Plastic anisotropy and composite slip: Application to uranium dioxide

The mechanical behaviour of UO$_2$ single crystal is under debate due to the unexpected multi-slip observations in the experiments that involve dislocations in $\frac{1}{2}$ <110> {100} slip systems but also in $\frac{1}{2}$ <110> {110} and $\frac{1}{2}$ <110> {111}. We propose a multi-scale model based on a composite slip in which, under the effect of cross-slip, part of the dislocation density in primary slip systems can be transferred in secondary systems with a lower propensity to glide but a more favourable orientation regarding the shear stress. This approach allows to describe the anisotropic mechanical response of UO$_2$ single crystal with an accuracy never reached up to now. After identifying the relevant slip systems depending on the orientation using a Schmid approach, dislocation dynamics simulations are used to assert if the cross-slip induces a composite slip and to quantify its effect on the flow stress which appears constrained by the activity of $\frac{1}{2}$ <110> {111} systems. In agreement with this result, the composite slip is adapted to couple the activity of slip systems with common Burger vectors in a crystal plasticity framework for a closer comparison to the experiment. This multi-scale approach significantly improves our current knowledge on the links between dislocation microstructures and mechanical properties in UO$_2$ . Composite slip mechanism appears as a candidate to explain unexpected plastic behaviours as often observed in complex materials with multiple slip modes underling that slip activation may be more complex than in usual constitutive laws.