Kink-limited Orowan strengthening and the brittle to ductile transition of bcc metals

It is well known that the fracture response of bcc metals and covalent materials undergoes a sharp brittle-to-ductile transition (BDT) with increasing temperature. The classic experiments of Hirsch and Roberts[1] measured the activation energy for the BDT with varying strain rate and provided compelling evidence that the BDT is controlled by dislocation mobility through preexisting microstructure. A critical task for structural nuclear materials science is to gain understanding into irradiation induced embrittlement, which in the Hirsch-Roberts interpretation is manifest as an increase in the BDT temperature due to irradiation defects acting as obstacles to dislocation motion. In this work[2], we use atomistic and mesoscale simulations alongside statistical mechanics to study the motion of kink-limited 1/2 screw dislocations in bcc metals through a field of obstacles, using a recently developed atomistic method[3] to determine the stress and temperture dependent activation free energy of kink pairs in atomistic simulations. Our model predicts the BDT activation energy of unirradiated, unworked bcc metals to be half the kink pair formation energy, a prediction which shows striking quantitative agreement between DFT kink energies and experimentally determined BDT activation energies of Fe, W, Mo and V. With increasing obstacle density under irradiation we predict the BDT temperature to at most double, a relationship we show is satisfied in BDT experiments on low-activation steels.

[1] P. B. Hirsch, S. G. Roberts, and J. Samuels, Proc. R. Soc. London, Ser. A 421, 25 (1989)
[2] T. D. Swinburne and S. L. Dudarev, Phys. Rev. Mat. 2, 073608 (2018)
[3] T. D. Swinburne and M.-C. Marinica, Phys. Rev. Lett. 120, 135503 (2018)