Effect of Nb Addition on the Phase Stability, Microstructure, and Mechanical Properties of Powder Metallurgy Ti-5Fe-xNb Alloys

Ti alloys contemplating the simultaneous addition of Fe and Nb are available in the literature as Fe enhances the strength and Nb improves the biological behaviour of Ti. Nevertheless, casting has been the main manufacturing process, the Nb content is normally ≥10 wt.%, and no tensile properties are available. In this study, Ti-5Fe-xNb alloys (x = 2, 6, and 9 wt.%) were produced via powder metallurgy, which is more energy efficient than casting, with the aim of understanding the relationship between the mechanical behaviour and the microstructural changes brought about by the progressive addition of a greater amount of Nb. This study shows that the increment of the Nb content reduces the densification of the alloys, as the relative density decreases from 98.2% to 95.0%, but remarkably increases the volume fraction of the stabilised β phase (14→36%). Accordingly, the Ti-5Fe-xNb alloys are characterised by Widmanstätten microstructures, which become finer for higher Nb contents, and progressively higher mechanical properties including yield stress (725–949 MPa), ultimate tensile strength (828–995 MPa), and hardness (66.5–67.6 HRA), but lower elongation to fracture (4.0–5.1%). It is found that the ductility is much more influenced by the presence of the residual pores, whereas the strength greatly depends on the microstructural changes brought about by the addition of the alloying elements.