The influence of crystallographic texture and interstitial impurities on the mechanical behavior of zirconium
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I. INTRODUCTION
PLASTIC deformation of polycrystalline metals occurs by either of two primary mechanisms: dislocation slip or twinning. Whether slip or twinning is the dominant deformation mechanism depends on which mechanism requires the least stress to initiate and sustain plastic deformation. This is not to say that either slip or twinning will operate at the expense of its competing mechanism, but rather that they are complementary mechanisms. In low-symmetry materials, which otherwise have too few slip systems to plastically deform solely via slip,[1,2,3] twinning may become a dominant mechanism controlling plasticity. If one hopes to develop a plasticity model based upon the operable deformation physics, knowledge of how twinning and slip operate in lowsymmetry metals is necessary. We have selected zirconium (Zr) as a model material for this study for a number of reasons. Among the crystallographic family of hexagonal close-packed (hcp) metals, few exhibit an ideal ratio of interatomic spacing in the c and a directions; cadmium (Cd) and zinc (Zn) have c/a ratios greater than !8/3, while magnesium (Mg), titanium (Ti), and zirconium are among those with c/a values less than !8/3. Of titanium and zirconium, one might expect that the two deform similarly, since their c/a ratios are nearly identical. Although both titanium and zirconium slip preferentially on {1010} prism planes,[4] slip on the {1011} pyramidal planes and the basal planes is usually observed only in titanium.[1] With respect to deformation by twinning, titanium favors {1124}, a twinning mode which is typically not observed in zirconium; twinning in zirconium is most commonly seen to occur on {1121} planes, a tensile mode, depending on texture.[1] These limiting cases make zirconium a model material to study if one’s intent is to deconvolute the various competing deformation G.C. KASCHNER, Technical Staff Member, MST-8, and G.T. GRAY III, Team Leader, Dynamic Properties, MST-8, are with the Los Alamos National Laboratory, Los Alamos, NM 87545. Manuscript submitted January 26, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
mechanisms controlling polycrystalline plasticity in zirconium. The interested reader is referred to the works of Partridge[2] and Tenckhoff[3] for greater detail about the deformation modes and mechanisms of hcp metals in general and zirconium in particular. Constitutive models, used to predict the mechanical behavior of a material subjected to a variety of stress states and loading environments, may be either empirical or physically based. Empirical models are often convenient to fit and implement, but may be limited in their predictive capacity. The most effective and robust constitutive models, we believe, are those rooted in an understanding of the physics of deformation. To that end, knowledge of the influence of temperature, strain rate, microstructure, and chemistry on mechanical response is critical to develop accurate constitutive models. Previous studies have demonstrated the influence of temperature and strain rate
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