Rearrangement Of Martensite Variants Research Articles

Tension induced detwinning of hierarchically twinned non-modulated martensite in Ni-Mn-Ga alloys

Detwinning of martensite accounts for the macroscopic deformation in shape memory alloys, yet the corresponding microstructural and crystallographic evolution remains less understood. Here, the detwinning behaviors of hierarchically twinned non-modulated (NM) martensite in a directionally solidified Ni54Mn24Ga22 alloy under uniaxial tension along the <001>A direction of austenite are systematically investigated. Based on the interrupted in-situ EBSD measurements, it is demonstrated that tensile loading leads to the thickening of favorable variants with the <001>NM parallel to the loading direction through intensive detwinning, which follows the route of detwinning of internal nanotwins as well as collective detwinning of internal nanotwins and inter-plate major-major variant pairs. Besides, the internal detwinning is also coupled with the rigid body rotation owing to the constraints of inter-plate boundaries, resulting in the refinements in the inter-plate correlation as the variation of thickness ratio of nanotwins. Consequently, the hierarchically twinned microstructure finally evolves into a single-variant state with the <001>NM along the loading direction. This work provides clear demonstrations on the detwinning behaviors of hierarchically twinned NM martensite induced by tensile loading, which allows us to gain deep crystallographic insights into the stress-induced rearrangement of martensite variants in association with the macroscopic deformation.

Direct evidence for stress-induced transformation between coexisting multiple martensites in a Ni–Mn–Ga multifunctional alloy

The structural response of coexisting multiple martensites to stress field in a Ni–Mn–Ga multifunctional alloy was investigated by the in situ high-energy x-ray diffraction technique. Stress-induced transformation between coexisting multiple martensites was observed at 110 K, at which five-layered modulated (5M), seven-layered modulated (7M) and non-modulated (NM) martensites coexist. We found that a tiny stress of as low as 0.5 MPa could trigger the transformation from 5M and 7M martensites to NM martensite and this transformation is partly reversible. Besides the transformation between coexisting multiple martensites, rearrangement of martensite variants also occurs during loading, at least at high stress levels. The present study is instructive for designing advanced multifunctional alloys with easy actuation.

Rearrangement of martensitic variants in Ni2MnGa studied with the phase-field method

A phase-field model is introduced to simulate the magnetic shape memory effect, i.e. the solid-state rearrangement of the boundaries of a martensitic microstructure using an external magnetic field, in the shape memory material Ni2MnGa. The model is derived from an existing phase-field model that has proven well in several applications in materials science, based on the interpolation of free energies. The micromagnetic and elastic energy contributions entering the constitutive free energy functional are given, and the coupled kinetic equations of motion for the phase fields that describe the microstructure geometry, the spontaneous magnetization and the elastic displacement field are derived from the principle of minimization of free energy. The concept of representative volume elements is applied for the microstructure simulations carried out to analyze the material behavior, and the relevant boundary conditions are discussed. Stress vs. strain and strain vs. applied magnetic field curves are shown for Ni2MnGa.

Application of EBSD to the Crystallographic Investigation on Ni-Mn-Ga Alloys

For off-stoichiometric Ni2MnGa ferromagnetic shape memory alloys, a large shape change could be induced through the rearrangement of martensitic variants under an external magnetic field. Insight into the orientation relationships of martensitic variants and the characteristics of variant boundaries is thus essential for understanding the magnetic shape memory performance. In this paper, a thorough crystallographic investigation was made on the incommensurate 7M modulated martensite in one polycrystalline Ni50Mn30Ga20 alloy by means of X-ray diffraction and SEM electron backscattered diffraction (EBSD). Locally, there are four differently-oriented martensitic variants, being twin related to one another. The twin interface planes are coherent and they are in coincidence with the respective twinning planes (K1). A primary exploration was performed to improve the microstructure by repeated magnetic field training during phase transition. The present investigation could offer useful guidance to develop specific technique for microstructure optimization.

In-Situ High-Energy X-Ray Diffuse-Scattering Study of the Phase Transition of Ni2MnGa Single Crystal under High Magnetic Field

The defects-related microstructural features connected to the premartensitic and martensitic transition of a Ni2MnGa single crystal under a high magnetic field of 50 KOe applied along the \( \left[ {1\bar{1}0} \right] \) crystallographic direction of the Heusler phase were studied by the in-situ high-energy X-ray diffuse-scattering experiments on the high energy synchrotron beam line 11-ID-C of APS and thermomagnetization measurements. Our experiments show that a magnetic field of 50 KOe applied along the \( \left[ {1\bar{1}0} \right] \) direction of the parent Heusler phase can promote the premartensitic transition of Ni2MnGa single crystal, but puts off martensite transition and the reverse transition. The premartensitic transition temperature (TPM) increases from 233 to 250 K (−40 to −23 °C). The martensite transition start temperature (Ms) decreases from 175 to 172 K (−98 to −101 °C), while the reverse transition start temperature (As) increases from 186 to 189 K (−87 to −84 °C). The high magnetic field leads to a rapid rearrangement of martensite variants below the martensite transition finish temperature (Mf). The real transition process of Ni2MnGa single crystal under the high magnetic field was in-situ traced.

Strain characteristics and superelastic response of [formula omitted] single crystals

The intermartensitic transformation, in a two-step complete thermoelastic martensitic transformation in Ni 53.2Mn 22.6Ga 24.2 single crystals, provides a much larger strain than that of the martensitic transformation. With a biasing magnetic field, the intermartensitic transformation strain is inhibited and the martensitic transformation strain is enhanced. Compressive stress–strain characteristics can be affected greatly by a static magnetic field. At low deformation temperature, the irreversible transformation strain induced by the stress becomes reversible, when a static magnetic field is applied. Further, the magnitude of the stress necessary for rearrangement of martensitic variants is dependent on the direction of the biasing magnetic field. Moreover, a well-defined character of the twin-boundary motion, similar to the soliton motion, has been observed upon loading or unloading.

Magnetocaloric effect in ferromagnetic Heusler shape-memory alloys

In this paper the magnetocaloric behaviour of Ni-Mn-based Heusler alloys is discussed in relation to their shape-memory and superelastic properties. We show that the magnetocaloric effect in these materials originates from two different contributions. The first, associated with the mechanism which makes feasible the magnetically induced rearrangement of martensite variants, controls the magnetocaloric effect at low applied fields, while the latter is dominant at higher fields and is essentially related to the possibility of magnetically inducing the martensitic transition. The occurrence of the inverse magnetocaloric effect associated with these two contributions is also considered.

Effect of magnetic field on solid-solid phase transformations in iron-based ferromagnetic alloys

We have investigated the effect of magnetic field on phase transformation of iron-based ferromagnetic alloys. Two systems are selected. One is Fe-Rh alloys exhibiting a diffusional transformation from an austenite phase to a ferrite phase on cooling. The field dependence of the transformation temperature is well explained thermodynamically. The other is an Fe-31.2Pd alloy exhibiting a martensitic transformation. In this system, rearrangement of martensite variants occurs by the application of the magnetic field, and this behaviour is quantitatively explained by considering magnetic shear stress, which is expressed by the magnetic energy difference between variants divided by twinning shear.

Magnetocaloric and Shape-Memory Properties in Magnetic Heusler Alloys

In this paper, we discuss the magnetocaloric behavior of Ni-Mn-based Heusler alloys in rela- tion to their shape-memory and superelastic properties. We show that the magnetocaloric effect in these materials originates from two different contributions: (i) the coupling that is related to a strong uniaxial magnetic anisotropy and takes place at the length scale of martensite variants and magnetic domains (extrinsic effect), and (ii) the intrinsic microscopic magnetostructural coupling. The first contribution is intimately related to the magnetically induced rearrange- ment of martensite variants (magnetic shape-memory) and controls the magnetocaloric effect at small applied fields, while the latter is dominant at higher fields and is essentially related to the possibility of magnetically inducing the martensitic transition (magnetic superelasticity). The possibility of inverse magnetocaloric effect associated with these two contributions is also considered.

Recent topics in microstructure control by magnetic field in Ni2MnGa and CoPt and new transformation behavior in Ti-Ni-Fe alloys

Three topics related to solid-solid phase transformations are presented. The first topic is related to ferromagnetic shape memory alloys. The general condition for rearrangement of martensite variants by magnetic field is discussed quantitatively. The second topic is related to microstructure control of CoPt (tetragonal L10-type structure) during ordering heat-treatment under a magnetic field. We show that a single variant state is realized by magnetic field, and magnetic field is especially effective at the early stage of ordering. The third topic is related to the so-called precursor phenomena in Ti-Ni-Fe shape memory alloys. In the topic we will show the existence of a commensurate phase, which inherits the microstructure of the incommensurate phase and is probably different from the R-phase.

Effect of Magnetic Field on Microstructure Evolution during Disorder-Order Transformation in an Fe-Pd Alloy

Control of microstructure through solid-solid phase transformation under magnetic field is a promising method to improve the properties and design new materials. In general, solid-solid phase transformations are classified into two groups from a viewpoint of atomic diffusion: diffusionless (martensitic) transformations and diffusional transformations. The characteristics of both diffusionless and diffusional transformations are known to be influenced by external fields such as magnetic field and pressure. In the case of the martensitic transformations, the effects of magnetic field on microstructure have been intensively studied. A well-known example is the rearrangement of martensite variants driven by magnetic field in ferromagnetic shape memory alloys. In these alloys the variant with the lowest magnetocrystalline anisotropy energy is selected to grow consuming others by twinning deformation, and the condition for realizing the rearrangement has been derived quantitatively. 1–4) On the other hand, concerning diffusional transformations, the effect of magnetic field on microstructure has not been so clarified yet although many interesting results have been reported until now. 5–10) Considering the effectiveness of magnetic field on selection of variants in ferromagnetic shape memory alloys, we can also expect a selective formation of a variant which is formed in a diffusive process in alloys with a large magnetocrystalline anisotropy. As for such materials, Co-Pt and FePd alloys can be a suitable system. The reason is as follows. The disorder-order transformation in CoPt and FePd is one of the representative diffusional transformation in which the microstructure formation is expected to be influenced by magnetic field because of their large magnetocrystalline anisotropies. The structure change associated with this disorder-order transformation is from an A1-type (cubic) disordered structure to an L10-type (tetragonal) ordered structure and the ordered phase has three lattice corresponding variants and is characterized by a high uniaxial magneto

Onset Magnetic Field for Rearrangement of Martensite Variants in Ni&lt;sub&gt;2&lt;/sub&gt;MnGa

The magnetic field strength, Hs, at which rearrangement of martensite variants initiates has been investigated in Ni2MnGa ferromagnetic shape memory alloy by magnetization measurements in the [001]P direction ("P" stands for the parent phase). We have also calculated Hs from the magnetocrystalline anisotropy constant Ku, spontaneous magnetization Ms, twinning shear s and twinning stress τreq by considering the condition for the rearrangement of martensite variants reported previously [Int. J. Appl. Electromagnetics and Mechanics, 23 (2006) 45]. The calculated value of Hs is in good agreement with the experimental value for all the examined temperatures. The agreement confirms the applicability of the reported condition.

Temperature dependence of rearrangement of martensite variants by magnetic field in 10M, 14M and 2M martensites of Ni–Mn–Ga alloys

We have examined the condition for the rearrangement of martensite variants (RMV) by magnetic field in a wide temperature range for three types of Ni–Mn–Ga ferromagnetic shape memory alloys: a Ni 2MnGa with the 10M martensite, a Ni 2.02Mn 1.09Ga 0.89 (at.%) with the 14M martensite and a Ni 2.14Mn 0.92Ga 0.94 with the 2M martensite structure. The RMV by magnetic field occurs at any temperature for the 10M martensite, while it occurs only in a limited temperature range for the 14M martensite and does not occur at any temperature for the 2M martensite. The reason of such difference is quantitatively explained by evaluating two shear stresses: the magnetic shear stress acting across a twinning plane τ mag and the stress required for the twinning plane movement τ req. That is, by comparing the values of τ mag and τ req, we have confirmed that the following condition is universal regardless of temperature, field direction and structure of the martenstie phase: the value of τ mag is larger than τ req when the RMV occurs by magnetic field and vice versa.

Magnetization rotation and rearrangement of martensite variants in ferromagnetic shape memory alloys

It is widely recognized that the rearrangement of martensite variants in a ferromagnetic shape memory alloy (FSMA) can lead to much higher strain than conventional magnetostriction. In this letter, the authors show that magnetization rotation can reduce ferromagnetic shape memory effect substantially when motion of the interface is severely hindered under a relatively large applied stress. This limits the energy density of FSMA actuators and suggests that hard magnetic anisotropy is necessary to achieve large actuation force. The analysis agrees well with experiments.

Magneto-mechanical evaluation for twinning plane movement driven by magnetic field in ferromagnetic shape memory alloys

Rearrangement of martensite variants under magnetic field has been investigated by using single crystals of three ferromagnetic shape memory alloys: a disordered Fe–31.2 at.% Pd alloy, an ordered (degree of order ∼0.8) Fe 3Pt, and stoichiometric Ni 2MnGa. Each alloy transforms from a cubic parent phase to a tetragonal or pseudotetragonal martensite phase whose tetragonality is about 0.94 at low temperatures. We evaluated the magnetic shear stress τ mag acting across the twinning plane under magnetic field, by using magnetic anisotropy constant and twinning shear, and compared it with the shear stress required for the rearrangement τ req, which is obtained from tensile or compressive tests. As a result, we confirmed that τ mag is larger than τ req when the rearrangement of variants occurs by magnetic field and vice versa regardless of temperature and field direction.

Influence of temperature on rearrangement of martensite variants by magnetic field in Fe 3Pt

We have investigated influence of temperature on magnetic field-induced strain due to rearrangement of martensite variants in Fe 3Pt with a degree of order of 0.8. Measurements were made in the field-cooling process (FC) and field application process after zero-field cooling (ZFC), where magnetic field was applied along three principal directions: [0 0 1] P, [1 1 0] P and [1 1 1] P (index are given by the parent phase). From the measured strain and lattice parameters, we evaluated the fraction f P of the preferable variant, which has the lowest magnetic energy among three variants. In the case of the [0 0 1] P and [1 1 0] P fields, the fraction f P obtained by FC process (3.2 MA/m) gradually increase with decreasing temperature while f P obtained by field application process after ZFC shows a maximum value around 40 K. On the other hand, the field-induced strain does not appear for the [1 1 1] P field regardless of both processes. A part of the magnetic field-induced strain is reversible and it shows a maximum value of 1% at 20 K under the [0 0 1] P field.

Magnetocrystalline anisotropy constant and twinning stress in martensite phase of Ni–Mn–Ga

We have examined rearrangement of martensite variants driven by magnetic field for 10M martensite in Ni 2MnGa and 2M martensite in Ni 2.14Mn 0.92Ga 0.94 in a wide temperature range below each transformation temperature. The following results are obtained: (i) the twinning plane of the 10M martensite moves by the application of magnetic field, but that of 2M martensite does not at any temperature, (ii) in order to explain such difference, we have evaluated the maximum of the magnetic shear stress acting across the twinning plane ( τ mag m ), which is calculated by using the magnetocrystalline anisotropy constant ( K u) measured in this study, and the twinning shear stress ( τ req) obtained by compressive test in this study for the 10M and 2M martensites, and have confirmed whether or not the relation of τ mag m > τ req is satisfied for 10M martensite and that of τ mag m < τ req is satisfied for 2M martensite. As a result, the above condition is satisfied at any temperatures for 10M and 2M martensites.

Influence of Magnetic Field Direction and Temperature on Rearrangement of Martensite Variants in Fe-31.2at.%Pd

We have measured temperature dependence of the uniaxial magnetocrystalline anisotropy constant Ku and the shear stress τr eq required for the twinning plane movement of an Fe-31.2Pd alloy in order to explain the field orientation dependence of rearrangement of martensite variants (RMV) of this alloy in the field-cooling process. Under the [001]P field (P represents parent phase), the RMV occurs perfectly at the temperatures where the measurement was performed; in this case, the magnetic shear stress acting across the twinning plane τmag, which is evaluated from Ku, becomes nearly twice as large as τr eq. On the other hand, under the [011]P field, the RMV occurs partially at all the measuring temperatures; in this case the maximum of τmag is nearly the same as τr eq. It is successful to evaluate the following relation quantitatively; τmag&gt; τr eq is satisfied when RMV occurs under magnetic field.

Magnetocrystalline Anisotropy and Twinning Stress of 10M and 2M Martensites in Ni-Mn-Ga System

Ni2MnGa alloy with 10M martensite exhibits rearrangement of martensite variants (RMV) by magnetic field, but Ni2.14Mn0.92Ga0.94 with 2M martensite does not. In order to explain the difference, we measured uniaxial magnetocrystalline anisotropy constant Ku and the stress required for twinning plane movement τreq in these alloys. Concerning the former alloy, the maximum value of magnetic shear stress acting across twinning plane τmag, which is evaluated as |Ku| divided by twinning shear, becomes larger than τr eq. On the other hand, concerning the latter alloy, the maximum of τmag is only one-tenth of τreq at any temperature examined. Obviously, the relation, τmag&gt; τr eq, is satisfied when RMV occurs by magnetic field and vice versa. In this martensite, the large twinning shear of 2M martensite is responsible for small τmag and large τreq.

Control of Microstructure Driven by Magnetic Field in Ferromagnetic Intermetallics

AbstractMagnetic field has been known to be effective in solidification processes. Recently, however, it has been revealed that the magnetic field is also effective for controlling the arrangement of variants, which are formed in association with a solid-solid transformation, in some ferromagnetic intermetallic-based alloys with a large magnetocrystalline anisotropy. In this presentation, we will show two of such cases: one is the rearrangement of martensite variants by magnetic field in intermetallic-based ferromagnetic shape memory alloys (Ni2MnGa and Fe3Pt), and the other is formation of mono-variant state by ordering heat-treatment under magnetic field in a L10-type CoPt. The former process is diffusionless and proceeds by the movement of twinning plane under a magnetic field. The latter is a diffusion controlled process. For both the cases, a large magnetocrystalline anisotropy is essentially important for controlling the arrangement of variants, although kinetics of the two processes is quite different each other.