure evolution of shock-loaded and quasi-statically def
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I.
INTRODUCTION
THE
combination of light weight and high specific modulus and strength of Li-containing A1 base alloys recently has attracted considerable attention in the aircraft industry. Wide ranges of research activity, including alloy production and processing, microstructure characterization, quasi-static mechanical properties, and corrosion resistance, all have been undertaken. The potential promise of these high strength A1 alloys for aircraft applications requires sufficient toughness under both conventional and high rate deformation. The latter is of concern when the airplane is subjected accidentally to an impact or shock wave during service. While the mechanical behavior of A1-Li alloys under quasi-static tension or compression has been the focus of many studies, the response of A1-Li base alloys to shock-loading (strain rates ~ > 106 s -1) remains unknown. One study o f the response of these alloys deformed at high strain rates has been reported by Chiem et a l . , m who studied the dynamic behavior of aged A1-Li-Mg-Cu (8090) alloys up to ~ = 104 s -1 utilizing a Split-Hopkinson Pressure Bar, hereafter SHPB. Based on the measurement of engineering shear stresses, the 8090 alloy was found to be highly rate-sensitive at k > 102 s -l under torsion loading, m Transmission electron microscopy J.C. HUANG, Research Fellow, and G.T. GRAY, III, Staff Member, are with Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545. Manuscript submitted May 23, 1988. METALLURGICAL TRANSACTIONS A
(TEM) observations m revealed "dislocation cells" in compression specimens tested between ~ = 102 and 104 s-l; however, the dislocation morphology seen from the published photographs at a strain of e > 9 pct appears more like dislocation bands. The substructure evolution of the 8090 alloy as a function of precipitate size and strain rate was not characterized systematically. The study of high-strain-rate or shock-loading deformation in precipitation-hardened alloys is not only important for A1-Li alloys, but is also essential from the viewpoint of fundamentally understanding the deformation of precipitation strengthening materials. Murr and his colleagues t2m have studied the effects of matrix inclusions on residual shock hardening in Chromel A, INCONEL 600,* TD-Ni, and TD-NiCr Ni base al*INCONEL is a trademark of the INCO family of companies. l o y s . [2'3] All
of the second phases in these materials were able to suppress dislocation cutting. The well-defined dislocation ceils typically observed in shock-loaded pure Ni were suppressed totally in the above Ni base alloys with various matrix particles. It was concluded from these studies that the second phases acted as sources for additional dislocation generation and as barriers to dislocation motion, t21 It also has been suggested that while the influence of a low density of very fine coherent precipitates on residual shock substructure might not be significant, the effect of a higher density of noncoherent particles definitely will
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