In situ visualization of Ni-Nb bulk metallic glasses phase transition
Résumé
We report the results of in situ investigation of the structural evolution and crystallization behavior of Ni-based bulk metallic glass. The X-ray diffraction, transmission electron microscopy, nanobeam diffraction, differential scanning calorimetry, radial distribution function and scanning tunneling microscopy (STM)/spectroscopy techniques were applied to analyze the structure and electronic properties of Ni63.5Nb36.5 glasses before and after crystallization. According to our STM measurements, the primary crystallization originally starts with the Ni3Nb phase formation as a leading eutectic phase. It was shown that surface crystallization differs drastically from bulk crystallization due to the possible surface reconstruction. The mechanism of Ni63.5Nb36.5 glass alloy two-dimensional crystallization was suggested, which corresponds to the local metastable (3 x 3) - Ni(1 1 1) surface phase formation. The possibility of different surface nanostructures developing by annealment of the originally glassy alloy in an ultrahigh vacuum at a temperature lower than the bulk crystallization temperature was shown. The increase of the mean square surface roughness parameter R-q while transforming from a glassy to a fully crystallized state can be caused by concurrent growth of Ni3Nb and Ni6Nb7 bulk phases. The simple empirical model for the estimation of Ni63.5Nb36.5 cluster size was suggested, and the value obtained (about 8 angstrom) is in good agreement with the corresponding STM measurements (8-10 angstrom). (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.