Oxidation of glassy Ni–Nb–Sn alloys and its influence on the thermodynamics and kinetics of crystallization

The best bulk metallic glass (BMG) forming systems are very viscous multi-component liquids, whose sluggish kinetics is the most important factor that impedes the nucleation and growth of crystals. However, it is usually the case that these BMG systems are a mixture of elements with very different nobility, where preferential oxidation and partitioning are common processes observed upon reaction with oxygen. The atomic partitioning induces a change in composition that thermodynamically destabilizes the amorphous phase and promotes crystallization. In this work, we show that adding a third element can suppress this oxidation-induced crystallization if it has a large size mismatch with respect to the base elements. The Ni–Nb-based BMG system is taken as a case study system to elucidate the thermodynamic and the kinetic reasons for the improved thermal stability of the amorphous phase upon addition of Sn. Due to the selective oxidation of Nb, atomic partitioning of Ni and Sn takes place underneath the oxide scale, and favors the nucleation of Ni3Sn instead of the otherwise thermodynamically stable Ni2NbSn ternary phase. Due to the sluggish diffusivity of the large Sn atoms ahead of the internal oxidation reaction front, the nucleation process is slow and the overall oxidation kinetics that are parabolic for the binary glass assume passive logarithmic behavior for the ternary glass counterpart. Microscopy experimental techniques include electron backscattering diffraction (EBSD) mapping and atom probe tomography (APT). [Display omitted]