Development of Low-Cost Manufacturing Process and Effects of Adding Small Amounts of Ta, O, and N on the Physical and Mechanical Properties of Highly Biocompatible Ti Alloys

Abstract

Zirconium (Zr), niobium (Nb), and tantalum (Ta) are important alloying elements of titanium (Ti) alloys for attaining superior biocompatibility. To develop low-cost manufacturing processes, we examined the effects of melting methods, Ta addition using a Ti–Ta mother alloy, and the addition of small amounts of oxygen (O) and nitrogen (N) on the continuous-hot-rolling, physical, and mechanical properties of biocompatible Ti alloys as well as their microstructure. Seven Ti–Zr-based alloys containing Zr, Nb, and Ta were subjected to vacuum arc melting, induction skull melting (ISM), and levitation induction melting. The use of Ti–30 mass% Ta mother alloy (hereafter, % represents mass%) was effective for melting the biocompatible Ti alloys. ISM was found to be a promising advanced method for biocompatible Ti alloys. A fine granular α-phase structure (approximately 1 µm), high strength, and excellent ductility were obtained in the continuous hot-rolling process. The Ti–Zr-based alloys were strengthened by adding small amounts of N and O. The observed mechanical properties were superior to those of artificial hip stems made of Ti–6Al–4V alloy. The Ti–Zr-based alloys started to melt at approximately 1620°C. The β-transus temperature (T_β) was in the range from 805 to 850°C. The microstructure was predicted using the temperature difference (ΔT) from T_β. The specific heat constant (C_p) and thermal conductivity (W) increased with increasing temperature up to T_β then decreased above T_β. The machinability of Ti–Zr-based alloy was similar to that of Ti–6Al–4V alloy.

Fig. 1 (a) Ti-Zr A alloy ingot and 100-mm-square billet after β- and α-β-forgings. (b) Schematic illustration of continuous hot rolling of Ti–Zr-based alloy to obtain rod specimens.