Role of crystal defects on brittleness of C15 Cr2Nb Laves phase

First-principles calculations, high-resolution transmission electron microscopy (HRTEM) investigations and geometrical phase analysis of lattice strain based on HRTEM images have been carried out on C15 Cr2Nb Laves phase. Asymmetrical nanoscale regions with severe lattice distortion were observed in the vicinity of Shockley partial dislocation cores. These disturbed regions are the result of synchroshear inside the Nb-Cr-Nb triple layers. Reactions of dislocations with twin boundaries (TBs) also result in severe lattice distortion. The resulting local lattice distortion and perturbed strain field deleteriously impact dislocation-governed plasticity. First-principles calculations show that cleavage occurs preferentially between the single Cr layer and the Nb-Cr-Nb triple layer in these materials. TBs, which are on single Cr layers, nucleate and propagate cracks readily in Cr2Nb, as evidenced by the lattice dilatation along a TB interacting with one 60° dislocation. The present study shows atomic level evidence on how crystal defects and their interactions influence the mechanical properties, especially the poor toughness at low temperatures, of C15 Cr2Nb. This provides new insights into the origin of low temperature brittleness of Laves compounds with complex structures.