Smart Metals with Memory?

Just when you thought Neil Degrasse Tyson ran out of edutainment and science became stale as the first box of Cheerios… Imagine a T-1000..

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Except the underlying properties are real!

Image result for shape memory alloys

What if I told you that this technology is applicable to a wide range of fields, from building robotic hands to innovating wing morphing technology for airplanes.

Image result for shape memory alloy robotic hand

You don’t have to be a futurist with a gleam in your eye to recognize this phenomenon of the shape memory alloy.

Enough with the corny hype and laymen dialogue.. let me direct you to a very informative, recent term paper written by Kirk Feeney (Engineering Student) on SMA for a University Class in Materials Engineering.

Journey into the Shape Memory Alloy by Kirk Feeney, December, 5th, 2018

Abstract

Metallurgy describes the study of engineering combined with material science, analyzing the chemical and physical behavior of metallic elements, compounds, and alloys. An alloy is simply a mixture of two or more different metallic elements or compounds.

Early in the studies of materials science it was discovered that pure metallic elements by themselves don’t contain ideal mechanical and or structural properties that are completely desirable. Starting around the third century the discovery of steel revolutionized material science and the strength of common materials from domestic tools to swords for battle. It was this discovery of steel, an alloy, where small amounts of carbon were added to the iron swords and tools to strengthen or harden the material, driving up the mechanical and structural properties.

History has led into the creation of modern technical instrumentation that allows scientists to create specific chemical compositions on a consistent basis thus obtaining a somewhat complete understanding of the processes and properties of such materials.

The modern development of “smart materials” where engineers have experimented, observed and exploited the laws of our universe to create useful materials that advance our technological capabilities and lead to and exciting future of science. Meet a smart material at the forefront of its class that contains unique properties including an immense amount of potential applications, the shape memory alloy.

Introduction

 

In the 1960’s William J. Buehler, a United States Naval scientist, was working on materials research at the United States Naval Ordinance Lab melting and casting bars of nickel titanium. While waiting for the bars to cool, he dropped one of the cooled bars on the concrete floor and noticed that it caused a dull thudding sound. He found this odd and dropped a bar that was still hot, which produced a lighter sound, like a bell. Worried that something had gone wrong in the casting process, Buehler ran to a drinking fountain and cooled the hot nickel-titanium bar under the water. When he dropped the now-cool bar, it produced the thudding sound. [1]

Thus, the discovery of the most dominant alloy within the shape memory family, an alloy synthesis of the metals Nickel and Titanium. Nickel-Titanium (NiTi) dominates the field due to its leading structural and functional properties. It should be stated that this journey will focus solely on NiTi, how it works, and the relevant applications of the material as NiTi is the number one contributor and most widely used material of the shape memory class.

The term Nitinol has been named for this alloy as due to the combination of fundamental elements Ni-Nickel, Ti-Titanium and the founding location being the Naval Ordinance Laboratory. Figure 1 best summarizes the main advantages and disadvantages of the alloy. While the advantages outweigh the disadvantages from a visual standpoint a major drawback of NiTi is the temperature dependent effect and poor fatigue properties under cyclic loading, which are important to be noted when considering the various applications for such a material. High mechanical performance, specific strength, damping capacity, corrosive resistance and large actuation force are some of the advantages presented.

Figure 1 (Advantages and Disadvantages of NiTi) [1]

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Body

 

Shape Memory Alloys differ from traditional metallic materials due to their ability to restore their shape after large deformations. Think of a material that can be bent at a relatively low temperature in any which way that will return to its pre-deformed state once the loading has been removed and the temperature increased. Think not fantasy yet modern scientific discovery of the last century. The Shape memory effect is that of a super elastic alloy capable of holding a memory. These materials are taking advantage of the principle of a diffusionless phase transformation, where the alloy is cycling between two different solid phases. For an effective thermodynamic analysis of phase transformations, Gibbs Free Energy is presented to describe the process.

Equation 1:Gibbs Free Energy

∆G= ∆H-T∆S

Delta H represents the change in Enthalpy of the system while T, the temperature, and Delta S represents the change in entropy. It can be assumed that a decrease in energy corresponds linearly with temperature and therefore there are preferred states of the system. The two important phases include Martensite at low temperatures and Austenite at the high temperature phase. Within figure 2, a binary phase diagram it should be noted that NiTi exists from about 45% through 52% Titanium with the rest Nickel. Therefore, NiTi exists with around the same atomic percent composition of the two elements, and by varying the composition of Nickel or Titanium and varying the temperature will correspond to a phase change. It should be noted from figure 2 that NiTi stability only resides within the region above 630 degrees Celsius.

Figure 2 (Binary Phase Diagram of NiTi) [2]

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Figure 3 represents a nice external representation of the shape memory effect (SME) with temperature being the driving force. It is important to understand the SME displayed below before proceeding to Superelasticity and any applications.

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Superelasticity refers to the ability of NiTi to return to its original shape upon unloading after substantial deformation, like stretching a rubber band. This phenomenon is based on stress-induced martensite formation. The application of an applied stress causes martensite to form at temperatures higher than the initial martensite beginning temperature.  With stress induced martensite, only one variant is formed that is parallel to the direction of the applied stress. When the stress is released, the martensite transforms back into austenite, and the specimen returns to its original shape. Thus, stress rather than temperature causes the phase transformation.

While most metals deform by slip or dislocation movement, NiTi responds to stress by simply changing the orientation of its crystal structure through the movement of twin boundaries. A NiTi specimen will deform until it consists only of the crystallographic orientation which produces maximum strain. In analyzing this behavior of the lattice, the superelastic phenomenon can be seen below where the lattice strain results in martensite formation and then returns to austenite, with no chemical or atomic compositional changes. [2] The SMA is under stable conditions in the Austenite phase and beginning at the yield point the transformation into martensite occurs. A constant linear addition of martensite occurs until the maximum strain is reached. Traditional stress-strain curves for a normal metallic specimen does not see the same recovery after strain as the shape memory alloy, it can be seen from the axial loading response below that a complete recovery post strain is shown.         

Figure 4 (Lattice Strain & Axial Loading response) [3]

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To get a better understanding of Nitinol it is important to understand how the alloy itself is manufactured and identify any drawbacks during the processes that could change key properties within the material. To start it is a difficult task to fabricate NiTi components because of the high reactivity and high ductility of the alloy which results in difficulties in the processing and machining. The working temperature is around 0 to 100 degrees Celsius when working to machine this material, a much lower different range than most metals which also causes problems and requires very special attention during the processes. There are two major processes that are usually implemented when manufacturing NiTi, first via a casting process and second through a variety of powder metallurgical methods.

First When considering the casting of NiTi the materials must be in a high purity form as the introduction of impurities and other nonmetallic elements can possibly disrupt the homogeneous nature of the alloy. By introducing impurities in the casting process the composition distribution of Nickel and Titanium will become uneven and potentially alter key properties of the material, possibly reducing the inherent superelastic effect. Some specific techniques that are used in industry include Vacuum Induction Melting (VIM), Vacuum Arc Melting (VAM) and Electron Beam Melting (EBM)

A second potential method is classified as the powder methods in metallurgy, where powder like materials are subject to pressure and forced into a mold where the material is then heated to finalize its shape. Some common techniques include Conventional Sintering (CS), Metal or Powder Injection Molding (MIM) and even additive manufacturing (3D Printing) Methods of Selective Laser Melting (SLM) and Electron Beam Melting (EBM). Power methods can lead to an increased homogeneity of the alloy and decreases losses during formation, with the drawback of increased potential for oxidation.

“This contamination can cause serious depletion of Ti from the matrix, forming oxides and consequently increase the grain size of the material. As a result, NiTi alloys fabricated this way tend to be more brittle which is generally unwanted. Additionally, transformation temperatures are also affected by the precipitation of oxides and intermetallic phases.” [4]

The most prominent limitation of applying Shape memory technology in industry is having to abide to the temperature dependent phase transformation property. In most mechanical systems temperature has a variance as result of complex moving components and the application. During any design process there is a finite number of design parameters that need to be accounted for and any mechanical engineers’ primary job is to make sense of these parameters and design for them. While temperature is a main limitation there are ways to engineer around this and reduce the fluctuations of temperature within the system. Many industries are developing design solutions that mitigate the limitations of the material and maximize its strengths, as we will see in a few applications to follow.

Applications

Actuation is a major application of NiTi where the idea is being experimented widely across these industries. Actuation simply refers to the ability to put something in to action. In an engineering systems case, it is what initiates motion within a subsystem, opening or closing a valve, flipping a switch, the possibilities are endless. NASA is one the many companies interested in the Shape memory effect and a team of engineers working within its Spanwise Adaptive Wing (SAW) project are experimenting with “the feasibility of bending or shaping portions of an aircraft’s wings in-flight, potentially increasing performance and efficiency by reducing weight and drag.”  This research is categorized by a term Wing morphing. Wing morphing is the ability of a wing to bend fold and adapt to flight conditions for optimal aerodynamic effects. Wing morphing is not a new term and is already present in a few manufactured aircrafts. Although these current systems are operated by primarily heavy power consuming hydraulic, electrical, and mechanical systems that actuate the wings motion.

NASA engineers have been developing an activation system that utilizes the shape memory effect as the driving force behind wing morphing. “For the SAW project, NASA is using SMA materials as torque-tube actuators. In this configuration, a single or group of trained SMA tubes are heated via internal heaters or external electrical coils, triggering them to twist and perform the desired actuation to drive a folding wing. Electrically induced temperature change is only one possible stimuli. SMA can also be activated by using bleed air from the aircraft’s engines or simply through the ambient temperature changes experienced during flight. This compact, lightweight application, which is also extremely quiet, allows the entire actuator package to be attached at the wing hinge point.  Conventional actuation approaches typically cannot fit in this area, leading to heavy and complex linkages or transmissions to drive a wing fold or similar aerodynamic surface. [5]

Outside the Aerospace field another application of nitinol resides within the civil engineering field, where civil engineers are experimenting with research using the materials superelastic properties as a possible replacement for bridge cables due to the attractive damping properties. Traditional Bridge cables are subject to cyclic oscillations due to the repeated use of transportation. Engineers are responsible for designing bridges for stability and safety for everyday use. The key component is in these bridge cables effectiveness damping the bridge oscillations. Another fatigue factor of bridges prestressed suspension cables is corrosion and their inherent response to the environment. Nickel Titanium has excellent corrosion resistance properties. When looking to the Galvanic Series of metals we can appreciate compared to the spectrum Titanium, and Nickel are quite inert. Inert meaning that these metals are less likely to experience corrosion when submerged in an electrolyte. It is no mystery that with a shape memory response and high corrosive resistance NiTi could be the future of such structures.

A French research group [6] carried out an experiment with NiTi testing its damping properties. Via a setup of laser sensors and an anchoring device, the research group held two NiTi springs in compression and applied a compression force to test the ability of the material to dampen the applied loading. “The cable oscillations were stopped after only 10 s while the cable without SMA damper is still vibrating after 120 s. The amplitude was reduced to a quarter by increasing the damping ratio. The influence of the position of the damping device on the efficiency of the SMA dampers has been evaluated. It is clearly shown that the result is better when the damping device is located near the maximum amplitude, typically near the force application point.”  For reference I have included below in figure 6 results of their experiment and highlighted the portion where the team had run a Fast Fourier transform of the displacement in response to a forcing of 4kN. The two displacements are comparing two cables one with Nitinol damping and one without the damping, a natural cable. In comparison of the experimental data the amplitude peaks are sizably reduced with the damper than without.

Figure 5 Nitinol Damping Response

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The biomedical industry is using the shape memory effect even inside the human body! Throughout history human life and quality and longevity of life has been increasing exponentially with the increase of new engineering tools. There has indeed been a barrier between the fields of engineering and human health in trying to coordinate proper materials and devices to operate within the working conditions of the human body. The goal of these such tools are to prevent disease and failure of primary human organs in the cardiovascular system. Nitinol is a material that is breaking this barrier.

The main biocompatible purpose of nitinol is within the procedure of angioplasties. Angioplasties are a very common procedure that addresses narrowed arteries where blood flow is not being sufficiently supplied. When any part of the human body is not getting enough blood flow deterioration of the organ begins and can lead to more serious diseases and even death. During an angioplasty a stent is placed into the artery or vein to widen the passageway and recirculate the blood to normal flow rates. “A martensitic stent is compressed in order to be guided to the desired location and when released from the catheter, at the temperature of the inner body, the SMA regains its original shape thus supporting the vessel’s wall and allowing normal circulation. Since the stent is not removed and must stay inside the patient’s body, biocompatibility is important and so Nitinol is used.” [7]   A visual for this process is succeeded in figure 6.

Figure 6: Nitinol Stent Visual [7]

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References

[1] Bidsorkhi, H. (2018). Advantages and disadvantages of NiTi SMA.. [online] Available at: https://www.researchgate.net/figure/Advantages-and-disadvantages-of-NiTi-SMA_tbl1_311202728 [Accessed 6 Dec. 2018].

[2] Chen, K. (2018). NiTi – Magic or Phase Transformations?. [online] Digitalcommons.calpoly.edu. Available at: https://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?referer=https://www.google.com/&httpsredir=1&article=1015&context=mate_fac [Accessed 6 Dec. 2018].

[3] Costa, B. (2018). The Elephants of Materials Science: SMAs Never Forget Their Shape. [online] COMSOL Multiphysics. Available at: https://www.comsol.com/blogs/the-elephants-of-materials-science-smas-never-forget-their-shape [Accessed 6 Dec. 2018].

[4]  Mie.uth.gr. (2018). [online] Available at: http://www.mie.uth.gr/ekp_yliko/TheUniqueProperties,ManufacturingProcessesandApplicationsofNearEquatomicNi-TiAlloys.pdf [Accessed 6 Dec. 2018].

[5] NASA. (2018). Metal with Memory: Shaping the Future of Aviation. [online] Available at: https://www.nasa.gov/feature/metal-with-memory-shaping-the-future-of-aviation [Accessed 6 Dec. 2018].

[6] Dieng, L. (2018). Use of Shape Memory Alloys damper device to mitigate vibration amplitudes of bridge cables. [online] Www-sciencedirect-com.gate.lib.buffalo.edu. Available at: https://www-sciencedirect-com.gate.lib.buffalo.edu/science/article/pii/S0141029613003428 [Accessed 6 Dec. 2018].

[7] Elahinia, M. (2015). Shape memory alloy actuators. John Wiley & Sons, Incorporated.

[8] The Measurement and Interpretation of Transformation Temperatures in Nitinol. (2018). Confluent Medical Technologies.

 

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