Vít Kopecký

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.

Structural Changes in Co-Based F-SMA

The Co38Ni33Al29 alloy as a potential ferromagnetic shape memory alloy was investigated. The method of preparation of the unidirectional solidified single-crystals from cast material is described. The high-temperature annealing and subsequent quenching was found to be necessary condition for the shape memory behavior. The martensitic transformation temperatures of annealed samples were about 200 K determined from magnetic measurement while as-cast sample did not exhibit any sharp transformation. All martensitic structures observed at room temperature by microscopic methods are thus stress induced. These results agree with pseudoelastic behavior observed in annealed and quenched samples.

Changes in magnetic domain structure during twin boundary motion in single crystal Ni-Mn-Ga exhibiting magnetic shape memory effect

The complexity of Ni-Mn-Ga single crystal originates from the interplay between ferromagnetic domain structure and ferroelastic twinned microstructure. Magnetic domain structure in the vicinity of single twin boundary was studied using magneto-optical indicator film and magnetic force microscopy technique. The single twin boundary of Type I was formed mechanically and an initial magnetization state in both variants were restored by local application of magnetic field (≈40 kA/m). The differently oriented variants exhibited either stripe or labyrinth magnetic domain pattern in agreement with the uniaxial magnetocrystalline anisotropy of the martensite. The twin boundary was then moved by compressive or tensile stress. The passage of the boundary resulted in the formation of granular or rake domains, respectively. Additionally, the specific magnetic domains pattern projected by twin boundary gradually vanished during twin boundary motion.

Magneto-optical Kerr effect of a Ni2.00Mn1.16Ga0.84 single crystal across austenite and intermartensite transitions

We carried out magneto-optical Kerr effect (MOKE) and magnetization measurements on a single crystal of Ni2.00Mn1.16Ga0.84 in a longitudinal geometry. While there had been reports of polar geometry measurements so far, we show that - against earlier predictions - surface magnetic states of the martensite and the austenite can be also probed efficiently via longitudinal MOKE. A single variant magnetic state, which occurs at room temperature, is characterized by ferromagnetic hysteresis loops similar to standard ferromagnets. Temperature dependencies of Kerr rotation were found to be linearly proportional to magnetization for martensitic phases. After passing through an inter martensitic structural transition at around 260 K in zero magnetic field, the coercive fields are more than doubled in comparison with the room temperature values. On the other hand, at higher temperatures above 338 K where an austenite structure is formed, the MOKE signals are dominated by quadratic contributions and the magnitude of Kerr rotation drops due to changes in the electronic and magnetic domains structure. After the sample is cooled across the austenite-martensite transition again, we observed a multiple beam reflected by the sample due to different types of structural domains present on its surface.

Magnetic Shape Memory Effect in Ni-Mn-Ga Single Crystal

Magnetic shape memory effect is general name for several effects in which the most visible feature is huge strain induced by magnetic field. Magnetic field-induced structure reorientation (MIR) occurs due to motion of twin boundaries in single phase. As the magnetic field is a relatively weak force compared with mechanical stress, very high mobility of twin boundaries is crucial. Here we study the properties of martensite relevant for this effect using X-ray diffraction, optical and electron microscopy, magnetic observation and mechanical testing. In 10M modulated martensite, two types of mobile twin boundary (type I and type II) are observed with complex layered microstructures consisting of a hierarchy of twinning systems. We search for analogue with non-magnetic Cu-Ni-Al shape memory alloy.