Constitutive behavior of tantalum and tantalum-tungsten alloys
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INTRODUCTION
THE microstructure/property relationships of tantalum and tantalum-based alloys continue to attract scientific and engineering interest due to their high density, melting point, excellent formability, good heat conductivity, good fracture toughness (even at low temperatures), corrosion resistance, and weldability.V] Since 1950, numerous studies have probed the microstructure-chemistry/property response of a large number of tantalum and tantalum-based alloys, both in single-crystal and polycrystalline form.[' 51 Tantalum, like all bcc metals, exhibits deformation behavior that is markedly influenced by impurities, alloying additions, crystallographic texture, temperature, and strain rate. t~81 Tantalum and its alloys are increasingly being utilized in defense-related applications where their mechanical properties under high strain-rate deformation are attractive. In this article, a wide range of data on unalloyed tantalum and Ta-W alloys subjected to high strain-rate compression at various temperatures will be presented. Yield and flow stresses are shown to be sensitive to the changes in temperature and strain rate at low temperatures and/or high strain rates. A large Peierls stress in bcc materials has been proposed as the rate-controlling mechanism in this temperature and strain-rate regime. [7.91 This large intrinsic lattice resistance results in restricted movement of screw dislocations; long straight screw segments are often observed in this class of materials after deformation,t3,7,~~This suppression of cross-slip of screw dislocations results in linear glide and, therefore, a lower degree of dynamic recovery. The strain-hardening rates in this class of materials at low temperature or high strain-rate loading states are seen to be temperature insensitive [2.71The addition of alloying solutes
to tantalum raises its yield and flow stresses through solid solution strengthening,tvl The overall work-hardening rates are increased relative to unalloyed Ta due to dislocationsolute interactions. The temperature and strain-rate dependence of the yield and flow stresses, as well as the strain-hardening rate, are changed upon solute additions in commercially pure tantalum. While a large number of studies have probed the mechanical behavior of a broad spectrum of tantalum alloys, details of the underlying deformation mechanisms remain poorly understood and, in some cases, controversial. The availability of modem high-speed computers makes it possible to develop more sophisticated material constitutive model descriptions capable of modeling complex problems. [1~,12,~3jAn accurate description of a materials response over a wide range of loading environments, as well as having predictive capabilities outside the measured range, is in great demand. The material properties unique to bcc metals and alloys bring many challenges for the development of physically based constitutive models. The influence of impurities and the effect of tungsten alloying on the constitutive behavior of Ta and Ta-W alloys will be pr
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