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We present the design, simulation, and characterization of a magnetic shape-memory alloy (MSMA) film actuator that transitions from bistable switching to resonant self-actuation when subjected to a stationary heat source. The actuator design comprises two Ni-Mn-Ga films of 10 µm thickness integrated at the front on either side of an elastic cantilever that moves freely between two heatable miniature permanent magnets and, thus, forms a bistable microswitch. Switching between the two states is induced by selectively heating the MSMA films above their Curie temperature Tc. When continuously heating the permanent magnets above Tc, the MSMA film actuator exhibits an oscillatory motion in between the magnets with large oscillation stroke in the frequency range of 50–60 Hz due to resonant self-actuation. A lumped-element model (LEM) is introduced to describe the coupled thermo-magnetic and magneto-mechanical performance of the actuator. We demonstrate that this performance can be used for the thermomagnetic energy generation of low-grade waste heat (T < 150 °C) with a high power output per footprint in the order of 2.3 µW/cm2. [ABSTRACT FROM AUTHOR]

As‐melt‐spun Ni50.2Mn28.3Ga21.5 ribbons are subjected to milling and subsequent annealing for various times. With progressing milling time, the martensitic transformation is gradually suppressed, magnetic moment deteriorates, whereas the crystal structure undergoes a body centered tetragonal (bct) into face centered cubic (fcc) change. High‐resolution transmission electron microscopy demonstrates a twin‐deformed zone in fcc powder particles, which works to improve circularity of as‐produced powders. Subsequent annealing of as‐milled powders restores martensitic transformation and magnetism, as well as it reverts the fcc into the original 5M structure. It is hence showcased that due to an allotropic transformation, brittle Heusler alloys are mechanically optimized for 3D printing without loss of their functional properties. [ABSTRACT FROM AUTHOR]

This work proposes a phenomenological level-set model for ferromagnetic shape-memory alloys (FSMAs), developed under consistent thermodynamic analysis. A set of adequate internal and external variables is chosen so as to describe the rate-independent magneto-mechanical response of a FSMA single crystal. Almost every phenomenological model in bibliography adopts martensitic volume fractions as internal variables. The novelty of this work is the introduction of a continuous level-set function as an internal variable of state, which accounts implicitly for the dissipative twin boundary motion in the evolution of inelastic reorientation of martensitic variants. Following the usual internal variable formalism within the framework of standard magnetomechanics, the evolution equations for the level-set function are derived for the forward and reverse variant reorientation processes. The model is implemented in a two-dimensional special case, where the reduced equations are numerically solved capturing the important effects of stress-induced and magnetic field-induced martensitic variant reorientation, such as magnetization hysteresis, pseudoelastic/partial pseudoelastic behavior and magnetic shape-memory effect. The proposed constitutive model is capable of explaining the constitutive response caused by the reorientation of martensitic variants in FSMAs and may be further improved so as to be implemented in even more complicated situations, such as dynamic and rate-dependent analysis. [ABSTRACT FROM AUTHOR]

This is the concluding part III of a series of papers. The aim of the current paper is to simulate and analyze the procedure of variant reorientation in a magnetic shape-memory alloy (MSMA) sample and to predict the response of the sample subject to various loading conditions. The sample to be considered in this paper has a 3D cuboid shape and is subject to typical magneto-mechanical loading conditions. Variant reorientation in the sample is realized through twin interface movements. To investigate the key features of twin interface movements, the properties of configurational forces on the twin interfaces are analyzed. For both the stress-assisted MFIS tests and the field-assisted quasi-elasticity tests, the magneto-mechanical behavior of the MSMA sample during the whole loading procedure is simulated by using the finite element method. The influence of the initial variant distribution in the sample on its global response is discussed. The obtained numerical results are compared with the experimental results. It can be seen that the model predictions can fit the experimental results both at a qualitative as well as at a quantitative level. [ABSTRACT FROM AUTHOR]

The microstructure evolution, i.e. reorientation of martensite variants, is an important deformation mechanism in shape-memory alloys. This microstructure evolution occurs by the motion of twin boundaries and the nucleation and annihilation of twins in the hierarchical microstructure. An appropriate discrete disclination model for the description of the internal elastic fields and microstructure evolution is introduced for representative volume elements. The model is applied to an experimentally characterized microstructure, i.e. conjugation boundary, and the predicted mechanical response is verified by comparison to experimental measurements. The influence of the twin microstructure on the homogenized stress-strain curve is studied. It is found that regular twinned microstructures have a low strain energy and a high resistance against deformation. These simulations also reason the origin of the microstructural stability of conjugation boundaries. [ABSTRACT FROM AUTHOR]

A series of Mn48- xTi xNi42Sn10 ( x = 1, 2, 3, and 4) ferromagnetic shape-memory alloys were prepared by the arc melting method. The results show that the martensitic transition (MT) temperatures and Curie temperature decrease clearly with Ti addition, but no obvious phase transition from weak-magnetic martensite to a ferromagnetic austenite can be observed for x = 4. Under an applied magnetic field of 30 kOe, the peak values of magnetic entropy changes are 26.6, 36.8, and 15.1 J/kg K, for x = 1, 2, and 3, respectively. The reasons for the large magnetic entropy change and adjustable MT temperatures are discussed in this paper. [ABSTRACT FROM AUTHOR]

A rate-independent model for the quasi-static magneto-elastic evolution of a magnetic shape-memory single crystal is presented. In particular, the purely mechanical Souza-Auricchio model for shape-memory alloys is here combined with classical micro-magnetism by suitably associating magnetization and inelastic strain. By balancing the effect of conservative and dissipative actions, a nonlinear evolution PDE system of rate-independent type is obtained. We prove the existence of so-called energetic solutions to this system. Moreover, we discuss several limits for the model corresponding to parameter asymptotics by means of a rigorous Γ-convergence argument. [ABSTRACT FROM AUTHOR]

Abstract: This work is concerned with the modeling of the stress- and magnetic field-induced reorientation of martensitic variants in magnetic shape-memory alloys. Of special concern is the loading path dependence of the associated nonlinear reorientation strain and magnetization response. A loading case of variable magnetic field in the direction of a constant uniaxial stress with a constant perpendicular magnetic field is analyzed. [Copyright &y& Elsevier]

Abstract: Atomic-force microscopy (AFM), magnetic-force microscopy (MFM), and nanoindentation experiments were performed on a Ni–Mn–Ga single crystal with orthorhombic (14M) martensite. The surface relief was characterized and used to identify the orientation of individual twin variants. It was shown that ab- and bc-twinning modes are present. The magnetic domains are elongated along the crystallographic c direction. Twin variants with the c direction perpendicular to the surface exhibit an out-of-plane magnetization which can be identified through a strong contrast in the MFM image. The deformation caused with nanoindentation is recoverable to a large degree. No trace of deformation twinning was found after unloading. The results are discussed in the light of pseudo-elastic twinning and dislocation activity. It is concluded that although the residual deformation is due to dislocation activity, it is likely that pseudo-elastic twinning took place during loading and unloading. The results indicate a size effect on deformation twinning under localized loading. [Copyright &y& Elsevier]

Elastocaloric cooling is currently under extensive study owing to its great potential to replace the conventional vapor-compression technique. In this work, by employing multiscale characterization approaches, including in situ neutron diffraction in a loading frame, in situ transmission electron microscopy observation at different temperatures, in situ synchrotron X-ray Laue microdiffraction, and high-resolution infrared thermal imaging, we have investigated the thermal and stress-induced martensitic transformation, the stability of superelastic behavior and the associated elastocaloric effect for a Heusler-type Ni 50.0 Fe 19.0 Ga 27.1 Co 3.9 single crystal. On the basis of transformation from cubic austenite into monoclinic martensite with a flexibly and reversibly transferring interface, this unique single crystal exhibits a giant elastocaloric effect of 11 K and ultralow fatigue behavior during above 12 000 mechanical cycles. The numerical simulation shows that the Ni 50.0 Fe 19.0 Ga 27.1 Co 3.9 alloy offers 18% energy saving potential and 70% cooling capacity enhancement potential compared to the conventional shape-memory nitinol alloy in a single-stage elastocaloric cooling system, making it a great candidate for energy-efficient air conditioner applications.