Neutron Diffraction Study of the Martensitic Transformation of Ni2.07Mn0.93Ga Heusler Alloy

The martensitic transition featuring the ternary Heusler alloy Ni2.09Mn0.91Ga was investigated by neutron diffraction. Differential scanning calorimetry indicated that structural transition starts at 230 K on cooling with a significant increase in the martensitic transformation onset compared to the classical Ni2MnGa. The low-temperature martensite presents the 5M type of modulated structure, and the structural analysis was performed by the application of the superspace approach. As already observed in Mn-rich modulated martensites, the periodical distortion corresponds to an incommensurate wave-like shift of the atomic layers. The symmetry of the modulated martensite at 220 K is orthorhombic with unit cell constants a = 4.2172(3) Å, b = 5.5482(2) Å, and c = 4.1899(2) Å; space group Immm(00γ)s00; and modulation vector q = γc* = 0.4226(5)c*. Considering the different neutron scattering lengths of the elements involved in this alloy, it was possible to ascertain that the chemical composition was Ni2.07Mn0.93Ga, close to the nominal formula. In order to characterize the martensitic transformation upon increasing the temperature, a series of neutron diffraction patterns was collected at different temperatures. The structural analysis indicated that the progressive change of the martensitic lattice is characterized by the exponential change of the c/a parameter approaching the limit value c/a = 1 of the cubic austenite.

Direct and Indirect Determination of the Magnetocaloric Effect in the Heusler Compound Ni1.7Pt0.3MnGa

Magnetic shape-memory materials are potential magnetic refrigerants, due the caloric properties of their magnetic-field-induced martensitic transformation. The first-order nature of the martensitic transition may be the origin of hysteresis effects that can hinder practical applications. Moreover, the presence of latent heat in these transitions requires direct methods to measure the entropy and to correctly analyze the magnetocaloric effect. Here, we investigated the magnetocaloric effect in the Heusler material Ni1.7Pt0.3MnGa by combining an indirect approach to determine the entropy change from isofield magnetization curves and direct heat-flow measurements using a Peltier calorimeter. Our results demonstrate that the magnetic entropy change ΔS in the vicinity of the first-order martensitic phase transition depends on the measuring method and is directly connected with the temperature and field history of the experimental processes.

Monoclinic–orthorhombic first-order phase transition in K2ZnSi5O12 leucite analogue; transition mechanism and spontaneous strain analysis.

Hydrothermally synthesised K2ZnSi5O12 has a polymerised framework structure with the same topology as leucite (KAlSi2O6, tetragonal I41/a), which has two tetrahedrally coordinated Al3+ cations replaced by Zn2+ and Si4+. At 293 K it has a cation-ordered framework P21/c monoclinic structure with lattice parameters a = 13.1773(2) Å, b = 13.6106(2) Å, c = 13.0248(2) Å and β = 91.6981(9)°. This structure is isostructural with K2MgSi5O12, the first cation-ordered leucite analogue characterised. With increasing temperature, the P21/c structure transforms reversibly to cation-ordered framework orthorhombic Pbca. This transition takes place over the temperature range 848−863 K where both phases coexist; there is an ~1.2% increase in unit cell volume between 843 K (P21/c) and 868 K (Pbca), characteristic of a first-order, displacive, ferroelastic phase transition. Spontaneous strain analysis defines the symmetry- and non-symmetry related changes and shows that the mechanism is weakly first order; the two-phase region is consistent with the mechanism being a strain-related martensitic transition.

Microstructural and Thermo-Mechanical Characterization of Cast NiTiCu20 Shape Memory Alloy

Among NiTi-based alloys, one of the most promising and exploited alloys is NiTiCu, since the addition of Cu in substitution of Ni in the binary equiatomic NiTi has a significant influence on the martensitic transformation and the thermomechanical properties of the system. A high content of Cu improves the damping properties at the expense of phase homogeneity and workability. The present study focuses on an alloy with a high copper content, i.e., 20 at.%. For this specific composition, the correlation between the thermal treatments, microstructure, formation of secondary phases, and damping properties are investigated by several analyses. The microscopic observation, together with the compositional analysis, allowed the determination of four different phases in the alloy. Both the calorimetry and dynamic thermo mechanical measurements, which confirmed the high damping ability of the alloy, provided a characterization of the martensitic transition. Finally, the electron backscatter diffraction (EBSD) analysis detected the different crystallographic structures (i.e., cubic austenite, orthorhombic martensite, and cubic (face-centered) NiTi2) and their orientation in the different phases. Therefore, the present work aims to improve the knowledge of the role of secondary phases in the optimization of the NiTiCu20 alloy as a valuable alternative to typical alloys used for damping purposes.

Impact of the Co Doping on the Structure, Magnetic Properties, and Phase Transition in Ni50-xCoxMn38Sn12 Ribbons

Three Heusler alloys, Ni50-xCoxMn38Sn12 (x = 1, 2, and 3), were elaborated by rapid solidification. The impact of the Co doping on the structure, magnetic properties, and phase transition in these alloys was studied. The structure of the Ni49Co1Mn38Sn12 and Ni48Co2Mn38Sn12 ribbons was martensite 14M monoclinic structure, while the Ni47Co3Mn38Sn12 sample structure was austenite cubic L21. The thermal analysis showed the impact of the substitution Ni by Co. It was noted that the temperatures of martensitic transition moved lower, and a decreases progressively of enthalpy and entropy changed. Likewise, an obvious increase in the temperature of Curie transition for austenite phase (TAC) was observed and a jump of magnetization change (ΔM) was detected, with increasing Cobalt content.

Kinetic Analysis of the Transformation from 14M Martensite to L21 Austenite in Ni-Fe-Ga Melt Spun Ribbons

In this study, the non-isothermal kinetics of the martensitic transition from 14M modulated martensite to austenite phase in Ni55Fe19Ga26 ribbons obtained by melt-spinning has been analyzed. The proximity of the martensitic transition to room temperature makes it very sensitive to pressure and subtle differences for different pieces of the ribbon (ascribed to stresses stored in the ribbon during its rapid solidification process). Despite the dispersion in the characteristic parameters of the transition, a general behavior is observed with a decreasing activation energy as the heating rate increases due to the nucleation driven character of the transition. It has been shown that a first-order autocatalysis can describe the temperature evolution of the austenite fraction using only two experimental temperatures. Predicted curves are in good agreement with experimental data.

Full Variation of Site Substitution in Ni-Mn-Ga by Ferromagnetic Transition Metals

Systematic doping by transition elements Fe, Co and Ni on each site of Ni2MnGa alloy reveal that in bulk material the increase in martensitic transformation temperature is usually accompanied by the decrease in ferromagnetic Curie temperature, and vice versa. The highest martensitic transformation temperature (571 K) was found for Ni50.0Mn25.4(Ga20.3Ni4.3) with the result of a reduction in Curie temperature by 55 K. The highest Curie point (444 K) was found in alloy (Ni44.9Co5.1)Mn25.1Ga24.9; however, the transition temperature was reduced to 77 K. The dependence of transition temperature is better scaled with the Ne/a parameter (number of non-bonding electrons per atom) compared to usual e/a (valence electrons per atom). Ne/a dependence predicts a disappearance of martensitic transformation in (Ni45.3Fe5.3)Mn23.8Ga25.6, in agreement with our experiment. Although Curie temperature usually slightly decreases while the martensitic transition increases, there is no significant correlation of Curie temperature with e/a or Ne/a parameters. The doping effect of the same element is different for each compositional site. The cascade substitution is discussed and related to the experimental data.

Microstructure and Magnetic Field-Induced Strain of a Ni-Mn-Ga-Co-Gd High-Entropy Alloy

The effect of a high-entropy design on martensitic transformation and magnetic field-induced strain has been investigated in the present study for Ni-Mn-Ga-Co-Gd ferromagnetic shape-memory alloys. The purpose was to increase the martensitic transition temperature, as well as the magnetic field-induced strain, of these materials. The results show that there is a co-existence of β, γ, and martensite phases in the microstructure of the alloy samples. Additionally, the martensitic transformation temperature shows a markedly increasing trend for these high-entropy samples, with the largest value being approximately 500 °C. The morphology of the martensite exhibits typical twin characteristics of type L10. Moreover, the magnetic field-induced strain shows an increasing trend, which is caused by the driving force of the twin martensite re-arrangement strengthening.