Effect of minor Si additions and cooling rate on the phase formation and properties of glass former Ni57Nb33Zr5Co5 alloy

Abstract

Ni57Nb33Zr5Co5 metallic glass is a promising candidate to be used as bipolar plates in Proton Exchange Membrane Fuel Cells. Nevertheless, this alloy presents a low glass-forming ability (GFA) and a relatively low supercooled liquid stability. A method well-known to improve the GFA of metallic glasses is via minor additions of alloying elements with a large difference in size and negative enthalpy of mixing relative to the elements in the glass. Using this approach, Si was identified to be a potentially good candidate to improve the GFA of the Ni57Nb33Zr5Co5. The aim of this work was to investigate the effect of Si additions and cooling rate on the formation and properties of this alloy. Ribbons and wedge shape samples of(Ni57Nb33Zr5Co5)100-xSix (x = 0.5, 1, 2 and 3 at.%) were prepared by melt spinning and suction casting, respectively, and characterized by X-ray diffraction, optical and scanning electron microscopy, differential scanning calorimetry and microhardness. The results show the addition of Si increases the supercooled region up to 84% mainly because of a significant decrease in the glass transition temperature but the additions do not improve the GFA of the Ni57Nb33Zr5Co5. The wedge samples showed that the critical diameter for amorphization is below 1 mm thickness and for larger sections, eutectic colonies of NiNb and Ni3Nb form together with primary Ni3Nb. It was observed that the addition of Si favors the formation of this primary phase, which increases in amount and size. High hardness values around 1150 HV were measured for regions with a fine eutectic structure and low amount of the primary phase.