Intermartensitic Transformation Research Articles

Constant plane shift model: Structure analysis of martensitic phases in Ni50Mn27Ga22Fe1 beyond non-modulated building blocks

Martensites of Ni-Mn-Ga-based alloys consist of hierarchical twinning domains spanning from micro- to nanoscale. This affects the diffraction pattern and thus can decrease the accuracy of the determination of the crystal structure. We propose a method to obtain different martensitic phases in Ni-Mn-Ga-Fe alloy with simplified variant microstructures and domain sizes of more than 2 micrometers. The use of simplified variant microstructures allows the influence of nanometer-scale domains on diffraction line position to be circumvented and enabls the comparison of the lattice parameters of non-modulated (NM), five-layered modulated (10M), and seven-layered (14M) phases in the same temperature range due to the large hysteresis of the intermartensitic transformations. It is found that the short crystallographic axes in NM, 14M, and 10M martensites at the same temperature have different lengths. As a result, equilibrium NM structure building blocks cannot be used to build the crystal structures of 14M and 10M martensites. Instead, we introduce a constant plane shift model with identical shift values of the nearest planes (110) along [1¯10] or [11¯0] as a replacement for the tetragonal building blocks model. The work demonstrates that plane shift values differ dramatically between martensites, which agrees with ab initio calculations. The application of the constant plane shift and hard sphere models in modulated lattices for atomic-level twinning considerations is discussed.

Magnetocaloric performance optimized by simple compression in directionally solidified Ni50Mn18Cu7Ga25 alloy

The Magnetocaloric Effect (MCE) of directionally solidified Ni50Mn18Cu7Ga25 alloy was improved based on texturation and introduction of extra phase transition in this paper. Magneto-structural transition is realized in directionally solidified Ni50Mn18Cu7Ga25 alloy, with a value of Adiabatic Temperature Variation (ΔTad) of − 1.35 K under magnetic field of 1.5 T. Through simple unidirectional compression, preferred orientation is formed and intermartensitic transformation is introduced in the present alloy. Preferred orientation strengthens the coupling of magnetic and structural transition, intermartensitic transformation makes transformation path more diverse, which both enhance ΔTad of the directionally solidified alloy. A value of ΔTad of − 2.34 K is obtained in the deformed alloy under magnetic field of 1.5 T, with a 73.3 % increase compared with the initial state. Moreover, the two incompletely overlapping structural transitions broadens the temperature range of MCE from 4.3 K to 7 K. The present work provides new insight on the MCE optimization of Ni-Mn-Ga alloy by simple and short-term unidirectional compression in view of merging different strengthening mechanisms.

Orientation dependent stress-induced intermartensitic transformations in Ni50.3Mn28.7Ga21.0 single crystal

Intermartensitic transformations (IMTs) between 10, 14, and 2M martensitic phases induced by the uniaxial compression and tension have been investigated using the same Ni50.3Mn28.7Ga21.0 single crystal. One series of thermomechanical measurements was carried out in a tensile configuration along the [001]c direction and another one under a compression load along the [110]c direction of the samples cut from the same single crystal. The obtained slopes of the “stress–temperature” phase diagrams of 10M → 14M and 14M → 2M IMTs are in satisfactory agreement with the Clausius–Clapeyron relationship applied for each IMT induced under compression and tension loadings. The results are important for the development of the theory of IMTs in magnetic shape memory alloys as well as for their practical applications given large superelastic strains summed over a sequence of martensite structures and a low twining stress noted uniquely for modulated martensites.

Intermartensitic transformation between modulated structures in Heusler Ni–Mn–Ga(–Fe) single crystals

Intermartensitic transformation between modulated structures in Heusler Ni–Mn–Ga(–Fe) single crystals

New Insights into the Intermartensitic Transformation and Over 11% Magnetic Field‐Induced Strain in 14 m Ni−Mn−Ga Martensite

Stress‐induced intermartensitic phase transformation from 10 m to 14 m modulated martensite structure is characterized in Ni50.5−Mn28.9−Ga20.6 at% single crystals, with particular focus on periodic atomic shuffling by in situ synchrotron radiation diffraction. Under compression, along the [001] direction, at 6 MPa, the self‐accommodated 10 m single crystal is found to transform to a nearly single‐variant state. Further stress increase up to 20 MPa triggers intermartensitic transformation to an intermediate, mixed martensite state. Intermartensitic transformation is hereby invoked at a considerably lower stress than previously reported for Ni−Mn−Ga alloys. However, the stress level applied along the [001] direction does not promote the complete 10 m→14 m martensite transformation. The 14 m structure is then fully achievable, when a consecutive compression along the [100] direction is applied. Such mechanically promoted 14 m single crystals are stable at room temperature and yield about 11.4% magnetic‐field induced strain—being the highest noted up to date for the Ni−Mn−Ga system.

Enhancing the elastocaloric effect in Ni–Mn–Ga alloys through the coupling of magnetic transition and two-step structural transformation

Elastocaloric effect driven by uniaxial stress in the Ni–Mn–Ga alloys can be greatly enhanced through introducing magnetic transition or inter-martensitic transformation to martensitic transformation. Here, we present large elastocaloric effect in a ⟨0 0 1⟩A textured Ni55Mn19Ga25Ti1 polycrystalline alloy prepared by directional solidification by exploiting the coupled multiple phase transformations, i.e., paramagnetic-ferromagnetic transition, martensitic transformation, and inter-martensitic transformation. Owing to such magneto-multistructural transformation, the transformation entropy change related to the inverse transformation is enhanced to 29.6 J kg−1 K−1. Consequently, on unloading from a compressive stress of 180 MPa, a large adiabatic temperature change of −12.9 K and specific adiabatic temperature change of −72 K GPa−1 are achieved, being much superior over those in the Ni–Mn–Ga based alloys obtained previously.

Effect of pressure on the phase stability and magnetostructural transitions in nickel-rich NiFeGa ribbons

Ribbons of a Ni55Fe19Ga26 Heusler alloy were prepared by melt spinning technique. The effect of pressure on magnetostructural transitions was studied comparing as-spun ribbons to ribbons previously submitted to an axial pressure and to powder obtained from manually grinded as-spun ribbons. The martensitic transformation present in the as-spun ribbon almost vanishes after pulverization. The structural change driven by mechanical treatment is a stress-induced intermartensitic transformation from a 14 M modulated to a non-modulated tetragonal (NM) structure. There is a progressive change from the structure of as-spun samples to that of the pulverized one in samples submitted to axial pressures as it increases.

Investigation of martensitic transformation behavior in Ni-Mn-In Heusler alloy from a first-principles study

Composition dependence of martensitic transformations as well as the magnetic properties for the Ni2Mn1+xIn1-x (0.25 ≤ x ≤ 0.58) alloys were investigated by using the first-principles calculations. Key results demonstrate that the stability of parent austenite (A) decreases gradually with increasing Mn content whilst it is opposite for the martensitic phase. This causes the total energy difference between the austenite and martensite phases increscent with increasing Mn contents. When x = 0.33, the martensite transformation during cooling is PA→FA→NM. When x ≥ 0.42, an intermartensitic transformation occurs from modulated 6 M martensite to non-modulated (NM) martensite with the martensite transformation sequence of PA→FA→6M→NM. The martensitic transformation from austenite to martensite accompanies the transition from ferromagnetic to ferrimagnetic state. This is a typical magneto-structural coupling transformation. The analysis of the density of states demonstrates that the Ni 3d state plays an important role in the phase stability.

Universality of Temperature Dependence of Twinning Stress in Ni-Mn-Ga 10M Martensite and Effect of Crystal Quality

We investigate experimentally the universality of the temperature dependences of twinning stress of type Ⅰ and type Ⅱ twins in Ni50Mn28Ga22 five-layered modulated martensite. Single crystals from different producers are compared to distinguish the universal behaviour and the effects of crystal quality. The twinning stress is both measured directly and calculated from magnetisation loops. In the vicinity of martensite transformation, the twinning stress is about the same for both types of twins. For type Ⅰ twins, it increases linearly with decreasing temperature with the slope of -0.045 MPa/K. For type Ⅱ twins, the twinning stress is almost temperature independent, but it increases rapidly near the intermartensite transformation temperature. In the crystal, which does not undergo an intermartensitic transformation and exhibits type Ⅱ twins, the magnetically induced reorientation of martensite is observed down to 2 K. Chemical inhomogeneity and mosaicity of the crystal results in mixed type twin boundaries with temperature dependences deviating from the universal behaviour of type Ⅰ and type Ⅱ twins.

Suppression of γ phase and its effect on mechanical behavior of melt-spun and annealed Ni–Mn–Ga high-temperature shape memory alloys

The suppression of the γ phase on the mechanical behavior of Ni58Mn25Ga17 and Ni60Mn25Ga15 melt-spun ribbons is investigated. The Ni60Mn25Ga15 ribbon, which contains the γ phase, generally exhibits a much higher fracture strain (average, 3.83 %) than the Ni58Mn25Ga17 ribbon, in which the γ phase is suppressed (average, 0.76 %). However, the reappearance of the γ phase after annealing under various conditions does not clearly increase the tensile fracture strain of the Ni58Mn25Ga17 ribbons. These results demonstrate that the proportion of precipitates associated with the original composition significantly affects the tensile strength. In particular, the distribution of the γ phase plays an important role in optimizing the tensile behavior. In addition, seven-layered modulated martensite and intermediate martensite structures appear in the ribbons owing to the fast cooling rates, inner stresses, and occurrence of the inter-martensitic transformation.

Enhanced magnetocaloric effects driven by two successive magneto-structural transformations in Ni55.5Mn17.8Ga26.7 alloy under hydrostatic pressure

The crystal structure, transformation property and magnetocaloric effect (MCE) were investigated in Ni55.5Mn17.8Ga26.7 alloy. The sample at room temperature exhibits an unmodulated martensitic structure with a small residual trace of 7 M modulated martensitic structure. Two successive magneto-structural transformations (MSTs) consisting of martensitic and intermartensitic transformations are observed. It is found that the maximum isothermal entropy change |ΔS|max of 7.65 J/kg K at 309 K and refrigeration capacity (RC) of 78.84 J/kg are attained for a field change μ0ΔH = 3 T under ambient pressure (p = 0 GPa). The enhanced MCE can be ascribed to the two successive MSTs. Much importantly, the |ΔS|max is basically independent of hydrostatic pressure and equals to 7.74 J/kg K at 314 K for p = 0.622 GPa. Besides, the application of hydrostatic pressure improve the width of phase transition and a large RC of 101.57 J/kg has been observed for p = 0.622 GPa under μ0ΔH = 3 T. Consequently, the response of RC to magnetic field increases from 26.86 J/kg T for p = 0 GPa to 34.13 J/kg T for p = 0.622 GPa. The reversible RC is also improved by pressure. Such an enhanced MCE and the non-toxic raw materials make this alloy particularly appealing for solid-state cooling technologies.

Deformation of Ni–Mn-Ga 7M modulated martensite through detwinning/twinning and forward/reverse intermartensitic transformation studied by in-situ neutron diffraction and interrupted in-situ EBSD

Shape memory alloys, especially the newly developed Ni–Mn-based heusler-type intermetallic compounds, exhibit specific mechanical responses to mechanical loading. Although the deformation behaviors have been studied for reducing the number of martensite variants, the mechanisms are not fully revealed. Thus in this work the compression process of twin-related 7M modulated martensite of Ni–Mn-Ga intermetallic compound was studied by in-situ neutron diffraction at macroscopic scale and by interrupted in-situ EBSD at microscopic scale. It is revealed that the mechanical response of the 7M martensite is featured by three states: a linear elastic-plastic state, a steady plastic state, and a second linear plastic state. The plastic deformation is initiated by the detwinning of the existing variants in the first linear state. It proceeds to the steady state by intensive detwinning of the these variants and by twinning of the remaining variants that result in the disappearance of the existing variants and the appearance of new variants, then by intermartensitic transformation to form non-modulated martensite (NM). These three shear processes are highly coordinated and compatible with the annihilation of the local incompatible strains by reverse intermartensitic transformation, which allows a steady progress of deformation and a continuous reorientation of the variants. The reorientation produces new twins with unfavorable orientations and limited deformation capacity, leading to a stress increase for further deformation. The present work provides comprehensive information on deformation mechanisms of Ni–Mn-Ga 7M martensite at each characteristic deformation step that is useful for mechanical simulation of deformation behaviors of intermetallic compounds.

Composition dependence of martensitic transformation and crystal structure in Ni50Mn25Ga25-xCux Heusler alloys

A detailed analysis of the influence of Cu addition on the martensitic transformation and the composition dependent transitions in Ni50-xMn25Ga25Cux Heusler alloys after two types of heat treatment was carried out. The chemical modification affected the martensitic transformation temperature, which increased from 202 to 691 K upon Cu addition. The chemical modification also affects the type of martensite structure, which changed from a ten-layered through a fourteen-layered and finally to a non-modulated crystal structure. At higher Cu content of ∼9 atomic percent precipitation of a second phase occurs enriched in Cu that disturbs the monotonic increase of the martensitic transformation temperature. It was also shown that water quenching at 1173 K affects the temperature and hysteresis of the martensitic transformation, as well as the chemical composition and temperature range of the 14 M → 2 M inter-martensitic transformation. Additionally, a narrow to wide transition of the martensitic transformation hysteresis was found to be independent of the applied heat treatments.

Martensitic transformation and giant magneto-functional properties in all-d-metal Ni-Co-Mn-Ti alloy ribbons

The all-d-metal Ni50-xCoxMn35Ti15 (x = 13,13.5) ferromagnetic shape memory alloy ribbons are successfully fabricated. The intermartensitic transformation and martensitic transformation arrest phenomenon are systematically investigated. In contrast to melt-spun ribbons, the annealed ribbons reveal an intermartensitic transformation from five-fold (5 M) modulated to non-modulated (L10) martensite below B2→5 M martensitic transformation. The XRD, TEM, DSC and temperature dependence of electrical resistivity measurements validate the occurrence of intermartensitic transformation. On the other hand, the magnetic-field-induced martensitic transformation arrest is achieved in these all-d-metal ribbons like other normal Ni-Mn-based Heusler alloys consisting of p-group and d-group elements. Strikingly, a kinetic de-arrest phenomenon is brought about in all-d-metal ribbons with the increasing heat treatment temperatures. The associated origins are discussed in these ribbons, which is very significant for deeply understanding the arrest and de-arrest phenomenon in ferromagnetic shape memory alloys. What's important, excellent magneto-functional and mechanical properties are obtained in these ribbons. Ni36.5Co13.5Mn35Ti15 annealed ribbon displays the best integrated properties, such as large magnetoresistance (∼34.9%), magnetic entropy change peak value (∼24.9 Jkg−1K−1) and effective refrigerant capacity (∼122.5 Jkg−1) under the field change of Δμ0H = 0–5 T, and large Vickers hardness [2.75 GPa (280.7HV)]. These make the all-d-metal Ni-Mn-based Heusler alloy ribbons attractive in the practical applications.

Large elastocaloric effect driven by stress-induced two-step structural transformation in a directionally solidified Ni55Mn18Ga27 alloy

The first-order inter-martensitic transformation induced by temperature change or external stress is frequently observed in Ni-Mn-Ga alloys. Here, the inter-martensitic transformation is exploited to strengthen the elastocaloric effect. Through the stress-induced two-step structural transformation, i.e., martensitic and inter-martensitic transformation, giant adiabatic temperature change up to −10.7K is obtained in a directionally solidified Ni55Mn18Ga27 polycrystalline alloy with 〈001〉A preferred orientation, representing the highest value so far in Ni-Mn-Ga alloys. It is demonstrated that microstructure texturing together with introduction of inter-martensitic transformation could be an effective route to improve the elastocaloric properties in polycrystalline NiMn-based alloys.

Deformation of Ni-Mn-Ga 7M Modulated Martensite Through Twinning/Detwinning and Forward/Reverse Intermartensitic Transformation Studied by In-Situ Neutron Diffraction and Interrupted In-Situ EBSD

Shape memory alloys, especially the newly developed Ni-Mn-based heusler-type intermetallic compounds, exhibit specific mechanical responses to mechanical loading. Although the deformation behaviors have been studied for reducing the number of martensite variants, the mechanisms are not fully revealed. Thus in this work the compression process of twin-related 7M modulated martensite of Ni-Mn-Ga intermetallic compound was studied by in-situ neutron diffraction at macroscopic scale and by interrupted in-situ EBSD at microscopic scale. It is revealed that the mechanical response of the 7M martensite is featured by three states: a linear elastic-plastic state, a steady plastic state, and a second linear plastic state. The plastic deformation is initiated by the detwinning of the existing variants (TrF-twins) in the first linear state. It proceeds to the steady state by intensive detwinning of the TrF-twins and by twinning of the remaining variants to form DeF-twins that result in the disappearance of the existing variants and the appearance of new variants, then by intermartensitic transformation to form non-modulated martensite (NM). The three shear processes are highly coordinated and compatible with the annihilation of the local incompatible strains by reverse intermartensitic transformation, which allows a steady progress of deformation and a continuous reorientation of the variants. The reorientation produces new twins with unfavorable orientations and limited deformation capacity, leading to a stress increase for further deformation. The present work provides comprehensive information on deformation mechanisms of Ni-Mn-Ga 7M martensite at each characterized deformation step that is useful for mechanical simulation of deformation behaviors of intermetallic compounds.

Evolution of phase transformation and magnetic properties with Fe content in Ni55−xFexMn20Ga25 Heusler alloys

A series of Ni55−xFexMn20Ga25 (0 ⩽ x ⩽ 5) Heusler alloys was prepared to investigate their phase transitions and magnetic properties. At room temperature, these alloys present various crystal structures, and the unit cell volume enlarges with increase of Fe content in both austenite and martensite. Multiple magneto-structural transformations were observed in the parent alloy (x = 0). In the process of cooling, it undergoes martensitic transformation (MT) from L21-type paramagnetic austenite to L10-type ferromagnetic martensite, accompanying an intermartensitic transformation (IMT, 7M → L10). By establishing a detailed phase diagram, we found that both MT and IMT shift to lower temperature simultaneously, while the ferromagnetic (FM) transition of austenite moves to higher temperature as Fe increases. With the further increase of Fe content beyond a critical value, both the IMT and the FM transitions split off from MT, and the former follows with the transforming sequence of 7M → 5M. Based on the experimental data, some key magnetic parameters have been obtained in this system. The calculated magnetocrystalline anisotropy constant () of martensite quickly increases as Fe increases, and then it almost reaches a saturated value (~5.5 × 105 J m−3) for the alloys with x > 3. However, the spontaneous magnetic moment () attains a peak value of about 4.2 μB/f.u. in the alloy with x = 4, which is not consistent with the linear increasing of effective magnetic moment (). Further magnetic measurements with hydrostatic pressure indicate that such a discrepancy could be ascribed to the competition between the magnetic exchange interaction and the volume change of unit cell governed by the dopant Fe content.

Multistep superelasticity of Ni-Mn-Ga and Ni-Mn-Ga-Co-Cu microwires under stress-temperature coupling

Martensitic and intermartensitic transformation induced by changing temperature and/or applying mechanical stress was investigated systematically in Ni50Mn30Ga20 and Ni46Mn24Ga22Co4Cu4 polycrystalline shape memory microwire. In particular, intermartensitic transformation induced by stress can be obtained in single crystal but difficult in polycrystalline materials that are usually intrinsically brittle due to the weak grain boundary cohesion. In this paper, two kinds of oligocrystalline microwires, containing free surface and bamboo-like grains, with austenite and martensite at room temperature exhibit multistep superelastic behavior with larger than 10% fully reversible strain from intermartensitic transformation. The two alloys undergo four kinds of phase transition sequence under the condition of stress-temperature coupling, and the later has smaller stress hysteresis and exhibits excellent high temperature mechanical properties during loading-unloading loops. Meanwhile, the transformation critical stress as a function of temperature phase diagram for the two kinds of alloys has been established. Utilizing the elements of Co and Cu replacing Ni and Mn in Ni50Mn30Ga20 alloys, phase transformation temperature is broadened and increased. For Ni-Mn-Ga microwire, continuous martensitic transformation process from 228 to 298 K is investigated by in-situ TEM observation, and the structure is confirmed to be five-layered modulated martensite at low temperature. Multiple martensites coexistence, including five-layered and seven layered modulated martensite, is explored in Ni-Mn-Ga-Co-Cu microwire at room temperature. Those multistep superelastic behavior and mechanical properties can be comparable but different from single crystal. This is an effective method to obtain small size, high performance shape memory alloys, which are of great significance for device miniaturization and intelligence.

Plastic deformation of Ni–Mn–Ga 7M modulated martensite by twinning & detwinning and intermartensitic transformation

Mechanical training (represented by cyclic loading and unloading) is frequently performed on Ni–Mn–Ga martensite to reduce undesirable variants for maximized shape memory performance. In this work, an effort was made to reveal the deformation mechanisms of 7M modulated (or 14M) martensite in a polycrystalline Ni–Mn–Ga alloy. It is demonstrated that during unidirectional compression, deformation bands form regularly and repeatedly through the sample. These bands are initiated from the colonies having martensite variants with high Schmid Factors (SFs) for the detwinning systems of the Type-I and II transformation twins (TrF-twins) and go through the colonies possessing variants with low SF for the detwinning systems of the TrF-twins. The deformation is realized by detwinning of the Type-I and Type-II TrF-twins in the high SF colonies but twinning and shuffling of the variants in the low SF colonies. The detwinning results in the thickening of the favorable variants, whereas the twinning gives rise to the formation of the new 7M variants and the shuffling induces intermartensitic transformation. Although these shear systems are activated individually, they act coordinately and form continuous strain paths through colonies, allowing the accommodation of the macroscopic deformation. The present results provide comprehensive information on the deformation mechanisms of Ni-Mn-Ga 7M martensite and their microstructural representation.

Internal friction behaviors of Ni-Mn-In magnetic shape memory alloy with two-step structural transformation

The internal friction (IF) behaviors of dual-phase Ni52Mn32In16 alloy with two-step structural transformation were investigated by dynamic mechanical analyzer. The IF peak for the martensite transformation (MT) is an asymmetric shoulder rather than those sharp peaks for other shape memory alloys. The intermartensitic transformation (IMT) peak has the maximum IF value. As the heating rate increases, the height of the IMT peak increases and its position is shifted to higher temperatures. In comparison with the IMT peak, the MT peak is independent on the heating rate. The starting temperatures of the IMT peak are strongly dependent on frequency, while the MT peak is weakly dependent. Meanwhile, the heights of both the MT and IMT peak rapidly decrease with increasing the frequency. This work also throws new light on their structural transformation mechanisms.