Development of Non-Equimolar CoCrCuFeNi High Entropy Alloys for Aerospace Brazing

The manufacturing of high temperature heat exchangers and precoolers within the aerospace industry often requires the joining of thin-walled components through brazing. Existing commercially available brazing alloys are often relatively hard with brittle failure modes despite ductility and strength being desirable properties of a brazed joint. High entropy alloys have been demonstrated to have desirable material properties such as high ductility and strength. Previous work on CoCrCuFeNi has demonstrated that the addition of melting point depressants such as Boron, are able to beneficially reduce the brazing temperatures sufficiently to allow brazing of steel components while maintaining a strong and ductile joint. The current work has focussed on finding new, non-equimolar, HEA compositions with a lower targeted melting point for a wider range of brazed substrate materials. Alloy compositions were down-selected through empirical thermodynamic classification and CALPHAD simulations. Identified potential compositions were synthesised using induction casting, and solidus and liquidus measured using DSC. Phases were confirmed using a combination of microscopy, hardness and XRD analysis. The best alloy candidates were then modified with the addition of Boron to further reduce the melting point to meet the required manufacturing temperatures of the joint. Finally, shear strength measurements were carried out on the samples which met the brazing temperature requirements.