Atomic scale compositional characterization of a nanocrystalline AlCrCuFeNiZn high entropy alloy using atom probe tomography

We have studied a nanocrystalline AlCrCuFeNiZn high entropy alloy synthesized by ball milling followed by hot compaction at 600 C for 15 min at 650 MPa. X ray diffraction reveals that the mechanically alloyed powder consists of a solid solution body centered cubic bcc matrix containing 12 vol. face centered cubic fcc phase. After hot compaction, it consists of 60 vol. bcc and 40 vol. fcc. Composition analysis by atom probe tomography shows that the material is not a homogeneous fcc bcc solid solution but instead a composite of bcc structured Ni Al , Cr Fe and Fe Cr based regions and of fcc Cu Zn based regions. The Cu Zn rich phase has 30 at. Zn a brass composition. It segregates predominantly along grain boundaries thereby stabilizing the nanocrystalline microstructure and preventing grain growth. The Cr and Fe rich bcc regions were presumably formed by spinodal decomposition of a Cr Fe phase that was inherited from the hot compacted state. The Ni Al phase remains stable even after hot compaction and forms the dominant bcc matrix phase. The crystallite sizes are in the range of 20 30 nm as determined by transmission electron microscopy. The hot compacted alloy exhibited very high hardness of 870 10 HV. The results reveal that phase decomposition rather than homogeneous mixing is prevalent in this alloy. Hence, our current observations fail to justify the present high entropy alloy design concept. Therefore, a strategy guided more by structure and thermodynamics for designing high entropy alloys is encouraged as a pathway towards exploiting the solid solution and stability idea inherent in this concept