On the Influence of Inclusions On the Fatigue Life of Pseudoelastic NiTi Shape Memory Alloys

In the present work, we investigate how oxide and carbide inclusions affect the fatigue behavior of pseudoelastic NiTi shape memory alloys (SMAs). Three types of NiTi SMA wires (high purity, O-rich and C-rich) with different oxygen and carbon impurity levels (both ranging from 0.004 to 0.05 wt.%), but very similar mechanical behavior, were prepared by arc melting and wire drawing in combination with heat treatments. Structural fatigue testing was carried out by bending rotation fatigue using electropolished samples to guaranty a good surface quality. The resulting fracture surfaces were characterized extensively through scanning electron microscopy. It was found that inclusions strongly affect fatigue performance, once other critical surface defects resulting from wire drawing are removed through electropolishing. For strain amplitudes above the transformation strain, the wire with the highest purity clearly outperforms the wires with high C- and O-levels. For strain amplitudes below the transformation strain, the high purity wire and the C-rich wire show higher fatigue lives than the material with a higher O-level. It was found that inclusions which are associated with voids (particle/void-assemblies) represent preferred crack initiation sites. The low fatigue life of oxygen-rich wires is directly related to the high frequency of this type of defect in the material.