Probing Phase Evolution Behavior during Nanocrystallization of Metallic Glass Using Positron Annihilation Spectroscopy

Positron annihilation lifetime spectroscopy and coincidence Doppler broadening (CDB) techniques have been used to study the nanocrystallization behavior of soft magnetic materials, using Metglas 2826MB as an example. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) characterization techniques revealed the presence of two phases, γ-(Fe,Ni) and (Fe,Ni,Mo)₂₃B₆, as well as the size and volume fraction of the phases following predetermined thermal annealing. Two distinct positron lifetime components have been observed in both amorphous and nanocrystallized samples. In the nanocrystallized samples, it has been demonstrated unambiguously that small and large lifetime components were due to positron annihilation with crystalline nanophases and with amorphous-crystalline or intercrystalline interfaces, respectively. First-principle calculation of the positron lifetime and electron momentum distribution in crystalline phases, supplemented by TEM and XRD studies, helped in the unambiguous interpretation of the experimental observation. This study throws new insights into positron behavior in metallic glasses, especially in the presence of single or multiple nanophases embedded in the amorphous matrix.