Research Progress and Aerospace Applications of Shape Memory Alloys

DU Yue,TU Gang

Aerospace Research Institute of Materials &Processing Technology,Beijing 10076

Abstract:As one of the newly developing intelligent/smart materials,shape memory alloys (SMAs) have become an important material and have broad application prospects.With smart structures,the integration of SMAs as actuators and sensors in structural components,has drawn significant attention and interest in the aerospace field.In this paper,the research status of SMAs at home and abroad in recent years was reviewed,including the characteristics,classification,investigation progress and applications in the aerospace field.Finally,the development trend prospects for SMAs was also presented.

Key words:shape memory alloys,smart material,aerospace applications

1 INTRODUCTION

Smart materials generally refer to materials that can adapt to a changing environment and perform intelligent functions due to their unique and superior properties,including self-sensing,self-diagnosing,self-driving,self-repairing and other capabilities.Shape memory alloy (SMA) is a kind of smart material or intelligent material with shape memory effect (SME),superelasticity(SE) and a high damping capability.The SMA has the ability to sense temperature changes and convert thermal energy into mechanical energy,and then output a force,displacement or store and release energy,which exhibits excellent dual functions of sensing and driving.In addition,SMA also has the characteristics of high power to weight ratio,good wear resistance and corrosion resistance,which can be utilized for miniaturization and intelligence enhancement of mechanical structures.Since then,SMAs have been used in a wide variety of applications in different fields,including biomedical,automotive,consumer products,robotics and especially in aerospace industry.In this paper,the characteristics and research status of SMAs are introduced,as well as the recent applications in aerospace field.The future research emphasis and development trends based on these materials are reviewed.

2 SHAPE MEMORY ALLOY OVERVIEW

2.1 Characteristics of SMA

As a category of intelligent material,SMA has been used as a functional material for a long time due to the exotic characteristic of SME.The SME is explained with a stress-strain diagram given in Figure 1.SME refers to that feature where a material regains its original shape in the presence of suitable stimulus which can be temperature,an electro-magnetic field,mechanical stress or vibration,so as to remember its shape.The SME of SMA is shown as follows:in the low temperature state,if the temperature remains unchanged,the SMA first produces elastic strain under external stress.When the external stress exceeds a certain value,the memory alloy will undergo plastic strain like other plastic materials;After the removal of external stress,there will be plastic strain residual in SMA.When heated to a certain temperature,the alloy will return to its original shape.

The SMAs exhibit SE as well,so when the alloy undergoes strain on the application of stress,it recovers significantly on unloading the stress,given in Figure 2.The SE of a SMA is as follows:at a certain temperature,when the alloy is subjected to external stress,an elastic strain is generated at first.When the external stress exceeds a certain value,elastic strain will increase with constant external stress.As the external stress is removed,the elastic strain will decrease to zero and the alloy will automatically return to its original shape.The SE effect makes the memory alloy produce large strain and has strong fatigue resistance.The recovery may be several hundred times greater than that of simple elastic strain.

Figure 1 Shape memory effect in SMA

Figure 2 Superelasticity in SMA

2.2 Mechanism of the Shape Memory Effect and Superelasticity

SMA exhibits an austenite phase and martensite phase,where the lattice structures of these two phases are different.The low temperature phase is known as martensite (‘M’ which has a noncubic lattice) and the high temperature phase as the austenitic phase or austenite ‘A’ or parent phase ‘P’ (with a cubic space lattice).The martensite structure is stable at low temperature and the austenite structure is stable at high temperature.The two phases can be transformed into each other under certain external load and temperature conditions,as shown in Figure 3.The martensitic transformation is an exothermic reversible and diffusionless reaction,in which one-toone correspondence between atoms does not change.These transformations occur at characteristic temperature intervals:during the heating process,when the temperature reaches As,which is called the beginning temperature of austenite,the SMA begins to transform from martensite to austenite.When the temperature reaches Af,transformation process from martensite to austenite is completed,and the volume fraction of martensite is 0.This temperature is called the austenite finish temperature,Af>As.In the same way,during the cooling process,the transformation starts to revert to the martensite at Ms and is completed when it reaches the Mf value,and the volume fraction of martensite is 1.This temperature is called the ending temperature of martensite,and Mf

Figure 3 Microstructural state showing shape memory effect

Eventually,the internal mechanisms of SME and SE for a SMA enable a solid phase transformation with different crystal structures.The SME of SMA can be divided into the following characteristics:the first is a one-way shape memory effect(OWSME) where the alloy retains a deformed state after the removal of an external force,and then recovers to its original shape only during heating;the second is the two-way shape memory effect (TWSME),that is,the alloy can remember its shape at both high and low temperatures.In case of TWSME,martensite can be restored,while in the case of full-SMA (FSMA)the alloy can remember the shape at Af and below Mf.On heating and cooling again it can completely reverse the respective shape.However,TWSME requires a special training process which is complex.In addition,the performance of TWSMA tends to decline quickly,especially at high temperatures.It usually produces about half of the recovery strain provided by OWSMA for the same material.These problems result in the fact that TWSMA is less applied commercially.Super-elasticity,that is,when the Af is generally lower than room temperature or the operating temperature,the SMA reverts to its original shape after an applied mechanical loading at temperatures between Af and Ms,without the need for any thermal activation.The loading stress induces A →M transformation,when the load is removed and single variant de-twinned martensite is developed.On unloading,the reverse M→A transformation takes place first partially and then entirely.

2.3 Development of SMA

The SME was first observed in the early 1930s as a solid-state phase transformation which produced rubber-like effect in Au-Cd samples.Since then,research has concentrated in this area,and a large number of alloys have been explored and widely used in the aerospace,automotive,petroleum engineering,civil engineering,biomedical and other fields.After 80 years of development,NiTi-based,Fe-based and Cu-based SMAs have been mainly formed.Among them,although Fe-based and Cu-based SMAs have lower cost,their applications are limited by their unstable properties,poor thermo-mechanical properties and corrosion resistance.NiTi-based SMAs with cold working and annealing treatments are considered most suitable for a number of engineering applications,given its better superelastic properties,higher temperature variation stability and higher resistance to corrosion and fatigue.

One disadvantage of the NiTi alloy in emerging applications is its low transformation temperatures (<100 ℃),which may lead to a sluggish response under ambient conditions and even unintentional actuation when in service in warmer environments.In addition,there is an increasing demand for high temperature SMA actuators in hot environments,where no other actuation system can operate,such as for uses in aerospace,aeronautics,automotive,energy and exploration.Here,the traditional NiTi alloy cannot meet the requirements for high temperature operation for the majority of envisioned applications.

In order to expand the application of SMAs,alloying with ternary elements has been successfully implemented to increase the transformation temperatures of SMAs,which has developed a category of SMAs called high temperature SMAs(HTSMAs).The HTSMAs are defined as SMAs that are operating at temperatures above 100℃,and can be categorized into three groups based on their martensitic transformation ranges:100?400℃,400?700℃ and >700℃.The operating temperature increases with the increase of Y content.For example,Cu-Al-Ni-Mn-x (X=Ti,B,V) alloys were developed on the basis of Cu-Al-Ni,MS is about 200℃;Ni-Al-Z (Z=Fe,Mn,B) alloys were developed on the basis of Ni-Al intermetallic compounds;Ru-Ta (Nb) shape memory alloys with an MS above 480℃ and recovery temperature above 1000℃,etc.

For a long time,research on HTSMAs with Ni-Ti-Y (Y=Hf,Pd,Pt,Au) alloys has been undertaken.NiTi-based HTSMAs carry most of the attractive features of binary NiTi,e.g.,high transformation strain,good work capability,reasonable fatigue life and excellent corrosion resistance,while exhibiting transformation temperatures as high as 500 ℃.In addition to the high MS point and acceptable strain recovery level,high longterm stability,resistance to plastic deformation and creep,oxidation resistance,machinability and affordable cost range are also required for HTSMAs.Therefore,alternative low-cost materials or compositions such as copper and cobalt have been researched.At present,only Ti-Ni-Pd,Ti-Ni-Pt,Ni-Ti-Hf,Ni-Ti-Zr and Cu-Al-Ni-Mn alloys are expected to be used within 100?300 ℃,while the properties of other alloys need to be further improved.The development of HTSMAs with good comprehensive properties has always been an important target in the aerospace field.Various strengthening methods including quaternary alloying,thermomechanical processing,and especially precipitate strengthening have been successfully employed to alleviate some of these inherent problems and improve the shape memory characteristics of HTSMAs.

3 APPLICATION IN AEROSPACE

SMA can obtain a reversible restoring strain of 4%– 8%in a narrow temperature range,and if the strain recovering is prevented by heating,the SMA can exhibit a larger resistance stress.In other words,by changing the temperature under certain conditions,SMA can output a force or displacement,which means SMA has both sensing and driving functions.The unique behavior of NiTi SMAs have spawned new innovative applications in the aerospace industries.Generally,the shape memory applications can be divided into three categories according to their primary function,where the SME can be used to generate motion and/or force,and the SE can store deformation energy.Since the 1970s,SMAs have gathered greater interest in the area of aerospace applications,which are subjected to high dynamic loads and geometric space constraints.Remarkable progress has been made in the case of aerospace actuators,release or deployment mechanisms,adaptive wing,noise suppression,inflatable structures,manipulators and other.

3.1 Space Deployable Mechanisms

Considering the carrying capacity of rockets and the need for large space structures (antennas,solar arrays,solar sails,space telescopes,etc.),deployable structures have been applied.Most deployable structures rely on electromechanical mechanisms or mechanical arms for deployment.Shape memory materials can be used as the driving components of new spatial deployable structures to realize the expansion driven by the shape memory properties of materials without complex mechanical devices.At the same time,the components manufactured with shape memory materials have the advantages of being light weight,having relatively high stiffness and strength,low cost,and high reliability.

The application of SMA in space deployable mechanisms is generally divided into two areas:one is to use SMA material to make say a self-deployable antenna indirectly,the other is that where the actuator is made of SMA material which is used as the device for unlocking or releasing and driving the deployable mechanism.In the 1960s,a satellite antenna was developed for the first time using the NiTi SMA wire in the United States.When the satellite antenna was cooled,its hrunk and folded into a 50 mm ball which was placed in the spacecraft.After the spacecraft entered the space,the satellite antenna underwent a reverse martensite transformation from solar radiation heating.The folded spherical antenna automatically opened in space and returned to its original shape,as shown in Figure 4.

Figure 4 SMA antennas

Today,spacecraft can realize connection and unlocking functions through a variety of connection and separation devices,such as large cabin separation,device and arrow separation,solar panel compression and unlocking,payload release,parachute cabin,parachute hatch cover and other kinds of hatch cover ejection.With the continuous development of space technology,spacecraft have become more and more complex,and there are a lot of separation devices used.Since the 1990s,a large number of space separation devices,based on SMA have developed,without weight penalty or stiffness loss compared to other conventional actuators.

SMAs are widely used in unlocking and releasing devices due to their high restoring force,high restoring deformation and repeatability.Large load bearing and fast response is the development trend for SMA based unlocking devices.At present,most SMA separation devices are divided into petal nut structure.The nut of the load bolt is composed of several pieces.Under the connection state,it is tightened by the locking device to ensure reliable connection with the bolt.During separation,the split nut is driven by SMA to separate and release the load.In 1993,with the support of the U.S.Air Force Laboratory,the Lockheed Martin Company developed two kinds of separation nuts,low force nut (LFN) and two stage nut (TSN) using SMA as actuators,and their structures are shown in Figure 5.In May 1999,LFN and TSN were used for the in-orbit Shape Memory Alloy Reusable Deployment Mechanism (SMARD) experiment onboard the MightySat satellite.

Figure 5 Structure of LFN device and TSN device [20]

NASA developed a compression release device for the separation of a carrier rocket and spacecraft and which was applied successfully.The cylindrical fixing sleeve was fixed on the lower connecting piece through a screw thread.When the device was compressed under the action of the piston,the three parts of the separation nut were tightly closed,which formed the complete thread.The separation nut was axial limited through the washer.When releasing,the SMA rod was heated and elongated,and the piston was driven to shear the pin and moved upward.The two bulges of the piston and the nut were separated from each other,and the connecting bolt was released after the radial constraint on the separating nut was removed.This device had a simple,mature structure with large bearing capacity,but it relied on a direct drive SMA rod,which resulted in a large volume,slow heating and long operation time.In addition,it would not be used more than once due to the fact that the pin was cut off during the operation process.

The Frangibolt device is a locking release device using a SMA driven slotted bolt,which was developed by TiNi Aerospace funded by the US Naval Research Laboratory in the early 1990s,as shown in Figure 6.The device features high load capacity,simple construction,light weight,with high safety and reliability,and the SMA actuator can be reused after cooling and compression to the length before triggering.However,the action time of it is slow (over 20 s),which is closely related to the power supply current,initial temperature,etc.It is not suitable for multipoint synchronous release and rapid separation at present.

Figure 6 Frangibolt lock and releasing device[22]

The Lockheed Martin Company developed a SMA-driven split nut device.The pre-stretched tubular SMA is heated to contract,which results in the driving of the bottom retaining ring and clamping ring which moves up,thereby releasing the radial restraint of the flapper nut.The separating block drives the flapper nut apart under the action of the piston spring,and the bolts are unlocked in the jacking rod.

In addition,Beihang University,Harbin Institute of Technology,Tsinghua University,the China Academy of Space Technology and other institutions in China have also carried out a lot of research work in shape memory alloy release device,and made a preliminary breakthrough.Among them,Beihang University has carried out an in-depth study on the performance of SMA wire.According to the applied aerospace environment,a variety of SMA compression release mechanisms have been developed,and four of them have completed on-orbit flight verification and many space missions.

3.2 Actuator

Elzey et al.have developed a shape morphing wing design for small aircraft by adopting an antagonistic SMA-actuated flexural structural form that enables the changing of the wing profile by bending and twisting,to improve the aerodynamic performance.A preliminary design study with finite element analysis presented by Icardi and Fer-rero verified that an adaptive wing for a small unmanned aerial vehicle (UAV).

Boeing has developed an active serrated aerodynamic device with SMA actuators,which is also known as a variable geometry chevron (VGC) and has been installed on a GE90-115B jet engine.Figure 8 shows the Boeing design for the variable geometry chevron.The SMA strips are installed on each side of the chevron centroid.This compact,light weight,and robust SMA actuators have proven to be very effective in optimize the noise reduction during take-off by maximizing the chevron deflection,and also increasing the cruise efficiency by minimizing the chevron deflection during the remainder of the flight.

Figure 7 Wing morphing with a bio-inspired,highauthority,shape morphing structures actuator [29]

Recently,NASA engineers have also demonstrated how a new SMA actuator folds the 136 kg wing from the F/A-18 Hornet supersonic fighter jet.The research is part of the NASA’s Spanwise Adaptive Wings program,which is investigating how adaptive wings can improve efficiency and control of aircraft.By simplifying the aircraft design,they will be able to replace complex mechanical,hydraulic and electric brakes with components or wings made of SMAs,which will enable a reduction in the weight of the actuator by 80%.A newly developed HTSMA torque tube actuator made of a Ni-Ti-Hf alloy was tested,which enables torque up to 564 Nm.NASA will continue to test the SMA actuators on the F/A-18 wing,with the goal of increasing the torque capacity to 2,260 N and applying it to the leading and trailing edges of the wing section.

In November 2017,scientists of the NASA Glenn Research Center successfully developed a non-inflatable tire made of NiTi SMA (see Figure 9),which was aimed to replace the old rover pneumatic tire.The NiTi alloy can be deformed up to 30%without permanent deformation or damage.In addition,it can be restored to its original shape after being extended by 10%,while ordinary metal material can only extend by 0.3%– 0.5%.It can be seen that SMA has obvious advantages in this respect.The risks of deflating or blowing current pneumatic tires can be avoided with SE properties with subtle waved structure.The non-inflatable tires are susceptible to temperature changes.The tires can be widely used in space cars or vehicles on Earth.

Figure 8 Boeing’s variable geometry chevron (VGC) [30]

3.3 Bearing

NASA is trying to make spherical bearings by using NiTi alloy.Traditional ball bearings are prone to rust and dent in extreme conditions,especially during aerospace flights.However,unlike traditional bearing steels,NiTi alloy with a highly elasticity that can be twisted and bent still returning to its original shape.

In addition,in the aerospace field,SMA has also been successfully used in actuators,vibration dampers,door sealing materials and manipulators.In the opening and closing of solar panels,cabin doors and other driving mechanisms,the application of SMA can effectively reduce the rebound force associated with automatic closing,ensure a large torque in the closing position,and avoid damage caused from the impact caused by excessive elastic force of composite materials around the cabin door.

Figure 9 SMA non-inflatable tire

4 PROSPECTS OF FUTURE SMA RESEARCH IN AEROSPACE FIELD

As a sensing and driving element,SMA has many incomparable advantages in terms of intelligent structure applications,such as strong functionality,various structural forms and miniaturization.Although SMA is a unique intelligent material with great potential,there are still many challenges in research and application process of intelligent structure integration.The contradiction between response speed,output force and output displacement of the SMA actuator is the most prominent issue that restricts its application.And there are two factors that leads to complex control problems:the complex mechanical properties of the SMA materials along with the stress,strain and temperature,and the macroscopic deformation behavior of them is sensitive to the components,grain size,preparation process,thermal force loading method and other factors.The response rate of the SMA actuator depends significantly upon heating and cooling strategy.At present,current heating and then natural cooling in environment are generally used.Rapid heating requires a large current and increases the diameter of the SMA wires,which will affect the performance of the device.

It can be predicted that with the continuous development of SMA technology,combined with modern design,electronic technology and feedback control systems,the application of SMA materials in intelligent structure for the aerospace industry will be extensively employed.