Special Issue "Shape Memory Alloys"

Guest Editor
Prof. Dr. Kurt R. Ziebeck
Department of Physics, Cavendish Laboratory, University of Cambridge, CB3, 0HE, UK
Website: http://www.tfm.phy.cam.ac.uk/directory/kraz2@cam.ac.uk
E-Mail: kraz2@cam.ac.uk
Interests: neutron scattering; low temperature physics; strongly correlated systems; magnetism; functional materials; spintronics; novel materials; spintronics

Dear Colleagues,

More than half a century has elapsed since research on Au-Cd alloys stimulated a world-wide interest in shape memory and super-elastic behaviour. Both the microscopic and mesoscopic transformation processes, associated with the essential martensitic phase transition have been extensively studied in a wide range of materials. Consequently these materials are finding use as sensors, actuators etc., in fields as diverse as medicine and aviation. Although the phase transition is usually thermally or mechanically driven, more recent research has focused on the ability to do this using magnetic fields in ferromagnetic alloys. However many of the materials studied are brittle which would appear to limit their application. This constraint maybe overcome by incorporating the materials in hybrid composites The possibility of doing this or incorporating shape memory alloys with other smart materials opens up new areas of fundamental and applied research.

Prof. Dr. Kurt R. Ziebeck
Guest Editor

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• shape memory
• phase transitions
• magnetism
• transport properties
• neutron
• xray and electron scattering
• smart materials

Type of Paper: Article
Title: Transformation Volume Effects on Shape Memory Alloys
Authors: V.A. Chernenko, V.A. L’vov, E. Cesari, A. Kosogor and J.M. Barandiaran
Affiliation: Dpto de Electricidad y Electronica, Universidad del Pais Vasco UPV/EHU, Apartado 644, E-48080 BILBAO, Spain;
E-Mail: vladimir.chernenko@gmail.com
Abstract: The volume change accompanying the first-order martensitic transformation (MT) attracted a special attention of researchers during the development of the ferromagnetic FeNi- and FeCo-based shape memory alloys (SMAs) some time ago. In spite of this fact it is generally accepted that the MTs in the traditional NiTi- and Cu-based SMAs are mainly characterized by the shear deformation of the crystal lattice () that arises in the course of MT, while a comparatively small volume change during MT () is considered as a secondary effect, which can be disregarded when the basic characteristics of MTs and functional properties of SMAs are analyzed. This point of view is a subject to change nowadays due to the new experimental and theoretical findings about multifunctional materials such as magnetic shape memory (MSM) alloys of Heusler type. In particular, one can find in recent publications dealing with the thermodynamic and elastic properties of real (containing defects) single crystalline MSM alloy specimens, that the volume variation owing to the defect reconfiguration changes noticeably the characteristic MT temperatures and storage elastic moduli. Moreover, the magnetically induced volume change (volume magnetostriction) is found to be responsible for the anomalies of thermodynamic, magnetic and elastic properties of MSM alloys in the vicinity of MT temperature and Curie point. In particular, the volume magnetostriction causes the appearance of the coupled magnetostructural transformation of the ferromagnetic Ni-Mn-Ga alloy if the MT temperature is close to the Curie point.

Type of Paper: Article
Titel: Mössbauer Spectroscopy Studies on Magnetic Properties for 57Fe-substituted Ni-Mn-Sn Metamagnetic Shape Memory Alloys
Authors: R.Y. Umetsu ¹, K. Fukushima ², K. Sano ², T. Kanomata ^3,4, Y. Taniguchi ⁵, Y. Amako ⁵ and R. Kainuma ⁴
Affiliations:¹ Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
² Faculty of Engineering, Tohoku Gakuin University, Tagajo 985-8537, Japan
³ Research Institute for Engineering and Technology, Tohoku Gakuin University, Tagajo 985-8537, Japan
⁴ Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan
⁵ Faculty of Science, Shinshu University, Matsumoto 390-8621, Japan
Abstract: In order to investigate the Fe substituted effects on the magnetic properties of the Ni-Mn-Sn metamagnetic shape memory alloys, Mössbauer spectroscopy experiments were performed with using 57Fe doped specimens, Ni50Mn37.5-x57FexSn13 (x = 0.5, 1.0 and 2.0). Singlet-type Mössbauer spectra were clearly observed for x = 0.5 and 1.0 just below the martensitic transformation temperature and above the Curie temperature in the parent phase. It was suggested that the paramagnetic region was kept in the martensite phase up to x = 1.0. However, existence of the hyperfine field was confirmed in whole temperature range in the martensite phase for x = 2.0. These results obtained from the Mössbauer spectra were consistent with the results of the magnetic measurements and reported phase diagram by Kikuchi et al.

Type of Paper: Article
Title: Towards a New Understanding of the Electrical Resistivity Measurements for Obtaining TiNi Transformation Temperatures
Author: Cristina Urbina Pons
Affiliation: Departament d'Enginyeria Mecànica, Universitat Rovira i Virgili, Spain
Abstract: The objective of this study is to clarify and to improve the interpretation of the ER measurements used to obtain NiTi SMA stress-free transformation temperatures for the austenite to martensite transformation process. To achieve this objective, the transformation temperatures of NiTi SMA wires are measured by ER using the Ling and Kaplow interpretation and are compared and complemented by weight fraction diagrams. The weight fractions are obtained from XRD profiles using the Parametric Rietveld refinement and adequate software. The study used different sets of TiNi wire. One set was heat treated at different temperatures to obtain different transformation paths, whereas the other set followed the same heat treatment followed by repeated thermal cycling at zero stress to develop the R-phase and thus, change the transformation path of the NiTi alloy. As a result of comparing both techniques, a new interpretation of the ER curves is proposed for obtaining R-phase and martensite transformation temperatures. This new interpretation is based on the shape of NiTi ER curve, which depends on the quantity of R-phase in the NiTi SMA. The quantity of the R-phase is obtained by weight fraction diagrams.

Type of Paper: Review
Author: Thermal Transformation Arrest Phenomenon in NiMn-based Heusler Alloys
Authors: Xiao Xu ¹, Rie Y. Umetsu ², Wataru Ito ³ and Ryosuke Kainuma ¹
Affiliations: ¹ Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan
² Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
³ Materials Science and Engineering, Sendai National College of Technology, Natori 981-1239, Japan
Abstract: In NiCoMnIn metamagnetic shape memory alloys, it has been reported that during field cooling under high magnetic field, the martensitic transformation is greatly suppressed by the magnetic field and does not transform to martensite phase even cooled to 4.2~K. If the magnetic field is removed at a low temperature, the parent phase will remain not to transform to martensite phase until heated up. This phenomenon is called the kinetic arrest phenomenon. However, during the investigation of the thermodynamic behaviors at low temperature, we have found in former researches that an obvious thermodynamic competition between the two phases exists, besides the kinetic behaviors. In order to reflect the nature of this phenomenon, which is the combination of both thermodynamic and kinetic effects, we suggest calling this phenomenon the thermal transformation arrest (TTA) phenomenon''. In this paper, we will briefly review the investigations on the TTA phenomenon, including both the thermodynamic and kinetic aspects.

Type of Paper: Article
Title: Influence of the Texture and Strain on the Behaviour of Ni_53.6Mn_27.1Ga_19.3 and Ni_54.2Mn_29.4Ga_16.4 Shape Memory Alloys
Authors: Alexandra Rudajevova ¹, Jiří Pospíšil ²
Affiliation: ¹ Charles University, Faculty of Mathematics and Physics, Department of Condensed Matter Physics, Ke Karlovu 5, 121 16 Praha 2, Czech Republic, E-mail: rud@mag.mff.cuni.cz
² Charles University, Faculty of Mathematics and Physics, Department of Condensed Matter Physics, Ke Karlovu 5, 121 16 Praha 2, Czech Republic, E-mail: jiri.pospisil@centrum.cz
Abstract: Polycrystalline samples of Ni_53.6Mn_27.1Ga_19.3 and Ni_54.2Mn_29.4Ga_16.4 shape memory alloys were investigated using dilatometry. The longitudinal axes of the samples were perpendicular to the columnar grains. Both alloys showed positive shape memory effects. The martensitic phase transformation occurred without hysteresis in both alloys with transformation temperatures of 174 ^oC for the Ni_53.6Mn_27.1Ga_19.3 alloy and 253 ^oC for the Ni_54.2Mn_29.4Ga_16.4 alloy. The dilatation characteristics for both alloys were determined in three perpendicular directions. The strain associated with the internal stress at the interface between the two martensitic structures and the two grains affected the dilatation characteristics in the y and z directions (perpendicular to the longitudinal axis of the sample). The microstructure was determined for all the directions investigated. To investigate the mechanical history, a round cross-section of the Ni_54.2Mn_29.4Ga_16.4 sample was machined using a milling machine along the longitudinal axis so that both sides of the sample were symmetrical. This sample treatment changed the dilatation characteristics of the martensite and austenite. The study and analysis of the dilatation characteristics of the thermal cycle showed the relaxation of internal stresses and the reorientation of the martensitic variants.

Type of Paper: Review
Title: Magneto-Structural Properties of the Ni2MnGa Type Ferromagnetic Shape Memory Alloys in Magnetic Fields
Authors: Takuo Sakon ¹, Yoshiya Adachi ² and Takeshi Kanomata ³
Affiliations: ¹ Ryukoku University, Otsu, Japan; ²Yamagata University, Yonezawa, Japan; ³ Tohoku Gakuin University, Tagajo, Japan
Abstract: The purpose of this review is to investigate the correlation between magnetism and crystallographic structures as it relates to the martensite transition of Ni2MnGa type alloys, which undergo the martensite transition below the Curie temperature. Especially, we focused on the physical properties in magnetic fields. Recent researches show that the martensite tenperature TM of Fe, Cu, or Co-doped Ni–Mn–Ga ferromagnetic shape memory alloys increase compared with Ni2MnGa. These alloys show large field dependence of the martensitic transition temperature, (dTM/dB). The results of thermal strain and magnetization indicate that a magneto-structural transition occurred at TM and magnetic field influences the magnetism and also the crystal structures.

Type of Paper: Article
Title: Experimental Study of Helical Shape Memory Alloy Actuators
Authors: Shane J. Yates and Alexander L. Kalamkarov *
Affiliation: Department of Mechanical Engineering, Dalhousie University, PO Box 15000, Halifax, Nova Scotia, B3H 4R2, Canada; E-Mails: sjyates@dal.ca (S.J.Y.); alex.kalamkarov@dal.ca (A.L.K.)
Abstract: Shape memory alloy actuators’ strokes can be increased at the expense of recovery force via heat treatment to form compressed springs in their heat-activated, austenitic state. Although there are models to explain their behaviour, few investigations present experimental results for support or validation. The aim of the present paper is not to support or discredit these models, but determine via experimentation how certain parameters affect a helical shape memory alloy actuator’s outputs: its transformation times and stroke. These parameters include wire diameter, spring diameter, transition temperature, number of active turns, bias mass, and direct current magnitude. Six investigations were performed: one for each parameter manipulation. Springs of varying geometries were annealed in the shape of compressed springs and activated with varying input parameter magnitudes. For repeatability and to observe thermo-mechanical training effects, the springs were cyclically activated. Heating times, cooling times, and strokes were measured throughout and patterns were observed for all parameter manipulations. The resultant patterns were compared with results predicted from one-dimensional behavioural models to elucidate the findings. Generally, it was observed that the transformation times and strokes converged as the internal shear stress increased or decreased; the convergence is likely the peak where the summation of elastic stroke and transformation stroke has reached its maximum. In addition, the actuators experience significant dynamic output changes as a result of training; cyclic loading should therefore be performed if the actuator is to be continuously used.
Keywords: shape memory alloys; helical shape memory alloy actuators; reaction times; strokes; electrical resistive heating

Type of Paper: Article
Title: Ab initio Study of Point Defects in NiTi-Based Materials
Authors: D. Holec, M. Friak, A. Dlouhy and J. Neugebauer
Affiliation: Max-Planck-Institut fuer Eisenforschung GmbH, Max-Planck-Strasse 1, D-402 37, Duesseldorf, Germany
Abstract: Changes in temperature or stress state may induce reversible B2 <-> (R) <-> B19' martensitic transformations and associated shape memory effects in close-to-stoichiometric nickel-titanium (NiTi) alloys. Recent experimental studies confirmed a considerable impact of the hydrogen-rich ageing atmosphere on the B2 austenite <-> B19' martensite transformation path. In this paper, we employ Density Functional Theory to study the energetics of Ar, He, O, and H interstitials in both the B2 austenite and B19' martensite phases. We show that O and H interstitials exhibit negative formation energies (stable defects), while Ar and He interstitials yield positive values (unstable defects). The extended concentration analysis of the austenite B2 phase doped with H shows that, independent of the H content (up to 6 at. %), a slight over-stoichiometry towards Ni-rich compositions in a range 51-52 a.t% is preferred. Finally, we discuss a possible role of interstitial H in the B2->B19' transformation based on local lattice distortions.