NiTi Shape Memory Research Articles

Ambient-Temperature Indentation Creep of Shape Memory NiTi Alloys: Additively Manufactured versus Cast

In this study, depth-sensing indentation creep response of cast and additively manufactured (laser powder bed fusion) NiTi alloys in heat-treated conditions have been investigated at ambient temperature. Indentation creep tests were evaluated with the help of a dual-stage approach comprising a loading segment with a subsequent constant load-holding stage and an unloading phase afterward. The investigation was carried out at a maximum load of 50 mN along with a holding time of 600 s. Different creep parameters comprising indentation creep displacement, creep strain rate, creep stress exponent as well as the indentation size effect have been analyzed quantitatively for the employed materials. In addition, microstructural analysis has been performed to ascertain the processing–microstructure–creep property correlations. A substantial indentation size effect was seen for both cast and printed NiTi samples in heat-treated conditions. According to the creep stress exponent measurements, the dominant mechanism of rate-dependent plastic deformation for all NiTi samples at ambient temperature is attributed to the dislocation movement (i.e., glide/climb). The outcome of this investigation will act as a framework to understand the underlying mechanisms of ambient-temperature indentation creep of the cast and printed NiTi alloy in conjunction with heat-treated conditions.

Microstructure and Properties of Shape Memory NiTi Alloy

Many years ago, the shape memory alloys properties of Nickel Titanium (NiTi) were first discovered in the early 1960s, as shape memory alloys had many applied applications, in which oxidization problems were not of concern for the most part. However, over the past decades, NiTi alloys have been increasingly considered in external and internal biomedical devices, for example cardiac stent wires, orthodontics, vascular and bone fractures, fixing plates and screws, self-expanding urinary tracts.. The aim of the research is to study the effect of the elements mobidium and zirconium on the shape memory alloys. When adding small amounts of zirconium leads to a smoothing of the granular size, when adding Mobidium improves the hardening process. The alloy was prepared from primary powders of nickel and titanium, using metallic powder technology, under pressure of 800 MPa. Then the sintering method was carried out in a 4-10 tor vacuum at 950 ° C. The results upon XRD analysis revealed that NiTi were completely changed into NiTi (both cubic and mono phases) and Ni3Ti phase. The samples are ground in a dry atmosphere and the samples are polished after sintering. Optical microscopy, x-ray diffraction techniques. hardness test was conducted using Vickers hardness machine. It was observed that as the Zr content is increased, the hardness values, in VPN, increased. For instance, equi at 30% of Ni Ti had a hardness of 127.05 which increased significantly as Zr content was increased to 20 at %. This is mainly observed due to precipitation hardening which occurs due to the presence of multiple phase in alloy D. Increase in hardness also suggests that the workability. It was observed that as the Mo content is increased, the hardness values, in VPN, increased. For instance, Ni Ti had a hardness of 127.05 which increased was increased to 40at%. This is mainly observed due to precipitation hardening which occurs due to the presence of multiple phase in alloy G. Due to molybdenum, Optical microscopy reveals surface characteristics such as open pores and grain borders, as well as the distinction between the phases NiTi and Ni3Ti.

Adiabatic kinetics of phase transformation in shape memory TiNi alloy subjected to shock loading

ABSTRACT In this paper, we study the adiabatic kinetics of martensitic phase transition induced by shock loading. Our main goals are to reveal the effect of thermal–mechanical coupling on phase interface propagation by demonstrating the wave profiles to the impact loading. First, a kinetics and constitutive model for shape memory TiNi alloy with thermo-mechanical coupling effect was preliminarily established based on the laws of thermodynamics. Then the numerical results show that phase transformation shock waves during shock loading and unloading would be aroused due to the nonlinear hardening since adiabatic temperature rise. The shock front and the temperature interface were coupled to each other. Moreover, the adiabatic temperature rise across the transformation shock front effectively reduces the driving force of phase transition and increases the propagation velocity of phase transition wave. Finally, the theoretical results are compared with the experimental results. The comparison results reflect the intrinsic thermo-mechanical coupling characteristic of shape memory TiNi alloy with strong nonlinear constitutive behaviour and kinetics, which will be helpful for temperature controlling for TiNi alloy material and structures subjected to blast or impact loading.

Investigation of In-Vitro Properties of NiTi Alloy After Micro Arc Oxidation

Malzeme biliminin diğer alanlarda olduğu gibi tıp bilimine entegre olmasıyla biyomalzemelerin de çeşitliliği, gelişimi ve kullanım alanları artmaktadır. Bu çeşitlilik içerisinde yer bulan biyomalzemelerden biride NiTi alaşımlarıdır. Bu çalışma; şekil hafızalı NiTi alaşımının yüzey ve mekanik özelliklerinin iyileştirilmesi amacı ile Mikro Ark Oksidasyon (MAO) yöntemi kullanılarak TiO2 kaplamasının büyütülmesi üzerine odaklanmıştır. Büyütülen kaplamaların yapısal özellikleri SEM ve XRD cihazları kullanılarak analiz edilmiştir. Biyoaktivite özellikleri yapay vücut sıvısı (SBF) içerisinde bekletme testi analizi ile tespit edilmiştir. SBF içerisinde bekletme testleri sonrasında kaplanmış ve kaplanmamış şekil hafızalı NiTi altlıkların, bulundukları ortama Ni+2 salınımı kontrolü ise ICP-MS cihazı ile tespit edilmiştir. Büyütülen oksit tabakasıyla NiTi alaşımlarının kullanımını sınırlayan özellikleri önemli derece giderildiği gözlenmiştir. MAO yöntemiyle TiO2 kaplanmış NiTi altlıkların biyoaktif özelliklerinin arttığı ve 72. saatin sonunda hücre canlılığında NiTi altlığına kıyasla istatistik olarak herhangi bir sitotoksisiteye sahip olmadığı tespit edilmiştir.

Architected material analogs for shape memory alloys

Architected material analogs for shape memory alloys

Using the finite element method and dimension analysis in modelling laser shock processing of titanium alloys with shape memory

The technology of laser shock processing of titanium alloys with the shape memory effect is simulated using the finite element method and dimensional analysis. In total, 16 independent dimensionless parameters characterizing the mechanical behavior of shape-memory NiTi alloys are analyzed. The dependences of the influence of the dimensionless duration of the laser pulse and the dimensionless peak pressure in the shock wave on the dimensionless depth of the plastic zone are obtained.

Innovative Bioactive Ag-SiO2/TiO2 Coating on a NiTi-Shape Memory Alloy: Structure and Mechanism of Its Formation.

In recent years, more and more emphasis has been placed on the development and functionalization of metallic substrates for medical applications to improve their properties and increase their applicability. Today, there are many different types of approaches and materials that are used for this purpose. Our idea was based on a combination of a chemically synthesized Ag-SiO2 nanocomposite and the electrophoretic deposition approach on a NiTi shape memory substrate. As a result, silver-silica coating was developed on a previously passivated alloy, which was then subjected to sintering at 700 °C for 2 h. The micrometer-sized coat-forming material was composed of large agglomerates consisting of silica and a thin film of submicron- and nano- spherical-shaped particles built of silver, carbon, and oxygen. Structurally, the coatings consisted of a combination of nanometer-sized silver-carbonate that was embedded in thin amorphous silica and siloxy network. The temperature impact had forced morphological and structural changes such as the consolidation of the coat-forming material, and the partial coalescence of the silver and silica particles. As a result, a new continuous complex ceramic coating was formed and was analyzed in more detail using the XPS, XRD, and Raman methods. According to the structural and chemical analyses, the deposited Ag-SiO2 nanocomposite material’s reorganization was due to its reaction with a passivated TiO2 layer, which formed an atypical glass-like composite that consisted of SiO2-TiO2 with silver particles that stabilized the network. Finally, the functionalization of the NiTi surface did not block the shape memory effect.

Embedded NiTi Wires for Improved Dynamic Thermomechanical Performance of Silicone Elastomers.

The extraordinary properties of shape memory NiTi alloy are combined with the inherent viscoelastic behavior of a silicon elastomer. NiTi wires are incorporated in a silicon elastomer matrix. Benefits include features as electrical/thermal conductivity, reinforcement along with enhanced damping performance and flexibility. To gain more insight of this composite, a comprehensive dynamic thermomechanical analysis is performed and the temperature- as well as frequency-dependent storage modulus and the mechanical loss factor are obtained. The analyses are realized for the composite and single components. Moreover, the models to express the examined properties and their temperature along with the frequency dependencies are also presented.

Dynamic thermo-mechanical behaviors of SME TiNi alloys subjected to shock loading

Dynamic thermo-mechanical coupling behaviors of shape memory TiNi alloy in the strain rate ranging from 300 s−1 to 2000 s−1 are investigated by the split Hopkinson pressure bar (SHPB) device with an infrared (IR) detection system. In stress–strain space, dynamic response shows a strong strain hardening property, however, in stress–temperature space, transformation path is particularly sensitive to the strain rate. The corresponding temperature evolution measured synchronously shows that local temperature increased associated with the forward phase transition, and it would keep the loading maximum value unchanged or decreased for unloading, depending on the strain rate. Besides, local temperature evolution was consistent with the transformation stress and its values at different points are the same. Temperature evolution and transformation deformation mechanism is then analyzed by a simple one-dimensional model. The results show that latent heat and dissipated energy are responsible for temperature variation. Furthermore, the temperature evolution with strain rate reveals that with the increase of strain rate, the phase transformation deformation mechanism undergoes a transformation from phase transition fronts propagation during lower strain rates to combination of local nucleation and front propagation during middle strain rate and to uniform nucleations during higher strain rates. The results are helpful for a passive shape memory alloy (SMA) micro-valve design.

Influence of electric current on the thermo-mechanical static and fatigue properties of shape memory NiTi wires

NiTi shape memory wires are fostered to be utilized as actuators due to their strain recovery capability involved with phase transformation and the high mechanical properties. Since large strains are involved with the phase transformation, fatigue is a crucial issue that needs to be carefully investigated. Fatigue of superelastic alloys is usually analyzed through S-N curves obtained by imposing alternated stresses. Differently, actuators based on shape memory alloys are usually subjected to a bias load and thermal cycles drive the actuation. This work presents an experimental characterization of the thermo-mechanical static and fatigue properties of shape memory NiTi wires intended for actuation purposes. A series of original results are presented. The wires’ stroke capability is found to depend on the heat rate and temperature associated with a varying magnitude of the current utilized to perform the thermal cycle exploiting the Joule’s effect. Alongside a reduction of the actuation time associated with increasing current, results show that the stroke increases with the current, and with it the alternated strain associated with a single thermal cycle. The ultimate stress under quasi-static loads is also found to be influenced by the current, as it linearly diminishes with it, hence with the operating temperature. Above all, the most interesting results relate to fatigue, as it is found that the influence of the applied current on thermo-mechanical fatigue is even larger than the effect of the applied bias load. Further, functional fatigue in terms of reduction of the recoverable strain upon thermal actuation has not been found for any loading condition.

Shape-memory NiTi alloy rebars in flexural-controlled large-scale reinforced concrete walls: Experimental investigation on self-centring and damage limitation

Economic damage and repair costs due to earthquakes are largely connected with permanent residual structural displacements. This study develops and tests the use of shape-memory Niti alloy rebars as a replacement to steel rebars in reinforced concrete walls. Large-scale cyclic quasi-static experimental tests on two units, predominantly behaving in flexure, show a clear reduction of residual displacements and damage. Additionally, the superelastic flag-shaped hysteretic response of the smart alloy in the wall boundary elements guarantees a sizeable energy dissipation, opening a promising avenue both for new construction and repair works. The experimental data is openly available (DOI: 10.14428/DVN/2QBQVJ).

Resin Injection Process in the Manufacture of a Polymer Composite Reinforced with NiTi Ribbons: A CFD Analysis

This work aims to numerically simulating the resin injection manufacturing process of a polymer composite,reinforced with ribbons of NiTishape memory alloy, using the software Ansys CFX®. The multiphase flow mathematical modeling was used to describe the transient and isothermal resin-air flow during the process. Results of the pressure fields, velocity andvolume fractionsof the involved phases are presented. The fluid flow inside the mold was compared withthe flow between parallel flat plates and showed to be consistent. Process parameters, such as resin volumetric flow rate, resin inlet and air outlet positions have a large influence in the mold filling time, volume and position of voids fractions inside de mold and final product quality.

Mathematical Modelling of Material Removal Rate During Electric Discharge Machining Using Buckingham’s Pi-Theorem

In the present work an attempt has been made to model material removal rate (MRR) by using Buckingham’s Pi-theorem during electric discharge machining (EDM) of shape memory NiTi alloy. Machining of Ni-Ti alloyis mostly attempted by non- conventional methods of machining, and, especially by EDM, as this advance material imposes machinability issues when cut by conventional processes. However, EDM process has low material removal rate.In present investigation EDM of NiTi alloy is carried out using electrolytic copper toolon die sink type of electric discharge machine. The experimental-based mathematical model to predict MRR is developed varying process parameters such as gap current, pulse on time, pulse off time, voltage, workpiece electrical conductivity and tool electrode electrical conductivity during the experiments. The model developed using dimensional analysis and Buckingham pie theorempredicts the value of MRR close to the experimental observations with the accuracy of 91 % shows that the developed model could be used to predict MRR during EDM of NiTi alloy within the domain of the parameters selected in the present study.

RETRACTED: Microstructural variation at interface during Fiber laser joining of NiTi/Ti6Al4V and effect of mechanical strength

Abstract Welding of nitinol (NiTi) shape memory alloy (SMA) and titanium-based alloy (Ti6Al4V) finds potential application in aerospace and medical industries. However, joining of these materials poses challenging issues because material properties of each material interacts to give rise to hybrid system performance. In the present study, copper was used as an intermediate layer to restrict the formation of brittle intermetallics during welding of NiTi with Ti6Al4V using laser welding. The formation of Cu4Ti3 and CuNi2Ti complex phases attempts to minimize the formation of brittle phases like Ti2Ni. It was observed that the maximum welding strength of 353 MPa could be achieved at 800 W laser power and 17 mm/s scan speed. Physical analysis of weldments indicates the presence of pores at nano-levels (

Functional Characteristics and Phase Transformations in Bar Semiproducts from Shape Memory NiTi Alloys

Functional characteristics of bar semiproducts from shape memory NiTi alloys obtained by equal channel angular pressing, worm forging, and their combination are determined. The functional characteristics are measured and the phase transitions are studied under the conditions of three-point bending and thermocycling at a constant load. It is shown that the bar specimens subjected to warm forging at 350°C and to a combination of equal channel angular pressing at 450°C and warm forging possess the best level of functional properties.

A Fuzzy Logic Control System for a Robotic Hand Driven by Shape Memory Alloy Wires

This paper presents a robotic hand using wires with shape memory (NiTi) as non-conventional actuators. The mechanical structure of the robot hand was first designed by means of a CAD computer program and afterwards 3D printed using ABS polymer. The robotic hand was designed according to the physiological characteristics of the human hand, with particular attention to the angles formed by the phalanges of the fingers. A mechanical system accommodates the thin NiTi wires compactly, thus forming an artificial muscle. A fuzzy logic based control system allows an accurate positioning of each phalanx. The contribution of the present work to science lies in the practical implementation of known techniques and materials.

A Fuzzy Logic Control System for a Robotic Hand Driven by Shape Memory Alloy Wires

This paper presents a robotic hand using wires with shape memory (NiTi) as non-conventional actuators. The mechanical structure of the robot hand was first designed by means of a CAD computer program and afterwards 3D printed using ABS polymer. The robotic hand was designed according to the physiological characteristics of the human hand, with particular attention to the angles formed by the phalanges of the fingers. A mechanical system accommodates the thin NiTi wires compactly, thus forming an artificial muscle. A fuzzy logic based control system allows an accurate positioning of each phalanx. The contribution of the present work to science lies in the practical implementation of known techniques and materials.

DSC and microstructure analysis of high temperature Ni-Ti-Hf, low hysteresis Ni-Ti-Cu and conventional super-elastic and shape memory Ni-Ti alloy ingots and wires

The progression of martensitic transformations in calorimetric experiments on NiTi-based shape memory alloys (SMA) depends significantly on the material composition, processing history and the thermo-mechanical treatment which is performed to prepare the material for use in application. The phase transformations in Ni-Ti alloys are first order transitions, i.e. the microstructure symmetry changes occur spontaneously and superheating and supercooling effects emerge. Thus, the thermal scanning rate in differential scanning calorimetry (DSC) affects the observation of important material parameters for the characterization of shape memory alloys like onset temperatures, hysteresis width and change in enthalpy of phase transformations. This work depicts how the transformation characteristics as observed by DSC measurements vary significantly by altering the alloy composition and processing conditions of the material. An in-depth calorimetric study on a wide range of NiTi-based alloys which were prepared as heat treated ingot and processed wire was performed. In order to evaluate also the rate effect on the transformation of distinct alloys, DSC rates from 1 to 30 °C/min were tested, highlighting the interplay between thermal scanning velocity and material parameters. Manifold results are presented on how chemistry, microstructure, DSC sample geometry and processing stresses in the materials contribute to award the alloys diverging sensitivity to the thermal scanning rate. Deviations in binary Ni-Ti composition alter the sensitivity to heating and cooling rate, e.g. martensitic transformation in annealed super-elastic (SE) wire is stronger affected by the rate than shape memory wire due to differences in the precipitation behavior. Ni-Ti-Hf high temperature shape memory alloys show a rather small enlargement of hysteresis, but a huge shift in transformation temperatures, while Cu addition to Ni-Ti causes very sensitive hysteresis changes up on modifying thermal rates which is assigned to B19 orthorhombic transformation. Transformation parameters of wires are less affected by DSC rates than ingots due to their high surface to volume ratio, while sample weight and surface finish is observed to have a minor impact. Small grains and residual strains owing to processing history of drawn wire tend to stabilize the transformation. In materials which exhibit R-phase transformation, the low heat capacity of this phase could be responsible for its minor sensitivity to variations in DSC rate.

Temperature evolution associated with dynamic phase transformation in shape-memory TiNi alloys

ABSTRACTIn this paper, temperature evolution in shape-memory TiNi alloys was in situ measured in the process of phase transition under shock loading tests. It was shown that the temperature increased during loading, while unloading, it would keep the maximum value constant or decreased, depending on the strain rate. The corresponding dynamic response indicated that the hardening property is related to phase transition strain and the transformation path has remarkable strain-rate effect. The reason for this phenomenon is essentially due to the temperature dependence of the phase transformation stress. Moreover, the evolution of temperature was consistent with the change of stress, suggesting that this dynamic phase transition deformation is uniform. The theoretical calculation shows that the mechanism of temperature evolution results from the release of latent heat and the hysteresis dissipated energy. The results of this investigation provide insight into intriguing strain-rate-dependent phenomena intrinsic of TiNi alloys.

Processing of nanotubes on NiTi-shape memory alloys and their modification with photografted anti-adhesive polymer brushes. Towards smart implant surfaces

Implants based on NiTi shape memory alloys may be overgrown with cells which is undesirable in traumatology and stenting. The processing of a self-organized nanotube (NT) layer and its functionalization with a polymer that resists protein and cell adhesion can help solving this issue. The necessary mechanical strength of the layer should arise from its 3D structure and inorganic-organic nanocomposite character. The self-organized NT layer is obtained via potentiostatic anodization of NiTi alloy sheets. Differential scanning calorimetry measurements show that the anodization process barely affects the transformation behavior of NiTi. Structural characterization reveals that the NT layer mainly consists of TiO2 after heat treatment at temperatures ≥450 °C. The functionalization of the NT layer is conducted via the photografting of a biocompatible, anti-adhesive zwitter-ionic polymer using soft UV light. The polymer layer is shown to cover the surface and the nanotube walls. Further, the high adhesion and mechanical strength of the functionalized anodic films together with their in vitro biocompatibility are demonstrated. Samples structured in functionalized and non-functionalized stripes show that the functionalized stripes are endowed with a high resistance to protein and bacteria adhesion. The results show that with the proposed surface design the aims set are attained.