Role of crystal defects on brittleness of C15 Cr_2Nb Laves phase

摘要

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 Cr_2Nb 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 syn-chroshear 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 Cr_2Nb, 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 Cr_2Nb. This provides new insights into the origin of low temperature brittleness of Laves compounds with complex structures.