High-power ultrasonic synthesis and magnetic-field-assisted arrangement of nanosized crystallites of cobalt-containing layered double hydroxideu

High-quality stoichiometric Co2Al–NO3 and Co2Al–CO3 layered double hydroxides (LDHs) have been obtained by precipitation followed by anion exchange, both high-power sonication assisted. Application of high-power ultrasound has been demonstrated to result in a considerable acceleration of the crystallization process and the anion-exchange reaction. Two independent approaches were used to form bulk and 2-D samples of Co2Al–NO3 with the oriented crystallites, namely uniaxial pressing of deposits from sonicated LDH slurries and magnetic field assisted arrangement of LDH crystallites precipitating on glass substrates. A convenient way of preparation of semi-transparent compacts with relatively big blocks of oriented crystallites have been demonstrated. Thin dense transparent films of highly-ordered crystallites of Co2Al–NO3 LDH have been produced and characterized.publishe

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

Anomalous Evolution of Strength and Microstructure of High‐Entropy Alloy CoCrFeNiMn after High‐Pressure Torsion at 300 and 77 K

Ultrafine and nanocrystalline states of equiatomic face‐centered cubic (fcc) high‐entropy alloy (HEA) CoCrFeNiMn (“Cantor” alloy) are achieved by high‐pressure torsion (HPT) at 300 K (room temperature, RT) and 77 K (cryo). Although the hardness after RT‐HPT reaches exceptionally high values, those from cryo‐HPT are distinctly lower, at least when the torsional strain lies beyond γ = 25. The values are stable even during long‐time storage at ambient temperature. A similar paradoxal result is reflected by torque data measured in situ during HPT processing. The reasons for this paradox are attributed to the enhanced hydrostatic pressure, cryogenic temperature, and especially large shear strains achieved by the cryo‐HPT. At these conditions, selected area electron diffraction (SAD) patterns indicate that a partial local phase change from fcc to hexagonal close‐packed (hcp) structure occurs, which results in a highly heterogeneous structure. This heterogeneity is accompanied by both an increase in average grain size and especially a strong decrease in average dislocation density, which is estimated to mainly cause the paradox low strength.© 2019 The Author