Discovery of a bulk C36-type MgZn2 structure step by step transformed from the C14 prototype laves phase structure

摘要

MgZn2 phase is the prototype of hexagonal C14 Laves phase, acting as an important intermetallic compound. Here we first observe the atomic-scale structure of C36-type MgZn2 Laves phase by high-angle annular dark-field scanning transmission electron microscopy. Solid atomic-scale evidence is provided for the finding that C14 structural precipitate can totally transform to C36 structure during extremely long-time over-aging at medium temperatures in an Al-Zn-Mg alloy, rather than C36 stacking faults as previously reported. Intermediate structures between C14 and C36 were observed along [0001] and [(1) over bar2 (1) over bar0] directions. Two possible transformation paths are illustrated based on the synchroshear mechanism, and their atomic resolution evidence is provided. Ab initio molecular dynamics calculations support our inference on the transformation process. This type of Laves phase transformation provides a new strategy for designing alloys that contain Laves phases as hardening precipitates.