COMPARISON BETWEEN THE DENDRITE AND INTER-DENDRITE REGIONS IN THE AS CAST AlCoCrFeNi HIGH ENTROPY ALLOY

The concept of the High Entropy Alloys (HEA) was first mentioned in [1]. Amid the myriad compositions of HEAs, the equiatomic AlCoCrFeNi (here after HE63) is among the most investigated compositions. According to previous reports HE63 tends to solidifies in a dendritic regime, with distinct compositional differences between the dendrite core (DR) and inter-dendrite (ID) region. Both regions are characterized by ordered BCC (B2) matrix, rich in Al+Ni, embedded by nano-metric interconnected BCC precipitates, rich in Fe+Cr. The morphology of the precipitates differs between the ID and DR regions. Up to date no report referred to differences in the phase content between the DR and ID regions.

Light microscope images of the as cast HE63 polished surfaces revealed a dendritic morphology. Differences in micro-hardness between the ID and DR regions, as the dendrite core was found to be softer than the inter-dendrite region. According to TEM analysis, in both regions the precipitates consist of BCC structure. Yet, while the matrix in the ID region consist of B2 structure, as previously reported, in the DR the matrix consist of FCC and B2 in an eutectic-like morphology. The differences in the phase composition was explained by differences in the Ni:Al ratio between the two regions: this ration is higher in the DR matrix and a higher Ni content promotes the formation of an FCC structure. The lower hardness measured in the DR region can be explained by the presence of the FCC phase, which is generally more ductile hence softer then a BCC structure.

References:

[1] Jien-Wei Yeh, Swe-Kai Chen, Advanced Engineering Materials. 6 (2004) 299.