Dislocation-Interstitial Interactions in Single-Crystal Tantalum

Abstract

Dislocation‐interstitial interactions have been investigated in zone‐melted single‐crystal tantalum by means of internal friction measurements. A Marx piezoelectric composite oscillator resonating at 50 kc/sec was used to measure the logarithmic decrement as a function of time, temperature, and strain amplitude. Damping measurements indicate that two separate pinning processes occur in the 0°–100°C temperature range. In the 0°–25°C range the amplitude‐independent damping measured immediately following deformation (∼1%) has been found to decrease with time. No change in the amplitude dependence has been detected. This has been called stage‐one pinning. The relaxation time for stage‐one pinning has been found to be too short to allow for the long‐range diffusion of interstitials required for Cottrell‐type pinning. The pinning mechanism is believed to be the reorientation of solute interstitial oxygen atoms in the dislocation strain field as proposed by Schoeck. The activation energy for the stage‐one process has been found to be 23 500±3000 cal/mole. This value is consistent with the diffusional energy of oxygen in tantalum. An increase in test temperature to the 75°–100°C range has been found to cause a further reduction of the amplitude‐independent damping with time. A decrease in the amplitude dependence has also been observed. This reduction in the damping has been called stage‐two pinning. Stage‐two pinning is explained satisfactorily by the Cottrell‐type pinning of the dislocation lines by solute interstitial oxygen atoms.