Samples prepared using various additive manufacturing methods were compared in terms of structure, texture, transformation temperature and superelastic properties. Samples manufactured using laser engineered net shaping (LENS) method showed texture several degrees deviated from the <001> build direction, however with composition near to the initial powder composition, enabling superelastic effect. The electron beam additive manufacturing (EBAM) samples showed martensitic structure at room temperature due to a shift of transformation temperatures to the higher range. This shift occurs due to a lower Ni content resulting from different processing conditions. However, EBAM method produced sharper <001> texture in the build direction and made it possible to obtain a good superelastic effect above room temperature. Intermetallic particles of size 0.5-2 mm were identified as Ti2Ni phase using EDS and electron diffraction analyses. This phase was often formed at the grain boundaries. Contrary to the LENS method, the EBAM prepared samples showed Ni-rich primary particles resulted from different processing conditions that reduce the Ni content in the solid solution thus increase the martensitic transformation temperature. Ageing at 500 degrees C allowed for shifting the martensitic transformation temperatures to the higher range in both, LENS and EBAM, samples. It resulted from the formation of Ni rich coherent precipitates. In samples prepared by both methods and aged at 500 degrees C, the presence of martensite B19' twins was observed mainly on {011} B19' planes.

Ni51.5Fe21.5Ga27.0 single crystals have been subjected to different heat treatments resulting in a different degree of L21 ordering. Superelastic response has been measured at different temperatures in compression mode. The mechanical behavior strongly depends on axis orientation. In the [001] direction, perfect superelasticity over a wide range of temperatures is found. For the [110] orientation, the material fails by brittle fracture short above austenite transformation finish temperature, Af. A linear dependence of the critical stress with temperature has been found in agreement with Clausius-Clapeyron equation. The slope does not significantly change with the degree of order, but it is notably affected by the crystal orientation. The microstructure of the samples after mechanical tests has been studied by transmission electron microscopy. The superelastic cycling produces dislocations with a Burgers vector that suggests local microplastic deformation of the martensitic phase. Finally, the adiabatic temperature change has been used to chacterize the elastocaloric effect in this alloy. The adiabatic cooling is found to be larger in the [110] than in the [001] orientation at 240 K. However, the brittleness of [110] samples avoid testing the adiabatic temperature change at room temperature. The adiabatic cooling in [001] orientation decreases systematically with temperature, which is related to decrease of the strain and entropy change of transformation.

The effect of atomic order on the martensitic and magnetic transformations undergone by Ni2Mn1-xCuxGa ferromagnetic shape memory alloys has been investigated. Different degrees of order have been induced by performing selected thermal treatments, and it has been found that, although both the structural (martensitic) and magnetic transformation temperatures increase with the improvement of the atomic order, each of these temperatures evolve with different kinetics. Reinforcing this result, during post-quench heating of Ni2Mn1-xCuxGa alloys two consecutive DSC exothermic peaks are observed, while the ternary Ni-Mn-Ga, Co-doped Ni-Mn-Ga and even Cu-doped Ni-Mn-Ga alloy when Cu replaces Ni show a single peak. The kinetics of the post-quench ordering processes have been studied using the Kissinger's method, and two different activation energies have been obtained: 1.16 eV for the process that is common to all alloys and 1.35 eV for the one that is only observed in Ni2Mn1-xCuxGa alloys. The former, responsible for the change in Curie temperature, is attributed to the improvement of L2(1) order mainly due to Mn diffusion; for the second, which underlies the rise of martensitic transformation temperatures, diffusion of Cu atoms, misplaced in the Ni sublattice after quench, towards their most favourable sites in the Mn sublattice is proposed as the responsible mechanism. (C) 2021 The Author(s). Published by Elsevier B.V. CC_BY_NC_ND_4.0

The production of μ-particles of Metamagnetic Shape Memory Alloys by crushing and subsequent ball milling process has been analyzed. The high energy involved in the milling process induces large internal stresses and high density of defects with a strong influence on the martensitic transformation; the interphase creation and its movement during the martensitic transformation produces frictional contributions to the entropy change (exothermic process) both during forward and reverse transformation. The frictional contribution increases with the milling time as a consequence of the interaction between defects and interphases. The influence of the frictional terms on the magnetocaloric effect has been evidenced. Besides, the presence of antiphase boundaries linked to superdislocations helps to understand the spin-glass behavior at low temperatures in martensite. Finally, the particles in the deformed state were introduced in a photosensitive polymer. The mechanical damping associated to the Martensitic Transformation (MT) of the particles is clearly distinguished in the produced composite, which could be interesting for the development of magnetically-tunable mechanical dampers.

The two-way shape memory effect (TWSME) induced in Ni49Fe18Ga27Co6 (at%) ferromagnetic shape memory single crystals is investigated in the present work. A short superelastic cycling in compression along the [001] direction at increasing temperatures up to 453 K, with a total of 18 cycles is enough to induce fully reversible TWSME with spontaneous strain up to 5.4% and maximum work output of 27 J.kg(-1) performed upon cooling through the martensitic transformation under antagonistic tensile stress. A very good correlation between martensite reorientation tests and thermal transformation under antagonistic stress has been obtained for TWSME trained samples.

We provide in-depth physical insight into the enhancement of the magnetic properties of metamagnetic shape memory alloys produced by thermal treatment and cobalt doping. We use neutron scattering to study the atomic order and magnetic structures in the austenitic phases of Ni50Mn34In16 and Ni45Co5Mn37In13 alloys in two different states induced by thermal treatments. The increase of the magnetization in the austenite phase, particularly by cobalt doping, is explained by the enhanced ferromagnetic coupling between the magnetic moments located in octahedral sites. The spin density maps obtained from polarized neutron diffraction reveal the magnetic interaction pathways responsible for this coupling scheme.

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

The relevance of the entropy and in particular the outstanding role of the magnetic contribution is analyzed in a non-equilibrium arrested austenite phase in a Ni45Mn36.7In13.3Co5 metamagnetic shape memory alloy. The Debye and Bragg-Williams approximations have been used to analyze and quantify the vibrational and magnetic contributions respectively, to the total entropy change linked to the martensitic transformation. The sign on the entropy change associated to the forward austenite to martensite transformation is different depending on whether it occurs at low or at high temperature as a consequence of the different vibrational and magnetic contributions.

Positron annihilation lifetime spectroscopy is used to experimentally demonstrate the direct relationship between vacancies and the shift of the martensitic transformation temperature in a Ni_{55}Fe_{17}Ga_{28} alloy. The evolution of vacancies assisting the ordering enables shifts of the martensitic transformation up to 50 K. Our results confirm the role that both vacancy concentration and different vacancy dynamics play in samples quenched from the L2_{1} and B2 phases, which dictate the martensitic transformation temperature and its subsequent evolution. Finally, by electron-positron density functional calculations V_{Ni} is identified as the most probable vacancy present in Ni_{55}Fe_{17}Ga_{28}. This work evidences the capability of vacancies for the fine-tuning of the martensitic transformation temperature, paving the way for defect engineering of multifunctional properties.

In this work, vacuum hot pressed Ni-Mn-Sn-In Heusler alloys with different concentration of In (0, 2 and 4 at.%), were investigated. The magneto-structural behaviour and microstructure dependencies on chemical composition and on heat treatment were examined. It was found that the martensite start transformation temperature increases with growing In content and to a lesser extent with increasing temperature of heat treatment. The high energy X-ray synchrotron radiation results, demonstrated that both chemical composition as well as temperature of heat treatment slightly modified the crystal structures of the studied alloys. Microstructural investigation performed by transmission electron microscopy confirmed chemical composition and crystal structure changes in the alloys.

Addition of Cu instead of Ga in stoichiometric Ni2MnGa have strong influence on crystal structure. At ambient temperature, four types of crystal structures were detected: L21 austenite and 5-layered modulated, 7-layered modulated and non-modulated martensite. The substitution of Ga for Cu in the Ni50Mn25Ga25-xCux system results in an increase of martensitic transformation temperature (TM) which in turn is proportional to the e/a ratio. Moreover, the quenching process has also an influence on the martensitic transformation temperature and crystal structure shifting TM to lower temperatures and stabilizing the L21 austenite and 7-layered modulated martensite phases. Electron microscopy observations revealed that the Cu addition significantly affects the martensite microstructure. At higher Cu concentration, i.e. 9 and 10 at.%, a so-called γ phase precipitates of face-centred cubic crystal structure forms being substantially enriched with Cu. This effect in turn decreases the e/a ratio of the martensite phase (as a matrix of the dual-phase microstructure) and consequently decreases the TM.

A single-crystalline specimen with the composition of Ni 49.5 Mn 38.4 Sn 12.2 shows a 4.9% recoverable transformation strain upon compressive loading. The critical compressive stress increases with temperature at the step of 5.6 MPa/K, whereas upon cycling it decreases by 18.1 MPa/cycle. The microstructure of the specimen undergoes considerable refinement upon superplastic training; however, it is only able to sustain a limited number of cycles (≤ 5). Martensite training, resulting in a single-variant microstructure, has a profound influence on the austenite start transformation temperature (Δ T = 29 K), resulting partially from the dissipation of the elastic strain energy. The Ni-Mn-Sn system is an interesting candidate for multiferroic applications given its mechano-magnetic properties and a huge value of the martensitic transformation entropy change (~ 50 J/kg K).

The temperature dependence of the macroscopic de-twining stress in Ni–Fe–Ga–Co shape memory alloy is proportional to the temperature dependence of the micro-yield stress. This proportionality holds for all values of the reversible anelastic strain derived from non-linear internal friction experiments and contradicts predictions of the Seeger model for the temperature dependence of the yield stress. The Seeger model requires two length scales which leads to the independence of two yield stress components. Instead, we find the self-affine behaviour of anelastic strain, which excludes all separations of lengthscales.

Metamagnetic shape memory alloys have aroused considerable attraction as potential magnetic refrigerants due to the large inverse magnetocaloric effect associated to the magnetic-field-induction of a reverse martensitic transformation (martensite to austenite). In some of these alloys, the austenite phase can be retained on cooling under high magnetic fields, being the retained phase metastable after field removing. Here, we report a giant direct magnetocaloric effect linked to the anomalous forward martensitic transformation (austenite to martensite) that the retained austenite undergoes on heating. Under moderate fields of 10 kOe, an estimated adiabatic temperature change of 9 K has been obtained, which is (in absolute value) almost twice that obtained in the conventional transformation under higher applied fields. The observation of a different sign on the temperature change associated to the same austenite to martensite transformation depending on whether it occurs on heating (retained) or on cooling is attributed to the predominance of the magnetic or the vibrational entropy terms, respectively.

An influence of the Cu doping on structural, magnetic and thermoelastic properties of the Heusler Ni44-xCuxCo6Mn39Sn11 (x = 1-4 at%) ribbons has been investigated. It is found that the addition of Cu stabilizes austenite phase. Martensite transformation (MT) temperatures generally decrease when Cu concentration increases, which is attributed to the atom size effect. The inverse magnetocaloric and elastocaloric effects in the vicinity of MT under moderate magnetic fields and stresses have been evaluated. Small Cu addition enhances both effects as compared to quaternary Ni-Co-Mn-Sn alloy. The magnetic entropy change under low magnetic field of 15 kOe increases from 2.9 J kg(-11)K(-1) for CuO to 6.3 J kg(-1)K(-1) for Cu2. (C) 2017 Elsevier B.V. All rights reserved.

Structural, magnetic and mechanical properties of Ni50Mn37.5Sn12.5-xInx ( x = 0, 2, 4, 6% at.) alloys sintered by vacuum hot pressing were investigated. The porosity of sintered alloys increases with increasing of In concentration. The addition of In leads T-C(A) to decrease while M-s is shifted to higher temperatures. Annealing sintered samples at 1020 K for 24 h allowed for lowering of the thermal hysteresis and narrows the temperature range of the forward and reverse martensitic transformations. Ternary Ni-Mn-Sn alloy exhibits the best mechanical properties and it can be plastically deformed up to about 5% while maintaining about 2000 MPa strength. TEM observations revealed unique microstructure observed in the case of polycrystalline Heusler alloys consisting of equiaxed grains of martensite and austenite with size of about 2 mu m. (C) 2017 Elsevier B.V. All rights reserved.

Ni49.4Mn38.5Sn12.1 near single crystal was obtained by the Bridgman method. At room temperature, it consisted of a mixture of the parent austenite phase with the cubic L21 Heusler structure (ac = 5.984 Å) and modulated, tetragonal martensite phase 4M (at = 4.337 Å, ct = 5.655 Å). Under the application of a magnetic field, the specimen undergoes field induced reverse martensitic transformation, which combined with the Curie transition in austenite leads to the coexistence of direct and inverse magnetocaloric effects. The maximum entropy change at 280 K and under 5 T amounts to 3.4 J·kg−1·K−1 for the structural transition and at 316 K reaches −2.7 J·kg−1·K−1 for the magnetic transformation. The magnetic entropy change occurs over a wide temperature span leading to improved refrigerant capacity of 101 J·kg−1 (5 T). Hysteretic losses are considerably reduced, which is promising with respect to improved cyclic stability of such a material.

The martensitic transformation entropy changes, ΔStr, measured from the stress-induced transformation in Ni–Co–Mn–In crystals with different degrees of long range L21 atomic order, follow a single temperature dependence and show good agreement with the theoretical Bragg–Williams approximation. The strong influence of the atomic order on ΔStr is attributed to the large shift in the transformation temperatures caused by the change in atomic order and the natural temperature dependence of the austenite magnetization. The effects of associated changes in magnetic order have a negligible contribution to ΔStr. Additional mechanical measurements performed under 1.6 T do not show any notable changes in ΔStr.

Transformation and multifunctional properties of a polycrystalline Ni42Co8Mn39Sn11 metamagnetic shape memory alloy have been investigated by extensive measurements of thermomagnetization, thermal expansion and thermoresistance under magnetic field. The martensitic transformation (MT) at zero field is near 300 K but changes moderately with field. Magnetization loops at room temperature show metamagnetic behavior and complete field-induced MT below 12 T. A non-linear phase diagram 'transformation temperatures versus field' has been established. The correlation between the transformation entropy, Delta S, with magnetic field, and temperature spat between MT and Curie temperature has been also established. A transformation volume effect of about 0.45% has been estimated at MT. It has been experimentally proved that the metamagnetic effect is at the origin of the giant effects of the studied alloy, such as volume magnetostriction and magnetoresistance, which have practical importance. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.