23 High entropy alloys based on the Ta-Nb-Mo-Cr-Ti-Al system are expected to possess high creep and 24 oxidation resistance as well as outstanding specific mechanical properties due to presumed high 25 melting points and low densities. However, we recently reported that arc-melted and subsequently 26 homogenized alloys within this system exhibit a lack of ductility up to 600 °C [H. Chen et al. in Metall. 27 Mater. Trans. A 49 (2018) 772-781 and J. Alloys Cmpd. 661 (2016) 206-215]. Thermodynamic

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... 28 calculations suggest the formation of a B2-type ordered phase below the homogenization temperature. 29 In the present article, we provide results of a detailed microstructural characterization of a series of 30 Ta-Nb-Mo-Cr-Ti-Al derivatives and evaluate if B2-type ordering could be the origin for the observed 31 lack of ductility. Backscatter electron (BSE) imaging, energy dispersive X-ray spectroscopy (EDX) and 32 atom probe tomography (APT) were used to verify uniform elemental distribution after 33 homogenization. X-ray diffraction (XRD) patterns indicate both, A2 or B2-type crystal structure, 34 whereas transmission electron microscopy (TEM) diffraction experiments unambiguously confirmed 35 B2-type order in the as-homogenized state of all investigated alloys. In MoCrTiAl, planar defects that 36 show antiphase boundary contrast with a {100}-type habit plane were detected by TEM dark field (DF) 37 imaging. They are wetted by a Cr-enriched and Ti-depleted layer as confirmed by scanning transmission 38 electron microscopy (STEM)-EDX line scans as well as APT analyses. The planar defects arise from a 39 disorder-order solid-state phase transformation during cooling, as indicated by differential scanning 40 calorimetry (DSC). 41 Keywords 42 high entropy alloys; ordering; thermal antiphase domain boundaries; phase transition; segregation. 43