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High entropy alloys based on the Ta-Nb-Mo-Cr-Ti-Al system are expected to possess high creep and oxidation resistance as well as outstanding specific mechanical properties due to presumed high melting points and low densities. However, we recently reported that arc-melted and subsequently homogenized alloys within this system exhibit a lack of ductility up to 600 °C [H. Chen et al. in Metall.

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A quenched-in state of thermal equilibrium (at 723 K) in a single crystal of Cr-Fe-Co-Ni close to equal atomic percent was studied. Atom probe tomography revealed a single-phase state with no signs of long-range order. The presence of short-range order (SRO) was established by diffuse x-ray scattering exploiting the variation in scattering contrast close to the absorption edges of the constituents: At the incoming photon energies of 5969, 7092, and 8313 eV, SRO maxima that result from the linear superposition of the six partial SRO scattering patterns, were always found at X position. Electronic structure calculations showed that this type of maximum stems from the strong Cr-Ni and Cr-Co pair correlations, that are furthermore connected with the largest scattering contrast at 5969 eV. The calculated effective pair interaction parameters revealed an order-disorder transition at approximately 500 K to a L1 2-type (Fe,Co,Ni) 3 Cr structure. The calculated magnetic exchange interactions were dominantly of the antiferromagnetic type between Cr and any other alloy component and ferromagnetic between Fe, Co, and Ni. They yielded a Curie temperature (T C) of 120 K, close to experimental findings. Despite the low value of T C , the global magnetic state strongly affects chemical and elastic interactions in this system. In particular, it significantly increases the ordering tendency in the ferromagnetic state compared to the paramagnetic one.

We aim on the model-based description of the strength of ferritic and austenitic oxide dispersion strengthened (ODS) steels in the temperature range from room temperature (RT) up to 800 °C. Therefore, we present two approaches for the synthesis of austenitic alloys by mechanical alloying Y2O3, namely with (i) elemental powders at RT and (ii) with a gas-atomized master-alloy. Consolidation of both powders by field assisted sintering technique leads to a more homogenous distribution of grain size and particles in specimens from elemental powders. In the entire temperature range, the compressive strength of the austenitic ODS steels is shown to be lower compared to the one of ferritic counterparts. Above approximately 500 °C, a strong decrease in strength was observed for all ODS variants due to the onset of creep-based deformation. Multi-scale materials characterization was performed to quantitatively assess microstructural materials parameters crucial for the modeling of the temperature dependent yield strength. These data were utilized to quantitatively describe the strength contribution by Hall-Petch and Orowan strengthening as well as dislocation strengthening at RT. Lower amounts of grain boundary and dislocation strengthening were found to be crucial for the lower strength of austenitic ODS steels. Meaningful calculation of materials strength is only achieved, when both interactions of strengthening contributions and experimental uncertainties are considered.Models describing diffusion-based creep (by Coble) and dislocation-based creep (by Blum and Zeng), which were shown to provide a more appropriate description of high temperature strength, are critically assessed for temperatures at and above the strength drop. It is shown that the deformation at high temperatures is possibly dominated by the formation and annihilation of dislocations at grain boundaries.

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The physical properties of FBS depend strongly on crystal structure, chemical composition and microstructure.