Titanium Shape Memory Alloy Research Articles

Laser welding of NiTi wires

The special properties of nickel–titanium shape memory alloys are currently used in micro-engineering and medical technology. In order to integrate NiTi components into existing parts and modules, they often need to be joined to other materials. For this reason, the present contribution deals with the laser welding of thin pseudoelastic NiTi wires (100 μm) with an Nd:YAG laser. Based on extensive parameter studies, faultless joints were produced. This study deals with the structural changes occurring in the fusion and heat-affected zones, the performance of the joints in static tensile tests and their functional fatigue. It can be shown that NiTi/NiTi joints reach about 75% of the ultimate tensile strength of pure NiTi wires. For welding NiTi to steel, no interlayer was used. The dissimilar NiTi/steel joints provide a bonding strength in the fusion and heat-affected zones higher than the plateau stress level. NiTi/steel joints of thin wires, as a new aspect, enable the possibility to benefit from the pseudoelastic properties of the NiTi component.

Nanofretting behaviors of NiTi shape memory alloy

Nanofretting refers to cyclic movements of contact interfaces with the relative displacement amplitude at the nanometer scale, where the contact area and normal load are usually much smaller than those in fretting. Nanofretting widely exists in microelctromechanical systems (MEMS) and may become a key tribological concern besides microwear and adhesion. With a triboindenter, the nanofretting behaviors of a nickel titanium (NiTi) shape memory alloy are studied under various normal loads (1–10 mN) and tangential displacement amplitudes (2–500 nm) by using a spherical diamond tip. Similar to fretting, the nanofretting of NiTi/diamond pair can also be divided into different regimes upon various shapes of tangential force–displacement curves. The dependence of nanofretting regime on the normal load and the displacement amplitude can be summarized in a running condition nanofretting map. However, due to the surface and size effects, nanofretting operates at some different conditions, such as improved mechanical properties of materials at the nanometer scale, small apparent contact area and single-asperity contact behavior. Consequently, different from fretting, nanofretting was found to exhibit several unique behaviors: (i) the maximum tangential force in one cycle is almost unchanged during a nanofretting test, which is different from a fretting test where the maximum tangential force increases rapidly in the first dozens of cycles; (ii) the tangential stiffness in nanofretting is three orders magnitude smaller than that in fretting; (iii) the friction coefficient in nanofretting is much lower than that in fretting in slip regime; (iv) no obvious damage was observed after 50 cycles of nanofretting under a normal load of 10 mN.

Effects of pulsing frequency on shape recovery and investigation of nickel out-diffusion after mechanical bending of nitrogen plasma implanted NiTi shape memory alloys

Nickel–titanium shape memory alloy bars were treated by nitrogen plasma immersion ion implantation at 50 and 100 Hz to investigate the effects of pulsing frequencies on the shape recovery ability. The shape recovery of the treated and untreated control samples was assessed using bending tests. It was found that the 50 Hz sample retained the same shape recovery ability similar to the untreated control sample but the 100 Hz sample could not return to the original shape. Immersion tests in simulated body fluids on the bent-and-recovered bars show that the amount of Ni leached from the treated NiTi bars was 18.2% of that of the untreated control samples. The results show that the nitrogen plasma treatment is effective in impeding the out-diffusion of nickel even after extensive shape deformation.

Auricle Reconstruction With a Nickel–Titanium Shape Memory Alloy as the Framework

The objective of this study is to explore the biocompatibility and implantability of a nickel-titanium (NiTi) alloy in auricle reconstruction. Twelve New Zealand rabbits underwent subcutaneous implantation with a NiTi alloy framework shaped like the human auricle under general anesthesia. The implant was inserted after skin expansion. Implant vascularization was evaluated at months 1, 3, 6, 9, and 12 after implantation by histologic analysis. Immunohistochemical methods were used to examine expression of vascular endothelial growth factor in tissue around the implant. The fibrovascular ingrowth rate of implants was determined by bone scanning using (99m)Tc-PYP. The surface of the NiTi alloy implant was examined microscopically with scanning electron microscopy. The implant harvested showed only partial vascularization at 1 month and completely vascularized at 3 months. The amount of vascular endothelial growth factor-positive cells was markedly increased at 6 months and reached the highest number at 3 months. The fibrovascular ingrowth rate of implant was assessed by (99m)Tc-PYP bone scan using ratios of (99m)Tc-PYP activity in placement regions versus the contralateral normal region. One rabbit had exposure of the NiTi alloy framework as a result of overlying skin flap necrosis. It was rescued with animal skin without the complete removal of the framework. All the other rabbits tolerated the implant well, and there were no complications. The NiTi alloy implant represents an alternative implant for auricular reconstruction.

Oxygen and sodium plasma-implanted nickel–titanium shape memory alloy: A novel method to promote hydroxyapatite formation and suppress nickel leaching

This study aims at modifying the surface bioactivity of NiTi by sodium and oxygen plasma immersion ion implantation (PIII). Sodium ions were implanted into oxygen plasma-implanted NiTi and untreated NiTi. X-ray photoelectron spectroscopy (XPS) revealed that more sodium was implanted into the oxygen pre-implanted sample in comparison with the untreated surface. Scanning electron microscopy (SEM) coupled with energy dispersive X-ray analysis (EDX) detected calcium and phosphorus rich deposits on both samples after immersion in simulated body fluids for 7 and 21 days. Inductively-coupled plasma mass spectrometry (ICPMS) conducted on the deposits dissolved in diluted hydrochloric acid showed more calcium on the oxygen PIII samples. The improved corrosion resistance of the oxygen PIII NiTi was retained after sodium PIII as evaluated by potentiodynamic polarization tests. Better spreading and proliferation of osteoblasts were also observed on the treated samples.

Measurement and modelling of the nanoindentation response of shape memory alloys

A nickel–titanium shape memory alloy was subjected to nanoindentation over a range of temperature (up to 200 °C), such that the starting material was either predominantly martensitic or largely composed of the parent phase. The load–displacement data were interpreted to give information about whether the imposed strain was being at least partly accommodated by the martensitic phase transformation, i.e. whether superelastic deformation was taking place. This interpretation was assisted by finite element simulation of the evolving strain field under an indenter, with or without the superelastic deformation mechanism being operative. It is concluded that the nanoindentation response can be used to determine whether the material is capable of exhibiting superelastic deformation, provided appropriate procedures are employed. Spherical indenters are more suitable than sharp tips. A relatively low value for the remnant indent depth ratio (depth after unloading/depth at peak load) is indicative that superelasticity is occurring. The procedure was found to be viable with a small radius (10 μm) spherical indenter, so it can be employed to explore local variations in superelastic response.

A new concept of a linear smart actuator

This paper discusses the utilization of deflected flexible beams to amplify the displacement of a nickel titanium (NiTi) shape memory alloy (SMA) actuator and to provide linear motion. The actuator is composed of a SMA wire fixed eccentrically along a flexible beam dividing it into equally spaced segments. It is designed mainly to overcome the problem of limited displacement upon using SMA wires as actuators and also due to the high force provided by the shape memory alloy wire upon heating. A preliminary testing for the actuator was performed which proves the benefit of using beam actuators to obtain longer amplified linear displacements. System identification techniques were used to model the SMA actuator upon heating and cooling. An ON/OFF control was applied to the actuator using the identification models, which showed closeness between simulation and experimental results.

Plasma surface treatment of artificial orthopedic and cardiovascular biomaterials

Plasma surface modification has become a popular method to modify the surface structure and biological properties of biomaterials. By modifying selective surface mechanical and biological properties, conventional materials can be redesigned with their favorable bulk attributes retained. Plasma surface modification can enhance the multi-functionality, mechanical properties, as well as biocompatibility of artificial biomaterials and medical implants. Here, our recent research work on plasma modification of orthopedic materials including titanium and nickel–titanium shape memory alloys as well as diamond-like carbon as cardiovascular materials is described. NiTi alloys that possess shape memory and super-elastic properties are of interest in spinal deformity correction. Shape recovery inside the human body allows for less traumatic gradual correction while obviating the needs for multiple surgeries. However, leaching of harmful nickel ions from the materials causes health hazards and plasma implantation is an excellent means to create a graded barrier layer to impede Ni out-diffusion and improve the corrosion resistance. Our latest results demonstrate that the shape recovery property is not compromised with the plasma treatment. Also described are our results on titanium implanted with calcium and sodium for enhancement of the surface biological properties. With regard to cardiovascular materials, the two main requirements are good surface mechanical properties and blood compatibility. Diamond-like carbon (DLC) is a potential material in artificial heart valve and our recent studies suggest that doping DLC with biological friendly elements such as nitrogen and phosphorus can improve the blood compatibility of the materials.

Nitrogen plasma-implanted nickel titanium alloys for orthopedic use

Nickel–titanium shape memory alloys (NiTi) have attracted much attention as orthopedic materials due to their shape memory effect and super-elasticity. However, this alloy consists of equal amounts of nickel and titanium and Ni is well known to cause allergy or other deleterious effects in living tissues. To improve the surface corrosion resistance and mitigate Ni leaching, we have modified the surface chemistry of this alloy with the aid of nitrogen plasma immersion ion implantation (PIII). The implanted surfaces were characterized by X-ray photoelectron spectroscopy (XPS). Electrochemical corrosion and nano-indentation tests were conducted to assess the corrosion resistance and surface hardness. Immersion tests were carried to investigate the extent of Ni leaching under simulated human body conditions and cell cultures employing enhanced green fluorescent protein mice osteoblasts were used to evaluate the cyto-compatibility of the materials. The XPS results reveal that a thin layer of TiN with higher hardness is formed on the surface after nitrogen-PIII. The corrosion resistance of the implanted sample is also superior to that of the untreated NiTi and SS. The release of Ni ions is significantly reduced compared to the untreated NiTi and both the treated and untreated NiTi alloys favor osteoblast attachment and proliferation. The sample with surface titanium nitride exhibits the largest degree of cell proliferation whereas stainless steel fares the worst.

Oxygen plasma treatment to restrain nickel out-diffusion from porous nickel titanium orthopedic materials

Porous nickel–titanium (NiTi) shape memory alloy is a prospective orthopedic biomaterial due to its porous structure that allows bone tissue and blood vessel ingrowth during fixation of implants. Unfortunately, the higher probability of Ni release from the increased surface area of a porous surface compared to conventional dense NiTi shape memory alloys causes more serious health concerns. In order to mitigate leaching of harmful Ni ions, we create a firm barrier layer on the surface by conducting oxygen plasma immersion ion implantation (PIII) into the porous structure. The non-line-of-sight capability of PIII allows more uniform treatment of all exposed areas compared to beam-line ion implantation. Our simulated body fluid immersion test indicates that Ni leaching is significantly reduced after oxygen PIII. X-ray photoelectron spectroscopy profiles illustrate that the thickness of the barrier layer is almost unchanged after immersion in SBF for 4 weeks, demonstrating the excellent durability of the layer in a biological medium.

Surface characteristics, mechanical properties, and cytocompatibility of oxygen plasma‐implanted porous nickel titanium shape memory alloy

Good surface properties and biocompatibility are crucial to porous NiTi shape memory alloys (SMA) used in medical implants, as possible nickel release from porous NiTi may cause deleterious effects in the human body. In this work, oxygen plasma immersion ion implantation (O-PIII) was used to reduce the amount of nickel leached from porous NiTi alloys with a porosity of 42% prepared by capsule-free hot isostatic pressing. The mechanical properties, surface properties, and biocompatibility were studied by compression tests, X-ray photoelectron spectroscopy (XPS), and cell culturing. The O-PIII porous NiTi SMAs have good mechanical properties and excellent superelasticity, and the amount of nickel leached from the O-PIII porous NiTi is much less than that from the untreated samples. XPS results indicate that a nickel-depleted surface layer predominantly composed of TiO(2) is produced by O-PIII and acts as a barrier against out-diffusion of nickel. The cell culturing tests reveal that both the O-PIII and untreated porous NiTi alloys have good biocompatibility.

Constitutive model of shape memory alloys: Theoretical formulation and experimental validation

The technological application of nickel–titanium shape memory alloys (SMA) requires a constitutive model that can be easily implemented into numerical methods. It should also be possible for the parameters of this model to be obtained experimentally. For these reasons, macroscopic constitutive models have gained ground in SMA designs. To develop a new macroscopic model that encompasses all the characteristics of these materials over the whole range of transformation temperatures, several macromechanical models have been analysed, evaluated and experimentally validated. From the discrepancies observed in these experimental validations, a model is proposed based on a specific critical stress–temperature diagram. In this model, various laws of evolution are formulated for the martensite fraction. Expressions for the elastic modulus and constitutive transformation tensor are also proposed. Experimental validation showed that this model predicts the behaviour of the material better than the previous models.

Stress-induced martensitic transformations and shape memory at nanometer scales

Nickel–titanium (NiTi) shape memory alloys undergo relatively large recoverable inelastic deformations via a stress-induced martensitic phase transformation. Although stress-induced phase transformations in shape memory alloys are well characterized and utilized at micrometer to meter length scales, significant opportunity exists to understand and exploit martensitic transformations at nanometer scales. Displacive stress-induced martensitic phase transformations may constitute an ideal nanometer-scale actuator, as evident in certain biological systems, such as the T4 bacteriophage. The present work uses nanoindentation to study the fundamentals of stress-induced martensitic phase transformations in NiTi shape memory alloys. The experimental results presented are the first to show evidence of discrete forward and reverse stress-induced thermoelastic martensitic transformations in nanometer-scaled volumes of material. Shape recovery due to indentation, followed by subsequent heating, is demonstrated for indent depths in the sub-10 nm range. The indentation results reveal that stress-induced martensitic phase transformations nucleate at relatively low stresses at nanometer scales, suggesting a fundamental departure from traditional size scale effects observed in metals deforming by dislocation plasticity. It is also shown that the local material structure can be utilized to modify transformation behavior at nanometer scales, yielding an insight into the nature of stress-induced martensitic phase transformations at small scales and providing an opportunity for the design of nanometer-sized NiTi actuators.

Atomic Dynamics and Energetics of Martensitic Transformation in Nickel–Titanium Shape Memory Alloy

Microscopic mechanism of martensitic transformation in nickel (Ni)–titanium (Ti) alloy is investigated by molecular dynamics (MD) simulation using embedded atom method (EAM) potentials. The computational parallelepiped specimen with nano-size dimension is surrounded by Ti-terminated free surfaces and constrained regions for loading. The detection method of martensite phase is newly exploited. The crystalographic B19 0 monoclinic crystal form can be identified as martensite phase by checking up atomic lengths and angles of neighborhood and by comparing them with possible values of lattice parameters already proposed. In tensile loading, the specimen shows a kind of stressinduced transformation from parent phase (B2 structure) to martensite phase (B19 0 structure). Outbreak of martensitic transformation occurs immediately after stress reaches maximum value. In outbreak of martensite, distortion of unit structure is observed as an actual change in atomic coordinates. It is found that there are two major transformation paths both resulting in martensite structure, each of which has contrary sequence of changes in atomic length or angle. There is also the other route of atomic movement for completing martensitic transformation with relatively long-range atomic migration. The EAM potential used in the present study is discussed as to crystalline energies of periodic B2 or B19 0 unit structures. Dynamic energies in transformation are also obtained from MD results and they show that there are energy barriers in martensitic transforming. Static evaluation of energy, on the assumption of uniform transformation, is carried out and is compared with energy change obtained by MD method.

Thin film NiTi microthermostat array

An autonomous flow restricting microthermostat array was successfully designed and fabricated using thin film nickel–titanium (NiTi) shape memory alloy (SMA). The device relies on changes in the environmental fluid temperature for closing without external electric sources. The array of 88 microthermostats was tested at pressures ranging from 6.9 to 34.5 kPa and water temperatures of room temperature and 80 °C. At 34.5 kPa and room temperature, the flow rate was 722.8 ml/min. As the temperature of the water increased to 80 °C, the flow rate was reduced by 90% depending on the pressure. Finite element predictions are in relatively good agreement with the measured flow rates.

Improvement on corrosion resistance of NiTi orthopedic materials by carbon plasma immersion ion implantation

Nickel–titanium shape memory alloys (NiTi) have potential applications as orthopedic implants because of their unique super-elastic properties and shape memory effects. However, the problem of out-diffusion of harmful Ni ions from the alloys during prolonged use inside a human body must be overcome before they can be widely used in orthopedic implants. In this work, we enhance the corrosion resistance of NiTi using carbon plasma immersion ion implantation and deposition (PIII&D). Our corrosion and simulated body fluid tests indicate that either an ion-mixed amorphous carbon coating fabricated by PIII&D or direct carbon PIII can drastically improve the corrosion resistance and block the out-diffusion of Ni from the materials. Results of atomic force microscopy (AFM) indicate that both C2H2-PIII&D and C2H2-PIII do not roughen the original flat surface to an extent that can lead to degradation in corrosion resistance.

Improvements of anti-corrosion and mechanical properties of NiTi orthopedic materials by acetylene, nitrogen and oxygen plasma immersion ion implantation

Nickel–titanium shape memory alloys (NiTi) are useful materials in orthopedics and orthodontics due to their unique super-elasticity and shape memory effects. However, the problem associated with the release of harmful Ni ions to human tissues and fluids has been raising safety concern. Hence, it is necessary to produce a surface barrier to impede the out-diffusion of Ni ions from the materials. We have conducted acetylene, nitrogen and oxygen plasma immersion ion implantation (PIII) into NiTi alloys in an attempt to improve the surface properties. All the implanted and annealed samples surfaces exhibit outstanding corrosion and Ni out-diffusion resistance. Besides, the implanted layers are mechanically stronger than the substrate underneath. XPS analyses disclose that the layer formed by C2H2 PIII is composed of mainly TiCx with increasing Ti to C concentration ratios towards the bulk. The nitrogen PIII layer is observed to be TiN, whereas the oxygen PIII layer is composed of oxides of Ti4+, Ti3+ and Ti2+.

Formation of titanium nitride barrier layer in nickel–titanium shape memory alloys by nitrogen plasma immersion ion implantation for better corrosion resistance

Nickel–titanium shape memory alloys (NiTi) are potentially useful in orthopedic implants due to their super-elasticity and shape memory properties. However, the materials are vulnerable to surface corrosion and the most serious issue is out-diffusion of toxic Ni ions from the substrate into body tissues and fluids. In this paper, we describe our fabrication of TiN barrier layers in NiTi by nitrogen plasma immersion ion implantation followed with vacuum annealing at 450 °C or 600 °C. Our results show that the barrier layer is not only mechanically stronger than the NiTi substrate, but also is effective in impeding the out-diffusion of Ni from the substrate. Among the samples, the 450 °C-annealed TiN barrier layer possesses the highest mechanical strength and best Ni out-diffusion impeding ability. The enhancement can be attributed to the consolidation of the Ti–N layer resulting from optimal diffusion at 450 °C.

Fretting wear behavior of superelastic nickel titanium shape memory alloy

The fretting behavior of superelastic nickel titanium (NiTi) shape memory alloy was studied at various displacement amplitudes on a serve-hydraulic dynamic test machine. The results showed that the superelastic properties of the material played a key role in the observed excellent fretting behavior of NiTi alloy. Due to the low phase transition stress (only 1/4 the value of its plastic yield stress) and the large recoverable phase transition strain (5%) of NiTi, the friction force of NiTi/GCr15 stainless steel pair is smaller than the value of GCr15/GCr15 pair and at the same time the Rabinowicz wear coefficient of NiTi plate is about 1/9 the value of GCr15 plate under the same fretting conditions. For NiTi/GCr15 pair, even NiTi has a much lower hardness than GCr15, the superelastic NiTi alloy exhibits superior fretting wear property than GCr15 steel. It was found that the weak ploughing was the main wear mechanism of NiTi alloy in the partial slip regime. While in the mixed regime and gross slip regime, the wear of NiTi was mainly caused by the abrasive wear of the GCr15 debris in the three-body wear mode.

Development of hydraulic linear actuator using thin film SMA

In this paper, thin film (10 μm) nickel–titanium (NiTi) shape memory alloy (SMA) was used to develop a compact actuator pump generating high output force at large velocities. The velocity was dependent on actuation frequency of the thin film NiTi membrane operated at 10, 50, and 100 Hz. The liquid flow also provided forced convection cooling of the NiTi membrane. The inlet and outlet pressure were always at equilibrium prior to actuation and the inlet pressure biased the NiTi membranes. Experimental results show that the optimum operating output load was 98 N, and the blocking force was 198 N. The peak velocity of 5.85 mm/s (2.96 cc/s) was obtained while operating the membrane at 100 Hz. Analytical predictions provide general trends observed experimentally. Actual measurements were smaller than predictions (i.e. peak velocity of 13.9 mm/s which corresponds to 7.03 cc/s) due to assumed constant pressure in the model.