Twin Boundary Motion Research Articles

Change of magnetic domain structure by mechanically induced twin boundary motion in Ni-Mn-Ga single crystal

The single variant state exhibits usual labyrinth and band magnetic domains depending on orientation of easy magnetization axis. By the passage of single twin boundary induced by mechanical stress the rake and granular domain patterns are formed. These domain patterns are further modified by repeated passage of the twin boundary resulting in similar domain patterns in the sample even though the orientation of the magnetization is different.

The effect of loading rate on characteristics of type II twin boundary motion in Ni-Mn-Ga

We measure the velocities of type II twins in Ni-Mn-Ga over four decades by force-driven pulsed magnetic tests and displacement-driven mechanical tests. Mechanical tests reveal that the twinning stress is a rate-sensitive property, and imply that displacement-driven twin boundary motion follows similar kinetics to those measured under force-driven conditions. All probability density plots of twin boundary velocities demonstrate normal distributions around an average value determined by the kinetic relation. The relative standard deviation of the measured histograms decreases monotonously with increasing velocity. The results imply that the kinetic relation is a valid input for modeling the actuation performance of Ni-Mn-Ga.

Regulating twin boundary mobility by annealing in magnesium and its alloys

We combine pre-compressive test, reverse tensile test, re-compressive test, in-situ electron back-scattered diffraction, and high-resolution transmission electron microscopy to systematically investigate the effect of annealing on the reciprocal motion of twin boundary (TB) in pure Mg and Mg alloys AZ31 and AZ91. We find that the twin boundary mobility (TBM) can be enhanced by decreasing the dislocation density and increasing the number of coherent TBs after annealing for a short time. On the other hand, after prolonged annealing in Mg alloys, TBM decreases since TBs are stabilized by segregated solute atoms and precipitates. As a result, the TBM significantly depends on both the alloying element content and the annealing time. We demonstrate, for the first time, that friction stress and back stress can be applied to clarify the variation of TBM during annealing in Mg alloys. Our findings show that the TBM can be regulated by annealing, opening up a novel avenue for developing Mg alloys with high damping capacity or enhanced mechanical properties.

Magnetic properties of Ni-Mn-Ga-Co-Cu tetragonal martensites exhibiting magnetic shape memory effect

Magnetic properties of Ni46Mn24Ga22Co4Cu4, (Ni45Mn23Ga22Co5Cu5, Ni46Mn22.5Ga23Co5Cu3.5) tetragonal martensites with c/a≈1.14 exhibiting near 10% magnetic field-induced strain (MFIS) were investigated between 10 and 500K and up to 9T field. The (001) easy plane magnetocrystalline anisotropy with K1(300K)=−1.95∙105J∙m−3 (K2<6%|K1|, K3<2%|K1|, MS(300K)=63Am2/kg) was about 30% lower than in the non-doped Ni50.5Mn30.4Ga19.1 reference, which can be related to a reduced c/a ratio in the doped material. In spite of different type of magnetocrystalline anisotropy compared to 10M martensite, the mechanism of MFIS is the same, i.e. with the typical first-quadrant magnetization hysteresis and {101} twin boundary motion.

Acoustic emissions associated with stress-induced twin boundary mobility in Fe7Pd3 ferromagnetic shape memory alloys

Twin boundary mobility is an essential prerequisite for the occurrence of magnetic field induced reversible strains in the ferromagnetic shape memory alloy Fe7Pd3. To study the behavior of twin boundaries, an abrupt movement of these structures is locally induced by nanoindentation while concomitant acoustic emissions are detected. In martensitic Fe7Pd3 bulk sample acoustic emissions can be correlated to pop-in events in the force-depth curves of the nanoindentation indicating twin boundary movement. In contrast, analogous experiments on freestanding Fe7Pd3 thin films indicate an insufficient hindering of twin boundary movements within the samples so that no abrupt dislocation displacement bursts take place.

Dynamic twinning stress and viscous-like damping of twin boundary motion in magnetic shape memory alloy Ni-Mn-Ga

We demonstrate a strong dependence of twinning stress (TS) on twin boundary (TB) velocity in Ni50Mn28Ga22 5M martensite. Different types of TBs exhibit substantially different dynamic properties under magnetic field actuation. For type 1 interface, the dynamic TS drastically increases from its quasi-static value of ~0.6 to ~3.2MPa, limiting the TB velocity to 3–4m/s at saturation magnetic driving force. Type 2 TBs show a gradual increase of the TS from ~0.1MPa to ~2MPa, reaching velocities of ~35–39m/s. Our findings bring to the fore type 2 interfaces for applications demanding high actuation speeds and frequencies.

Temperature dependence of twinning and magnetic stresses in Ni46Mn24Ga22Co4Cu4 alloy with giant 12% magnetic field-induced strain

We study magnetic shape memory alloy Ni46Mn24Ga22Co4Cu4 showing up to 12% magnetic field-induced strain (MFIS). The MFIS is mediated by motion of the compound twin boundaries (CTBs) in non-modulated tetragonal crystal lattice. We present temperature dependences of the twinning stress σTW(T), tetragonality ratio, and magnetic anisotropy energy density KU(T), used for magnetic stress σMAG(T) calculations. With temperature increasing, the twinning stress for CTBs decreases with the slope of ‐0.02MPa/K, twice slower than observed for type I twin boundaries in 10M Ni-Mn-Ga martensite. We found and explained existence of temperature window, within which the giant MFIS is observed.

Microstructure of as-cast single and twin roller melt-spun Ni2MnGa ribbons

Stoichiometric Ni2MnGa alloys are processed by two rapid solidification techniques – single-roller (SR) and twin-roller (TR) melt spinning – and the resulting microstructures and magnetic properties determined. Samples processed at tangential wheel speeds of 10m/s (V10) and 15m/s (V15) are studied in the as-cast condition to analyze the influence of the production methods on the microstructure. Important aspects like the resulting phases, their crystallographic texture, magnetic properties, martensitic transformation temperatures and Curie temperatures are compared. In addition, the magnetization mechanism involving twin boundary motion is explored. Our results indicate that the TR method provides lower cooling rates, thicker samples, higher internal stresses and larger MnS precipitates. However, the quenching rate is mainly determined by the tangential wheel velocity. TR samples also exhibit [100] texture normal to the ribbon plane but in a lesser extent than SR ribbons. Martensitic transformation temperatures are higher in samples V15 (~150K) than in V10 (~100K), with no clear difference between the SR and TR modes. This behavior is explained by considering distinct degrees of disorder in the L21 austenite phase resulting from quenching. The hysteresis of the transformation, defined as the difference Af−MS, takes similar values in the four samples analyzed. Pre-martensitic transformation temperatures are also slightly higher in samples V15, (230±3) K, than in samples V10, (222±3) K, as the magnitude of the Hopkinson effect, in good agreement with a higher residual stress level in TR ribbons. In the martensitic state, all ribbons exhibit hysteresis loops characteristic of a magnetization mechanism involving twin boundary motion. The switching magnetic fields for the onset of Type I twin boundary motion result between 220mT and 365mT, values equivalent to twinning stresses of about 1MPa. It is concluded that both procedures, SR and TR melt spinning, provide microstructures favoring magnetic field induced twin variant reorientation.

Large recovery of six-fold twinned nanowires of α-Fe

Six-fold twinned nanowires sustain very large twists under torsion. The twist is generated by mobile twin boundaries. In contrast, torsion in un-twinned nanowires is carried by dislocations and leads to failure for small torsion angles. The twisted, twinned nanowire can be totally “untwisted” by the reverse motion of twin boundaries, leading to huge pseudo-elasticity. The movement of twin boundaries and dislocations is non-smooth. It progresses by jerks and avalanches. Molecular dynamics simulations show that twin boundary jerks are power law distributed and the size exponent is compatible with mean field theory.

Magnetic Shape Memory Effect in Ni-Mn-Ga Single Crystal

Magnetic shape memory effect is general name for several effects in which the most visible feature is huge strain induced by magnetic field. Magnetic field-induced structure reorientation (MIR) occurs due to motion of twin boundaries in single phase. As the magnetic field is a relatively weak force compared with mechanical stress, very high mobility of twin boundaries is crucial. Here we study the properties of martensite relevant for this effect using X-ray diffraction, optical and electron microscopy, magnetic observation and mechanical testing. In 10M modulated martensite, two types of mobile twin boundary (type I and type II) are observed with complex layered microstructures consisting of a hierarchy of twinning systems. We search for analogue with non-magnetic Cu-Ni-Al shape memory alloy.

The effect of low-temperature aging on the microstructure and deformation of uranium- 6 wt% niobium: An in-situ neutron diffraction study

The mechanical properties of uranium-niobium alloys evolve with aging at relatively low temperatures due to subtle microstructural changes. In-situ neutron diffraction measurements during aging of a monoclinic U-6Nb alloy at temperatures to 573 K were performed to monitor these changes. Further, in-situ neutron diffraction studies during deformation of U-6Nb in the as-quenched state and after aging for two and eight hours at 473 K were completed to assess the influence of microstructural evolution on mechanical properties. With heating, large anisotropic changes in lattice parameter were observed followed by relaxation with time at the aging temperature. The lattice parameters return to nearly their initial values with cooling. The active plastic deformation mechanisms including, in order of occurrence, shape-memory de-twinning, mechanical twinning, and slip-mediated deformation do not change with prior aging. However, the resistance to motion of the as-quenched martensitic twin boundaries increases following aging, resulting in the observed increase in initial yield strength.

A discrete twin-boundary approach for simulating the magneto-mechanical response of Ni–Mn–Ga

The design and optimization of ferromagnetic shape memory alloys (FSMA)-based devices require quantitative understanding of the dynamics of twin boundaries within these materials. Here, we present a discrete twin boundary modeling approach for simulating the behavior of an FSMA Ni–Mn–Ga crystal under combined magneto-mechanical loading conditions. The model is based on experimentally measured kinetic relations that describe the motion of individual twin boundaries over a wide range of velocities. The resulting calculations capture the dynamic response of Ni–Mn–Ga and reveal the relations between fundamental material parameters and actuation performance at different frequencies of the magnetic field. In particular, we show that at high field rates, the magnitude of the lattice barrier that resists twin boundary motion is the important property that determines the level of actuation strain, while the contribution of twinning stress property is minor. Consequently, type II twin boundaries, whose lattice barrier is smaller compared to type I, are expected to show better actuation performance at high rates, irrespective of the differences in the twinning stress property between the two boundary types. In addition, the simulation enables optimization of the actuation strain of a Ni–Mn–Ga crystal by adjusting the magnitude of the bias mechanical stress, thus providing direct guidelines for the design of actuating devices. Finally, we show that the use of a linear kinetic law for simulating the twinning-based response is inadequate and results in incorrect predictions.

Temperature-dependent magnetostriction as the key factor for martensite reorientation in magnetic field

The magnetic field application leads to spatially inhomogeneous magnetostriction of twinned ferromagnetic martensite. When the increasing field and magnetostrictive strain reach certain threshold values, the motion of twin boundaries and magnetically induced reorientation (MIR) of twinned martensite start. The MIR leads to giant magnetically induced deformation of twinned martensite. In the present article, the threshold field (TF) and temperature range of observability of MIR were calculated for the Ni–Mn–Ga martensite assuming that the threshold strain (TS) is temperature-independent. The calculations show that if the TS is of the order of 10−4, the TF strongly depends on temperature and MIR can be observed only above the limiting temperature (~220 K). If the TS is of the order of 10−6, the TF weakly depends on temperature and MIR can be observed at extremely low temperatures. The obtained theoretical results are in agreement with available experimental data.

Effects of demagnetization on Magnetic-Field-Induced Strain and microstructural evolution in Ni-Mn-Ga Ferromagnetic Shape Memory Alloy by phase-field simulations

Demagnetization effect plays a vital role in ferromagnetic materials. However, its influence on Magnetic-Field-Induced Strain (MFIS) in Ni-Mn-Ga Ferromagnetic Shape Memory Alloy has not yet been fully quantified. This study investigates demagnetization effects on MFIS and microstructural evolution in Ni-Mn-Ga for different demagnetization factors (i.e. vector N) by phase-field simulations. The investigation reveals that demagnetization can exert substantial impact on both macroscopic and microscopic behavior of Ni-Mn-Ga through the influence of internal magnetic field on twin boundary movement and domain evolution. The switching field and the saturating field as well as their difference increase with Nf (component of N parallel to easy axis of field-favored martensitic variant). For larger Nf values, the increase of strain appears to be less sharp and more stable (hardening-like). The stable increase of strain is ascribed to the steady movement of twin boundary. The velocity of magnetic domain wall motion during MFIS process decreases as Nu (component of N parallel to easy axis of field-unfavored martensitic variant) increases and their relationship obeys the power law. However, Nu does not seem to affect strain behavior or twin boundary movement. The dependence of switching field and saturating field on Nf is established for application design purposes.

Direct observation of fast-moving twin boundaries in magnetic shape memory alloy Ni–Mn–Ga 5 M martensite

We report a direct observation of fast Type II twin boundary motion under a pulsed magnetic field excitation in Ni–Mn–Ga 5M martensite, utilizing high-speed camera technique. Our results demonstrate saturation velocities up to ~39m/s measured along [100] direction for individual boundaries moving at RT, showing an order of magnitude larger values as compared to the earlier studies. Actuation response times of ~1μs were observed using laser vibrometry technique. The reported results show a minor role of magneto-static forces and emphasize the role of inertia and internal friction in twin boundary dynamics at the measured speeds.

Real time monitoring of twin boundary motion in polycrystalline magnetocaloric Ni2+xMn1−xGa by in situ focused ion beam microscopy

Polycrystalline Ni-Mn-Ga intermetallics were studied for their potential in magnetic refrigeration near room temperature. Previous research has shown an interrelation between entropy change and magnetocrystalline anisotropy. In this work, the influence of mechanical stress on magnetocrystalline anisotropy was studied. Both, uncontrolled processing and defined in situ mechanical loading with focused ion beam imaging were used to show the influence of initial directional solidification and the relationship between compressive stress and movement in twin boundaries in the martensite phase.

Phase-field simulations of partial pseudoelastic stress-strain behavior and microstructure evolution of Ni-Mn-Ga

The partial pseudoelastic stress-strain behavior and the underlying microstructure evolution of Ni-Mn-Ga ferromagnetic shape memory alloy under different values of constant (assisted) magnetic field H and demagnetization factor N are studied by phase-field simulations. The results show that both H and Nfield (component of N parallel to H) have significant influence on the partial strain recovery in the compressive stress loading-unloading cycle, while Nstress (component of N parallel to the stress) only affects the formation of magnetic domains in the stress-preferred variant and has no influence on the strain recovery behavior. The recovered strain can be increased by raising H or reducing Nfield. Moreover, the range of H for the partial strain recovery increases with Nfield. It is shown that Ni-Mn-Ga exhibits the partial pseudoelasticity at moderate/high H under a large Nfield rather than just at low H as observed in existing experiments, demonstrating the significant influence of Nfield on the strain recovery behavior. The work reveals that both H and Nfield affect the strain recovery behavior through the effective magnetic field *Hfield (component of the internal magnetic field parallel to H) during the stress cycle. *Hfield decreases with the twin boundary motion during the reverse martensite reorientation and eventually becomes insufficient to trigger the twin boundary motion, which is responsible for the partial strain recovery.

Changes in magnetic domain structure during twin boundary motion in single crystal Ni-Mn-Ga exhibiting magnetic shape memory effect

The complexity of Ni-Mn-Ga single crystal originates from the interplay between ferromagnetic domain structure and ferroelastic twinned microstructure. Magnetic domain structure in the vicinity of single twin boundary was studied using magneto-optical indicator film and magnetic force microscopy technique. The single twin boundary of Type I was formed mechanically and an initial magnetization state in both variants were restored by local application of magnetic field (≈40 kA/m). The differently oriented variants exhibited either stripe or labyrinth magnetic domain pattern in agreement with the uniaxial magnetocrystalline anisotropy of the martensite. The twin boundary was then moved by compressive or tensile stress. The passage of the boundary resulted in the formation of granular or rake domains, respectively. Additionally, the specific magnetic domains pattern projected by twin boundary gradually vanished during twin boundary motion.

Influence of the twin microstructure on the mechanical properties in magnetic shape memory alloys

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.

Molecular dynamics simulation of effects of twin interfaces on Cu/Ni multilayers

We perform molecular dynamics simulation of the indentation on pure Cu and Ni films and Cu/Ni multilayered films with a cylindrical indenter, aimed to investigate the effects of the cubic-on-cubic interface and hetero-twin interface on their mechanical properties. We also investigate systematically the formation of twin boundary in the pure metals and the effects of the cubic-on-cubic and hetero-twin interface on mechanical properties of the multilayers. We find that the slip of the horizontal stacking fault can release the internal stress, resulting in insignificant strengthening. The change in the crystal orientation by horizontal movement of the atoms in a layer-by-layer manner is found to initiate the movement of twin boundary, and the hetero-twin interface is beneficial to the hardening of multilayers. Moreover, we also find that increasing number of hetero-twin interfaces can harden the Cu/Ni multilayers.