Commercially available nitinol is currently manufactured using classic casting methods that produce blocks, the processing of which is difficult and time consuming. By continuous casting, wherein molten metal solidifies directly into a semi-finished product, the casting and processing of ingots can be avoided, which saves time and expense. However, no reports on continuous casting of nitinol could be found in the literature. In this work, Φ 12 mm nitinol strands were continuously cast. Using a graphite crucible, smelting of pure Ni and Ti in a medium frequency induction furnace is difficult, because it is hard to prevent a stormy reaction between Ni and Ti and to reach a homogeneous melt without a prolonged long holding time. Using a clay-graphite crucible, the stormy reaction is easily controlled, while effective stirring assures a homogeneous melt within minutes. Strands of nearly equiatomic chemical compositions were obtained with acceptable surface quality. The microstructure of strands containing over 50 at. % Ni, consisted of Ti2Ni and cubic NiTi, whereas the microstructure of strands containing less than 50 at. % Ni consisted of TiNi3 and cubic NiTi. This is consonant with the results of some other authors, and indicates that the eutectoid decomposition NiTi → Ti2Ni + TiNi3 does not take place.

In this paper we present research that is connected to the performance of a series of experiments combined with the vacuum-induction melting and continuous vertical casting of a NiTi alloy in order to produce the strand. The theoretical chosen parameters made it possible to obtain a continuously cast strand with a diameter of 11 mm. The strand microstructures were investigated with a light and scanning electron microscope, while the chemical composition of the single phase was identified with the semi-quantitative micro-analysis energy-dispersive X-ray spectroscopy and inductively coupled plasma -optical emission spectrometry. The research showed that the microstructure is dendritic, where in the inter-dendritic region the eutectic is composed of a dark NiTi phase and a bright TiNi3-x phase. In some areas we found Ti carbides and phases rich in Fe. The micro-chemical analysis of the NiTi strand showed that the composition changed over the cross and longitudinal sections, which is proof that the as-cast alloys are inhomogeneous. In the final part, the electrochemical behaviours of NiTi strand samples were compared to a commercially available NiTi cast alloy with the same composition. Keywords: NiTi alloy, continuous vertical casting, microstructure, potentiodynamic and impedance test V tem prispevku predstavljamo raziskavo, ki je povezana z izvedbo niza preizkusov vakuumskega pretaljevanja in so~asnega kontinuirnega vertikalnega litja NiTi zlitine s ciljem odliti palico. Teoreti~no izbrani parametri so omogo~ili, da smo uspeli kontinuirno odliti NiTi palico s premerom 11 mm. Dobljeno mikrostrukturo palice smo raziskali s svetlobnim in vrsti~nim elektronskim mikroskopom, kemijsko sestavo posameznih faz pa smo identificirali s semi-kvantitativno mikro-kemi~no analizo Energijsko disperzijsko spektrometrijo in z opti~nim emisijskim spektrometrom z induktivno sklopljeno plazmo. Preiskave so pokazale, da je mikrostruktura dendritska, medtem ko s v meddendritskem prostoru nahaja evtektik, sestavljen iz temne NiTi faze in svetle TiNi3-x faze. Mestoma smo identificirali tudi Ti karbide in fazo bogato s Fe. Mikro-kemi~na analiza NiTi palice je odkrila, da se sestava spreminja po prerezu in po dol`ini, kar nakazuje, da je zlitina po strjevanju nehomogena. V zaklju~nem delu smo primerjali elektrokemijsko obna{anje vzorcev NiTi palice s komercialno dostopno valjano NiTi zlitino enake sestave. Klju~ne besede: NiTi zlitina, vertikalno kontinuirno litje, mikrostruktura, potenciodinami~ni in impedan~ni test

Nitinol is a group of nearly equiatomic alloys composed of nickel and titanium, which was developed in the 1970s. Its properties, such as superelasticity and Shape Memory Effect, have enabled its use, especially for biomedical purposes. Due to the fact that Nitinol exhibits good corrosion resistance in a chloride environment, an unusual combination of strength and ductility, a high tendency for self-passivation, high fatigue strength, low Young’s modulus and excellent biocompatibility, its use is still increasing. In this research, Atomic Layer Deposition (ALD) experiments were performed on a continuous vertical cast (CVC) NiTi rod (made in-house) and on commercial Nitinol as the control material, which was already in the rolled state. The ALD deposition of the TiO2 layer was accomplished in a Beneq TFS 200 system at 250 °C. The pulsing times for TiCl4 and H2O were 250 ms and 180 ms, followed by appropriate purge cycles with nitrogen (3 s after the TiCl4 and 2 s after the H2O pulses). After 1100 repeated cycles of ALD depositing, the average thickness of the TiO2 layer for the CVC NiTi rod was 52.2 nm and for the commercial Nitinol, it was 51.7 nm, which was confirmed by X-ray Photoelectron Spectroscopy (XPS) and Scanning Electron Microscope (SEM) using Energy-dispersive X-ray (EDX) spectroscopy. The behaviour of the CVC NiTi and commercial Nitinol with and without the TiO2 layer was investigated in a simulated body fluid at body temperature (37 °C) to explain their corrosion resistance. Potentiodynamic polarisation measurements showed that the lowest corrosion current density (0.16 μA/cm2) and the wider passive region were achieved by the commercial NiTi with TiO2. Electrochemical Impedance Spectroscopy measurements revealed that the CVC NiTi rod and the commercial Nitinol have, for the first 48 h of immersion, only resistance through the oxide layer, as a consequence of the thin and compact layer. On the other hand, the TiO2/CVC NiTi rod and TiO2/commercial Nitinol had resistances through the oxide and porous layers the entire immersion time since the TiO2 layer was formatted on the surfaces.

This article presents the information of the microstructure and methods for determination of mechanical and functional properties of the Continuous Vertical Cast (CVC) NiTi rod. We prepared special samples (discs, cylinders). The results of the microhardness measurements with HV0.1 show the average Vickers hardness value 583 HV0.1 in the cross-section of the CVC NiTi rod measured from the edge toward the middle of CVC NiTi rod. The results show that Vickers hardness slightly increases towards the center of the rod as a consequence of the formatted microstructure. The results of the tensile test show a typical curve for brittle materials. The obtained results indicate that the CVC NiTi rod does not pass into a non-linear plastic region. Therefore, CVC NiTi rod is unsuitable for strain hardening. The results of the compressive test also follow the conclusions that CVC NiTi rod is brittle material. Fracture of the CVC NiTi rod sample occurs before the material enters the plastic deformation region. The cytocompatibility of the CVC NiTi rod was tested with the submersion of the samples to the HUVEC cell solution, where HUVEC cells adhered to the samples surface. A commercial NiTi alloy was used to monitor the cytocompatibility. The results show that the number of adhered HUVEC cells on the samples surface of CVC NiTi rod is smaller than on commercial NiTi. The number of dead HUVEC cells on the samples surface of CVC NiTi rod is slightly larger than on commercial NiTi. The proportion of viable HUVEC cells on the samples surface of CVC NiTi rod is 96,6 %, while on commercial NiTi 98,4 %.