Low-temperature Martensitic Phase Research Articles

Direct Measurements of Magnetocaloric Effect in a Single Crystalline Ni<sub>2.13</sub>Mn<sub>0.81</sub>Ga<sub>1.06</sub> Heusler Alloy

Magnetocaloric effect (MCE) in the vicinity of first order martensitic transformation and second order magnetic transition in a single crystalline Ni2.13Mn0.81Ga1.06 Heusler compound was studied by a direct method. The obtained results revealed that, for the applied magnetic field strength μ0H = 1.9 T, MCE is irreversible in the vicinity of the first order martensitic transformation only when the MCE measurements are performed under cooling protocol. Plot of the experimentally measured adiabatic temperature change ΔTad as a function of temperature T indicated that ΔTad has a negligible benefit from the magnetic field-induced conversion of the high-temperature austenitic phase into the low-temperature martensitic phase and is mainly determined by the paraprocess of the austenitic phase around both direct and reverse martensitic transformations.

Deciphering M–T diagram of shape memory Heusler alloys: reentrance, plateau and beyond

We present our recent results on temperature behaviour of magnetization observed in Heusler alloys. Three regions can be distinguished in the M–T diagram: (I) low temperature martensitic phase (with the Curie temperature K), (II) intermediate mixed phase (with the critical temperature K) exhibiting a reentrant like behavior (between and ) and (III) high temperature austenitic phase (with the Curie temperature K) exhibiting a rather wide plateau region (between and ). By arguing that powerful structural transformations, causing drastic modifications of the domain structure in alloys, would also trigger strong fluctuations of the order parameters throughout the entire M–T diagram, we were able to successfully fit all the data by incorporating Gaussian fluctuations (both above and below the above three critical temperatures) into the Ginzburg–Landau scenario.

Study of Non-Homogeneity of Magnetic Field Distribution in Single-Crystal Ni-Mn-Ga Magnetic Shape Memory Element in Actuators due to its Anisotropic Twinned Microstructure

Magnetic shape memory (MSM) alloys are relatively new and very promising “smart” materials that respond to magnetic fields and exhibit the shape memory effect at room temperature. Maximum strain varies from 6% to 12% of the MSM element’s length depending on its microstructure. The shape memory effect and the magnetic field-induced reorientation of MSM twin variants in low-temperature martensite phase have been subject to ongoing research for almost two decades. However, the magnetic field distribution in the MSM elements and effects of its varying magnetic permeability on bias magnetic field are not well studied. In this paper, we present an extension to the existing modeling approach for MSM elements applicable to actuator design. The effects arising from single-crystal anisotropy and demagnetization effects due to non-homogeneous multi-variant MSM microstructure are studied and discussed. The proposed approach is validated by comparing computational results with the previously reported measurement data.

Effect of interfacial pinning on the thermo-remanent magnetization of disorder Mn50Ni38.5Sn11.5 alloy

In this report we show that field cooling through the irreversibility temperature (Tirr) only has more significant effect on the thermo-remanent magnetization (TRM) of martensite phase than the system cooled through its martensite transformation (MT). The large TRM observed at low temperature martensite phase is not associated with the arrest of higher magnetic austenite phase. The pinning of uncompensated spins at the interfaces of magnetically inhomogeneous phases is mainly contributing to the large irreversibility between the TRM curves. This strong pinning results to a huge exchange bias field (HEB). A giant entropy change (ΔStr) of 18.1J/kgK and a large negative magneto-resistance (MR) of 27% for field change (ΔH) of 50kOe are estimated across the MT. We have comprehensively discussed these observed behaviours in the paper.

Structural and Magnetic Properties of Ni-Mn-Al Heusler Alloys: A Review

We reviewed structural and magnetic properties of Ni-Mn-Al Heusler alloys which are a representative of ferromagnetic shape memory alloy system. Special attention was given to a variety of crystal structures of the low-temperature martensitic phase formed in this alloys and to the influence of disorder and Co alloying on magnetic properties of Ni-Mn-Al. Magnetic properties strongly depends on the chemical composition, crystal structure and heat treatment. In present review works concerning magnetic properties dependence on treatment are made out. A two way shape memory effect observed recently in Ni-Mn-Al melt spun ribbons is briefly described. The aim of the review is to sum up the information about Ni-Mn-Al family.

Structural and magnetic transformations in Ni51 − x Mn36 + x Sn13 alloys

The structural and magnetic phase transitions in Ni-Mn-Sn-based alloys have been studied. The temperature dependences of the martensitic and magnetic phase transformations in Ni51 − x Mn36 + x Sn13 alloys (0 ≤ x ≤ 4) have been determined in the case where manganese atoms substitute for nickel atoms. For the studied alloys, the concentration phase diagram has been constructed and the temperature regions of the existence of the high-temperature austenite phase L21 and the low-temperature martensite phase in different magnetic states have been determined.

Magnetocaloric effect, magnetostructural and magnetic phase transformations in Ni50.3Mn36.5Sn13.2 Heusler alloy ribbons

Thermomagnetic properties and magnetocaloric effect (MCE) in Ni50.3Mn36.5Sn13.2 Heusler alloy ribbons are reported. Large magnetocaloric response has been obtained for μ0H=3T in both the reversible adiabatic change in temperature ΔTad (−6.3 and +4.7K) and the isothermal change in the total entropy ΔST (+11.8 and −2.2JK−1kg−1), respectively at the temperature Tstr=271K at which the magnetostructural phase transition from the low-temperature Martensite phase (MP) to the high-temperature Austenite phase (AP) takes place, and at TCA=311K at which the magnetic phase transition in the AP occurs. The MCE has been studied through the Stoner’s magnetocaloric parameter ξS (Stoner EC. Phil Mag 19 (1935) 565), and a generalized expression for the equations of state for magnetic materials is presented. The phase coexistence of the MP and AP has been studied via their respective sublattice with opposite-aligned magnetic moments by means of the effective magnetic anisotropy density eA induced by the supercooled (superheated) secondary AP (MP) in the MP (AP). Magnetic fields higher than the critical value of μ0HR=0.5T produce alignment of the two magnetic moments and hence a change in the magnetic ordering in the MP from antiferromagnetic to ferromagnetic, resulting in a thermomagnetic behavior in which the field-cooling and field-heating magnetizations become coincident, the induced anisotropy vanishes eA=0 and the metastability disappears. New tentative methods to obtain the spontaneous magnetization of both MP and AP and their respective fractions in the phase coexistence range are reported.

A phase-field model of phase transitions and domain structures of NiCoMnIn metamagnetic alloys

We developed a computational model to investigate the magnetic and structural phase transitions in metamagnetic shape memory alloys. The model combined the phase-field method with micromagnetic simulations. The model was used to calculate the transition temperature from ferromagnetic austenite to antiferromagnetic martensite, the Curie temperature, and their response to an external magnetic field, for the typical metamagnetic alloy NiCoMnIn. The calculated magnetization curves at different temperatures are consistent with reported experimental measurements. The simulations show that the walls of martensite twins are superimposed with the 90° magnetic domain walls of the low-temperature martensite phase, because of magnetostructural order parameter coupling.

Characterization of microstructure and residual stress in a 3D H13 tool steel component produced by additive manufacturing

Abstract

Self-Accommodating Nature of Martensite Formation in Shape Memory Alloys

Shape memory effect is a peculiar property exhibited by certain alloy system. This behavior is facilitated by martensitic transformation, and shape memory properties are intimately related to the microstructures of alloys; in particular, the morphology and orientation relationship between the various martensite variants. Martensitic transformation occurs in thermal manner, on cooling the materials from high temperature parent phase region. Thermal induced martensite called self-accommodated martensite or multivariant martensite occurs as multivariant martensite in self-accommodating manner and consists of lattice twins. Shape memory alloys are deformed in low temperature martensitic phase condition, and deformation proceeds through a martensite variant reorientation. Copper based alloys exhibit this property in metastable β - phase region.

Extended investigation of intermartensitic transitions in Ni-Mn-Ga magnetic shape memory alloys: A detailed phase diagram determination

Martensitic transitions in shape memory Ni-Mn-Ga Heusler alloys take place between a high temperature austenite and a low temperature martensite phase. However, intermartensitic transformations have also been encountered that occur from one martensite phase to another. To examine intermartensitic transitions in magnetic shape memory alloys in detail, we carried out temperature dependent magnetization, resistivity, and x-ray diffraction measurements to investigate the intermartensitic transition in Ni50Mn50–xGax in the composition range 12≤x≤25 at. %. Rietveld refined x-ray diffraction results are found to be consistent with magnetization and resistivity data. Depending on composition, we observe that intermartensitic transitions occur in the sequences 7M→L10, 5M→7M, and 5M→7M→L10 with decreasing temperature. The L10 non-modulated structure is most stable at low temperature.

Neutron diffraction evidence for kinetic arrest of first order magneto-structural phase transitions in some functional magnetic materials

Neutron diffraction measurements, performed in the presence of an external magnetic field, have been used to show structural evidence for the kinetic arrest of the first order phase transition from (i) the high temperature austenite phase to the low temperature martensite phase in the magnetic shape memory alloy Ni37Co11Mn42.5Sn9.5, (ii) the higher temperature ferromagnetic phase to the lower temperature antiferromagnetic phase in the half-doped charge ordered compound La0.5Ca0.5MnO3 and (iii) the formation of glass-like arrested states in both compounds. The cooling and heating under unequal fields protocol has been used to establish phase coexistence of metastable and equilibrium states, and also to demonstrate the devitrification of the arrested metastable states in the neutron diffraction patterns. We also explore the field–temperature dependent kinetic arrest line TK(H), through the transformation of the arrested phase to the equilibrium phase. This transformation has been observed isothermally in reducing H, as also on warming in constant H. TK is seen to increase as H increases in both cases, consistent with the low-T equilibrium phase having lower magnetization.

Microstructure and magnetic properties of as-cast Ni2MnGa alloys processed by twin roller melt spinning

The magnetism and the microstructure of Ni2MnGa alloys, processed for the first time by twin roller melt spinning at tangential wheel speeds of 10m/s (V10), 15m/s (V15) and 20m/s (V20) are investigated. At room temperature, the major phase in the as-cast alloys is the cubic L21 Ni2MnGa ordered austenitic phase, with a lattice parameter only ∼0.1% larger than the tabulated value. The order domain size in the austenitic phase decreases from (40±1) nm in samples V10 to (19±1) nm in V20. Mn(S,Se) small precipitates are also found uniformly embedded in the ribbons with mean size of (26±2) nm (V10) and (7±2) nm (V20), exhibiting a definite orientation relation with the austenitic matrix, <100>P// <100>a. The as-cast alloys transform to an intermediate cubic phase I at about 220–230K depending on the quenching rate and to a martensitic phase at about 130K. The high temperature austenitic phase and the low temperature martensitic phase are ferromagnetic; in both cases the saturation polarization is lower in samples quenched at higher rates. The demagnetization curves measured from saturation in the martensitic state show two marked steps: a first one for positive fields in V15 and V10 (∼46mT) and a larger second one for relatively large inverse fields (∼130–250mT) in all the samples. These steps are likely to arise from a demagnetization mechanism involving a field induced twin boundary motion in the few martensite variants selected by the crystallographic texture and the quenched stresses in the ribbons.

Determination of the normal and anomalous hall effect coefficients in ferromagnetic Ni50Mn35In15 − x Si x Heusler alloys at the martensitic transformation

The magnetization, the electrical resistivity, the magnetoresistance, and the Hall resistivity of Ni50Mn35In15 − xSix (x = 1.0, 3.0, 4.0) Heusler alloys are studied at T = 80-320 K. The martensitic transformation in these alloys occurs at T = 220–280 K from the high-temperature ferromagnetic austenite phase into the low-temperature martensite phase having a substantially lower magnetization. A method is proposed to determine the normal and anomalous Hall effect coefficients in the presence of magnetoresistance and a possible magnetization dependence of these coefficients. The resistivity of the alloys increases jumpwise during the martensitic transformation, reaches 150–200 μΩ cm, and is almost temperature-independent. The normal Hall effect coefficient is negative, is higher than that of nickel by an order of magnitude at T = 80 K, decreases monotonically with increasing temperature, approaches zero in austenite, and does not undergo sharp changes in the vicinity of the martensitic transformation. At x = 3, a normal Hall effect nonlinear in magnetization is detected in the immediate vicinity of the martensitic transformation. The temperature dependences of the anomalous Hall effect coefficient in both martensite and austenite and, especially, in the vicinity of the martensitic transformation cannot be described in terms of the skew scattering, the side jump, and the Karplus-Lutinger mechanisms from the anomalous Hall effect theory. The possible causes of this behavior of the magnetotransport properties in Heusler alloys are discussed.

The study of structure, magnetism, electricity, and their correlations at martensitic transition for magnetostriction system Cu1−xMnxNMn3 (0 ≤ x ≤ 0.5)

The antiperovskite functional compound CuNMn3 exhibits magnetostriction below the non-collinear ferromagnetic (FM)-paramagnetic (PM) transition (accompanied by a martensitic transformation). After the substitution of Mn for Cu, martensitic transformation is decoupled from FM transition. Meanwhile, the tetragonality of the martensitic phase and the magnetostriction are weakened. The combined analysis indicates that the spin, charge, and lattice are closely coupled around martensitic transformation temperature (TMS). The low-temperature martensitic phase is associated with a metastable magnetic state characterized by small magnetocrystalline anisotropy. Additionally, the depression of TMS with introducing the Mn dopant is revelatory for the development of low-temperature negative thermal expansion material.

Thermomechanical training and characterization of Ni–Ti–Hf and Ni–Ti–Hf–Cu high temperature shape memory alloys

Nickel–titanium (NiTi) is the most commonly used shape memory alloy (SMA) for actuator applications, though its usefulness is limited to temperature ranges below 100 °C. High temperature SMAs are formed by adding ternary elements to NiTi, but their usefulness as actuators is still in question. The purpose of this research was to characterize and train two high temperature SMAs, NiTi29.7Hf20 and NiCu5Ti29.7Hf20, to determine their effectiveness as linear actuators. Low temperature martensitic phase and high temperature austenitic phase stress–strain tests were performed to characterize the materials’ behavior followed by temperature cycling under constant stress. Temperature cycling under constant stress is known as thermomechanical training and resulted in small amounts of plastic strain growth and the development of two-way shape memory (TWSM). The results from these tests support the conclusion that hafnium distorts slip planes within the martensitic material phase, and that (Ti,Hf)2Ni and (Ti,Hf)3Ni4 particulates form during aging and annealing. The distorted slip planes cause slip and martensite reorientation to occur simultaneously, which develops a strong internal stress field during training within the first few cycles. The internal stress field develops TWSM, but limits further plastic growth. The particulate formation also embrittles the material. The transformation temperatures of both alloys were below creep and annealing temperatures making them ideally suited for high temperature actuators.

Influence of buffer layer on structural, electrical and mechanical properties of PZT/NiTi thin film heterostructures

Ti and TiN layers of 8–10nm thickness were employed as a buffer layer to improve the structural, electrical and mechanical properties of Pb(Zr0.52Ti0.48)O3(PZT)/NiTi heterostructures synthesized using dc/rf magnetron sputtering. The buffer layers were deposited on NiTi prior to the growth of PZT films. It was observed that the incorporation of buffer layer favored the formation of single perovskite PZT phase with no evidence of pyrochlore phase. Further, the properties of these heterostructures were observed to change with corresponding change in crystalline orientation of top PZT layer. The PZT/NiTi heterostructures with additional buffer layer exhibited enhanced remnant polarization (Pr) and dielectric constant (ε). A sudden increment in the dielectric constant was observed for PZT/buffer/NiTi heterostructures near room temperature. This could be attributed to the increasing ionic displacement of Ti ion with respect to the O ion in PZT films due to the stress induced by the martensite variants of NiTi layer during phase transformation from a low temperature martensite phase to high temperature austenite phase. This transformation promotes the strain-induced polarization and henceforth enhancing the dielectric constant of PZT films. Nanoindentation studies indicate the maximum hardness of 7.6GPa and indentation modulus of 206GPa for PZT(111)/TiN/NiTi heterostructure.

Size effect on shape recovery and induced strain of NiTi nanowires

The nanomechanical properties, the mechanism of fracture, and the shape recovery of NiTi nanowires under the nanobending test by an atomic force microscope tip are studied using molecular dynamics simulations. The effects of the nanowire diameter and heat treatment are evaluated in terms of atomic trajectories, loading force, adhesion, stress, slip vector, centro-symmetry parameter, Young's modulus, and yield strength. Simulation results show that the occurrence of initial adhesion deformation depends on the nanowire diameter. The nanomechanical strength of a NiTi nanowire can be enhanced by decreasing its diameter. NiTi nanowires in the high-temperature austensite phase have better nanomechanical strength than those of nanowires in the low-temperature martensite phase. During deformation, dislocations glide along the {110} planes inside the nanowire. The shape recovery after loading on the (100) surface is faster than that on the (110) surface.

Effect of magnetic field on the morphology and fine structure of low-temperature martensite phase in a ferromagnetic Ni2.08Mn0.96Ga0.96 alloy

The morphology and fine structure of high-temperature austenitic and low-temperature martensitic phases in a ferromagnetic Ni2.08Mn0.96Ga0.96 alloy and the effect of magnetic field on the peculiarities of the martensite formation have been studied. The Ni2.08Mn0.96Ga0.96 alloy in the initial cast and annealed states was found to undergo martensitic and magnetic phase transformations at temperatures M s = 280 K, M f = 265 K, A s = 280 K, A f = 295 K, and T C = 375 K, respectively. Above M s, the alloy is in a metastable premartensitic state, which leads to a characteristic diffuse scattering and tweed contrast when studying by electron microscopy. An applied magnetic field of up to 600 kA/m was found to affect the martensitic transformation in the alloy. The field application leads to changes in the morphology and fine structure of martensite due to orientation along the field direction of the magnetizations of the energetically advantageous (in terms of the directions of the magnetization vectors) c domains of the existing orientation variants of the martensite crystals having a packet pairwise-twinned morphology. The martensitic structure of the Ni2.08Mn0.96Ga0.96 alloy already formed previously during cooling is not affected by an external magnetic field with a strength of up to 600 kA/m.

Basic Properties of Magnetic Shape-Memory Materials from First-Principles Calculations

The mutual influence of phase transformations, magnetism, and electronic properties of magnetic-shape memory Heusler materials is a basic issue of electronic structure calculations based on density functional theory. In this article, we show that these calculations can be pursued to finite temperatures, which allows to derive on a first-principles basis the temperature versus composition phase diagram of the pseudo-binary Ni-Mn-(Ga, In, Sn, Sb) system. The free energy calculations show that the phonon contribution stabilizes the body-centered-cubic (bcc)-like austenite structure at elevated temperatures, whereas magnetism favors the low-temperature martensite phase with body-centered-tetragonal (bct) or rather face-centered-tetragonal (fct) structure. The calculations also allow to make predictions of magnetostructural and magnetic field induced properties of other (new) magnetic Heusler alloys not based on NiMn such as Co-Ni-(Ga-Zn) and Fe-Co-Ni-(Ga-Zn) intermetallic compounds.