We investigate the magnetic properties of disconnections in magnetic shape memory alloys (MSMAs), focusing on their role in deformation via twinning. Deformation occurs through the motion of twin boundaries, which is mediated by disconnections-defects with both step and dislocation components. These move under applied stress, and in MSMAs, magnetic fields provide an additional force. The paper derives the magnetic charge associated with disconnections, finding that both components contribute, though the step component is typically dominant. This magnetic charge gives rise to a repulsive interaction that decreases inversely with distance at long range. The interaction depends more weakly on distance at short range. Elastic interactions show a similar repulsion at long range but become attractive at short distances. Magnetic interactions are particularly significant in materials with high saturation magnetization and elastic softening, such as Ni-Mn-Ga. These interactions influence fracture behavior under dynamic loading: without a magnetic field, they reduce susceptibility to crack formation, resulting in extremely long fatigue life under mechanical loading. However, under strong rotating magnetic fields, this protective effect is eliminated, and Ni-Mn-Ga fails after significantly fewer load cycles. These theoretical findings align with experimental results, offering insights into fatigue mechanisms in MSMAs.

In the present work, Ni38Co12Mn41Sn9 melt-spun ribbons were annealed under different conditions to make samples free of secondary & gamma; phase precipitates or get them with various average grain sizes. A comparative study was carried out to demonstrate how & gamma; phase formation affects the martensitic transformation (MT) and the magnetocaloric properties linked to the first-order phase transition. The & gamma; phase significantly reduces the structural transformation temperatures, while slightly increases the thermal hysteresis. The & gamma; phase also de-creases the maximum entropy change (|& UDelta;S|max) and the effect is more significant for smaller & gamma; phase precipitates. As compared with those with coarse & gamma; phase grains, the ribbons containing smaller ones possess a lower |& UDelta;S|max but a smaller maximum magnetic hysteresis loss.

This study investigates the impact of alloying Mn-Al-Ga with 3 at.-% Ni and the stability and formation mechanisms of the τ phase and the resulting magnetic properties. The stabilizing effect of Ga on the τ phase was verified, and the ternary alloy’s magnetization was measured up to M2T=482kA/m−1. The phase transformation from γ2 to τ in ternary Mn-Al-Ga was demonstrated microscopically. The solubility limit of Ni into the τ phase was exceeded at 3 at.-% and a primitive cubic κ phase formed. The Ni addition stabilized the τ phase. The highest magnetization at 2 T for the Mn52Al39.4Ga5.6Ni3 alloy was M2T=416kA/m−1 . A new transformation pathway was demonstrated by first annealing the Mn-Al-Ga-Ni alloy at 800 °C for 24 h, which forms a nearly single κ phase, which is followed by a second anneal at 500 °C for 24 h at which the phase τ formed with some remaining κ phase. This is a new transformation mechanism since it involves a phase reaction from κ to τ. The energy product of the Mn-Al-Ga-Ni alloy exceeded that of the ternary Mn-Al-Ga alloy by a factor of 4.5. The κ-phase particles in the Mn-Al-Ga-Ni alloy hinder magnetic domain boundary motion, thus providing a method for magnetic hardening and increasing the energy product.

The characteristic crystallography of "classical" twins was set out over 60 years ago by Bilby and Crocker: a twin boundary is an invariant plane, and the adjacent crystals are interrelated by a homogeneous simple shear. In subsequent work, Crocker and Bevis introduced the notion of "non-classical" twinning, where, as before, the adjacent crystals are interrelated phenomenologically by a homogeneous simple shear, but where the misfit-free interface plane is not invariant. Later experimental studies revealed that "classical" twins actually grow by glissile motion of interfacial line-defects with both dislocation and step character, now known as disconnections. This inhomogeneous shear mechanism has been expounded in the topological model of interfaces developed by Pond and Hirth. In the present paper, experimental observations of "non-classical" twins in Ni2MnGa by Seiner et al. are analysed using the phenomenological and topological approaches. It is concluded that the descriptor "non-classical" is not appropriate because physically feasible glissile disconnections cannot arise in these boundaries. Instead, the boundaries are immobile, misfit-free grain boundaries formed at the intersection of "classical" compound twins.

We anticipate and demonstrate experimentally the mechanical-to-electrical energy conversion through the variation of magnetization resulting from a periodical stress-induced martensitic transformation in the single crystalline Co-Ni-Ga ferromagnetic shape memory alloy. Dynamic loading in a stress-induced austenite-martensite two-phase state was performed with the peak-to-peak strain amplitude from 0.002 to 0.024 in a frequency range of 25 - 150 Hz. A reproducible superelastic behavior occurred at all frequencies. Alongside this remarkable result, we found that the electrical power output depends quadratically on the peak-to-peak strain. This study suggests a new route for the development of broad-band vibration energy harvesting devices.

In the theory of twinning proposed by Bilby and Crocker, the mechanism of twin formation is envisaged as a homogeneous simple shear parallel to η 1 across a plane, K 1. This phenomenological theory predicts the crystallographic forms of classical twinning modes where K 1 is an invariant plane (type I, type II, and compound). Subsequently, Bevis and Crocker also predicted non-classical modes, where the interface is not invariant and both η 1 and K 1 may be irrational. In a later treatment by Pond and Hirth, known as the topological theory, the mechanism of twin growth is modeled as inhomogeneous shear by the motion of line-defects called disconnections along the K 1 plane. For classical twins, the disconnection mechanism leads to twinning parameters consistent with phenomenological predictions, and extensive experimental evidence supporting the disconnection mechanism has been reported in the literature. By contrast, the mechanism of non-classical twin formation has not been elucidated. In the present work, we use the topological model to analyze recent experimental observations of non-classical twins in Ni2MnGa. It is demonstrated that glissile disconnections cannot form in these boundaries, so their formation must involve a novel mechanism. Here, we describe a model in which the active disconnections of two compound twins interact at their intersection, forming a misfit-free non-classical interface.

We studied the mechano-electric energy conversion for Ni-Mn-Ga alloys with dynamic experiments under a bias magnetic field. At low and at high magnetic fields, the magneto-crystalline anisotropy energy and the Zeeman energy dominate the formation of magnetic domains, respectively. At lower fields and when the bias field is tilted against the twin boundary, the formation of 180 & DEG; magnetic domains reduces the net magnetization parallel to the load axis. However, at low strains and in a compressed state and when the bias field is tilted along the twin boundary, the majority of the volume saturates parallel to the load axis. Further, the evolution of the magnetic domains with increasing the magnetic field is different in each twin domain; a lower magnetic field is needed to eliminate magnetic domain boundaries in the twin domain with the direction of easy magnetization closely aligned with the field than in the twin domain with the direction of easy magnetization at a large angle to the field. Therefore, due to increased net magnetization parallel to the load axis, the magnetic structure generated at lower bias fields tilted parallel to the twin boundary is more favorable to maximize power conversion. However, a minimum bias field is required to expand the sample against the axial load and this field must be higher than the switching field. Therefore, in order to optimize electric power output, the energy conversion has to take place at lower bias magnetic fields and on samples with low twinning stress with the field direction inclined nearly parallel to the twin boundaries.

The local actuation of a magnetic shape memory (MSM) element as used in an MSM micropump is considered. This paper presents the difference between an electromagnetic driver and a driver that uses a rotating permanent magnet. For the magnetic field energy of the permanent magnetic drive, the element takes in a significant stray field. In a particular case, energy reduction was 12.7 mJ. For an electromagnetic drive with an identical size of the MSM element, the total magnetic field energy created by the system was 2.28 mJ. Attempts to experimentally nucleate twins in an MSM element by energizing an electromagnetic drive failed even though the local magnetic field exceeded the magnetic switching field. The energy variation is an order of magnitude smaller for the electromagnetic drive, and it does not generate the necessary driving force. It was assumed in previous work that the so-called magnetic switching field presents a sufficient requirement to nucleate a twin and, thus, to locally actuate an MSM element. Here, we show that the total magnetic field energy available to the MSM element presents another requirement.

Additive manufacturing (AM) of magnetic shape-memory alloys (MSMAs) allows fuller use of geometry in the design of MSMA parts and avoids the segregation and high cost associated with single crystal production. While most research effort in AM of MSMAs pursues functional foams or polycrystals, epitaxial growth during liquid-phase AM may enable fully-dense single-crystalline MSMA parts, with associated availability of the full blocking stress. We melted a Ni51Mn24.4Ga24.6 single crystal with a moving laser spot under several process parameter combinations of laser power and velocity. While tracks created with lower laser travel velocity were almost entirely epitaxial, the track created with highest velocity (10 mm/s) included non-epitaxial columnar grains and grains at the top of the track. Synchrotron-based high-energy diffraction microscopy (HEDM) experiments revealed that mosaic spread of epitaxial material was slightly higher than that of surrounding non-re-solidified material. Our results demonstrate epitaxial growth of Ni-Mn-Ga with minimal grain content using full-melting laser processing.

Low twinning stress (TS) is a prerequisite for magnetic shape memory functionality in ferromagnetic martensites. We compare Ni 50 Mn 28 Ga 22 (nominal at.%) single crystals from four different producers to reveal the effect of crystal quality on the TS. Near the reverse martensite transformation, the TS is generally low, about 1 MPa, regardless of mosaicity of up to 1.7° and chemical composition deviations of up to 2 at.% of Mn. Pure type I and type II twin boundaries occur in crystals with smooth chemical composition gradients. The corresponding temperature dependences of TS follow universal linear trends with the slopes of −0.045 MPa/K for type I twins and −0.001 MPa/K for type II twins, enabling a very low TS down to 2 K for the latter. An intermediate slope of −0.023 MPa/K appears for mixed type I/type II twin boundaries in a crystal with sharp local chemical deviations. We conclude that the crystal quality affects the magnitude of the TS indirectly, through its impact on the character of the twin boundaries. The effect is weak near the martensite transformation temperature and strong at low temperatures.

Twinning is an important mechanism of deformation in various crystalline materials, and in particular in shape memory alloys, where it is inherent to the shape memory and super-elasticity effects. This paper presents a generalized methodological approach for analyzing and modeling twin boundary dynamics with particular relevance for shape memory alloys. This approach combines the topological model description of the interface structure at the atomistic/lattice scale with analytical analysis of energy barriers and mechanisms of motion that provide macro-scale kinetic laws for the twin boundary motion. We emphasize the main differences between the topological structures of different types of twin interfaces and their implications for the mobilities of the different twin types. In particular, we elaborate on the relaxed topological structure of type II twin boundaries that contains a coherently facetted structure, where the facets are rational planes that accommodate misfit strain. Then, we clarify the lattice barriers' role in determining the different regimes of the kinetics of twin boundary motion. Further, we develop models leading to analytical expressions for the activation energies of various nucleation processes that dictate the overall kinetics of twin boundary motion, and identify of the rate-limiting process for the different twin types. In the case of compound and type I twins, the analysis leads to an explicit expression for the magnitude of the twinning stress, revealing a strong dependency on the shear modulus and the twinning shear, which is in excellent quantitative agreement with experimental values reported for BaTiO3, Ni-Ti, Cu-Al-Ni, and 10M and NM Ni-Mn-Ga. Moreover, our analysis reasons the different temperature dependencies of the twinning stress exhibited by the different twin types, and in particular the very low temperature sensitivity of type II twins.

Active materials couple a stimulus (electrical, magnetic, and thermal) with a mechanical response. Typical materials such as piezoelectrics strain as bulk materials to the stimuli. Here we consider an undulation created by heterogeneous deformation within a magnetic shape memory alloy (MSM) transducer. We study the mechanical response of an MSM element vs two surface treatments: a polished state with minimal surface stresses, and a micropeened state with compressive surface stress. The polished element had a sharp-featured, faceted trough shape. The micropeened element had a smooth trough shape and an additional crest. The undulation was created by a rotating localized magnetic field, which caused heterogeneous variation of the twin-microstructure. For the polished and micropeened elements, the twin-microstructures were coarse and fine, respectively. For the polished element, the undulation moved by the nucleation of a few twin boundaries, which traveled along the entire element. For the micropeened sample, the twin boundaries moved back and forth over a short distance, thereby creating a dense twin lamellar, which formed the trough. The motion of the lamellar approximated the single thick twin while allowing additional degrees of freedom due to increased mobile interface density and different initial conditions of domain volume fraction. The dense twin microstructure also smoothed the magnetic flux pattern. The undulation amplitude was about 40 μm for the sample in both treatments.

We fabricated a Ni45Co6.4Mn37In11.6 polycrystalline sample with a specific texture by directional solidification. The alloy is characterized by wide successive structural and magnetic phase transitions that considerably extend their temperature intervals with the increase in the applied magnetic field. Under a magnetic field change μoΔH of 5 T, the working temperature window obtained from the magnetic entropy change curve for the structural and magnetic transition is as large as 66 and 52 K, respectively. As a result, the corresponding refrigeration capacity RC reached high values of 118 and 95 J kg−1, respectively, irrespective of the considerably small magnetic entropy changes.

Here we report a systematic research on effects of Fe and Cu upon properties relevant for the magnetic shape memory effect of Ni–Mn–Ga ferromagnetic shape memory alloys. Fe and Cu were identified as elements with potential synergism to increase the martensite transformation temperature of Ni–Mn–Ga magnetic shape memory (MSM) alloys. Eighteen Ni–Mn–Ga–Fe–Cu alloys with different systematic trends in substituting the ternary elements with Cu and Fe have been investigated. We found a method to describe the effectiveness of Ni, Mn, and Cu upon raising the martensitic transformation temperature, lowering the saturation magnetization, and varying the Curie temperature. We find the martensite transformation temperature most influenced by the Ni content, followed by Mn, with a smaller effect of Cu. The saturation magnetization decreases with similar coefficients for Mn and Cu alloying. The Curie temperature monotonously decreases with Mn, but not Cu. The 10M martensite structure is stable for the composition Ni46.5Mn25+XGa25−X−YFe3.5CuY with X and Y range of 0–5.7, and 0.8–3.0. Used in combination with the total e/a, the elemental e/a-ratio gives some insight into the complex behavior of quinary MSM alloys and is a useful method of analyzing MSM alloys for improved functional properties.

The structure of type II twins in 10M Ni-Mn-Ga is modeled using the topological method. This method predicts the same twinning parameters as the kinematic model of Bevis and Crocker. Furthermore, topological modeling provides mechanistic insight into boundary migration rates, the twinning stresses and their temperature dependence. A type II twin is envisaged to form from a precursor, which is its type I conjugate. Disconnections on the precursor k1 plane align into a tilt wall, which, after the relaxation of the rotational distortions, forms the type II boundary parallel on average to the k2 plane. The component defects may align into a sharp wall or relax by kinking into a less orderly configuration. Both interfaces can host additional glissile disconnections whose motion along a boundary produces combined migration and shear. The ease of motion of these defects increases with their core width, and this, in turn, decreases with increasing sharpness of the boundary. Some experimental evidence in other materials suggests that type II twins can reduce their interfacial energy by adopting a configuration of low-index facets, which reduces twin boundary mobility. Topological modeling suggests that such a coherently faceted structure is unlikely in 10M Ni-Mn-Ga, in agreement with the high mobility of type II twin boundaries.

The stress required for the propagation of twin boundaries in a sample with fine twins increases monotonically with ongoing deformation. In contrast, for samples with a single twin boundary, the stress exhibits a plateau over the entire twinning deformation range. We evaluate the twin boundary and magnetic domain boundary interactions for increasing twin densities. As the twinned regions get finer, these interaction regions result in additional magnetic domains that form magnetoelastic defects with high magnetostress concentrations. These magnetoelastic defects act as obstacles for twinning disconnections and, thus, harden the material. Whereas in a low twin density microstructure, these high-energy concentrations are absent or dilute and their effectiveness is reduced by the synergistic action of many twinning disconnections. Therefore, with increasing twin density, the interaction of twin boundaries and magnetic domain boundaries reduces the twin boundary mobility. The defect strength has a distribution such that twinning disconnections overcome soft obstacles first and harder obstacles with ongoing deformation. The width of the distribution of obstacle strength and the density of obstacles increase with increasing twin density and, thus, the hardening coefficient increases with increasing twin density.

: When applying a magnetic field parallel or perpendicular to the long edge of a parallelepiped Ni-Mn-Ga stick, twin boundaries move instantaneously or gradullay through the sample. We evaluate the sample shape dependence on twin boundary motion with a micromagnetics computational study of magnetic domain structures and their energies. Due to the sample shape, the demagnetization factor varies with the direction of external magnetic field. When the external magnetic field is applied perpendicular to the long edge of the sample, i.e. in the direction in which the demagnetizing field is highest, the magnetic energy intermittently increases when the strength

Medical micropumps that utilize Magnetic Shape Memory (MSM) alloys are small, powerful alternatives to conventional pumps because of their unique pumping mechanism. This mechanism—the transfer of fluid through the emulation of peristaltic contractions—is enabled by the magneto-mechanical properties of a shape memory alloy and a sealant material. Because the adhesion between the sealant and the alloy determines the performance of the pump and because the nature of this interface is not well characterized, an understanding of sealant-alloy interactions represents a fundamental component of engineering better solid state micropumps in particular, and metal-polymer interfaces in general. In this work we develop computational modeling techniques for investigating how the properties of sealant materials determine their adhesive properties with alloys. Specifically, we develop a molecular model of the sealant material polydimethylsiloxane (PDMS) and characterize its behavior with a model Ni-Mn-Ga surface. We perform equilibrium molecular dynamics simulations of the PDMS/Ni-Mn-Ga interface to iteratively improve the reliability, numerical stability, and accuracy of our models and the associated data workflow. To this end, we develop the first model for simulating PDMS/Ni-Mn-Ga interfaces by combining the Optimized Potentials for Liquid Simulations (OPLS) [21] force field with the Universal Force Field [5], and show promise for informing the design of more reliable MSM micropumps. We also reflect on the experiences of Blue Waters Supercomputing intern Guevara (the first author) to identify key learning moments during the one-year internship that can help guide future molecular simulation training efforts. ∗Corresponding author Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. Copyright ©JOCSE, a supported publication of the Shodor Education Foundation Inc. © 2020 Journal of Computational Science Education https://doi.org/10.22369/issn.2153-4136/11/2/3