Invited
WHAT ARE THE CAUSES FOR THE DIFFERENCES IN PHASE TRANSFORMATION BETWEEN THE DENDRITE AND INTER-DENDRITE REGIONS IN AlCoCrFeNi HIGH-ENTROPY ALLOY?

In the dendritically solidified AlCoCrFeNi high-entropy alloy (HEA) the dendrite core (DR) and inter-dendrite (ID) regions consist of Cr, Fe-rich body-centered cubic (BCC) particles and Al, Ni-rich B2 ordered BCC matrix. In the DR region BCC particles are nanosized cuboids while in the ID regions their morphology is “Chinese letters”-like. The differences between the regions affect phase transformations occurring upon exposure to high temperatures. In the ID region the BCC phase transforms to FCC and σ phase, while only minor transformation to FCC is detected in the DR region. The current research was aimed to understand these differences.

Both B2 or BCC phases display identical compositions in the DR and ID regions. Thus, the causes for the differences in phase transformations are not compositional by nature. On the other hand, antiphase boundaries (APBs) were detected in the B2 matrices of both regions but with different ranges of order: short-range in the ID region and long-range in the DR region. Strain assessment near particles/matrix interfaces reveals further differences between DR and ID regions. To our understanding, range of APB order and the strain associated with them, are dominant factors influencing the phase transformations. This understanding is supported by Differential Scanning Calorimetry (DSC) measurements whereas an exothermic event, that may be related to a stress release, was detected at 700^oC, after the completion of the transformation to FCC and σ. Furthermore, TEM examination of samples after heat treatment at 850^oC revealed long-range ordering of the B2 domains in both the DR and ID regions, which implies that the stress release detected by DSC measurement origins from the ordering of the domains.