Processing, crystallization and oxidation of the Ni-Nb(-Sn) refractory alloy glass system

摘要

In this work, the topics of processing, crystallization and isothermal oxidation, below the glass transition temperature, of Ni-Nb(-Sn) based bulk metallic glasses are studied. In addition to the as-cast amorphous structures, the thermally treated amorphous structures, as well as the nanocrystalline and microcrystalline structures are studied. Characterizations were conducted by employing various experimental techniques, such as differential thermal analysis, x-ray diffraction, thermogravimetry, electron microscopy and atom probe tomography. The results show that the microcrystalline structures obey purely parabolic laws. The as-cast amorphous structures of Ni59.35Nb34.45Sn6.2, Ni57Nb34Sn9 and Ni58.85Nb34.45Sn6.2Si0.5 oxidize in a paralinear manner, while Ni70Nb30, Ni62Nb38 and Ni60Nb36Sn3B1 oxidize parabolically. The microcrystalline samples have lower parabolic oxidation rates than the as-cast amorphous structures with the same composition. Thermally treated amorphous- and especially the nanocrystalline- structures show significantly better oxidation resistance than the as-cast structures. The oxidation of the amorphous structure is accompanied by crystallization of Ni-rich intermetallics, among which Ni3Sn is found. Ni3Sn is not found as a product of devitrification in Ar. The results are discussed in terms of thermodynamics and kinetics of oxidation and crystallization.