Microstructural evolution and mechanical properties of bulk and porous low-cost Ti–Mo–Fe alloys produced by powder metallurgy

Abstract

A series of low-cost Ti–10Mo-xFe (x = 1, 5, 9 wt%) alloys were fabricated using conventional press and sinter powder metallurgy to investigate the effect of Fe content on their phase stability, sintering response, microstructure and mechanical properties. Phase analyses indicated that all alloys were composed of α, β and intermetallic phases. However, Fe additions increased the proportion of β and intermetallic phases and reduced the propensity of the alloy system to form α phase during slow furnace cooling. Densification of the Ti–10Mo-xFe alloys was subjected to two contradicting effects; a strong sintering response caused by the fast diffusion rate of Fe atoms which promotes densification and formation of Kirkendall pores related to the fast diffusion rate of Fe atoms in conjunction with comparatively slower diffusion of Ti and Mo. Nonetheless, the porosity level of the alloys was less than that of the sintered CP-Ti. Depending on the content of α, β and intermetallic phases, the alloys exhibited varied mechanical properties. It was found that Ti–10Mo–5Fe presented the best combination of mechanical properties including the highest compressive strength (2392 MPa) and strain (43%) and low elastic modulus (91 GPa) superior to the corresponding ones for the commonly used CP-Ti and some other Ti-based alloys. Porous Ti–10Mo–5Fe alloy was also fabricated by addition of 30 and 60 vol% ammonium hydrogen carbonate (NH4HCO3) space holder which generated sintered scaffolds with 25% and 52% porosity, respectively, with an increased effective pore size at higher porosity. This reduced the compressive strengths to 649 MPa and 168 MPa and the elastic moduli to 34 GPa and 16 GPa, respectively. This study demonstrates that Ti–Mo–Fe alloys offer significant savings on raw materials compared to current commercial and many recently developed biomedical Ti alloys. It also shows that porous Ti–10Mo–5Fe scaffolds are promising for hard tissue engineering applications offering mechanical properties which closely match with the human bone and optimal pore sizes essential for bone ingrowth.