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Method for Improving the Structural Integrity using SMA Revolutionary Technology

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An Innovative Method for Improving the Structural Integrity using SMA Revolutionary Technology

D1.1 – State of the art of materials, technologies and systems / Appropriate FE software

Table of Contents

  1. INTRODUCTION

1. SHAPE MEMORY ALLOYS

1.1 General considerations

1.2 Martensitic Transformation

1.3 Thermomechanical properties of SMAs

1.4 Transformation Induced Plasticity (TRIP)

1.5 Thermal Cycling

1.6 Transformation Induced Fatigue in SMAs

1.7 Types of SMA materials

2. PATENT SEARCH ON SMAS

3. SMA APPLICATIONS

4. METALLIC COATINGS AND SMA THIN FILMS

5. SURVEY OF TECHNOLOGIES FOR DEPOSITION OF METALLIC COATINGS: A STATE OF THE ART OVERVIEW

5.1 Physical Vapour Deposition

5.2 Chemical Vapour Deposition

5.3 Thermal and Cold Spray deposition

5.4 Electrochemical deposition

6. SURVEY OF TECHNOLOGIES FOR SURFACE PREPARATION: A STATE OF THE ART OVERVIEW

6.1 Hand and power tool cleaning

6.2 Abrasive cleaning systems: Dry Blast Cleaning and Wet Blasting

6.3 Solvent cleaning

6.4 Alkaline cleaning

6.5 Acid cleaning

6.6 Ultrasonic cleaning

6.7 Pickling

7. SURVEY OF TECHNOLOGIES AND COMMERCIAL SYSTEMS FOR SURFACE AND COATING QUALITY INSPECTION

7.1 Introduction

7.2 Contact and Intermittent Contact profilometry

7.3 Non-Contact Non Optical techniques

7.4 Non-Contact and Optical Profilometry: State of the Art

7.5 Review of commercial Profilometers for surface inspection

8. REVIEW ON IMAGE PROCESSING TECHNIQUES FOR SURFACE INSPECTION USING LASER PROFILOMETRY

8.1 Pixel Intensity based Segmentation Method

8.2 Texture Analysis based Segmentation Method

8.3 Edge filtering using gradient information

8.4 Image processing techniques based on fringe images

8.5 Conclusions and challenges of current laser profilometer based surface inspection, novelty introduced in Innosmart project

9. SURVEY OF COMMERCIAL MANIPULATORS AND ROBOTS FOR SURFACE AND COATING QUALITY INSPECTION, SURFACE PREPARATION AND DEPOSITION OF METALLIC COATINGS

9.1 Introduction

9.2 Required specification and selection criteria of the manipulating device

9.3 Commercial manipulators and robots for surface and coating quality inspection

9.4 Commercial manipulators and robots for surface preparation

9.5 Commercial manipulators and robots for deposition of metallic coatings

9.6 Results and General Discussion

CONCLUSIONS

REFERENCES

 

INTRODUCTION

This report contains nine sections. The first section contains a detailed study of the state-of-the-art of shape memory alloys (SMAs), including the mechanisms for the shape memory effect and the various types of materials. The second section includes a patent search on shape memory alloys. The third section discusses the various applications related to the use of SMAs. The fourth section presents the metallic coatings and SMA thin films. Section 5 presents a state of the art overview of technologies for deposition of metallic coatings. Section 6 presents a state of the art overview of technologies for surface preparation. Section 7 describes a survey of commercial systems and technologies for surface and coating quality inspection. Section 8 presents a state-of-the-art of image processing techniques. Finally, Section 9 presents a survey of commercial manipulators and robots for surface and coating quality inspection, surface preparation and deposition of metallic coatings. The conclusions presented at the end of this report offer a guideline for SMA material selection, scenario selection and coating deposition/inspection selection.

 

1. SHAPE MEMORY ALLOYS

1.1 General considerations

When a regular metallic alloy is subjected to an external force greater than its elastic limit, it deforms plastically, i.e. the deformation persists after returning to the unloaded state. The Shape Memory Alloys (SMAs) do not follow this behaviour. At low temperatures, an SMA specimen may undergo a plastic deformation of about few percent, and then fully recover its initial shape that had at higher temperature by simple heating above a threshold temperature. Their ability to recover their form when temperature is raised, makes this class of materials unique. This phenomenon has been discovered in 1938 by researchers working on the gold-cadmium alloys [Gilbertson, 1994]. The shape memory phenomenon remained a laboratory curiosity until 1963, when the first industrial and medical applications appeared.

In SMA materials, two main phases exist: austenitic and martensitic. The austenite phase (parent phase) has a cubic lattice. Austenite has in general a cubic crystal lattice, while martensite is of tetragonal, monoclinic, or orthorhombic crystal lattice, subject to the composition the alloy material. For example, in equiatomic NiTi SMAs the martensite that is formed has monoclinic lattice. By adding an element like Cu or Pd it changes the martensitic microstructure to orthorhombic or rhombohedral phase [Saburi, Nenno, 1981].

1.2 Martensitic Transformation

The shape memory effect is based on the existence of a reversible phase transformation of thermoelastic martensitic type [Kurdjumov, Khandros, 1949; Kumar, Lagoudas, 2008], between a microstructural state at high temperature (austenite phase) and a microstructural state at low temperature (martensite phase) [Patoor et al., 2006; Lagoudas et al., 2006]. The transformation from austenite to martensite take place by shear distortion of the lattice (atoms move in respect to their initial position) and not by atoms diffusion. This type of transformation is called martensitic transformation [Perkins (1975); Funakubo, 1987; Otsuka, Wayman, 1999]. In reality, the matrenitic transformation in SMAs is a phase transformation of the first order, where there is co-existence of several phases, and there is presence of interfaces between the phases [Guénin, 1986].

Historically, the term martensitic transformation describes the transformation of the austenite of steels (iron-carbon alloys) to martensite during a quenching. By extension, this term has been generalized to a large number of alloys whose phase transformations have certain characteristics typical of the transformation of steels [Rosa, 2013].

Two mechanisms can help the matrensite to be formed with negligible change in volume of the material: (a) through slip (where atoms move by some atomic spaces), or (b) by twinning (where atoms move by a portion of atomic space) [Saburi, Nenno, 1981; Miyazaki, Otsuka, 1986]. In shape memory alloys, lattice shear occurs through twinning. In detwinning, inelastic strains are generated through relative motion of atoms, causing a macroscopic change in shape of the material. This process allows for reversible behaviour to the initial crystallography structure when the specimen is heated to austenitic phase [Kumar, Lagoudas, 2008].

During martensitic transformation of a SMA, the crystal lattice of the material changes its shape. The microstructure of martensite is characterized by a change in shape and by the difference in volume, which exists between matrensitic and austenitic phase [Duerig et al., 1990]. Therefore, internal strains arise during the emergence of martensitic areas within the austenite. The internal strains can be partially relaxed by the formation of several areas of self-accommodated martensite crystals that minimize the overall deformation induced. These areas called variants and are oriented in different crystallographic directions [Kumar, 2008].

In the absence of external stress, martensite variants are formed by twinning and are equally possible. These twins are arranged in specific patterns to minimize strain energy (self-accommodation) and the distribution of self-accommodated groups allows the material to be transformed in order to retain its original shape. Therefore, the formation of the martensite results in elastic (reversible) deformations [Funakubo, 1987]. At constant temperature, the martensite-austenite interfaces are in steady state. A change in temperature in one direction or the other results in moving these interfaces to the benefit of one or the other phase structure. The interfaces can also move under the action of an imposed strain. A specimen can therefore be distorted not by sliding, which is the usual mechanism of plastic deformation, but by the appearance and disappearance of martensite variants [Kumar, 2008].

Therefore, during martensitic transformation atoms in the structure move on very small distances leading to deformation of the crystal lattice. This causes a small variation in volume with shearing of the structure in a specific direction. During the transformation process, the growth of martensite crystals occur in form of platelets to minimize the energy at the interface.

The martensitic variants can occur in two different types: twinned martensite (formed by combination of self-accommodated martensite variants) and detwinned martensite (reoriented martensite) where a particular variant dominates [Liu, Xie, 2007]. The characteristic behaviour of SMAs is based on the reversible transformation of phase from austenitic to martensitic and the opposite. By cooling under zero loading, the crystal sructure changes from austenitic to martensitic phase (forward transformation to twinned martensitic phase). This transformation is resulting in the development of a number of martensitic variants, which are arranged in a way that the mean change in shape macroscopically is insignificant, causing a twinned martensite [Leclercq, Lexcellent, 1996]. When the material’s temperature raises at the martensite, the crystalline structure is transforming to austenitic (reverse transformation from detwinned martensitic to austenitic phase), leading to recovery of shape [Saburi, Nenno, 1981; Shimizu, Otsuka, Perkins, 1975]. The above process is called Shape Memory Effect (SME) [Schetky, 1979; Wayman, Harrison, 1989].

The martensitic transformation is characterized by four temperatures (Figure 1) [Gotthard, Lehnert, 2001]:

  • MS: Temperature below which the martensite appears (martensite start)
  • MF: Temperature below which the entire sample is transformed into martensite (martensite finish)
  • AS: Temperature above which the austenite appears (austenite start)
  • AF: Temperature above which the entire sample is transformed into austenite (austenite finish)

The transformation begins at the cooling to the temperature MS. This transformation is completed to the temperature MF. Between these two temperatures, there is coexistence of two phases, which is a characteristic of transformation of the first order. If the cooling is interrupted, the material will not change. To go back to the initial shape, the temperature is increases so that the inverse transformation begins at the temperature AS and finishes to temperature AF, which is higher than M[Massalski et al., 1990]. If the trace on a diagram (Figure 1) the volume fraction of material processed as a function of temperature, there is a hysteresis loop, due to the presence of an irreversible energy corresponding to dissipation of mechanical energy transformed into heat [Ortin, Planes, Delaey, 2006; Wei, Yang, 1988].

Figure 1 Martensitic transformation temperatures [Gotthard, Lehnert, 2001]

The thermoelastic reversibility of the crystal lattice is certain in the case of an ordered alloy [Otsuka, Shimizu, 1977]. The relation between atomic order and manifestation of martensitic transformation was experimentally shown in Fe-Pt SMA materials [Dunne, Wayman, 1973]. Nevertheless, in disordered alloys, such as Fe-Pd, Mn-Cu and In-TI, can occur thermoelastic transformation too. The atomic order is, therefore, a sufficient condition for manifestation of thermoelastic transformation, but not necessary [Otsuka, Shimizu, 1977].

1.3 Thermomechanical properties of SMAs

Several effects specific to the SMAs appear through the transformations of the crystal lattice as a function of the field of stresses applied on the material and temperature [Duerig, Melton, Stöckel, 2013].

1.3.1 Pseudoelastic Effect

In general, by pseudoelasticity we describe both the material’s superelastic behaviour, as well as rubble-like behaviour. Superelastic behaviour is called the reversible phase transformation produced by thermo-mechanical loading. Rubber-like effect refers to the reversible martensitic re-orientation. The stress-strain curve during this process resamples to the superelastic behaviour, which is similar to rubber’s nonlinear elastic behaviour [Otsuka, Wayman, 1999].

Therefore, a part from inducing phase transformation thermally, martensitic transformation can also be prompt by applying on the material appropriately high mechanical loading, resulting in creating a martensitic phase from austenitic. When the temperature of SMA goes more than AF, the shape recovery is resulted while unloading. Such behaviour of the material is termed pseudoelastic effect [Kumar, 2008].

Stress-induced martensite, is generally forming from austenite when external stress is present. The process of forming stress-induced martensite can occur through different thermomechanical loading routes [Miyazaki, Otsuka, 1986]. One form of stress-induced martensite is the detwinned (re-oriented) martensitic phase formed from austenitic after application of external stress. The material, during the stress-induced martensitic transformation and the reversed process, shows nonlinear elastic behaviour described by closed σ-ε curves. This nonlinear elastic behaviour is called pseudoelastic transformation [Otsuka, Shimizu, 1981]. Shape recovery occurs because of the crystallographic reversibility of the transformation, like in the SME. Hence, the two phenomena, transformation SME and pseudoelasticity are practically the same except the fact that reverse transformation is produced by warming the specimen to temperature above AF. In reality, an alloy that undergoes thermoelastic martensitic transformation exhibits both transformation pseudoelasticity and shape memory effect [Otsuka, Shimizu, 1981].

Nevertheless, in order to happen a pseudoelastic transformation, the necessary stress for slip should be greater than that for stress-induced martensite transformation. As an example, we can refer to equiatomic Ti-Ni alloys which are exposed to slip and do not exhibit any transformation pseudoelasticity, regardless of their Ni content. It was shown, however, that Ni-rich Ti-Ni alloys subjected to annealing after cold working, causing refining of their grain size, leads in raising critical slip stress, which results in any transformation pseudoelasticity [Miyazaki et al., 1982; Saburi, Tatsumi, Nenno, 1982; Saburi, Yoshida, Nenno, 1984]. The existence of transformation pseudoelasticity is affected by crystalline orientation, composition of the alloy, and direction of applied stresses [Miyazaki, Otsuka, 1986].

1.3.2 One-Way SME

Another property of SMAs is the one-way shape memory effect. It takes place in four steps:

  1. The material is cooled to a temperature lower than MF (the parent austenitic phase) to obtain self-accommodated martensite.
  2. Re-orientation of variants of the martensite is obtained via application of stress.
  3. The stress is released at constant temperature T < MF. The material remains to a shape depending on the stress field.
  4. The sample is heated at a temperature T > AF making re-appear the austenitic phase and the material gets its original shape, as shown in Figure 2.

Figure 2 One-way shape memory effect [Miyazaki, Otsuka, 1986]

Two conditions are necessary for occurring shape recovery by shape memory effect. Firstly, the transformation should be reversible, and second, slip should not occur during the entire deformation process. Martensitic transformations in ordered alloys are reversible in nature [Miyazaki, Otsuka, 1986; Arbuzova, Khandros, 1964], so the entire shape memory effect mainly occurs in this type of alloys. The second condition is necessary because in the case of high stress and every type of deformation mode (stress-induced martensitic transformation in parent phase, twinning in the martensitic phase) slip can be induced, resulting in plastic strain and, not completed recovery of shape.

In the one-way shape memory effect, the shape in memory by the SMA is the one of the parent phase.

1.3.3 Two-Way SME

The two-way shape memory effect is the reversible transition from a shape at a high temperature to another shape at low temperature under stress.

The two-way shape memory effect is achieved when a sample under deformation is subjected to heating [Nagasawa et al., 1974]. Training of SMAs consists of temperature cycling at constant stress or stress cycling at constant temperature. During training, microstructural defects (i.e. dislocations) lead to internal stresses and therefore promote oriented martensite. A SMA subjected to training can then move from austenitic phase to oriented martensite under zero load by simple change of temperature [Schroeder, Wayman, 1977]. It has then a shape in memory for each of the two phases. However, there is limit on how much strain could be recovered in the two-way SME, which is about 2%.

Various methods that cause two-way shape memory effect have been suggested, such as, large deformation in stress-induced martensite transformation at temperatures > MS [Delaey et al., 1974], shape memory effect training [Schroeder, Wayman, 1977], stress-induced martensite training [Schroeder, Wayman, 1977], training involving both of shape memory effect as well as stress-induced martensite [Perkins, Sponholz, 1984] remaining in martensite state while heating at a temperature > AF [Takezawa, Shindo, Sato, 1976], as well as using precipitates [Tadaki, Otsuka, Shimizu, 1988].

1.4 Transformation Induced Plasticity (TRIP)

Several experimental studies have shown the development of nonlinear plastic (irreversible) strain when phase transformations occur [Greenwood, Johnson, 1965; Abrassart, 1972; Magee, 1966; Desalos, 1981; Olson, Cohen, 1986; Denis et al., 1982]. This mechanism of deformation is termed Transformation Induced Plasticity (TRIP), resulting from internal stress rising from the change in volume related to the transformation, as well as from the associated change in shape [Marketz, Fischer, 1994]. TRIP differs from classical plasticity. Although plasticity is caused from the applied stress or variation in temperature, TRIP is triggered by phase variations, and occurs even at low and constant stress levels [Gautier et al., 1989; Leblond et al., 1989; Gautier, 1998; Tanaka, Sato, 1985; Fischer et al., 1996 & 2000]. TRIP takes place because of two separate mechanisms. The first, refers to a process of accommodation of micro-plasticity related to volume change [Greenwood, Johnson, 1965]. The other, refers to an orientation caused by shear internal stresses, favoring the direction of preferred orientation for the formation of martensite when and external stress is present, which involves change in shape [Magee, 1966]. TRIP is due to the difference in compactness of the lattice structure between the austenite (parent) and the martensite (product) phase [Greenwood, Johnson, 1965]. During martensitic transformation, this difference has produces a change in volume as well as internal stresses causing plasticity in the phase with less yield stress, which is weaker  [Paiva, Savi, Pacheco, 2005].

1.5 Thermal Cycling

Thermoelastic martensitic transformation cannot be considered as perfectly reversible, since thermal cycling, through which the transformation (direct and reverse) is recurring, finally will result in accumulating of a great number of dislocations in parent phase. This change in the microstructure of the parent phase of the SMA will cause shifting of transformation temperature [Tadaki, Otsuka, Shimizu, 1988]. For example, when single crystal Cu-Al-Ni SMA is subjected to thermal cycling with a large number of cycles (more than 100), the MS temperature is significantly lowered [Nakata, Tadaki, Shimizu, 1985]. In other SMAs, such as Cu-Zn-Al, is was shown that the effect of thermal cycling effects depend on the crystallographic order of the parent phase (i.e MS temperature of an alloy with a B2 order raises with several thermal cycles, but MS temperature of an alloy with a D03 order lowers). Such modifications in the MS temperature may be described if one considers the disorder in parent phases [Tadaki, Takamori, Shimizu, 1987].

The effect of thermal cycling in TiNi SMA is determined on the thermo-mechanical treatments [Miyazaki, Igo, Otsuka, 1986]. SMAs that have been treated by solution are more influenced by thermal cycling than those that are annealed after cold work. Moreover, extending thermal cycling to more than 100 cycles, causes important shifts to the transformation temperature of R phase, in addition to MS temperature. This is also true for aged Ni-rich TiNi SMA, where MS temperature reduced, while the transformation temperature of R phase raised [Jara et al., 1982]. Such change in the transformation temperature should be considered when accounting for the relaxation of tensile stresses near Ti3Ni4 precipitation [Tamura, Suzuki, Todoroki, 1986].

1.6 Transformation Induced Fatigue in SMAs

SMA materials often used as sensors and actuators, applications requiring multiple cycles. In such cases, it is important to examine the cycling effect to fatigue behaviour induced by transformation [Kumar, Lagoudas, 2008]. The behaviour of SMAs in fatigue is depending on processing conditions of the material (such as the process of fabrication and heat treatments), on the loading condition (such as stress/strain levels environmental conditions, and variations in temperatures), and microstructural changes induced by transformation (such as grain boundary defects) [Miller, Lagoudas, 2001; Rodriguez, Guénin, 1991; McNichols, Brookes, Cory, 1981]. In sensor/actuator applications of SMAs, many transformation cycles applied via repetition of loading configuration exhibiting phase transformation induced thermally or pseudoelastic effect under loading. The repetition of loading produces progressive changes in the microstructure. Such changes cause the SMA behaviour to degrade in a low cycle fatigue regime (differently to what happens in loading configurations at the elastic regime of the material, where degradation is caused by high cycle fatigue) [Melton, Mercier, 1979; Tobushi et al., 1998; Miller, 2000].

The characteristics of the microstructure (such as crystallographic orientation and precipitation size) also the fatigue life of SMA materials [Gall, Maier, 2002]. Another factor that influences SMA fatigue behaviour is the heat treatment of the material, which, under optimum conditions, can increase SMA resistance to fatigue. Nevertheless, elevated temperatures of annealing can promote oxidation or corrosion, leading to cracking, thereby decreasing SMA fatigue life [Tobushi et al., 1997; Bertacchini et al., 2003; Eggeler et al., 2004; Predki, W., M. Klönne, A. Knopik, 2006].

1.7 Types of SMA materials

The three main SMA material groups are:

  • Iron-based SMAs
  • Copper-based SMAs
  • Nickel-based SMAs

1.7.1 Iron-based Shape Memory Alloys

The two main iron-based SMAs are Fe-Ni-Co-Ti and Fe-Mn-Si. are the main ferrous SMAs. The first, exhibits SME (hysteresis of about 150oC) after it is subjected to thermos-mechanical treatment. The second contains silicon for improving the SME and increase slip stress in austenite [Tamarat et al., 1991; Kajiwara, 1999]. The transformation strains in ferrous SMAs is about 2.5-4.5% [Otsuka, Wayman, 1999]. This type of SMAs finds application mainly in civil engineering. The amount of iron (60-65%) in this type of alloy offers a combination of low cost with increased Young’s modulus and strength. Fe-Mn-Si alloys are suitable for using the one-way SME.

The Fe-Mn-Si alloys have a face-centered cubic crystal structure in the austenitic state and hexagonal close packed (HCP) in the martensitic state. Although the crystal structure of ternary Fe-Mn-Si is exhibiting no notable thermoelastic properties, it is possible to bring about the formation of martensite by an external stress [Sato, Yamaji, Mori, 1986]. For this reason, the cooling of the alloy below MS is necessary in order to provide the metastable state as a prerequisite for the transformation, but not a sufficient condition. The transformation occurs only after a certain external stress has been applied. This stress is responsible for the formation of exactly one martensite variant within the hcp structure. This type of martensite formation is elementary to recover the pseudoplastic deformations upon heating above AF. During the recent years, Fe-Mn-Si alloys with a variety of supplementary alloying compounds were developed, in order to surpass their weak SME and low resistance to corrosion. By adding up to 10% of nickel and chromium, corrosion properties equivalent of those of stainless steel could be obtained [Li, Dunne, 1997]. By adding other compounds (such as Al, Nb, Cu, Co, C, V, etc.) researchers could improve the SME [Farjami, Hiraga, Kubo, 2004; Yakovenko et al., 2003]. Moreover, several treatments were used to impove the SME, such as thermos-mechanical training, precipitation hardening, etc. [Yoneyama et al., 2003].

Most of iron-based alloys are non-thermoelastic. Nevertheless, the Fe-Ni-Co-Ti alloy exhibits thermos-elastic transformation with recovery stress up to 1 GPa [Wei, Sandstroröm, Miyazaki, 1998], and is also resistant to corrosion.

1.7.2 Copper-based Shape Memory Alloys

Copper-based SMAs offer some attractive properties such as excellent thermal and electric conductivity. These materials have transformation temperatures that depend on their composition. A change of the order of 10−3 – 10−4 (at.%) maybe needed to get transformation temperatures that are reproducible in a range of 5 oC. The most common copper-based SMAs are Cu-Al and Cu-Zn systems [Kumar, Lagoudas, 2008].

Cu-Al-Ni SMAs are less affected by aging. A change in Al or Ni content will influence the transformation temperature of Cu-Al-Ni. If the Al content changes by 14 – 14.5 (at.%), the MS shifts from -140 to 100 oC. Manganese is frequently added for improving ductility and Ti is added for refining the microstructure. The main shortcoming in using the Cu-Al-Ni SMA is its weak ductility because of intergranular fracture behaviour [Funakubo, 1987]. The phenomenon influences also the mechanical behaviour so that the material fails at stress levels approximately 280MPa. The transformation strain in those materials is typically less than 3%. The material also behaves poorly in cyclic loading [Funakubo, 1987].  In recent years, numerous other copper-based SMA are being developed (Cu-Al-Mn, which exhibits improved ductility, Cu-Al-Nb, which is suited for high-temp applications) [Torra et al., 2009].

Cu-Zn SMAs are ductile and resistant to intergranular cracking, compared to other copper-based SMA [Otsuka, Wayman, 1999]. These SMAs exhibit matrensitic transformation below the room temperature. The transformation temperatures can be raised by adding Al to the binary system. If the Al content changes by 5-10 (wt.%) the MS changes from -180 to 100 oC.  Nevertheless, this SMA is affected by overheating or aging. Therefore, the temperatures of operation are restricted to about 100 oC. In addition, the transformation temperatures of this SMA are very sensitive to the composition, hence its fabrication process must be controlled accurately. Cu-Zn-Al SMAs are also sensitive to heat treatment. They have mediocre mechanical properties (stress level limited to about 200 MPa). However, inside its stress range, it has a good SME and pseudoelastic effect. The transformation strains for those materials is typically less than 3-4% [Otsuka, Wayman, 1999].

1.7.3 Nickel-Titanium-based Shape Memory Alloys

The SME in a nickel-titanium (Ni-Ti) alloy was reported in 1963 [Otsuka, Kakeshita, 2002]. Binary Ni-Ti as well as ternary Ni-Ti-X alloys are certainly the most researched types of SMAs (X stands for possible additional alloying elements). These alloys can change between austenite and martensite and vice versa just by changing temperature. Therefore, they belong to the group of thermoelastic alloys. Reviews on Ni-Ti alloys can be found in [Duerig et al., 1990; Otsuka, Xu, Ren, 2003] and with special regard to fatigue in [Predki, Klönne, Knopik, 2006].

The crystal lattice of Ni-Ti is body centered cubic (BCC) in the austenite phase and face centered cubic (FCC) in the martensite phase. A composition with about 49 to 51 (atomic) % Ni in the binary alloy is appropriate in order to avoid precipitates of undesirable intermetallic phases of nickel and titanium [Duerig et al., 1990]. The composition can be adjusted in order to use the alloy in actuator, superelastic, shape memory and martensite-damping applications. Such applications can then be used at the usual ambient temperatures in civil structures. Self-accommodated martensite is formed in Ni-Ti when cooling the material below MS. This is an important prerequisite for an excellent pseudoplastic deformability. Strains up to 8% can then be recovered by shape memory effect. Recovery stresses up to 900 MPa for constrained recovery are possible [Otsuka, Xu, Ren, 2003].

Due to its excellent deformation behaviour and a very good fatigue resistance, Ni-Ti was successfully used in several studies on damping. However, less costly SMAs, like Cu-Zn-Al and Cu-Al-Ni, have showed very good damping properties as well [Huang, 2002; Van Humbeeck et al., 1995]. The following statements for superelastic damping behaviour of usual Ni-Ti alloys can be found in the literature [Hodgson, 2002]. Strains up to 8% for tension and 12% for torsion can be recovered. The first 1.5 to 2% of strain are Hookian elasticity. Several hundreds or thousands of superelastic cycles can be carried out if the strains are limited to values of 3 to 6%. The martensitic damping capacity of Ni-Ti rises with increasing strains [Van Humbeeck, 2001]. First cycle effects occur; thus the modification of stress-strain curve, in particular within the initial 10 to 20 cycles [Saadat et al., 2002]. The addition of Cu (copper) as X to the Ni-Ti-X system results in a very small temperature hysteresis of only a few Kelvin, which is good for actuator applications. When adding Nb (niobium), one can widen the temperature hysteresis up to 145K. This can be desirable for applications aimed at permanent recovery stress [Otsuka, Wayman, 1999].

The fact that the Ni-Ti SMA system has strong pseudoelasticity effect, SME, and TWSME under specific conditions, makes this alloy attractive for many applications. In addition, the corrosion resistance is an important advantage of Ni-Ti SMAs. Their thermo-mechanical behaviour, heat treatment effects, as well as the transformation temperature variation with composition changes, are better understood compared to other SMAs [Kumar, Lagoudas, 2008].

The Ni-Ti SMA in equiatomic composition (50 at% Ni – 50 at% Ti) has maximum AF (120 oC) compared to any other composition [Buehler, Gilfrich, Wiley, 1963]. If the Ni content decreases from equiatomic composition, will not affect the transformation temperature. By increasing the Ni content above 50 (at.%), it will cause the transformation temperature to reduce, and AF will be as low as -40oC when the Ni content is at 51 (at.%). By varying the alloy composition, the SMA characteristics at room temperature can change from shape memory effect to pseudoelastic effect [Richardson, 2001]. The Ni-Ti SMAs have recoverable transformation strains (about 8%) and can have a variety of complex forms and shapes because they do not need cold working [Clingman, Calkins, Smith, 2003; Mabe, Ruggeri, Calkins, 2006]. In addition, Ni-Ti SMAs have excellent corrosion resistance (more than that of stainless steel in chloride environments [Richardson, 2001]. The transformation temperatures of Ni-Ti SMAs can be easily controlled using heat treatment.

Ni-rich SMAs can form Ti3Ni4 precipitates, through temperature aging at 400 oC [Miyazaki et al., 1982; Tadaki et al., 1986]. The formation of precipitates generate stresses that can lead in forming a phase of rhombohedral lattice (R-phase) between the austenitic phase and the martensitic phase. This phase is disappearing with the heat treatment at high temperature [Dunne, Wayman, 1973].

Intermetallic materials are usually brittle, however, Ti-Ni SMAs are quite ductile, since an elongation greater than 50% can be easily achieved. The high ductility of these materials is directly related to martensite transformation [Ohba, Emura, Otsuka, 1992].

By adding other compounds to the binary Ni-Ti system to replace Ni, other ternary SMAs can be obtained. The addition of copper forms Ni-Ti-Cu alloys. This can reduce the pseudoelastic hysteresis (advantage for actuation devices), but also reduces the transformation strain [Nam, Saburi, Shimizu, 1990; Saburi et al., 1989], and lowers the detwinning stress level [Matveeva, Khachin, Shivokha, 1985]. The addition of Niobium, however, will cause to broadening the thermal hysteresis of the binary Ni-Ti [Melton, Simpson, Duerig, 1986; Melton, Proft, Duerig, 1988]. An example of such material is Ni47Ti44Nb9 [Zhao et al., 1990]. This can have advantages for applications requiring the material to exhibit small response to a wide change in temperature. This will allow the SMA to deform at small temperatures, and also to be transported safely at room temperature. Lately, this type of SMAs have been developed with lower Niobium content (3 at.%), which exhibit good SME behaviour [He et al., 2004].

1.7.4 Other SMA systems

Except the three main groups of SMA materials there are other, such as:

Co-Ni-Al SMAs (such as the CoNi33Al29), which have good corrosion resistance at high temperature, interesting magnetic properties, but a brittle nature [Enami, Menno, 1971; Morito et al., 2002]. In recent years, there are efforts to control the intermetallic phases using heat treatment in an effort to improve its ductility [Kainuma et al., 1996; Schryvers et al., 2002].

Other class of SMA materials is the magnetic SMAs, such as the Ni-Mn-Ga [Webster et al., 1984; Miyazaki et al., 1999], Fe-Pd [Cui, Shield, James, 2004; James, Wuttig, 1998; Shield, 2003; Yamamoto et al., 2004], Co-Ni-Ga, and Ni-Mn-Al [Murray et al., 1999; Fujita et al., 2000; Wuttig, Li, Craciunescu, 2001]. These materials, however, have important limitation, which is their low blocking stress, which is about 6-10 MPa (i.e stress where magnetic re-orientation strain entirely suppresses). Ni2MnGa SMAs exhibit pseudoelastic transformation when subjected to compressive strains about 4% [Karaca et al., 2007].

High-Temperature SMAsis another class of materials, such as Ni-Ti-X SMAs (X can be Au) [Bozzolo et al., 2006], Pd [Wu, Tian, 2003; Golberg et al., 1995a; Golberg et al., 1995b; Shimizu et al., 1998; Shirakawa, Morizono, Nishida, 2000; Tian, Wu, 2002a; Tian, Wu, 2002b; Tian, Wu, Cheng, 2003; Noebe et al., 2006; Bigelow et al., 2007; Lo, Wu, Wayman, 1999; Yang, Mikkola, 1993; Cai, Otsuka, 2001], Zr [Firstov, Van Humbeeck, Koval, 2004; Sawaguchi, Sato, Ishida, 2004], Hf [Meng et al., 2006; Kockar et al., 2006], Pt [Noebe et al., 2005; Noebe et al., 2008], that have various limitations, problems, unknown yet critical characteristics (precipitate properties, trainability, low cycle fatigue behaviour, two-way SME, etc.), since the published work on these is rather limited [Bigelow et al., 2011; Meng et al., 2006; Meng, 2008].

 

 

2. PATENT SEARCH ON SMAS

More than 20,000 patents have been issued in the world until now on SMA materials and applications, more than half of them in the United States alone. It must be noted, however, that the realization of sustainable products from all these patents has been rather limited [61; 80; 81]. This is mainly due to absence of basic understanding by engineers and scientists on the technical shortcomings of SMA materials as well as the methods for applying SMAs in a reliable way to attain technical requirements for sustainable development [69; 80; 81; 82; 83].

In particular, patents issued the past 30 years related to NiTi SMAs respond to increasing market demands for more compact and lightweight actuators, particularly for biomedical applications.

Figure 3 presents an analysis has been performed in [Mohd Jani, 2014] using the “United States Patent and Trademark Office (USPTO) search engine”, using the keywords “nitinol” or “shape memory alloy” for associated application areas. It is interesting to observe that that the great majority of all patents issued is for biomedical applications, while aerospace applications represents less than 5% of the patents, and automotive and robotics applications about 2% and 3%, respectively.

Figure 3 United States patents (1990-2013) on SMAs (source USPTO) [Mohd Jani, 2014]

A patent search performed for the years 2014-2017 did not alter the trend presented in Figure 3.

Selected, representative SMA patents are presented and shortly described below, in particular concerning SMA thin films and NiTi material, for the aerospace, automotive, biomedical, and robotics fields.

[1] Patent 4,864,824 “Thin film shape memory alloy and method for producing” (1989)

The invention relates to the area of micromechanics, particularly to using SMAs to miniaturize devices and actuators. An actuating device made from thin film SMA material is mounted on a substrate. The device made from thin film is deformed, and then heated to reinstate its initial shape. During deformation and restauration of shape, motion is created resulting to producing certain work.

This is a typical example of using SMAs in a thin film configuration for developing actuators, but not for structural element applications.

[2] Patent 6,358,380 B1 “Production of binary shape-memory alloy film by sputtering using a hot pressed target” (2002)

This patent concerns a method for depositing NiTi SMA thin films, by ion-sputtering process using metal powder hot pressed on the target.

This is an interesting method for thin film deposition on selected substates at the micro-scale.

[3] Patent 5,061,914 “Shape-memory alloy micro-actuator” (1991)

This patent concerns an actuating device for achieving motion of a MEM using SMA materal a actuator. The patent also describes a method for developing thin films of SMAs. This is another example of developing SMA thin films for actuators.

[4] Patent 5,825,275 “Composite shape memory micro actuator” (1998)

This patent concerns SMA multi-morph actuators in the micro-scale made in the form of SMA thin film deposited on a substrate film. This patent aims in manufacturing better switches by using stress-compensating film. This patent is representative to the development of improved, miniaturized actuators.

[5] Patent 5,325,880 “Shape memory alloy film actuated microvalve” (1994)

This patent aims in developing micro-valves for controlling fluids, gas or liquid. The purpose of the patent is to miniaturize electrically actuated valves using SMA thin films. The rather large displacement and force attained using SMA thin films allows for higher flow compared to equally sized valves. Using SMAs for miniaturization and actuation purposes is again the objective of this patent.

[6] Patent WO2001053559 “Thin-film shape memory alloy devices and method” (2011)

This patent is relating to fabricate intravascular stents made by SMA thin films, using sputtering process. This is typical example of developing thin films for biomedical applications for miniaturized devices.

[7] Patent 4,505,767 “Nickel/titanium/vanadium shape memory alloy” (1985)

This patent concerns the development of an improved NiTi-based SMA by adding vanadium at an amount of 4.6-25 (at.%). The patent, however, is not related to developing thin films or coatings from such ternary alloy systems.

[8] Patent 4,770,725 “Nickel/titanium/niobium shape memory alloy” (1988)

This patent concerns the development of an improved NiTi-based SMA by adding niobium at an amount of 2.5-30 (at.%). The patent, however, is not related to developing thin films or coatings from such ternary alloy systems.

[9] Patent 4,565,589 “Nickel/titanium/copper shape memory alloy” (1986)

This patent concerns the development of a stabilized NiTi-based SMA by adding copper at an amount of 7.5-14 (at.%), improving machinability and workability. Such SMAs find application in bulk material forms and are not suited for thin films or coatings.

[10] Patent 8,966,893 “Shape memory alloy actuators with sensible coatings” (2012)

This patent involves SMA linear actuators, like strips, cables, or wires. Methods of depositing sensible coatings to SMA actuators are disclosed. These (non-SMA) coatings make the linear SMA materials able to be used as sensors. These actuators when are couples with a controller device they can be operated with proportional control. This is an interesting patent on applying non-SMA coatings to SMAs with a linear shape, however, the patent is not related to the technology developed under the InnoSMART project.

[11] “Patent 6,509,094 B1 Polyimide coated shape-memory material and method of making same” (2003)

This patent deals with depositing polymeric (polyimide) coatings on SMA materials. These coatings are able to endure higher temperatures, which is necessary to form SMA materials in the anticipated configuration. Again, this is an example of a patent of applying a non-SMA coating to SMA materials.

[12] Patent 4,965,545 “Shape memory alloy rotary actuator” (1990)

This patent concerns rotary actuators producing rotating movement with important mechanical motion. Applications include circuit breakers, switches, controls, etc. This is another example of using SMA materials as actuators.

[13] Patent 5,092,781 “Electrical connector using shape memory alloy coil springs” (1992)

This patent also concerns an SMA actuator device. The patent deals with an electrical connector that uses a spring system made from SMA material, operated upon variation of temperature.

[14] Patent 6,151,897 “Shape memory alloy actuator” (2000)

This patent concerns again a remote miniaturized actuator device that is optically controlled and uses SMA material activated by the presence of thermal energy induced by an optical source or high temperature gas. This can find application in aerospace, since in aircraft powered by gas turbines the compressor (or sections in the turbine) can be a hot source that controls the device.

[15] Patent 4,930,494 “Apparatus for bending an insertion section of endoscope using a shape memory alloy” (1990)

This patent concerns biomedical application of SMAs. The device is based on the principle that a change in temperature produces a change in shape of an SMA, thereby driving loads or causing self-operation of the apparatus.

[16] Patent 4,984,581 “Flexible guide having two-way shape memory alloy” (1991)

This patent also concerns biomedical application of SMAs. It respond to a need from flexible and elongated guides for medical operations for precise endoscopic access to areas of the human body without surgery.

[17] Patent 5,645,520 “Shape memory alloy actuated rod for endoscopic instruments” (1997)

This patent also concerns an endoscopic apparatus made from an SMA, which changes shape with temperature changes.

[18] Patent US 2004/0093017 A1 “Medical devices utilizing modified shape memory alloy” (2004)

This patent is also a representative of the many SMA-related inventions for biomedical applications.

[19] Patent 5,114,504 “High transformation temperature shape memory alloy” (1992)

This patent concerns application of a new material, a Ni-Ti SMA containing 0.1 (at.%) hafnium. The advantage of the new material is its elevated transformation temperatures. This SMA can be easily cold and hot worked to be used in wire or spring form. Our application for SMA coatings under the InnoSMART project does not concern high temperature SMAs.

[20] Patent 5,843,244 “Shape memory alloy treatment” (1998)

This patent concerns a method for treating components made of Ti-Ni SMAs in order to exhibit superelasticity (i.e. improved elastic property). The component is subjected to cold work followed by annealing, while it is retained at a restrained state, then exposed to a higher temperature than annealing, causing AF to decrease. This patent is an example of inventions dealing with material treatment, however, it finds application mainly in bulk material forms.

[21] Patent US 2014/0026554 A1 “Superelastic shape memory alloy overloading and overheating protection mechanism” (2014)

This patent concerns methods and devices for protecting an actuator from overheating or overloading, by using superelastic SMA actuation. The disclosure is representative of the numerous inventions and applications dealing with using SMA materials as actuators.

[22] Patent US 9,581,145 “Shape memory alloy actuation system for variable area fan nozzle” (2017)

This patent concerns an aerospace application. It describes an actuation system that can deform its shape using SMA material (rather than mechanical movement) to reduce aerodynamic drag. Due to the fact that this actuation system has no moving or rotating parts, there is less maintenance needed due to limited wear.

In conclusion:

  1. Some patents deal with the production process of SMA thin films [patents 1, 2], other deal with using thin film SMA as micro-actuators [patents 3, 4 and 5], and a recent patent deals with using thin film shape memory alloys as intravascular stents for medical applications [patent 6].
  2. Related to Ni-Ti SMA systems, there are a few older patents dealing with the development of Ni-Ti-based alloys with improved properties. In this regard, a patent deals with the nickel/titanium/vanadium system [patent 7], another deals with the Nickel/titanium/niobium SMA system [patent 8], and a third with the nickel/titanium/copper SMA system [patent 9].
  3. Recent patents deal with the application of coatings on shape memory alloy actuators [patents 10 and 11].
  4. Some patents deal with using SMA as actuators for a variety of applications [patents 12 and 13], or developing a microactuator control apparatus using shape memory alloy [patent 14].
  5. Other patents deal with using SMA to build medical devices such as endoscopes, etc. [patents 15, 16, 17 and 18].
  6. A patent deals with the development of a nickel-titanium based shape memory alloy for high temperature applications [patent 19].
  7. Another patent deals with a method of treating a component formed from a Ni-Ti based shape memory alloy [patent 20].
  8. Finally, a patent deals with the overloading and overheating protection mechanism of a SMA actuator [patent 21] and another using shape memory alloy actuation system for variable area fan nozzle [patent 22].

The extensive patent search on Shape Memory Alloys led to the following interesting observations:

  • The vast majority of SMA-related patents deals with using SMAs in actuating devices. Very limited invention disclosures exist for using SMA materials in bulk form in structural components to enhance mechanical or fatigue properties.
  • There are few patents related to the use of shape memory alloys as thin films. Thin films are deposited on substrates but they are not considered as coatings, since their purpose is to function alone and not as part of a film/substrate structure (i.e. are not intended for structural applications by design). SMA thin films are used as miniaturized actuators.
  • There is no patent dealing with using shape memory alloys as coating materials (i.e.  as part of a coating/substrate structure for structural applications).


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