Strain rate dependency of dynamic flow stress of FCC metals at very high strain rate and high temperature

摘要

Strain rate sensitivity in FCC metals is known to increase dramatically when the strain rate exceeds about 5×10{sup}3/s. The phenomenon has been interpreted by transition in rate controlling mechanism of dislocation motion from thermal activation to viscous phonon drag. It is generally known that the phonon drag increases with temperature. Usually, however, the experimental flow stress in the viscous flow range shows opposite temperature dependency. In order to clarify the above contradiction andthe mechanism, high strain rate tests are performed for high-purity polycrystalline aluminum and copper in the strain rate range from about 1×10{sup}3～2×10{sup}4/s and at temperatures ranging up to 600K. A simplified model for the dislocation kineticsunder dynamic plastic deformation is used to consider the deformation mechanism in the above strain rate and temperature ranges. The flow stress calculated in consideration of the temperature dependency of the mobile dislocation density shows fairly goodagreement with the flow stress directly measured. The increase in mobile dislocation density with increasing temperature lowers the flow stress and shifts the transition region to the higher strain rate side.