Abstract
Fatigue crack propagation behavior and stress-induced martensitic transformation behavior in two types of TiNi were investigated: One is TiNi with shape memory effect, FML4, the other is superelastic TiNi, FML6, at room temperature. In addition to the observation of fatigue cracks by optical and scanning electron microscopes, the crystallographic orientation image microscope was utilized to investigate the distribution of martensitic phase around the crack. Particular emphasis was placed on the effect of the stress-induced martensitic transformation measured by neutron and X-ray diffraction in a smooth and notched specimens. The results show that the crack propagation rate, da/dN, in FML4 was smaller than that in FML6 when compared at the same stress intensity range, ΔK. In the range over da/dN of 1x10^<-9> m/cycle, the value was about 1/5 of that in FML6. The threshold value of ΔK was also higher in FML4. Even when da/dN is compared at the same effective stress intensity range, ΔK_<eff>, the difference remains remarkable and the threshold value of ΔK_<eff> in FML4 was found to be larger. The difference of the phase surrounding the fatigue crack was found to be a dominant factor for difference between FML4 and FML6. Based on the result of EBSP measurement, the residual martensitic phase was hardly found at the crack tip and crack wake in FML6. The distribution of the stress-induced martensitic phase at notch root was measured in FML4 and found to be the effective method to measure the stress-induced martensitic phase at a crack tip.
