Self-accommodated defect structures modifying the growth of Laves phase

Abstract

Laves phases, with the topologically close-packed structure and a chemical formula of AB2, constitute the largest single class of intermetallics. Planar defects in Laves phases are widely investigated, especially for stacking behavior transformations through synchroshear. Here, we report the coexistence of C14, C36 and C15 structures in MgZn2 precipitates by using atomic resolution scanning transmission electron microscopy, verifying the previously predicted Laves phase transformation sequence of C14 → C36 → C15 also applies to MgZn2. One type of stacking fault couple in precipitates has been found to alone reduce the lattice mismatch with matrix, while some other stacking fault couples need to self-accommodate with irregular planar defects (rhombic units and flattened hexagonal units), or with five-fold symmetry structures to relieve the strain concentration. Precipitates thus grow towards an equiaxed or even round morphology, rather than the plate morphology as conventionally believed. Molecular dynamics calculations are performed to support our analysis. These findings reveal the principles governing the concurrent occurrence of various defects in laves structures, acting as an update of the widely accepted perception of random occurrence of defects during crystal growth.