Modifications of the Response of Materials to Shock Loading by Age Hardening
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INTRODUCTION
THE response of materials to high velocity impact scenarios is of interest to a number of industries, including armor and armor defeat applications from the military, and satellite protection. However, to gain an understanding of materials response under these conditions is nearly impossible given the complex nature of the strain state under the impact site. Therefore, it is more usual to generate the appropriate strain rates under conditions where the resultant state of strain is precisely known. In general, the technique of plate impact is employed, where the impact of a flat flyer plate onto an equally flat target assembly generates a planar shock front, behind which conditions of one-dimensional strain apply (i.e., all strain is accommodated along the impact axis). Further discussion of the basics of shock compression is beyond the scope of this report, but the interested reader is directed to the text books of Meyers[1] and Bourne.[2] The mechanical response of materials within the weak shock regime (i.e., below the pressure where the shock velocity (US) becomes greater than the ambient pressure elastic sound speed (cL)), like lower strain-rate loading, is governed by the material microstructure. This will include crystalline structure, grain size, distribution, and balance of additional phases and prior history (dislocation and twin density). The most widely understood class of materials under shock loading conditions is the face-centered cubic (fcc) metals and alloys (mentioned here as they are the base metals for the alloys under JEREMY C. F. MILLETT, Senior Scientist, is with AWE, Aldermaston, Reading RG7 4PR. U.K. Contact e-mail: Jeremy.millett @awe.co.uk Manuscript submitted April 28, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
investigation in this report). With no other factors such as additional phases, the mechanical response of these materials is driven by the stacking fault energy (SFE c), which controls the motion and generation of dislocations. The operative slip system in this case is a/2h110i dislocations, slipping on the {111} planes. However, it is often energetically favorable for the unit dislocation to split into two partials, of type a/6h112i. The stacking fault energy controls the separation distance d of these partials: d¼
la2 ; 24pc
½1
where l is the shear modulus and a is the lattice parameter. As the partials must move as a pair, it is clear that increasing their separation (by reducing the stacking fault energy) will impede their ability to overcome obstacles, either by cross-slip or by climb, potentially to the point where dislocation activity may be replaced by other mechanisms of plasticity such as twinning. In the case of metals with moderate or high stacking fault energies, (copper ca. 80 mJ m2, aluminum ca. 135 mJ m2, and nickel ca. 200 mJ m2),[3] recovered microstructures have been seen to consist largely of welldeveloped dislocation cells,[4–7] along with vacancy loops observed in pure aluminum.[5] In addition, the post-shock mechanical response in
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