Introduction: Recently, Ti-29Nb-13Ta-4.6Zr (TNTZ) with low Young’s modulus composed of alloying elements with high biocompatibility, has been developed in Japan[1]. However, as-solutionized TNTZ with the equiaxed single β phase has around twice larger Young’s modulus than that of cortical bone, and the mechanical strength is poorer than that of annealed Ti-6Al-4V ELI (Ti64). Therefore, the effect of frictional stir processing (FSP), which is one of mechanical surface modifications, under various working conditions on the microstructure and mechanical strength of nearly developed TNTZ and Ti64 as a comparison material was investigated for improvement the mechanical strength with low Young's modulus in this study.
Experimental Procedures: Materials used in this study were coin-shaped TNTZ with a thickness of 3.0 mm and a diameter of 30 mm and Ti64 with a thickness of 3.0 mm and a diameter of 35 mm. The coin-shaped TNTZ were conducted with a solution treatment (ST) at 1063 K for 3.6 ks followed by water quench (WQ). Those of Ti64 were done with annealing at 978 K and 3.6 ks followed by air cooling (AC). Friction stir processing (FSP) were done on both coin-shaped samples under various working conditions such as the pressure and atmosphere. Optical microscopy (OM) and X-ray diffraction (XRD) spectroscopy were carried out on each specimen to evaluate the microstructure. Vickers hardness (HV), Young’s modulus measurement and tensile test were performed to examine the mechanical properties.
Results and Discussion: Microstructure near subsurface of FPB worked surface on TNTZ shows the recrystallization area by the frictional heating with very fine equiaxed β phase with an average grain diameter of 3.0 μm as shown in Fig.1, while that of as-solutionized TNTZ was around 20 μm. The refinement ratio of microstructure is around 15%. The microstructure of Ti64, where almost all plastic flowing region was removed, showed mainly heat affected region with needle like α phase. Thus, it is considered that the heat affected region was heated over β transus more than 1000 K followed by cooling.
Figure 2 shows HV of TNTZ/70.7(Air) and Ti64/6.37(Air) from FSP worked surface to around center of specimen. HV at very edge of FSP worked surface of TNTZ/70.7(Air) shows around 75% higher than that at around the center of specimen. This is attributed to fine β phase by severe deformation and frictional heating as mentioned above, and the precipitation of α phase during cooling after FSP. HV from FSP worked surface to inside of specimen shows an inclinable change with the diameter of β phase and the decrease in the amount of precipitation of α phase. On the other hand, HV in the FSP worked surface of Ti64/6.37(Air) don’t increase significantly as compared with that of TNTZ/70.7(Air). The results were attributed to the refinement of needle like α phase, the precipitation of α phase in β phase and the diffusion of gas elements such as oxygen and nitrogen.
Conclusions:
- Surface layer of TNTZ subjected to FSP was composed of a recrystallization area and a heat affected area with a large amount of α phases in fine or relatively coarse β phases, respectively. On the other hand, that of Ti64 was occupied by mainly the heat affected area, which was composed of relatively small acicular type α/β phases in the prior β phases.
- Vickers hardness of TNTZ and Ti64 subjected to FSP showed the maximum value at subsurface of FSP worked surface, although the value decreased sharply as a fanction of depth from the surface.
References:
[1] M. Niinomi: Metall. Mater. Trans. A, 33, (2002), 477-486.