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Abstract:
The main goal of the proposed paper is to present the results of the nitrogen ion implantation effects on mechanical and corrosion properties of NiTi shape memory alloy. Local pseudoelasticity phenomena of NiTi were determined using the ultra-low load applied system. The load–penetration depth curves show that lower nitrogen fluence improves mechanical properties in the near surface layer but higher ion fluence leads to degradation of pseudoelasticity properties. Corrosion resistance of NiTi in the Ringer solution was evaluated by means of electrochemical methods. The results of potentiodynamic measurements in the anodic range for implanted NiTi indicate a decrease of passive current density range in comparison with non-treated NiTi, without any signs related to Ni release. The results of impedance measurements recorded at the corrosion potential show a capacitive behaviour for all samples without clear predominance of one of them. It can be explained by the fact that this result concerns the first stage of corrosion exposition. It is shown that nitrogen ion implantation leads to formation of modified surface of improved physicochemical properties

Abstract:
Due to superelasticity phenomena, shape memory effect and durability in many environments NiTi shape memory alloys (SMAs) are being intensively adapted for biomedical applications. However, high Ni concentration and its possible release from the surface create a problem of negative effects on biological environment. Therefore a number of surface treatments has been proposed to stabilize a surface passive layer and inhibit Ni dissolution. The main goal of the proposed article is to present results of the effect of nitrogen ion implantation on mechanical and corrosion properties of equiatomic commercial NiTi SMA. Mechanical properties (local superelasticity phenomena) of NiTi samples were determined using ultra-low load indentation system. The differential scanning calorimetry was applied to measure the change of characteristic temperatures due to ion implantation treatment. The load–penetration depth curves show that the lower of applied nitrogen doses improve mechanical properties in the near-surface layer of NiTi. Corrosion resistance of implanted samples carried out in Ringer solution was evaluated by means of electrochemical methods. The results of potentiodynamic measurements in anodic range of polarization curves for implanted samples indicated a decrease of current density in passive range in comparison with non-treated NiTi sample. Therefore an inhibition of Ni release from the alloy can be expected. The results of impedance measurements recorded at the corrosion potential show a capacitive behaviour for all samples without clear predominance of one of them. It can be explained by the fact that this result concerns the case of general corrosion in the beginning of exposition. Prolonged exposition gives rise to the occurrence of local corrosion. It is shown in this work that nitrogen ion implantation (fluences 1017- 1018 ion/cm2, energy 50 keV) leads to a creation of modified surface of improved physicochemical properties with unchanged shape memory effect.

Keywords:
shape memory alloy(SMA), nitinol, ion implantation, superelasticity, corrosion resistance