Do substitutional solutes soften or embrittle cracktips in α-Fe?

We employ atomistic modeling to examine the influence substitutional solutes have on the activated process of nucleating dislocations from cracktips in α-Fe. We demonstrate a strong correspondence between the energy landscape of the solute during an idealized block-like sliding process and the influence on the activation energy barrier for dislocation emission near a idealized cracktip. Using a series of solute potentials developed with input from ab initio calculations, these models predict that Cu and Ni both lower the activation energy barriers for emission, while solutes such as Mo and W raise the barriers. For solutes that raise or lower the emission barriers, the implications on the overall nucleation frequency turns out to be very different.