Phase composition, microstructure, corrosion resistance and mechanical properties of molten salt electrochemically synthesised TiNbSn biomedical alloys

Abstract

TixNbySn alloys with different Nb and Sn contents, of x = 24, 35, 42 mass% and y = 4, 7.9 mass%, were synthesised directly from TiO 2 , Nb 2 O 5 and SnO 2 mixtures via the FFC-Cambridge process. Compacted powder discs were employed as the cathode versus graphite as the anode in molten CaCl 2 as the electrolyte. XRD analysis of the as-synthesised alloys showed that the two alloys with Nb content of 24 mass% were dual-phase ¡/¢-Ti whereas the other four alloys with Nb contents of 35 and 42 mass% were single-phase ¢-Ti. SEM analysis showed that the alloys were highly porous, and that particle size decreased with increase in Nb and Sn contents. Alloy samples of each composition were subjected to short-term and long-term immersion tests in Hanks’ simulated body fluid solution. XPS studies then identified a passive oxide film on the surface of the alloy, and a hydroxyapatite layer on top of the oxide. Potentiodynamic polarisation studies revealed excellent corrosion resistance with very small corrosion current densities despite high open porosities. Furthermore, alloy samples were subjected to heat treatment in vacuum. Mechanical testing of these identified a substantial increase of elastic modulus and Vickers hardness. Overall, the experimental programme has brought out that the properties of the TiNbSn alloys prepared are influenced markedly by their Nb and Sn contents, and that single-phase ¢-Ti35Nb4Sn holds promise as a candidate material for body implant applications.