Papers by Keyword: Heusler Alloy

Abstract: The most extensively studied Heusler alloys are those based on the Ni-Mn-Ga system. However, to overcome the high cost of Gallium and the usually low martensitic transformation temperature, the search for Ga-free alloys has been recently attempted, particularly, by introducing In, Sn or Sb. In this work, Mn50Ni40In10, Mn50Ni34In16, Ni50Mn36-xIn14+x (x = 0, 0.5, 1, 1.5) and Ni50Mn37Sn13 ribbons has been obtained by melt spinning. We outline their structural and thermomagnetic behavior. Columnar grains and preferential orientation has been obtained. The martensitic, Tm, and the Curie, TC, temperatures of the ribbons are lower than those of the bulk samples with similar compositions. This effect is probably due to the ribbons small and constrained grains. For it, a large under-cooling is necessary for the martensitic transformation. The decrease of TC in the ribbons could be associated with the increased degree of quenched-in short-range disorder around defects.

Abstract: In this paper, we discuss the magnetocaloric behavior of Ni-Mn-based Heusler alloys in rela- tion to their shape-memory and superelastic properties. We show that the magnetocaloric effect in these materials originates from two different contributions: (i) the coupling that is related to a strong uniaxial magnetic anisotropy and takes place at the length scale of martensite variants and magnetic domains (extrinsic effect), and (ii) the intrinsic microscopic magnetostructural coupling. The first contribution is intimately related to the magnetically induced rearrange- ment of martensite variants (magnetic shape-memory) and controls the magnetocaloric effect at small applied fields, while the latter is dominant at higher fields and is essentially related to the possibility of magnetically inducing the martensitic transition (magnetic superelasticity). The possibility of inverse magnetocaloric effect associated with these two contributions is also considered.

Abstract: In this paper, we discuss the possibility of inducing a martensitic transition by means of an applied magnetic field or hydrostatic pressure in Ni-Mn based Heusler shape memory alloys. We report on the shift of the martensitic transition temperatures with applied magnetic field and applied pressure and we show that it is possible to induce the structural transformation in a Ni50Mn34In16 alloy by means of both external fields due to: (i) the low value of the entropy change and (ii) the large change of magnetization and volume, which occur at the martensitic transition.

Abstract: The giant magnetocaloric properties of NiMnGa alloys can be enhanced by suitable composition changes that make structural and magnetic transition temperatures to coincide. In this paper we report results on critical temperatures, magnetic anisotropy, and magnetocaloric effect in Ni- and Mn-rich alloys as a function of composition. A phenomenological phase diagram, useful for the identification by thermomagnetic analysis of magnetic and structural transitions in the vicinity of their coincidence, is proposed. Particular emphasis is given to the discussion of giant magnetocaloric effect of those alloys showing a first order magnetostructural transition, the method of its determination, and the potentialities for applications in the field of magnetic refrigeration.

Abstract: In this work with the help of the phenomenological Ginzburg-Landau theory of structural and magnetic phase transitions the phase diagrams of Heusler Ni-Mn-X (X = In, Sn, Sb) alloys with the inversion of exchange interaction are investigated. The investigation shows that the type of the phase diagrams in Heusler Ni-Mn-X (X = In, Sn, Sb) alloys depends on the value and sign of the free energy parameters. As it is seen from the analysis of the phase diagrams with the determined values and signs of the parameters of Landau functional there are thermodynamic paths which allow to explain experimental phase transitions in Heusler alloys Ni-Mn-X (X = In, Sn, Sb) and Ni-Co- Mn-X (X = In, Sn, Sb) qualitatively.

Abstract: X-ray powder diffraction and magnetization measurements were done on the magnetic shape memory alloys Ni2Mn1+xSn1-x. The alloys with 0≤x≾0.4 crystallize in the cubic L21 structure and exhibit the ferromagnetic behavior. X-ray diffraction patterns indicate that the excess Mn atoms occupy the Sn sites. Furthermore, magnetization measurements make clear that the Mn atoms, which substitute for Sn sites, are coupled antiferromagnetically to the ferromagnetic manganese sublattices. The alloys with 0.4≾x≤0.6 undergo a martensitic transition from the high temperature L21 structure to the orthorhombic 4O one. These alloys show a variety of magnetic transitions. A magnetic phase diagram of Ni2Mn1+xSn1-x system is discussed qualitatively on the basis of the interatomic dependence of the exchange interactions.

Abstract: Magnetic shape-memory alloys owe their exceptional properties primarily to the accompanying effects of a martensitic phase transformation. The twinning disconnection as elementary carrier of magnetic-field-induced deformation is the starting point of the present study. A disconnection is a line defect similar to a dislocation but located at an interface and exhibiting a step character besides a dislocation character. The mutual interaction of disconnections is fully tractable by the theory of dislocations. Due to the martensitic transformation, a hierarchical twin microstructure evolves, details of which are controlled through disconnection-disconnection interaction. Depending on the mutual orientation of twin boundaries on different hierarchical levels, twinning disconnections are incorporated in higher hierarchical twin boundaries forming disclination walls, or they stand off individually from those interfaces. Disconnections which stand off from interfaces contribute to magnetoelasticity, i.e. recoverable magnetic-field-induced deformation. Disconnections in disclination walls contribute to magnetoplasticity, i.e. permanent magnetic-field-induced deformation, if the twin thickness is large. In self-accommodated martensite with very thin twins, resulting from a martensitic transformation without training, the deformation is fully magnetoelastic and small. In single-domain crystals, resulting from effective thermo-magnetomechanical training, the deformation is fully magnetoplastic and large. Between these limiting cases, there is a continuous spectrum where, as a rule, the fraction of magnetoplastic strain and the total strain increase with increasing effectiveness of training.

Abstract: The initials results on growth and structural properties of Ni-Mn-Al full Heusler alloy thin films on silicon substrates deposited by RF magnetron sputtering is reported in this paper. Good crystallinity in the film is obtained by optimizing the sputtering parameters. The as-deposited film was post-annealed in vacuum in the temperature range between 150 °C, 250 °C and 450 °C for 60 min. It is observed that as deposited film shows nanocrystalline in nature. The film annealed at 450 °C shows L21 structure. The magnetic properties of the NiMnAl films at room temperature are measured by vibrating sample magnetometer [VSM]. It is found that the annealed samples shows clear saturating loop whereas the as prepared film is paramagnetic in nature.

Abstract: A strong need exists to develop new kinds of high-temperature shape-memory alloys. In this study, two series of CoNiGa alloys with different compositions have been studied to investigate their potentials as high-temperature shape-memory alloys, with regard to their microstructure, crystal structure, and martensitic transformation behavior. Optical observations and X-ray diffractions confirmed that single martensite phase was present for low cobalt samples, and dual phases containing martensite and γ phase were present for high cobalt samples. It was also found that CoNiGa alloys in this study exhibit austenitic transformation temperatures higher than 340°C, showing their great potentials for developing as high-temperature shape-memory alloys.