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Short range order and local compositional fluctuations in refractory high entropy alloys

One of the still mysterious effects in high entropy alloys (HEAs) is how atoms in highly supersaturated solid solutions locally arrange in the given crystal lattice. Recent investigations indicate that chemical short range order (SRO) and local compositional fluctuations are characteristic for HEAs, which can significantly affect the mechanical properties. In this project, the characteristics of short range order and local compositional fluctuations in refractory high entropy alloys are revealed at atomic resolution and are correlated to micro- and macroscopic mechanical properties.

Aberration-corrected high angle anular dark-field (HAADF) STEM is an ideal technique to direclty image local lattice imperfections and their compositional fingerprint simultaneously. Figure 1 shows an atomic resolution high angle anular dark-field (HAADF) STEM image of an as-cast refractory Ti4Zr4Hf3Ta HEA. We observed strong contrast modulations on different length scales within the HEA matrix. In correlation with near atomic resolution energy-dispersive X-ray spectroscopy, it is confirmed that the bright and dark contrast modulations are related to compositional fluctuations with a wave length of ~2-4 nm. On an even smaller scale, the contrast differences of neighboring atomic columns indicate that atoms are short range ordered within the body-centered cubic (BCC) lattice. After deformation, the alloy partially transforms to the hexagonal-closed packed (HCP) structure as shown in Figure 2. The results from electron backscatter imaging confirm that a dual-phase BCC-HCP microstructure evolves after room temperature compression.

In future work, we aim to correlate the impact of SRO and local compositional fluctuations on the BCC-HCP phase transformation and the corresponding mechanical properties. Resolving the interaction of dislocations with short range ordered domains or nanoscale regions with different chemical composition will provide a mechanistic understanding of the bulk mechanical properties.

Figure 1: Atomic resolution HAADF-STEM image of an as-cast refractory Ti4Zr4Hf3Ta high entropy alloy.

Figure 2. EBSD results of the refractory Ti4Zr4Hf3Ta high entropy alloy after room temperature compression. (a) inverse pole figure + image quality map; (b) phase map.

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