Mobility Of Twin Boundaries Research Articles

Thermal and structural properties of the martensitic transformations in Fe7Pd3 shape memory alloys: an ab initio-based molecular dynamics study

Ferromagnetic shape memory alloys, including the Fe7Pd3 system, constitute an upcoming class of functional materials, whose atomic-scale physical foundations are still insufficiently understood. The present work employs molecular dynamics simulations, based on ab initio derived embedded atom method potentials, to study martensitic transformations and twin variant reorientation. We address thermal and stress induced austenite-martensite transitions, twinning, as well as twin boundary mobility. While the predicted thermal properties are in accordance with experimental observations, we explore the detailed crystallography underlying transformation as well as twin boundary motion.

Microstructure evolution and kinetic laws for the motion of multiple twin boundaries in Ni–Mn–Ga

The motion of individual twin boundaries in ferromagnetic Ni–Mn–Ga alloys is the basic mechanism by which large reversible strains are generated. The resulting mechanical response makes these alloys potential candidates for magneto-mechanical actuation applications. In this work, we characterize the relations between the magnetically induced strain response of Ni–Mn–Ga single crystals and their internal twin boundary arrangements and kinetics. The macroscopic response is measured under constant magnetic fields, while the kinetics of individual twin boundaries is measured by the pulsed magnetic field method. By testing two different crystals with different twinning microstructures, we show that the main characteristics of the macroscopic response can be controlled by the number of mobile twin boundaries. In addition, we show that the magnitudes of controlling kinetic parameters of different twin boundaries vary with their location within the sample.

Study of reversible motion of [formula omitted] tensile twin boundaries in a magnesium alloy during strain path changes

Twinning process involving reversible motion of twin boundaries was examined and quantified in a rolled Mg-3Al-1Zn magnesium alloy compressed along two perpendicular directions. The evolution of twinning is analyzed by quasi in-situ electron backscatter diffraction technique, and the detailed structure of the twin boundary is analyzed using high-resolution transmission electron microscopy technique. The results suggest that both twinning and detwinning are attributed to the mobility of twin boundaries. Low-angle boundaries were identified at the prior twin boundaries after detwinning due to the effect of alloying elements, and these low-angle boundaries will affect subsequent deformation. Twin boundary is serrated and consists of 101¯2 coherent twin boundaries and prism-basal boundaries that control twin boundary migration.

Martensitic transformation, twin boundary and phase interface mobility of directionally solidified Ni-Mn-Ga alloys during compression by EBSD tracing

Directionally solidified Ni-Mn-Ga alloy with orderly arrayed austenite and non-modulated martensite (NM) was formed due to microsegregation between dendrite arm and interdendritic region, leading to preferred orientation of <001>A and <110>M coexistence at ambient temperature. The stress-induced martensitic transformation took place due to the increase of martensitic transformation temperature under step-wise uniaxial compression. The detwinning process accompanied with dislocation mobility was easy to carry out on the twin planes 45° inclined to the compression axis, compared with the twin planes parallel to the compression axis in the dominant twinned groups. The martensitic transformation and reorientation of variants were investigated by electron backscatter diffraction.

High damping capacity of single crystalline iron-palladium alloy exhibiting a weak first-order martensitic transformation

We have investigated the influence of twin boundary properties on the damping behavior of single crystalline Fe-31.2Pd (at%) alloy, which exhibits a weak first-order martensitic transformation. Different static tensile stress was applied by dynamic mechanical analyzer to adjust the twin boundary density and hydrogen was doped to modify the twin boundary mobility. A high damping capacity (tanδ > 0.1) in a wide temperature range from about 140 K to 230 K was observed under a low static stress of about 0.3 MPa. Hydrogen doping further increased the maximum value of the damping capacity by introducing a relaxation tanδ peak. The damping capacity decreased with increasing static stress probably due to the decrease in twin boundary density.

Damping capacity of pre-compressed magnesium alloys after annealing

We systematically studied the effect of annealing on damping capacity of pre-deformed magnesium alloy and demonstrated the close relationship between damping capacity and twin boundary mobility. The damping capacity of pre-compressed AZ31 alloy can be improved by annealing for short period as a result of increased twin boundary mobility. In contrast, the damping capacity will be deteriorated after prolonged annealing in pre-compressed AZ91 alloy, which is ascribed to the stabilization of twin boundary by significant segregation and precipitation. In addition, we firstly succeeded to distinguish the damping capacity contributed by TB motion from that by dislocation motion in Mg alloys. This study provides the strategy to enhance damping capacity by improving twin boundary mobility after appropriate annealing, the time of which depends on the alloying element concentration.

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.

Pulsed magnetic field-induced changes in the meso- and nanostructure of Co49Ni21Ga30 martensite

A near single-variant martensitic state of Co[Formula: see text]Ni[Formula: see text]Ga[Formula: see text] alloy was induced by a static axial stress of 66[Formula: see text]MPa. Upon application of a magnetic field pulse, the single-variant state of stressed specimen was transformed into a hierarchic martensitic structure at micron, submicron and nanometer scales. The martensitic structures, which were induced during the magnetic field pulse, did not disappear after the field was switched off. This stability of the field-induced martensitic structure is attributed to the appearance of additional twinning systems with hardly mobile twin boundaries.

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.

Temperature dependence of twinning stress – Analogy between Cu–Ni–Al and Ni–Mn–Ga shape memory single crystals

To obtain a direct non-magnetic analogy to Ni–Mn–Ga 10M martensite with highly mobile twin boundaries, we present the recalculation of twinning systems in Cu–Ni–Al martensite. In this approach, the twinning planes denoted as Type I, Type II and compound have similar orientations for both alloys (Ni–Mn–Ga and Cu–Ni–Al). In Cu–Ni–Al, compound twinning exhibits the twinning stress of 1 to 2 MPa comparable to twining stress of Ni–Mn–Ga. In contrast Type II twinning stress of Cu–Ni–Al is approximately 20 MPa, i.e. much higher than twinning stress for Type II in Ni–Mn–Ga (0.1 to 0.3 MPa). Similarly to Ni–Mn–Ga, the twinning stress of Type II in Cu–Ni–Al is temperature independent. Moreover, no temperature dependence was found also for compound twinning in Cu–Ni–Al.

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.

An ‘ex situ’ electron backscattered diffraction study of nucleation and grain growth in pure magnesium

In the present study, the microstructure and texture evolution of pure magnesium during annealing was captured through ‘ex situ’ electron backscattered diffraction (EBSD). Pure magnesium was subjected to plastic deformation through CSM (Continuous Stiffness Measurement) indentation followed by annealing at 200°C for 5min and 30min of soaking time respectively. The nucleation mechanism was attributed to the formation of sub-grains independent of the orientation of the grains. However, the growth of the grains was dependent on the difference in GOS values between the neighboring grains. When the difference in GOS values was more, growth of the grain with low GOS value was noticed during short term annealing. Otherwise, a strain free sub-grain was formed in the deformed microstructure which subsequently consumed the neighboring grain. A rotation of nearly 10–30° with respect to c-axis of parent grain was also noticed during the growth of the grains. Further, high mobility of contraction or double twin boundaries leading to grains >10μm were also observed during static recrystallization.

Impact of solute elements on detwinning in magnesium and its alloys

By combining finite element analysis, compressive tests, reverse tensile tests, in-situ electron-backscattering diffraction, and high-resolution transmission electron microscopy, we investigate systematically twinning and detwinning behaviors of {101¯2} twins in pre-compressed pure Mg and AZ31 and AZ91 alloys. We find that the solute elements Al and Zn and the resultant precipitates disrupt the synchronous motion of atoms in twinning boundaries (TBs) during twin formation, yielding a large amount of incoherent TBs and low-angle boundaries from the pre-existing TBs after detwinning. The detwinning in pure Mg and its alloys is found to be linked to both twin boundary (TB) mobility and the interaction of their boundaries with dislocations. At a low strain level (0–1%), comparable detwinning activities in both pure Mg and its alloys are observed due to the selective detwinning in grains with the highest SF, while at high strain level (1–4%), the contribution of detwinning in Mg alloy is higher. This is because a small twin size in Mg alloys leads to strong interactions between basal slip and TBs, and also because in pure Mg, twin size is much larger and its tip is pinned by grain boundaries. In addition, the secondary twins are also found to be formed remarkably in the interior of the primary twins in Mg alloys with a high solute-element concentration owing to the low mobility of incoherent TB and the variation of local stress state by precipitates. The ultimate strength of reverse tensile test is found to increase more substantially in the pre-compressed Mg alloys in comparison to pure Mg, especially in the AZ91 alloy, which is attributed to the hardening effect caused by the formation of many low-angle and secondary TBs and also for the reduced Schmid factor for the basal slip in Mg alloys during detwinning.

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.

High damping capacity of a Ni-Cu-Mn-Ga alloy in wide ambient-temperature range

The damping capacity of Ni55−xCuxMn25Ga20 (x = 0, 2, 4, 6) shape memory alloys is studied in details. The results show an increasing damping peak capacity with increasing Cu content. Especially, a high damping peak with the peak value of 0.08 is found in Ni49Cu6Mn25Ga20 alloy, which spans over a wide temperature range of 173 K–425 K. The first-principles calculations indicate the substitution of Cu for Ni weakens the p-d orbital hybridization of Ni55−xCuxMn25Ga20 and reduces its martensite modulus, which thus enhances its twin boundary mobility and the damping capability. Our study sheds light on developing ambient-temperature high damping alloys.

The mobility of growth twins synthesized by sputtering: Tailoring the twin thickness

The current work presents a protean twin thickness contour zone map that illustrates how the nucleation and the mobility of twin boundaries affects the twin thickness of sputtered films. The twin thickness contour zone map can be used as a versatile guide to synthesize fully nanotwinned films with tailored twin thicknesses in materials with a wide range of stacking fault energies. The nucleation and mobility of twin boundaries was studied in four Cu alloys of different compositions (Cu-6wt.%Al, Cu-4wt.%Al, Cu-2wt.%Al, and Cu-10wt.%Ni), having stacking fault energies ranging from 6 mJ/m2 to 60 mJ/m2. The films were synthesized by magnetron sputtering and characterized by transmission electron microscopy, where the twin thickness varied from 2 nm to 35 nm. Our experimental results show that it is possible to control the twin thickness. Three main mechanisms are explained to describe twin nucleation and twin boundary mobility, which are correlated to the interplay of specific sputtering conditions and the deposition temperature.

Mobility of Twin Boundaries in Fe-Pd-Based Ferromagnetic Shape Memory Alloys

Mobility of Twin Boundaries in Fe-Pd-Based Ferromagnetic Shape Memory Alloys

Stress-induced martensitic transformation, twin boundary mobility and elastic properties of vapor deposited Fe70Pd30 ferromagnetic shape memory alloy thin films

Within an in situ study, we explore mechanical properties of Fe70Pd30 ferromagnetic shape memory alloy thin films. While performing tensile measurements on freestanding films, we accomplish to observe the appearance of twin boundaries due to the stress-induced austenite–martensite transition. Changing distances between twin boundaries during further straining indicate the growth of single variant regions at the expense of others. Young’s moduli of 2.2–8.3GPa were determined. For comparison, temperature dependent surface acoustic waves were applied on as-deposited films exhibiting much higher Young’s moduli. These findings are discussed considering the influence of phase stability and phase transition characteristics.

A phase field study focuses on the transverse propagation of deformation twinning for hexagonal-closed packed crystals

The thickening mechanism of the deformation twinning (DT) has been frequently studied in numerous researches and the transverse propagation of that is beginning to trigger the attention of scholars. Recently, some researchers report that the twin front of {101¯2} mode of Magnesium is composed of a conjugate twin plane and prismatic/basal (PB) planes, and the combined mobility of these planes rule the overall kinetics of twin propagation. Focusing on that, a continuum phase field model is proposed to investigate the equilibrium shape of tensile twins and the kinetics of the twin front. A new form of surface free energy is introduced in this model for the purpose of describing the orientation-dependent properties of twin boundaries. The simulations well reproduce the PB interfaces and the results indicate that the anisotropic surface energy plays a dominant role in forming the irregular facets on the twin front. A generalized energy-momentum tensor is derived and analyzed for shear loading in order to investigate the equilibrium and mobility of twin boundaries, and the simulations show that the configurational forces distributed on the PB interfaces are smaller than that on the other twin planes, which implies that the growth of twin is beneficial for the formation of PB interfaces. The simulations also indicate that the anisotropic twin boundary energy is not responsible for the large aspect ratio nature of twins, which may be governed by the competition between thickening mechanism and transverse propagation mechanism of the DT.

Texture evolution in deformed AZ31 magnesium sheets: Experiments and phase-field study

Experimental and phase-field studies are performed to investigate mechanisms of preferential growth which lead to improved formability in AZ31 Mg sheets. A compression/annealing treatment is specialized to modify the initial texture in thin sheets. The texture and stress states of materials are studied via electron back scattered diffraction (EBSD) technique before and after annealing. Using the EBSD data on microstructure and residual stresses, a phase-field model is constructed to simulate the texture evolution after initial compression. The results suggest that the residual-stresses induced by in-plane compression are the main driving force for recrystallization and grain growth. The inhomogeneous stress distribution leads to preferential growth of {21¯1¯0} texture along the normal to the sheet, which are at lowest stress state, at the expense of initial basal texture. Limited mobility of twin boundaries changes the mixture of textures but the non-basal textures are still preferred. The formability tests confirm a significant enhancement of the final product compared to as-received sheets.