A. Sozinov

12% magnetic field-induced strain in Ni-Mn-Ga-based non-modulated martensite

Magnetic field-induced strain (MFIS) of 12% is reported in ferromagnetic Ni 46Mn24Ga22Co4 Cu 4 martensite exhibiting non-modulated (NM) tetragonal crystal structure with lattice parameter ratio c / a > 1 . The strain was measured at ambient temperature in a magnetic field of the order of 1 T. The twinning stress σ T W and the magnetic stress σ M A G were also measured and the condition for a giant MFIS observation σ T W < σ M A G was confirmed. The MFIS was achieved in NM Ni 46Mn24Ga22Co4 Cu 4 martensite by considerable lowering of the σ T W value as compared to the values for NM martensites in ternary Ni-Mn-Ga system.

Highly mobile type II twin boundary in Ni-Mn-Ga five-layered martensite

Twin relationships and stress-induced reorientation were studied in Ni2Mn1.14Ga0.86 single crystal with five-layered modulated martensite crystal structure. Very low twinning stress of about 0.1 MPa was found for twin boundaries which deviated a few degrees from the (011¯) crystallographic plane. However, twin boundaries oriented exactly parallel to the (011¯) plane exhibited considerably higher level of twinning stress, above 1 MPa. X-ray diffraction experiments and calculations based on approximation of the martensite crystal lattice as a tetragonal lattice with a slight monoclinic distortion identified the two different kinds of twin interfaces as type II and type I twin boundaries.

Ni-Mn-Ga single crystals with very low twinning stress

Twinning stress or mechanical hysteresis associated with the twin boundary motion is one of the most essential parameters which determine the actuating performance of magnetic shape memory alloys. Recent effort at AdaptaMat Ltd. to decrease the twinning stress resulted in a consistent production of Ni-Mn-Ga magnetic shape memory single crystals with the twinning stress of about 0.1 MPa, which is much lower than previously reported. In this work, the mechanical and magnetomechanical response of the developed crystals is discussed in detail and the importance of adjustment of the twin microstructure for obtaining an optimal actuating behavior is illustrated.

Effect of intermartensite transformation on twinning stress in Ni-Mn-Ga 10?M martensite

Intermartensite transformations (IMTs) and temperature dependence of very low twinning stress associated with Type II twin boundaries were studied experimentally in four magnetic shape memory alloys with composition Ni50Mn25 + xGa25 – x (at. %), where x was between 3.2 and 3.9. The slightly different x or Mn content resulted in slightly different martensite transformation temperatures between 309 and 328 K, but in significantly different intermartensite transformation temperatures between 10 K and 251 K, with the transformation temperature being approximately proportional to x. In all four alloys, the initial twinning stress, about 0.1 MPa at room temperature, increases sharply when approaching the IMT at lower temperatures, up to 1 MPa and more. The effect is explained by the instability of 10M martensite near the IMT from which it follows that the twinning stress dependence shall be flat for x≤2.7. That is experimentally confirmed by an additional measurement on Ni50Mn27.7Ga22.3 alloy (x = 2.7), which e...

Magnetic shape memory effect at 1.7 K

Magnetic shape memory effect or magnetically induced structure reorientation (MIR) occurred down to 1.7 K in 10 M martensite with composition of Ni50.0Mn27.5Ga22.5 exhibiting no intermartensite transformation. The reorientation of the martensite microstructure was mediated by the motion of single Type II twin boundary. In contrast with weak thermal dependence of Type II boundary, MIR with Type I boundary in the same alloy showed strong thermal dependence resembling normal thermal activation process and the effect disappeared below 220 K. Thus the type of the boundary is decisive for MIR at low temperatures.

Diffraction study of bending-induced polysynthetic twins in 10M modulated Ni-Mn-Ga martensite

Detailed conventional and synchrotron x-ray and electron backscatter diffraction experiments were performed to investigate bending-induced polysynthetic twins in a Ni−Mn−Ga single crystal with five-layered (10M) martensite crystal structure. It was proved that the twin microstructure produced in such a way is composed of type I twins. The type I twins exhibit a relatively high twinning stress of about 0.8 MPa, which limits the performance of bending-induced twins in magnetic actuation.

Ab initio prediction of stable nanotwin double layers and 4O structure in Ni2MnGa

The ab initio electronic structure calculations of the

Low temperature a / b nanotwins in Ni 50 Mn 25+x Ga 25−x Heusler alloys

{\mathrm{Ni}}_{2}\mathrm{MnGa}

Transformation Paths from Cubic to Low-Symmetry Structures in Heusler Ni 2 MnGa Compound

alloy indicate that the orthorhombic 4O structure exhibits the lowest energy compared to all known martensitic structures. The 4O structure is formed by nanotwin double layers, i.e., oppositely oriented nanotwins consisting of two (101) lattice planes of nonmodulated martensitic structure. It exhibits the lowest occupation of density of states at the Fermi level. The total energy 1.98 meV/atom below the energy of nonmodulated martensite is achieved within structural relaxation by shifting Mn and Ga atoms at the nanotwin boundaries. The same atomic shift can also be found in other martensitic nanotwinned or modulated structures such as 10M and 14M, which indicates the importance of the nanotwin double layer for the stability of these structures. Our discovery shows that the nanotwinning or modulation is a natural property of low-temperature martensitic phases in Ni-Mn-Ga alloys.

Highly mobile twinned interface in 10 M modulated Ni–Mn–Ga martensite: Analysis beyond the tetragonal approximation of lattice

We have found low temperature a/b nanotwins having (110) twinning plane in a five-layered modulated martensite phase of Ni50Mn25+xGa25−x (at. %) Heusler alloys and identified the particular region in phase diagram where the nanotwinning occurs. Evolution of the structure with decreasing temperature was studied by X-ray diffraction using single crystals exhibiting magnetic shape memory effect. The merging of (400) and (040) lines upon cooling for 2.6 < x < 3.5 indicated a/b nanotwinning originating from the refinement of initially coarse a/b twins. Refinement of the twins with decreasing temperature was observed directly using scanning electron microscopy. The prerequisite for nanotwinning is an extremely low twin boundary energy, which we estimated using first-principles calculations to be 0.16 meV/Å2. As the nanotwinning distorts the relation between the crystal lattice and the X-ray diffraction pattern, it should be taken into consideration in structural studies of Ni-Mn-Ga Heusler alloys.