Uncovering the short range order and local distortions in high entropy alloys using EXAFS

Local distortions and short-range order are the key to understand the solid solution mechanical behavior of the novel material class of High Entropy Alloys, more than the individual properties of alloying elements, such as atomic radius or shear modulus. Low energy X-ray absorption spectroscopy performed at PHOENIX beamline, together with BM08 (ESRF) data allowed obtaining local structure information on a chosen compositionally complex alloy, Al₈Cr₁₇Co₁₇Cu₈Fe₁₇Ni₃₃, especially highlighting the role of Al. XAS provided qualitative information about the electronic structure and quantitative information on local structure and partial chemical ordering: Al short-range chemical ordering and local lattice distortions in Al₈Cr₁₇Co₁₇Cu₈Fe₁₇Ni₃₃ were identified and quantified experimentally for the first time in a compositionally complex alloy. EXAFS analysis identified local distortions and a higher affinity of Al for heavier 3d metals, with Cu-Al being the preferred pair. The amount of local distortions in Al₈Cr₁₇Co₁₇Cu₈Fe₁₇Ni₃₃ cannot result in lattice distortions in the fcc unit cell larger than ~0.04 Å. These values are much smaller than those predicted solely by atomic radii considerations. The Al metallic radius in the Al₈Cr₁₇Co₁₇Cu₈Fe₁₇Ni₃₃ is estimated to be of the same order than the average transition metal one, i.e. 1.26 Å, much lower than the tabulated value of 1.43 Å of pure Al. Al reduces its metallic radius in order to accommodate into the lattice, lessening its distortion. Two important points in the characterization of HEA, namely local chemical ordering and local lattice distortions, were clarified providing a tool to answer the open questions reported in recent reviews.

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