High Entropy Alloy Deposition from an Aqueous Bath

Abstract

A new alloy design approach based on the concept of high entropy alloy (HEA) has attracted attention during the last few years. HEA is defined as an alloy composed of five or more principal elements with a concentration between 5 and 35 at% of each element. Depending on the combinations of the elements, HEA could show high resistance to corrosion, fatigue and fracture toughness besides unique electrical and magnetic responses. The outstanding characteristic of HEA is attributed to the high mixing entropies and the severe lattice distortion related to the addition of multiple components. Due to this unique combination of properties and the potential for designing a wide variety of new materials, interests have been directed toward high entropy alloy films and coatings for applications where corrosion and oxidation protection are strongly required. This study attempted to deposit five elements (Ni-Co–Cu–Mo-W) from an aqueous bath on a low-carbon steel substrate by the direct current. The goal was to combine the properties of Ni-Co-Cu with Ni-W and Ni-Mo coatings. In this case, a HEA coating can be produced, which might maintain corrosion and wear resistance even at high temperatures and be suitable for application in aggressive environments. To understand better the role of entropy on the coatings properties, three electroplating systems were designed, including electroplating of five elements (Ni-Co-Cu-Mo-W), which was compared with electroplating of four elements (Ni-Co-Cu-W) and (Ni-Co-Mo-W). In this case, systems with different configuration entropy can be produced, and by characterizing them, the relationship between the entropy and the properties of the coating might be cleared.Electrodeposition was carried out in Hull-cell to find the alloy deposition's proper working window. The role of different parameters such as pH, current density, and complex agent on the coating's composition was studied. The morphology and composition of the Hull-cell coatings were checked by scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy along with X-ray fluorescence spectroscopy. Finally, the hardness and corrosion resistance of the coatings was studied by the nanoindentation technique and polarization in NaCl. This study developed a single aqueous bath for the electrodeposition of Ni-Co-Cu-Mo-W, Ni-Co-Cu-W and No-Co-Mo-W, and suitable deposition conditions were determined. It was shown that besides the metal concentration ratio, pH was the most influential parameter in the codeposition of elements with different deposition mechanisms. Ni and Co were the main inducing elements in the deposition of Mo and W than Cu. In addition, Ni, Co, Mo and W deposited uniformly, while Cu showed a different behavior. A competition between Mo and W deposition was seen, and Mo deposited more preferentially than W. The coatings showed weak passivity behavior and comparison between coating’s corrosion resistance was a challenge since their corrosion behavior does not only depend on coating composition but also on coatings compactness and integrity. By comparing the nano-hardness of the coatings, it was seen that the Ni-Co-Cu-W system with even the lowest configurational entropy has the highest hardness value. Keywords: Electrodeposition; High entropy alloy; Alloy deposition; hardness; Corrosion resistance