The magnetic state of low temperature martensite phase in Co-substituted Ni-Mn-Sn-based ferromagnetic shape memory alloys (FSMAs) has been investigated, in view of numerous conflicting reports of occurrences of spin glass (SG), superparamagnetism (SPM) or long range anti-ferromagnetic (AF) ordering. Combination of dc magnetization, ac susceptibility and small angle neutron scattering (SANS) studies provide a clear evidence for AF order in martensitic phase of Ni45Co5Mn38Sn12 alloy and rule out SPM and SG orders. Identical studies on another alloy of close composition of Ni44Co6Mn40Sn10 point to presence of SG order in martensitic phase and absence of SPM behavior, contrary to earlier report. SANS results do show presence of nanometre-sized clusters but they are found to grow in size from 3 nm at 30 K to 11 nm at 300 K, and do not correlate with magnetism in these alloys.
Modulated martensites play an important role in magnetic shape memory alloys, because all functional properties are closely connected to the twin microstructure and the phase boundary. The nature of the modulated martensites is still unclear. One approach is the concept of adaptive martensite, which regards all modulated phases as nanotwinned microstructures. In this article, we use the Ni-Mn-based shape memory alloys as an example to show the geometric rationale behind this concept using analytic equations based on the phenomenological theory of martensite. This could enhance discussions abou…
Ni57.5Mn17.5Ga25 shape memory alloy exhibits a complex transformation behavior, appearing after aging. Aging in the austenitic state resulted in an ordinary decrease of the martensitic transformation temperature. Contrary to this, aging in the martensitic state brought about unusual features of the martensitic transformation observed so far only in Ni-Ti alloys.
A polycrystalline and single-crystal samples of near-stoichiometric Ni2MnGa alloys have been investigated. It was confirmed that martensite short crystallographic axis (c-axis) is the easy axis of magnetization. The reversible reorientation of the easy magnetization direction of martensite samples after 5% compression is found. The same reversible reorientation of the easy magnetization direction under applied magnetic field occurs in the single-crystal samples. The field-induced reorientation of the easy magnetization direction is accompanied with more than 4% of sample's dimension change. Ab…
The large magnetocaloric effect in Heusler alloys showing martensite phase transformation puts them forward as efficient materials for magnetic refrigeration. However, irreversibility of the magnetocaloric cooling cycle is a major challenge for real applications. This irreversibility is directly linked to the thermal hysteresis at the first-order martensite phase transition. Therefore, minimizing the hysteresis is essential in order to achieve reversibility. Here we show a large reduction in the thermal hysteresis at the martensite transition in the Ni$_{2}$Mn$_{1.4}$In$_{0.6}$ and Ni$_{1.8}$C…
In the present work, a combined theoretical and experimental study on Co$_{2}$PtGa Heusler alloy shows that it exhibits a martensite transition around 1320 K with a small thermal hysteresis (10 K). Dynamical stability of Co$_2$PtGa in the tetragonal phase has been established by the theoretically calculated phonon dispersion curves. Magnetization measurements suggest that this alloy is a ferromagnetic material with a saturation magnetic moment of 2.83 $μ_B$/f.u. at 2 K, which is in excellent agreement with the value obtained from \textit{ab-initio} calculations (2.87 $μ_B$/f.u.). Our present s…
The irreversibility of the martensite transition in magnetic shape memory alloys (MSMAs) with respect to external magnetic field is one of the biggest challenges that limits their application as giant caloric materials. This transition is a magneto-structural transition that is accompanied with a steep drop in magnetization (i.e., 'delta M') around the martensite start temperature (Ms) due to the lower magnetization of the martensite phase. In this communication, we show that 'delta M' around Ms in Mn rich Ni-Mn based MSMAs gets suppressed by two orders of magnitude in crushed powders due to t…
This book is the first monograph in the scientific literature, dedicated to the FCC-BCC transformation in iron-based alloys, in which the dynamical approach is used for the explanation of the martensite growth stage. The rapid growth of a cooling martensite crystal is considered as a self-organized process controlled by the quasi-longitudinal lattice displacement waves (DW). The regime of the DW initial excitation is rigid. DW have the frequencies ~$10^{10} sec^{-1}$ from the hypersound band and the amplitudes providing the level of deformation ~$10^{-3}$. The conditions that are necessary for…
Modulated phases occur in numerous functional materials like giant ferroelectrics and magnetic shape memory alloys. To understand the origin of these phases, we review and generalize the concept of adaptive martensite. As a starting point, we investigate the coexistence of austenite, adaptive 14M phase and tetragonal martensite in Ni-Mn-Ga magnetic shape memory alloy epitaxial films. The modulated martensite can be constructed from nanotwinned variants of a tetragonal martensite phase. By combining the concept of adaptive martensite with branching of twin variants, we can explain key features …
Tailoring the phase stability and magnetic structure by combining the antiferromagnetic MnNiGe martensite with ferromagnetic MnFeGe austenite, a magnetic-field-induced martensitic transformation has been achieved in MnNi1-xFexGe system. The doped Fe enables the isolated structural and magnetic transitions of MnNiGe to coincide at room temperature and also establishes ferromagnetic couplings in six-Mn-centering-one-Fe-atom clusters. The ferromagnetic clusters transit the antiferromagnetic state in martensite to a ferromagnetic state and the alloys are further induced to a stronger ferromagnetic…
We consider a recently introduced 2-D square-to-rectangle martensite model that explains several unusual features of martensites to study ferromagnetic martensites. The strain order parameter is coupled to the magnetic order parameter through a 4-state clock model. Studies are carried out for several combinations of the ordering of the Curie temperatures of the austenite and martensite phases and, the martensite transformation temperature. We find that the orientation of the magnetic order which generally points along the short axis of the rectangular variant, changes as one crosses the twin o…
Magnetic, transport, and x-ray diffraction measurements of ferromagnetic shape memory alloy Ni$_{2.16}$Mn$_{0.84}$Ga revealed that this alloy undergoes an intermartensitic transition upon cooling, whereas no such a transition is observed upon subsequent heating. The difference in the modulation of the martensite forming upon cooling from the high-temperature austenitic state [5-layered (5M) martensite], and the martensite forming upon the intermartensitic transition [7-layered (7M) martensite] strongly affects the magnetic and transport properties of the alloy and results in a large thermal hy…
We have studied the phase volume fraction related magnetoresistance (MR) across the first order martensite transformation (MT) of Ni44Cu2Mn43In11 alloy. Within the metastability of MT, an isothermal application of magnetic field converts the martensite into austenite. The field induced austenite phase fraction (fIA) at any temperature depends on the availability and instability of martensite phase fraction (fM ) at that temperature. This fIA is found to contribute most significantly to the observed giant MR while the contribution from pure martensite and austenite phase fraction is negligible.…
We present here results of temperature dependent high resolution synchrotron x-ray powder diffraction study of sequence of phase transitions in Ni2MnGa. Our results show that the incommensurate martensite phase results from the incommensurate premartensite phase, and not from the austenite phase assumed in the adaptive phase model. The premartensite phase transforms to the martensite phase through a first order phase transition with coexistence of the two phases in a broad temperature interval (~40K), discontinuous change in the unit cell volume as also in the modulation wave vector across the…
Using first-principles calculations based on density functional theory, we have studied the mechanical, electronic, and magnetic properties of Heusler alloys, namely, Ni$_{2}BC$ and Co$_{2}BC$ ($B$ = Sc, Ti, V, Cr and Mn as well as Y, Zr, Nb, Mo and Tc; $C$ = Ga and Sn). On the basis of electronic structure (density of states) and mechanical properties (tetragonal shear constant), as well as magnetic interactions (Heisenberg exchange coupling parameters), we probe the properties of these materials in detail. We calculate the formation energy of these alloys in the (face-centered) cubic austeni…
Besides permitting an accurate determination of the ferromagnetic-to-paramagnetic phase transition temperature and the characteristic temperatures for the beginning and end of the growth of martensite (austenite) phase at the expense of austenite (martensite) phase while cooling (heating), the results of an extensive ac susceptibility, sound velocity and internal friction investigation of the thermoelastic martensitic transformation in melt-quenched (site-disordered) Ni55Fe20Al25 alloy provide a clear experimental evidence for the following. Irreversible thermoelastic changes (thermal hysteres…
A three-dimensional phase-field model is proposed for simulating the magnetic martensitic phase transformation. The model considers a paramagnetic cubic austenite to ferromagnetic tetragonal martensite transition, as it occurs in magnetic Heusler alloys like Ni2 MnGa, and is based on a Landau 2-3-4 polynomial with temperature dependent coefficients. The paramagnetic-ferromagnetic transition is recaptured by interpolating the micromagnetic energy as a function of the order parameter for the ferroelastic domains. The model is numerically implemented in real space by finite element (FE) method. F…
Fe doping in Ni$_2$Mn$_{1.5}$In$_{0.5}$ results in suppression of the martensitic phase via two contrasting routes. In Ni$_2$Mn$_{1.5-x}$Fe$_{x}$In$_{0.5}$, the martensitic phase is converted to a strain glassy phase, while in Ni$_{2-y}$Fe$_y$Mn$_{1.5}$In$_{0.5}$, a cubic ferromagnetic phase results at the expense of the martensite. Careful studies of magnetic and structural properties reveal the presence of the impurity $γ-$(Fe,Ni) phase as the reason for the emergence of non-ergodic strain glassy phase when Fe is sought to be doped at Y/Z (Mn) sites of X$_2$YZ Heusler alloy. Whereas attempts…
This article presents a brief overview of our recent work on the nature of long period modulation in the premartensite and martensite phases of Ni2MnGa ferromagnetic shape memory alloy using high resolution synchrotron x-ray powder diffraction patterns. The commensurate structure model using the Pnnm space group is unable to account for the peak positions of the satellite reflections that appear due to modulations correctly. LeBail and Rietveld refinements using the (3+1)-D super space group $Immm(00γ)s00$ show that the peak positions of all the reflections, including the satellites, can be ex…
The martensitic start temperature ($M_{\text{s}}$) is a technologically fundamental characteristic of high-temperature shape memory alloys. We have recently shown [Phys. Rev. B 94, 224104 (2016)] that the two key features in describing the composition dependence of $M_\text{s}$ are the $T=0$ K phase stability and the difference in vibrational entropy which, within the Debye model, is directly linked to the elastic properties. Here, we use density functional theory together with special quasi-random structures to study the elastic properties of disordered martensite and austenite Ti-Ta alloys a…
The crystal structures, martensitic structural transitions and magnetic properties of MnCo1-xFexSi (0 <= x <= 0.50) alloys were studied by differential scanning calorimetry (DSC), x-ray powder diffraction (XRD) and magnetic measurements. In high-temperature paramagnetic state, the alloys undergo a martensitic structural transitions from the Ni2In-type hexagonal parent phase to the TiNiSi-type orthorhombic martensite. Both the martensitic transition temperature (TM) and Curie temperatures of martensite (T_C^M) decrease with increasing Fe content. The introduced Fe atoms establish ferromagnetic …
A systematic study of magnetic properties of Ni2+xMn1-xGa (0 \le x \le 0.19) Heusler alloys undergoing structural martensite-austenite transformations while in ferromagnetic state has been performed. From measurements of spontaneous magnetization, Ms(T), jumps ΔM at structural phase transitions were determined. Virtual Curie temperatures of the martensite were estimated from the comparison of magnetization in martensitic and austenitic phases. Both saturation magnetic moments in ferromagnetic state and effective magnetic moments in paramagnetic state of Mn and Ni atoms were estimated and the i…
In this paper, we present temperature and field dependent neutron diffraction (ND) study to unravel the structural and the magnetic properties in Ni50Mn38Sb12 Heusler system. This alloy shows martensitic transition from high temperature austenite cubic phase to low temperature martensite orthorhombic phase on cooling. At 3 K, the lattice parameters and magnetic moments are found to be almost insensitive to field. Just below the martensitic transition temperature, the martensite phase fraction is found to be 85%. Upon applying the field, the austenite phase becomes dominant, and the field induc…
Stabilization of Martensite and Austenite Phases and Realization of Two-way Martensitic Transition in Co-Ni-Ga Ferromagnetic Shape Memory Alloy Nanoparticles
The crystal structure and magnetoresponsive properties of Fe-doped MnCoGe alloys have been investigated using x-ray diffraction (XRD) and magnetic measurements. By alloying the Fe-containing isostructure compounds into MnCoGe, a magnetostructural transition from paramagnetic austenite to ferromagnetic martensite with large magnetization difference can be realized in a temperature window between the Curie temperatures of the austenite and martensite, resulting in magnetic-field-induced martensitic transformations and large magnetic-entropy changes. A structural and magnetic phase diagram of Fe-…
The magnetic and thermodynamic properties of a Ni2.19Mn0.81Ga alloy with coupled magnetic and structural (martensitic) phase transitions were studied experimentally and theoretically. The magnetocaloric effect was measured by a direct method in magnetic fields 0-26 kOe at temperatures close to the magnetostructural transition temperature. For theoretical description of the alloy properties near the magnetostructural transition a statistical model is suggested, that takes into account the coexistence of martensite and austenite domains in the vicinity of martensite transformation point.
The summary of the models offered by the author revealing features of the physical mechanisms controlling processes of martensite crystal formation is resulted. The rapid growth of a cooling martensite crystal is considered as a self-organized process controlled by the quasi-longitudinal lattice displacement waves (DW). It is shown, that processes of the heterogeneous nucleation and wave growth have the genetic connection in case of spontaneous fcc-bcc martensitic transformation. The exposition of strain martensite formation is considered in the context of a cryston model.
The first order martensitic transition in the ferromagnetic shape memory alloy Ni45Co5Mn38Sn12 is also a magnetic transition and has a large field induced effect. While cooling in the presence of field this first order magnetic martensite transition is kinetically arrested. Depending on the cooling field, a fraction of the arrested ferromagnetic austenite phase persists down to the lowest temperature as a magnetic glassy state, similar to the one observed in various intermetallic alloys and in half doped manganites. A detailed investigation of this first order ferromagnetic austenite (FM-A) to…
The microstructure of a CoNiAl FSMA was determined by conventional transmission electron microscopy, electron diffraction studies together with advanced microscopy techniques and in-situ Lorentz microscopy. 10 to 60 nm sized rod-like precipitates of hcp $ε$-Co were confirmed to be present by HRTEM. The orientation relationship between the precipitates and B2 matrix is described by the Burgers orientation relationship. The crystal structure of the martensite obtained after cooling is tetragonal L10 with a (1-11) twinning plane. The magnetic domain structure was determined during an in-situ cool…
Influence of disorder, antisite defects, martensite transition and compositional variation on the magnetic properties and electronic structure of Mn$_2$NiGa and Mn$_{1+x}$Ni$_{2-x}$Ga magnetic shape memory alloys have been studied by using full potential spin-polarized scalar relativistic Korringa-Kohn-Rostocker (FP-SPRKKR) method. Mn$_2$NiGa is ferrimagnetic and its total spin moment increases when disorder in the occupancy of Mn$_{\rm Ni}$ (Mn atom in Ni position) is considered. The moment further increases when Mn-Ga antisite defect[1] is included in the calculation. A reasonable estimate o…
Exploiting solute segregation and partitioning to the deformation-induced planar defects and nano-martensite in designing ultra-strong Co-Ni base alloys
The phonon dispersion relations and elastic constants for ferromagnetic Ni$_2$MnGa in the cubic and tetragonally distorted Heusler structures are computed using density-functional and density-functional perturbation theory within the spin-polarized generalized-gradient approximation. For $0.9<c/a<1.06$, the TA$_2$ tranverse acoustic branch along $[110]$ and symmetry-related directions displays a dynamical instability at a wavevector that depends on $c/a$. Through examination of the Fermi-surface nesting and electron-phonon coupling, this is identified as a Kohn anomaly. In the parent cubic pha…
We study the branching of twins appearing in shape memory alloys at the interface between austenite and martensite. In the framework of three-dimensional non-linear elasticity theory, we propose an explicit, low-energy construction of the branched microstructure, generally applicable to any shape memory material without restrictions on the symmetry class of martensite or on the geometric parameters of the interface. We show that the suggested construction follows the expected energy scaling law, i.e., that (for the surface energy of the twins being sufficiently small) the branching leads to en…
In the present work, the aspects of magnetic and structural properties of Ni-Mn-Ga alloys are described in the framework of fist-principles approach and mapped into ternary composition diagrams. The stable atomic arrangement and magnetic alignment for compositions with cubic austenite and tetragonal martensite structures across phase diagrams are predicted. It is shown that Ni- and Ga-rich compositions possess the regular Heusler structure in contrast to Mn-rich compositions with inverse Heusler structure as favorable one. Compositions with unstable austenite structure are concentrated in the …
Precursor phenomena observed prior to the martensite phase transition plays a critical role towards the understanding of important technological properties of shape memory and magnetic shape memory alloys (MSMAs). The premartensite (PM) phase, considered as the precursor state of the martensite is proven to be a thermodynamically stable phase recently (Nature Commun. 8, 1006 (2017)), necessitates a critical investigation of precursor effects in these materials. We present here an evidence for the existence of a precursor state of the PM phase in Ni2MnGa MSMA using high energy synchrotron pair …
Materials which can undergo extremely fast displacive transformations as well as very slow diffusive transformations are studied using a Ginzburg-Landau framework to understand the physics behind microstructure formation and time-temperature-transformation (TTT) diagrams. This simple model captures the essential features of alloys such as steels and predicts the formation of mixed microstructures by an interplay between diffusive and displacive mechanisms. The intrinsic volume changes associated with the transformations stabilize mixed microstructures such as martensite-retained austenite (res…
The large magnetocaloric effect (MCE) observed in Ni-Mn based shape-memory Heusler alloys put them forward to use in magnetic refrigeration technology. It is associated with a first-order magnetostructural (martensitic) phase transition. We conducted a comprehensive study of the MCE for the off-stoichiometric Heusler alloy Ni$_{2.2}$Mn$_{0.8}$Ga in the vicinity of its first-order magnetostructural phase transition. We found a reversible MCE under repeated magnetic field cycles. The reversible behavior can be attributed to the small thermal hysteresis of the martensitic phase transition. Based …
The premartensite phase of shape memory and magnetic shape memory alloys (MSMAs) is believed to be a precursor state of the martensite phase with preserved austenite phase symmetry. The thermodynamic stability of the premartensite phase and its relation to the martensitic phase is still an unresolved issue, even though it is critical to the understanding of the functional properties of MSMAs. We present here unambiguous evidence for macroscopic symmetry breaking leading to robust Bain distortion in the premartensite phase of 10% Pt substituted Ni2MnGa. We show that the robust Bain distorted pr…
We present here the improved crystallographic/geometric compatibility and magnetocaloric reversibility by measurement of magnetic entropy change using different protocols in 10% Pt substituted Ni2Mn1.4In0.6 magnetic shape memory alloy. The substitution of Pt reduces the thermal hysteresis about 50% to the Ni2Mn1.4In0.6. The origin of the reduced thermal hysteresis is investigated by the crystallographic compatibility of the austenite and martensite phases. The calculated middle eigenvalue of the transformation matrix turned out to be 0.9982, which is very close to 1 (deviation is only 0.18%) s…
Heusler alloys exhibiting magnetic and martensitic transitions enable applications like magnetocaloric refrigeration and actuation based on the magnetic shape memory effect. Their outstanding functional properties depend on low hysteresis losses and low actuation fields. These are only achieved if the atomic positions deviate from a tetragonal lattice by periodic displacements. The origin of the so-called modulated structures is the subject of much controversy: They are either explained by phonon softening or adaptive nanotwinning. Here we used large-scale density functional theory calculation…
We use time-resolved optical reflectivity to study the laser stimulated dynamics in the magnetic shape memory alloy Ni_2MnGa. We observe two coherent optical phonons, at 1.2 THz in the martensite phase and at 0.7 THz in the pre-martensite phase, which we interpret as a zone-folded acoustic phonon and a heavily damped amplitudon respectively. In the martensite phase the martensitic phase transition can be induced by a fs laser pulse on a timescale of a few ps.
We use the phase-field method to study the martensitic transformation at the nanoscale. For nanosystems such as nanowires and nanograins embedded in a stiff matrix, the geometric constraints and boundary conditions have an impact on martensite formation, leading to new microstructures --such as dots aligned on a square lattice with axes along <01>-- or preventing martensite formation altogether. We also perform tension tests on the nanowires. The stress-strain curves are very different from bulk results. Moreover, they are weakly affected by microstructures -- the mechanical response of nanowi…
It is shown that a temperature window between the Curie temperatures of martensite and austenite phases around the room temperature can be obtained by a vacancy-tuning strategy in Mn-poor Mn1-xCoGe alloys (0 <= x <= 0.050). Based on this, a martensitic transformation from paramagnetic austenite to ferromagnetic martensite with a large magnetization difference can be realized in this window. This gives rise to a magnetic-field-induced martensitic transformation and a large magnetocaloric effect in the Mn1-xCoGe system. The decrease of the transformation temperature and of the thermal hysteresis…
Stress-induced martensitic transformations enable metastable alloys to exhibit enhanced strain hardening capacity, leading to improved formability and toughness. As is well-known from transformation-induced plasticity (TRIP) steels, however, the resulting martensite can limit ductility and fatigue life due to its intrinsic brittleness. In this work, we explore an alloy design strategy that utilizes stress-induced martensitic transformations but does not retain the martensite phase. This strategy is based on the introduction of superelastic nano-precipitates, which exhibit reverse transformatio…
The thermodynamic stability of the premartensite (PM) phase has been a topic of extensive investigation in shape memory alloys as it affects the main martensite phase transition and the related physical properties. In general, the PM phase is stable over a rather narrow temperature-composition range. We present here evidence for chemical pressure induced suppression of the main martensite transition and stabilization of the PM phase over a very wide temperature range from 300 K to around 5 K in a magnetic shape memory alloy (MSMA) Ni50Mn34In16 using magnetic susceptibility, synchrotron X-ray p…
Neutron diffraction measurements, performed in presence of an external magnetic field, have been used to show structural evidence for the kinetic arrest of the first-order phase transition from the high temperature austenite phase to the low temperature martensite phase in the magnetic shape memory alloy Ni37Co11Mn42.5Sn9.5 and the formation of a glass-like arrested state (GLAS). The CHUF (cooling and heating under unequal fields) protocol has been used to establish phase coexistence of metastable and equilibrium states of GLAS in the neutron diffraction patterns. We also explore the field-tem…
The martensitic transformation is one of the most important phenomena in metals science due to its essential contribution to the strength of steels and most engineering alloys. Yet the basic, atomistic mechanisms leading to martensite nucleation and twin morphology are not yet known. A detailed picture in this regard is required if the strengthening effects of martensite are to be properly understood. This work presents molecular dynamics (MD) simulations of the martensitic transformation using a model fcc/bcc semi-coherent interface with Nishiyama-Wasserman orientation relationship. Significa…
Molecular dynamics simulations are performed to provide a detailed understanding of the functional degradation of shape memory alloys at small scale. The origin of the experimentally reported accumulation of plastic deformation and the anomalous sudden increase of the residual strain under cyclic mechanical loading are explained by detailed insights into the relevant atomic scale processes. Our work reveals that the mechanical response of shape-memory-alloy pillars under cyclic compression is significantly influenced by the presence of an amorphous-like surface region as experimentally induced…
Thermal hysteresis is recognized as one of the main drawbacks for cyclical applications of magnetocaloric and ferromagnetic shape memory materials with first order transformations. As such, the challenge is to develop strategies that improve the compatibility between the phases involved in the transitions and study its influence on thermal hysteresis. With this purpose, we explore the thermal, structural and magnetic properties of the Ni2Mn1-xCuxGa0.84Al0.16 Heusler alloys. The alloys present a thermal hysteresis reduction of ~60% when the Cu content in the compound varies from x = 0.10 to x =…
We study the temperature dependence of strain under constant magnetic-fields in Ni-Mn based ferromagnetic Heusler alloys in the form Ni-Mn-$X$ ($X$: Ga, In, Sn, Sb) which undergo a martensitic transformation. We discuss the influence of the applied magnetic-field on the nucleation of ferromagnetic martensite and extract information on the easy-axis of magnetization in the martensitic state.
Shape memory alloys inherit their macroscopic properties from their mesoscale microstructure originated from the martensitic phase transformation. In a cubic to orthorhombic transition, a single variant of marten- site can have a compatible (exact) interface with the austenite for some special lattice parameters in contrast to conventional austenite/twinned martensite interface with a transition layer. Experimentally, the phase compat- ibility results in a dramatic drop in thermal hysteresis and gives rise to very stable functional properties over cycling. Here, we investigate the microstructu…
Diffusionless phase transitions are at the core of the multifunctionality of (magnetic) shape memory alloys, ferroelectrics and multiferroics. Giant strain effects under external fields are obtained in low symmetric modulated martensitic phases. We outline the origin of modulated phases, their connection with tetragonal martensite and consequences for their functional properties by analysing the martensitic microstructure of epitaxial Ni-Mn-Ga films from the atomic to macroscale. Geometrical constraints at an austenite-martensite phase boundary act down to the atomic scale. Hence a martensitic…
The origin of incommensurate structural modulation in Ni-Mn based Heusler type magnetic shape memory alloys (MSMAs) is still an unresolved issue inspite of intense focus on this due to its role in the magnetic field induced ultra-high strains. In the archetypal MSMA Ni2MnGa, the observation of non-uniform displacement of atoms from their mean positions in the modulated martensite phase, premartensite phase and charge density wave as well as the presence of phason broadening of satellite peaks have been taken in support of the electronic instability model linked with a soft acoustic phonon. We …
An effective scheme of isostructural alloying was applied to establish a Curie-temperature window in isostructural MnNiGe-CoNiGe system. With the simultaneous accomplishment of decreasing structural-transition temperature and converting antiferromagnetic martensite to ferromagnetic state, a 200 K Curie-temperature window was established between Curie temperatures of austenite and martensite phases. In the window, a first-order magnetostructural transition between paramagnetic austenite and ferromagnetic martensite occurs with a sharp jump in magnetization, showing a magnetic entropy change as …
