High-strain, high-strain-rate behavior of tantalum
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I.
INTRODUCTION
IRON is the most common body-centered cubic (bcc) metal, and its mechanical behavior has been exhaustively studied. Tantalum has also received considerable attention; both exhibit a high temperature and strain-rate sensitivity, and their mechanical properties are strongly affected by interstitial solutions. Armstrong et al.m have attributed this behavior to the rate-controlling mechanism of the thermal component of the flow stress; whereas, in face-centered cubic (fcc) metals, the rate-controlling mechanism is the overcoming of dislocation forests by individual dislocations, it is considered to be the overcoming of Peierls-Nabarro stresses (through a ledgewise motion of dislocation kinks) for the bcc metals. A specific double-kink mechanism has been proposed by Seegert21 and extended by Dora and Rajnak/31 Detailed analyses on thermally activated plastic deformation in tantalum have been carried out by Gypen and Deruytteret< (single crystals) and Werner~51 (solid solutions). Koizumi et al. t61 calculated the effect of the barrier shape on the flow stress vs temperature for a number of bcc metals, including tantalum. Anomalies in the flow stress decay with temperature were successfully correlated with barrier shape changes. It has been shown that the activation volume (or area) decreases with plastic strain for fcc metals and is constant for bcc metals. An increase in strain rate should only therefore produce a translation (upward or downward) of the stress-strain curve for bcc metals, whereas the work-hardening curves should "fan out" in fcc metals. The activation volume for bcc metals is much smaller than for fcc metals, yielding a much higher temM.A. MEYERS, Professor, Department of Applied Mechanics and Engineering Sciences, Y.-J. CHEN, Graduate Student, Institute for Mechanics and Materials, and D.S. KIM, Post-Doctoral Research Scientist, Center of Excellence for Advanced Materials, are with the University of California, San Diego, La Jolla, CA 92093. F.D.S. MARQUIS, Visiting Professor, is with the Institute for Mechanics and Materials, University of California, San Diego, and the Metallurgical Engineering Department, South Dakota School of Mines and Technology, Rapid City, SD 57701. This article is based on a presentation made in the symposium "Dynamic Behavior of Materials," presented at the 1994 Fall Meeting of TMS/ASM in Rosemont, Illinois, October 3-5, 1994, under the auspices of the TMS-SMD Mechanical Metallurgy Committee and the ASM-MSD Flow and Fracture Committee. METALLURGICAL AND MATERIALSTRANSACTIONS A
perature and strain-rate sensitivity. This basic difference between the activation volumes of fcc and bcc metals has served as the basis for the Zerilli-Armstrongt7,81constitutive equations. The constitutive behavior of tantalum has been studied by Bechtold, tgl Gilbert et al., t1~ Mitchell and Spitzig,V~] Mordike and Rudolf, t12] and Hoge and Mukherjee. v31 More recently, the shock response of Ta has been explored by Lassila and Gray. tl4] Gurevitch et al.[~51 and Shih et al. [16] have
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