10M Martensite Research Articles

Anharmonic incommensurate structure modulation in Ni-Mn-Ga martensite exhibiting highly mobile twin boundaries

We perform neutron diffraction on a bulk Ni50.0Mn27.5Ga22.5 single crystal to investigate the evolution of its five-layered modulated (10M) martensite structure, from 300 K down to 10 K. Close to martensite transformation, the modulation is nearly commensurate, with q = 0.402 in a modulation vector (q, q, 0). Upon cooling, the shift in diffraction satellites indicates a transition to an incommensurate modulation with increasing q. However, the observed fifth-order diffraction satellites below 260 K and even more complex satellite landscape at 10 K cannot be explained by incommensurability alone. Our analysis reveals that the modulation function is highly anharmonic, encompassing Fourier components up to the eighth order. We have developed a single-parameter description of the evolution of modulation across the entire temperature interval of the 10M phase existence. Surprisingly, the structure evolution from commensurate to incommensurate modulation has no significant effect on twin boundary mobility.

Crystal Structure and Properties of Heusler Alloys: A Comprehensive Review

Heusler alloys, which were unintentionally discovered at the start of the 20th century, have become intriguing materials for many extraordinary functional applications in the 21st century, including smart devices, spintronics, magnetic refrigeration and the shape memory effect. With this review article, we would like to provide a comprehensive review on the recent progress in the development of Heusler alloys, especially Ni-Mn based ones, focusing on their structural crystallinity, order-disorder atoms, phase changes and magnetic ordering atoms. The characterization of the different structures of these types of materials is needed, where a detailed exploration of the crystal structure is presented, encompassing the influence of temperature and compositional variations on the exhibited phases. Hence, this class of materials, present at high temperatures, consist of an ordered austenite with a face-centered cubic (FCC) superlattice as an L21 structure, or body-centered cubic (BCC) unit cell as a B2 structure. However, a low-temperature martensite structure can be produced as an L10, 10M or 14M martensite structures. The crystal lattice structure is highly dependent on the specific elements comprising the alloy. Additionally, special emphasis is placed on phase transitions within Heusler alloys, including martensitic transformations ranging above, near or below room temperature and magnetic transitions. Therefore, divers’ crystallographic defects can be presented in such types of materials affecting their structural and magnetic properties. Moreover, an important property of Heusler compounds, which is the ability to regulate the valence electron concentration through element substitution, is discussed. The possible challenges and remaining issues are briefly discussed.

Microstructure, martensitic transformation kinetics, and magnetic properties of (Ni50Mn40In10)100−xCox melt-spun ribbons

AbstractThe effect of Co-doping on the structure, microstructure, martensitic phase transformation kinetics, and magnetic properties of the melt-spun (Ni50Mn40In10)1−xCox (x = 1, 2, and 3) Heusler ribbons, named hereafter Co1 (x = 1), Co2 (x = 2), and Co3 (x = 3), was assessed using X-ray diffraction, scanning electron microscope, energy-dispersive spectroscopy, X-ray fluorescence, differential scanning calorimetry, and vibrating sample magnetometer. The XRD results reveal the formation of a 14M martensite structure alongside the face-centered-cubic (fcc) γ phase. The crystallite size ranges between 50 and 98 nm for the 14M martensite and from 9 to 16 nm for the γ phase. The mass fraction of the γ phase lies between 36.4 and 44.2%. Co-doping affects the lattice parameters and the characteristic temperatures (martensite start, martensite finish, austenite start, and austenite finish). The calculated activation energy values for the non-isothermal martensitic transformation kinetics are 257 kJ mol−1 and 135.6 kJ mol−1 for the Co1 and Co2, respectively. The produced ribbons show a paramagnetic behavior. The variation in the coercivity can be related to the crystallite size and mass fraction of the γ phase. The produced ribbons exhibit an exchange bias at room temperature that decreases with increasing the Co content.

Adaptive Phase or Variant Formation at the Austenite/Twinned Martensite Interface in Modulated Ni–Mn–Ga Martensite

AbstractThe crystal structure and transformation path from austenite to 10M martensite in Ni–Mn–Ga single crystal are examined employing high‐energy synchrotron radiation, scanning, and transmission electron microscopy. Using temperature gradient, an austenite/twinned martensite interface is stabilized revealing the crystal structure and microstructure of both phases and the transformation sequence across the interface. Depending on the distance from the interface, three distinct types of martensite crystal lattice, namely simple tetragonal and two monoclinic modulated ones, that is, 10M′ and 10M are confirmed. In situ measurements show that lattice mismatch formed at the habit plane is compensated by the formation of micro‐twinned and branched martensite along with an elastic change in lattice parameters. It is shown that the characteristics for periodic shuffling twin boundaries, such as modulation twins or inverted stacking faults, are installed in later stages of transformation. In other words, large microstructure elements that most efficiently accommodate the strain are installed first, and then smaller elements are operable. Overall, the experimental results show that the crystal lattice does not adapt modulated phase at the habit plane and cannot be built up from simple non‐modulated tetragonal blocks but rather a specific sequence of phase transformations using shear/shuffling deformation takes place.

Increase of twinning stress in single crystalline Ni-Mn-Ga micropillars

We have investigated the effect of characteristic size decrease on the magneto-mechanical properties of Ni-Mn-Ga single crystalline 10M martensite, using micropillars prepared by focused-ion-beam milling. Following the electro-chemical surface treatment, the twinning stress in a focused-ion-beam milled micropillar with a characteristic size of about 17 µm was reduced from 17 MPa to below 2 MPa, allowing the magnetically induced reorientation of twin variants. We found that twinning stress universally increases with the decrease of characteristic size. In combination with the data for bulk material, our results suggest a power law dependence of twinning stress on characteristic size, with an exponent of -0.5 for both Type I and Type II single twin boundaries. Extrapolation of the fit suggests the characteristic size limit for magnetically induced reorientation to be 30 µm and 2 µm for Type I and Type II twin boundaries, respectively.

Anharmonic modulation and its interplay with twinned microstructure in Ni–Mn–Ga–(Fe) 10M martensite

Anharmonic modulation and its interplay with twinned microstructure in Ni–Mn–Ga–(Fe) 10M martensite

Exceptionally small Young modulus in 10M martensite of Ni-Mn-Ga exhibiting magnetic shape memory effect

A Ni50Mn28Ga22 single crystal exhibiting 10M martensitic structure at room temperature was studied by X-ray diffraction measured in situ under uniaxial tensile stress up to 20 MPa and in different initial orientations of the sample. When the c axis lies along the direction of the applied stress, the sample is elongated primarily by the stress-induced reorientation. In comparison, with a or b axis lying along the stress direction, the overall elongation happens primarily due to the change of the lattice parameters and also the modulation amplitude of 10M martensite rapidly decreases with the applied tension. The initial tensile Young modulus calculated from the refined lattice parameters is very low and exhibits gradual stiffening with the increasing tensile stress. Close to zero stress the modulus has an extremely low value of about 0.3 GPa.

Investigation of the martensite variant reorientation of the single crystal Ni-Mn-Ga alloy via training processes and a modification with a silicone rubber

The shape changes of the Ni-Mn-Ga alloys, which could be driven via a manipulation of temperature, stress, and/or magnetic-field, are considered as promising materials for the application of future technologies, such as robots and smart medical devices. The single crystal (SC) Ni49Mn30Ga21 (at.%) alloy, which possessed a 7-modulated (7M) martensite phase, was prepared by a floating-zone (FZ) technique. The fundamental thermal analysis, phase identification, crystallographic orientation determination, magnetic properties, mechanical properties, and microstructure observations were carried out for the evaluations of the SC Ni-Mn-Ga alloy. Prior to the training processes via an introduction of certain compressive stress cycles, no martensite variant reorientation (MVR) could be observed in the magnetization-magnetic field (M–H) curves. During the training processes, it was obviously found that the stress for the commencement of the MVR was reduced gradually. An apparent MVR was thereafter realized after some specific training procedures. This could be attributed to the less pinning effect at the twin boundaries of the SC Ni-Mn-Ga alloy. In addition, obvious shape changes were revealed after the introduction of an external magnetic field to the SC Ni-Mn-Ga alloy. It was further found that the imposed MVR could be constrained by integrating the SC Ni-Mn-Ga alloy with a silicone rubber.

Twin boundary mobility in additive manufactured magnetic shape memory alloy 10M Ni-Mn-Ga

The additive manufacturing of magnetic shape memory (MSM) alloys is attracting increasing interest as a promising method for manufacturing the active parts of small devices with complex geometries and tailored properties. Recently, laser powder bed fusion (L-PBF) was demonstrated as a successful approach for manufacturing magnetically actuatable samples from Ni-Mn-Ga MSM alloys, presenting a giant repeatable magnetic-field-induced strain of 5.8%. The most important characteristics of an MSM material that define its application potential include its magneto-structural and twin boundary (TB) mobility properties. This research presents for the first time a detailed characterization of TB mobility in a mm-sized single crystalline grain of five-layered (10M) martensite of Ni-Mn-Ga manufactured via L-PBF. Both type 1 and type 2 TBs, which are characteristic of 10M Ni-Mn-Ga, were identified and their mobility was characterized under mechanical stress and magnetic actuation. The results show that the TBs of both types exhibit behavior similar to that observed in conventionally grown single crystals. However, their mobility is decreased, which is attributed to keyhole porosity defects characteristic of the L-PBF process. The average twinning stress for type 1 and type 2 TBs was measured as 1.4 and 0.6 MPa, respectively, resulting in a maximum output stress of about 1.3 MPa for type 1 and 2.1 MPa for type 2 TBs. The presented results demonstrate that L-PBF has good potential for manufacturing active parts from Ni-Mn-Ga. The stabilization of desired TB types and patterns in an active region to ensure reliable and repeatable device operation is required.

Microstructure, Critical Behavior and Magnetocaloric Properties of Melt-Spun Ni51.82Mn32.37In15.81

Heusler alloy with an atomic composition of Ni51.82Mn32.37In15.81 was prepared by melt spinning from arc-melted ingots. X-ray diffraction, scanning electron microscopy and magnetic measurements were used to study the structural, microstructural and magnetic properties. The crystal structure consists of a mixture of B2 austenite (~50%) and 14M martensite (~50%). The alloy undergoes a second order magnetic transition at a Curie temperature of TcA=194.2 K. The hysteresis loop reveals the occurrence of exchange bias phenomenon at room temperature. The critical exponents β, γ and δ were estimated using modified Arrott plots, Kouvel–Fisher curves and critical isothermal analysis. The respective values are β=0.500±0.015, γ=1.282±0.055 and δ=3.003±0.002. The critical behaviour in ribbons is governed by the mean field model with a dominated long-range order of ferromagnetic interactions. The maximum entropy change, ∆SMmax, for an applied magnetic field of 5 T reaches an absolute value of 0.92 J/kg·K. The experimental results of entropy changes are in good agreement with those calculated using Landau theory.

Correlation between Acoustic Emission and Stress Evolution during Single Twin Boundary Motion in Ni–Mn–Ga Magnetic Shape Memory Single Crystal

Stress evolution and related acoustic emission (AE) in modulated 10Mmartensite are simultaneously studied during compression forced motion of Type I and Type II single twin boundary (TB) resulting in microstructure reorientation and large deformation. The single TB of both types is formed in the same sample and traveled the same volume. The single boundary exhibits random jerky movement and for Type I the stress fluctuation can be directly correlated to AE signal. The AE during the single TB motion is an avalanche‐like process surmounting the obstacles – crystal defects. The amplitudes of AE are approximately five times higher for Type I compared to Type II. The energy amplitudes of AE events follow the universal power law with different coefficients for Type I and Type II TB. The peak rate well correlates with stress fluctuation during TB motion. Large differences observed for the same sample indicate a different mechanism of pinning and ultimately a different mechanism of Type I and Type II TB motion.

Reverse transformation crystallography of deformed martensite in Ni-Mn-Ga shape memory alloys

Shape memory alloys exploit the crystallographically reversible martensitic transformation to achieve the significant shape memory effect. However, the crystallography of reverse transformation from deformed martensite to austenite that specifies the strain recovery remains less understood. In this work, based on the detailed microstructural and crystallographic examinations on the deformed seven-layered modulated (7M) martensite in a Ni50Mn30Ga20 alloy through uniaxial tension, it is revealed that the reverse transformation from deformed martensite to austenite may not conform to the transformation crystallography of self-accommodated martensite, but involve more crystallographic routes and higher lattice discontinuity to resume the crystallographic orientation of austenite, owing to the activation of new twinning systems in the deformed martensite. Comprehensive knowledge on the reverse transformation of deformed martensite is of practical importance for understanding the intrinsic characteristics of shape memory behavior and theoretical interest for insights into the crystallographic reversibility of martensitic transformation.

Martensitic Transition and Superelasticity of Ordered Heat Treatment Ni-Mn-Ga-Fe Microwires

The preparation of Ni-Mn-Ga and Ni-Mn-Ga-Fe master alloy ingots and microwires was completed by high vacuum electric furnace melt melting furnace and melt drawing liquid forming equipment, and the lattice dislocations and defects formed inside the microwires during the preparation process were corrected by stepwise ordered heat treatment. The micro-structure and phase structure were characterized using a SEM field emission scanning electron microscopy and an XRD diffractometer combined with an EDS energy spectrum analyzer; the martensitic phase transformation process of the microwires was analyzed using a DSC differential scanning calorimeter; and the superelasticity of the microwires was tested by a Q800 dynamic mechanical analyzer. The results indicate that Fe doping can refine the grain, transform the phase structure from parent phase to single 7M martensite, reduce the number of martensitic variants, and increase the mobility of the twin grain boundary interface. The MT phase transition temperature (MS) is substantially increased in the martensite transition (MT) process by the increase of the number of free electrons in its lattice. During the superelasticity (SE) test, both microwires displayed superior recover-ability of SE curves, and the Fe doping curves showed similar characteristics of “linear superelasticity”, showing higher critical stress values and complete SE in the experiment. The critical stress satisfies the Clausius-Clapeyron equation and exhibits higher temperature sensitivity than Ni-Mn-Ga microwires.

Nanotwinned (inter)martensite transformation interfaces in Ni50Mn25Ga20Fe5 magnetic shape memory single crystal foil

Using common and high resolution transmission electron microscopy (TEM), we study the magnetic and crystal phase structures as well as their evolution in Ni50Mn25Ga20Fe5 magnetic shape memory material with high Curie point. The particular alloying by Fe and changing thickness of the TEM foil enable us to observe all martensitic phases known in Ni-Mn-Ga Heusler alloy system and their respective transitions simultaneously. Starting from cubic austenite at about 10 nm foil thickness, the structure evolves via peculiar interleaved stripes of austenite and five-layered modulated 10M martensite to pure 10M phase with a low density of stacking faults at 40 nm thickness. With further increasing thickness, the 10M phase transforms gradually to seven-layered modulated 14M martensite with an increased density of stacking faults. Finally, the non-modulated tetragonal NM phase appears within the 14M phase by detwinning of nanotwins forming the modulated phases. High resolution TEM further confirms that nanotwinning and stacking faults are inherent structure features tightly connected with the lattice modulation and intermartensite transformations. Overall, the evolution of average lattice shows the same general trend, an increasing simple shear with (11¯0) shuffling plane, across the whole phase sequence. Additionally, we found large local variation of lattice parameters in all phases, which is is ascribed to strong lattice softening in the vicinity of the martensitic transformation and high density of stacking faults in 14M martensite lattice.

Unraveling the Phase Stability and Physical Property of Modulated Martensite in Ni2Mn1.5In0.5 Alloys by First-Principles Calculations.

Large magnetic field-induced strains can be achieved in modulated martensite for Ni-Mn-In alloys; however, the metastability of the modulated martensite imposes serious constraints on the ability of these alloys to serve as promising sensor and actuator materials. The phase stability, magnetic properties, and electronic structure of the modulated martensite in the Ni2Mn1.5In0.5 alloy are systematically investigated. Results show that the 6M and 5M martensites are metastable and will eventually transform to the NM martensite with the lowest total energy in the Ni2Mn1.5In0.5 alloy. The physical properties of the incommensurate 7M modulated martensite (7M–IC) and nanotwinned 7M martensite () are also calculated. The austenite (A) and phases are ferromagnetic (FM), whereas the 5M, 6M, and NM martensites are ferrimagnetic (FIM), and the FM coexists with the FIM state in the 7M–IC martensite. The calculated electronic structure demonstrates that the splitting of Jahn–Teller effect and the strong Ni–Mn bonding interaction lead to the enhancement of structural stability.

Experimental Observations versus First‐Principles Calculations for Ni–Mn–Ga Ferromagnetic Shape Memory Alloys: A Review

This review summarizes recent advances in first‐principles simulations of the structural, mechanical, and magnetomechanical properties of Ni–Mn–Ga‐based ferromagnetic shape memory alloys from the perspective of experimental characterization. It focuses on calculations that can capture the main features of the low‐temperature phases, particularly the appearance of spatial modulations, the occurrence of intermartensitic transitions, and the high mobility of the twin boundaries, which allows for magnetically induced reorientation. Although the calculations can only be used to interpret some of these features and are not yet ready to be used as a tool for designing new alloys with the required properties, the gap between experimental observations and simulations is narrowing. When experiments and theory are discussed together, new challenges arise for both. In addition, new results on first‐principles‐determined elastic constants of the 10M martensite of Ni–Mn–Ga are presented and discussed with respect to the high mobility of the twin boundaries.

New Metastable Baro- and Deformation-Induced Phases in Ferromagnetic Shape Memory Ni2MnGa-Based Alloys.

Structural and phase transformations in the microstructure and new metastable baro- and deformation-induced phases of the Ni50Mn28.5Ga21.5 alloy, typical of the unique class of ferromagnetic shape memory Heusler alloys, have been systematically studied for the first time. Phase X-ray diffraction analysis, transmission and scanning electron microscopy, and temperature measurements of electrical resistivity and magnetic characteristics in strong magnetic fields were used. It was found that in the course of increasing the pressure from 3 to 12 GPa, the metastable long-period structure of martensite modulated according to the 10M-type experienced transformation into a final non-modulated 2M structure. It is proved that severe shear deformation by high pressure torsion (HPT) entails grainsize refinement to a nanocrystalline and partially amorphized state in the polycrystalline structure of the martensitic alloy. In this case, an HPT shear of five revolutions under pressure of 3 GPa provided total atomic disordering and a stepwise structural-phase transformation (SPT) according to the scheme 10M → 2M → B2 + A2, whereas under pressure of 5 GPa the SPT took place according to the scheme 10M → 2M → B2 → A1. It is shown that low-temperature annealing at a temperature of 573 K caused the amorphous phase to undergo devitrification, and annealing at 623–773 K entailed recrystallization with the restoration of the L21 superstructure in the final ultrafine-grained state. The size effect of suppression of the martensitic transformation in an austenitic alloy with a critical grain size of less than 100 nm at cooling to 120 K was determined. It was established that after annealing at 773 K, a narrow-hysteresis thermoelastic martensitic transformation was restored in a plastic ultrafine-grained alloy with the formation of 10M and 14M martensite at temperatures close to those characteristic of the cast prototype of the alloy.

Crystallography and Microstructure of 7M Martensite in Ni-Mn-Ga Thin Films Epitaxially Grown on (1 1 2¯ 0)-Oriented Al2O3 Substrate.

Epitaxial Ni-Mn-Ga thin films have been extensively investigated, due to their potential applications in magnetic micro-electro-mechanical systems. It has been proposed that the martensitic phase in the <1 1 0>A-oriented film is much more stable than that in the <1 0 0>A-oriented film. Nevertheless, the magnetic properties, microstructural features, and crystal structures of martensite in such films have not been fully revealed. In this work, the <1 1 0>A-oriented Ni51.0Mn27.5Ga21.5 films with different thicknesses were prepared by epitaxially growing on Al2O3(1 1 0) substrate by magnetron sputtering. The characterization by X-ray diffraction technique and transmission electron microscopy revealed that all the Ni51.0Mn27.5Ga21.5 films are of 7M martensite at the ambient temperature, with their Type-I and Type-II twinning interfaces nearly parallel to the substrate surface.

Unconventional twin deformation of Ni-Mn-Ga 7M martensite under tension mediated by the collective lattice reorientation from a-c twin to b-c twin

Deformation of martensite twins is a prerequisite to obtain the shape memory effect in shape memory alloys, which is conventionally conceived to be realized by twinning/detwinning based on the simple shear. Here, we report the unconventional twin deformation of seven-layered modulated (7M) martensite under tension in a directionally solidified Ni50Mn30Ga20 alloy. Based on the neutron diffraction and EBSD measurements, it is evidenced that the deformation of 7M martensite in the variant colonies with unfavorable Schmid factors is achieved by the collective lattice reorientation from four twin-related variants (a-c twin) to another type of four twin-related variants (b-c twin), rather than the detwinning of existing variants. Such deformation is not caused by the simple shear, but originated from the local atomic rearrangements that convert the initial lattice to the reoriented lattice. Moreover, the collective conversion from the initial self-accommodated variants to the reoriented variants follows a specific orientation relationship, where the deformation of individual variant yields the consistent dilatational strain along the loading direction and effectively accommodates the macroscopic strain. The present study provides clear evidence on the unconventional twin deformation mediated by the collective lattice reorientation, which is expected to deepen the understanding of martensite deformation behaviors in shape memory alloys.

Coexisting Multiple Martensites in Ni57−xMn21+xGa22 Ferromagnetic Shape Memory Alloys: Crystal Structure and Phase Transition

A comprehensive study of the crystal structure and phase transition as a function of temperature and composition in Ni57−xMn21+xGa22 (x = 0, 2, 4, 5.5, 7, 8) (at. %) magnetic shape memory alloys was performed by a temperature-dependent synchrotron X-ray diffraction technique and transmission electron microscopy. A phase diagram of this Ni57−xMn21+xGa22 alloy system was constructed. The transition between coexisting multiple martensites with monoclinic and tetragonal structures during cooling was observed in the Ni51.5Mn26.5Ga22 (x = 5.5) alloy, and it was found that 5M + 7M multiple martensites coexist from 300 K to 160 K and that 5M + 7M + NM multiple martensites coexist between 150 K and 100 K. The magnetic-field-induced transformation from 7M martensite to NM martensite at 140 K where 5M + 7M + NM multiple martensites coexist before applying the magnetic field was observed by in situ neutron diffraction experiments. The present study is instructive for understanding the phase transition between coexisting multiple martensites under external fields and may shed light on the design of novel functional properties based on such phase transitions.