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You may further deposit the accepted manuscript version in any repository, provided it is only made publicly available 12 months after official publication or later and provided acknowledgement is given to the original source of publication and a link is inserted to the published article on Springer's website. The link must be accompanied by the following text: "The final publication is available at link.springer.com”. 1 23 Author's personal copy Shap. Mem. Superelasticity https://doi.org/10.1007/s40830-018-0159-y SPECIAL ISSUE: A TRIBUTE TO PROF. JAN VAN HUMBEECK – A LIFETIME OF CONTRIBUTIONS TO UNDERSTANDING MARTENSITE, INVITED PAPER Straightforward Downsizing of Inclusions in NiTi Alloys: A New Generation of SMA Wires with Outstanding Fatigue Life Alberto Coda 1 • Andrea Cadelli 1 • Matteo Zanella1 • Luca Fumagalli 1 Ó ASM International 2018 Abstract One of most debated aspects around Nitinol Introduction quality is microcleanliness, nowadays considered as the main factor affecting fatigue life. Recent results demon- NiTi-based shape memory alloys (SMA) are functional strate that fatigue is undoubtedly associated with inclusions materials with a multitude of properties interesting for which can act as crack initiators. However, type, size, and technological applications. These properties depend on the distribution of such particles have been observed to peculiar deformation mechanisms, accounting for the shape strongly depend on Ni/Ti ratio as well as melting and memory effect, the superelastic behavior and the damping thermo-mechanical processes. Therefore, if a general capacity of these materials [ 1]. SMAs are used in different reduction of non-metallic inclusions is expected to generate fields, like thermo-mechanical devices [ 2], anti-loosening a beneficial effect in improving lifetime of Nitinol, on the systems [ 3], biomedical applications [ 4], mechanical other hand this necessarily involves a hard review of both damping systems, and in some cases employed for large- material melting and processing. In this work, the charac- scale civil engineering structures [ 5]. NiTi shows a very terization of the fatigue behavior of SMA wires with good combination of properties, especially in terms of diameter below 100 lm is presented. The wires were pre- energy density and a large amount of recoverable strain pared by a peculiar, non-standard combination of melting compared to other actuator principles. The obvious sim- and thermo-mechanical processes (Clean Melt technology). plicity of mechanical design and minimum number of Thermo-mechanical cycling was carried out and the frac- moving parts is amazing for an actuator and make SMAs ture surfaces of all failed wires were investigated by particularly attractive for microsystems applications. Con- scanning electron microscopy. A robust set of data was siderable progress has been made thanks to the integration collected and analyzed by using the statistics of extremes. and miniaturization of sensors, control electronics and the Results clearly demonstrate that in the new NiTi Clean implementation of intelligence using integrated microcon- Melt alloy the maximum inclusion size and area fraction trollers and specific softwares. are significantly reduced compared to standard Nitinol. However, there are several design criteria that must be This offers meaningful improvement in fatigue resistance controlled to guarantee a widespread diffusion of SMAs to over standard wires. technological fields. For instance, SMAs display a narrow dependence of the shape-memory related properties, like Keywords NiTi shape memory wires Fatigue transition temperatures, on their actual composition. For Á Á Inclusions Extreme value this reason, a great care in the production steps, mainly Á based on melting and casting processes, is required. Another design criterion lies in the strong influence of thermo-mechanical history on their properties. This may disclose interesting perspectives of application to smart & Alberto Coda devices in which different aspects of the shape memory [email protected] phenomenology, like one- and two-way shape memory 1 SAES Getters S.p.A, 20020 Lainate, MI, Italy effect, pseudoelasticity, damping capacity, etc., are used. 123 Author's personal copy Shap. Mem. Superelasticity Currently, studies are mainly devoted to some aspects technology). The ‘‘improved’’ wires were thus compared related to the material quality and behavior enhancement. with standard materials. Among these, the improvement of thermo-mechanical fatigue represents with no doubt the most important one [6]. Experimental Fatigue of SMAs is related to microcleanliness [ 7]. Ò Microcleanliness of NiTi alloys is rated by the presence of SmartFlex trained wires (Ti 51.0 ± 0.05 at.% As- non-metallic inclusions and porosity. The size and distri- = ? 95 °C) with diameter of 25, 76, and 100 lm were bution of inclusions can play a critical role in affecting the prepared with standard VIM-VAR approach and with a fatigue behavior and the quality of NiTi alloys used in new combination of melting and thermo-mechanical pro- components or products [ 8]. Typical non-metallic inclu- cess, modified to have smaller inclusions size and area sions are carbides (TiC) and oxides (Ti 4Ni 2Ox). Through fraction (Clean Melt technology). the diligence of the alloy manufacturers, virtually all Drawing recipe, final cold work (35% ± 10), surface inclusions in NiTi alloys are indigenous. The type of finish (standard commercial light amber), straight anneal- inclusions, the amount, and the size distribution depend on ing, and training procedures were kept constant and similar the raw materials, the melting method, and the thermo- to the standard production to neutralize as much as possible mechanical processes used in making wrought product. secondary effects on functional and fatigue behavior. The indigenous inclusions are uniformly distributed on a Minor Carbon and Oxygen concentrations were detected macroscopic scale but segregated on a microscopic scale on ingots by LECO analyzers, using the combustion [9]. infrared absorption method as per suggestions of standard The reduction of both size and amount of inclusions is practice ASTM F2063-12 [ 14 ]. The transformation tem- key to increase lifetime of NiTi-based SMA alloys. The peratures were measured at ingot level by differential benefits of high purity NiTi alloys are yet to be completely scanning calorimetry (DSC), as prescribed by the ASTM quantified. There is evidence that fatigue cracks often F2004-05 [ 15 ]. nucleate at subsurface inclusions; however, the differences The inclusions analysis was carried out in the longitu- in carbon and oxygen contents between VIM, VIM-VAR, dinal direction of semi-finished hot worked coils and multiple VAR processes do not result in a measurable (Ø = 6.35 mm) according to a consolidated internal pro- difference in fatigue resistance. cedure described by Sczerzenie et al. [ 16 ] and following Recent results [10 –12 ] claim that fatigue is undoubtedly the guidelines indicated within ASTM F2063-12. In par- associated with inclusions which can act as crack initiators. ticular, the samples were taken from head, center, and tail Further works also demonstrate a strong correlation of full size production spools. Metallographic centerline between the fatigue limit and the size of extreme, or samples of the materials were prepared by standard prac- maximum inclusions, while the gas impurities amount do tice and analyzed by scanning electron microscopy (SEM). not show any evident relationship with the fatigue behavior The analysis was carried out by scanning the length of each [13 ]. However, type, size, and distribution of such particles sample in three regions of the cross section: the centerline, have been observed to strongly depend on Ni/Ti ratio as the mid-radius, and the near edge lines. Nine fields of view well as melting and thermo-mechanical processes. There- and over one hundred particles per sample were analyzed. fore, if a general reduction of non-metallic inclusions can The shape memory behavior of wires was characterized have a beneficial effect in improving lifetime of Nitinol, on by uniaxial constant force thermal cycling (UCFTC) under the other hand this necessarily should involve a hard different loading conditions (from 100 to 400 MPa) in a review of both material melting and processing. special equipment. From this general background, efforts have been Constant force thermal cycling fatigue was conducted undertaken to reduce the inclusions size, especially for on three different wire diameters (25, 76, and 100 lm) by production of very thin shape memory