Dynamic Plastic Response of Aluminum at Temperatures Approaching Melt
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INTRODUCTION
TEMPERATURE and strain rate are significant parameters that affect material response. Frutschy and Clifton[2] conducted high-temperature, pressure-shear plate impact tests on oxygen-free high-conductivity (OFHC) copper at three elevated temperatures and two ultra-high strain rates to characterize such responses. The results of that work (Figure 1) show that the shear flow stress decreases with increasing temperature and increases with increasing strain rate over the full range of temperatures and strain rates examined. However, the flow stresses observed experimentally were substantially greater than those predicted by three popular models for OFHC copper (Figure 2). One of those models was the Follansbee/Kocks (with Johnson/Tonks)[3,4] model, which is based on ratecontrolling processes involving the thermally activated motion of dislocations past obstacles. While conclusive evidence of a change in rate-controlling mechanism was not obtained, the response suggests that the influences of temperature and strain rate may be changing at the highest temperatures. Adiabatic heating is assumed during plate impact tests, and it is estimated that 90 pct of plastic work is converted to heat. Test ‘‘C’’ on OFHC copper (from Figure 1) was tested up to 1119 K (846 C) or 82 pct of its ambient melting temperature, 1356 K (1083 C). However, during the pressure-shear plate impact tests, the sample is subjected to large pressures and the melting STEPHEN E. GRUNSCHEL, Graduate Student, and RODNEY J. CLIFTON, Professor, are with the Engineering Division, Brown University, Providence, RI 02912, USA. Contact e-mail: grunschel@ yahoo.com This article is based on a presentation made in the symposium entitled ‘‘Dynamic Behavior of Materials,’’ which occurred during the TMS Annual Meeting and Exhibition, February 25–March 1, 2007 in Orlando, Florida, under the auspices of The Minerals, Metals and Materials Society, TMS Structural Materials Division, and TMS/ASM Mechanical Behavior of Materials Committee. Article published online October 9, 2007 METALLURGICAL AND MATERIALS TRANSACTIONS A
temperature for most metals, including OFHC copper and aluminum, is higher at larger pressures. Therefore, it is relevant to think of the testing temperature as a percentage of the absolute melting temperature at the current pressure and not the ambient melting temperature. The OFHC copper sample in test C experienced a pressure of 7.8 GPa and had a melting temperature of 1670 K (1397 C) when it reached 1119 K (846 C). Therefore, OFHC copper was sheared at temperatures up to 67 pct of its current melting temperature. In the current study, temperatures that are larger fractions of the melting temperature are accessible because of the lower melting point of aluminum, 933 K (660 C). So far, the shearing resistance has been measured at temperatures up to 903 K (630 C), which is 81 pct of the melting temperature at the concurrent pressure.
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EXPERIMENTAL PROCEDURE
The basic configuration for a pressure-shear plate impact experiment is shown in
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