Facile Electrochemical Synthesis of Nanoscale (TiNbTaZrHf)C High-Entropy Carbide Powder.

High-entropy alloys and compounds are becoming an important class of new materials due to their outstanding refractory and high-temperature properties. However, preparation in bulk quantities and in powder form via classical metallurgical methods is challenging. Here, we report the first synthesis of an ultra-high-temperature high-entropy carbide, (TiNbTaZrHf)C, via a facile electrochemical process. In this, a mixture of the individual metal oxides and graphite is deoxidised in a melt of CaCl2 at a temperature of only 1173 K. The (TiNbTaZrHf)C prepared is single-phase fcc and has a powdery morphology with a particle-size range of 15-80 nm. Such materials are in demand for modern additive manufacturing techniques, while preliminary tests have also indicated a possible application in supercapacitors. The successful synthesis of (TiNbTaZrHf)C powder may now guide the way towards establishing the electrochemical route for the preparation of many other entropy-stabilised materials.

Electrochemical synthesis of porous Ti-Nb alloys for biomedical applications.

Porous titanium­niobium alloys of composition Ti-24Nb, Ti-35Nb and Ti-42Nb were synthesised by electro-deoxidation of sintered oxide discs of mixed TiO2 and Nb2O5 powders in molten CaCl2 at 1173 K, and characterised by XRD, SEM, EDX and residual oxygen analysis. At the lower Nb content a dual-phase α/ß-alloy was formed consisting of hexagonal close-packed and body-centred cubic Ti-Nb, whereas at the higher Nb contents a single-phase ß-alloy was formed of body-centred cubic Ti-Nb. The corrosion behaviour of the alloys prepared was assessed in Hanks' simulated body fluid solution at 310 K over extended periods of time. Potentiodynamic polarisation studies confirmed that the alloys exhibited passivation behaviour, and impedance studies revealed that the passive films formed on the surface of the alloys comprised a bi-layered structure. XPS analysis further proved that this contained hydroxyapatite at the top and native metal oxide underneath. The mechanical properties of the alloys were evaluated, and the elastic moduli and the Vickers hardness were both found to be in the range of that of bone. Overall, Ti-35Nb is proposed to be the best-suited candidate of the materials studied in regard to biomedical applications.