N. Lanska

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.

Twin microstructure dependent mechanical response in Ni–Mn–Ga single crystals

Ni–Mn–Ga single crystals with a twinning stress of about 0.1 MPa were studied. They showed a tendency to stay in a single variant state and to retain only one or very few twin boundaries during martensite reorientation induced by an external stress or magnetic field. This makes the crystals problematic for application in a magnetic actuator. To solve the issue, we introduced many parallel twin boundaries into the crystals by bending. However, this twin microstructure was not stable under cycling load. Additionally, it exhibited a twinning stress of 0.8 MPa—about ten times higher than a crystal with a single boundary.

Temperature dependence of magnetic anisotropy in Ni–Mn–Ga alloys exhibiting giant field-induced strain

Temperature dependence of structure and magnetic anisotropy of single crystalline Ni48.8Mn28.6Ga22.6 alloy exhibiting giant field-induced strain or magnetic shape memory (MSM) effect was studied in the temperature range 80–420 K. Upon cooling the alloy transforms from cubic austenite at 307 K to the martensite which exhibits five-layered (modulated) tetragonal structure (5M) with a=0.595 nm and c=0.559 nm. Reverse transformation occurs at 317 K. An additional intermartensitic transition takes place at about 95 K. The basic mechanism of the MSM effect was corroborated by direct simultaneous measurements of strain and magnetization as a function of magnetic field. The magnetic anisotropy of the martensite exhibiting the giant strain was determined from the magnetization curves measured by a vibrating sample magnetometer at different temperatures. The anisotropy of the single variant 5M martensite is uniaxial with easy axis along the tetragonal c axis. The first magnetic anisotropy constant is Ku1=2.0×105 J/...

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.

Twin interaction and large magnetoelasticity in Ni-Mn-Ga single crystals

We demonstrate experimentally the existence of triple twins in Ni-Mn-Ga magnetic shape memory single crystals with a modulated five-layered martensite structure using optical observations andx-ray diffraction. Subsequently, we investigate the response of the crystals with triple-twin segments to compressive loading up to several MPa. Such loading typically resulted in an abrupt rearrangement of the twin microstructure to a configuration with many fine twins (1–10 µm in size) ending at a twin boundary. This type of twin microstructure exhibited recoverable deformation with up to 0.3% macroscopic strain and an estimated 2.5% local strain, while the recoverable strain was much smaller for other studied microstructure configurations. The results indicate that by the creation of a suitable twin microstructure, the originally pseudoplastic or magnetoplastic material can be made rubberlike elastic or magnetoelastic with the macroscopic recoverable strain comparable to 2.5%.