Straightforward Downsizing of Inclusions in NiTi Alloys: A New Generation of SMA Wires with Outstanding Fatigue Life
Recent results demonstrate that fatigue is undoubtedly associated with inclusions which can act as crack initiators. However, type, size, and distribution of such particles have been observed to strongly depend on Ni/Ti ratio as well as melting and thermo-mechanical processes. Therefore, if a general reduction of non-metallic inclusions can have a beneficial effect in improving lifetime of Nitinol, on the other hand this necessarily involves a hard review of both material melting and processing.
In this work the characterization of the fatigue behavior of NiTi-based shape memory and superelastic wires with diameter below 100µm is presented. The wires were prepared by a peculiar, non-standard combination of melting and thermo-mechanical processes. Cycling was carried out using a fatigue tester and the fracture surfaces of all failed wires were investigated by scanning electron microscopy. A robust set of data was collected and analyzed by the most common statistical methods. The improved wires were thus compared with standard materials. Results demonstrate that in the new NiTi alloys the maximum inclusion size and area fraction are significantly reduced compared to standard Nitinol. The inclusions analysis shows that the size of more than 99% of non-metallic particles is below 3µm with the average centered around or below 1µm. This offers significant (10X to 100X) improvement in fatigue resistance over standard wires.