Laser oxidation of NiTi for improving corrosion resistance in Hanks' solution

NiTi plates were surface treated to form an oxide film with a pulsed Nd:YAG laser in ambient air using a carefully selected set of processing parameters. The oxidized samples had a yellowish interference color. The temperature rise of the sample bulk was only about 20 °C though the surface temperature reached could be much higher. Atomic force microscopy (AFM) recorded an average surface roughness of 12.5 nm for the oxidized surface. X-ray photoelectron spectroscopy (XPS) showed that the surface Ni/Ti atomic ratio was reduced from 0.30 for the mechanically polished samples to 0.17 for the oxidized samples, indicating improved biocompatibility. Measurements using electrochemical impedance spectroscopy (EIS) at open-circuit potential recorded a 15.8-fold increase in corrosion resistance in Hanks' solution. In cyclic polarization tests, the laser oxidized NiTi samples did not show pitting up to an applied potential of 1550 mV SCE, similar to bare NiTi, but the passive current density was an order of magnitude lower. The present study positively indicates the feasibility of employing laser oxidation for improving the corrosion resistance of NiTi.     

Tribological behavior of NiTi alloy against 52100 steel and WC at elevated temperatures

The dry tribological behavior of a Ti–50.3 at.% Ni alloy at temperatures of 25 °C, 50 °C and 200 °C was studied. The wear tests were performed on a high temperature pin-on-disk tribometer using 52100 steel and tungsten carbide pins. The worn surfaces of the NiTi alloy were examined by scanning electron microscope. The results showed that in the wear tests involving steel pins, the wear rate of the NiTi decreased as the wear testing temperature was increased. However, for the NiTi/WC contact, a reverse trend was observed. There was also a large decrease in the coefficient of friction for the NiTi/steel contact with increasing wear testing temperature. The formation of compact tribological layers could be the main reason for the reduction of the wear rate and coefficient of friction of the NiTi/steel contact at higher wear testing temperatures.     

High strain and long duration cycling behavior of laser welded NiTi sheets

The use of NiTi in complex shaped components for structural applications is limited by the material cost and machinability and adequate joining techniques have been investigated to minimize the thermal cycle effect on the superelastic and shape memory effects exhibited by NiTi. Laser welding is the most used joining process for this material. However, existing studies mainly address the functional properties of laser welded NiTi wires, and the superelastic cycling tests are limited to either a low number of cycles (maximum 100) or to low strains (below 6%). This paper discusses the results of the cycling behavior exhibited by laser butt welded 1 mm thick NiTi plates, when tested to high strains (up to 10%) and for a large number of cycles (600). The superelastic effect was observed and the microstructural changes induced by the laser welding procedure, namely the extension of the thermal affected regions, were seen to influence the evolution of the accumulated irrecoverable strain. Thus, it is possible, by controlling the heat input introduced during welding, to tune the maximum superelastic recovery presented by NiTi laser welds.     

In vitro mesenchymal stem cell responses on laser-welded NiTi alloy

The biocompatibility of NiTi after laser welding was studied by examining the in vitro (mesenchymal stem cell) MSC responses at different sets of time varying from early (4 to 12 h) to intermediate phases (1 and 4 days) of cell culture. The effects of physical (surface roughness and topography) and chemical (surface Ti/Ni ratio) changes as a consequence of laser welding in different regions (WZ, HAZ, and BM) on the cell morphology and cell coverage were studied. The results in this research indicated that the morphology of MSCs was affected primarily by the topographical factors in the WZ: the well-defined and directional dendritic pattern and the presence of deeper grooves. The morphology of MSCs was not significantly modulated by surface roughness. Despite the possible initial Ni release in the medium during the cell culture, no toxic effect seemed to cause to MSCs as evidenced by the success of adhesion and spreading of the cells onto different regions in the laser weldment. The good biocompatibility of the NiTi laser weldment has been firstly reported in this study.     

Microstructure of selective laser melted nickel–titanium

In selective laser melting, the layer-wise local melting of metallic powder by means of a scanning focused laser beam leads to anisotropic microstructures, which reflect the pathway of the laser beam. We studied the impact of laser power, scanning speed, and laser path onto the microstructure of NiTi cylinders. Here, we varied the laser power from 56 to 100 W and the scanning speed from about 100 to 300 mm/s. In increasing the laser power, the grain width and length increased from (33 ± 7) to (90 ± 15) μm and from (60 ± 20) to (600 ± 200) μm, respectively. Also, the grain size distribution changed from uni- to bimodal. Ostwald-ripening of the crystallites explains the distinct bimodal size distributions. Decreasing the scanning speed did not alter the microstructure but led to increased phase transformation temperatures of up to 40 K. This was experimentally determined using differential scanning calorimetry and explained as a result of preferential nickel evaporation during the fabrication process. During selective laser melting of the NiTi shape memory alloy, the control of scanning speed allows restricted changes of the transformation temperatures, whereas controlling the laser power and scanning path enables us to tailor the microstructure, i.e. the crystallite shapes and arrangement, the extent of the preferred crystallographic orientation and the grain size distribution.     

The influence of laser welding parameters on the microstructure and mechanical property of the as-jointed NiTi alloy wires

The Nd:YAG laser welding was used to join the binary NiTi alloy wires with different compositions(Ti-50.0 at.%Ni and Ti-50.9 at.%Ni) which had the same diameter of 1 mm. The wires were welded with different parameters, including impulse width and welding current. The aim was to assess the influence of the laser-welding process on the microstructure and mechanical properties of the welded joint of binary NiTi wires. The optical microscopy (OM) and the metallographic microscopy (MM) were used to analyze the microstructure of the welded joints. The tensile test and the differential scanning calorimetry (DSC) were carried out to examine the ultimate tensile strength and the reverse martensitic transformation temperatures of the welded joints. It was found that the welding current and the impulse width had great influence on the quality of the welded joints, an optimal parameter combination would remove the pores and micro-cracks appeared in the fusion zone, and result in good mechanical properties such as higher fracture strength and elongation. The laser welding had a few effect on the reverse martensitic transformation temperatures of the welded joints.     

Determination of corrosion rate of orthodontic wires based on nickel‐titanium alloy in artificial saliva

The goal of this study was to determine corrosion behavior of three orthodontic wires based on nickel‐titanium alloy (NiTi) in artificial saliva at temperature of 37 °C as function of immersion time. Following orthodontic wires were used: uncoated (NiTi), rhodium coated (Rh NiTi) and nitrified (N NiTi) orthodontic wires. Corrosion of investigated orthodontic wires were monitored by measuring of Ni²⁺ and Ti⁴⁺ ions released in artificial saliva by inductively coupled plasma‐optical emission spectroscopy (ICP‐OES) after 3, 7, 14, 21 and 28 days of immersion. Obtained results indicate that corrosion reaction of the NiTi wires in artificial saliva follows the parabolic rate law. According to the obtained values of parabolic corrosion rate constants, corrosion susceptibility of orthodontic wires decreases in the following order: Rh NiTi wire (K_p = 2.48 μg²/cm⁴ h) > NiTi wire (K_p = 1.6 × 10^–3 μg²/cm⁴ h) > N NiTi wire (K_p = 6.0 × 10^–4 μg²/cm⁴ h). These results indicate that in comparison with uncoated NiTi wire, rhodium coating significantly increases corrosion susceptibility, while nitrification effectively suppresses the release of Ni²⁺ and Ti⁴⁺ ions.     

Improving the wear resistance of AA 6061 by laser surface alloying with NiTi

To improve the wear resistance of a popular aluminum alloy AA 6061, a 1.5 mm thick hard surface layer consisting of Ni–Al and Ti–Al intermetallic compounds was synthesized on the alloy by laser surface alloying technique. NiTi powder was preplaced on the aluminum alloy substrate and irradiated with a high-power CW Nd:YAG laser in an argon atmosphere. With optimized processing parameters, a modified surface layer free of cracks and pores was formed by reaction synthesis of Al with Ni and Ti. X-ray diffractometry (XRD) confirmed the main phases in the layer to be TiAl₃ and Ni₃Al. The surface hardness increased from below 100 HV for untreated AA 6061 to more than 350 HV for the laser-treated sample. Accompanying the increase in hardness, the wear resistance of the modified layer reached about 5.5 times that of the substrate.     

Effect of porous NiTi alloy on bone formation: A comparative investigation with bulk NiTi alloy for 15 weeks in vivo

The purpose of this study is to investigate the effect of porous NiTi alloy on bone formation with a bulk NiTi alloy as a contrast. The porous NiTi alloy prepared by element powder sintering under Ar protection has a porosity of 45% and a mean pore size of 130 μm, and the pores are highly interconnected. The porous and bulk NiTi alloys were bilaterally implanted into the femurs of rabbits for 15 weeks. The bone-implant interface and bone ingrowth were evaluated by undecalcified histological examination under light and fluorescent microscope as well as environmental scanning electron microscope (ESEM). The results show: osteoblasts are very active with fast proliferation and no adverse tissue reaction occurs for the porous NiTi alloy after 15 weeks implantation; porous NiTi alloy has better osteoconductivity and osteointegration than the bulk one; the osteoblasts can ingrow the pores of porous NiTi implant, and direct bone-implant interface can be observed by fluorescent light microscope.     

Cavitation erosion resistance of heat-treated NiTi

NiTi (Ti–50.8 at.% Ni) specimens were solution-treated at 1000 °C, followed by aging between 200 and 600 °C. The cavitation erosion resistance of all the aged specimens in deionized water was improved relative to the solution-treated specimen, with a maximum increase of about 8.7 times, which was achieved by aging at 500 °C. The results also indicate that both austenite and martensite contribute to the high cavitation erosion resistance of NiTi. It is also shown that a simple macro-indentation test employing a Rockwell indenter may be used for preliminary screening of heat-treated NiTi with respect to cavitation erosion resistance.     

Nickel-titanium shape memory alloys made by selective laser melting:a review on process optimisation

Selective laser melting (SLM) is a mainstream powder-bed fusion additive manufacturing (AM) process that creates a three-dimensional (3D) object using a high power laser to fuse fine particles of various metallic powders such as copper, tool steel, cobalt chrome, titanium, tungsten, aluminium and stainless steel. Over the past decade, SLM has received significant attention due to its capability in producing dense parts with superior mechanical properties. As a premier shape memory alloy, the nickel-titanium (NiTi) shape memory alloy is attractive for a variety of biomedical applications due to its superior mechanical properties, superelasticity, corrosion resistance and biocompatibility. This paper presents a comprehensive review of the recent progress in NiTi alloys produced by the SLM process, with a particular focus on the relationship between processing parameters, resultant microstructures and properties. Current research gaps, challenges and suggestions for future research are also addressed.The full text can be downloaded at https://link.springer.com/article/10.1007/s40436-021-00376-9     

Improvement in corrosion resistance of NiTi by anodization in acetic acid

NiTi was galvanostatically anodized in acetic acid to increase the oxide film thickness for improving corrosion resistance. The galvanostatic anodization behavior of NiTi in acetic acid differed markedly from that of Ti. In particular, the anode potential reached was much lower for NiTi, and thus only thin oxide films could be obtained. With a suitable choice of anodizing conditions, the anodic oxide film formed had a thickness of 20–25 nm, as determined by profilometric measurement. Atomic force microscopy (AFM) revealed that the surface roughness was increased after anodization. Analysis by X-ray photoelectron spectroscopy (XPS) showed a low Ni/Ti ratio of 0.04 at the anodic oxide surface versus a value of 0.30 for bare NiTi. Electrochemical impedance measurements of the anodized sample in Hanks' solution at 37 °C recorded a 9-fold increase in polarization resistance, and cyclic polarization tests also recorded a matching reduction in the passive current density. These observations indicate that anodization of NiTi can serve as a simple low-temperature method to enhance the corrosion resistance of NiTi when used as an implant material.     

Photoemission measurements of temperature in pulsed laser heating of various materials

The possibilities for the photoemission method of measuring the temperature of various materials heated by millisecond laser pulses have been investigated. The temperature of graphite, tungsten, tantalum, silicon plates, and silicon dioxide films was determined experimentally with a time resolution of 1 μs within the range 1200–2900 K.     

Mechanical properties of additive laser-welded NiTi alloy

Laser welding would be a suitable joining technique for NiTi shape memory alloy if the mechanical properties of laser weld were improved. With this purpose, effects of additive on mechanical properties of laser-welded NiTi alloy have been experimentally studied. Welding specimens used in this study were 2 mm thick hot-rolled plates with a chemical composition of Ni50.9Ti49.1. (Ni50.9Ti49.1)-Ce2 (at.%) alloy foil or Ni47Ti44Nb9 plate was used as filler metal to add Ce or Nb element into NiTi laser weld metal. Both tensile strength and the toughness of additive-welding specimens were improved significantly compared with non-additive-welding specimen. The mechanical property improvement was attributed to the fine solidification NiTi grains and good grain-linking in weld center. The microstructure control mechanisms of these two additive welds were discussed.     

The biocompatibility and mechanical properties of cylindrical NiTi thin films produced by magnetron sputtering

Superelastic nickel titanium shape memory alloys (NiTi–SMA) are of biomedical interest due to the large obtainable strains and the constant stress level. Production of NiTi–SMA thin films by magnetron sputtering was developed recently. NiTi sputtered tubes have a high potential for application as vascular implants, e.g. stents. Magnetron sputtering, three dimensional lithography and wet etching were used in order to produce Ti and NiTi stent devices (thickness: 5–15 μm; diameter: 1–5 mm). For tensile tests, specimens were prepared in radial and axial directions in order to compare the mechanical properties of the film in both directions. The specimens – produced for cell culture experiments – were incubated with human mesenchymal stem cells (hMSC) for 7 days. Cell viability was analyzed via fluorescence microscopy after live/dead staining of the cells. Cytokine release from cells was quantified via ELISA. Cylindrical NiTi films showed a strain up to 6%. Tensile parameters were identical for both directions. Best material properties were obtained for deposition and patterning in the amorphous state followed by an ex-situ crystallization using rapid thermal annealing in a high vacuum chamber. First biological tests of the Ti and NiTi–SMA samples showed promising results regarding viability and cytokine release of hMSC.     

Mechanical properties of single crystal tungsten microwhiskers characterized by nanoindentation

The nanoindentation results in this work showed that the one-dimensional single crystal tungsten microwhiskers fabricated by vapor deposition possess unique yielding behavior. The average hardness of the microwhiskers measured on their (1 1 1) surfaces was 8.44 GPa, significantly higher than that of the bulk tungsten ranged from 3.4 to 5.8 GPa. The hardness increase was attributed to the lacking of dislocation avalanche in the 1D single crystal tungsten that was often observed in the nanoindentation of the bulk tungsten. However, the values of elastic modulus of the microwhiskers measured on the (1 1 1) surfaces were considerably scattered, whose average value is much lower than the reported value of 410 GPa for the bulk tungsten.     

Combustion synthesis/quasi-isostatic pressing of TiC–NiTi cermets: processing and mechanical response

TiC–NiTi composites were produced by a technique combining self-propagating high-temperature synthesis (SHS) of elemental powders of Ni, Ti, and C with densification by quasi-isostatic pressing (QIP). In order to create a one-step synthesis/densification process, the Ti + Ni + C reactant material was surrounded in a bed of graphite and alumina particulate before initiation of the combustion reaction. The sample was ignited within the particulate and subjected to a uniaxial load immediately after passage of the combustion wave. The constitutive response, composition and resulting structures of the composites with varying volume fractions of NiTi are characterized. Powder mixtures prepared anticipating the formation of stoichiometric TiC result in the formation of composites with a eutectic matrix of Ni₃Ti and NiTi. This titanium impoverishment of the matrix is consistent with the formation of nonstoichiometric TiC _x during the combustion reaction. The Ni₃Ti phase can be suppressed by anticipating the formation of TiC_0.7 and adjusting the chemical content of the reactant mixture to include additional titanium. These cermets combine the high hardness of the ceramic phase with the possible shape memory and superelastic effects of NiTi.     

Changes in mechanical properties of dental alloys induced by saliva and oral probiotic supplements

The effects of the saliva and oral probiotic supplements on roughness, friction and microhardness of the stainless steel (SS) and nickel-titanium (NiTi) alloys used in dentistry was studied. The specimens of stainless steel, uncoated, rhodium-coated and nitrided NiTi were exposed to artificial saliva with pH 4.8 and artificial saliva with addition of probiotic supplements containing bacteria Lactobacillus reuteri Prodentis through 28 days. First 5 days specimens were subjected to thermocycling to simulate intraoral conditions, 2500 cycles from 5 °C to 50 °C and the following days to the temperature of the 37±2 °C. Analyses demonstrated that oral probiotic supplements do not influence microhardness, roughness or friction of stainless steel above the influence of saliva. Probiotics increase roughness in NiTi, but without significant influence on friction, while microhardness in NiTi is not influenced. Surface nitriding reduces the influence of probiotics on roughness while rhodium coating increases it.     

Deposition of TiN coatings on shape memory NiTi alloy by plasma immersion ion implantation and deposition

An investigation has been carried out to study the effect of pulse negative bias voltage on the morphology, microstructure, mechanical, adhesive and tribological properties of TiN coatings deposited on NiTi substrate by plasma immersion ion implantation and deposition. The surface morphologies were relatively smooth and uniform with lower root mean square values for the samples deposited at 15 kV and 20 kV negative bias voltages. X-ray diffraction results demonstrated that the pulse negative bias voltage can significantly change the microstructure of TiN coatings. The intensity of TiN(220) peak increased with the increase of negative bias voltage in the range of 5-20 kV. When the negative bias voltage increased to 30 kV, the preferred orientation was TiN(200). Nanoindentation test indicates that hardness and elastic modulus increased with the increase of the negative bias voltage (5 kV, 15 kV and 20 kV), and then dropped sharply at 30 kV. The adhesion between the TiN and NiTi alloy and tribological properties of TiN coated NiTi alloy depend strongly on the bias voltage parameter; the sample deposited at 20 kV possesses good adhesion strength and excellent tribological property.     

Porous NiTi by creep expansion of argon-filled pores

NiTi powders are densified in the presence of argon gas, whose initial pressure is varied between 1 and 33 atm, to create NiTi billets containing isolated Ar-filled pores. Upon vacuum annealing, the pressurized pores expand by creep of the surrounding NiTi matrix at rates which are in agreement with a simple analytical model up to 16% porosity. Beyond this porosity, foaming becomes very slow, as pores connect with each other and with the specimen surface where the gas escapes. This is due to failure of previous NiTi powder boundaries weakened by oxides insoluble in NiTi; this mechanism does not occur in Ti foams which dissolve their oxides at high temperature, allowing higher levels of pore expansion and foam porosity. NiTi with 10–16% porosity exhibits Young's moduli of 48–57 GPa, and may be useful for high-strength, low stiffness biomedical implants with superelastic or shape-memory properties.