Effect of V content on the microstructure and mechanical properties of HPT nanostructured CoCrFeMnNiV_x high entropy alloys

The paper presents investigations of microstructure and low-temperature mechanical properties of nanostructured alloys CoCrFeMnNiVx (x = 0.15 – 0.75), processed by high-pressure torsion (HPT) at temperatures of 300 and 77 K. While at × ≥ 0.5 the values of microhardness (Hv) and compression yield stress (σ0.2) in samples after HPT at 77 K are larger than those in samples after HPT at 300 K, for × ≤ 0.2 surprisingly the opposite effect is observed. As in case of the undeformed CoCrFeMnNiVx alloys, the behaviour for vanadium concentrations × ≥ 0.5 can be related to the formation of tetragonal σ-phase in addition to fcc matrix, while the anomalous behaviour for × ≤ 0.2 arises from the formation of HPT-induced hexagonal martensitic phase. In the low temperature ranges, i.e. 20 – 300 K in case of HPT nanostructured CoCrFeMnNiV0.2, and 150 – 300 K in case of HPT nanostructured CoCrFeMnNiV0.5, dependences of σ0.2(T) show characteristics of thermally-activated dislocation movement. For the first time in HEAs - anomalous dependences of σ0.2(T) at temperatures 4.2 – 20 K for CoCrFeMnNiV0.2, and at 80 – 150 K for CoCrFeMnNiV0.5 are found which indicate at the occurrence of non-thermal inertial dislocation movement

Tribological Properties at 20 and 500°C of TiN and CrN Cathodic ARC Coatings Deposited on Ti-6Al-4V Alloy

Tribological properties of TiN and CrN coatings deposited by cathodic arc method at three different bias potentials -50, -150 and -300V on Ti-6Al-4V alloy in pair with alumina have been investigated. X-ray diffraction analysis showed that single-phase textured cubic nitrides of TiN and CrN were formed in these coatings. It is shown that the friction coefficient of the coatings is practically equal to that established for the Ti6Al4V alloy, but the wear rate is more than an order of magnitude lower than for the titanium alloy substrate. Coatings deposited at a potential of -50 V show optimal tribological properties at temperatures 20 and 500°C. Friction coefficients for TiN coatings are 0.4-0.8 at 20°C and 0,75 at 500°C; for CrN coatings they are 0.5 at 20°C and 0,7 at 500°C. Wear rates for TiN coatings are 0.86·10-5 мм3/Нм at 20°C and 3.56·10-5 мм3/Нм at 500°C; for CrN coatings they are 1.43·10-5 мм3/Нм at 20°C and 7.13·10-5 мм3/Нм at 500°C