Isothermal Treatments of AlCoCrFeNi High Entropy Alloy Produced via Binder Jetting and Direct Energy Deposition: Microstructural Modification and Analysis

Thermally-induced transformation is a vital component of the processing-structure-property relationship in alloys. The influence of this type of phenomenon during and after material fabrication is fundamental to microstructural evolution in high entropy alloys. In this study, the response of additively manufactured high entropy alloy (AlCoCrFeNi) samples to isothermal heat treatment was investigated and compared with the as-printed samples. The equiatomic AlCoCrFeNi high entropy alloy was fabricated via two additive manufacturing routes: binder jetting 3D printing (BJ3DP) and direct energy deposition (DED). The microstructural evolution was studied using backscattered electron images from scanning electron microscopy (SEM). Energy dispersive spectroscopy (EDS) was used to probe the constituent’s composition and distribution. The different phases present in the microstructure were investigated using X-ray diffractometer (XRD) and electron-backscattered diffraction (EBSD) techniques. Moreover, the mechanical properties data captured via nanoindentation were evaluated and mapped to show a representation of how they vary through the bulk alloy. Interplays of these essential parameters are responsible for the various mechanical properties exhibited by the alloy. The isothermal heat treatments significantly modified the microstructures of the alloy. However, the BJ3DP and DED samples respond differently, as diffusion mechanisms like grain boundary precipitation, precipitate-free zones (PFZ), and Widmanstatten structure were more observable in the BJ3DP samples. The BJ3DP samples exhibit higher values of mechanical properties than the DED samples. The higher mechanical properties in BJ3DP samples are also linked to intermetallics such as the sigma phase formed between 450°C - 1000°C and ordered BCC (B2) which is stable up to the melting point of the alloy. The ongoing exploits provide significant insight into how to exploit the immense opportunities the thermally modified AlCoCrFeNi high entropy alloy has for fields of applications such as the tools, automobile, structures, and aerospace industries.