Direct Production of Ti-29Nb-13Ta-4.6Zr Biomedical Alloy from Oxide Mixture in Molten CaCl2

Shogo Osaki, Hiroshi Sakai, and Ryosuke O. Suzuki
Faculty of Engineering, Hokkaido University

A new process based on the principle of the OS process involving electrolysis and the calciothermic co-reduction of an oxide mixture consisting of TiO2, Nb2O5, Ta2O5, and ZrO2 for producing Ti-29Nb-23Ta-4.6Zr (TNTZ alloy) was proposed.
TiO2 + Nb2O5 + Ta2O5 + ZrO2 + Ca TNTZ + CaO (1)
Ca was produced by the electrolysis of CaO in molten CaCl2 by applying a voltage of 3.2 V between a carbon anode and Ti cathode at 1173 K, as illustrated in Figure 1. The current during electrolysis exhibited a comparatively large value at the initial stage; it then gradually decreased, and approached a low and constant value.

The residual oxygen concentration in the obtained powder decreased monotonically with an increase in the supplied electric charge, Q, as shown in Figure 2. An oxygen concentration of 0.19 mass% was attained by supplying the electrical charge around three times larger than Q0 necessary to generate a stoichiometrically required amount of Ca.

Figure 3 shows the phase identifications by X-ray diffraction measurements, where and correspond to HCP and BCC solid solutions, respectively. -phase generated effectively during electrolysis. However, TiO2 and ZrO2 would react with CaO dissolved in the molten salt to form the composite oxides CaTiO3 and CaZrO3. This implies that the reduction behavior (reduction rate) could be classified into two groups, based on the free energy change in the chemical reaction (oxygen removal from the oxide): a group of Ta and Nb and another of Ti and Zr. The elements in the latter group are thermodynamically more stable than those in the former one. When the electric charge reached 165% of Q0, only the and phase were found The smaller intensity of the peak indicated the transformation of to . At 323%, TiC precipitation was observed. This is due to the generation of carbon via the reduction of CO and CO2 as a parasite reaction. At all stages, a residual phase was detected; in contrast, it was reported that the TNTZ alloy had a single phase.

At Q/Q0 = 262% (Figure 4), the distribution of the four elements was measured by SEM. The homogeneous distribution over a wide area was found, but the measured compositions at eight locations exhibited slight variations. A residual phase probably exists due to differences in the reduction behavior of the constituent oxides. The differences in the reduction rate lead to variations in the elements. This is because the initial product Nb and Ta metals form the blocks, whereas the delayed product Ti and Zr cannot diffuse well in the blocks. Zr acts as an HCP stabilizing element in Ti alloy, whereas Nb and Ta act as BCC stabilizing elements. Therefore, the variability of elements causes the residual phase.

Several reduced samples were sintered for future application in powder metallurgy. It was pressed into a pellet (diameter: 10mm, thickness: of the order of a few millimeters) at 450 MPa and then sintered in Ar atmosphere. After sintering at 1300 K for 54 ks, for example, the Ti- and Zr-rich HCP phase disappeared and the composition achieved the targeted value, as shown in Figure 5.

The results of this study indicate that the OS process can be applied to directly produce TNTZ alloy from an oxide mixture.

[Published in Journal of the Electrochemical Society, Vol.157, No.8 (2010) pp.E117-E121.]

Fig. 1 Principle of OS process.
Fig. 2 Change in oxygen concentration versus supplied charge.
Fig.3  XRD patterns of reduced samples.
Fig. 4  SEM image and elemental concentration mappings of the sample obtained at Q/Q0 = 262 %. The time dependency of current was inserted for this sample.
Fig. 5  SEM image and elemental concentration mappings of the pellet after sintering.