The question of the role of grain boundaries on toughness has not yet been resolved.
On the one hand, grain boundaries act as obstacles for dislocation motion and can confine the plastic zone. Based on these considerations, a decrease of the grain size would lead to a decrease of toughness.
On the other hand, the intersection points of grain boundaries with the crack front are preferred dislocation nucleation sites. It is speculated that the toughness increases with decreasing distance of the spacing of dislocation nucleation sites along the crack front, λ, i.e. with decreasing grain size.
It can be expected that both mechanisms (i) blocking of gliding dislocations and (ii) acting as dislocation nucleation site is true for grain boundaries. The question is just on how to weight the single contributions.
In this presentation we show the results of a comparison of data obtained from modelling crack tip plasticity by means of crack dislocation dynamics with data derived from experiments.
In a first step, the impact of the parameters (i) dislocations source spacing and (ii) mean free path for dislocations glide (pile up) on toughness was assessed in isolation. In a second step, both contributions of grain boundaries viz. (i) dislocation nucleation sites and (ii) obstacles for gliding dislocations were considered in parallel and weighted by the force of the obstacle.
Our results provide a clear picture: a decrease in grain size increases strength and increases toughness.