Local lattice distortions, phase stability, and mechanical properties of NbMoTaWHfx alloys : A combined theoretical and experimental study

• Refractory high-entropy alloys (RHEAs) are of growing interest due to their potentially superior mechanical performances at elevated temperatures. Inherent lattice distortions are believed to be a major contributor to strength in solid solution phases of RHEAs. Here, we investigate the NbMoTaWHfx alloy series (x = 0, 0.27, 0.57, 0.92, 1.33, 1.82) using first-principles simulations, thermodynamic modeling, and experimental techniques. The first-principles results suggest that Hf alloying is an effective means to enhance atomic-scale lattice distortions in BCC NbMoTaWHfx solid solutions. X-ray diffraction on prepared as-cast samples shows that the alloys with Hf content x <= 0.92 are single phase BCC alloys, whereas a dual BCC phase microstructure is observed for x = 1.33 and 1.82. Elemental mappings from scanning electron microscopy for the dual-phase alloys are checked with predictions from thermodynamic modeling for equilibrium and non-equilibrium solidifications. Room-temperature compressive mechanical tests reveal that yield and ultimate strengths increase strongly with the addition of Hf and saturate for x > 0.92, whereas the compressive plasticity is slightly improved by Hf but remains limited. We predict the compositional effects on poly-crystal elastic moduli for the constituent BCC phases and the dual-phase composites and find a linear behavior between modulus-normalized yield strength and lattice distortion on average.

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TEKNIK OCH TEKNOLOGIER  -- Materialteknik -- Metallurgi och metalliska material (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Materials Engineering -- Metallurgy and Metallic Materials (hsv//eng)

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