The Effect of Nonmetallic Inclusions of Ti-Ni Alloys on Rotary Beam Fatigue Properties

Ti-Ni alloy has superelasticity but also excellent corrosion resistance and biocompatibility, and thus is used in medical devices, such as guidewires in addition to stents. In particular, applied for stents are which have been growing more prevalent in recent years.

Stents placed inside the body experience cyclical deformation loads over a long period of time, such as those due to the pulse. Accordingly, there are concerns over fatigue fractures originating from nonmetallic inclusions. For this reason, the ASTM F2063 series, which is a set of standards for Ti-Ni alloys for medical use, specifies the sizes and area ratios of nonmetallic inclusions, for instance titanium carbides (TiC) and titanium oxides (Ti₄Ni₂O_x).

Although the two types of nonmetallic inclusions known in Ti-Ni alloy are TiC and Ti₄Ni₂O_x as described above, there has been little research on the effect of the types of these phases on fatigue properties [1][2], there have been no detailed reports on which inclusions fail preferentially. In this research, the wires containing nonmetallic inclusions are mixed phases of carbides and oxides in the Ti-Ni alloy. The effect of the types of the nonmetallic inclusions were investigates by the rotary beam fatigue test.

[1] Hara et al., Proceed. 24th Int. Conf., Processing and Fabrication of Advanced Mat., (2015).

[2] Yamashita et al., Metals, 9 (2019), 999.