Passive Layer on Some Titanium Alloys

It is well known that surface sensitive properties like corrosion and hardness are dependent on the chemical composition of the surface. In this paper Ti, Ti-6Al-4V and Ti–6Al–3.5Fe were evaluated; the microstructure and microhardness were determined. From metallographic photos can be observed that both titanium alloys, Ti-5Al-4V and Ti-6Al-4Fe have an alpha-beta structure. Aluminium is an alpha phase stabilizer while V and Fe are beta phase stabilizers. The beta phase appears dark and the alpha phase light. Alpha phase was the dominant phase in these alloys. From Vickers microhardness measurements can be concluded that the alloys formed a hard layer on their surface which greatly improves their wear resistance in comparation with titanium. As the load increases, the values of microhardness are increasing (the layer became more compact). With a load of 200 grams it can be seen that the microhardness is decreasing which mean that the indenter reach the base metal. From the corresponding depth of penetration, it was found that passive film on the implant surface has a two-layer structure: a thin barrier-type inner layer (about 3 µ) and a porous outer layer (about 1.5 µ). The proposed model for the passive layer formed on the surface of the implants, deduced from the metalographical observations and microhardness measurements is shown in figure. The results were confirmed by mechanical approach, in terms of two-layer model of the oxide film, consisting of a thin barrier type inner layer and a porous outer layer. The pronounced porous outer layer is expected to facilitate the incorporation of mineral ions and to improve the resistance to electrochemical corrosion over the potential of relevance for implant conditions.