Effect of Surface Roughness and Ti Base Precipitate on Fatigue Strength of Ni-Ti-Nb Shape Memory Alloy

The fatigue strength of Ni-Ti-Nb shape memory alloy was investigated focusing on the influence of surface roughness. The specimens had the surface roughness between 20 and 50 μm and a notch with depth of 250 μm, and the cyclic axial stresses of different stress ratios were applied at 293 K. When the surface roughness was below 50 μm, the alternating fatigue limit leveled off at 285 MPa. However, when the notch depth was 250 μm, it decreased to 145 MPa. The microscopic observation of the fracture suface revealed that the crack started at the Ti base precipitates formed in the subsurface of the specimens with roughness below 50 μm. In contrast the crack began at the surface for notch specimens. Fracture mechanics analysis shows that Ti-rich precipitates affect the fatigue limit of the 50 μm sample, which is four to five times thicker than that of the precipitates. These results indicate that the influence of surface roughness on the fatigue strength of Ni-Ti-Nb shape memory alloy can be disregarded surface roughness less than 50 μm.