Original Articles

Chemical ordering in Ni-Mn-Ga Heusler alloys

Nickel-Manganese-Gallium-based Heusler alloys have been extensively studied for magnetic cooling applications with different compositions. However, the large hysteresis and narrow range of working temperatures associated with the first-order phase transition make limit their practical application. In this direction, the magnetocaloric effect was analyzed for three different off-stoichiometric compositions of Ni_50+y+zMn_25+x-yGa_25‐x‐z alloys, where x, y, and z vary once at a time. Ni₅₀Mn₂₇Ga₂₃ (NMG 1) and Ni₅₄Mn₂₁Ga₂₅ (NMG 2) samples show large magnetocaloric effects across the second-order ferromagnetic to paramagnetic phase transition. Another sample Ni₅₄Mn₂₅Ga₂₁ (NMG 3) undergoes coupled magneto-structural transition near room temperature with very low hysteresis loss (⁓4 J/kg) and a very high sensitivity of the martensite transition temperature (⁓3.6 K/T) with the field. By cyclic cooling protocol reliable magnetic entropy changes are found to be maxima of − 1.98 ± 0.09, − 1.90 ± 0.11 and − 3.53 ± 0.14 J/kg-K for 3 T field change with large effective RCP values of 235.8 ± 0.1, 161.5 ± 0.1 and 144.7 ± 0.3 J/kg for NMG 1, NMG 2 and NMG 3, respectively. The presence of inter-martensite transition along with the magnetic transition or magneto-structural phase transition widens the range of working temperature which effectively enhances the magnetic cooling performance of these materials. The T_C and T_M are tuned in Ni-Ga-Mn-based alloys by changing the chemical composition with considering the electron concentrations.

Actuators and sensors are core components in the modern devices and techniques, such as internet of things (IoT) and robots. Ferromagnetic shape memory alloys (FSMAs), particularly prototype Ni-Mn-Ga Heusler compounds, exhibiting giant periodic strains in response to the alternating magnetic field, are promising materials for such components. In the present work, a Bi-doping effect on the grain structure of the Ni-Mn-Ga polycrystalline alloy and properties of the grain particles obtained by a mechanical crushing using a well-controlled technique were studied in detail. Phase constituents, surface morphologies, chemical composition analysis, grain size distributions, thermal behaviors, and interfacial properties of the size-controlled Ni-Mn-Ga-xBi (x = 0.00–0.05 at.%) single crystalline (SC) particles have been investigated. Furthermore, a 20 vol.% of the optimized Ni-Mn-Ga-0.03Bi SC particles were regularly distributed within a silicone polymer. The resultant composite was elastically compressed up to 50% producing a large and recoverable deformation of the embedded individual particles, that was in-situ measured by an X-ray microcomputed tomography (μCT) 3-dimensional (3D) imaging. The observed variations in the strain behavior of particles were attributed to the difference in their martensitic phase states and their concentration in composite. The results are important for a design of Ni-Mn-Ga/polymer composites for vibration damping, magnetostrain actuators, and haptic applications.

The effect of atomic order on the martensitic and magnetic transformations undergone by Ni₂Mn_1−xCu_xGa 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 Ni₂Mn_1−xCu_xGa 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 Ni₂Mn_1−xCu_xGa alloys. The former, responsible for the change in Curie temperature, is attributed to the improvement of L2₁ 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.

Magnetic shape memory (MSM) alloys have a high potential as an emerging class of actuator materials for a new generation of fast and simple digital components. In this study, the MSM alloy Ni_50.5Mn_27.5Ga_22.0 was built via laser powder bed fusion (L-PBF) using gas atomized powder doped with excess Mn to compensate for the expected evaporation of Mn during L-PBF. The built samples were subjected to stepwise chemical homogenization and atomic ordering heat treatments. The experiments followed a systematic experimental design, using temperature and the duration of the homogenization treatment as the varied parameters. Overall, the produced samples showed only a minor variation in relative density (average density ~98.4%) and chemical composition from sample to sample. The as-built material showed broad austenite-martensite transformation and low saturation magnetization. The crystal structure of the as-built material at ambient temperature was a mixture of seven-layered modulated orthorhombic (14 M) and five-layered modulated tetragonal (10 M) martensites. Notably, ordering heat treatment at 800 °C for 4 h without homogenization at a higher temperature was enough to obtain narrow austenite-14 M martensite transformation, Curie temperature, and saturation magnetization typical for bulk samples of the same composition. Additionally, homogenization at 1080 °C stabilized the single-phase 14 M martensite structure at ambient temperature and resulted in considerable grain growth for homogenization times above 12 h. The results show that post-process heat treatment can considerably improve the magneto-structural properties of Ni-Mn-Ga built via L-PBF.

A series of single crystal Ni₂MnGa_1-xIn_x samples, varying in compositions between x = 0 to 0.15, were grown by Bridgman method. Critical temperatures were obtained from the magnetisation and electrical resistivity measurements. A Curie temperature and the temperatures of pre-martensitic and martensitic transformation decrease with increasing indium content. The decrease rate is faster than the rate published previously on polycrystalline samples, since no martensitic transformation was observed in our samples with the indium content x > 0.05. The results of X-ray diffraction measurements of low temperature martensitic phase show significant decrease of monoclinic lattice parameter a and increase of monoclinic lattice parameter c and γ angle with respect to increasing indium content. Composition dependence was observed also for the modulation vector in martensitic and pre-martensitic phase, where its x and y coordinates gain values around 3/7 and 1/3 respectively.

The interplay between antiphase boundaries (APBs), magnetic domain structure and functional properties was investigated in the martensitic state of Ni–Mn–Ga single crystals which showed magnetically induced martensite reorientation (MIR) with 6% strain. The APB density was controlled by different heat treatments. The APBs and magnetic domains were observed by Lorentz transmission electron microscopy (LTEM). Slow cooling at ~1 K/min resulted in a low density (<1/µm), air quenching in a medium density (≈8/µm), and water quenching in a high density (≈15/µm) of APBs. Abundant pinning of domain walls on APBs was observed, which resulted in close correspondence between the magnetic domain walls and antiphase boundaries (APBs), magnetic domain memory, and finer domain patterns for the high APB density. For low APB density, a labyrinth domain structure was established between domain walls pinned on APBs. For low and medium density of APBs the magnetization was oriented mostly parallel to the out-of-plane easy magnetization axis. For high APB density the magnetization switched to the in-plane orientation, indicating that the effective magnetic anisotropy decreased and became lower than the stray field energy. In addition, magnetic vortices appeared. The novel functionalities based on a combination of MIR and interactions of the magnetic structure with APBs are feasible since the MIR was observed even for the highest density of APBs.