AbstractsEngineering

The structural evolutions during the devitrification of three bulk metallic glasses, including Zr52.5Cu17.9Ni14.6Al10Ti5 (BAM-11), Zr50Cu40Al10 and Mg65Cu25Tb10, have been studied in-situ by neutron and synchrotron X-ray scattering. By measuring simultaneously the wide-angle X-ray scattering (WAXS) patterns and the small-angle X-ray scattering (SAXS) patterns, a good comparison of the structural evolutions at the atomic scale and nano scales during the different transformation stages are achieved. Also the evolutions of pair distribution functions (PDFs) in real space have been measured by neutron and X-ray total scattering. These results are further complemented by ex-situ experiments including DSC, conventional XRD, TEM and SEM. The results show all three samples are amorphous in the as-cast state, and that phase separation precedes crystallization for Zr-based BMGs. BAM-11 devitrifies in two stages; the first stage is related to topological changes within a 2 nm range, and a metastable fcc NiZr2 phase is formed, which transforms to the tetragonal NiZr2 phase in the second stage. Above 690 K, Zr50Cu40Al10 decomposes to form the Cu10Zr7 phase and CuZr2 phase, which transform to the CuZr phase eutectoidly at higher temperature. Mg65Cu25Tb10 devitrifies in three stages, the first stage is related to the formation of icosahedral clusters, the second and third stages are the formation of quasicrystalline phase and further decomposition. Kinetic model and SAXS model are proposed and applied to refine the patterns. The topologies of conventional oxide glasses and BMGs are compared and summarized. The nonperiodic order plays an important role in nanometer scale for BMGs.