Laves phase and equiaxed grains formation in directed energy deposited AlCuFeNiTi high entropy alloy

Abstract

In this study, an equiatomic AlCuFeNiTi high entropy alloy (HEA) was successfully fabricated for the first time using pre-alloyed powder with the directed energy deposition method on a Ti-6Al-4V substrate. The alloy was characterized using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), X-ray diffraction (XRD), transmission electron microscopy (TEM), and nanoindentation to provide insight into the relationship between its microstructural and nanoscale mechanical properties. The microstructure has three distinct regions, resulting from variation in the alloy's local elemental and crystallographic composition that influences its mechanical properties. The alloy was found to be multiphase with a mostly ordered Heusler structure (L21) and a small amount of FCC structure with traces of C14 Laves in them. Needle-like and spherical-shaped Cu-rich precipitates were also observed in the dendrites. During the solidification process, evidence of dendritic fragmentation was observed, contributing to the alloy having mostly equiaxed grains rather than the commonly observed columnar grains in previously reported additively manufactured (HEAs). Nanoindentation results showed that the dendrites constitute a hard zone, whereas the inter-dendritic regions typify a continuous soft zone, with the dispersed Laves phase having the highest hardness. The presence of Laves phase inside the grain boundaries and its distribution as small-size grains strengthens the grain boundaries. Results highlight the synthesis of an equiaxed HEA with multiple distinct regions that influence its overall mechanical properties.