Introduction: In recent years, a relatively low elastic modulus of around 30 GPa nearly equal to the bone has been demanded on the metallic biomaterial. However, in the case of the application with the spinal fixture, the metallic material needs a relatively high elastic modulus in order to suppress the spring back during deformation process. Therefore, the control of the elastic modulus by stress-induced transformation on newly developed biomedical β-type Ti-Cr system alloy has been proposed by preset authors. During the local plastic deformation, the elastic modulus rises locally by the precipitation of ω phase.
Therefore, the change in the mechanical properties of Ti-Cr system alloy for the spinal fixture subjected to heat treatments and surface modification processing was investigated in this study.
Experimental Methods: Materials used in this study were hot forged Ti-12Cr bars (φ: around 8mm) and Ti-29Nb-13Ta-4.6Zr bars (φ: around 8mm). Ti-12Cr and Ti-29Nb-13Ta-4.6Zr (TNTZ) were solutionized at 1003 K and 1063 K for 3.6 ks in a vacuum followed by water quenching (WQ). As-solutionized both alloys are designated as Ti-12CrST and TNTZST. Some Ti-12CrST were subjected to aging treatment (AT) at 573 K, 673 K and 723 K up to their peak aging. Aged Ti-12CrST was classified by aging temperature (e.g. Ti-12CrSTA573K). The surface modification used by a fine particle bombarding (FPB) was done on the specimen surface of Ti-12CrST at the pressure surface of 0.6 MPa, which is designated as e.g. Ti-12CrST/FPB (0.6 MPa / 0.18 ks).
For the microstructure evaluation, the optical microscope (OM), the scanning electron microscope (SEM) and the X-ray diffraction (XRD) spectroscopy were carried out on each specimen. For the mechanical properties, a Vickers hardness, tensile properties and fatigue properties were investigated on each smooth specimen.
Results and Discussion: Ti-12Cr alloy subjected to aging at all temperatures after solution treatment (ST) showed a significant embrittlement trend. Therefore, a solo ST was suitable method for Ti-12Cr because of an excellent balance between strength (and 294 MPa) and ductility (and 23%) as composed with aging after ST.
Ti-12Cr alloy subjected to FPB shows that each HV of very edge of specimen surface and the center is the 360 HV and 294 HV as shown in Fig. 1. From this results, an increase in HV near the specimen surface is attributed to the precipitation of ω phase and the work hardening, which is enhanced by the ω phase through FPB.
In addition, the fatigue strength of Ti-12Cr alloy subjected to FPB increases by 70 MPa as compared with that of Ti-12Cr in ST condition as shown in Fig. 2. From this results, an increase in fatigue strength is attributed to the formation of the working layer and the occurance of the compressive residual stress through FPB.
Conclusions:
- An increase significantly in HV near the specimen surface was attributed to precipitate ω phase and the work hardening through FPB.
- An increase in fatigue strength was attributed to the formation of the working layer and the occuranse of the compressive residual stress through FPB.