Toshikazu Akahori1, Mitsuo Niinomi1, Tatsuya Higuchi1, and Koichi Morii2
1Production Systems Engineering, Toyohashi University of Technology, Toyohashi 441-8580
Ti-Ni alloys show superior mechanical properties, especially fatigue strength as compared with other shape memory alloys, so that Ti-Ni alloys are used for practical applications. Ti-Ni alloys are mainly used as wires and plates for dental and medical applications under a cyclic load. Relationship between the shape memory characteristics and fatigue properties is important and should be understood to improve the safety usages of products made of Ti-Ni alloys. Recently, the third alloying elements have been attempted to add in Ti-Ni alloys, and the effects of the transformation temperature on the mechanical properties have been investigated due to achieve better mechanical properties. However, there are a few reports on the mechanical properties, in particular, fatigue strength with different Ni and third element contents. In present study, fatigue properties were investigated for Ti-54.2 mass%Ni, Ti-55.8 mass%Ni, Ti-55.4 mass%Ni-0.5 mass%Co and Ti-54.4 mass%Ni-1.49 mass%Co heat-treated at 723 K and 873 K for 3.6 ks followed by water quenching.
Transformation temperatures of Ti-Ni and Ti-Ni-Co alloys tend to decrease with increasing Ni contents and Co/Ni content ratio. Stress-strain curves of Ti-Ni and Ti-Ni-Co alloys show two-step deformation behavior except for Ti-54.4 mass%Ni-1.49 mass%Co. Fatigue ratio, which means the ratio of maximum cyclic stress to tensile strength, of Ti-Ni and Ti-Ni-Co alloys increases with increasing heat treatment temperature in low cycle fatigue (LCF) and high cycle fatigue (HCF) life regions except for Ti-55.4Ni-0.5Co. The fatigue ratio of Ti-54.4 mass%Ni-1.49 mass%Co heat-treated at 873 K shows the highest value in LCF and HCF life regions. The fatigue limit of Ti-54.4 mass%Ni-1.5 mass%Co is around 0.62. The reason for the increment of fatigue strength is that the transformation temperatures are much lower and the tensile strengths are relatively higher as compared with those of other Ti-Ni and Ti-Ni-Co alloys.
(Received February 24, 2003)
titanium-nickel alloy, titanium-nickel-cobalt alloy, transformation temperature, tensile properties, fatigue properties
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