We present an experimental approach for revealing the impact of lattice distortion on solid solution strengthening in a series of body-centered (bcc) Al-containing, refractory high entropy alloys from the Nb-Mo-Cr-Ti-Al system. By systematically varying the Nb and Cr content, a wide range of atomic size difference as a common measure for the lattice distortion was obtained. Single phase, bcc solid solutions were achieved by arc-melting and homogenization as well as verified by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD). The atomic radii of the alloying elements for determination of atomic size difference were recalculated on the basis of the mean atomic radii in and the chemical compositions of the solid solutions. Microhardness at room temperature correlates well with the deduced atomic size difference. Nevertheless, the mechanisms of microscopic slip lead to pronounced temperature dependence of mechanical strength. In order to account for this particular feature, we present a combined approach, using microhardness, nanoindentation and compression tests. The athermal proportion to the yield stress of the investigated equimolar alloys is revealed. These parameters support the universality of this aforementioned correlation. Hence, the pertinence of lattice distortion for solid solution strengthening in bcc high entropy alloys is proven.

In this work, we present the results of the thermodynamic assessment of two equiatomic refractory High Entropy Alloys (HEAs), namely TaMoCrTiAl and NbMoCrTiAl, in the temperature range between 700 and 1500°C. Particular attention is paid on the constitution of the intermetallic phases stable in these alloy systems. Thermodynamic calculations were performed using a self-developed thermodynamic database based on the CALPHAD (Calculation of Phase Diagram) approach. The details of the thermodynamic modelling and particular characteristics of the relevant phases within the Ta-Nb-Cr-Ti-Al system are presented. To verify the new database, the phase formation and stability of both quinary alloys in near-equilibrium conditions were studied experimentally by utilizing scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD) as well as X-ray powder diffraction (XRD). Both equiatomic alloys reveal a complex microstructure including several intermetallic phases at intermediate temperatures. The alloy NbMoCrTiAl consists of an ordered B2 phase, Al(Mo, Nb)3 and two polytypes (C14 and C15) of the Cr2Nb Laves phase. Precipitations of Cr2Ta Laves phase (C14, C15 and C36-type) in the B2 matrix were observed in the alloy TaMoCrTiAl. Based on the results of thermodynamic calculations, it was concluded that: (i) Nb stabilizes the AlMo3 A15 phase in the alloy NbMoCrTiAl, (ii) Al and Ti play a crucial role in the formation of the ordered B2 phase in both alloys and (iii) the concentrations of Cr and/or Ta/Nb should be dramatically reduced to decrease the Laves phase volume fraction.

Highlights (3 to 5 Bullet points, max. 85 Characters per Bullet point) Disordered A2 crystal structures are obtained in MoCrTi-xAl (x = 3, 5 at%)  The A2 crystal structure leads to higher ductility compared to B2 ordered alloys  Appearance of order is appropriately described by thermodynamic calculations  PLC effect is detected in all investigated compositions at elevated temperatures

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