Pseudoelastic Alloy Research Articles

Corrosion behavior of two Cu‐based shape memory alloys in NaCl solution: An electrochemical study

AbstractThe corrosion behavior of two different Cu–Al–Mn–Ni alloys, pseudoelastic and pseudoplastic, was studied in a 0.6 M sodium chloride aqueous solution by monitoring the open circuit potential for 100 h and characterizing the resulting corrosion products. Electrochemical impedance spectroscopy analysis detected three processes related to the electrochemical double layer, a passive film and a diffusive contribution associated with the dissolution/precipitation of corrosion products. Potentiodynamic scans revealed a cathodically controlled corrosion mechanism and the presence of active–passive behavior at anodic potentials for both alloys studied. Polarization of the samples at selected potentials in the anodic branch allowed the investigation of the reactions involved, highlighting an improved corrosion resistance of the pseudoelastic alloy. The corrosion rates of the pseudoelastic and pseudoplastic alloys, after 100 h of immersion, were determined to be 0.007 and 0.011 mmpy, respectively. The post‐experiment characterization was carried out by means of scanning electron microscopy, micro‐Raman spectroscopy and X‐ray diffraction, supporting the electrochemical results.

Micromechanism of Propagation of Fatigue Cracks in Pseudoelastic NiTi Shape-Memory Alloy

We analyze the kinetic features and micromechanism of propagation of fatigue cracks in pseudoelastic nitinol alloy with shape-memory effect under the conditions of constant-amplitude cyclic loading with stress ratios R = 0.2 and 0.5. It is established that, independently of the parameter R, fatigue cracks grow by the quasicleavage mechanism, which is explained by the austenite-martensite deformation transformation at the crack tip. This also explains the fact that the maximal value of the stress intensity factor in a loading cycle (but not its range) is responsible for the kinetics of fracture observed as the stress ratio R changes.

Assessment of small-scale deformation characteristics and stress-strain behavior of NiTi based shape memory alloy using nanoindentation

The study is focused to understand the micro-mechanisms that govern the deformation behavior of a Ni-rich NiTi alloy at the sub-micron scale. This alloy undergoes reversible stress induced martensitic transformation leading to pseudoelasticity. Correspondingly, significant strain recoverability is noted. In this investigation, nanoindentation is utilized as the primary experimental tool. Various parameters including the indenter tip configuration, size and applied load levels are varied systematically. It is observed that compared to the most commonly used sharp Berkovich indenter tip, blunt spherical tip imposes reduced amount of strain and strain-gradient within the indentation volume. This opens up the window for systematically varying the sequential deformation modes in NiTi. Further in-depth analysis revealed that optimum combination of indenter tip configuration, size and applied load level is prerequisite to appreciate the pseudoelastic mechanism in NiTi. Most importantly, a tailored and simplified protocol is formulated for converting nanoindentation load-displacement response to corresponding indentation-stress-strain behavior for pseudoelastic alloy. These indentation-stress-strain curves featuring the signature deformation characteristics of NiTi alloy are realized without utilizing any specialized experimental modes. In a nutshell, this study highlights the importance of choosing adequate nanoindentation parameters for assessing the functional characteristics of pseudoelastic NiTi system at sub-micron scale. Accordingly, overall localized deformation behavior of this unique alloy is investigated.

Assessment of Small-Scale Deformation Characteristics and Stress-Strain Behavior of NiTi Based Shape Memory Alloy Using Nanoindentation

The study is focused on appreciating the micro-mechanisms that govern the deformation behavior of a Ni-rich NiTi alloy at the sub-micron scale. This alloy undergoes reversible stress-induced-martensitic-transformation leading to pseudoelasticity. Correspondingly, significant strain recoverability is noted. In this investigation, nanoindentation is utilized as the primary experimental tool. Various parameters including the indenter tip configuration, size and applied load levels are varied systematically. It is observed that compared to the most commonly used sharp Berkovich indenter tip, blunt spherical tip imposes reduced amount of strain and strain-gradient within the indentation volume. This opens up the window for systematically varying the sequential deformation modes in NiTi. Further in-depth analysis revealed that optimum combination of indenter tip configuration, size and applied load level is prerequisite to appreciate the pseudoelastic mechanism in NiTi. Most importantly, a tailored and simplified protocol is formulated for converting nanoindentation load-displacement response to corresponding indentation-stress-strain behavior for pseudoelastic alloy. These indentation-stress-strain curves featuring the signature deformation characteristics of NiTi alloy are realized without utilizing any specialized experimental modes. In a nutshell, this study highlights the importance of choosing adequate nanoindentation parameters for assessing the functional characteristics of pseudoelastic NiTi system at sub-micron scale. Accordingly, overall localized deformation behavior of this unique alloy is appreciated.

Growing Fe-Mn-Al-Ni single crystals by combining directional annealing and thermal cycling

In pseudoelastic alloys, a fine-grained microstructure implies restrictions on the propagation and shrinkage of the martensitic phase. Thus, the pseudoelastic effect is adversely affected. For this reason, thermomechanical methods that accelerate grain growth are sought in these alloys. In this paper, we present a novel and extremely fast method to obtain single crystals of centimeter-order lengths. The process consists of combining two methods of abnormal crystal growth: directional annealing; and thermal cycling. This technique is applicable to pseudoelastic alloys that have a parent single-phase high-temperature region and a two-phase, low-temperature region. The crystallographic orientations measured in single crystals generated by this process are also shown. A full-strain recovery pseudoelastic behavior was observed in single crystalline specimens tested under compression.

Single segment actuator using shape memory alloy for three-directional bending endoscopic tip

Shape Memory Alloy (SMA) is a specially fabricated alloy which is capable of altering its shape based on external stimuli. With this mechanism, SMA can be used as actuator with highly controlled maneuverability and flexibility. This study was aimed to design, fabricate, test and analyze an SMA actuator with three-directional bending capabilities. The design consisted of a set of SMA actuators with a pseudo-elastic alloy bias wire. A silicone rubber sheathe was fabricated to act as a heat buffer. The actuator was activated with a DC current between 0-900mA at 70% PWM duty cycle. Bending and heat tests resulted in positive bending of about 30° consistently while the instrument without the silicone rubber sheathe on the instrument. Temperature readings on the thermal camera are 40°C. With the silicone rubber sheathe, the bending angle is halved over the entire instrument, but with a smaller decrease in recorded temperature of the main bodies. This SMA-based actuator is prospectively used to improve the maneuverability and flexibility of an endoscopic tip.

Structural transitions in a NiTi alloy: a multistage loading-unload cycle

NiTi shape memory alloys (SMAs) are increasingly used in many engineering and medical applications, because they combine special functional properties, such as shape memory effect and pseudoelasticity, with good mechanical strength and biocompatibility. However, the microstructural changes associated with these functional properties are not yet completely known. In this work a NiTi pseudo-elastic alloy was investigated by means of X-ray diffraction in order to assess micro-structural transformations under mechanical uniaxial deformation. The structure after complete shape recovery have been compared with initial state.

Crack-tip thermal and mechanical hysteresis in Shape Memory Alloys under fatigue loading

Crack tip stress-induced phase transformation mechanisms in nickel–titanium alloys (NiTi), subjected to fatigue mechanical loads, have been analyzed by full field measurement techniques. In particular, Infrared thermography (IR) and Digital Image Correlation (DIC), have been applied to analyze the cyclic temperature and displacement evolutions in the crack tip region of a commercial pseudoelastic alloy, together with the associated thermal and mechanical hysteresis, by using Single Edge Crack (SEC) specimens. IR investigations revealed a global temperature variation of the specimen due to crack formation and propagation mechanisms, which is similar to common engineering metals, i.e. surface temperature rises quickly in an initial phase, then it reaches an almost constant value, and finally it increases rapidly as a consequence of the fatigue crack growth. In addition, cyclic thermal variation in the crack tip region and related hysteresis (temperature vs load) has been measured, which can been directly related to the thermal effects of the reversible stress-induced phase transformations. Furthermore, a proper experimental setup has been made, based on a reflection microscope, for direct measurements of the crack tip displacement field by the DIC technique. Furthermore, a fitting procedure has been developed to calculate the mode I Stress Intensity Factor (SIF), starting from the displacement field, and the related mechanical hysteresis (SIF vs load).

Fracture mechanics of pseudoelastic NiTi alloys: review of the research activities carried out at University of Calabria

This paper reports a brief review of the research activities on fracture mechanics of nickel-titanium based shape memory alloys carried out at University of Calabria. In fact, this class of metallic alloys show an unusual fracture response due to the reversible stress-induced and thermally phase transition mechanisms occurring in the crack tip region as a consequence of the highly localized stresses. The paper illustrates the main results concerning numerical, analytical and experimental research activities carried out by using commercial NiTi based pseudoelastic alloys. Furthermore, the effect of several thermo-mechanical loading conditions on the fracture properties of NiTi alloys are illustrated.

AFM-FIB-TEM Combo Analysis of Surface Relief Caused by Various Types of deformation bands in the Ti-Mo-Based Pseudoelastic Alloys

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Fracture behavior of the compact tension specimens in NiTi shape memory alloys

In the present work, the fracture behavior of a NiTi pseudoelastic alloy has been investigated. The fracture toughness is obtained by experiments on compact tension (CT) specimens, which are pre-cracked at a low fatigue load. The fracture toughness of the studied material is K IC = 39.4 MPa m 1/2. The fracture surface of selected specimen is studied by scanning electron microscope (SEM). It is found that the fracture mechanism is dominated by cleavage fracture. With the increasing crack velocity, the fracture surface at the fast fracture regime exhibits a more brittle response. The finite element simulation on both specimens with and without pre-crack is carried out to better understand the fracture mechanisms of the NiTi alloys. It is shown that a pre-crack results in phase transformation at the crack tip under very low load. However, due to the low fatigue load and short pre-crack length, the material is always partly phase transformed, even at the crack tip, during the fatigue pre-crack process. There is always a small area around the crack tip fully transformed during the fast fracture process.

Martensite Transformations and Fatigue Behavior of Nitinol

Abstract In addition to excellent biocompatibility and corrosion resistance, the unique flow behavior of NiTi pseudo-elastic alloys renders them ideally suited for several medical device applications utilizing minimally invasive technologies. For example, self-expanding stents are indicated chronic therapies for treatment of a variety of vascular diseases. Long-term device integrity is thus critical with the FDA recommending a minimum 10-year life. Historically, the majority of published fatigue data on NiTi alloys have been generated under strain-controlled conditions using wire or single-diamond cell samples designed to replicate the behavior of the repetitive unit within a stent. Resulting data are then presented on modified Goodman or Soderberg diagrams in order to define regions of acceptable device life. Consistent with their unusual monotonic flow characteristics these alloys are found to exhibit unique cyclic behavior in fatigue: published data clearly demonstrate that increasing mean strain often increases high-cycle fatigue life. This anomalous behavior is generally attributed to any of a number of possible microstructural or mechanical effects associated with the reversible stress-assisted austenite-to-martensite phase transformation. Although the majority of published work has focused on these macroscopic properties in order to characterize and adequately define device life, various efforts are currently underway to better understand the more fundamental metallurgical and mechanical aspects controlling fatigue life in this alloy. Such studies include crack initiation and propagation rates, determining the roles of absolute and relative inherent flow behavior of the transforming structures, mechanical instabilities resulting from the heterogeneous nature of the transformation, latent heat effects during cyclic transformation, as well as the effects of testing frequency, crystallographic texture, grain-shape anisotropy, and stress-state on fatigue life. A review of existing literature delineating these behaviors and current efforts is presented in light of known beneficial effects of transformation plasticity and toughening in enhancing fatigue and fracture properties of a variety of metallic and nonmetallic materials.

Experimental and FEM Analysis of the Fracture Behavior in NiTi Shape Memory Alloys

In the present work, the fracture toughness of a NiTi pseudoelastic alloy has been obtained by experiments on CT specimens, which is KIC =39.38MPa·m1/2. Then the stress induced phase transformation behavior in front of the crack tip of the CT specimen is simulated by a micromechanical model considering the different elastic properties between martensite and austenite. The results show that the pre-crack promotes phase transformation at the crack tip. And the phase transformation is localised near the crack tip. It is also shown that phase transformation reduces the Mises stress around the crack tip.

Pseudoelasticity at elevated temperatures in [0 0 1] oriented Co 49Ni 21Ga 30 single crystals under compression

Pseudoelasticity as a function of temperature of as-grown Co49Ni21Ga30 [0 0 1] oriented single crystals was studied under compression. These alloys demonstrate a large pseudoelastic temperature range, greater than 385 °C, with a maximum recoverable pseudoelastic strain of about 4%. We provide an insight into the evolution of stress–strain behavior and corresponding stress–hysteresis with test temperatures, and the ramifications on the potential use of CoNiGa alloys as high temperature pseudoelastic alloys are presented.

放電・通電焼結によって作製したＴｉＢ２分散ＴｉＮｉ超弾性合金の機械的性質と耐摩耗性

Mixtures of Ti, Ni and TiB2, TiC or SiC powders were spark- and resistance-sintered to obtain TiB2, TiC or SiC dispersed TiNi pseudoelastic alloy. The alloys were annealed, solution treated and aged after the sintering. Nano-indentation tests, bending tests, and abrasion wear tests were carried out on these alloys. Nano-indentation tests showed that the TiNi alloy and ceramics-dispersed TiNi alloys exhibited larger displacement recovery than the reference materials such as steels and titanium alloy at the same indentation displacement. The solution treated and aged specimens exhibited larger displacement recovery than the annealed ones. Vickers hardness of the annealed alloys increased with increasing ceramics content, while relative density and bending strength decreased. TiB2 dispersed TiNi alloy has higher hardness than TiC dispersed TiNi alloy and larger bending strength than SiC added alloy. Abrasive wear resistance of TiB2 dispersed TiNi alloy was much larger than that of the steels and titanium alloys, which can be explained by the stress relaxation of pseudoelastic matrix surrounding ceramics particles. In addition, the TiNi alloys annealed after sintering exhibited better wear resistance than the solution treated and aged ones. The wear resistance of TiB2 dispersed TiNi alloy was larger than that of TiC dispersed TiNi alloy perhaps because of larger interfacial strength between TiB2 particles and TiNi matrix than that of TiC particles and TiNi matrix.

Design and Experimental Verification of Pseudoelastic-Based Constant-Force Springs

Constant-force springs were designed with Ni-Ti pseudoelastic alloy. Through local concentration of flexural deformations, the design produces “pseodoelastic hinges” under relatively small deformations. Constant moments at these “pseudoelastic hinges” produce a relatively constant level of spring force under increasing deformations. Local concentration of flexural deformations are achieved in this design through bracing the spring element outside the locations where deformations concentrate. Experimental data verified the desirable constant-force behavior of the springs and their desirable fatigue life, relaxation attributes, and corrosion resistance. The pseudoelastic spring design presented here is subject of approved and pending U.S. patent applications.

A new type of wear-resistant material: pseudo-elastic TiNi alloy

A number of investigations have demonstrated that TiNi alloy exhibits high resistance to wear. As a new type of wear-resistant material, TiNi alloy has attracted increasing interest. Different from conventional wear materials, the high wear resistance of TiNi alloy is mainly attributable to its unique mechanical behaviour, the so called pseudo-elasticity, resulting from a reversible martensitic transformation. In this article, the wear behaviour and relevant properties of TiNi alloy are briefly introduced, including wear resistance, reversible martensitic transformation, pseudo-elasticity, and microstructural effects. Future exploration of the pseudo-elastic alloy as a wear-resistant material is discussed.

Materialtechnische Besonderheiten orthodontischer Nickel-Titan-Drähte

Since their introduction in 1971 nickel-titanium wires have been widely used in orthodontics. Today, there is a multitude of new NiTi-alloys, whose properties are described. Beside the memory effect, these alloys have particular elastic properties, which can be characterized by a low modulus of elasticity, excellent springback, and pseudoelasticity (superelasticity). These properties are a consequence of the fact that depending on temperature and mechanical stress NiTi-alloys have two crystalline structures: martensite and austenite. The transition between these two phases, called martensitic transformation, is responsible for the memory effect, where a one way and a two way effect can be distinguished. For orthodontic applications pseudoelasticity is regarded as a highly favourable property. Pseudoelastic behavior is caused by stress induced martensite. Analysing the elastic properties of the available wires two categories can be distinguished: "work hardened martensite" and "pseudoelastic alloy". The biocompatibility of NiTi is sufficient, it can be used as an implant material.

Potentiel pseudoelastique et plasticite de transformation martensitique dans les monoet polycristaux metalliques

The subject of this study is the behaviour of single and polycrystals undergoing a thermoelastic martensitic transformation which leads to creation of different variants of martensite. The overall behaviour is the superposition of contributions from the phase transformation and from other mechanisms of deformation—for example plasticity. Two classes of behaviour can then be distinguished: pure transformation plasticity exclusively due to phase transformation (the martensitic transformation); and coupled transformation plasticity in which plastic flow couples with the intrinsic (martensitic) contribution. For the first, a thermomechanical analysis based on the Gibbs-free energy gives the transformation criterion and the associated flow rule. The analysis, for the case of a single crystal, introduces the interaction matrix between the variants of the created martensite. This matrix determines the different classes of interaction between the variants. A phenomenological constitutive relation (transformation threshold for the rule of strain hardening) has been proposed for a polycrystal. It is similar to that proposed by Drücker and Prager. The results of theoretical analyses have been compared with the experimental data, for both single and polycrystals, obtained for pseudoelastic alloys of the Cu-Zn-Al type.

Nonpalpable breast lesion localization using a curved-end retractable wire.

A new needle-wire system has been developed for localization of nonpalpable breast lesions. The curved-end wire, which is made of a tough pseudoelastic alloy, eliminates many of the pitfalls of this procedure for both the radiologist and the surgeon. Advantages of this new system include the ability to anchor the needle during filming, the ability to easily reposition the needle if necessary, the option of injecting dye, the inability to transect the wire during surgery, and the option of leaving both the needle and wire in place in the subject to allow easier dissection during biopsy.