NiMn-based Shape Memory Alloys Research Articles

Microstructure and correlated mechanical properties study of Ni–(Fe, Co)–Mn–(Al, In) as-spun ribbons

The Ni–Mn–based shape memory alloys as a promising candidate of elastocaloric material has been reported in many literatures, especially on bulk samples. The as-spun ribbon, which has a larger surface area and is more efficient for heat transfer, is rarely studied and hence of importance. In the present work, we succeeded in producing very long as-spun Ni–Fe–Mn–(Al, In) ribbons, with around 300 mm in length. The microstructure and mechanical properties of these as-spun ribbons were thoroughly investigated by scanning electron microscopy and electron backscattered diffraction (SEM-EBSD), nanoindentation and 3-points bending experiments. Through SEM-EBSD analyses, the microstructure and texture of the as-spun ribbons were studied. A gradient in microstructure exists along the thickness direction (TD) of the ribbon, which is induced by the temperature gradient during fast rate solidification, resulting in fine equiaxed grains along the surface contacted with the rotating wheel in melt spinning process and elongated grains, respectively. Both equiaxed and elongated grains possess a strong {001} fiber texture (<001>//TD). Nanoindentation analyses show little variation of hardness between the two different microstructures. The ductility index of both Ni–Fe–Mn–Al and Ni–Fe–Mn–In ribbons are within the range of intermetallic materials. The substitution of In by Al allows to increase very slightly the ductility index, which can reach 0.75. The fine equiaxed grains show better tensile resistance than the elongated grains in 3-points bending test. The substitution of In by Al improves the maximum bending strain by a factor of 3. The maximum strain for Ni–Fe–Mn–Al as-spun ribbons can reach 3% before fracture. Fractography shows that, the intergranular fracture is the main damage mechanism in these as-spun ribbons.

Anomalous pressure effect on the magnetic properties of Ni-Mn based shape memory alloys

The magnetic behavior of three Ni-Mn-(Sn,In) based shape memory alloys are investigated under hydrostatic pressure. Among them, Ni51.2Mn32.8In16 (NM-In16) and Ni51Mn35Sn14 (NM-Sn14) have their martensitic transition close to room temperature and undergo several magnetic transitions at lower temperatures. They order ferromagnetically at TCA, which is just above room temperature. However, the ferromagnetism is destroyed by the martensitic transition at TMS (&amp;lt;TCA). The ferromagnetic order is revoked further at a lower temperature TCM (&amp;lt;TMS) in the martensite phase. The third alloy, namely, Ni50Mn34Sn16 (NM-Sn16), has a relatively stable ferromagnetic state (Curie point TCA=347 K), and it survives below the martensitic transition temperature, TMS. Our magnetic study under hydrostatic pressure indicates that TMS and TCM (only for the first two alloys) increase systematically with pressure. However, the saturation magnetization at base temperature is found to be an increasing function of pressure for NM-Sn14 only, and a decreasing trend is observed for the other two alloys. Most interestingly, NM-Sn14 shows a large enhancement in magnetocaloric effect (MCE) with pressure, whereas the other two compositions show a drop in MCE with pressure. A subtle competition between the change in Ni-Mn hybridization strength and the Mn-Mn magnetic interaction with pressure can be responsible for the observed anomalous behavior.

Giant magnetoresistance and large inverse magnetocaloric effect in Ni2Mn1.36Sn0.64 alloy

The magnetoresistance (MR) behaviour and the magnetocaloric effect (MCE) of the ferromagnetic shape memory alloy of nominal composition Ni2Mn1.36Sn0.64 have been investigated. We observe giant negative MR (∼ − 48% at 90 kOe) around the martensitic transition (MT) temperature. The observed MR was found to be irreversible with respect to the applied magnetic field, particularly in the vicinity of MT. The sample exhibits a large but positive magnetocaloric effect (a peak value of 5.1 J kg−1 K−1 for the magnetic field changing from 0 to 20 kOe) around MT. A comparison with the other Ni–Mn-based shape memory alloys indicates that the observed MR scales with the magnitude of the MCE.

Hybridization Effects in Ni-Mn Based Shape Memory Alloys: XAFS Study

Martensitic and magnetic properties of ferromagnetic shape memory alloys are known to depend up on structural modulations and associated changes in the Fermi surface. These modulations although periodic and spanning over multiple unit cells, involve movement of atoms typically of the order of 0.01Å. Therefore X-ray Absorption Fine Structure (XAFS) is an ideal tool to map both, local atomic movements and changes in density of states (DOS) due to changing hybridization as the system transforms from austenitic to martensitic phase. This paper presents a compilation of our XAFS studies on the Ni-Mn based shape memory alloys. A complete description of the changes in local structure around the constituent metal ions in the following alloy compositions: Ni2+xMn1-xGa, Ni2Mn1.4Sn0.6 and Ni2Mn1.4In0.6 in the austenitic and martensitic phases have been obtained. The results give the new experimental evidence for the crucial hybridization component that influences and leads to structural transition in these Ni-Mn based Heusler alloys.

Kinetic Model of Thermoelastic Martensite Transformation in Niti and NiMn Based Shape Memory Alloys

ABSTRACTA new, quantitative model was developed to describe the martensite transformation kinetics of thermoelastic shape memory alloys (SMAs). In addition, a series of experiments were conducted to study the Kinetics of thermoelastic martensite transformation in four SMA systems: NiTi, NiTi-15at%Hf, NiTi-20at%Zr and NiMn-7.5at%Ti alloys. Comparisons between data of the kinetic of martensite transformation with the present theoretic models show that the proposed model is in good agreement and concurs with the experimental data. Also, a comparison of data from the proposed model with data from existing kinetic models, such as Liang's and Magee's [1,7], indicates that the proposed model can better describe the experimental data, including the relationship between dξ(T)/dT and ξ, and dξ(T)/dT and T.