The high-temperature oxidation mechanism of a series of refractory high entropy alloys: TaMoCrTiAl, NbMoCrTiAl, NbMoCrAl and TaMoCrAl at 1000°C in air was studied. A complex protective oxide layer consisting of Al2O3, Cr2O3 and CrTaO4 oxides was observed for the quinary Ta-containing alloy. The formation of CrTaO4 in this alloy after a short incubation period decreased the oxidation kinetics from a parabolic to a quartic rate law. Ti was found to support the formation of CrTaO4. In the Nbcontaining alloys, the formation of different Nb2O5 polytypes near the metal/oxide interface caused a highly porous oxide scale and severe oxide spallation.

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The high temperature oxidation behavior of a refractory high-entropy alloy (HEA) 20Nb-20Mo-20Cr-20Ti-20Al at 900°C, 1000°C and 1100°C was investigated. The oxidation kinetics of the alloy was found to be linear at all temperatures. Oxide scales formed are largely inhomogeneous showing regions with thick and porous layers as well areas with quite thin oxide scales due to formation of discontinuous chromium-rich oxide scales. However, the oxidation resistance can be moderately improved by the addition of 1 at.% Si. The thermogravimetric data obtained during oxidation of the Si-containing alloy at 1000°C and 1100°C reveal pronounced periods of parabolic oxidation that, however, change towards linear oxidation after prolonged exposure times. Microstructural investigations using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) document that the Si addition gives rise to a nearly continuous alumina-rich layer which seems to be responsible for the good protection against further oxidation. Pronounced zones of internal corrosion attacks consisting of different oxides and nitrides were observed in both alloys. In order to determine the chemical composition of the corrosion products and their mass fraction, quantitative Xray diffraction (XRD) analysis was performed on powdered oxide scales that formed on the alloys after different oxidation times. Rutile was identified as the major phase in the oxide scales rationalizing the relatively high mass gain during oxidation.

In the present contribution, we describe the successful development of two ternary Mo-Si-Ti alloys with two-phase eutectic and eutectoid microstructure, respectively. In the case of Mo-20.0Si-52.8Ti (at.%), a fully eutectic microstructure consisting of body-centered cubic (bcc) solid solution (Mo,Ti,Si) and hexagonal (Ti,Mo) 5 Si 3 can be obtained in very good agreement with thermodynamic calculations. A fully eutectoid decomposed microstructure is observed subsequent to heat-treatment at 1300 °C for 200 h in the case of Mo-21Si-34Ti (at.%). For this alloy, bcc (Mo,Ti,Si) and tetragonal (Mo,Ti) 5 Si 3 appears after decomposition from the A15-type (Mo,Ti) 3 Si. Besides that, a small amount of hexagonal (Ti,Mo) 5 Si 3 forms in the silicide lamellae, too, which is attributed to Ti segregations in the as-cast microstructure. In addition to the focus on microstructure, both oxidation and creep behavior were preliminarily investigated and compared to other state-of-the-art Mo-based alloys. In the case of the eutectic alloy, a promising and unexpected oxidation resistance at 800 °C is observed whereas the eutectoid alloy exhibits catastrophic oxidation; a behavior that is typically observed under these conditions in alloys containing Mo-rich solid solution. The eutectic alloy shows an approximately one order of magnitude higher creep rate within the investigated temperature and stress range as compared to the eutectoid decomposed counterpart. This is attributed to the rather low intrinsic creep resistance of the hexagonal (Ti,Mo) 5 Si 3 and generally lower melting point of the former alloy, whereas in the latter case, creep seems to be controlled by the deformation of the bcc solid solution (Mo,Ti,Si) and the tetragonal (Mo,Ti) 5 Si 3 .

In the present investigation, we provide results on the casting, homogenization, and deformation behavior of a new Al-containing refractory high-entropy alloy, namely the equiatomic Nb-Mo-Cr-Ti-Al. The alloy shows a dendritic microstructure after arc melting. The dendrites completely dissolve due to a heat treatment at 1300 °C for 20 h. Besides a major phase in the form of a solid solution of W prototype structure, identified by X-ray diffraction (XRD) measurements as well as electron backscatter diffraction (EBSD), additional phases of small volume fraction within the grains and at the grain boundaries were observed. Quasistatic compression tests, performed between room temperature and 1200 °C, reveal sustaining and high yield strength up to 800 °C and an increasing ductility with increasing test temperature. The dominant deformation mechanism for quasistatic compression loading between 800 °C and 1200 °C is the 〈111〉 pencil glide of dislocations within the solid solution which was proven by the according fiber texture components, evolving during deformation.

The high temperature oxidation behavior of a new family of refractory high entropy alloys (HEAs) with compositions of W-Mo-Cr-Ti-Al, Nb-Mo-Cr-Ti-Al and Ta-Mo-Cr-Ti-Al was studied at 1000°C and 1100°C. Based on these equimolar starting compositions, the main incentive of this study was to select the most promising alloy system whose properties may then be successively improved. Despite the high amount of refractory elements, all HEAs studied here showed good oxidation resistance at least during 48h of air exposure at 1000°C and 1100°C. Moderate values of mass gain and complex oxidation kinetics were observed for the W-and Nb-containing HEAs. These alloys formed inhomogeneous oxide scales possessing regions with thick and porous layers as well as areas revealing quite thin oxide scales due to the formation of discontinuous Cr-and Al-rich scales. The most promising behavior, though, was shown by the alloy Ta-Mo-Cr-Ti-Al which followed the parabolic rate law for oxide growth due to the formation of a thin and compact Al-rich layer.

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.

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.