Microstructural Evolution and Mechanical Properties of Ni57Nb33Zr5Co5 Metallic Glass

The Ni57Nb33Zr5Co5 metallic glass is a promising alloy for bipolar plates in proton exchange membrane fuel cells. It is important to know which phase forms in this alloy under different cooling rates in order to understand its influence on the thermal stability and mechanical properties. In this work, melt-spun ribbons and rod samples with 1, 2 and 3 mm diameters were prepared and their phase formation, microstructure and mechanical properties were investigated by X-ray diffraction, differential scanning calorimetry, optical microscopy, scanning electron microscopy and microhardness. It is found that a completely crystalline structure forms in the lower cooling rate samples (2 and 3 mm diameter rods) with the presence of the equilibrium phases Ni-3 (Nb, Zr) and Nb7Ni6 as primary phases or as a very fine eutectic structure, while a fully glassy structure is attained in the samples with the highest cooling rate (ribbons). For the sample with an intermediate cooling rate (1 mm diameter rod), polymorphically crystals of an unknown metastable phase with spherical morphology precipitate in the glassy matrix with virtually the same composition as the matrix. The 2 mm diameter sample exhibits higher hardness than the other samples, which is attributed to its very fine eutectic colonies.