Ductile-brittle transition at edge cracks (001)[110] in BCC iron under different loading rates in mode I: a 3D atomistic study

Abstract

3D atomistic simulations via molecular dynamics ( MD) at temperature of 0 K and 295 K (22 °C) with a high quasi-static loading rate dP/dt of 2.92 kN s ^−1 show that cleavage fracture is supported by surface emission of oblique dislocations $\left\langle 11\bar{1}\right\rangle \left\{011\right\}$ and by their subsequent cross slip to {112} planes, which increases separation of the (001) cleavage planes inside the crystal. Under the slower loading rate by a factor 5, the crack growth is hindered by twin generation on oblique planes {112} and the fracture is ductile. The MD results explain the contribution of the crack itself to the ductile-brittle transition observed in our fracture experiments on Fe-3wt%Si single crystals of the same orientation and geometry, loaded at the same rates dP/dt as in MD . The loading rates are equivalent to the cross head speed of 5 mm min ^−1 and 1 mm min ^−1 used in the experiment. The MD results also agree with the stress analysis performed by the anisotropic LFM and comply with experimental observations.

Keywords

• bcc iron
• cracks
• ductile-brittle transition
• molecular dynamics
• loading rate effect