Layer-stacking irregularities in C36-type Nb-Cr and Ti-Cr Laves phases and their relation with polytypic phase transformations

Specific layer-stacking irregularities have been identified in C36 (4H) Nb-Cr and Ti-Cr Laves phases on the basis of X-ray diffraction line-profile analysis and high-resolution transmission electron microscopy. Domain boundaries and transformation errors within domains could be distinguished. The layer-stacking irregularities in both C36-NbCr2 and C36-TiCr2 can be associated with a preceding C14 (2H) C36 (4H) phase transformation carried out by glide of mobile synchro-Shockley partial dislocation dipoles in an ordered fashion. The stacking irregularities observed can be interpreted as deviations from such perfect oordered glideo. The interpretation is supported by the observation that, in the case of C36-NbCo2, where no preceding C14 C36 transformation occurs, different layer-stacking irregularities are observed.