Modelling the plasticity of zirconium hydrides

The presence of hydrides is a major concern for the safety of Zr-alloys for nuclear applications. Hydrides are usually considered as a brittle phase embedded in the ductile metallic matrix. Nevertheless, there is an interesting size effect to be considered. Experimental studies in titanium and zirconium from the literature show that thin micro-hydrides withstand significant plastic deformation without cracking. In both materials slip bands are observed to cross the interface, the critical thickness below which this happens being hundreds of nanometres.

However, the reasons behind this turning point are not understood. In this work, planar discrete dislocation plasticity (DDP) is deployed to investigate this thickness effect. Given the zirconium-hydride orientation relationships, there are two possibilities, as both screw and edge type dislocations are able to cross the interface while satisfying the criteria for slip transmission. Both cases are analysed. Key parameters are the strength of the interface to dislocation crossing and the resistance offered by the hydride to their motion. This controls where dislocations accumulate, and therefore where cracks are likely to be initiated.

It is argued that as the hydride thickness increases, the higher plastic strain to be accommodated causes an increase in the number of internal dislocations. This forest hardening leads to very high internal glide dislocation densities. Furthermore, interfacial dislocations that come from the misfit at the semi-coherent interface may promote the formation of glide dislocation pile-ups in front of the hydride. The two scenarios are explored varying the hydride thickness.

Novel aspects of this study are: the implementation of a screw version of 2D DDP, and its combination with conventional edge 2D DDP to feed into a three-dimensional stress analysis.