Institute of Fundamental Technological Research

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

The Ni/Al multilayer coating of λ ≈100 nm was deposited onto (001)-oriented monocrystalline silicon substrate using double target magnetron sputtering system equipped with rotating sample holder. The thicknesses of alternating layers were adjusted in the way to preserve the chemical composition ratio close to 50%Al:50%Ni (at.%). The in situ X-ray diffraction and in situ transmission electron microscopy heating experiments were carried out at relatively low heating rates (20°C/min) in order to study the phase transformation sequence. The investigations revealed that the reaction between Ni and Al multilayers starts at ≈200°C with precipitation of Al₃Ni phase, while above 300°C dominates precipitation of Ni₃Al and NiAl intermetallic phases. Both the X-ray and electron diffractions acquired at 450°C confirmed presence of the Ni₃Al and NiAl intermetallics, but the former pointed at still lasting traces of Ni(Al) solid solution.

transmission electron microscopy, multilayers

The results of X-ray diffraction studies on structural changes in the near-surface layers in the NiTi alloy caused by nitrogen-ion implantation with the energy E = 50 keV and the fluence D = 1018cm°2 are presented. X-ray diffractometry, using the Philips diffractometer type X’Pert in the Bragg–Brentano geometry, was used to identify the phase composition of NiTi alloy. For layer by layer analysis of structural changes in the near-surface layers, the D8 Discover Bruker diffractometer with polycapilar beam optics was used. The ion-implanted NiTi alloy in the near-surface layer exhibits five phases: the dominating austenite phase, two martensitic phases and a small amount of the Ni4Ti3 and NTi phases. Along with the decreasing thickness of the near-surface layer investigated in material an increasing fraction of the Ni4Ti3 and NTi phases was observed. With the thickness of this layer about 340 nm, besides still existing the austenite, Ni4Ti3 and NTi phases, only one martensitic phase is present in the alloy. Further decrease of the thickness of the near-surface layer to about 170 nm leads to the increasing fraction of the Ni4Ti3 and NTi phases.

X-ray diffraction, ion implantation, shape memory alloy, nitinol, phase composition

In the present study we report the changes in the modified near-surface layer on NiTi shape memory alloy, caused by ion implantation as well as their influence on the structure and mechanical properties of this material. Experimental results of an inhomogeneous structure and tribological properties of implanted NiTi are discussed in this paper.

shape memory NiTi alloy, Ion implantation, DSC, X-ray diffractometry, TEM

This paper reports the application of nitrogen ion implantation for modification of a shape memory alloy. It is known that the problem of creating a protective surface coating for the shape memory alloy is the most acute for potential applications of this material. Thus, the problem of increasing surface protective properties and, at the same time, simultaneous preservation of functional properties of shape memory materials is a subject of research and development [Pelletier H, Muller D, Mille P, Grob J. Surf Coat Technol 2002;158:309.]. The surface characterization of nitrogen implanted (fluence 1018 cm−2 and energy 50 keV) equiatomic commercial NiTi alloy samples was performed with the assistance of high resolution transmission electron microscopy (HTEM) techniques and modifications of phase composition before and after irradiation are studied at room and martensitic transformation temperatures by X-ray diffraction methods. Differential scanning calorimetry (DSC, TA Instruments) was used to characterize the transformation sequence and transformation temperatures for the initial and surface-modified materials. Experimental results of an inhomogeneous structure of near-surface layers in the ion-implanted NiTi alloy are discussed in this paper

Shape memory NiTi alloy, Ion implantation, DSC, X-ray diffractometry, TEM