Materials Science Forum Vols. 738-739

Abstract: It is shown that deformability of Ti50.0Ni50,0 alloy at the rolling with current is considerably above than at the cold rolling. The influence of deformation with a current and without a current on feature of martensitic transformations was investigated. Recovery strain and recovery stress were studied on heating the sample subjected to preliminary strain by a three-point bending and optimal regimes of electroplastic rolling were defined.

Abstract: Magnetic shape-memory properties refer to the ability of certain materials to show strong response in strain to an applied magnetic field. This strain is caused by either inducing the martensitic transition or rearranging martensitic variants. In the first, case a superelastic effect is possible, while in the second, the system is able to show the shape-memory effect. The complex behaviour displayed by these materials is mainly a consequence of a strong interplay between magnetism and structure which is driven by a martensitic transition. This interplay is the source of many other observed effects such as giant magneto-resistance, exchange bias and magnetocaloric effects. In this paper, we will overview the present state of the art, discuss present challenges and outline some future perspectives in the field.

Abstract: This paper explains the magnetic-field induced martensite reorientation in Ferromagnetic Shape Memory Alloys (FSMA) through a simple energy analysis from which the role of the martensite’s magnetic anisotropy is emphasized. In particularly, with a three-dimensional (3D) energy analysis, we study the switching between the three tetragonal martensite variants driven by a rotating magnetic field (with a constant magnitude) and a non-rotating magnetic field (with a fixed direction but varying magnitudes). Finally, a simple planar phase diagram is proposed to describe the martensite reorientation in general 3D loadings.

Abstract: Abstract. Present publication gives a general theoretical concept and also presents the relevant experimental results concerning the effect of the magnetostatic coupling between the twin layers on the magnetic-field-controlled superelastic behavior during the mechanical cycling in magnetic field in Ni-Mn-Ga.

Abstract: This paper presents a miniature energy harvesting device that makes use of stress-induced cyclic martensite variant reorientation in a Ni-Mn-Ga single crystal of 0.3x2x2 mm³ size. The stress- and magnetic field-induced reorientation is investigated for single crystalline Ni_50.2Mn_28.4Ga_21.4 specimens of 0.3 mm thickness that are cut along the (100) direction and subjected to uniaxial compressive loading. A demonstrator is presented consisting of a FSMA specimen placed in the gap of a magnetic circuit to guide and enhance the field of biasing permanent magnets. The cyclic motion of a piezoelectric bimorph actuator is used to mechanically load the FSMA specimen. The corresponding change of magnetic flux induces an electrical voltage in a pick-up coil (N=2000 turns). The effects of biasing magnetic field, strain amplitude and strain velocity are investigated. An optimum magnetic field of 0.4 T exists, where the output voltage reaches 120 mV at a strain velocity of 0.006 ms^-1.

Abstract: 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.

Abstract: Experiments of biaxial compression were conducted to study the energy dissipation (stress-hysteresis) of martensite reorientation in NiMnGa single crystals. From the experiments, the observed stress-hysteresis consists of two parts ― the material intrinsic friction due to martensite reorientation, and the external friction between the loading clampers and the sample surfaces. It is found that the former one is independent of the 2D stress state while the latter one strongly depends on the 2D stress levels. Both kinds of friction are important and need to be considered in real applications.

Abstract: The results of the computer simulations of certain electronic properties of the austenite phase of Ni-Mn-Ga alloy are presents. Cluster approach was used and both self-consistent and density functional methods were exploited.

Abstract: Microstructures of the Fe-29.6at%Pd alloy ribbons were observed with an X-ray diffractometer and a transmission electron microscope. The X-ray diffraction profiles at room temperature showed that the ribbon consists of FCT martensitic phase and FCC parent phase. Moreover, the ribbon exhibits a strongly 200-oriented texture analyzing with pole figure measurements. TEM bright field images for the cross section of the ribbon showed high-density striation in the FCT martensite variants. Twin-related two sets of reflections were observed in the SAED patterns taken of the FCT martensite variants. _FCT was nearly parallel to the thickness direction according to the analysis of the SAED patterns. This result corresponds to the pole figure measurements.

Abstract: In present paper two ribbons of the Ni₄₄Co₆Mn₃₆In₁₄ (at.%) were prepared under different melt-spinning technique conditions. Microstructure of the ribbons was studied by scanning electron microscopy (SEM). Depending on the liquid ejection overpressure two types of ribbons microstructures were observed. Ribbon T1 for which ejection overpressure was 1.5 bar showed typical melt-spun ribbon microstructure consisting of a top layer of small equi-axial grains and columnar grains below. For T2 ribbon (ejection overpressure 0.2 bar) only a small fraction of the columnar grains were observed. Structure analysis of the ribbons performed by XRD showed that at room temperature both ribbons have B2 parent phase superstructure. No gamma phase precipitates were observed. In order to determine the orientation of the grains the EBSD technique was applied.