Eckhard Quandt University of Kiel
Ludwig Schultz IFW Dresden
Manfred Wuttig University of Maryland
Tomoyuki Kakeshita Osaka University
BB1: MSM and Magnetocaloric Effects
Monday PM, November 26, 2007
Room 209 (Hynes)
9:30 AM - **BB1.1
Magnetoelastic and Magnetocaloric Properties Related to Martensitic Transformations in Ni-Mn-based Heusler Alloys.
Eberhard Wassermann 1 , Thorsten Krenke 2 , Seda Aksoy 1 , Mehmet Acet 1 , Markus Gruner 3 , Peter Entel 3 Show Abstract
1 Experimental Physics, Duisburg-Essen University, Duisburg Germany, 2 Thyssen Krupp, Electrical Steel F&E Ge, Gelsenkirchen Germany, 3 Theoretical Physics, Duisburg-Essen University, Duisburg Germany
Ni-Mn-X based off-stoichiometric Heusler alloys undergo martensitic transformations below and around room temperature from a high-symmetry austenite cubic phase to a tetragonal or modulated martensite phase of lower symmetry. In general, the martensite start temperatures increase almost linearly with increasing valence electron concentration, but the slope is X-dependent. In the ferromagnetic martensitic state, some of the systems (X=Ga), due to strong magneto-elastic coupling, show field induced twin-boundary motion – the magnetic shape-memory effect accompanied by large strains. On the other hand, the systems (X = In, Sn, Sb) exhibit a field-induced reverse transformation from martensite to austenite also accompanied by large strains. The behavior is similar to the one-way shape memory effect in non-magnetic systems like Ni-Ti. Because of the first-order nature of the transitions, large magneto-caloric effects are also observed. The cause of the transition is related to the internal electronic pressure provided by the Ni 3d-electrons in different atomic environments due to different X-species. We discuss basic systematics related to the interplay between structural and magnetic properties and give experimental evidences for the cause of the martensitic transition in these systems.
10:00 AM - **BB1.2
Magnetocaloric Effect and Its Relation to Shape-Memory Properties in Ferromagnetic Heusler Alloys.
Antoni Planes 1 , Lluis Manosa 1 , Xavier Moya 1 Show Abstract
1 Estructura i Constituents de la Matèria, Universitat de Barcelona, Barcelona Spain
Magnetic shape-memory properties are a consequence of the coupling between magnetism and structure in ferromagnetic materials undergoing a martensitic transformation. In these materials, large deformations occur either by inducing the transition (magnetic superelasticity) or by rearranging martensitic variants (magnetic shape-memory effect) by means of an applied magnetic field. A number of Heusler alloys have been reported that show this property. In connection with the martensitic transition, these materials also exhibit a giant magnetocaloric effect. In this work we will discuss the magnetocaloric behavior of Ni-Mn-based Heusler alloys in relation to their shape-memory and superelastic properties. The magnetocaloric effect will be quantified by the isothermal entropy change induced by the application/removal of an applied field (obtained either from magnetization data or directly from calorimetric measurements under an applied magnetic field). We will show that the magnetocaloric effect in these Heusler materials originates from two different contributions: (i) the coupling that is related to a strong uniaxial magnetic anisotropy and takes place at the length scale of martensite variants and magnetic domains (extrinsic effect), and (ii) the intrinsic microscopic magnetostructural coupling. The first contribution is intimately related to the magnetically induced rearrangement of martensite variants and controls the magnetocaloric effect at small applied fields, while the latter is dominant at higher fields and is essentially related to the possibility of magnetically inducing the martensitic transition. We will show that the relative importance of both contributions depends on composition. The possibility of inverse magnetocaloric effect associated with these two contributions will also be considered.
10:30 AM - BB1.3
Manipulating Strain and Magnetocaloric Properties in Ni-Mn-In-based Ferromagnetic Heusler Shape Memory Alloys.
Seda Aksoy 1 , Thorsten Krenke 3 , Xavier Moya 2 , Mehmet Acet 1 , Lluis Manosa 2 , Antoni Planes 2 , Zhuhong Liu 1 , Eberhard Wassermann 1 Show Abstract
1 Experimental Physics, Duisburg-Essen University, Duisburg Germany, 3 Thyssen Krupp, Electrical Steel F&E Ge, Gelsenkirchen Germany, 2 Estructura i Constituents de la Matèria, University of Barcelona, Barcelona Spain
Magnetic-field induced structural modifications lead to the occurrence of large strains in ferromagnetic Ni-Mn-based Heusler alloys undergoing martensitic transformations. These modifications can be related to twin-boundary motion occurring within the martensitic state (as in Ni-Mn-Ga) or in the form of a magnetic-field-induced structural transformation. Such field-induced transformations lead to large recoverable strains and inverse magnetocaloric effects. The Ni(50)Mn(34)In(16) system having a martensitic transition temperature (Ms) of about 150 K is typical in showing both properties. Replacing In by small amounts of Ga causes Ms to increase, while the ferromagnetic Curie temperature remains essentially unchanged. In this manner, it is possible to shift the favorable magnetocaloric and strain properties to a desired temperature. We find for the case of Ni(50)Mn(34)In(14)Ga(2) that Ms shifts to about 275 K, and the size of the magnetocaloric effect remains almost unchanged. A field-induced transformation and an accompanying strain is observed as in the case of the alloy without Ga. Such possibilities of manipulating Ms to shift favorable strain and magnetocaloric properties to desired temperatures is also possible in a number of other Heusler based systems such as Ni-Mn-In-Sn, for which the magnetocaloric effect is enhanced up to 22 J/kgK under an applied magnetic-field of 5 T.
10:45 AM - BB1.4
Acoustic-Assisted, Magnetic-Field-Induced Strain of a Ni–Mn–Ga Single Crystal.
Ratchatee Techapiesancharoenkij 1 , Bradley Peterson 2 , Jorge Feuchtwanger 3 , David Bono 1 , Jesse Simon 4 , Jari Kostamo 6 , Samuel Allen 1 , Robert O'Handley 1 Show Abstract
1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 , NSWC/CD, Carderock, Maryland, United States, 3 , University of Basque Country, Bilbao Spain, 4 , Ferro Solutions, Cambridge, Massachusetts, United States, 6 , Helsinki University of Technology, Helsinki Finland
The effect of acoustic energy input from a piezoelectric actuator on the magnetic-field-induced strain (MFIS) of Ni-Mn-Ga ferromagnetic shape memory alloys (FSMAs) is observed to enhance the MFIS performance in FSMAs. The threshold field required to induce twin boundary motion is reduced by this new technique by up to one kOe and the strain can be increased by an order of magnitude in some range of the magnetic field. The twin-boundary yield stress is also reduced the acoustic-assistance. All of these phenomena should have beneficial impact the size and efficiency of a FSMA actuator. Here we report the results of more extensive studies of the acoustic-assist effect on both the magnetic threshold field Hth and the twinning-yield stress σtw of a Ni–Mn–Ga single crystal (100-cut measuring 15 x 5 x 2 mm). For the threshold field and MFIS measurement, a quasi-static magnetic field was applied along the sample length. For the twin-yielding stress measurement, a compressive load was slowly applied along the sample length direction. For both measurements, a piezoelectric stack was attached at one end of the sample to generate a longitudinal acoustic vibration along the sample length. The piezo stack was driven at different frequencies and amplitudes to vary acoustic-assist magnitudes. The reduction of Hth and σtw increases with increasing piezo drive frequency up to about 2 to 5 kHz. Beyond this frequency range, the Hth and σtw do not decrease further with increasing piezo frequency. The maximum reductions of the Hth and σtw by this acoustic-assisted technique are approximately 1 kOe and 0.5 MPa, respectively. The absence of an increase in acoustic-assisted effect at frequencies above 5 kHz appears to be due to degraded output displacement of the piezo above 5 kHz. The piezo assist effect on FSMA actuation can be understood in terms of the alternating stress waves provided to facilitate twin boundary motion. Acoustic wave theory, based on elastic and isotropic assumption, is used to estimate the amplitude of stress waves. The applied stress values from acoustic wave theory are comparable to the measured yield stress reductions. The empirical stress wave amplitudes are generally lower than the calculated ones, because the actual stress waves generated in FSMA are limited by inelastic and anisotropic nature of the FSMA samples.References: B. Peterson et al., “Acoustic assisted, field-induced strain in ferromagnetic shape memory alloys,” J. of Appl. Phys., 95, 6963-6964 (2004).
BB2: Twin Microstructure and Mobility
Monday PM, November 26, 2007
Room 209 (Hynes)
11:30 AM - **BB2.1
Between Magnetoelasticity and Magnetoplasticity.
Peter Mullner 1 Show Abstract
1 Materials Science and Engineering, Boise State University, Boise, Idaho, United States
Magnetic-field-induced deformation may recover upon removal of the magnetic field (magnetoelasticity) and it may be permanent (magnetoplasticity). Many experimental results display characteristics of both magnetoelasticity and magnetoplasticity. Depending on thermo-magneto-mechanical treatment (training), the response to magnetic actuation of a given sample varies from magnetoelastically dominated to magnetoplastically dominated. Thermo-magneto-mechanical training affects the twin-microstructure. Effective training leads to a coarse twin-microstructure with a low density of internal boundaries. In-effective training leads to a fine twin-microstructure with a high density of internal boundaries. Twinning disconnections, which are the carriers of magnetoelasticity and magnetoplasticity, interact mutually and also with interfaces. These interactions are modeled in terms of dislocations and disclinations. A numerical analysis shows that high and low densities of twin boundaries result in magnetoelastically and magnetoplastically dominated magnetic-field-induced deformation, respectively.
12:00 PM - **BB2.2
Twin Boundary Mobility in Shape Memory Alloys.
Bob Pond 1 , Hassan Khater 1 , David Bacon 1 Show Abstract
1 Engineering, University of Liverpool, Liverpool United Kingdom
Successful technological application of ferromagnetic shape memory alloys requires twin boundaries separating the martensitic variants in these materials to be highly mobile. Twin boundaries, being low energy structures, are not intrinsically mobile unless they contain line defects known as disconnections, i.e. defects exhibiting both dislocation and step character. Motion of a disconnection along a twin boundary causes the interface to migrate a distance equal to its step height, and simultaneously effects the twinning shear equal to its Burgers vector. Thus, the fundamental origin of twin mobility is the generation and motion of disconnections. These aspects are investigated here by atomic-scale computer simulation of disconnections in twins subject to applied mechanical stresses. Disconnection motion in otherwise perfect twins is studied, as well as twins containing other defects like captured crystal dislocations. The operation of a continuous source of disconnections is also illustrated. At present, simulations can only be carried out on relatively simple crystal structures, such as the hexagonal-close-packed metals investigated here. Additional considerations likely to affect disconnection generation and motion arising in the case of Heusler alloys are outlined.
12:30 PM - BB2.3
Twin Stabilization in a Ferromagnetic Shape Memory Alloy.
Liyang Dai 1 , Manfred Wuttig 1 , Emmanouel Pagounis 2 Show Abstract
1 Department of Materials Science and Engineering, University of Maryland, College Park, College Park, Maryland, United States, 2 , AdaptaMat Ltd., Helsinki Finland
The modulus and damping of martensitic Ni49Mn26Ga25 (wt%) cantilevers that deform by twin motion when bent have been investigated at room temperature. The modulus increases logarithmically as a function of aging time while the damping decreases commensurably. Both changes signal a gradual reduction of the twin boundary mobility. The logarithmic time dependence indicates a wide distribution of relaxation times. This distribution results from the volume dependence of the adaptation kinetics of nano-sized short range ordered clusters to the martensitic symmetry.
12:45 PM - BB2.4
Effects of Magnetic Field and Uniaxial Stress on Twinning Plane Movement and Martensitic Transformation in Some Ferromagnetic Shape Memory Alloys.
Takashi Fukuda 1 , Tomoyuki Kakeshita 1 Show Abstract
1 Department of Materials Science and Engineering, Graduate School of Engineering, Osaka University, Suita, Osaka, Japan
Three topics related to ferromagnetic shape memory alloys (FMSMAs) will be presented. The first topic is related to the condition for realizing the rearrangement of martensite variants (RMV) by a magnetic field. In order to realize this, the magnetic shear stress, which is given by magnetic energy difference between variants divided by twinning shear, should be larger than the twinning stress. We have confirmed propriety of this condition in three ferromagnetic shape memory alloys of Fe-31.2Pd, Fe3Pt and Ni2MnGa by measuring magnetocrystalline anisotropy constant, twinning stress and the twinning shear. The second topic is related to effect of magnetic field on the reverse martensitic transformation temperature As of Ni2MnGa. When the easy axis of the martensite phase (M-phase) is parallel to the magnetic field, the As temperature increases linearly with increasing magnetic field. However, when the field direction is not parallel to the easy axis, the As temperature decreases in a low field region and then increases with increasing magnetic field. Such results have been quantitatively explained by considering magnetocrystalline anisotropy energy. The third topic is related to stress-induced martensitic transformation in Ni2MnGa. When a compressive stress is applied to the intermediate phase (I-phase) in the P direction, it transforms to the M-phase via a new phase, which is termed as the X-phase. There is a triple point at which the I-phase, M-phase and X-phase coexist. In addition it is found that thermally-induced transformation in Ni2MnGa in the cooling process is P→X→I→M, suggesting that there is not a triple point at which the I-phase, X-phase and P-phase coexist.
Monday PM, November 26, 2007
Room 209 (Hynes)
2:30 PM - BB3.1
First-principles Calculations of Magnetic Field-induced Effects in Magnetic Shape Memeory Alloys.
Peter Entel 1 , Markus Gruner 1 , Alfred Hucht 1 , Georg Rollmann 1 , Waheed Adeagbo 2 , Alexey Zayak 3 , Mario Siewert 1 , Eberhard Wassermann 1 , Mehmet Acet 1 Show Abstract
1 Physics Department, University of Duisburg-Essen, Duisburg Germany, 2 Chemistry Department, Ruhr-University Bochum, Bochum Germany, 3 Department of Physics , University of Austin, Austin, Texas, United States
We discuss the properties of the magnetic shape memory alloys and related nanoclusters of Ni_2MnZ (with Z = Ga, In, Sn and Sb)on the basis of first-principles calculations including the effects of an external magnetic field in comparison to the experimental observations. We find that for the prototype Ni-Mn-Ga alloy the artensitic instability arises from the strong coupling of the Ni-states and Ni-Mn-states to small displacement fields in -direction leading to the nucleation of martensite and the opening of a pseudogap in the Ni-density of states at the Fermi energy . We supplement this picture by discussing the associated changes of the Fermi surface nesting behavior. For all four compounds Ni_2MnZ, the magnetic field-induced changes are discussed on the basis of fixed spin moment claculations , which allow to establish the fundamental (c/a, M)-phase diagram, where c/a defines the tetraganal distortion and M is the magnetic moment per formula unit. This phase diagram allows to draw conclusions of the finite-temperature behavior of the alloys.  P. Entel et al., J. Magn. Magn. Mater. 310, 2761 (2007). P. Entel et al., in press (http://dx.doi.org/10.1016/j.mesa.2007.01.191).
2:45 PM - BB3.2
First Principles Determination of Phase Transitions in Magnetic Shape Memory Alloys.
Tilmann Hickel 1 , Matthe Uijttewaal 1 , Blazej Grabowski 1 , Joerg Neugebauer 1 Show Abstract
1 Computational Materials Design, Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf Germany
Ni2MnGa is a typical example of a Heusler alloy that undergoes a martensitic transformation. In the high-temperature austenitic phase it has a cubic L21 structure, whereas below 200 K the symmetry is reduced by an orthorhombic distortion. Despite lattice deformations of more than 6% and large strains connected to this change, it is completely reversible. Therefore, Ni2MnGa serves as a shape memory compound. The fact that Ni2MnGa additionally orders ferromagnetically below 360 K makes the material particularly attractive for applications as actuators and sensors. Nevertheless, its structural details in the martensitic phase are still a subject of much debate. Several shuffling structures have been observed experimentally. The temperature and magnetic field dependent transformations between these structures need to be understood for an improvement of the magnetic switching (e.g. operation with higher reliability and smaller magnetic fields). Our approach to identify the stable structures and the low energy transition paths is the calculation of free energy surfaces, F(T), as function of key reaction coordinates (e.g. c/a-ratio) in density functional theory. The different (meta)stable phases lead to characteristic minima at this surface with temperature dependences obtained by the quasiharmonic approximation. Particular care has been taken to determine those phases which are characterized by shuffling structures. Here, we systematically analyzed the phonon spectra obtained by the quasiharmonic approximation and extracted detailed information about the type of this lattice instability from the eigenvectors of the unstable phonon modes. Based on the structures for the austenite, martensite and pre-martensite, we successfully determined transition temperatures from the intersection of the F(T) curves belonging to these phases. The results allow to assign and to interpret the experimental observations.
3:00 PM - BB3.3
A Theoretical Model of Variant Reorientation in Magnetic Shape Memory Alloys.
Vesselin Stoilov 1 Show Abstract
1 Mechanical, Automotive and Materials Engineering , University of Windsor , Windsor, Ontario, Canada
This paper presents a new model of reorientation of martensitic variants in ferromagnetic shape memory alloys subjected to combined magneto-mechanical loading. The model is based on the multiscale model introduced by Stoilov (Smart Mat. Str., 2007), and allows for complete three dimensional description of the second order phase transformations in FSMA under multiaxial loading. The magnetic and crystallographic aspects of the twin-boundary motion responsible for this phenomenon were described. Closed form expressions for the twin boundary velocity and overall macro displacements were derived. Nucleation of variants and propagation of twin boundaries were investigated under combined magneto-mechanical loading and compared to recent experiments. The model showed that phase boundary motion can result in significant deformation and allowed estimation of the overall deformation in a magnetic shape memory material. The tetragonal to tetragonal transformation was simulated and the results were consistent with those in the literature. The reorientation behavior and macroscopic response of a single crystal subjected to combined magneto-mechanical loading was also simulated and the results showed very good agreement with the experimental observations of Karaca et al. Acta Mat. 2006. Thus this model provides a viable tool for exploring the microscale dynamics of martensitic variants and their implications on macroscopic behavior, and it may be very valuable in the design of advanced microdevices.
3:15 PM - BB3.4
A Thermo-Magneto-Mechanical Free Energy Model for NiMnGa Single Crystals.
Phillip Morrison 1 , Stefan Seelecke 1 , Berthold Krevet 2 , Manfred Kohl 2 Show Abstract
1 Department of Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 Institute of Microstructure Technology, Forschungszentrum Karlsruhe, Karlsruhe Germany
The paper extends the authors’ recent model for one-dimensional rate-dependent magneto-mechanical behavior of NiMnGa single crystals to account for temperature-dependent effects including austenite/martensite and ferro-/paramagnetic phase transitions. The magneto-mechanical model consists of a constitutive Helmholtz free-energy landscape constructed for a meso-scale lattice element with strain and magnetization as order parameters. This two-dimensional energy landscape includes three paraboloidal wells representing the two easy-axis and one hard-axis martensite variants relevant for the one-dimensional case. Phase transformations resulting from applied stresses and magnetic fields follow from a system of evolution laws based on the Gibbs free energy equations and the theory of thermally-activated processes. The phase fractions subsequently determine the macroscopic strain and magnetization of a sample of NiMnGa by a standard averaging procedure.To account for phase transitions to austenite, additional wells representing the stable states of austenitic NiMnGa are added to the Helmholtz energy landscape. The transition from ferromagnetic to paramagnetic states is modeled as a second-order transformation. The temperature evolution of the sample follows from an energy balance equation, incorporating joule heating, heat transfer with the environment, and latent heats of transition between martensite and austenite, thus accounting for a full thermo-magneto-mechanical coupling.
3:30 PM - BB3.5
Micromagnetic Theory of Ferromagnetic Shape Memory Alloys.
Jiangyu Li 1 , Yunfei Ma 1 Show Abstract
1 , Univ of Washington, Seattle, Washington, United States
It is widely recognized that the rearrangement of martensite variants in a ferromagnetic shape memory alloy (FSMA) can lead to much higher strain than conventional magnetostriction. In this work, we develop micromagnetic theory to study the magnetic field induced strain in FSMA from energy minimization point of view. For single crystalline FSMA, we show that magnetization rotation can reduce ferromagnetic shape memory effect substantially when motion of the interface is severely hindered under a relatively large stress, which seriously limits the energy density of FSMA actuators. A solution to overcome this deficiency is also proposed. For polycrystalline FSMA, we estimate the magnetic field induced strain from Taylor bound, and identify the optimal textures for maximum actuation strain in FSMA polycrystals of various crystallographic symmetries. These analyses agree well with experiments.
3:45 PM - BB3.6
Phase Field Modeling of Domain Microstructure Processes in Ferromagnetic Shape Memory Alloys.
Yongmei Jin 1 Show Abstract
1 Department of Aerospace Engineering, Texas A&M University, College Station, Texas, United States
Phase field microelastic micromagnetic model is developed to simulate the domain microstructure processes in ferromagnetic shape memory alloys (FMSMAs). Magnetic field-induced deformation results from coupled ferromagnetic and ferroelastic domain evolutions. The crystalline anisotropy indirectly couples the transformation strain and magnetization and determines the ability of magnetic field-induced deformation under given thermal, magnetic and stress conditions. The model takes into account the domain microstructure-dependent long-range magnetostatic and elastostatic interactions. The simulations show that 180 magnetic domain walls play important role not only in magnetization process but also in deformation through its interaction with martensite twin boundaries, involving competition of various energetic terms and frustrations in domain microstructures. The difference in domain microstructure mechanisms between FMSMAs and conventional magnetostrictive materials is discussed.
Monday PM, November 26, 2007
Room 209 (Hynes)
4:30 PM - **BB4.1
Technical Advances Enhance Prospects for Applications of Magnetic Shape Memory Alloys.
Robert O'Handley 1 , Ratchatee Techapiesancharoenkij 1 , Samuel Allen 1 , Jari Kostamo 1 2 , David Bono 1 , Jorge Feuchtwanger 3 , Manikam Mahendran 4 , Marc Richard 5 , Bradley Peterson 6 , Thomas Lograsso 7 , Deborah Schlagel 7 Show Abstract
1 , M.I.T., Cambridge , Massachusetts, United States, 2 , Helsinki University of Technology, Helsinki Finland, 3 , University of Basque Country, Bilbao Spain, 4 , Tiagarajar College of Engineering, Madurai India, 5 , Richard Stockton College, Pomona, New Jersey, United States, 6 , NSWC/CD, Carderock, Maryland, United States, 7 , Ames Lab, Iowa State University, Ames , Iowa, United States
Ni-Mn-Ga FSMAs have the potential to generate work of order 100 kJ/m^3 (comparable to or greater than that of piezoelectric and magnetostrictive materials) and to operate at frequencies up to 1 or 2 kHz. One of the major impediments to application of Ni-Mn-Ga or other FSMAs as active materials is the relatively large magnetic field needed (approximately 3 kOe or 240 kA/m) for actuation of twin-boundary motion that brings with it strains up to 6% and output stresses in excess of 2 MPa. Recent research in the field of ferromagnetic shape memory alloys (FSMAs) has identified specific defects that play a role in pinning twin boundaries, elucidated changes in chemical site selection in off-stoichiometry Ni-Mn-Ga alloys, confirmed the role of twin boundary motion for energy absorption in Ni-Mn-Ga/polymer composites and provided a stronger understanding of how a simultaneous acoustic signal facilitates field-induced or stress-induced twin boundary motion. The potential impact of these scientific advances on the prospects for applications of FSMAs is assessed. A correlation has been established between twin boundary pinning and the concentration and size of sulfide precipitates. This has led to improved methods of crystal growth that give samples having reduced twin-boundary pinning (lower threshold fields and twin yield stress). The twin-boundary yield stress and magnetic threshold field for actuation can be further reduced by the application of a longitudinal acoustic wave that resolves as a shear stress across twin planes. This high-frequency (5 kHz) alternating stress field (of order 1 MPa) facilitates twin boundary motion particularly for magnetic fields or applied quasistatic stresses just below actuation threshold levels. These advances have led to new actuator designs that have the potential for enhanced actuation efficiency with lower magnetic fields. Finally, the large plastic loss associated with stress-induced twin boundary motion (absent acoustic assist) in Ni-Mn-Ga composites is now better understood to be the source of mechanical energy absorption in such composites up to frequencies of several hundred Hz.
5:00 PM - **BB4.2
Ni-Mn-Ga Thin Film Microactuators.
Manfred Kohl 1 , Fadila Khelfaoui 1 , Berthold Krevet 1 Show Abstract
1 IMT, University of Karlsruhe, Karlsruhe Germany
Microactuators of ferromagnetic shape memory alloys make use of the thermoelastic, ferromagnetic and magnetomechanical material properties, which enables a new level of multifunctionality and, as a consequence, particularly compact designs. However, designing the functional parts of such microactuators becomes rather complicated due to the complex coupling of the different physical properties. In the presence of a magnetic field, in particular, several magnetomechanical effects may be of importance such as ferromagnetic forces, Lorentz forces, the conventional magnetostriction and magnetic field-induced reorientation of martensite variants. The paper addresses the design, simulation and fabrication technology of selected NiMnGa thin film microactuators being representative for a number of typical applications. A method for coupled finite element simulation is presented, which is used as a design tool. The technologies of thin film deposition, micromachining and integration in a microsystem environment are discussed.
BB5: Poster Session
Tuesday AM, November 27, 2007
Exhibition Hall D (Hynes)
9:00 PM - BB5.1
Electronic Structure of Ni-Mn-Ga.
Sudipta Roy Barman 1 , Soma Banik 1 , Ajay Shukla 1 , Chinnathambi Kamal 2 , Chhayabrita Biswas 1 , Rajendra Dhaka 1 , Aparna Chakrabarti 2 Show Abstract
1 , UGC-DAE Consortium for Scientific Research, Indore, Madhya Pradesh, India, 2 , Raja Ramanna Centre for Advanced Technology, Indore, Madhya Pradesh, India
9:00 PM - BB5.2
Focused Alloy Design and Characterization of their Christallographic and Mechanical Properties for Actuating Devices.
Katharina Rolfs 1 , Arno Mecklenburg 1 2 , Jan-Magnus Guldbakke 2 , Jürgen Hesselbach 2 , Rainer Schneider 1 Show Abstract
1 SF1, Hahn-Meitner-Institut, Berlin Germany, 2 Institut für Werkzeugmaschinen und Fertigungstechnik, TU Braunschweig, Braunschweig Germany
9:00 PM - BB5.3
Development of a Miniaturized Gripper Driven by a Magnetic Shape Memory Single Crystal Actuator.
Jan Guldbakke 1 , Katharina Rolfs 2 , Arno Mecklenburg 2 , Annika Raatz 1 , Rainer Schneider 2 , Juergen Hesselbach 1 Show Abstract
1 Institute of Machine Tools and Productioh Technlogy, TU Braunschweig, Braunschweig Germany, 2 SF1, Hahn-Meitner-Institute, Berlin Germany
The development of grippers for the handling of microparts must solve special requirements, for example very high preciseness, adhesion, sensitive parts, moderate dynamics and clean room conditions. Because only few conventional grippers can meet all of these requirements, a mechanical miniature gripper having flexure hinges and a magnetic shape memory (MSM) actuator is developed. The design of the gripper is simple, but it is capable of gripping small objects with high precision. This could be realized by using a frictionless mechanism.The gripping mechanism is a so-called compliant mechanism. This term has been defined as a mechanism which gains the whole or a part of its mobility from the relative flexibility of its members rather than from rigid body motion. This means that conventional rotational bearings which connect two rigid bodies are replaced by flexure hinges. A flexure hinge is a type of bending beam which is used in mechanical systems as a revolute joint. Compliant mechanisms are strongly recommendable for precise applications because in opposite of conventional bearings they do not have undesired friction and wear effects. Further on the mechanisms consist of only one part and can easily be machined in small dimensions. However, there are some restrictions one has to take into account for a successful design of a compliant mechanism. First, flexure hinges reach only small angular displacement, limiting the motion of the whole compliant mechanism. This is not a general drawback for microgrippers, since the objects to be handled are only small in size. Second, flexure hinges have restoring forces. Therefore the actuator has to exert not only the desired gripping force, but additionally the restoring forces of the mechanism. Third, unlike rigid-body-links the instantaneous center of velocity is not fixed but changes its position during the movement of the link.As an actuator a 5M-Ni-Mn-Ga single crystal is used. This MSM crystal was produced at the Hahn-Meitner-Institute Berlin. Before the integration of the single crystal as the actuation element into the gripper, the properties of the MSM alloy have to be examined. Therefore test devices were developed and set-up. With these devices temperature dependent stress-strain curves of the single crystals can be determined as well as the magneto-stress-strain behaviour of MSM alloys. The integrated crystal works at temperatures up to 65°C, has a stress plateau under 1 MPa and exhibits a stress induced strain of approximately 7 %. Furthermore the MSM sample can be activated with magnetic fields less than 0.6 T. For the layout of the coil system to generate the required magnetic field the finite element program ANSYS was used.
9:00 PM - BB5.4
Preparation of Textured Ni48Mn30Ga22.
Martin Poetschke 1 , Uwe Gaitzsch 1 , Stefan Roth 1 , Bernd Rellinghaus 1 , Ludwig Schultz 1 Show Abstract
1 IMW, IfW Dresden, Dresden Germany
NiMnGa alloys have gained large research interest because of their possible application as magnetic shape memory materials. This effect is caused by the movement of twin boundaries in a magnetic field. So far this effect has only been shown in single crystals. The preparation of single crystals is a long time and cost intensive process and there can be compositional changes along the crystal and segregations. That is why for technical applications there is a great interest in polycrystals. To expand this effect to polycrystals, directional solidification was applied in order to get a coarse grained, textured sample. The technique of stationary casting in a preheated ceramic mold mounted on a copper plate was chosen in order to have a heat flow from the sample to the bottom and therefore a directional solidification in the opposite direction. To achieve a higher mold temperature at cast an additional heater was built and used for the casting process. The transformation temperature was checked by DSC. Since the transformation temperature strongly depends on composition, chemical homogieniety along the sample axis was approved by using this method. The preferred growth direction was determined by EBSD. Further annealing, which is necessary for chemical homogeneity, coarsening of grains and stress relaxation, influences the texture. Investigations on the texture development during annealing were done.
9:00 PM - BB5.5
Magnetic-Field-Induced Strain of Fe-based Ferromagnetic Shape-Memory Alloy in a Pulsed Magnetic Field.
Takuo Sakon 1 , Takashi Fukuda 2 , Tomoyuki Kakeshita 2 Show Abstract
1 Faculty of Enginerring and Resource Science, Akita University, Akita, Akita Prefecture, Japan, 2 Graduated School of Science, Osaka University, Suita, Osaka Prefecture, Japan
9:00 PM - BB5.6
Neutron Diffraction Study on Crystal Structure of Martensite Phases in Ni2MnGa.
Hiroaki Kushida 1 , Tomoyuki Terai 1 , Takashi Fukuda 1 , Tomoyuki Kakeshita 1 , Takuya Ohba 2 , Toyotaka Osakabe 3 , Kazuhisa Kakurai 3 Show Abstract
1 Department of Materials Science and Engineering, Graduate School of Engineering, Osaka University, Suita, Osaka, Japan, 2 Department of Material Science, Shimane University, Matsue, Shimane, Japan, 3 Quantum Beam Science Directorate, Japan Atomic Energy Agency, Tokai, Ibaraki, Japan
Ni2MnGa is known to transform from an L21-type parent phase to a martensite (M-) phase via an intermediate (I-) phase in the cooling process. Recently, we found that a new phase (X-phase) is induced from the I-phase under uniaxial stress applied in the P (index is given on the basis of the parent lattice) direction [Scr. Mater. 54 (2006), 585]. In this paper, we have investigated crystal structures of the M-phase, I-phase and X-phase in stoichiometric Ni2MnGa by neutron diffraction using a single crystal specimen. The neutron diffraction profile of the M-phase at T = 4.2 K exhibits four satellite reflections between fundamental reflections corresponding to (0 2 0)P and (2 0 0)P. They appear at incommensurate positions of (h 2-h 0) P with h = 0.428, 0.863, 1.136 and 1.572, which are near 3/7, 6/7, 8/7 and 11/7, respectively. Intensities of four satellite reflections relative to the fundamental ones are approximately 1/10, 1/1000, 1/1000 and 1/10, respectively. With increasing temperature, these four satellite reflections move toward the fundamental reflection. The profile of the I-phase at T = 210 K exhibits two satellite reflections between fundamental reflections corresponding to (0 2 0)P and (2 0 0)P. They appear at incommensurate positions of (h 2-h 0)P with h = 0.342 and 1.658, which are near 1/3 and 5/3, respectively. Intensities of these satellite reflections relative to the fundamental ones are approximately 1/100. Temperature dependence of the satellite positions is negligibly small, while their intensities decrease with increasing temperature and disappear at T = 250 K. When a compressive stress is applied to the I-phase, additional satellite reflection of the X-phase appears in the diffraction profile. Under a compressive stress of approximately 10 MPa, the reflection of the X-phase appears at the position of (h 2-h 0)P with h = 0.360 near the satellite reflection of the I-phase with h = 0.338. The reflection of the X-phase shifts to h = 0.369 when the stress increases to approximately 25 MPa. In addition, the intensity of satellite reflection of the X-phase increases with increasing compressive stress. With increasing temperature, the position of satellite reflection of the X-phase moves toward fundamental reflections, and the intensity of that decreases.
Eckhard Quandt University of Kiel
Ludwig Schultz IFW Dresden
Manfred Wuttig University of Maryland
Tomoyuki Kakeshita Osaka University
Tuesday AM, November 27, 2007
Room 209 (Hynes)
9:30 AM - **BB6.1
Current Challenges in Magnetic Shape Memory Research.
Oleg Heczko 1 2 Show Abstract
1 , Helsinki University of Technology, Helsinki Finland, 2 , IFW Dresden, Dresden Germany
Since the first observation of 0.2% magnetic-field-induced deformation called magnetic shape memory (MSM) effect in Ni-Mn-Ga Heusler alloys by Ullakko in 1996, the intensive research of this very promising phenomenon has been conducted in Helsinki University of Technology. In 2000 we reported 6% field-induced deformation in moderate field of about 0.5 T. Even larger field-induced deformation about 10% was observed in 2002. These values are very close to theoretical maximum deformation permitted by particular crystal lattice of Ni-Mn-Ga alloys. The MSM effect is due to redistribution of martensitic twin variants resulting in microstructure reorientation and giant strain. The redistribution is driven by the difference of magnetic energy in differently oriented martensitic variants. Although the largest effort was devoted to study Ni-Mn-Ga alloys, the phenomenon is no means restricted to these alloys and indeed not even to ferromagnetic martensites. Basic requirements for the MSM effect are high mobility of the twin boundaries described by twinning stress and high magnetic anisotropy. These will be evaluated and discussed for various martensite phases existing in Ni-Mn-Ga alloys. Three types of martensite were found in Ni-Mn-Ga but only 5M and 7M structures exhibit the effect. Experimental simultaneous determination of magnetization reversal M = M (H, σ, T) and magnetic-field-induced strain ε = ε (H, σ, T) as a function of magnetic field, H, and external stress, σ, in temperature T gives full macroscopic description of the phenomenon. There are two complementary strain effects; firstly the usual MSM effect, in which large field-induced strain occurs as a function of increasing magnetic field under constant external stress and secondly magnetically induced superelasticity, in which the reversible strain occurs as a function of the external stress under constant magnetic field. Simple model will be presented which describes reasonably well the observed MSM behaviour. This presentation reviews recent experiments on Ni-Mn-Ga bulk alloys extending from magnetic and magneto-elastic properties to the quality of the crystal, magnetic fatigue and damping. Emerging new forms of the MSM material as thin films and composites will be discussed. From our research it is clear that the major impediment to full utilization of the effect is the non-uniform MSM response for different samples and alloys, narrow temperature range and low fatigue resistance. These are main challenges for future research.
10:00 AM - BB6.2
Efficient Manufacture of MSM Alloy Single Crystals using a Synthetic Slag Cover.
Arno Mecklenburg 1 , Katharina Rolfs 1 , Jan Guldbakke 1 , Rainer Schneider 1 Show Abstract
1 SF1, Hahn-Meitner-Institute, Berlin Germany
Common techniques for the growth of single crystals from the melt suffer from numerous drawbacks when applied to the growth of Ni-Mn-Ga. Well known is the loss of Mn from the metallic melt during the growth due to its high vapour pressure. In order to minimize this loss, solidification is often performed under high inert gas pressures. This measure cannot suppress Mn volatilization completely as only some Nernst diffusion layer is established over the surface of the melt and diffusion through this layer always takes place. Moreover, high inert gas pressures provide insufficient degassing of the metallic melt leading to a high gaseous porosity of the as-grown crystals.Another drawback is that impurities from the ingredients, such as oxides and sulfides, are not removed from the melt but solidified together with the alloy derogating the properties of the crystal e.g. by the formation of hard oxide inclusions.Finally, in the Bridgman technique interactions between the alloy and the crucible are persistent, for example chemical reduction of crucible material or unintentional nucleation of crystal growth.We wish to present an elegant modification of the Bridgman technique which solves all of the above problems. This technique is named SLag Refinement and Encapsulation (SLARE).Centre of the idea is the use of high purity molten salts (“synthetic slag”) to fully encapsulate the metallic melt and hinder any exposure to the atmosphere or the crucible walls. Ionic impurities are efficiently dissolved in the slag according to their partitioning coefficient and are thus removed from the melt prior to solidification. 5M and 7M Ni-Mn-Ga single crystals grown by the SLARE technique are characterised by the absence of solid inclusions, very small mosaic spreads, very low detwinning stresses (< 0.5 MPa in 7M crystals), and low porosity. However, it has to be mentioned, that slag composition and crucible material must be carefully chosen for each alloy with respect to chemical and physical interactions. Adopting the SLARE technique to alloys other than Ni-Mn-Ga, high quality Ni-Mn-Sn, Ni-Co-Mn-Ga and CeCu2 were successfully grown in the first attempt.
10:15 AM - BB6.3
Combination of 10M and 14M Martensite Plates in Ni-Mn-Ga Alloy.
Minoru Nishida 1 , Mitsuhoro Matsuda 2 , Yoshihiro Yasumoto 2 , Takashi Fukuda 3 , Tomoyuki Kakeshita 3 Show Abstract
1 Applied Science for Electronics and Materials, Kyushu University, Kasuga, Fukuoka, Japan, 2 Materials Science and Engineering, Kumamoto University, Kumamoto, Kumamoto, Japan, 3 Materials Science and Engineering, Osaka University, Suita, Osaka, Japan
It has been widely recognized that the lattice invariant shear of martensite with layered structure such as 9R or 18R martensites is stacking faults on (001) basal plane. Therefore, internal twins are boundaries of martensite plate variants, which are so-called “variant accommodation twins”. The variant accommodation twin is introduced as a result of mutual accommodation of shear strains between variants in the martensite. For 9R and/or 18R martensites, many researchers have reported that there are four plate variants commonly designated as A, B, C and D, and three fundamental plate variant combinations can be identified in a given plate variant; designated as A : B (C : D), A : C (B : D) and A : D (B : C) types. The intervariant boundaries of these three types are in Type II, Type I and compound twin relations, respectively. These are also classified to three features, i. e., spear, wedge and fork-like morphologies, respectively. However, there is no investigation on the variant combinations in the 10M and the 14M martensites in the Ni-Mn-Ga alloys as far as we know. In the present paper we will present the variant combinations in the layered martensites in the Ni-Mn-Ga alloy.
10:30 AM - BB6.4
Magnetostrain of Polycrystalline 5M and 7M Ni-Mn-Ga Magnetic Shape Memory Alloys.
Uwe Gaitzsch 1 , Martin Potschke 1 , Stefan Roth 1 , Bernd Rellinghaus 1 , Ludwig Schultz 1 Show Abstract
1 Inst metal mater, IFW Dresden, Dresden Germany
Magnetic shape memory materials gained a large research interest owing to their capability to deform by some percent via twin boundary motion under the influence of a magnetic field. Concurrently, they are supposed to react faster than conventional shape memory materials because neither heating nor cooling are involved. The predominant material system is Ni-Mn-Ga with compositions around Ni2MnGa. In our case Ni50Mn30Ga20 and Ni50Mn29Ga21 alloy samples are used to produce polycrystalline textured samples. Upon cooling these alloys transform martensitically at 100 °C and 55 °C, respectively. The evolving martensitic structure is either orthorhombic (7M) or tetragonal (5M, NM) and depends on the thermomechanical history of the sample as well as the composition. Since only two of the three possible structures are suitable of providing the mandatory highly moblile twin boundaries, it is important to understand and control the phase formation process by appropriate thermal and mechanical treatment. Once the sample is given a suitable structure, samples for training are hot mold cast for directional solidification. X-ray diffraction techniques are applied to investigate the texture in these samples. After being cut erosively they are trained thermomechanically and in successive compression tests to lower the (de)twinning stress. The stresses in the training process have to be kept as low as possible to avoid brittle fracture of the samples during the cyclic compression process. The magnetically achievable strain can be measured in the training device, which is an Instron testing machine. That has been modified in order to allow for magnetomechanical tests in magnetic fields of up to 0.8 Tesla.
10:45 AM - BB6.5
Dynamic Measurements of Twin Boundary Mobility in Ni—Mn—Ga /polymer Composites by Strobe-mode Neutron Diffraction.
Jorge Feuchtwanger 1 , Jose Barandiaran 1 , Jon Gutierrez 1 , Patricia Lazpita 1 , Robert O'Handley 2 , Samuel Allen 2 , Thomas Hansen 3 , Claudia Mondelli 4 Show Abstract
1 Electricidad y electronica, Universidad del Pais Vasco, Leioa, Bizkaia, Spain, 2 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 , Institute Laue-Langevin, Grenoble France, 4 CNR-INFM and CRS-SOFT, Institute Laue-Langevin, Grenoble France
The use of Ferromagnetic Shape Memory Alloy (FSMA)/ polymer composites as energy dampers has been proposed due to the large amounts of energy that can be dissipated by the pseudo-plastic deformation of the FSMA particles. Static X-Ray diffraction measurements, as well as magnetic measurements, have shown that it is possible to change the variant distribution of the particles by applying a stress to the composite[1-3]. The composites used for the experiment were cured under a magnetic field, yielding a pseudo 3:1 composite with the particles aligned in chains and a preferential crystallographic texture along the direction of the field applied while curing. The motion of twin boundaries in the particles alters the texture and results in a relative change in the areas of peak pairs that stem from a single austenitic peak. Those measurements have now been extended to show the dynamic response of the composites to a time varying strain. Strobe mode neutron diffraction was carried out on FSMA/polymer composites at the D20 diffractometer in the Institute Laue Langevin in Grenoble France. The motion of twin boundaries in the particles alters the texture and results in a relative change in the areas of peak pairs that stem from a single austenitic peak. A custom built dynamic testing machine was mounted in the diffactometer. The neutron diffraction collection was synchronized to the mechanical testing of the sample and 100 slices of 20 ms were collected during each deformation cycle. The data from consecutive cycles was added to obtain 100 diffraction patterns along one deformation cycle.The shape of the stress-strain loop measured during the neutron measurements show a departure from ellipticity, while the polymer matrix on its own shows an elliptical trace. The additional loss can therefore be directly related to the observed twin variant redistribution along the cycle. The area ratio the (020) to (112), the two peaks, indexed in the martensite, that split form the (112) austenite peak, varies as a function of strain in the cycle. The area ratio tracks the strain applied to se sample, but it shows a lag. Additionally the shape of the area ratio curve is not fully sinusoidal and the zero crossing halfway through the cycle is shifted to the compressive side of the loop.The measurements allow a direct knowledge of the texture of the Ni-Mn-Ga particles in the composite and therefore get insight into the actual processes taking place during the macroscopic deformation cycle. References J. Feuchtwanger, K. Griffin, J.K. Huang, D. Bono, R.C. O’Handley and S.M. Allen, J Magn Magn Mater 272–276 (2004), p. 2038 J. Feuchtwanger, M.L. Richard, Y.J. Tang, A.E. Berkowitz, R.C. O’Handley and S.M. Allen, J Appl Phys 97 (2005), p. 10M319 Scheerbaum, N.; Hinz, D.; Gutfleisch, O.; Mueller, K.-H.; Schultz, L. Acta Materialia (2007), 55(8), 2707-2713
BB7: Alternative MSM Materials
Tuesday PM, November 27, 2007
Room 209 (Hynes)
11:30 AM - **BB7.1
Influence of Homogeneity on the Magnetic Properties of Ni-Mn-Sn Heusler Alloys.
Thomas Lograsso 1 , Deborah Schlagel 1 , Ralph McCallum 1 Show Abstract
1 Materials and Engineering Physics, Ames Laboratory, Ames, Iowa, United States
With the discovery of large magnetic field induced strain and more recently, giant magnetocaloric effect (MCE) in Ni-Mn-Ga Heusler alloys, there have been many investigations to identify new ferromagnetic shape memory alloys which exhibit these special properties. For a particular alloy system it is therefore necessary to adjust alloy composition to tune the electron concentration (electron to atom ratio), thereby altering the magnetic ordering and structural transformation temperatures to achieve the desired field induced response. This tuning of the composition can have dramatic effects on the microstructural development during solidification and crystal growth. These solidification effects and their influences on the shape memory transformations and magnetic ordering transitions are demonstrated using the Ni-Mn-Sn alloy system. Chemical fluctuations introduced during solidified, when not fully equilibrated, have been found to mask the intrinsic magnetostructural behavior in Ni50Mn37Sn13. The use of structural homogeneity (phase purity) was found to be an insufficient descriptor for the degree of homogeneity. Once both structural and chemical homogenizations are achieved, the sequence of transitions (both structural and magnetic) on cooling is more distinct and, for the low temperature martensite phase, the magnetic measurements reflect its intrinsic behavior (i. e., ferromagnetic ordering and Curie Weiss behavior on heating above its magnetic transition temperature). Further the nature and sequence of the transitions on cooling were found to be; 1) austenite paramagnetic to ferromagnetic transition (second order); 2) coupled ferromagnetic to paramagnetic and austenite-to-martensite crystallographic transition (first order); and martensite paramagnetic to ferromagnetic transition (second order).
12:00 PM - BB7.2
Resistivity at the Martensitic Transformation of Ni-Fe-Ga Alloys.
Jose Manuel Barandiaran 1 , Patricia Lazpita 1 , Jon Gutierrez 1 , Jorge Feuchtwanger 1 , Inaki Orue 1 , Volodymyr Chernenko 2 Show Abstract
1 Depto. Electricidad y Electrónica, Universidad del País Vasco, Bilbao Spain, 2 , Institute of Magnetism, Kyiv Ukraine
Fe base Ferromagnetic Shape Memory Alloys (FSMA) have some attractive properties, as compared with the classical Ni-Mn-Ga ones, mainly because its great ductility. Some compositions show martensitic temperatures close to room temperature and can be of interest for technical applications as magnetic sensors or actuatorsIn this work we present a Resistivity study of the martensitic transformation in alloys with nominal composition Ni55-xFe19+xGa26 (x = 0, 1, 2) transforming close to room temperature, with special insight into the effect of a Magnetic Field in the transformation. Resistivity is well suited for assessing the effect of a magnetic field in the transformation temperature because, unless magnetization, maintains a great level of accuracy irrespective of the value of the applied field. On the other hand the measurement of resistivity under field is a standard one in contrast with calorimetric or diffraction measurements where the application of a magnetic field is highly restricted. Measurements were performed in a SQUID platform with a liquid He refrigerated superconducting magnet up to 7 Tesla field, and in a closed cycle refrigeration magnet up to 14 Tesla.The zero field transition is shown to be narrower (≈30K for x=0 to 10K for x=2) and slightly less hysteretic as the Fe content increases, as already reported by the authors (1). On the other hand, changes in the resistivity at the transition increase from 10% (x=0) to 20% (x=2). However, no variation in this values, nor in the transformation temperatures, to +/-3K, has been observed up to the maximum applied field. This is in sharp contrast with the reported behavior of NiMnGa alloys, that can show a 6K/Tesla change (2).REFERENCES(1) J.M. Barandiarán, J. Gutierrez, P. Lázpita, V. A. Chernenko, C. Seguí, J. Pons, E. Cesari, K. Oikawa, T. Kanomata, “Martensitic transformation in Fe-Ni-Ga alloys” Materials Science and Engineering A, 2007, in print(2) S. Jeong, K. Inoue, S. Inoue, K. Koterazawa, M. Taya, K. Inoue ”Effect of magnetic field on martensite transformation in a polycrystalline Ni2MnGa”, Materials Science and Engineering A, 359 (2003) 253-260
12:15 PM - BB7.3
Temperature Dependence of the Field Induced Phase Transformation in Ni50Mn37Sn13.
D. Schlagel 1 , W. Yuhasz 1 , R. McCallum 1 2 , T. Lograsso 1 Show Abstract
1 Material and Engineering Physics Progam, Ames Laboratory, Ames, Iowa, United States, 2 Materials Science & Engineering, Iowa State University, Ames, Iowa, United States
We investigated field induced phase transformation from martensite (M) to austenite (A) in Ni50Mn37Sn13. Polycrystalline Ni-Mn-Sn alloys were prepared by arc melting and chill casting into copper molds. The samples were then fully homogenized at 1225K for 672 hours such that the structural and magnetic transitions were clearly separated from each other in temperature. The nature and sequence of the transitions on cooling are; 1) austenite paramagnetic to ferromagnetic transition (second order); 2) austenite-to-martensite crystallographic and coupled ferromagnetic to paramagnetic transition (first order); and martensite paramagnetic to ferromagnetic transition (second order). The shape memory transformation start and finish temperatures; Ms = 290K, Mf = 283K, As = 300K, Af = 308K, were determined by differential scanning calorimetry. From magnetization curves, where the sample was zero field cooled then warmed to temperatures ranging from just below to just above the transformation temperature, the field induced transformation from the fully paramagnetic martensitic state to the ferromagnetic austensitic state was observed beginning at a temperature of 290K, 10K below the As temperature under a field of 9 Tesla (T). As the temperature approaches As, the critical field to induce transformation decreases and the amount of austenite formed at 9T increases. At 297.5K and 0.0T field, the sample is 95% martensitic and the minimum field to transform additional austenite is found to be ~1.5T. The sample then becomes fully transformed to the austenite phase at a field of 8.9T. At 295K and 0.0T field, the sample is 100% martensitic and the minimum field to transform to austenite is found to be ~4T. The sample transforms to 85% austenite phase at a field of 9.0T. At all temperatures, the transformation from martensite to austenite was found to be irreversible as a function of field.
12:30 PM - BB7.4
Crystallographic Orientation Dependence of Magnetic Field-Induced Phase Transformation in NiMnCoIn Single Crystals.
Ibrahim Karaman 1 , Haluk Karaca 1 , Burak Basaran 1 , Yuriy Chumlyakov 2 , Hans Maier 3 Show Abstract
1 Department of Mechanical Engineering, Texas A&M University, College Station, Texas, United States, 2 , Siberian Physical Technical Institute, Tomsk Russian Federation, 3 , University of Paderborn, Paderborn Germany
Magnetic Shape Memory Alloys (MSMAs) can undergo large shape changes upon application of magnetic field. NiMnGa alloys are the most well-known MSMAs in which the magnetic-field induced martensite reorientation is the responsible mechanism for this shape change. NiMnCoIn MSMAs, on the other hand, utilize Metamagnetic Phase Transition (MPT) mechanism where martensite with very low saturation magnetization transforms into ferromagnetic austenite when magnetic field is applied. The large Zeeman energy difference between the transforming phases in NiMnCoIn alloys can result in similar magnetic field-induced strain (MFIS) levels thru phase transformation to the MFIS levels in NiMnGa alloys obtained via martensite reorientation. However, in addition, higher actuation stress levels than NiMnGa alloys are possible in the former. Employing Zeeman energy instead of magnetocrystalline anisotropy energy as the main magnetic energy source for phase transformation can also offer the possibility of inducing phase transformation magnetically in polycrystals. Although Zeeman energy is orientation independent, shape memory properties such as transformation strain level, critical stress for phase transformation, stress and temperature hysteresis, so does the critical magnetic field for the field induced phase transformation, are all orientation dependent. We will present and discuss our recent experimental findings on NiMnCoIn single crystals to reveal the effects of crystal orientation and magnetic field on the shape memory properties including transformation strain levels, critical stress for phase transformation, magnetostress, and temperature and stress hysteresis. Magneto-thermal measurements and in situ high energy synchrotron X-ray techniques are employed to demonstrate the reversible field-induced phase transformation in these alloy systems. It was found that cooling under magnetic fields above 5 Tesla change the structure and magnetic order of martensite, i.e. a ferromagnetic martensite forms instead of an antiferromagnetic one. This was attributed to the change in the amount of manganese atom separation in different martensite structures. Necessary magnetic and mechanical conditions, and materials design and selection guidelines are proposed to search for field-induced phase transformation in new alloys or improve MSMA properties of existing ones to increase actuation stress, MFIS, and operating temperature range, and decrease required magnetic fields for phase transformation.
12:45 PM - BB7.5
Characterization of a FePd Single Crystal for Sensor Applications.
Christoph Bechtold 1 , Andreas Gerber 1 , Manfred Wuttig 2 , Tomoyuki Kakeshita 3 , Takashi Fukuda 3 , Eckhard Quandt 4 Show Abstract
1 , CAESAR Research Center, Bonn Germany, 2 Department of Material Science, University of Maryland, College Park, Maryland, United States, 3 Department of Material Science and Engineering, Graduate School of Engineering, Osaka University, Osaka Japan, 4 Chair for Inorganic Functional Materials, Christian-Albrechts-University, Kiel Germany
MSM materials have received much attention in actuator applications due to their large strains and in comparison to thermal shape memory alloys short response times. Their potential as an alternative magnetostrictive component in sensor systems has however been studied less exhaustively. The aim of this project is the development of strain sensors based on FePd as well as the combination of this MSM material with piezoelectric materials for magnetic field sensing. As a first step, a FePd single crystal was characterized by EDX and temperature dependent XRD analysis. The composition of the 2.75 x 2.45 x 1.06 mm3 crystal was Fe69Pd31 and the XRD measurements showed a transition from the austenitic state at room temperature to the martensitic state below -20°C. Furthermore, the blocking stress of the material and the field-strain curves under different compressive loads were studied at temperatures below -20°C. These results will be discussed in respect of the above mentioned applications. Funding via the DFG priority programme 1239 “Change of Microstructure and Shape of Solid Materials by External Magnetic Fields” is thankfully acknowledged.
BB8: MSM Thin Films
Tuesday PM, November 27, 2007
Back Bay D (Sheraton)
2:30 PM - **BB8.1
Epitaxial Ni-Mn-Ga and Fe-Pd films: What is Different Compared to Bulk Single Crystals?
Sebastian Faehler 1 Show Abstract
1 , IFW Dresden, Dresden Germany
Single crystals of Ni-Mn-Ga and Fe-Pd are well known to exhibit magnetically induced reorientation (MIR) of martensitic variants. Epitaxial Ni-Mn-Ga and Fe-Pd films grown on MgO (100) are used to analyze in detail the effect of constraints imposed by the substrate. This results in significantly different behavior compared to bulk single crystals.Epitaxial Ni-Mn-Ga films which are martensitic and ferromagnetic at room temperature fulfilling two key requirements for applications, were studied. It is shown that the substrate influences several properties. First the film orientation is controlled by epitaxy. Second the biaxial stress with respect to the substrate induces the martensitic transition. Both, composition and stress analysis suggest that the martensitic transition temperature of a released film should be about 50 K lower. Third, the boundary condition towards a rigid substrate reduces the number of observed variants and appears to slightly broaden the thermal hysteresis. Fourth, the additional free parameter of an orthorhombic structure allows a magnetically induced microstructure reorientation within a film even though its overall dimensions are constrained by the substrate. Independent polarization and pole figure measurements are in reasonable agreement with the suggested model. As no field dependent change in several μm large twins was observed it is suggested that reorientation occurs only within small twins with coherent length of several tens of nanometers. Reorientation in these small twins is more likely as they are significantly smaller compared to the film thickness. Though no macroscopic strain is expected in constrained films these measurements show that MIR or magnetic shape memory effect in thin epitaxial NiMnGa films is possible in quite moderate magnetic fields. These Ni-Mn-Ga films are compared with epitaxial Fe-Pd films grown at room temperature. Due to epitaxial growth film structure and physical properties can be studied more precisely compared with polycrystalline films. Epitaxial Fe-Pd films allow an uncomplicated phase determination, which is not possible in fiber-textured Fe-Pd films. Furthermore, epitaxial growth at room temperature makes Fe-Pd films a promising candidate for future MSM microsystems. The epitaxial growth observed through the whole composition range examined appears to benefit from martensite instability, allowing adopting large misfits. The existence range of the fct martensite in thin films is significantly broader compared to bulk, which is attributed to stabilization by the compressive film stress. For thicker films (111)fcc twinning as a second mechanism to relaxing stress was observed.
3:00 PM - BB8.2
Martensite Transition and Microscopic Magnetism of Epitaxial Ni2MnGa Films.
Gerhard Jakob 1 , Tobias Eichhorn 1 , Michael Kallmayer 1 , Hans-Joachim Elmers 1 Show Abstract
1 Institute of Physics, University of Mainz, Mainz Germany
Ni2MnGa undergoes a magnetically induced shape memory effect, which can lead to huge magnetostrictive effects of several percent. We use x-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD) in order to get information of element specific magnetic moments and electronic structure of single crystalline Ni2MnGa(110) films on Al2O3(11-20) substrates and correlate the data to information from temperature dependend x-ray diffraction and magnetisation measurements. The thin films have been grown by dc-sputtering from a stoichiometric target onto single crystalline sapphire substrates. Four circle diffraction shows epitaxial L21 ordered film growth. The in-plane growth direction is 90° rotated from the expected alignment considering lattice constants. In these films the martensite transition is shifted to higher temperatures T(MS)=280K due to film-substrate interaction compared to the bulk value of 200K. The phase transition is visible in resistivity, magnetization, x-ray diffraction and spectroscopic measurements. The temperature dependence of the structural transition was investigated in detail by temperature dependend four-circle diffractometry. A seemingly different behavior of the (400) reflection and the (220) reflection of the austenite phase can be understood considering the formation of twin boundaries along the family of equivalent (110) lattice planes.The Ni XAS shows a significant modification on undergoing the martensite transition in agreement with published ab-initio calculations. The cubic symmetry in the austenite phase leads to a high density of unoccupied states just above the Fermi-edge, which give rise to a special absorption feature in XAS. On entering the martensite state the crystal symmetry changes and the degeneracy of d-orbital states is lifted. Using XAS in transmission geometry on our thin film samples we observe the corresponding reduction of the absorption feature as predicted by theoretical calculations. By XMCD measuremnets we can evaluate the atomic magnetic orbital and spin moments. For our films on Al2O3 the temperature dependence of the Mn magnetic moment is stronger than that of the Ni magnetic moment, which is not in-line with theoretical expectations.
3:15 PM - BB8.3
Stress-induced Twin Boundary Motion in Ni2MnGa - Polymer Composites.
Oliver Gutfleisch 1 2 , Nils Scheerbaum 2 , Jian Liu 2 , Dietrich Hinz 2 , Ludwig Schultz 2 Show Abstract
1 , IFW Dresden, Dresden Germany, 2 Institute of Metallic Materials, Leibniz Institute of Solid State and Materials Research Dresden, Dresden Germany
3:30 PM - BB8.4
Structural, Magnetic and Phase Transformation Properties of Ferromagnetic Shape Memory Thin Films Based on Fe70Pd30.
Sven Hamann 1 2 , Hayo Brunken 1 2 , Alan Savan 1 , Alfred Ludwig 1 2 Show Abstract
1 Combinatorial Materials Science Group, caesar, Bonn Germany, 2 Institute for Materials, Ruhr-University Bochum, Bochum Germany
Thin films in a composition range around Fe70Pd30 were fabricated by magnetron sputtering and annealing. Different processing routes were applied to study the influences of fabrication on the phase transformation properties of the thin films. Depositions were performed as (i) co-deposition, (ii) multilayer deposition, (iii) deposition from an alloy target. The as-deposited thin films were annealed in different furnaces (including micro-hotplates) with various heating and cooling rates, temperatures and annealing times. The films were characterized by energy dispersive X-ray spectroscopy, temperature-dependent X-ray diffraction, scanning and transmission electron microscopy, temperature-dependent resistance measurements and temperature-dependent vibrating sample magnetometry. Thin films in the composition range from Fe70Pd30 to Fe72Pd28 show a reversible phase transformation after heat treatment at 850°C for 15 minutes and with sufficient cooling rates (quenching). The highest transformation temperatures (up to 65°C) can be achieved for co-deposited thin films, annealed at 850°C, 15 min. The hysteresis of the phase transformation is low (< 6 K), but typically spread across a wide temperature range. Based on the optimized processing of binary Fe-Pd thin films, ternary Fe-Pd-X (X=Co, Ni, …) thin films were deposited, annealed and characterized. First results indicate that the transformation temperatures can be further increased by choosing appropriate alloying elements.
3:45 PM - BB8.5
A Combinatorial Investigation of Magnetostriction in Fe-Co-X Alloys.
Jason Hattrick-Simpers 1 , Dwight Hunter 1 , Kyu Sung Jang 1 , Makoto Murakami 1 , Samuel Lofland 2 , Manfred Wuttig 1 , Ichiro Takeuchi 1 Show Abstract
1 Materials Science and Engineering, University of Maryland, College Park, Maryland, United States, 2 Department of Physics and Astronomy, Rowan University, Glassboro, New Jersey, United States
The discovery of large magnetostricion in Fe-Ga alloys in 2000 stimulated renewed interest in magnetostritive alloys. Most of this work has focused on alloying ternary additions into Fe-Ga to improve its mechanical properties without degrading its magnetic properties. An alternative system with similar magnetostrictive properties, inherently better mechanical properties, and a similar phase diagram is Fe-Co. Here, the combinatorial approach to materials science has been applied to search for promising magnetostrictive materials in Fe-Co ternaries. Using a newly developed an optical high-throughput magnetostriction measurement technique the change of magnetostriction across the ternary phase diagram has been monitored. The effect of the addition of Al and Ga to the structure and magnetostriction of Fe-Co will be discussed.
BB9: Microstructural Characterization
Tuesday PM, November 27, 2007
Room 209 (Hynes)
4:30 PM - BB9.1
Observation and Modelling of Magnetic Domain Structure in Bulk NiMnGa.
Ryan Yiu Wai Lai 1 , Jeffrey McCord 1 , Rudolf Schaefer 1 , Ulrich Roessler 1 , Aristide Traian Onisan 1 , Ludwig Schultz 1 Show Abstract
1 , IFW Dresden, Dresden Germany
A study of the magnetic domain structure in bulk NiMnGa magnetic shape memory single crystals is presented. Polarization microscopy, using a magneto-optical indicator film technique, is employed to obtain the static magnetic domain patterns at all surfaces of bulk crystals. Different complexity of domain patterns is revealed in different twinning states (e.g. single variant state, two-variant state). Special focus is put on the domain structure around the structural twin boundary which shows a characteristic pattern that matches conflicting influences from both variants. Domain models explaining the observation will be discussed in detail. Funding through the DFG priority program SPP1239 is gratefully acknowledged.
4:45 PM - BB9.2
Neutron Scattering Studies of Ferromagnetic Shape Memory Alloy Ni2MnGa.
Stephen Shapiro 1 , Peter Vorderwisch 2 Show Abstract
1 , Brookhaven National Lab, Upton, New York, United States, 2 , Hahn-Meitner Institute, Berlin Germany
Ni2MnGa is the most studied ferromagnetic shape memory alloy. By use of neutron scattering techniques we have shown that there are phonon anomalies that are strongly temperature dependent and are precursors to the transitions to the intermediate and martensitic phases. These anomalies indicate that there is strong electron-phonon coupling that is the driving mechanism of the transformations. By applying a magnetic field one produces a single variant of the martensite phase, which exhibits an incommensurate charge density wave. By monitoring the diffraction peaks, the H-T phase diagram is explored.
5:00 PM - BB9.3
Cast and Rolling Textures of NiMnGa Alloys.
Robert Chulist 1 , Martin Poetschke 2 , Andrea Boehm 3 , Heinz-Gunter Brokmeier 4 , Ulf Garbe 5 , Carl-Georg Oertel 1 , Werner Skrotzki 1 Show Abstract
1 Institut für Strukturphysik, Technische Universität Dresden, Dresden Germany, 2 Institut für Metallische Werkstoffe, Leibniz-Institut für Festkörper- und Werkstoffforschung, Dresden Germany, 3 , Fraunhofer-Institut für Werkzeugmaschinen und Umformtechnik, Dresden Germany, 4 , GKSS Forschungszentrum, Geesthacht Germany, 5 GKSS Aussenstation, ZWE FRM II, Garching Germany
5:15 PM - BB9.4
In-Situ Neutron Scattering Studies of Magnetic Shape Alloys Under Stress, Temperature and Magnetic Fields.
Donald Brown 1 , Yandong Wang 2 3 , Hahn Choo 2 , Peter Liaw 2 , Michael Benson 2 Show Abstract
1 Materials Science and Technology, Los Alamos National Lab, Los Alamos, New Mexico, United States, 2 Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee, United States, 3 , Northeastern University, Shenyang China
5:30 PM - BB9.5
Systematic Neutron Diffraction Analysis of the Chemical Order and Magnetic Properties of a Series Off-stoichiometric Ni–Mn–Ga Alloys.
Marc Richard 1 , Ratchatee Techapiesancharoenkij 2 , Jorge Feuchtwanger 3 , Thomas Lograsso 4 , Thomas Proffen 5 , Samuel Allen 2 , Robert O'Handley 2 Show Abstract
1 Chemistry, The Richard Stockton College of New Jersey, Pomona, New Jersey, United States, 2 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Dpto. Electricidad y Electrónica, Universidad del Pais Vasco UPV/EHU, Bilbao Spain, 4 Materials and Engineering Physics, Ames Laboratory, Ames, Iowa, United States, 5 , Los Alamos Neutron Science Center, Los Alamos, New Mexico, United States
The chemical order of several off-stoichiometric Ni–Mn–Ga compositions has been systemically analyzed in the austenitic phase using powder neutron diffraction at the Los Alamos National Laboratory Pulsed Neutron Source. The gallium content of all samples was fixed at approximately 22 at%, while the nickel content ranged from 48 to 53 at%, which includes compositions with excess and deficiency in nickel (and manganese) with respect to stoichiometry (Ni = 50 at%, Mn = 25 at%). The site occupancies determined through Rietveld refinement were found to be consistent with previous work (1): in nickel-rich compositions, the excess nickel was found to occupy Mn-sites, while the excess and displaced manganese occupied Ga-sites, and for compositions deficient in nickel, excess manganese occupied both Ni and Ga sites.Lattice constants determined from the refined patterns display two unique dependences on the sample composition. As expected, the unit cell size increases as the Ni:Mn ratio is decreased. The increase is most rapid as the nickel content is reduced from 53 at% towards the stoichiometric value of 50 at%. As the nickel content decreases further into the nickel-deficient range, the rate of change in lattice parameter is reduced markedly. The antiferromagnetic interactions between Mn atoms occupying correct sites as well as nearest neighbor anti-site Mn could account for the observed changes in lattice constant. Finally, saturation magnetization measurements of the compositions studied with neutron diffraction along with the calculated site occupancies have been used to determine the magnitude of the magnetic moment on the anti-site Mn atoms. The anti-site atoms couple antiferromagnetically with correctly sited Mn atoms, leading to a reduction in the total moment with respect to the stoichiometric composition. These results are consistent with previous theoretical modeling of possible antiferromagnetic interactions reported by Enkovaara, et al. (2). (1) M. Richard, et al. Phil. Mag. In press, 2007(2) J. Enkovaara, et al. , Phys. Rev. B 67, 212405 (2003).