Abstract
The eutectic high entropy alloy Nb0.73CoCrFeNi2.1 was manufactured by means of arc smelting and the obtained ingots were cut into 300-μm-thick foils, which were used as filler alloys to braze Crofer 22 APU to Hf-metallized yttria-stabilized zirconia (3YSZ). The brazing process was conducted in a vacuum furnace at 1200 °C for 5 min at a vacuum of 4.3·10–4 mbar. In order to minimize the intense diffusion and erosion of the steel substrate, a heating and cooling rate of 50 K/min was applied. Sound joints without any pores or flaws were obtained. The microstructure of the joints consisted of an HfO2 reaction layer at the ceramic interface and the same eutectic structure consisting of a Laves phase and a solid solution that was already detected in the smelted foil. The average hardness of the microstructure in the joint seam amounted to 352 ± 17 HV0.01 and the joints reached strength values up to 97 ± 7 MPa while the fracture area was always located at the ceramic interface in the HfO2 layer. Comparable joints, with AgCuTi3 as filler metal, brazed at 920 °C, only reached a shear strength of ~ 52 ± 2 MPa.
Highlights
Construction materials are based on one principal element or binary compound that is doped with small amounts of alloying elements in order to enhance the material properties
This research revealed that eutectic high-entropy alloys are applicable as high temperature filler metals tor seal materials in solid oxygen fuel cells (SOFCs)
Sound joints could be obtained for Crofer 22 APU and hafnium-metallized yttria-stabilized zirconia (YSZ) bonds by using Nb0.73CoCrFeNi2.1 as a filler metal
Summary
Construction materials are based on one principal element or binary compound that is doped with small amounts of alloying elements in order to enhance the material properties In this regard, most alloying elements that are used tend to form intermetallic phases when added in larger. Recommended for publication by Commission XVII - Brazing, Soldering and Diffusion Bonding In this regard, the thermodynamic stability of a phase at given environmental conditions is determined by the Gibbs energy, which depends on the enthalpy as well as the product of the entropy and temperature. This indicates that a maximization of the configurational entropy can be achieved by increasing the amount of elements added in an equimolar ratio While taking this into consideration, the Gibbs energy of solid solutions can be higher than the energy of intermetallic phases, especially at high temperatures. This study includes the production of the filler alloy, the characterization of the microstructure, and the determination of the shear strength of the joint
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