R. Chulist

Over 7% magnetic field-induced strain in a Ni-Mn-Ga five-layered martensite

A Ni-Mn-Ga single crystal with a modulated five-layered martensite structure is reported, demonstrating giant magnetic field induced strain (MFIS) of 7.1% at room temperature and of 6% at temperatures close to the austenite transformation (TA = 71 °C). The room temperature MFIS clearly exceeds the best results of around 6% measured earlier in 10M martensites. The larger MFIS is connected to the huge (>1%) change in the lattice distortion of the 10M structure, obtained within a narrow temperature interval of 47 K, which has been previously observed only during intermartensitic transformation. The present material shall effectively reduce the size of magnetic shape memory actuators.

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.

Large magnetic field-induced work output in a NiMnGa seven-layered modulated martensite

We report the performance of a Ni-Mn-Ga single crystal with a seven-layered lattice modulation (14M martensite), demonstrating large actuation work output driven by an external magnetic field. A magnetic field-induced strain of 11.2%, a twinning stress of 0.64 MPa, and a magneto-crystalline anisotropy energy of 195 kJ/m3 are measured at room temperature, which exceed the best results reported in Ni-Mn-Ga 14M martensites. The produced magnetically induced work output of about 70 kJ/m3 makes the material attractive for actuator applications. Detailed XRD investigation reveals that the studied 14M martensite is stress-induced. With increasing compression stress, the stress-induced intermartensitic transformation sequence 10M → 14M → NM was demonstrated.

Sn and Nb modified ultrafine Ti-based bulk alloys with high-strength and enhanced ductility

Sn and Nb modified ultrafine eutectic Ti−Fe alloys with high strength and plasticity prepared by cold crucible levitation melting were tested in compression at room temperature. (Ti70.5Fe29.5)93.15Sn3.85Nb3 alloy exhibited an ultimate compressive strength of 2.36 GPa at 15% plastic strain. Electron microscopy revealed that lamellar structures in Ti70.5Fe29.5 alloy could be tailored by the addition of Sn and Nb to obtain a globular structure. The microstructural refinement, morphology of phase constituents, and their relationships to the mechanical properties are discussed.

Structural modification and twinning stress reduction in a high-temperature Ni-Mn-Ga magnetic shape memory alloy

Mechanical and synchrotron diffraction experiments were performed to investigate the temperature dependent structural changes in a Ni-Mn-Ga single crystal. The initial sample exhibits a mixture of seven-layered (7M) and non-modulated (NM) martensites at room temperature. Compression along ⟨100⟩ resulted in a strain of 18%, indicating a stress-induced intermartensitic transformation from the 7M to the NM phase. The thermally induced intermartensitic transformation follows the sequence 5M→7M→NM during cooling from the austenite phase. The structural changes are quantitatively reflected in the mechanical response. A twinning stress of 3.8 MPa is measured at 90 °C, which is the lowest reported in high-temperature Ni-Mn-Ga structures.

Magnetostructural transition and magnetocaloric effect in highly textured Ni-Mn-Sn alloy

Ni49.4Mn38.5Sn12.1 near single crystal was obtained by the Bridgman method. At room temperature, it consisted of a mixture of the parent austenite phase with the cubic L21 Heusler structure (ac = 5.984 A) and modulated, tetragonal martensite phase 4M (at = 4.337 A, ct = 5.655 A). Under the application of a magnetic field, the specimen undergoes field induced reverse martensitic transformation, which combined with the Curie transition in austenite leads to the coexistence of direct and inverse magnetocaloric effects. The maximum entropy change at 280 K and under 5 T amounts to 3.4 J·kg−1·K−1 for the structural transition and at 316 K reaches −2.7 J·kg−1·K−1 for the magnetic transformation. The magnetic entropy change occurs over a wide temperature span leading to improved refrigerant capacity of 101 J·kg−1 (5 T). Hysteretic losses are considerably reduced, which is promising with respect to improved cyclic stability of such a material.

Self-accommodated and pre-strained martensitic microstructure in single-crystalline, metamagnetic Ni-Mn-Sn Heusler alloy

Metamagnetic shape memory alloys are a unique class of materials capable of large magnetic field-induced strain due to reverse martensitic phase transformation. A precondition for large shape change is martensite deformation, which heavily depends on microstructure. Elucidation of microstructure is therefore indispensable for strain control and deformation mechanics in such systems. The current paper reports on a self-accommodated martensitic microstructure in metamagnetic Ni50Mn37.5Sn12.5 single crystal. The microstructure here is hierarchically organised at three distinct levels. On a large scale, martensite plate colonies, distinguished by intercolony boundaries, group individual martensitic plates. Plates are separated by interplate boundaries and deviate by 2.2° from an ideal twin relation. On the lower scale, plates are composed of subplate twins. Conjugation boundaries separating two pairs of twins arise in relation to a subplate microstructure. Modulation boundaries separating two variants with perpendicular modulation directions and with parallel c-axes also appear. Mechanical training frees larger plates from fine subplate microtwins bringing macro-lamellae into twin relation, what then permits further detwinning until a single variant state.

Asymmetric distribution of martensitic variants in non-modulated NiMnGa single crystals

A strong asymmetric distribution of martensitic variants in non-modulated NiMnGa single crystals with respect to austenite is produced during martensitic transformation. A cubic-to-tetragonal transformation occurs with 24 possible variants divided into two groups. The first group with a misorientation of about 6° is composed of the so-called major variants separated by inter-plate boundaries, while the latter comprises minor variants with misorientation of 12.2°. The 6° rotation associated with major variants can also be observed at each inclination point where conjugation boundaries (CBs) occurs. The removal of CBs straightens out the inclined inter-plate boundaries confirming the Müllner–King mechanism.

Multiphase Microstructure and Extended Martensitic Phase Transformation in Directionally Solidified and Heat Treated Ni 44 Co 6 Mn 39 Sn 11 Metamagnetic Shape Memory Alloy

Directionally solidified Ni–Co–Mn–Sn alloy shows a multiphase solidification microstructure relatable primarily to the varying Co–Mn/Sn ratio. Thermal treatment at 1220 K lasting for 72 h encourages chemical homogeneity with average stoichiometry of Ni45.1Co6.2Mn37.2Sn11.5. At room temperature, despite the chemical uniformity, the as-homogenized alloy shows a multiphase microstructure with coexisting L21 austenite and 6M and 4O martensite phases. The martensite phase preferentially locates at grain boundaries. The onset of the martensitic transition temperature is estimated at 402 K, which overlaps with the Curie transition of austenite. The martensitic transition appears to initially take place at the grain boundaries and then it extends to low temperature as the volume of the grains transforms to martensite.

Effect of heat treatment on the precipitation hardening in FeNiCoAlTaB shape memory alloys

Abstract In order to obtain optimal mechanical properties, the effect of heat treatment on the precipitation hardening in multicomponent Fe-based shape memory alloys (containing Ni, Co, Al, Ta, B) was studied. The polycrystalline material was investigated after application of three different processing schemas: slowly cooled, quenched and subsequently annealed with various aging conditions. The study was carried out using synchrotron X-ray diffraction along with mechanical tests, revealing the evolution of strengthening phases. As a result an optimum heat treatment for 10 h at 700°C was established yielding an optimal mechanical response.