Plasma Arc Melting (PAM) and Corrosion Resistance of Pure NiTi Shape Memory Alloys

Shape Memory and Superelasticity, Mar 2015

Plasma arc melting (PAM) as a suitable non-contaminating melting route for manufacturing high-quality NiTi alloy was successfully examined. The corrosion resistance of PAM Nitinol was evaluated by both potentiodynamic and potentiostatic tests and compared with lower purity NiTi produced by vacuum induction melting (VIM). For the electro-polished surfaces, excellent corrosion resistance of NiTi comparable with the Ti alloys was found with no pitting up to 800 mV versus saturated calomel electrode in simulated body fluid at 37 °C. Potentiostatic results of PAM Nitinol indicate slightly better corrosion resistance than the lower quality VIM alloy.

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Plasma Arc Melting (PAM) and Corrosion Resistance of Pure NiTi Shape Memory Alloys

Shap. Mem. Superelasticity Plasma Arc Melting (PAM) and Corrosion Resistance of Pure NiTi Shape Memory Alloys A. Tuissi 0 1 G. Rondelli 0 1 P. Bassani 0 1 0 CNR IENI Unit of Milan, National Research Council of Italy - Institute for Energetics and Interphases , Via R. Cozzi 53, 20125 Milan , Italy 1 CNR IENI Unit of Lecco, National Research Council of Italy - Institute for Energetics and Interphases , Corso Promessi Sposi 29, 23900 Lecco , Italy Plasma arc melting (PAM) as a suitable noncontaminating melting route for manufacturing high-quality NiTi alloy was successfully examined. The corrosion resistance of PAM Nitinol was evaluated by both potentiodynamic and potentiostatic tests and compared with lower purity NiTi produced by vacuum induction melting (VIM). For the electro-polished surfaces, excellent corrosion resistance of NiTi comparable with the Ti alloys was found with no pitting up to 800 mV versus saturated calomel electrode in simulated body fluid at 37 C. Potentiostatic results of PAM Nitinol indicate slightly better corrosion resistance than the lower quality VIM alloy. NiTi processing; Corrosion resistance; Plasma arc melting; Microstructure Introduction Due to the highly reactive nature of titanium, the synthesis of NiTi shape memory alloy (also known as Nitinol) requires non-contaminating processing methods. The gathering of C, N, and O on melting causes the formation of inclusions such as TiC, TiO2, and Ti2NixOy on bulk level and hot-working oxidation may leave distributed inclusions at surface level of semi-finished products. A recent observation, the surface particulates of NiTi and in biomedical devices, is reported by Shabalovskaya et al. [ 1 ]. The inclusions, and their sizes, could affect both the NiTi functional properties and the processing aspects such as the workability of low dimension semi-finished products. Nowadays, the role and the amount of inclusions affecting specific properties such as the fatigue life, biocompatibility, or corrosion resistance of devices made in NiTi are not univocally explained. Regarding TiC inclusions, which are found typically in VIM processed Nitinol due to carbon crucibles employed in this kind of melting plant, their presence is particularly detrimental to pitting corrosion resistance, as asserted in Kimura and Sohmura [ 2 ], Shabalovskaya et al. [ 3 ]. Also, regarding oxide-type inclusions (i.e., Ti2NiOx) Liang and Huang [ 4 ] have found that their presence is the main reason for pitting. Coda et al. [ 5 ] characterized the main types of inclusions of commercial Nitinol produced by VIM and vacuum arc melting (VAR) routes; by means of extreme value statistics, Urbano et al. [ 6 ] showed that the maximum inclusion dimension in the highly strained volume of a fatigue test contributes to a decrease in the fatigue performance of 0.3mm-diameter superelastic wires. Morgan et al. [ 7 ] reported about extra low inclusions content (ELI) alloys appeared to have improved fatigue performances. Rahim et al. [ 8 ] investigated the effect of C and O impurity levels on fatigue lives, of pseudoelastic NiTi, reporting that the fatigue resistance is affected mainly by surface aspects and that lower C and O contents will not result in greater fatigue lives. However, cleaner processing decreases the probability for fatigue crack initiation due to inclusions, but it cannot suppress it completely. It is a matter of fact that non-contaminating melting route should be used for the production of high-quality NiTi alloys for biomedical applications, according to the ASTM standard specification F 2063-12 [ 9 ] that limits both the impurity contents as well as the amount of particulates. Among non-contaminating melting methods, PAM represents one of the most important achievements in the field of high-quality alloy production over the last three decades. The elevated working pressure of PAM prevents the selective evaporation of the high-vapor pressure elements avoiding chemical alterations of final alloys composition. Moreover, inert operating conditions of PAM, in conjunction with the transfer of a high energy density, permit alloying of a great number of metallic elements. For over 20 years, industrial PAM furnaces have been employed for the production of reactive, high melting points, refractory metals, and superalloys, as reported in [ 10–15 ]. Thus, PAM seems particularly suitable for melting ‘‘cleaner,’’ near equiatomic NiTi shape memory alloys (SMA), not only on laboratory scale NiTi-based button preparations [ 16, 17 ], but also for Nitinol industrial production. The aim of this work is the evaluation of the corrosion resistance of a Ni50.8Ti49.2 alloy produced by a pilot scale PAM furnace in comparison to one prepared through a conventional VIM melting technique by using the same starting Ni and Ti metals. Microstructures of PAM and VIM NiTi alloys were investigated by FEG-SEM, and the corrosion resistance was determined by potentiodynami (...truncated)


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A. Tuissi, G. Rondelli, P. Bassani. Plasma Arc Melting (PAM) and Corrosion Resistance of Pure NiTi Shape Memory Alloys, Shape Memory and Superelasticity, 2015, pp. 50-57, Volume 1, Issue 1, DOI: 10.1007/s40830-015-0011-6