Applications of Nickel-Titanium Alloy

Additive manufacturing of NiTi shape memory alloy and its industrial applications

NiTi shape memory alloys are the prime choice for many engineering and biomedical applications due to their unique response to environmental/external stimuli. The capability of laser powder bed fusion (LPBF) to additively manufacture high-quality density NiTi alloy with intricate geomaterial configuration, good surface quality, and chemical homogeneity makes the LPBF process the preferred choice among other additive manufacturing (AM) methods for manufacturing the NiTi alloy. The AM process parameters have a decisive effect on the functional and mechanical properties of NiTi alloy. There is a need to understand the resultant effect of the interrelationship between the process parameters on the final NiTi additive manufactured structures. The inherent high rate of melting and cooling of the LPBF process resulting in high internal stress could cause adverse effects such as cracks and Ni-loss which are detrimental to the phase transformation temperature of the NiTi alloy. Despite the current challenges, the literature reveals that LPBF NiTi components demonstrated functional and mechanical properties according to the ASTM standard and have been used widely for biomedical applications due to its stress-strain hysteresis, which is similar to bone tissues. The alloy is also used extensively for high-value engineering applications such as the automotive and aerospace industries due to its actuation properties.

Shape Memory Alloys in Textile Platform: Smart Textile-Composite Actuator and Its Application to Soft Grippers

In recent years, many researchers have aimed to construct robotic soft grippers that can handle fragile or unusually shaped objects without causing damage. This study proposes a smart textile-composite actuator and its application to a soft robotic gripper. An active fiber and an inactive fiber are combined together using knitting techniques to manufacture a textile actuator. The active fiber is a shape memory alloy (SMA) that is wire-wrapped with conventional fibers, and the inactive fiber is a knitting yarn. A knitted textile structure is flexible, with an excellent structure retention ability and high compliance, which is suitable for developing soft grippers. A driving source of the actuator is the SMA wire, which deforms under heating due to the shape memory effect. Through experiments, the course-to-wale ratio, the number of bundling SMA wires, and the driving current value needed to achieve the maximum deformation of the actuator were investigated. Three actuators were stitched together to make up each finger of the gripper, and layer placement research was completed to find the fingers' suitable bending angle for object grasping. Finally, the gripping performance was evaluated through a test of grasping various object shapes, which demonstrated that the gripper could successfully lift flat/spherical/uniquely shaped objects.

Molten Salt Synthesis of Intermetallic Compound TiNi Nanopowder Passivated by TiOx Shell Prepared from NiTiO3 for Catalytic Hydrogenation

Titanium-nickel alloy is an attractive material due to its unique properties of shape memory effect, superior elasticity, and biocompatibility. Generally, Ti-Ni alloy powders are prepared from pure elemental powders of Ti and Ni as starting materials, but it is an energy-intensive process to obtain pure titanium. In this study, intermetallic compound TiNi powder passivated by TiOx shell was prepared by directly reducing a commercial NiTiO3 using CaH2 reducing agent in a molten LiCl at 650 °C. Analyses by X-ray diffraction, scanning electron microscopy/transmission electron microscopy with energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy revealed that the powder had a core-shell structure, with the core of TiNi and the shell of TiOx-rich composition with scarce metallic Ni nicely catalyzing hydrogenation reactions with good recyclability and stability.

Corrosion behavior, in vitro and in vivo biocompatibility of a newly developed Ti-16Nb-3Mo-1Sn superelastic alloy

The biocompatibility of a recently developed Ni-free Ti-16Nb-3Mo-1Sn (at.%) superelastic alloy was investigated both in vitro and in vivo. In addition, static water contact angle (WCA) and electrochemical tests were carried out. Commercial purity Ti (cp-Ti), which is already being used as a clinical material, was used as the control material. The alloy showed a stable corrosion behavior similar to that of the cp-Ti. The WCA measurements showed that the alloy exhibited hydrophilic properties that contributed to cell attachment to implants, as evident by the cytocompatibility tests. According to the in vivo implantation tests conducted on 30 adult BALB/c rats for periods up to 12 weeks, the tissue reaction around the implants was similar for both the cp-Ti and the alloy, and no significant difference was found in almost all parameters analyzed. Due to its stable superelastic properties accompanied with excellent biocompatibility and high corrosion resistance, we believe that this alloy is considered as a promising substitute for the biomedical materials containing Ni or other toxic elements.

Hydrogen Effect on the Cyclic Behavior of a Superelastic NiTi Archwire

In this work, we are interested in examining the strain rate effect on the mechanical behavior of Ni–Ti superelastic wires after hydrogen charging and ageing for 24 h. Specimens underwent 50 cycles of loading-unloading, reaching an imposed deformation of 7.6%. During loading, strain rates from 10−4 s−1 to 10−2 s−1 were achieved. With a strain rate of 10−2 s−1, the specimens were charged by hydrogen for 6 h and aged for one day showed a superelastic behavior marked by an increase in the residual deformation as a function of the number of cycles. In contrast, after a few number of cycles with a strain rate of 10−4 s−1, the Ni-Ti alloy archwire specimens fractured in a brittle manner during the martensite transformation stage. The thermal desorption analysis showed that, for immersed specimens, the desorption peak of hydrogen appeared at 320 °C. However, after annealing the charged specimens by hydrogen at 400 °C for 1 h, an embrittlement took place at the last cycles for the lower strain rates of 10−4 s−1. The present study suggests that the embrittlement can be due to the development of an internal stress in the subsurface of the parent phase during hydrogen charging and due to the creation of cracks and local zones of plasticity after desorption.

Polymeric nanoparticles-based multi-functional coatings on NiTi alloy with nickel ion release control, cytocompatibility, and antibacterial performance

Photo-cross-linked quaternized copolymer coatings on NiTi alloy show prominent inhibition of nickel ion release and antibacterial properties.

A Review of Selective Laser Melted NiTi Shape Memory Alloy

NiTi shape memory alloys (SMAs) have the best combination of properties among the different SMAs. However, the limitations of conventional manufacturing processes and the poor manufacturability of NiTi have critically limited its full potential applicability. Thus, additive manufacturing, commonly known as 3D printing, has the potential to be a solution in fabricating complex NiTi smart structures. Recently, a number of studies on Selective Laser Melting (SLM) of NiTi were conducted to explore the various aspects of SLM-produced NiTi. Compared to producing conventional metals through the SLM process, the fabrication of NiTi SMA is much more challenging. Not only do the produced parts require a high density that leads to good mechanical properties, strict composition control is needed as well for the SLM NiTi to possess suitable phase transformation characteristics. Additionally, obtaining a good shape memory effect from the SLM NiTi samples is another challenging task that requires further understanding. This paper presents the results of the effects of energy density and SLM process parameters on the properties of SLM NiTi. Its shape memory properties and potential applications were then reviewed and discussed.