On the isotropy of the dynamic mechanical and failure properties of swaged tungsten heavy alloys
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I. INTRODUCTION
THE alloys referred to as “heavy alloys” generally consist of the alloy families containing elements of very high density, such as W, Ta, and U. Tungsten-base heavy alloys are produced by liquid-phase sintering of elemental powders, W, Ni, and Fe. The high density of these materials (typically in excess of 17 Mg/m3) makes these alloys suitable for armor piecing purposes (kinetic penetrators).[1,14] The typical microstructure of a tungsten-base heavy alloy consists of nearly spherical single crystals of tungsten (bcc) surrounded by a binding phase (fcc matrix), which is a solid solution of W, Ni, and Fe. The mechanical properties of these alloys have been extensively studied, both quasi-statically (e.g., References 2 through 5) and dynamically (e.g., References 6 through 12), most recently in the examination of the alloy’s ability to undergo shear localization in the form of adiabatic shear banding.[8–11] These heavy alloys can be produced in a variety of near-final shapes, but cylindrical rods are the most commonly produced. Various thermomechanical treatments are used to improve the properties of the rods, with swaging operations being the most common, to obtain the final desired cylindrical rod shape. Past research has shown that heavy alloys are strain-rate sensitive.[9,10] At low strain rates, the material exhibits some strain hardening that disappears gradually as the strain rate is # increased ( e 103s1 ). As the strain rate increases further, strain softening starts to occur, possibly due to the increase in local temperature and thermomechanical coupling effects.[5] When the components of the composite material are considered individually, they have different characteristics in terms of their high strain-rate behavior. A lack of strain hardening
D. RITTEL, Professor, and R. LEVIN and A. DOROGOY, Doctors, are with the Faculty of Mechanical Engineering, Technion Mechanical Engineering, Israel Institute of Technology, 32000 Haifa, Israel. Contact e-mail: [email protected] Manuscript submitted February 24, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
has been reported for dynamically loaded and cold-worked pure tungsten.[6] In addition, dynamic strain softening of textured tungsten has also been reported for the 001 orientation.[13] Zhou and Clifton[11] tested an alloy whose composition is representative of the binding matrix, and they observed no high strain rate related softening effects. All of the studies report that tungsten heavy alloys will ultimately fail at high strain rates through a mechanism involving adiabatic shear bands, first modeled by Zhou et al.[10] Ramesh[9] reports a critical shear strain to failure of c 0.08 to 0.13, while Zhou and Clifton report much higher values of c 1 to 1.5. For the most part, none of the previous studies have investigated the possible contribution of anisotropy to the mechanical properties and the resulting adiabatic shear band formation. The issue of adiabatic shear band formation has not been related to material orientation, e
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