Influence of Structural Porosity and Martensite Evolution on Mechanical Characteristics of Nitinol via In-Silico Finite Element Approach

Nitinol (NiTi) alloys are gaining extensive attention due to their excellent mechanical, superelasticity, and biocompatibility properties. It is difficult to model the complex mechanical behavior of NiTi alloys due to the solid-state diffusionless phase transformations, and the differing elasticity and plasticity presenting from these two phases. In this work, an Auricchio finite element (FE) model was used to model the mechanical behavior of superelastic NiTi and was validated with experimental data from literature. A Representative Volume Element (RVE) was used to simulate the NiTi microstructure, and a microscale study was performed to understand how the evolution of martensite phase from austenite affects the response of the material upon loading. Laser Powder Bed Fusion (L-PBF) is an effective way to build complex NiTi components. Porosity being one of the major defects in Laser Powder Bed Fusion (L-PBF) processes, the model was used to correlate the macroscale effect of porosity (1.4–83.4%) with structural stiffness, dissipated energy during phase transformations, and damping properties. The results collectively summarize the effectiveness of the Auricchio model and show that this model can aid engineers to plan NiTi processing and operational parameters, for example for heat pump, medical implant, actuator, and shock absorption applications.     

High maneuverability guidewire with functionally graded properties using new superelastic alloys

Nitinol shape memory alloys (SMAs) are attracting considerable attention as core materials for medical guidewires because of their excellent flexibility and shape retention. However, since Nitinol guidewires possess low rigidity, the pushability and torquability of the guidewires are insufficient. On the other hand, although guidewires made of stainless steel have high pushability, plastic deformation occurs easily. We have developed a new class of superelastic guidewires with functionally graded properties from the tip to the end by using new SMA core materials such as Cu‐Al‐Mn‐based or Ni‐free Ti‐Mo‐Sn SMAs. The tip portion of the guidewire shows excellent superelasticity (SE), while the body portion possesses high rigidity. These functionally graded characteristics can be realized by microstructural control. These guidewires with functionally graded properties show excellent pushability and torquability and are considerably easier to handle than conventional guidewires with Nitinol or stainless steel cores. Moreover, a metallic catheter using a Ni‐free Ti‐based SMA with high biocompatibility is introduced.     

正畸弓丝在模拟牙矫治实验中的力学性能测定

本文通过模拟上颌前牙矫治实验，测定了目前国内常用几种弓丝的弹性性能。结果显示不锈钢类弓丝刚度大，其中澳丝刚度最大。镍钛丝和带垂直曲的弓丝刚度小，能提供较持续的矫治力。0．014中国镍钛丝具有超弹性性能，而0．016中国镍钛丝在0～250g范围内未显示出超弹性。     

Two-Way Bending Properties of Shape Memory Composite with SMA and SMP

A shape memory composite (SMC) was fabricated with a shape memory alloy (SMA) and a shape memory polymer (SMP), and its two-way bending deformation and recovery force were investigated. The results obtained can be summarized as follows: (1) two kinds of SMA tapes which show the shape memory effect (SME) and superelasticity (SE) were heat-treated to memorize the round shape. The shape-memorized round SMA tapes were arranged facing in the opposite directions and were sandwiched between the SMP sheets. The SMC belt can be fabricated by using the appropriate factors: the number of SMP sheets, the pressing force, the heating temperature and the hold time. (2) The two-way bending deformation with an angle of 56 degrees in the fabricated SMC belt is observed based on the SME and SE of the SMA tapes during heating and cooling. (3) If the SMC belt is heated and cooled by keeping the bent form, the recovery force increases during heating and degreases during cooling based on the two-way properties of the SMC. (4) The development and application of high-functional SMCs are expected by the combination of the SMA and the SMP with various kinds of phase transformation temperatures, volume fractions, configurations and heating-cooling rates.     

An overview of superelastic stent design

Summary

The purpose of this paper is to contrast the performance of self-expanding and balloon-expandable stents. While both approaches to stenting have proven to be successful in treating a wide range of vascular disease, there are significant differences in the philosophy behind and properties of the two types of stents. Many of these differences, such as strength, stiffness (or compliance), recoil, dynamic scaffolding, vessel conformity and fatigue resistance will be highlighted by studying the mechanics of the stent alone, and then of a stent within a vessel. These differences can be summarised by observing that self-expanding stents provide more anatomically-correct scaffolding, while balloon-expandable stents provide rigid and uncompromising reinforcement. Other differences, such as corrosion resistance, placement accuracy and visibility, will also be briefly summarised.     

Optimisation of processing and properties of medical grade Nitinol wire

The purpose of this paper is to review the current processing and resultant properties of standard Nitinol wire for guide-wire applications. Optimised Ti-50.8at%Ni wire was manufactured according to industry standards by precise control of the composition, cold work and continuous strain-age annealing. Mechanical properties of this wire are reported from -100°C to 200°C to demonstrate the effects of test temperature. Within the ‘superelastic window’ the plateau stresses are linearly related to test temperature. Additional ageing treatments can be used as a tool to fine-tune transformation temperatures and mechanical properties. A review of the fatigue properties of thermomechanically-treated Nitinol wire shows that they are affected by test temperature, stress and strain.     

Non‐medical applications of NiTinol

Shape memory alloys based on the intermetallic system Nickel‐Titanium (NiTi or NiTinol) are widely used in various kinds of industries. At present, the most popular market is medical industry, in which NiTinol finds a perfect environment with a narrow temperature specification ranging from room to body temperature. In this area we find a multitude of different applications, products and important patents. The awareness of engineers in R&D departments of the existence of NiTinol and the knowledge about the material properties has remarkably increased over the years.

However, the penetration of other markets in non‐medical industries with shape memory alloys, such as automotive industry, did not show a similar development. For many of those applications the use of NiTinol suffers from the fact that the relevant material properties are strongly related to the environmental temperature while some temperature specifications in such markets are remarkably wide. Thus, the number of commercially exploited applications is smaller than in the medical industry with its tightly defined requirements for the working temperature range. The purpose of this paper is to increase the awareness of R&D engineers for use of NiTinol by explaining the strengths and weaknesses of NiTinol, especially by describing and promoting the less well‐known properties and effects of the material. These effects do not depend so much on the environmental temperature. In particular, the deformability of the martensitic phase and the linear superelastic effect will be subject of this paper together with some recent applications in non‐medical areas.     

Softening Effects in Biological Tissues and NiTi Knitwear during Cyclic Loading

Samples of skin, tendons, muscles, and knitwear composed of NiTi wire are studied by uniaxial cyclic tension and stretching to rupture. The metal knitted mesh behaves similar to a superelastic material when stretched, similar to soft biological tissues. The superelasticity effect was found in NiTi wire, but not in the mesh composed of it. A softening effect similar to biological tissues is observed during the cyclic stretching of the mesh. The mechanical behavior of the NiTi mesh is similar to the biomechanical behavior of biological tissues. The discovered superelastic effects allow developing criteria for the selection and evaluation of mesh materials composed of titanium nickelide for soft tissue reconstructive surgery.     

The Effects of Temperature and Time of Heat Treatment on Thermo-Mechanical Properties of Custom-Made NiTi Orthodontic Closed Coil Springs

Nickel-Titanium (NiTi) springs have been increasingly used in orthodontics; however, no optimum condition of heat treatment has been reported. Therefore, this research was conducted to determine the optimum heat-treatment temperature and duration for the fabrication of NiTi-closed coil springs by investigating their effects on thermo-mechanical properties. As-drawn straight NiTi wires of 0.2 mm diameter were used to fabricate closed coil springs of 0.9 mm lumen diameter. The springs were heat-treated at three different temperatures (400, 450, and 500 °C) with three different durations (20, 40, and 60 min). Electron Probe Micro-Analysis (EPMA) and Differential Scanning Calorimetry (DSC) were used to investigate element composition and thermo-mechanical properties, respectively. Custom-made NiTi closed coil springs composed of 49.41%-Ti and 50.57%-Ni by atomic weight, where their DSC curves of 500 °C presented the obvious endothermic and exothermic peaks, and the austenite finish temperature (A_f) were approximately 25 °C. With increasing temperature, deactivation curves presented decreased plateau slopes generating higher superelastic ratios (SE ratios). At 500 °C, closed coil springs showed superelastic tendency with lower stress hysteresis. The thermo-mechanical properties were significantly influenced by heat-treatment temperature rather than duration. The optimum parameter appeared to be 500 °C for 40 min to produce appropriate force delivery levels, relatively low plateau slope, and lower hysteresis for orthodontic use.