Non-modulated Tetragonal Structure Research Articles

Deformation induced martensite stabilization in Ni45Mn36.7In13.3Co5 microparticles

A Ni45Mn36.7In13.3Co5 alloy has been deformed by hand crushing and ball milling, both in martensite and in austenite, to analyze the effect of the induced plastic deformation on the relative stability between structural phases. A huge martensite stabilization, with shifts on the temperature of the first reverse martensitic transformation of up to 100 K, is observed not only in the deformed martensite but also in the grinding-induced martensite. It is also found that, regardless of the degree of plastic deformation, the stabilized martensite displays a non-modulated tetragonal structure, which accommodates the generated internal stresses better than the thermal induced modulated monoclinic martensite. In all cases, the destabilization process is shown to finish around the Curie temperature of the austenite, which actually represents a high temperature limit for the mechanically-stabilized martensite. This phenomenon is analyzed and explained in terms of the effect of magnetic entropy contribution to the driving force for reverse MT.

Cooling field dependent exchange bias in Mn2Ni1.4Ga0.6: A reentrant spin glass system with short range ferromagnetic ordering

The intermetallic alloy, Mn2Ni1.4Ga0.6, has been investigated by x-ray diffraction, dc magnetization, and ac susceptibility measurements. The material exhibits a non-modulated tetragonal structure, and undergoes a ferromagnetic transition near room temperature. Ac susceptibility and dc magnetization measurements show that below the Curie temperature, short range ferromagnetic ordering and reentrant spin glass state coexist in the material. A cooling field dependent exchange bias effect, with a maximum exchange bias field of 730 Oe, for a cooling field of 1 kOe, has also been observed in the material. The experimental observations suggest that the exchange bias effect in Mn2Ni1.4Ga0.6 is a result of the interaction between the coexisting spin glass and short range ferromagnetic ordering in the material.

Effect of platinum substitution on the structural and magnetic properties ofNi2MnGaferromagnetic shape memory alloy

${\mathrm{Ni}}_{2}\mathrm{MnGa}$ exhibits ideal ferromagnetic shape memory properties, however, brittleness and a low-temperature martensite transition hinder its technological applications motivating the search for novel materials showing better mechanical properties as well as higher transition temperatures. In this work, the crystal structure, phase transitions, and the magnetic properties of quaternary ${\mathrm{Ni}}_{2\ensuremath{-}x}{\mathrm{Pt}}_{x}\mathrm{MnGa}(0\ensuremath{\le}x\ensuremath{\le}1)$ shape memory alloys were studied experimentally by x-ray diffraction, magnetization measurements, and neutron diffraction and compared to ab initio calculations. Compositions within $0\ensuremath{\le}x\ensuremath{\le}0.25$ exhibit the cubic austenite phase at room temperature. The $x\ensuremath{\approx}0.3$ composition exhibits a seven-layer modulated monoclinic martensite structure. Within $0.4\ensuremath{\le}x\ensuremath{\le}1$, the system stabilizes in the nonmodulated tetragonal structure. The martensite transition has very narrow thermal hysteresis $0\ensuremath{\le}x\ensuremath{\le}0.3$, which is a typical characteristic of a shape memory alloy. By increasing $x$, the temperature of the martensite transition increases, while that of the magnetic transition decreases. The $x=1$ composition ($\text{NiPtMnGa}$) in the martensite phase undergoes a para-to-ferrimagnetic transition. The saturation magnetization exhibits a nontrivial behavior with increasing up to $x\ensuremath{\approx}0.25$, above which, it suddenly decreases. Powder neutron diffraction reveals the presence of antisite disorder, with about 17% of the original Ga sites being occupied by Mn. Computations suggest that the antisite disorder triggers an antiferromagnetic coupling between two Mn atoms in different crystallographic positions, resulting into a sudden drop of the saturation magnetization for higher $x$.

Magnetoelastic Anomalies Exhibited by Ni–Fe(Co)–Ga Polycrystalline Ferromagnetic Shape Memory Alloy

The martensitic transformation (MT) as well as the structure, magnetic and magnetostrain properties of a polycrystalline Ni50.1Fe18.6Ga27.2Co4.1 (at%) ferromagnetic shape memory alloy are investigated. The studied alloy exhibits a martensitic ferromagnetic phase near room temperature that shows a non-modulated tetragonal structure with a tetragonality ratio of c/a > 1. Strain measurements reveal a complex spontaneous change at MT amounting to about 0.15% that increases up to 0.22% at 5T of the applied magnetic field indicating orientation influence of the field on the martensitic variants growth. Magnetostrain measurements enable evaluation of the magnetic fieldinduced strain effect in the vicinity of MT. The sample shows moderate deformation achieving values of 70ppm related to the magnetization rotation and volume magnetostriction that does not saturate at applied magnetic field of 5T. [doi:10.2320/matertrans.M2013154]

Origin of magnetic interactions and their influence on the structural properties of Ni2MnGa and related compounds

In this work, we perform first-principles DFT calculations to investigate the interplay between magnetic and structural properties in Ni2MnGa. We demonstrate that the relative stability of austenite (cubic) and non-modulated martensite (tetragonal) phases depends critically on the magnetic interactions between Mn atoms. While standard approximate DFT functionals stabilize the latter phase, a more accurate treatment of electronic localization and magnetism, obtained with DFT+U, suppresses the non-modulated tetragonal structure for the stoichiometric compound, in better agreement with experiments. We show that the Anderson impurity model, with Mn atoms treated as magnetic impurities, can explain this observation and that the fine balance between super-exchange RKKY type interactions mediated by Ni d and Ga p orbitals determines the equilibrium structure of the crystal. The Anderson model is also demonstrated to capture the effect of the number of valence electrons per unit cell on the structural properties, often used as an empirical parameter to tune the behavior of Ni2MnGa based alloys. Finally, we show that off-stoichiometric compositions with excess Mn promote transitions to a non-modulated tetragonal structure, in agreement with experiments.

Magnetic structure of the modulated martensite phase in Ni–Mn–Ga ferromagnetic shape memory alloys

The magnetic structure of the seven layered (7 M) modulated martensite phases in Mn-rich Ni–Mn–Ga alloys was studied using Mössbauer spectroscopy. The Mössbauer results clearly demonstrate that in contrast to the non-modulated tetragonal structure two new magnetic sublattices exists for the 7 M orthorhombic martensite phase. Based on the unit cell symmetry and atomic coordination, the additional magnetic sublattices have been assigned to the Ni site. The variation in the magnetic properties of the martensite phases has been related to the underlying magnetic structure.

Designing shape-memory Heusler alloys from first-principles

The phase diagrams of magnetic shape-memory Heusler alloys, in particular, ternary Ni-Mn-Z and quarternary (Pt, Ni)-Mn-Z alloys with Z = Ga, Sn, have been addressed by density functional theory and Monte Carlo simulations. Finite temperature free energy calculations show that the phonon contribution stabilizes the high-temperature austenite structure while at low temperatures magnetism and the band Jahn-Teller effect favor the modulated monoclinic 14M or the nonmodulated tetragonal structure. The substitution of Ni by Pt leads to a series of magnetic shape-memory alloys with very similar properties to Ni-Mn-Ga but with a maximal eigenstrain of 14%.

Twin relationships of 5M modulated martensite in Ni–Mn–Ga alloy

For Ni–Mn–Ga ferromagnetic shape memory alloys, the characteristic features of modulated martensite (including the number/shape of constituent variants, the inter-variant orientation relationship and the geometrical distribution of variant interfaces) determine the attainability of the shape memory effect. In the present work, a comprehensive microstructural and crystallographic investigation has been conducted on a bulk polycrystalline Ni 50Mn 28Ga 22 alloy. As a first attempt, the orientation measurements by electron backscatter diffraction (EBSD) – using the precise information on the commensurate 5M modulated monoclinic superstructure (instead of the conventionally simplified non-modulated tetragonal structure) – were successfully performed to identify the crystallographic orientations on an individual basis. Consequently, the morphology of modulated martensite, the orientation relationships between adjacent variants and the characters of twin interfaces were unambiguously determined. With the thus-obtained full-featured image on the configuration of martensitic variants, the possibility of microstructural modification by proper mechanical “training” was further discussed. This new effort makes it feasible to explore the crystallographic/microstructural correlations in modulated martensite with high statistical reliability, which in turn provides useful guidance for optimizing the microstructure and shape memory performance.

A low temperature anomaly observed in off-stoichiometric Ni–Mn–Ga system studied by higher harmonic ac-susceptibility measurements

Results of fundamental and third order ac susceptibility measurements in a Ni49Mn29Ga22 single crystal demonstrate that higher harmonics of ac-susceptibility measurement is an excellent tool to identify transitions/anomalies originating from different magnetic substructures that remain undetected by usual dc susceptibility measurements. Anomaly observed at low temperature (∼150 K) has also been explored by differential scanning calorimetry, temperature dependent x-ray diffraction, and transmission electron microscopy measurements. The results demonstrate that the magnetic subsystem in Ni–Mn–Ga is unstable at low temperatures not only for metastable modulated crystal structure but also for very stable nonmodulated tetragonal structure.

Microstructures of a sputtered Ni56Mn27Ga17 ferromagnetic shape memory alloy thin film

Abstract A Ni 56 Mn 27 Ga 17 ferromagnetic shape memory alloy thin film was deposited onto a thermal substrate of 673 K by using radio frequency (RF) magnetron sputtering technique. The crystallographic structure and microstructures of the thin film are investigated by X-ray diffraction, transmission electron microscopy and high-resolution transmission electron microscopy, respectively. The results show the thin film is in martensitic state with a non-modulated tetragonal structure at room temperature. The martensite variants in the thin film are twinned relationship with (2 1 2) I type twin. The nanotwinned structures for non-modulated martensite variants are revealed at atomic level.