Premartensitic Transition Research Articles

Martensitic and magnetic transitions in Ni2+xMnGa1−x ferromagnetic shape memory alloys

This study experimentally investigates a series of phase transitions of ferromagnetic shape memory alloys, Ni2+xMnGa1−x, in the composition range 0.02 ≤ x ≤ 0.27. An experimental phase diagram of these alloys was constructed by the use of electrical resistivity, thermomagnetization, and thermoanalysis measurements, and compared with both the Ni-rich Ni2+xMn1−xGa and Mn-rich Ni2Mn1+yGa1−y alloy systems which have been extensively studied as proto-materials for ferromagnetic Heusler alloys. The comparison makes it clear that there are remarkable composition dependencies for the following characteristic temperatures. All three systems have the common property that Curie temperature decreases and martensitic transition temperature increases with increasing x till the both temperatures coincide each other. As for the magnetostructural coupling temperature, however, the Ni-rich system has a wide temperature range, and the Mn-rich system does not show signs of the magnetostructural coupling, whereas the magnetostructural coupling was observed in Ni2+xMnGa1−x alloys in the range 0.10 ≤x≤ 0.12 which has similar characteristics between the former two systems. The premartensitic transition temperature was also observed in Ni2+xMnGa1−x alloys; it reached the ambient temperature at x = ~0.04, the highest of the three alloy systems. In addition, an intermartensitic transition appeared in Ni2+xMnGa1−x alloys in the composition range 0.06 ≤ x ≤ 0.10, which showed a drastic change especially in the electrical resistivity. These characteristic temperatures are also compared in terms of the number of valence electrons among the Ni–Mn–Ga Heusler alloy systems, and the differences are clarified and discussed in detail.

Temperature dependence of the anomalous Nernst effect in Ni2MnGa shape memory alloy

We report a detailed investigation of the Ni$_{2}$MnGa shape memory alloy through magnetic, electronic, and thermal measurements. Our measurements of the anomalous Nernst effect (ANE) reveal that this technique is very sensitive to the onset of the pre-martensitic transition in sharp contrast to other transport measurements. With the ANE being sensitive to changes at the Fermi surface, we infer on the link between the structural modulations and the modulation of the Fermi surface via its nesting features, with the magnetic field induced strain being the mediating mechanism.

The Characteristic Properties of Magnetostriction and Magneto-Volume Effects of Ni2MnGa-Type Ferromagnetic Heusler Alloys.

In this article, we review the magnetostriction and magneto-volume effects of Ni2MnGa-type ferromagnetic Heusler alloys at the martensitic, premartensitic, and austenitic phases. The correlations of forced magnetostriction (ΔV/V) and magnetization (M), using the self-consistent renormalization (SCR) spin fluctuation theory of an itinerant electron ferromagnet proposed by Takahashi, are evaluated for the ferromagnetic Heusler alloys. The magneto-volume effect occurs due to the interaction between the magnetism and volume change of the magnetic crystals. The magnetic field-induced strain (referred to as forced magnetostriction) and the magnetization are measured, and the correlation of magnetostriction and magnetization is evaluated. The forced volume magnetostriction ΔV/V at the Curie temperature, TC is proportional to M4, and the plots cross the origin point; that is, (M4, ΔV/V) = (0, 0). This consequence is in good agreement with the spin fluctuation theory of Takahashi. An experimental study is carried out and the results of the measurement agree with the theory. The value of forced magnetostriction is proportional to the valence electron concentration per atom (e/a). Therefore, the forced magnetostriction reflects the electronic states of the ferromagnetic alloys. The magnetostriction near the premartensitic transition temperature (TP) induces lattice softening; however, lattice softening is negligible at TC. The forced magnetostriction at TC occurs due to spin fluctuations of the itinerant electrons. In the martensitic and premartensitic phases, softening of the lattice occurs due to the shallow hollow (potential barrier) of the total energy difference between the L21 cubic and modulated 10M or 14M structures. As a result, magnetostriction is increased by the magnetic field.

Elastic Properties of TiNi Shape Memory Alloys Subjected to Various Heat Treatments

Comparative experimental studies for elastic characteristics of commercial nitiline TiNi shape memory alloys subjected to various heat treatments have been performed nitinol in the temperature range 80–300 K, including the premartensitic and martensitic transitions. The distortion of the crystal structure during the premartensitic transition in the as-prepared sample is found to be accompanied by pronounced anomalies of the Young’s modulus and the internal friction near the phase transition temperature TS ≈ 270 K both on h-eating and cooling. An additional heat treatment causes slight shifts of the premartensitic transition temperatures on heating TSu and cooling TSd and an increase in the hysteresis, but the character of the elastic anomalies is changed only slightly. Conversely, anomalies of the elastic properties during the martensitic transition at T ~ 220 K are observed only on cooling, and the temperature and the character of the anomalies is significantly changed after a heat treatment.

Magnetic Domain Walls and Macroscopic Magnetization-Related Elastic and Anelastic Effects during Premartensitic Transition in Ni₂MnGa.

The temperature and field dependences of internal friction and Young’s modulus are studied using a high-resolution ultrasonic (90 kHz) technique in stoichiometric ferromagnetic Ni2MnGa shape memory alloy close to the premartensitic transformation temperature, TPM, in the demagnetized state and under moderate fields. Several new effects observed like an apparent Young´s modulus softening close to TPM under moderate fields, instead of the hardening outside this range, as well as existing controversies in the apparent elastic and anelastic properties of Ni2MnGa close to TPM are explained by microeddy and macroeddy current relaxations that to date have been disregarded.

Упругие свойства сплавов TiNi с памятью формы с различной термообработкой

Comparative experimental studies of the elastic characteristics of commercial shape memory TiNi alloys (Nitinol) exposed to different heat treatment have been carried out within the (80 - 300) K temperature range, including the points of premartensitic and martensitic transitions. It was found that the distortion of the crystal structure at the premartensitic transition in the “as prepared” sample is accompanied by pronounced anomalies of the Young's modulus and internal friction. This transition occurs at TS ≈ 270 K both upon heating and cooling. After an additional heat treatment, the premartensitic transition temperatures TSu and TSd upon heating and cooling respectively, shift slightly and the hysteresis turns out to be more pronounced. However, the form of elastic anomalies remains nearly the same. In contrast, for the martensitic transition at T ≈ 220 K, the elastic anomalies are observed only upon cooling, and the temperature and shape of these anomalies undergo significant changes under effect of the heat treatment.

Strain glass transition in high damping Mn-22Cu-5Ni-2Fe alloy

It has been recognized that the high damping capacity of MnCu based alloys is closely related to the formation of elastic twins as a result of antiferromagnetic transition and martensitic transformation. However, the mechanism for these transitions in this kind of alloy has not been fully understood. For example, previous investigations ascribed the elastic modulus softening prior to martensitic transformation to antiferromagnetic or pre-martensitic transitions, although no convincing evidence was reported. In this paper, an intensive experimental study was made of the modulus softening and microstructural evolution in a typical high damping Mn-22Cu-5Ni-2Fe alloy, and the evidence of strain glass transition was reported. Such a transition, characterized by a continuous formation of nano-martensite domains, is accompanied by a gradual frequency-dependent modulus softening over a broad temperature range prior to martensitic transformation.

Pronounced pre-martensitic anomaly in the magnetization on Ni2MnGa thin films

We have prepared [110]-textured Ni2MnGa thin films exhibiting an unusual pre-martensitic transition accompanied by an extremely large magnetization change. The thin films were grown by molecular beam epitaxy directly on epi-ready GaAs(111)B. Crystalline structure was investigated in situ by reflection high-energy electron diffraction (RHEED) and ex situ by x-ray diffraction (XRD) and transmission electron microscopy (TEM). The results show that the film exhibits cubic crystalline structure (L21) at room temperature with lattice parameter a = 5.88 Å which undergoes martensitic transition. Magnetic characterization shows ferromagnetic behavior at room temperature with Curie temperature higher than room temperature. Martensitic transformation occurs at TM ∼ 185 K. A phenomenological model based on Landau theory of phase transformation was developed to explain the anomalous pre-martensitic transition at ∼285 K.

Strain and order-parameter coupling in Ni-Mn-Ga Heusler alloys from resonant ultrasound spectroscopy

Resonant ultrasound spectroscopy and magnetic susceptibility experiments have been used to characterize strain coupling phenomena associated with structural and magnetic properties of the shape-memory Heusler alloy series Ni$_{50+x}$Mn$_{25-x}$Ga$_{25}$ ($x=0$, 2.5, 5.0, and 7.5). All samples exhibit a martensitic transformation at temperature $T_M$ and ferromagnetic ordering at temperature $T_C$, while the pure end member ($x=0$) also has a premartensitic transition at $T_{PM}$, giving four different scenarios: $T_C>T_{PM}>T_M$, $T_C>T_M$ without premartensitic transition, $T_C\approx T_M$, and $T_C<T_M$. Fundamental differences in elastic properties i.e., stiffening versus softening, are explained in terms of coupling of shear strains with three discrete order parameters relating to magnetic ordering, a soft mode and the electronic instability responsible for the large strains typical of martensitic transitions. Linear-quadratic or biquadratic coupling between these order parameters, either directly or indirectly via the common strains, is then used to explain the stabilities of the different structures. Acoustic losses are attributed to critical slowing down at the premartensite transition, to the mobility of interphases between coexisting phases at the martensitic transition and to mobility of some aspect of the twin walls under applied stress down to the lowest temperatures at which measurements were made.

Premartensitic and martensitic phase transitions in magnetic shape-memory alloys

Premartensitic and martensitic phase transitions in magnetic shape-memory alloys

Magnetostriction of Ni2Mn1−xCrxGa Heusler Alloys

Among the functionalities of magnetic Heusler alloys, magnetostriction is attracting considerable attention. The alloy Ni2MnGa has a premartensite phase, which is a precursor state to the martensitic transition. Some researchers have observed magnetostriction in this alloy in the premartensite phase. We performed magnetostriction studies on the premartensite phase of related Cr-substituted Ni2Mn1−xCrxGa alloys and measured the thermal strain, permeability, magnetisation, and magnetostriction of polycrystals. Our thermal expansion measurements show an anomaly that indicates the occurrence of lattice deformation below the premartensitic transition temperature TP. Our permeability measurements also showed an anomaly at the premartensitic transition. From our magnetisation results, we obtained the magnetic-anisotropy constant K1. In the martensite phase, we found that the magnetic-anisotropy constant of the x = 0.00 alloy is larger than that of the x = 0.15 alloy. At 0.24 MA/m, we obtained a magnetostriction of −120 ppm for the x = 0.15 alloy. Magnetostriction in the premartensite phase is larger than that in the austenite and martensite phases at low magnetic-field strength, thus indicating that it is related to lattice softening in the premartensite phase. The e/a is proportional to the magnetostriction and TP, which indicates that the electron energy, the magnetostriction, and the Tp are correlative each other.

Premartensitic transition and relevant magnetic effects in Ni50Mn34In15.5Al0.5 alloy.

Resistance measurement, in situ optical microscopic observation, thermal and magnetic measurements have been carried out on Ni50Mn34In15.5Al0.5 alloy. The existence of a pronounced premartensitic transition prior to martensitic transition can be characterized by microstructure evolution as well as exothermic peak and smooth decrease of resistance and magnetization with obvious hysteresis over a wide temperature range upon cooling. Consequently, the alloy undergoes two successive magneto-structural transitions consisting of premartensitic and martensitic transitions. Magnetoelastic coupling between magnetic and structural degrees of freedom would be responsible for the appearance of premartensitic transition, as evinced by the distinct shift of transitions temperatures to lower temperature with external applied field of 50 kOe. The inverse premartensitic transition induced by magnetic field results in large magnetoresistance, and contributes to the enhanced inverse magnetocaloric effect through enlarging the peak value and temperature interval of magnetic entropy change ΔSm.

Theory and experimental evidence of phonon domains and their roles in pre-martensitic phenomena

AbstractPre-martensitic phenomena, also called martensite precursor effects, have been known for decades while yet remain outstanding issues. This paper addresses pre-martensitic phenomena from new theoretical and experimental perspectives. A statistical mechanics-based Grüneisen-type phonon theory is developed. On the basis of deformation-dependent incompletely softened low-energy phonons, the theory predicts a lattice instability and pre-martensitic transition into elastic-phonon domains via ‘phonon spinodal decomposition.’ The phase transition lifts phonon degeneracy in cubic crystal and has a nature of phonon pseudo-Jahn–Teller lattice instability. The theory and notion of phonon domains consistently explain the ubiquitous pre-martensitic anomalies as natural consequences of incomplete phonon softening. The phonon domains are characterised by broken dynamic symmetry of lattice vibrations and deform through internal phonon relaxation in response to stress (a particular case of Le Chatelier’s principle), leading to previously unexplored new domain phenomenon. Experimental evidence of phonon domains is obtained by in situ three-dimensional phonon diffuse scattering and Bragg reflection using high-energy synchrotron X-ray single-crystal diffraction, which observes exotic domain phenomenon fundamentally different from usual ferroelastic domain switching phenomenon. In light of the theory and experimental evidence of phonon domains and their roles in pre-martensitic phenomena, currently existing alternative opinions on martensitic precursor phenomena are revisited.

Premartensite transition in Ni2FeGa Heusler alloy

Martensitic phase transformation of Ni2FeGa Heusler alloy was studied by differential scanning calorimetry. Atomic ordering induced in the austenite structure by quenching from high temperature plays a significant role on martensitic phase transformation. Higher magnetization and larger magneto-crystalline anisotropy of martensite phase than that of austenite phase are noticed. Tweed contrast regions observed in the transmission electron microscopy were correlated to premartensite phenomena. A shift in premartensitic transition temperature prior to martensitic transformation as measured by differential scanning calorimetry is being reported for the first time in this system.

Magnetic Phase Diagram of Heusler Alloy System Ni2Mn1-xCrxGa

The temperature dependence of the electrical resistivity ρ of Ni2Mn1-xCrxGa (x=0.05∼0.25) was measured. Two anomalies corresponding to the magnetic and structural phase transitions at TC and TM were observed on the ρ-T curves for each sample, respectively. The kinks corresponding to the premartensitic transition at Tp were observed for all samples except x=0.25. On the basis of the experimental results, the T vs. x phase diagram of Ni2Mn1-xCrxGa was determined.

Elasticity and magnetism of Ni2 MnGa premartensitic tweed

Magnetic, magneto‐elastic and elastic measurements were used to characterize the thermal evolution of the premartensite phase of Ni2 MnGa. The premartensitic transition is shown to correspond to a sharp maximum of magnetostriction and a pronounced minimum of the (110) shear stiffness (the elastic coefficient); no additional softening of the coefficient prior to the martensitic transformation was observed. In contrast, a significant softening of the (100) stiffness (the elastic coefficient) was observed in the vicinity of the martensitic transition, while this coefficient is fully unaffected by the premartensitic transition. A simple two‐dimensional model of the tweed is presented to explain this mutual independence between the and shears and the effect of tweed formation on both of them.

Ellipsometry applied to phase transitions and relaxation phenomena in Ni2MnGa ferromagnetic shape memory alloy

The temperature dependences of the refractive index, n(T), calculated from ellipsometric data during the premartensitic and martensite transitions in a Ni2MnGa ferromagnetic shape memory alloy allow to detect all critical temperatures of the sample. The performed measurements reveal the behavior of a surface layer of a few tens nanometers depth due to the light absorption. Optical measurements reveal the earlier onset of premartensitic transition at the sample surface and shows unknown new features like time and temperature dependent effects related to the crystal surface. The underlying mechanisms of the observed temperature changes of the equilibrium n0 may be related to temperature dependent interactions between free electrons and phonons as well as between electrons and defects. A giant isothermal creep (up to 20%) of the refractive index was found for temperatures below T0 = 315 K. Below T0 the creep amplitude grows with the temperature difference T0-T. The creep disappears and the relaxation time becomes zero at temperatures above T0.

Effect of Sn doping on the martensitic and premartensitic transitions in Ni2MnGa

The effect of Sn doping at the Ga site of Ni2MnGa is investigated through magnetic and magneto-transport measurements. Clear signatures of martensitic and premartensitic transitions are observed in the pure as well as in 5% Sn doped alloy. For 10% Sn doping, the martensitic transition vanishes, while the premartensitic transition remains visible at low temperature. All the samples are found to have a ferromagnetic ground state with saturation moment decreasing with increasing Sn concentration. The magnetocaloric effect near the martensitic transition in the pure and 5% Sn doped samples is found to be positive. However, the entropy change is found to decrease with increasing magnetic field, which is particularly prominent in the undoped sample. The samples also show negative magnetoresistance with anomalies at the martensitic and premartensitic transition points.

Strain glassy behavior and premartensitic transition in Au7Cu5Al4alloy

Strain glassy behavior (SGB) in correlation with premartensitic tweed (PMT) morphology was observed in an Au(7)Cu(5)Al(4) alloy, which is different from previously observed SGB that is incompatible with PMT in doped Ti-Ni alloys. Additionally, an inverse freezing transition from martensite to premartensitic strain glass upon heating was experimentally discovered, confirming a recent theoretical prediction. Investigations of rich phenomena in nonmagnetic Au-Cu-Al alloys are expected to shed light on the origin of both premartensitic transition and glasses.

Ni2MnGa(100) ferromagnetic shape memory alloy: A surface study

Ni2MnGa(100) single crystal studied using low energy electron diffraction (LEED) and ultraviolet photoemission spectroscopy (UPS) exhibits interesting modification of the surface properties that are mainly influenced by surface composition as well as intrinsic effects. In the martensite phase, the LEED spot profiles show presence of an incommensurate modulation for the stoichiometric surface. In contrast, a commensurate modulation is observed for Mn-excess Ni–Mn–Ga surface. A pre-martensite phase is identified at the surface. Both the surface martensitic and pre-martensitic transition temperatures decrease as the Mn content increases. The UPS spectra in the austenite phase exhibit systematic change in shape as a function of surface composition that can be related to changes in the hybridization between Ni and Mn 3d states. The spectra in the martensite phase exhibit interesting modifications near the Fermi level, which has been compared to density of states calculated for a modulated structure by ab-initio density functional theory. Intrinsic surface properties dissimilar from the bulk are enhanced hysteresis width of the martensite transition and increased pre-martensitic transition temperature.