The joint tensile strength and metallurgical properties of a friction welded joint of commercially pure Ti and pure Ni has been investigated in as welded and post-weld heat treated conditions. While friction pressure did not significantly impinge on joint tensile strength, joint tensile strength was affected by friction time. A 1-1?5 mm thick interlayer is essential to join pure Ti and pure Ni using friction welding. A maximum joint tensile strength of 450 MPa was achieved and the joint fractured in the Ti original (not heat affected zone) substrate, i.e. the joint efficiency was approximately 112% relative to Ti substrate and 94?5% relative to Ni substrate. The joint tensile strength abruptly decreased as heating temperature was increased to 873 K and/or the LarsonMiller parameter was increased to approximately 19-20610 3 . The joint tensile strength rapidly decreased with increasing interlayer thickness up to approximately 10 mm, and then remained constant for further increase in interlayer thickness. Four layers occurred at the interface of joints heated to more than 873 K, namely Ti 2 Ni, TiNi, TiNi 2 , TiNi 3 . The fracture of heated joints propagated mainly in the Ti 2 Ni layer and/or at the interface between the TiNi and TiNi 3 layers.

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Photocatalytic activity from the reaction of titanium oxide with ultraviolet light has recently gained much attention. In particular, there is scientific interest in inducing photocatalytic reactions on Ti-Ni alloy, a material widely used in orthodontic applications. However, it is believed that inducing a photocatalytic reaction with an amorphous oxide film on the alloy is a difficult challenge. In this study, therefore, we sought to induce a photocatalytic reaction on Ti-Ni alloy by subjecting the latter to electrolytic and heat treatments. Then, an antibacterial test was used to examine whether a photocatalytic reaction had indeed been induced. By thickening the titanium oxide film with electrolytic treatment and then applying heat treatment, the surface oxide film of Ti-Ni alloy was thus modified from amorphous structure to rutile crystal. Furthermore, it was revealed that Ti-Ni alloy had an antibacterial effect by virtue of the photocatalytic reaction.

Shape memory characteristics of Ti-18 mol%Nb alloys containing 3, 4, 5 and 6 mol%Cu (termed Ti18Nb3Cu, Ti18Nb4Cu, Ti18Nb5Cu and Ti18Nb6Cu, respectively) were investigated and effects of Cu addition on shape memory behavior was clarified. The alloys fabricated by Ar arc-melting method were cold-rolled with 98% reduction thickness and solution-treated at 1273 K for 1.8 ks followed by quenching into water. It was found by -2 X-ray diffraction analysis (XRD) at room temperature (RT) that Ti18Nb3Cu is composed of 00 (c-centered tetragonal)martensite phase and (bcc) parent phase. The other alloys with higher Cu contents are single phase. These results indicate that 6 mol%Cu is completely dissolved in Ti-18 mol%Nb alloys. Besides, the lattice parameter of phase is decreased by Cu addition with a rate of 0:2 Â 10 À3 nm/mol%Cu, and then, the atomic radius of Cu in Ti-18 mol%Nb alloys is estimated to be 0.130 nm. By tensile test it was found that (1) shape recovery strain of Ti18Nb3Cu reaches 3% by heating after deformation, (2) Ti18Nb4Cu exhibits superelasticity at RT, and (3) either shape memory effect or superelasticity does not appear at RT for Ti18Nb5Cu and Ti18Nb6Cu. Besides, the stress for slip deformation is increased by Cu addition with a rate of 50 MPa/mol%Cu. By tensile tests at cryogenic temperatures, the martensitic transformation start temperature (M s ) of Ti18Nb5Cu is determined to be 75 K, and the Cu addition to Ti-18 mol%Nb alloys decreases M s with a rate of 100 K/ mol%Cu. Moreover, more than 5% in transformation strain is observed for Ti18Nb5Cu at 173 K. It was concluded that Cu is an effective additional element in order to improve shape memory and superelastic properties of Ti-Nb alloys.

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