Effect of low-temperature shock compression on the microstructure and strength of copper
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6/30/04
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Effect of Low-Temperature Shock Compression on the Microstructure and Strength of Copper DAVID H. LASSILA, TIEN SHEN, BU YANG CAO, and MARC A. MEYERS Copper with two purities (99.8 and 99.995 pct) was subjected to shock compression from an initial temperature of 90 K. Shock compression was carried out by explosively accelerating flyer plates at velocities generating pressures between 27 and 77 GPa. The residual microstructure evolved from loose dislocation cells to mechanical twins and, at the 57 and 77 GPa pressures, to complete recrystallization, with a grain size larger than the initial one. The shock-compressed copper was mechanically tested in compression at a strain rate of 103 s1 and temperature of 300 K; the conditions subjected to lower pressures (27 and 30 GPa) exhibited work softening, in contrast to the conventional workhardening response. This work softening is due to the uniformly distributed dislocations and the formation of loose cells, evolving, upon plastic deformation at low strain rates, into well-defined cells, with a size of approximately 1 m. The 99.995 pct copper subjected to the higher shock-compression pressures (57 and 77 GPa) exhibited a stress-strain response almost identical to the unshocked condition. This indicates that the residual temperature rise was sufficient to completely recrystallize the structure and eliminate the hardening due to shock compression. Thermodynamic calculations using the Hugoniot– Rankine conservation equations predict residual temperatures of 570 and 1000 K for the 57 and 77 GPa peak pressures, respectively.
I. INTRODUCTION
THIS contribution honors R.W. Armstrong and his seminal contributions to our understanding of the mechanical behavior of materials. Of great importance to the dynamic behavior of materials is the Zerilli–Armstrong equation, which has found application in computational simulations worldwide. It is anchored in the physical processes occurring in a material and has been instrumental in elucidating numerous phenomena. This is but one example of the broad contributions of R.W. Armstrong. Shock-recovery experiments have been used to assess the effects of shock waves on the postshock microstructure and related mechanical behavior of copper and many other materials. These experiments are performed such that the imparted shock passes through the test material and is then dissipated in momentum traps. Shock-release waves from free surfaces in the momentum traps can prevent spurious plastic deformation from passing through the test material. This allows the test material to be recovered and then examined to assess only the effects of the passage of a uniaxial-strain shockwave rise and release. Several types of loading (flyer-plate impact, laser-shock ablation, etc.) and momentum-trap arrangements have been employed. In general, shock-recovery investigations performed on Cu and other fcc metals have shown that shock loading results in DAVID H. LASSILA and TIEN SHEN, Research Scientists, are with the Lawrence Livermore
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