Shock-Induced Defects in Bulk Materials
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The magnitude of the imposed shock on a material determines whether: 1) a "purely" elastic wave, 2) an elastic plus a plastic wave traverse a sample, or 3) in the case of some materials, such as Fe, Ti, Zr, Bi, Sn, or Hf, upon achieving a specified shock pressure a pressure-induced phase transformation to a higher density phase occurs with a second plastic wave[4,6,7]. In this paper, examples of how the structure / property effects of planar shock waves on metals and alloys due to the manifestations of Plastic I wave propagation through a material are reviewed. Results are also presented of an on-going study at Los Alamos National Laboratory combining wave profile measurements, shock-recovery experiments, and post-mortem substructure evaluation of the a-co pressure-induced phase transformation in titanium, due to the passage of a Plastic II wave. The influence of shock-induced defect generation due to shock prestraining below and above the a-co phase transition in Ti on post-shock mechanical behavior in two Ti-alloys containing different interstitial oxygen contents is also discussed. DEFECT GENERATION DURING SHOCK LOADING In a truly isotropic homogeneous material the passage of an elastic shock through a bulk material should by definition leave behind no lattice defects or imperfections. However experimental observations have shown that this may not be the case in complex engineering materials, in particular composites and/or brittle solids such as ceramics, where significant local elastic anisotropy's may result in local plasticity, and/or cracking, although the global stress state remains elastic[12]. With increasing shock amplitude the yield strength of the material is exceeded and a plastic wave is initiated. Accommodation of the imposed plastic strain rate and peak shock pressure upon the passage of a plastic shock is known to result in the generation of a variety of defects. Microstructural examinations of shock recovered samples have characterized the differing types of lattice defects (dislocations, point defects, stacking faults, deformation twins, and in some instances high-pressure phase products) generated during shock loading[4,6,7]. The specific type of defect or defects activated, their density, and morphology within the shockrecovered material have in turn been correlated to: 1) the details of the starting materials chemistry, microstructure, and initial mechanical behavior or hardness, and 2) the post-mortem mechanical behavior of the shock prestrained material[2,3,5,6,8-10]. Deformation Substructures - Dislocations The deformation substructures observed following shock prestraining in a material which deforms predominately by slip are very similar to that seen following deformation at very low temperatures or due to a decrease in stacking fault energy (SFE). In materials where the thermally activated motion of dislocation slip is rate controlling, this inverse dependency between temperature and strain rate is well established[13]. Low-rate deformation at cryogenic temperatures has been shown to r
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