Kinetics of dislocations in pure Fe. Part II. In situ straining experiments at low temperature

In situ straining experiments have been carried out at low temperature in pure Fe, in order to study the change of mechanism occurring at around 250K. The local stress necessary to move individual screw dislocations is in good agreement with the macroscopic yield stress at various temperatures. In the lower temperature range, straight screw segments have a jerky motion in {110} planes, at variance from the steady motion observed near room temperature. The distributions of waiting times in locked positions, and jump distances, the temperature variation of the average jump distance, and the stress/temperature variation of the macroscopic activation areas, are inconsistent with the kink-pair mechanism observed above 250K. They have been interpreted in terms of a locking–unlocking mechanism, already proposed in hexagonal-closed-packed metals. Under such conditions, the change of mechanism at 250K can account for the surprisingly low value of the flow stress extrapolated to 0K (much lower than the theoretical Peierls stress).