2018 年 59 巻 8 号 p. 1296-1303
Ultrafine-grained pure titanium (Ti) plates with the thickness of 1 mm were fabricated by a combined process of multi-directional forging (MDFing) and cold rolling (here after referred as MDFed pure Ti for simplicity) aiming at the dental implant application. The plates exhibited higher tensile strength than the conventional cold-rolled pure Ti plates due to the ultrafine-grained structure with an average size of 200 nm. The axial fatigue tests were conducted in laboratory air and in Ringer’s solution to investigate long-term durability as dental implants. The fatigue strengths of the MDFed pure Ti plates in laboratory air were higher than those of the cold-rolled pure Ti plates as well as the tensile properties. In the high cycle fatigue (HCF) regime, sub-surface crack initiation with fish-eye fracture surface was observed in the MDFed pure Ti plates, while surface crack initiation was dominant in the cold-rolled pure Ti plates. Sub-surface crack generally initiated at the mid-thickness of the thin plates. Inclusions were not recognized at the crack initiation sites, while microstructural analyses revealed that some coarse grains with the size of a few µm distributed around the crack initiation sites. Consequently, the sub-surface crack initiation mechanism was attributed to the inhomogeneity of the microstructure near the mid-thickness of the plates. The corrosion fatigue strengths in Ringer’s solution were comparable to those in laboratory air, where sub-surface crack initiation occurred in the HCF regime even in corrosive environment. That indicates the high corrosion resistance of the MDFed pure Ti plates.
