In situ visualization of Ni-Nb bulk metallic glasses phase transition

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 Ni_63.5Nb_36.5 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 Ni_63.5Nb_36.5 glass alloy two-dimensional crystallization was suggested, which corresponds to the local metastable (3 7times; 3)-Ni(111) 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 Ni_63.5Nb_36.5 cluster size was suggested, and the value obtained (about 8Å) is in good agreement with the corresponding STM measurements (8.10Å).