ON THE QUESTION OF FLOW STRESS AT HIGH STRAIN RATES CONTROLLED BY DISLOCATION VISCOUS FLOW

Moving dislocations experience a damping force because of their interactions with lattice phonons and electrons. Except at velocities close to the transverse sound velocity this force is proportional to the dislocation velocity. At high plastic strain rates dislocations may be required to move so quickly that dislocation damping forces determine the flow stress. In this note an estimate is made of strain rate at which dislocation damping becomes important. In the plastic flow stress range of 10 MPa to 100 MPa (for aluminum) the critical strain rate is of the order of 4 x 10(5) s(-1) to 10(6) s(-1) if almost all the dislocations within a piece of metal are mobile during plastic straining. If only a small fraction of the dislocations during plastic straining is mobile, the critical strain rate is reduced by an amount equal to this fraction.