Effects of Al Addition on the Corrosion and Mechanical Properties of NbTaTiV High Entropy Alloy

High-entropy alloys (HEAs) are known for their excellent microstructural stability, mechanical properties, and corrosion resistance owing to the high configurational entropy by multiple principal elements. NbTaTiV, a refractory HEA, is particularly interesting because of its high melting point (2275°C), good ductility with high compressive fracture strain (&gt;40%), high yield strength and wear resistance. Alloying with other elements, such as Al, Cr, and Zr, has the potential to further improve these properties. In this study, we investigated the impact of Al addition at an equimolar ratio on the NbTaTiV HEA system, as it not only has the potential to improve the corrosion resistance and mechanical properties but also reduces the alloy’s density and production cost.<br/>The alloys were prepared by casting and homogenization. X-ray diffraction (XRD), backscattered electron scanning electron microscopy (BSE-SEM) and energy dispersive X-ray spectroscopy (EDS) techniques were used for the characterization of the alloys. To evaluate the corrosion resistance, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization experiments were performed in a 3.5wt% NaCl solution at room temperature. Optical microscopic photographs of the surface were taken before and after the corrosion experiments to detect the presence of pitting or crevice corrosion. The localized mechanical properties of the alloys, such as hardness and modulus, were measured using nanoindentation.<br/>The XRD results revealed that both alloys are BCC solid solutions. The BSE-SEM and EDS images revealed a lower level of elemental segregation in the AlNbTaTiV HEA in contrast to the NbTaTiV HEA. The EIS results indicated that the addition of Al leads to an increase in the charge transfer resistance (~40%), which means higher corrosion resistance. However, the potentiodynamic test revealed that above 1.5V, the AlNbTaTiV HEA’s passive film dissolves leading to pitting corrosion, whereas the NbTaTiV HEA formed a second metastable passive film that protected it from pitting. Nanoindentation tests showed that adding Al increases the hardness (~10%) and modulus (~10%) of the matrix. Al addition also increased the density of titanium precipitation, especially along grain boundaries, which may enhance the alloy's strength, hardness, creep resistance, and wear resistance.<br/>In summary, adding Al to the NbTaTiV HEA can improve both the mechanical properties and corrosion resistance of the alloy, except in scenarios where pitting corrosion might occur. These findings contribute to the ongoing development of HEAs for better performance in various industrial applications.<br/>Acknowledgements:<br/>This research was funded by the U.S. National Science Foundation (NSF), Award Number: 2138674