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The effects of Zr addition on martensitic transformation and the lattice parameters of α” (orthorhombic) martensite and β (bcc) phase were investigated in Ti-3mol%Mo-6mol%Sn based alloys containing up to 4mol%Zr using θ-2θ X-ray diffraction measurement (XRD) and differential scanning calorimetry (DSC). It was found by XRD that orthorhombic α” martensite phase is formed when Zr content is 0 to 2mol% while bcc β phase also existed in the alloy containing 2 to 4mol%Zr. Based on the lattice parameters in α” martensite and β parent phases evaluated, the transformation strains between α” and β phase calculated become slightly small with increasing Zr content. DSC revealed that, with increasing Zr content, reverse martensitic transformation start and finish temperatures decreased down to 410K with a rate of-30K/mol%Zr. It is concluded in the Ti-Mo-Sn alloy system that Zr addition stabilizes β phase and that Zr addition is effective to control martensitic transformation temperature without changing the transformation strains largely.

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Shape memory alloys (SMAs) including superelastic alloys have unique properties such as shape memory and superelasticity, thus they are recognized as very useful biomaterials. These properties are very advantageous for medical use, and actually the SMA wires have been widely used in medical field. However, biocompatibility of nickel-titanium (Ni-Ti) alloy, which is the only practical SMA at present, has been questioned because of its high nickel content. The aim of this study was to evaluate the biocompatibility of a newly developed Ni-free Ti-based SMA for medical use. The newly developed SMA made of Ti-Mo-Sn-Zr system was processed into a disk of 15.1 mm in diameter. Pure titanium of the same shape was prepared as control. All the disk surfaces were polished using emery papers, #120, #400, and #600. Scanning electron microscopy and a 3D optics profiler were used to evaluate the surface of the materials. In vitro evaluations included colony examination for evaluation of the cell cytotoxicity, DNA quantification for the cell proliferation, Alamar blue assay for metabolic activity, FDA staining for the live cell imaging, and cell cycle analysis, using Chinese hamster fibroblastic V-79 cells and mouse osteoblastic MC3T3-E1 cells. In colony examination and DNA quantification, there was no significant difference between the Ti-Mo-Sn-Zr and the pure titanium. In FDA staining, cultured cells on the Ti-Mo-Sn-Zr alloy showed the same biocompatibility as those on the pure titanium. The present results suggest that the newly developed Ti-Mo-Sn-Zr alloy showed the high biocompatibility comparable to pure titanium and can be used as efficient biomaterial for medical use.

Nanoindentation measurements on various grain boundaries were performed to clarify the effects of the geometry of neighboring grains and the addition of boron (B) on plasticity resistance in the vicinity of grain boundaries in interstitial free (IF) steels. We define a parameter, ¡, which is measured by the slope of a P/hh curve with load, P, and displacement, h, to estimate the resistance against plastic deformation in the grain interior and at the grain boundaries. The value of ¡ is almost constant with the geometric compatibility factor of the grain boundaries. This result shows that the geometry of the neighboring grains does not affect the plasticity resistance of the grain boundaries. However, the value of ¡ increases by the addition of B due to the segregation of B and Ti to the grain boundaries. It is indicated that the elemental segregation to the grain boundaries enhances the resistance to the plastic deformation in the vicinity of the grain boundaries.

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Effects of Zr addition on the phase constitution and mechanical properties of Ti-5Mo-3Sn (mol%) based shape memory alloys were investigated by X-ray diffraction measurements and tensile tests. It was found that the phase is stabilized by Zr addition and that the maximum lattice deformation strain of Ti-5Mo-3Sn alloy is 6.6%. Ti-5Mo-3Sn alloy exhibited the shape memory effect with shape recovery strain of 4.2 %, while Ti-5Mo-3Sn-6Zr alloy exhibited the superelasticity at room temperature with the shape recovery strain of 4.3 %. However, the shape memory effect and the superelasticity did not appear in 12mol%Zr addition. The slip stress increases from 306 MPa in Ti-5Mo-3Sn alloy to 455 MPa in Ti-5Mo-3Sn-6Zr alloy. Therefore, 6mol%Zr addition is effective to obtain the room temperature superelasticity and to increase the slip stress of Ti-5Mo-3Sn based alloys without degrading transformation strain.