Abstract
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 Ni3(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.
Highlights
Ni-based metallic glasses have generated substantial recent interest for their potential use as bipolar plates in Proton Exchange Membrane Fuel Cells (PEMFCs)[1,2,3], primarily due to their higher corrosion resistance, compared with conventional stainless steels (i.e., 316L, SUS316L), their high mechanical strengths, and their easy of processing in the supercooled liquid state
In order to control and improve the properties of bipolar plates, an investigation of the influence of cooling rate on the phase formation and microstructure of the Ni57Nb33Zr5Co5 metallic glass composition was initiated with the goal of characterizing its potential use as bipolar plates in PEMFCs
The 2 and 3 mm diameter rods exhibit sharp Bragg peaks, whereas the 1 mm diameter rod exhibits an almost fully amorphous structure confirmed by a presence of a broad diffraction peak around 42o and a lower intensity sharp diffraction peak around 31o
Summary
Ni-based metallic glasses have generated substantial recent interest for their potential use as bipolar plates in Proton Exchange Membrane Fuel Cells (PEMFCs)[1,2,3], primarily due to their higher corrosion resistance, compared with conventional stainless steels (i.e., 316L, SUS316L), their high mechanical strengths, and their easy of processing in the supercooled liquid state. According to Hu et al.[4], among the Ni-based metallic glasses, the Ni57Nb33Zr5Co5 alloy shows high glass forming ability (GFA) and high corrosion resistance in 6 M HCl, open to air at room temperature. While Inoue et al.[1] have investigated the use of Ni-based glasses as bipolar plates under simulated conditions of PEMFCs, no studies exist on the use this particular alloy under PEMFC working conditions and no systematic investigation of the correlation between phase formation, microstructure and properties of this Ni57Nb33Zr5Co5 metallic glass have been reported. The characteristics of phase formation and microstructure, which may affect the thermal stability and mechanical properties of samples have been determined, for different rapidly solidified Ni57Nb33Zr5Co5 samples (i.e., melt-spun ribbons and chill-cast rods of 1, 2 and 3 mm diameter) using X-ray diffraction (XRD), differential scanning calorimetry (DSC), optical microscopy, scanning electron microscopy (SEM) and microhardness
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