Dislocation mobility in pure copper single crystals

Abstract

This thesis presents the results of an experimental investigation of the stress dependence of dislocation velocity in 99.999 per cent copper single crystals. Dislocation displacements were detected by etch pitting dislocation sites on {100} planes. Stress pulses of microsecond duration were applied to single crystal test specimens by means of a torsion impulse machine. Maximum applied, resolved shear stresses ranged from 29 g/mm[superscript 2] to 236 g/mm[superscript 2], and calculated dislocation velocities ranged from 160 cm/sec to 710 cm/sec. The dislocations were presumably predominantly edge-oriented. The growth of copper single crystals, the spark and chemical machining of single crystal test specimens, and the behavior of the etchants which reveal dislocation sites on {100} planes are also discussed. The experimental data have been found to obey a linear relation between dislocation velocity and applied, resolved shear stress. This finding does not correlate with the explanation of the low strain rate sensitivity of the flow stress in copper as proposed by Cottrell (46)*, which predicts that dislocation velocity should be proportional to stress raised to a power of about 200. The low strain rate sensitivity of the flow stress in copper is explained by the high velocity of dislocations at low stresses and the strong stress dependence of the mobile dislocation density. This high velocity is interpreted as enabling the strain essentially to achieve its equilibrium value even at relatively high strain rates. *One- and two-digit numbers appearing in parentheses indicate references listed at the end of the thesis.