Mobility Of Twin Boundaries Research Articles

Friction in ferroelastic and martensitic materials

Friction during ferroelastic and martensite phase transformations and under external elastic forcing is related to the appearance of jerks as fingerprint for avalanches of moving domain boundaries. It is argued that such avalanches can be observed if the time scale of the experiment is sufficiently long to allow a statistical evaluation and have a time resolution compatible with the characteristic duration of an avalanche. Typical experimental methods include the observation of acoustic emission, resonance elastic spectroscopy, calorimetry, and advanced phonon spectroscopy. The changes of the internal structure of mobile twin boundaries contribute to changes in their mobility and their friction. The local structures of jammed twin boundary patterns are elucidated by computer simulation, their time evolution is similar to that of observed avalanche dynamics.

Ambient-temperature high damping capacity in TiPd-based martensitic alloys

Shape memory alloys (SMAs) have attracted considerable attention for their high damping capacities. Here we investigate the damping behavior of Ti50(Pd50−xDx) SMAs (D=Fe, Co, Mn, V) by dynamic mechanical analysis. We find that these alloys show remarkably similar damping behavior. There exists a sharp damping peak associated with the B2–B19 martensitic transformation and a high damping plateau (Q−1~0.02–0.05) over a wide ambient-temperature range (220–420K) due to the hysteretic twin boundary motion. After doping hydrogen into the above alloys, a new relaxation-type damping peak appears in the martensite phase over 270–360K. Such a peak is considered to originate from the interaction of hydrogen atoms with twin boundaries and the corresponding damping capacity (Q−1~0.05–0.09) is enhanced by roughly twice that of the damping plateau for each alloy. Moreover, the relaxation peaks are at higher temperatures for the TiPd-based alloys (270–370K) than for the TiNi-based alloys (190–260K). We discuss the influence of hydrogen diffusion, mobility of twin boundaries and hydrogen–twin boundary interaction on the temperature range of the relaxation peak. Our results suggest that a martensite, with appropriate values for twinning shear and hydrogen doping level, provides a route towards developing high damping SMAs for applications in desired temperature ranges.

Magnetic shape memory effect and highly mobile twin boundaries

The magnetic shape memory effect can be classified as an example of the multiferroic effect combining ferro-elasticity and (ferro)magnetism. After short overview of all known effects the focus is on magnetic field induced structure reorientation (MIR) in the martensite of Ni–Mn–Ga. In this material giant deformations of up to 12% have been observed in moderate magnetic fields. The phenomenology of the effect is first discussed and a model presented. The properties of Ni–Mn–Ga relevant to MIR are then considered. One necessary condition for MIR is a highly mobile twin boundary or interface between two differently oriented martensite variants (ferroelastic domains). 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. The boundaries strongly differ according to the magnitude and temperature dependence of the twinning stress. Finally, the nature of these boundaries and their different behaviour is reviewed.

In-situ observations and quantification of twin boundary mobility in polycrystalline magnesium

In-situ microtensile experiments were conducted on commercially pure polycrystalline magnesium to examine the motion of twin boundaries (TBs) during deformation in a scanning electron microscope. The nature of the boundaries is identified using electron back scattering diffraction (EBSD) analysis. TB displacement is continuously recorded during the deformation and the TB velocity is computed from the real time images and time data. The TB velocity is observed to monotonically increase with the applied stress and eventually reach a speed of 35 nm/s.

Highly mobile vortex structures inside polar twin boundaries in SrTiO3

Abstract

Local elasticity and mobility of twin boundaries in martensitic films studied by atomic force acoustic microscopy

Nanoscale elastic properties of twinned martensite NiMnGa films were characterized by means of atomic force acoustic microscopy using cantilever contact-resonance spectra to measure the local contact stiffness k* and the local damping Q−1, which contains information on the crystallographic anisotropy of martensitic twin variants and the dissipative motion of twin boundaries (TBs). Images of k* and indentation modulus maps were obtained. Similar to topography images measured by conventional atomic force microscopy in contact mode, they show the nature of the twin structure and thus a regular variation in local elastic modulus. A correlation between k* and Q−1 was observed and mirrors the motion of the TB accompanied by a viscoelastic procedure. The k*-image and the topography image measured are opposite in contrast, which likely arises from mobile and immobile TBs depending on the geometry of twinning. Multi-resonance spectra were measured, which can be related to martensitic multivariants and are explainable as different types of nanotwins. A critical stress, defined as the starting point of softening due to TB movement was determined to be about 0.5 GPa for a thick film (1 μm) and 0.75 GPa for a thin film (0.15 μm), respectively. The values are much larger than that measured for bulk materials, but reasonable due to a large internal stress in the films.

Nanoindentation response of substrate-attached and freestanding single-crystalline Fe7Pd3 ferromagnetic shape memory thin films around the martensite transition: The impact of constraints and beyond

Ferromagnetic shape memory alloys offer great potential in the fields of engineering and medical sciences as integrated actuators or sensors. However, their physical properties, when miniaturized and connected to a substrate or mounted as active elements, are still insufficiently understood. The present work explores the impact of miniaturization and external boundaries on one of the most central features, namely twin boundary mobility. By measuring the nanoindentation response of substrate-attached films and freestanding foils around the austenite↔martensite transformation temperature in classical indentation, as well as dynamical quasi-continuous stiffness-measurement mode, dramatic softening and increasing recovery after film lift-off are discovered. The atomistics of these findings are explored with the help of classical multimillion-atom molecular dynamics simulations on indentation into martensite and austenite films on rigid or flexible substrates, as well as freestanding or mounted thin foils. They clearly demonstrate how substrate or lateral constrains hinder twin boundary motion, while complete untwinning only prevails in the presence of a flexible substrate or completely free foils. Experimentally observed pop-in events can be rationalized as local austenite→martensite transitions. Surface softness, which is observed by low indentation moduli, when compared to predictions from the bulk elastic constants, might indicate a more fundamental scenario close to the martensite transformation.

Differently mobile twin boundaries and magnetic shape memory effect in 10 M martensite of Ni–Mn–Ga

We investigated twin boundaries with sharply different mobility or twinning stress in five-layered modulated (10M) martensite of Ni–Mn–Ga exhibiting magnetic shape memory effect or magnetically induced reorientation. Different mobility is ascribed to the different microstructures of the macrotwin planar interface. In monoclinic approximation the mobile boundaries are of Type I and Type II with complex microstructure of adjoining variants. These boundaries respond differently to magnetic field. For Type II boundary the reorientation takes place at very low field 250Oe.

Twinning in shear and uniaxial loading in five layered martensite Ni-Mn-Ga single crystals

Five-layered martensite Ni-Mn-Ga single crystals are known for their exceptionally mobile twin boundaries allowing a shape change under mechanical stress and by magnetic field. The mechanically measured twinning stress has usually been studied in uniaxial mode, however the twinning and detwinning is generally accepted to be resulted by the shear component. We have studied the twinning behavior at uniaxial and shear stress. In addition we have applied the shear stress at different angles in relation to the expected twinning direction \(\left[ {10\bar 1} \right]\). The results show that the onset of twinning lays at similar stress levels in both uniaxial and shear modes.

Elastic excitations in BaTiO3single crystals and ceramics: Mobile domain boundaries and polar nanoregions observed by resonant ultrasonic spectroscopy

The dynamic properties of elastic domain walls in BaTiO${}_{3}$ were investigated using resonance ultrasonic spectroscopy (RUS). The sequence of phase transitions is characterized by minima in the temperature dependence of RUS resonance frequencies and changes in $Q$ factors (resonance damping). Damping is related to the friction of mobile twin boundaries (90\ifmmode^\circ\else\textdegree\fi{} ferroelectric walls) and distorted polar nanoregions (PNRs) in the cubic phase. Damping is largest in the tetragonal phase of ceramic materials but very low in single crystals. Damping is also small in the low-temperature phases of the ceramic sample and slightly increases with decreasing temperature in the single crystal. The phase angle between the real and imaginary part of the dynamic response function changes drastically in the cubic and tetragonal phases and remains constant in the orthorhombic phase. Other phases show a moderate dependence of the phase angle on temperature showing systematic changes of twin microstructures. Mobile twin boundaries (or sections of twin boundaries such as kinks inside twin walls) contribute strongly to the energy dissipation of the forced oscillation while the reduction in effective modulus due to relaxing twin domains is weak. Single crystals and ceramics show strong precursor softening in the cubic phase related to polar nanoregions (PNRs). The effective modulus decreases when the transition point of the cubic-tetragonal transformation is approached from above. The precursor softening follows temperature dependence very similar to recent results from Brillouin scattering. Between the Burns temperature (\ensuremath{\approx}586 K) and ${T}_{c}$ at 405 K, we found a good fit of the squared RUS frequency [\ensuremath{\sim}\ensuremath{\Delta} (${C}_{11}$\ensuremath{-}${C}_{12}$)] to a Vogel--Fulcher process with an activation energy of \ensuremath{\sim}0.2 eV. Finally, some first-principles-based effective Hamiltonian computations were carried out in BaTiO${}_{3}$ single domains to explain some of these observations in terms of the dynamics of the soft mode and central mode.

Different microstructures of mobile twin boundaries in 10 M modulated Ni–Mn–Ga martensite

The morphology and microstructure of a single, macroscopically straight twin interface with a twinning stress of about 1MPa was analysed in detail by differential interference contrast optical microscopy and X-ray diffraction. The interface was identified as a Type I macrotwin boundary between two variants with a/b-laminates and constant modulation direction, in contrast with a highly mobile twinned interface consisting of Type II macrotwin boundary segments with changing modulation direction and a/b-laminate reported earlier. Theoretical analysis using elastic continuum theory shows that only pure Type I or Type II boundaries are fully compatible with a/b-laminate. Other hypothetical twin microstructures combining these two mobile interfaces are shown to be incompatible to various degrees. A weakly incompatible combination of Type I and II boundaries was experimentally observed. The large difference in mobility between Type I and Type II macrotwin boundaries created at the same location of the same sample indicates that the mobility depends on the internal structure of these boundaries. A possible origin of this different mobility is discussed.

Why Magnetic Shape Memory Alloys?

Why Magnetic Shape Memory Alloys?

The Role of Adaptive Martensite in Magnetic Shape Memory Alloys

AbstractMagnetic shape memory materials require a high twin boundary mobility and low hysteresis for applications mainly as actuators, sensors, and magnetocaloric cooling elements. Usually, outstanding properties are found only in samples with a modulated martensitic structure. Here, we analyze the question why a modulated structure is beneficial and show evidence that the modulated martensite is not an equilibrium phase but a nanoscale microstructure of non‐modulated (NM) martensite. In this review, we combine results from continuum and atomistic theory, as well as local and integral measurements on the model system Ni–Mn–Ga. Following the concept of adaptive martensite the modulated phase forms to minimize elastic energy near the phase boundary by introducing low‐energy twin boundaries between lamellae of the NM martensite that have widths of a few unit cells.

Nanoscale surface properties of a Ni–Mn–Ga 10M magnetic shape memory alloy

Highly mobile twin boundaries allow straining of the 10M Ni–Mn–Ga martensite single crystals with low as 0.15 MPa uniaxial stresses. This is clearly lower than what is possible to magnetically generate in this structure. Therefore, this material can be used for magnetically controlled actuation. In cyclic deformation occurring by twin variant reorientation, fatigue life up to 2 × 10 9 cycles has been demonstrated without decrease of the magnetic-field-induced strain or without failure of the specimen. However, the actuation behavior can be strongly impaired by unfavorable twin variant configuration or surface constraints which decrease the twin mobility and increase the twinning stress. In the present work, the surface properties of the 10M Ni–Mn–Ga are studied in details. The existence of an ultra-thin surface layer having chemical composition different from the bulk crystal is established with the XPS analysis. The influence of this layer on the nano-mechanical properties of the surface is investigated using instrumented nanoindentation both on electropolished crystals and crystals where surface deformation has been introduced by mechanical polishing. The results show that the surface layer appears both in the electropolished and mechanically polished surface. However, the nano-mechanical properties of the surface are distinctly different in the two cases. In the case of electropolished surface essentially elastic loading occurs first followed by a notable pop-in of the indenter, while in the case of mechanically polished surface minor or no pop-in is observed. This behavior is attributed to the differences of the initiation of plastic deformation in the two types of surfaces. The contribution of the layer to the performance and actuation properties of the material is discussed. © 2012 Elsevier B.V. All rights reserved.

Modulated martensite: why it forms and why it deforms easily

Diffusionless phase transitions are at the core of the multifunctionality of (magnetic) shape memory alloys, ferroelectrics and multiferroics. Giant strain effects under external fields are obtained in low symmetric modulated martensitic phases. We outline the origin of modulated phases, their connection with tetragonal martensite and consequences owing to their functional properties by analysing the martensitic microstructure of epitaxial Ni–Mn–Ga films from the atomic to the macroscale. Geometrical constraints at an austenite–martensite phase boundary act down to the atomic scale. Hence, a martensitic microstructure of nanotwinned tetragonal martensite can form. Coarsening of twin variants can reduce twin boundary energy, a process we could observe from the atomic to the millimetre scale. Coarsening is a fractal process, proceeding in discrete steps by doubling twin periodicity. The collective defect energy results in a substantial hysteresis, which allows the retention of modulated martensite as a metastable phase at room temperature. In this metastable state, elastic energy is released by the formation of a ‘twins within twins’ microstructure that can be observed from the nanometre to the millimetre scale. This hierarchical twinning results in mesoscopic twin boundaries. Our analysis indicates that mesoscopic boundaries are broad and diffuse, in contrast to the common atomically sharp twin boundaries of tetragonal martensite. We suggest that the observed extraordinarily high mobility of such mesoscopic twin boundaries originates from their diffuse nature that renders pinning by atomistic point defects ineffective.

Temperature dependence of single twin boundary motion in Ni–Mn–Ga martensite

Magnetic-field-induced reorientation in Ni–Mn–Ga five-layered martensite (10 M) mediated by the motion of single twin boundary was evaluated from magnetization measurements between 20 and 300 K. At 300 K, the single twin boundary moved in an exceptionally small field of 25 kA/m. Twinning stress, as a measure of the twin boundary mobility, was determined from the magnetization curves using a magnetic-energy-based model; it increased from ≈0.1 MPa at 300 K to ≈0.8 MPa at 20 K. The dependence is discussed in terms of thermal activation and the effect of intermartensitic transformation is considered.

Effect of the quenching temperature on the structure and deformation mechanism of a VT22I alloy

The effect of the temperature of heating for quenching on the phase composition, structure, and deformability of a high-strength VT22I titanium alloy is studied. After quenching from a critical temperature or above, an unstable β phase forms in the structure of the alloy; upon further deformation up to 3%, this phase transforms into the α” martensite. This process occurs at low stresses (150–200 MPa) and is characterized by an increase in the internal friction of the material, which is related to the high mobility of interfacial and twin boundaries of β-phase and martensite crystals. In VT22I alloy samples quenched from below the critical temperature, a mechanically stable β phase forms; it is deformed according to a slip mechanism at sufficiently high stresses (550 MPa).

Frequency response of acoustic-assisted Ni–Mn–Ga ferromagnetic-shape-memory-alloy actuator

A prototype of Ni–Mn–Ga based ferromagnetic-shape-memory-alloy (FSMA) actuator was designed and built; an acoustic-assist technique was applied to the actuator to enhance its performance. A piezoelectric stack actuator was attached to the Ni–Mn–Ga sample to generate acoustic energy to enhance twin-boundary mobility and, hence, reduce the magnetic threshold field required for activating twin-boundary motion. The dynamic response of the acoustic-assist FSMA actuator was measured up to 1 kHz actuation. The acoustic assistance improves the actuator performance by increasing the reversible magnetic-field-induced strain (MFIS) by up to 100% (increase from 0.017 to 0.03 at 10 Hz) for drive frequencies below 150 Hz. For frequencies above 150 Hz, the acoustic-assist effect becomes negligible and the resonant characteristic of the actuator takes over the actuator response. Even though the acoustic assist does not improve the actuation at high frequencies, the MFIS output of 5% can be obtained at the resonant frequency of 450 Hz without acoustic assistance. The FSMA actuator is shown to be ideal for applications that require large strain at a specific high frequency

Effects of twin boundary mobility on domain microstructure evolution in magnetic shape memory alloys: Phase field simulation

Effects of twin boundary mobility on domain microstructure evolution during magnetic field-induced deformation in magnetic shape memory alloys are studied by phase field micromagnetic microelastic modeling. The simulations show that different twin boundary mobilities lead to drastically different domain microstructures and evolution pathways, yielding very different magnetization and strain responses, even with opposite signs. The study also reveals complex domain phenomena in magnetic shape memory alloys.

Crystal quality boosts responsiveness of magnetic shape memory single crystals

Magnetic shape memory alloys are promising materials to replace giant magnetostrictive materials and piezoelectrical ceramics in actuating devices due to the large magnetically induced strains. Ni–Mn–Ga is the most intense studied system due to its relatively high operational temperatures and the huge magnetically induced strains reported. Up to now the application of these materials is still limited by the operational temperature range. Additionally twin boundary mobility suffers from structural defects increasing the magnetic fields needed for significant and reproducible strains. The sample quality is affected by crystal inhomogeneity, porosity and impurities. Here new results are reported for the Ni–Mn–Ga class based on a set of single crystals grown by the SLARE method, recently developed by Mecklenburg et al. Single crystalline samples of Ni 49.7 Mn 29.3 Ga 21 of tetragonal martensitic structure exhibit a magnetic field induced strain of more than 4% below 170 mT and 6.5% at only 340 mT. Furthermore the operational temperature regimen could be expanded up to 65 °C.