Twin Boundary Motion Research Articles

Effect of B4C, TiB2 and ZrSiO4 ceramic particles on mechanical properties of aluminium matrix composites: Experimental investigation and predictive modelling

This paper focuses on the influence of processing temperature and inclusion of micron-sized B4C, TiB2 and ZrSiO4 on the mechanical performance of aluminium matrix composites fabricated through stir casting. The ceramic/aluminium composite could withstand greater external loads, due to interfacial ceramic/aluminium bonding effect on the movement of grain and twin boundaries. Based on experimental results, the tensile strength and hardness of ceramic reinforced composite are significantly increased. The maximum improvement is achieved through adding ZrSiO4 and TiB2, which has led to 52% and 125% increase in tensile strength and hardness, respectively. To predict the effect of incorporating ceramic reinforcements on the mechanical properties of composites, experimental data of mechanical tests are used to create 3 models named Levenberg–Marquardt Algorithm (LMA) neural networks. The results show that the LMA- neural networks models have a high level of accuracy in the prediction of mechanical properties for ceramic reinforced-aluminium matrix composites.

The effects of magnetic and mechanical microstructures on the twinning stress in Ni-Mn-Ga

The ferromagnetic 10M Ni-Mn-Ga alloy exhibits complex magnetic and mechanical microstructures, which are expected to form barriers for motion of macro twin boundaries. Here, the contributions of both microstructures to the magnitude of the twinning stress property are investigated experimentally. A series of uniaxial loading-unloading curves are taken under different orientation angles of a constant magnetic field. The different 180° magnetic domains microstructures that are formed across the twin boundary in each case are visualised using a magneto optical film. Analysis of the different loading curves and the corresponding magnetic microstructures show that the latter does not contribute to the barriers for twin boundary motion. In accordance, the internal resisting stress for twin boundary motion under any magnetic field can be taken as the twinning stress measured in the absence of an external field. In addition, a statistical analysis of the fine features in the loading profiles reveals that the barrier for twinning is associated with a μm sized characteristic length scale. This length scale corresponds to the typical thickness of micro-twinning laminates that constitute a mechanical microstructure. These findings indicate that the magnitude of the twinning stress in 10M Ni-Mn-Ga is determined by the characteristic fine twinned mechanical microstructure of this alloy.

The relation between lattice parameters and very low twinning stress in Ni50Mn25+xGa25−x magnetic shape memory alloys

In search of the origins of the extraordinary low twinning stress of Ni-Mn-Ga 10M martensite, we studied the temperature induced changes in lattice parameters of Ni50Mn25+xGa25−x (x = 2.7–3.9) single crystal samples and compared them with twinning stress dependences. The alloys exhibited transformation to five-layered (10M) martensite structure (cubic to monoclinic) between 297 to 328 K and exhibited the magnetic shape memory effect in martensite. The structural changes were monitored using x-ray diffraction in the temperature range 200–343 K. The 10M structure was approximated by monoclinic lattice, a = b > c, γ > 90° with the coordinates derived from the cubic unit cell of the parent L21 phase. The lattice parameters γ and c/a correlate well with the universal linear increase of twinning stress of type 1 twins with decreasing temperature. On the contrary, the twinning stress is not affected by differences between a and b and thus a/b twins seem to play no role in a − c twin boundary motion resulting in magnetically induced reorientation.

Pulsed magnetic field-induced single twin boundary motion in Ni–Mn–Ga 5M martensite: A laser vibrometry characterization

Pulsed magnetic field-induced motion of single twin boundaries in Ni–Mn–Ga 5M martensite has been studied using laser vibrometry technique. Maximum actuation velocities of 0.25m/s for Type I and 2m/s for Type II twin boundary motion were directly observed, which corresponds to velocities of 4.2m/s and 33m/s for these types of twin boundaries moving inside the crystal. Actuation response time of 2μs and actuating acceleration of about 2×105m/s2 for Type II boundary were achieved. The difference between Type I and Type II boundaries and the limiting role of inertia in twin boundary motion are discussed.

Magnetic Domains and Twin Microstructure of Single Crystal Ni–Mn–Ga Exhibiting Magnetic Shape Memory Effect

Magnetic Shape Memory Effect or more precisely magnetic-field-induced-structural reorientation (MIR) generate large strain (up to 12 %) and fast response (around 1 kHz) in a moderate magnetic field below 1 T. The strain is caused by twin microstructure reorientation in which the maximum generated strain is determined by difference of lattice constants of the pseudotetragonal structure a = b > c, i.e. e MAX = e 0 = 1−c/a. The reorientation is mediated by twin boundary motion. Very high mobility is, therefore, necessary condition for the existence of the effect. Recently considering more precise structural description of 10M martensite of Ni-Mn-Ga alloys as monoclinic, i.e. a > b > c and γ π 90, we showed that there are two different kinds of mobile a-c twin boundary, Type I and II with different microstructure [1]. The boundaries differ not only in magnitude of twinning stress needed to move twin boundary, i.e. in mobility, but also in temperature dependence of the mobility or twinning stress [2]. Despite of the simplicity of the moving interface the twinned structure is complex [3]. The model of the movable Type II twin boundary is shown in the figure. The twinned structure consists apart of the moving a-c twin boundary the modulation twinning bands and a-b twinning.

Properties of the pulsed electric current sintered Ni–Mn–Ga–Co–WC composites

Two double dispersion composites were prepared by pulsed electric current sintering (PECS) from two powders on Ni–Mn–Ga–Co/WC–Co system. Composite 1 consists of 90 vol% WC–12Co and 10 vol% of non-modulated Ni–Mn–Ga–Co martensite and Composite 2 of 18 vol% of WC–12Co and 82 vol% of 10 M Ni–Mn–Ga–Co martensite. Damping capabilities of both composites were studied with DMA and cavitation erosion resistance tests. Both composites show promise as candidates as damping element materials when high hardness and wear resistance is required but Composite 2 had a higher damping capability. The high damping ability of composites are proposed to be derived from the combined effect of energy dissipation by twin boundary movement in the Ni–Mn–Ga matrix combined with the high hardness and wear resistance arising from the tungsten carbides.

The exploration of the effect of microstructure on crackling noise systems

A wide variety of physical systems respond to changing external conditions through discrete impulsive events called jerks, typically leading to collective “crackling noise” behaviour. Statistical distributions of jerky events often exhibit a universal scale-invariant power law, regardless of the specific mechanisms that are responsible for crackling noise processes and microstructural features that affect them. Here, we analyse uniaxial compression loading curves of two different physical systems that exhibit jerky behaviour: a martensitic NiMnGa single crystal and a stack of corrugated fiberboards. The jerky response is attributed to a non-uniform twin boundary motion along the NiMnGa crystal and to a local buckling of individual fiberboard layers. In both cases, our analysis reveals that different variables exhibit different statistical distributions. While the velocity of temporal processes within jerky events exhibits scale invariant distribution, the irreversible displacements induced throughout complete events are distributed around a characteristic value. In the case of NiMnGa, the displacement of a twin boundary is directly related to the length-scale of the internal magneto-mechanical microstructure. Similarly, the displacement of the fiberboard stack corresponds to the thickness of individual board layers. These observations reveal the effect of the internal microstructure on crackling noise systems and demonstrate an analysis approach for uncovering the details of the jerk mechanism.

Transmission Electron Microscope In Situ Straining Technique to Directly Observe Defects and Interfaces During Deformation in Magnesium

In situ straining was used to study deformation behavior of hexagonal close-packed (hcp) metals. Twinning and dislocation motion, both essential to plasticity in hcp materials, were observed. Typically, these processes are characterized postmortem by examining remnant microstructural features after straining has occurred. By imposing deformation during imaging, direct observation of active deformation mechanisms is possible. This work focuses on straining of structural metals in a transmission electron microscope (TEM), and a recently developed technique that utilizes familiar procedures and equipment to increase ease of experiments. In situ straining in a TEM presents several advantages over conventional postmortem characterization, most notably time resolution of deformation and streamlined identification of active deformation mechanisms. Drawbacks to the technique and applicability to other studies are also addressed. In situ straining is used to study twin boundary motion in hcp magnesium. A $$ \left\{ {10\bar{1}2} \right\} $$ twin was observed during tensile and compressive loading. Twin-dislocation interactions are directly observed. Notably, dislocations are observed to remain mobile, even after multiple interactions with twin boundaries; this result suggests that Basinki’s dislocation transformation mechanism by twinning is not present in hcp metals. The coupling of in situ straining with traditional postmortem characterization yields more detailed information about material behavior during deformation than either technique alone.

An in-situ study of magnetic domain structures in undercooled Fe-29.5 at. %Pd magnetostrictive alloys by Lorentz microscopy and electron holography

Understanding of correlation between magnetic domain structure and functional properties is of importance for the magnetic field driven phase transition (e.g., martensitic transformation) or microstructure variation (e.g., twin boundary motion) materials. In this work, we report a Fe-29.5 at. %Pd shape memory alloy treated by undercooling processing upon a degree of 320 K below the liquid temperature. The effect of high undercooling on the solidified microstructure and martensitic transformation temperatures was investigated. By in-situ Lorentz transmission electron microscopy and electron holography, magnetic domain structure and the field-induced domain wall motion behavior in as-cast and undercooled samples have been schematically studied. The irregular domain structure can be observed in these alloys. On the application of a field up to 300 Oe, the domain walls for both samples are able to move along the direction of the external magnetic field, but structural transition and rearrangement of variants are not observed in the undercooled alloy. The large magnetostriction of Fe-29.5 at. %Pd undercooled alloy originates from the irregular domain walls motion instead of the rearrangement of martensitic twin variants.

Temperature rise of high-frequency martensite reorientation via Type II twin boundary motion in NiMnGa Ferromagnetic Shape Memory Alloy

We measured the temperature rise of the high-frequency magnetic-field-induced martensite reorientation in Ni–Mn–Ga Ferromagnetic Shape Memory Alloy and observed the morphology of the associated twin boundaries. It was found that this temperature rise is mainly caused by low-friction Type II twin boundary motion, even though high-friction Type I twin boundary motion is dominant in quasi-static stress-induced martensite reorientation. A simple formula considering the frictional twin boundary motion and heat transfer is derived to describe this temperature rise.

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.

De-twinning and Texture Change in an Extruded AM30 Magnesium Alloy during Compression along Normal Direction

Twinning and de-twinning are the salient deformation mechanisms in hexagonal close-packed (hcp) metals. The aim of this study was to examine and quantify the de-twinning process involving a reversible motion of twin boundaries in an extruded AM30 magnesium alloy after re-compression along the normal direction (ND) of pre-compressed samples along the extrusion direction (ED). {10 1 ¯ 2} extension twins were first introduced at a compressive strain of 3.7% along the ED. The subsequent compressive deformation along the ND induced a gradual shrinkage of twins with increasing cumulative true strain, and the complete de-twinning occurred at a strain of ∼7.7%. The twin width decreased linearly with increasing true strain. Texture measurements verified the rotation of c-axes of hcp unit cells towards the anti-compression direction due to {10 1 ¯ 2} extension twinning after compression along the ED, and a gradual return of c-axes to the initial orientation due to twin shrinking or de-twinning during the following compression along the ND. The {10 1 ¯ 2} twinning corresponded to the formation of new texture components C{ 1 ¯ 2 1 ¯ 0} and D{01 1 ¯ 0} and a decrease in the initial texture components A{0001} 1 ¯ 1 ¯ 0> and B{0001} 1 ¯ 0>, while the twin shrinking or de-twinning was characterized by a gradual vanishing of components C{ 1 ¯ 2 1 ¯ 0} and D{01 1 ¯ 0} and an increase in the components A{0001} 1 ¯ 1 ¯ 0> and B{0001} 1 ¯ 0>.

A macroscopic constitutive model of temperature-induced phase transition of polycrystalline Ni2MnGa by directional solidification

Directional solidification technology has been widely used to improve the properties of polycrystalline Ni2MnGa materials. Mechanical training can adjust the internal organizational structures of the materials, reduce the stress of twin boundaries motion, and then result in larger strain at lower outfield levels. In this paper, we test the microscopic structure of Ni2MnGa polycrystalline ferromagnetic shape memory alloy produced by directional solidification and compress it along two axes successively for mechanical training. The influences of pre-compressive stresses on the temperature-induced strains are analyzed. The macroscopic mechanical behaviors show anisotropy. According to the generating mechanism of the macroscopic strain, a three-dimensional constitutive model is established. Based on thermodynamic method, the kinetic equations of the martensitic transformation and inverse transformation are presented considering the driving force and energy dissipation. The prediction curves of temperature-induce strains along two different directions are investigated. And the results coincide well with the experiment data. It well explains the macroscopic anisotropy mechanical behaviors and fits for using in engineering.

Structural, Magnetic and Mechanical Properties of Oligocrystalline Ni-Mn-Ga Shape Memory Microwires

In single-crystalline Ni-Mn-Ga alloys, high reversible strain (∼10%) induced by an external magnetic field due to twin variants reorientation can be achieved. However, twin boundary motion is hindered by grain boundaries in polycrystalline Ni-Mn-Ga alloys, resulting in a low strain output. In this work, oligocrystalline Ni-Mn-Ga microwires with near bamboo-like grain structure (1-3 grains across the wire diameter of 30-80μm) were prepared by melt-extraction technique and subsequent annealing treatment. Single twin variant, with twin lamella thickness up to 6μm, spans across the entire diameter of the microwire. The oligocrystalline microwires are of ferromagnetic nature at ambient temperature. Twining boundary motion was demonstrated by tensile test performed on a Q800 DMA where different twining stresses corresponding to various grain orientations in a single microwire were detected. A maximum strain of ∼1.57% was obtained and related to the orientation of a subset grains. The martensite twin variant orientations after tensile test were observed using a SEM.

Properties of NiMnGa Alloys Ultra Rapidly Solidified by Suction Casting

Ni2MnGa alloys are obtained by suction casting in water chilled cylindrical copper moulds 50mm long and 1-4mm in diameter; the microstructure and the magnetic properties are then investigated as functions of the cylinder diameter. The solidification substructures are characterized by scanning electron microscopy and confocal microscopy. The martensitic transformation temperatures, determined by magnetization vs. temperature measurements, are lower than those measured in bulk Ni2MnGa single crystals (202K). The demagnetization curves measured from saturation in the martensitic state show two steps: the first is at about+130 mT and the larger second one at relatively large inverse fields (about -300 mT), except in cylinders 4mm in diameter, where these steps overlap at low inverse fields. These steps are likely to arise from a demagnetization mechanism involving field induced twin boundary motion in a few martensite variants,indicating a ferromagnetic shape memory effect in the cylinders.

Modeling of Single Crystal Magnetostriction Based on Numerical Energy Relaxation Techniques

AbstractIn this work, an incremental energy minimization technique is proposed to simulate the magnetomechanically‐coupled, nonlinear, anisotropic and hysteretic response of single crystalline magnetic shape memory alloys (MSMA). The model captures the three key physical mechanisms that cause this characteristic behavior, namely the field‐ or stress‐induced martensite variant reorientation (twin boundary motion), magnetic domain wall motion, and local magnetization rotation, through an (incremental) energy minimizing evolution of internal state variables. Representative numerical response predictions are presented, compared to experimental observations, and discussed with respect to the associated microstructure evolution. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Development of microstructure in submicron particles reinforced magnesium matrix composite processed by room temperature deformation

Magnesium matrix composite reinforced with submicron SiC particles was subjected to room temperature deformation. A stepped tensile method was adopted to observe the development of microstructure at different tensile strain state. The microstructure evolution determined by transmission electron microscopy showed that the existence of submicron SiC particles could promote dislocation multiplication as well as impede dislocation motion. Dislocation density around SiC particles increased with the increase of tensile strain. Compared with the matrix alloy, the composite could withstand greater external loads, which can be related to the pinning effect caused by the submicron SiC particles on the movement of grain and twin boundaries. The study of the interface between the submicron SiC particles and matrix in the composite suggested that single submicron SiC particle bonded well with the matrix alloy after tensile process. Initiation of micro-cracks usually generated in the submicron particle dense zone during tensile process, which could lead to the fracture of composite.

Understanding Motion of Twin Boundary—A Key to Magnetic Shape Memory Effect

High mobility or the ease of movement of twin boundary in magnetic field, necessary for magnetically induced reorientation (MIR) is studied. In modulated 10M martensite two types of a-c twin boundaries, Type I and Type II are mobile in magnetic field. Here, we focus on the behavior of a single twin boundary in magnetic field at different temperatures. Using Type II, twin boundary MIR resulting in 6% field-induced deformation can occur in the field ~10 kA/m. For Type I, in contrast, the field more than ten times larger is needed. The effect of demagnetization is evaluated. In addition, the temperature dependence of the field needed to initiate twin motion is flat for Type II compared with sharp increase for Type I. The explanations for this sharply differing behavior are proposed.

Dynamics of twin boundaries in ferromagnetic shape memory alloys

Combined experimental and theoretical investigation of discrete twin boundary motion in ferromagnetic shape memory alloys 10M NiMnGa is presented. Motion of individual boundaries is studied through mechanical tests and pulsed magnetic field experiments. Analysis of the experimental results leads to identification of two intrinsic energy barriers that dominate twinning kinetics and possible mechanism for overcoming these barriers. At low velocities, the twinning stress property is dictated by a long range (micrometre scale) periodic potential. At higher rates, a short range (nanometre scale) potential, which is related to the periodicity of the lattice, is responsible for the transition in kinetic behaviour from slower thermally activated to faster athermal motion. These observations enable formulation of different kinetic relations that are valid at different ranges of the driving force. The derived relations show a very good fit to the experimental results, allowing quantitative extraction of fundamental nanoscale properties of the twin boundary.

Rapid actuation and response of Ni–Mn–Ga to magnetic-field-induced stress

The mechanism that is responsible for the large strains in the magnetic shape memory (MSM) alloy Ni–Mn–Ga is the movement of twin boundaries caused by an internal magnetic-field-induced stress. This is the primary property that makes Ni–Mn–Ga such an attractive material for use as an actuator. Hence, a deeper understanding of the movement and dynamics of twin boundaries in Ni–Mn–Ga would enable the development of applications that could take full advantage of the material’s properties. In this study, a novel experimental method was developed that could observe, in situ, the movement of a single twin boundary within a sample of Ni–Mn–Ga. A twin boundary velocity of 82.5ms−1, an actuation response time of 2.8μs and an actuation acceleration of 1.6×106ms−2 were experimentally observed. These experimental results have also been validated by an independently developed theoretical model. This is the most rapid actuation and twin boundary movement of all actuating materials on this scale and these results may have a significant impact on future applications, particularly in microtechnology where speed and precision are essential.