Development of new high entropy alloys for brazing of Ni-base superalloys

Abstract

High entropy alloys (HEAs), also called multi-principal component alloys, are novel alloys containing at least five elements with an atomic percent ranging between 5% and 35%. HEAs have attracted attention due to their unique properties such as high strength, good ductility, and high resistance to corrosion and wear. However, applications of HEAs still need to be explored. In this study, a new HEA filler metal (Fe5Co20Ni20Mn35Cu20) was designed for brazing of Inconel 600 alloy. Thermodynamic simulation, using Thermo-Calc software, was performed to design the composition of the HEA. A face-centered cubic (FCC) crystal structure was achieved that balanced strength, ductility and a melting range lower than the solidus temperature of Inconel 600. Differential thermal analysis (DTA) confirmed that the as-cast HEA had a solidus temperature of 1080 °C and a liquidus temperature of 1150 °C, which were in good agreement with the melting range predicted by Thermo-Calc using the newly published “HEA 1.0” database. Only a 10 °C discrepancy was identified. Through wetting angle tests at various temperatures, the optimum brazing temperature for this HEA was determined to be 1200 °C. The as-cast HEA button was cold rolled into foils with different thickness ranging from 50 to 300 µm for brazing tests. The brazing time was varied from 15 to 120 min. Effect of brazing time and foil thickness on the shear strength of the brazed joint were evaluated. A maximum shear strength of 530 MPa was achieved with a brazing time of 90 min and a foil thickness of 300 µm. With a fixed brazing time of 90 min, the shear strength reduced continuously to a minimum value of 302 MPa with decreasing foil thickness. Microscale digital image correlation (DIC) was used to map localized strain distribution at the brazed joints during the shear test. Moreover, metallurgical characterizations including optical microscopy (OM), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) were carried out at the interface to explain the observed mechanical behaviors. In addition, interface reactions between as-cast, oxidized FeCoNi(AlSi)0.2 HEA and aluminum melt at 700 °C were investigated to evaluate the alloy’s potential for use as mold material for casting of aluminum alloys. It was found that, compared with Fe, the as-cast and oxidized HEA have a thinner reaction products layer. Oxidized HEA had the thinnest layer. Pre-oxidation treatment of HEA is an effective and economical way to improve the mold material’s resistance to aluminum melt attack.