Novel Research for Development of Shape Memory Alloys

Shape memory alloys have attracted much attention due to their attractive properties for applications as well as their basic aspects of deformation and transformation in structural and magnetic behavior. In 1951, the Au–Cd alloy was discovered [1]. After that, numberless shape memory alloys have been developed. A lot of applications of shape memory alloys were realized after the Ti–Ni alloy was found in 1963 and developed extensively in 1980s [2]. Recently, ferromagnetic shape memory alloys (FSMA) have been studied as candidates for highly functional materials [3]. Among FSMA, Ni2MnGa is the most renowned [4]. The magnetostriction, or magnetic field induced strain (MFIS) is most featured phenomena. Few % MFIS were found for some Ni2MnGa type single crystals. MFISs of 6.0% have been produced at room temperature in single crystals of Ni49.8Mn28.5Ga21.7 (TM = 318 K) by application of fields of order 400 kA/m (=0.50 T) under an opposing stress of order 1 MPa [5]. Fe–Pd and Fe–Pt alloys are also famous FSMA for MFIS. The strain is the result of field-induced twin boundary motion under an atmospheric pressure. A disordered Fe-31.2%Pd (at %) alloy (A1-type cubic) [6,7], and an ordered Fe3Pt (L12-type cubic) ferromagnetic alloy [8,9], have attracted much interest due to the large MFISs. The alloys which was doped other elements are also studied. The magnetostriction studies on the premartensite phase of related Cr-substituted Ni2Mn1−xCrxGa alloys were studied and robust 120 ppm magnetostriction was observed [10]. New alloys in the Ni–Mn–In, Ni–Mn–Sn, and Ni–Mn–Sb Heusler alloy systems that are expected to be ferromagnetic shape memory alloys have been studied [11]. A re-entrant magnetism was observed in some alloys [12,13]. These alloys are promising as a metamagnetic shape memory alloys with a magnetic field-induced shape memory effect and as magnetocaloric effect [14]. Consequently, these materials are finding use or are candidates as materials for sensors, actuators, magnetic refrigerator, etc. [15]. The dynamical functionality of the nickel–titanium (NiTi)-based alloy comes into the limelight. NiTi is an attractive alloy due to its unique functional properties, for example, shape memory effect (SMA), elastic deformation, super-elasticity, low stiffness, and damping characteristics [16,17]. In this special issue, physical properties are also shown for industrial objects, as joining between the different kind of metals, SMA bolts, and tubes. In our department in Ryukoku Univertsity, we are focused on robotics and applications (mechanical actuators). Therefore, our colleagues shed light on dynamic applications for SMAs.


Introduction and Scope
Shape memory alloys have attracted much attention due to their attractive properties for applications as well as their basic aspects of deformation and transformation in structural and magnetic behavior.In 1951, the Au-Cd alloy was discovered [1].After that, numberless shape memory alloys have been developed.A lot of applications of shape memory alloys were realized after the Ti-Ni alloy was found in 1963 and developed extensively in 1980s [2].Recently, ferromagnetic shape memory alloys (FSMA) have been studied as candidates for highly functional materials [3].Among FSMA, Ni 2 MnGa is the most renowned [4].The magnetostriction, or magnetic field induced strain (MFIS) is most featured phenomena.Few % MFIS were found for some Ni 2 MnGa type single crystals.MFISs of 6.0% have been produced at room temperature in single crystals of Ni 49.8 Mn 28.5 Ga 21.7 (T M = 318 K) by application of fields of order 400 kA/m (=0.50 T) under an opposing stress of order 1 MPa [5].Fe-Pd and Fe-Pt alloys are also famous FSMA for MFIS.The strain is the result of field-induced twin boundary motion under an atmospheric pressure.A disordered Fe-31.2%Pd(at %) alloy (A1-type cubic) [6,7], and an ordered Fe 3 Pt (L12-type cubic) ferromagnetic alloy [8,9], have attracted much interest due to the large MFISs.The alloys which was doped other elements are also studied.The magnetostriction studies on the premartensite phase of related Cr-substituted Ni 2 Mn 1−x Cr x Ga alloys were studied and robust 120 ppm magnetostriction was observed [10].New alloys in the Ni-Mn-In, Ni-Mn-Sn, and Ni-Mn-Sb Heusler alloy systems that are expected to be ferromagnetic shape memory alloys have been studied [11].A re-entrant magnetism was observed in some alloys [12,13].These alloys are promising as a metamagnetic shape memory alloys with a magnetic field-induced shape memory effect and as magnetocaloric effect [14].Consequently, these materials are finding use or are candidates as materials for sensors, actuators, magnetic refrigerator, etc. [15].
The dynamical functionality of the nickel-titanium (NiTi)-based alloy comes into the limelight.NiTi is an attractive alloy due to its unique functional properties, for example, shape memory effect (SMA), elastic deformation, super-elasticity, low stiffness, and damping characteristics [16,17].In this special issue, physical properties are also shown for industrial objects, as joining between the different kind of metals, SMA bolts, and tubes.In our department in Ryukoku Univertsity, we are focused on robotics and applications (mechanical actuators).Therefore, our colleagues shed light on dynamic applications for SMAs.

Contributions
Xu et al. prepared Cu 71.5 Al 17.5 Mn 11 shape memory alloy by directional solidification by means of unique homemade equipment [18].A large maximum recoverable strain of more than 11% was maintained due to the retained beneficial grain characteristics.Good superelastic behavior was observed.
Hu et al. investigated microstructural evolution of NiTi shape memory alloy (SMA) on the basis of heat treatment and severe plastic deformation (SPD) [19].Consequently, SPD and subsequent aging contribute to enhancing the transformation temperature of martensite.The relation between softening of elastic constants and martensitic transformation has attracted considerable attention for many years and has been discussed [20].Fukuda et al. investigated the relation between the softening of elastic constants and martensitic transformation in Fe 3 Pt [21].The softening in shear elastic coefficient C = (C 11 −C 12 )/2 is probably most strongly related to the formation of the face-centered-tetragonal with c/a < 1 (FCT1) martensite.This result implies that softening is most strongly related to the formation of the martensite.
Zhang et al. studied about physical mechanism for dynamic recrystallization of NiTi shape memory alloy subjected to local canning compression at various temperatures, 600, 700, and 800 • C [22].In the case of 600 and 700 • C, continuous dynamic recrystallization and discontinuous dynamic recrystallization coexist in NiTi shape memory alloy.In the case of discontinuous dynamic recrystallization, the recrystallized grains are found to be nucleated at grain boundaries and even in grain interiors.
Jiang et al. studied about NiTi shape memory alloy (SMA) tube, which was coupled with mild steel cylinder in order to investigate deformation mechanisms of NiTi SMA tubes undergoing radial loading [23].Microstructure was characterized by transmission electron microscope.When NiTi SMA tube is subjected to radial loading, strain induced martensite transformation is of great significance in the superelasticity of NiTi SMA, and reorientation and detwinning of twinned martensite lays a profound foundation for the shape memory effect of NiTi SMA.
Shiue et al. studied the infrared dissimilar joining Ti 50 Ni 50 and 316L stainless steel using Cu foil in between Cusil-ABA and BAg-8 filler metals [24].This study indicates great potential for industrial applications.
Jiang et al. studied the deformation behavior and microstructure evolution of NiTiCu SMA, which possesses martensite phase at room temperature based on a uniaxial compression test at the temperatures of 700~1000 • C [25].Dislocations become the dominant substructures of martensite phase in NiTiCu SMA compressed at 700 • C. Martensite twins are dominant in NiTiCu SMA compressed at 800 and 900 • C. The microstructures resulting from dynamic recovery or dynamic recrystallization significantly influence the substructures in the martensite phase of NiTiCu SMA at room temperature.
Liang et al. studied NiTiFeNb and NiTiFeTa SMAs [26].The microstructure, mechanical property, and phase transformation of NiTiFeNb and NiTiFeTa SMAs were investigated.As compared to NiTiFe SMA, quaternary NiTiFeNb and NiTiFeTa SMAs possess the higher strength, since solution strengthening plays a considerable role.
The processing map of Ni 47 Ti 44 Nb 9 (at %) shape memory alloy (SMA), which possesses B2 austenite phases and Nb phases at room temperature, is established by Wang et al. in order to optimize the hot working parameters [27].
Hu et al. investigated the texture evolution of NiTi shape memory alloy during uniaxial compression deformation at 673 K by combining crystal plasticity finite element method with electron back-scattered diffraction experiment and transmission electron microscope experiment [28].Using the fitted material parameters, a crystal plasticity finite element method is used to predict texture evolution of NiTi shape memory alloy during uniaxial compression deformation.The simulation results agree well with the experimental ones.With the progression of plastic deformation, a crystallographic plane of NiTi shape memory alloy gradually rotates to be vertical to the loading direction, which lays the foundation for forming the <111> fiber texture.
Mitsui et al. observed field-induced reverse transformation in Co-doped Ni-Mn-In film by means of high field X-ray diffraction experiments under magnetic fields up to 5 T and temperature ranging from 293 to 473 K [29].The reverse martensitic transformation induced by magnetic fields was directly observed in situ HF-XRD techniques.Superelastic SMA bolts, which have a recentering capability upon unloading, are fabricated by Seo et al. as to solve drawbacks, and utilized by replacing conventional steel bolts in the partially restrained bolted T-stub connection [30].[31].The Cu-xZn-11Al SMAs are promising for practical high-damping applications under isothermal conditions because they possess good workability, low cost, acceptable mechanical properties, and the high damping capacities.

Wu et al. investigated damping characteristics of inherent and intrinsic internal friction of Cu-Zn-Al Shape Memory Alloys
Adachi et al. made the phase diagram of Ni 2 MnGa 1-x Fe x on the basis of the experimental results [32].The magnetostructural transitions were observed at the Fe concentrations x = 0.275, 0.30, and 0.35.The obtained phase diagram was very similar to that for Ni 2 Mn 1−x Cu x Ga by Kataoka et al. [33].The theoretical analysis of the phase diagram for Ni 2 Mn 1−x Cu x Ga showed that the biquadratic coupling term of the martensitic distortion and magnetization plays an important role in the interplay between the martensitic phase and ferromagnetic phase.Further theoretical investigation of the phase diagram for Ni 2 MnGa 1-x Fe x is needed.
Umetsu et al. performed the specific heat measurements at low temperatures for Ni 50 Mn 50−x In x alloys to determine their Debye temperatures θ D and electronic specific heat coefficients γ, and investigated the change in the density of states during the martensitic phase transformation [34].The γ was slightly larger in the parent phase, in good agreement with the reported density of states around the Fermi energy obtained by the first-principle calculations.The martensite (M) phase (x ≤ 15), θ D decreases linearly and γ increases with increasing Indium content.
Anelastic properties of Ti-Ni-based alloys are widely explored, both for microstructural characterization of the alloys and for their application as high-damping materials [35].Sapozhnikov et al. investigated the linear and non-linear internal friction, effective Young's modulus, and amplitudedependent modulus defect of a Ti 50 Ni 46.1 Fe 3.9 alloy after different heat treatments, affecting hydrogen content [36].They found that the internal friction maximum is associated with a competition of two different temperature-dependent processes affecting the hydrogen concentration in the core regions of twin boundaries.
Carl et al. investigated the SMA with high martensitic transformation temperature [37].Small changes in at % Ni have a dramatic effect on the transformation temperatures of the NiTi-20 at % Zr system, even more so than in binary NiTi.The transformation temperature can be tuned for a given application through aging treatments.Ni-Ti-based shape memory alloys (SMAs) have now become an important technological material for a wide array of applications not only for specifically medical devices [38] but actuators above room temperature [39].
As introduced here, many fundamental research results are shown in 17 articles.There are many SMA materials in which there is possibility of the application for mechanical and/or magnetic actuators, springs, bolts, magnetic refrigeration, sensors, etc.
I hope the results of the research in this special issue contribute to further development of SMAs.