How in situ straining experiments can contribute to refine DDD calculations

October 18, 2024

Heavy plastic deformation hardens materials through a process described by the Taylor law, which assumes an average internal stress. In our study, using in situ transmission electron microscopy we found that the law fails at low temperatures. Our results show that dislocation motion is driven by localized peak stresses, and cooperative motion between screw dislocations can lead to unexpectedly high velocities and negative hardening.

Metals and other crystalline materials are considerably hardened by heavy plastic deformation. This “cold-working” process results from the intense multiplication of dislocations which are mutually blocked as a result of their elastic interactions. Hardening is often described by an average “internal stress” increasing with plastic deformation (Taylor law). This rough description has been recently improved by the discrete dislocation dynamics (DDD) method which involves the computation of all individual interactions and subsequent evolution of the whole substructure under stress.

In our work based on in situ straining experiments in a transmission electron microscope that allow one to directly observe the motion of dislocation under stress between 95K and 1500K, we have shown that even such complex calculations are not sufficient in metals with a body-centered-cubic (BCC) structure like iron and ferritic steels strained at low temperature.

We have indeed studied all possible dislocation interactions in BCC iron at various temperatures, and the results show that the concept of average internal stress is unsuitable to describe real situations. For instance, the velocity of a screw dislocation is not determined by its average driving stress, but by the highest local stress along its whole line. Interacting screw dislocations of different types can also move cooperatively at a high velocity up to ten times their individual uncoupled velocities (figure), leading to a strongly negative hardening effect, resulting from tangential components of interaction stresses not taken into account before. Such unexpected behaviors will be used to improve future DDD calculations.

Fig. : Dislocations moving under stress in iron at 110K. The two pictures (a) and (b) are separated by 12s. The difference-image (c)=(a)-(b) shows that the cross formed by the two coupled dislocations 1 and 2 has moved over a much larger distance (arrowed) than all other individual dislocations.

Contact:
Daniel Caillard | daniel.caillard[at]cemes.fr

Publication:
Elastic interactions between screw dislocations in iron
D. Caillard
Modelling Simul. Mater. Sci. Eng. 32 (2024) 035027
https://doi.org/10.1088/1361-651X/ad29b0