Strain rate dependency of dynamic flow stress of FCC metals at very high strain rate and high temperature
Sakino Kiyotaka
Abstract:Strain rate sensitivity in FCC metals is known to increase dramatically when the strain rate exceeds about 5x103/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 and the mechanism, high strain rate tests are performed for high purity polycrystalline aluminum and copper in the strain rate range from about 1x103 - 2x104/s and at temperatures ranging up to 600K. A simplified model for the dislocation kinetics under 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 good agreement with the flow stress directly measured. The increase in mobil dislocation density with increasing temperature lowers the flow stress and shifts the transition region to the higher strain rate side. Key Words:FCC metals, strain sensitivity, rate controlling mechanism, viscous phonon drag, temperature dependency of mobile dislocation density