119 research outputs found
A two dimensional model for ferromagnetic martensites
We consider a recently introduced 2-D square-to-rectangle martensite model
that explains several unusual features of martensites to study ferromagnetic
martensites. The strain order parameter is coupled to the magnetic order
parameter through a 4-state clock model. Studies are carried out for several
combinations of the ordering of the Curie temperatures of the austenite and
martensite phases and, the martensite transformation temperature. We find that
the orientation of the magnetic order which generally points along the short
axis of the rectangular variant, changes as one crosses the twin or the
martensite-austenite interface. The model shows the possibility of a subtle
interplay between the growth of strain and magnetic order parameters as the
temperature is decreased. In some cases, this leads to qualitatively different
magnetization curves from those predicted by earlier mean field models.
Further, we find that strain morphology can be substantially altered by the
magnetic order. We have also studied the dynamic hysteresis behavior.
The corresponding dissipation during the forward and reverse cycles has
features similar to the Barkhausen's noise.Comment: 9 pages, 11 figure
Homogenization in magnetic-shape-memory polymer composites
Magnetic-shape-memory materials (e.g. specific NiMnGa alloys) react with a
large change of shape to the presence of an external magnetic field. As an
alternative for the difficult to manifacture single crystal of these alloys we
study composite materials in which small magnetic-shape-memory particles are
embedded in a polymer matrix. The macroscopic properties of the composite
depend strongly on the geometry of the microstructure and on the
characteristics of the particles and the polymer.
We present a variational model based on micromagnetism and elasticity, and
derive via homogenization an effective macroscopic model under the assumption
that the microstructure is periodic. We then study numerically the resulting
cell problem, and discuss the effect of the microstructure on the macroscopic
material behavior. Our results may be used to optimize the shape of the
particles and the microstructure.Comment: 17 pages, 4 figure
First-principles study of lattice instabilities in the ferromagnetic martensite NiMnGa
The phonon dispersion relations and elastic constants for ferromagnetic
NiMnGa in the cubic and tetragonally distorted Heusler structures are
computed using density-functional and density-functional perturbation theory
within the spin-polarized generalized-gradient approximation. For
, the TA tranverse acoustic branch along and
symmetry-related directions displays a dynamical instability at a wavevector
that depends on . Through examination of the Fermi-surface nesting and
electron-phonon coupling, this is identified as a Kohn anomaly. In the parent
cubic phase the computed tetragonal shear elastic constant,
C=(CC)/2, is close to zero, indicating a marginal
elastic instability towards a uniform tetragonal distortion. We conclude that
the cubic Heusler structure is unstable against a family of energy-lowering
distortions produced by the coupling between a uniform tetragonal distortion
and the corresponding modulation. The computed relation between the
ratio and the modulation wavevector is in excellent agreement with
structural data on the premartensitic ( = 1) and martensitic ( =
0.94) phases of NiMnGa.Comment: submitted to Phys. Rev.
XRD Characterization of the Structure of Graphites and Carbon Materials Obtained by the Low-Temperature Graphitization of Coal Tar Pitch
The structure of some commercial graphites and carbon materials (CMs) obtained by the low-temperature catalytic graphitization of coal tar pitch with iron salt, needle coke, foamed graphite as the catalysts has been studied. The study was performed using the X-ray diffraction technique with reflections from base plane and their decomposition into two components corresponding to the structural phases of graphite which have different XRD characteristics. Various CMs were compared with respect to the structural phase ratio, distance between polyarene layers in these phases, and sizes of the coherent scattering regions. The (004) reflection provided a better fit of some properties of graphites to the calculated XRD characteristics as compared to calculation from the (002) reflection. In the case of carbonization of coal tar pitch with investigated catalyst additions, prepared carbon materials have a higher degree of graphitization and a crystallite size greater than in the other case of carbonization of the individual pitch. The highest catalytic activity is shown by foamed graphite. It was found that the use of foamed graphite as the catalyst at 800-900 ºC produced carbon materials possessing a crystalline structure with interplanar spacing close to that in commercial graphites, while in the absence of catalyst the coal tar pitch material has an amorphous structure
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Review of properties of magnetic shape memory (MSM) alloys and MSM actuator designs
Magnetic shape memory alloys are a new group of "smart" materials that exhibit large strain of 6-12% when subjected to magnetic fields. This indicates their enormous potential to be used in different electromagnetic (EM) devices such as actuators, sensors, energy harvesters and dampers. Shape change in MSM materials is controlled by magnetic field and doesn't involve phase transformation, allowing it to overcome a number of disadvantages of conventional shape memory alloys (SMAs). MSM devices are capable of producing large force and stroke output in considerably small dimensions. At the same time they can have fast response and potentially very long lifetime. This paper discusses different modern designs and approaches to MSM actuator design with their advantages and disadvantages. An overview on characteristics of MSM alloys is also presented in order to highlight how different properties of the material influence the total output of a device
Effect of Co on the martensitic transformation, crystal structure and magnetization of Ni52.5Mn23.5Ga24 based ferromagnetic shape memory alloys
Characterization of Ni-Mn-Ga magnetic shape memory alloys using electron holography and Lorentz microscopy
Stress-induced a/b compound twins redistribution in 10M Ni-Mn-Ga martensite
[eng] Slight orthorhombic and monoclinic distortion of approximately tetragonal lattice in five-layered modulated (10M) martensite of Ni-Mn-Ga leads to the formation of laminates consisting of a/b compound twins. Here, we present the first direct optical observation and x-ray diffraction confirmation of the stress-induced redistribution of a/b twins of ~1-15 μm in size. We demonstrate that a/b twins are highly mobile, exhibiting twinning stress as low as 0.1 MPa, which is comparable to the twinning stress of Type 2 a/c twin boundaries. We assume that a local rearrangement of the a/b laminate may occur during the motion of a/c twin boundaries
Ultrahigh damping and Young´s modulus softening due to a/b twins in 10M Ni-Mn-Ga martensite
[eng] We study Young's modulus and damping of 10M martensite in Ni50.0Mn28.4Ga21.6 magnetic shape memory alloy using a sample without a/c twin boundaries (TBs) and maximum or zero shear stress in a/b twinning planes. Damping is ~300 times higher and modulus ~3.5 times lower when shear stress acts on a/b TBs. Ultrahigh damping and low modulus are insensitive to magnetic field, show strong strain amplitude nonlinearity and are attributed to stress-induced motion of a regular system of mobile a/b TBs. Near the reverse transformation, E001 modulus without contribution of TBs is ~10 times higher than reported previously for 10M polyvariant sample
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