The effect of laser beam welding on the structure and properties of a Ti_1.89NbCrV_0.56 refractory high entropy alloy was studied. In particular, the effect of different pre-heating temperatures was examined. Due to the low ductility of the material, laser beam welding at room temperature resulted in the formations of hot cracks. Sound butt joints without cracks were produced using pre-heating to T ≥ 600 °C. In the initial as-cast condition, the alloy consisted of coarse bcc grains with a small amount of lens-shaped C15 Laves phase particles. A columnar microstructure was formed in the welds; the thickness of the grains increased with the temperature of pre-heating before welding. The Laves phase particles were formed in the seams after welding at 600 °C or 800 °C, however, these particles were not observed after welding at room temperature or at 400 °C. Soaking at elevated temperatures did not change the microstructure of the base material considerably, however, “additional” small Laves particles formed at 600 °C or 800 °C. Tensile test of welded specimens performed at 750 °C resulted in the fracture of the base material because of the higher hardness of the welds. The latter can be associated with the bcc grains refinement in the seams.

Ti(50-x)Zr38NbxTa8Sn4 high-entropy alloys with x = 0, 10, and 20 at.% were produced by vacuum arc melting in a high-purity argon atmosphere. The initial microstructures consisted of equiaxial bcc grains with sizes of 115 ± 30 µm, 250 ± 60 µm, and 280 ± 70 µm for the Ti30Nb20, Ti40Nb10, and Ti50Nb0 alloys, respectively. The Ti30Nb20 and Ti40Nb10 alloys showed untypical mechanical behavior with a short strain-hardening stage followed by a gradual decrease in flow stress after reaching the yield point. Although these two alloys had some inclination toward macroscopic strain localization, their tensile elongation was similar to that obtained in the Ti50Nb0 alloy, which had a more extended stage of uniform deformation. The differences were associated with distinct microstructures observed after deformation to fracture. The formation of dislocation bands and the activation of cross-slip at the microscale, as well as the appearance of kink bands at the mesoscale, can result in plastic instability. In contrast, a lamellar-like microstructure with parallel dislocation bands, such as the one observed in the Ti50Nb0 alloy, can ensure a more stable mechanical behavior. The developed alloys (Ti30Nb20 and Ti40Nb10) have properties that make them highly attractive for biomedical application due to a combination of very high yield strengths (1090 and 930 MPa, respectively), low Young’s moduli (~78 and ~69 GPa, respectively), reasonable ductility, and excellent biocompatibility.

Herein, the oxidation behaviour of refractory (HfCo)_100−x(NbMo)_x (x = 0; 10; 25; 40; 75; 100 (at.%)) high-entropy alloys with a bcc+B2 structure subjected to cyclic oxidation at 1000 °C was studied. The single-phase B2-ordered HfCo alloy demonstrated the best spallation resistance and retained a pristine form after 100 h. The oxidation kinetics of the HfCo alloy was near-parabolic, accompanied by the formation of external HfO₂ or CoO layers after 1 or 100 h, respectively. Additions of (NbMo)_x deteriorated the spallation resistance (x ≤ 25 at.%) or led to complete disintegration (x > 25 at.%). Among the (NbMo)-containing alloys, the (HfCo)₉₀(NbMo)₁₀ alloy with the dual-phase bcc+B2 structure showed the most promising oxidation resistance. This alloy withstood cyclic oxidation up to 15 h with a mass gain close to the HfCo alloy and survived 100 h without changes in geometry of the specimen. Unlike the HfCo alloy, in the (HfCo)₉₀(NbMo)₁₀ alloy, the external CoO layer was found already after 1 h. The effect of chemical and phase compositions on the formation of certain oxides was discussed. Comparison with the other refractory high-entropy alloys was also presented.

Refractory high-entropy alloys (HEAs) are a new class of metallic materials based on group 4–6 elements of the periodic table with possible additions of Al, Si, Re, C, or B. Some single-phase refractory HEAs can maintain high strength up to 1600°C, while multiphase compositions have more attractive specific properties at temperatures up to 1200°C. Here we examine the structure and mechanical properties of refractory HEAs Nb₃₀Mo₃₀Co₂₀Hf₂₀, Nb₃₀Mo₃₀Co₂₀Zr₂₀, and Nb₃₀Mo₃₀Co₂₀Ti₂₀ (at %). The alloys consisted of an intermetallic B2 matrix and particles of a disordered bcc phase, as well as a minor volume fraction of additional bcc (Nb₃₀Mo₃₀Co₂₀Hf₂₀ and Nb₃₀Mo₃₀Co₂₀Zr₂₀) or fcc (Nb₃₀Mo₃₀Co₂₀Ti₂₀) phases. When tested for uniaxial compression, Nb₃₀Mo₃₀Co₂₀Ti₂₀ alloy showed a higher yield strength in the temperature range of 22–1000°C than Nb₃₀Mo₃₀Co₂₀Hf₂₀ and Nb₃₀Mo₃₀Co₂₀Zr₂₀ alloys. Nb₃₀Mo₃₀Co₂₀Zr₂₀ alloy did not fail at temperatures of 22–800°C to a given 50% strain, while Nb₃₀Mo₃₀Co₂₀Ti₂₀ alloy turned out to be brittle. All alloys demonstrated high strain hardening in the temperature range of 22–800°C, and they can compete in terms of specific strength with commercial nickel and cobalt superalloys.