Cross-correlative TEM and atom probe analysis of partial crystallisation in NiNbSn metallic glasses

Devitrification in a Ni60Nb35Sn5 bulk metallic glass alloy is investigated below the glass transition temperature using cross-correlative TEM and atom probe tomography. As cast and post-anneal (500 degrees C for 2 h) specimens were analysed in both the TEM and the atom probe to determine the extent and nature of any phase separation in the as cast material or devitrification in the annealed material. Furthermore, it was of interest to determine the accuracy of the atom probe reconstructions of both types of material. No significant compositional partitioning was observed in the as cast state, suggesting an amorphous material, and the atom probe reconstructions of these specimens were found to be consistent with the TEM data. The annealed specimens contained Nb and Nb oxide rich regions, Ni3Sn nanocrystals, and a significant build-up of oxygen at the boundaries between these nanocrystals and the remaining amorphous matrix regions. These oxygen enriched regions are shown to result in anomalous atom probe reconstructions that are inconsistent with the TEM analyses. It is theorised that the initial crystallisation of Nb and Nb oxide rich regions results in NiSn rich regions, thereby resulting in the formation of Ni and Ni3Sn crystalline precipitates.