Influence of peak pressure and temperature on the structure/property response of shock- loaded Ta and Ta-10W
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INTRODUCTION
THE passage of shock waves through materials has been known since the 1940s to alter to varying degrees the structure/property response of a broad range of metals and alloys. Specific examples of postshock response in a wide variety of materials as a function of the applied shock parameters have been reviewed previously by a number of authors, ft~ These effects have been particularly well documented for a large number of face-centered-cubic (fcc) metals, such as copper, nickel, and aluminum, and fcc alloys including brass and austenitic stainless steels. Shock response studies on body-centered-cubic (bcc) metals have preferentially focused on iron and ferritic steels due to extensive interest in the c~-e pressure-induced phase transition.r2,31 Considerably fewer studies have been undertaken on other pure bcc metals, such as niobium, molybdenum, tantalum, and tungsten.t~,3J Shock-loaded fcc metals and alloys have been repeatedly shown to exhibit increased yield strengths and hardness values in mechanical tests following shock prestraining compared to the same metal deformed at a low strain rate to an equivalent plastic strain level.t~ 6~ Figure 1 illustrates an example of the substantially increased reload yield strength response for shock-prestrained high-purity copper (Cu) and nickel (Ni-270). tT,sl In this
figure, the stress-strain responses of the shock-prestrained Cu and Ni-270, measured quasistatically after recovery, have been offset with respect to the annealed starting material responses at a low strain rate by an amount equal to the transient strain generated by the shock. The shock transient strain is defined here as 4/3 In (V/Vo), where V and Vo are the final and initial volumes of Cu and Ni-270 during the shock cycle as determined by their known equation-ofstate.[9~ While fcc, bcc, and hexagonal close-packed (hcp) metals exhibit a large number of similarities in their general properties as metals, significant differences in their conventional mechanical responses are recognized. For example, annealed bcc and hcp metals and alloys exhibit pronounced strain-rate and temperature-dependent yield and flow stresses, unlike annealed fcc metals, whose yield strengths show a weak dependence on these variables, tl~ This dependence in bcc and hcp metals is due to their strong inherent lattice resistance (called the Peierls-Nabarro force or Peierls stress) to dislocation motion compared to fcc metals 600
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:Ni-270, Fine-Grain . I "--,, 10 GPa Shock . . . . . . . .
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GEORGE T. GRAY III is Technical Staff Member and Team Leader-Dynamic Properties, Materials Research & Processing Science, with the Los Alamos National Laboratory, Los Alamos, NM 87545. KENNETH S. VECCHIO, Associate Professor, is with the Materials Science Group, Department of Applied Mechanics and Engineering Sciences, University of California-San Diego, La Jolla, CA 92093-0411. This article is based on a presentation made in the symposium "Dynamic Behavior of Materials," presented at the 1994 Fall Meeting
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