Jingmin Wang

Search for transformation from paramagnetic martensite to ferromagnetic austenite: NiMnGaCu alloys

Search for transformation from paramagnetic martensite to ferromagnetic austenite in ferromagnetic shape memory alloys is performed through designing NiMnGaCu alloys. The composition dependence of the martensitic transformation temperature TM, the magnetic transition temperatures TCA of the austenite and TCM of the martensite is systematically investigated. The sequence of the martensitic transformation and magnetic transition is determined. The diagram on the structural and magnetic transition in a specific system Ni46Mn25+xGa25−xCu4 is outlined, in which a transformation from paramagnetic martensite to ferromagnetic austenite is predicted, exhibiting TCM<TM<TCA. Such a transformation is then experimentally achieved in Ni46Mn33Ga17Cu4 alloy.

Temperature dependence of the giant magnetostrain in a NiMnGa magnetic shape memory alloy

The temperature dependence of the magnetostrain was investigated in the Ni50Mn27.5Ga22.5 magnetic shape memory alloy with a five-layer martensitic (5M) structure in the temperature range from 110Kto300K. A temperature threshold at 166K was found for the magnetostrain. A giant magnetostrain of 6.3% was achieved above the temperature, while no magnetostrain was monitored below the temperature. No intermartensitic transformation was detected around the temperature threshold. The lattice parameter a slightly increases, c largely decreases, and the tetragonality (a∕c−1) drastically increases with decreasing the temperature. The increase of the tetragonality is thought to be related to the temperature threshold of the magnetostrain by inducing a change of the electronic structure, twin structure, or the type of the variant with the same 5M martensitic structure below the temperature threshold. The interpretation is reasonably understood by the fact that only few samples with the same 5M martensitic structure ex...

Tailoring the magnetostructural transition and magnetocaloric properties around room temperature: In-doped Ni-Mn-Ga alloys

Some off-stoichiometric Ni-Mn-Ga alloys undergo a coupled magnetostructural transition from ferromagnetic martensite to paramagnetic austenite, giving rise to the large magnetocaloric effect. However, the magnetostructural transitions of Ni-Mn-Ga alloys generally take place at temperatures higher than room temperature. Here, we report that by the partial substitution of In for Ga, the paramagnetic austenite phase is well stabilized, and the magnetostructural transition can be tailored around room temperature. Sizable magnetic entropy change and adiabatic temperature change were induced by magnetic field change in the vicinity of the magnetostructural transition of the In-doped Ni-Mn-Ga alloys.

Magnetic field-induced reverse martensitic transformation in NiMnGaCu alloy

The reverse martensitic transformation was monitored by testing the x-ray diffraction patterns with increasing temperatures in an Ni46Mn33Ga17Cu4 alloy. A large magnetization change from a weak-magnetic martensite phase to a ferromagnetic austenite phase has been found on the M–T curves of heating cycle in this alloy. The reverse martensitic transformation temperatures have been lowered by 7 K under the magnetic field of 90 kOe. The magnetic field-induced reverse martensitic transformation has been confirmed at a fixed temperature in this alloy, demonstrating that another alloy performing the MFIRMT is discovered: NiMnGaCu alloy.

COMPOSITION TRIANGLE DIAGRAMS OF Ni-Mn-Ga MAGNETIC SHAPE MEMORY ALLOYS

A statistical work has been done to collect the composition ranges of Ni-Mn-Ga alloys exhibiting different structures and martensite start temperature (M s ), large magnetostrain or the co-existence of magnetic and structural transitions. The alloys with five-layered (5M), seven-layered (7M) modulated and non-modulated (T) martensitic structures were mapped in the graph. An empirical formula has been presented to reflect the effect of elements nickel (Ni), manganese (Mn) and gallium (Ga), on the martensite start temperature (M s ). The martensitic structure is sensitive to the composition and the martensitic transformation temperature is most drastically affected by the Ni content. The alloys with large magnetostrain or co-existence effect of the magnetic and structural transitions were also listed in a limited area.

Internal friction associated with the premartensitic transformation and twin boundary motion of Ni50+xMn25−xGa25 (x = 0−2) alloys

The internal friction (IF) during the martensitic transformation (MT) has been extensively studied in NiMnGa alloys. In this paper, temperature dependence of the IF associated with the premartensitic transformation (PMT) and twin boundary motion (TBM) was investigated in Ni50+xMn25−xGa25 (x = 0−2) alloys. Both the composition and frequency had no obvious effect on the IF peak position of the PMT and TBM. With increasing frequency from 0.1 Hz to 5 Hz, the IF peak height corresponding to the TBM was significantly decreased, but was kept constant during the PMT. The observed phenomena were discussed in terms of the different microscopic mechanisms of the TBM and PMT.

Magnetostructural coupling near room temperature in Ni46-xFexCu4Mn34Ga16 alloys

We report the magnetostructural coupling near room temperature in Ni46-xFexCu4Mn34Ga16 (0 ≤ x ≤ 10) alloys. The martensitic transformation temperature was detected over the whole composition range and was decreased by the substitution of Fe for Ni. The martensitic and austenitic Curie temperatures, TCM and TCA, were observed for 0 ≤ x ≤ 6 and 4 ≤ x ≤ 10, respectively. With the increasing Fe content, TCA was slightly increased and TCM was more rapidly increased. The paramagnetic state of the martensite phase collapsed for x > 6 with the presence of the ferromagnetic austenite phase. The magnetostructural coupling transition from paramagnetic martensite to ferromagnetic austenite was obtained within the temperature range of 300–350 K which was near room temperature.

Influence of cooling rate on magneto-structural transition and magnetocaloric effect of Ni30Cu8Co12Mn37Ga13 alloy

The influence of heat treatment with different cooling rates on phase transition behaviors and magnetocaloric effect is systematically studied. Difference in atomic order is induced by changing cooling rates, where ordered phase is obtained in the furnace cooled (FC) sample while disordered phase is reserved in the water quenched (WQ) sample. The coupled magneto-structural transition is detected in both samples but the characteristic temperature significantly shifts to lower temperatures with increasing atomic order. Giant magnetic entropy change (ΔSmag) derived from magnetic field induced martensitic transformation is confirmed for both samples, and can be remarkably enhanced by the atomic ordering. The largest ΔSmag of 20.9 J/(kg · K) is obtained at 307.5 K under 5 T in the FC sample.

Effect of isoelectronic substitution on phase transition and magnetic properties of Ni57Mn18Ga25−xInx (0 ≤ x ≤ 8) alloys with Ni excess

Effect of isoelectronic substitution of In for Ga on the phase transition and magnetic properties was studied in Ni57Mn18Ga25−xInx (0 ≤ x ≤ 8) alloys. With the increasing In content, the room-temperature phase structure evolved from tetragonal martensite to cubic austenite. The martensitic transformation temperatures were significantly decreased by the substitution of In for Ga, but the austenitic Curie temperature was only slightly decreased. Especially, both thermal and isothermal magnetization measurements revealed that the Curie temperature and saturation magnetization of the martensite were independent on the isoelectronic substitution. The results were discussed by considering the phase structures and the atomic interactions.

Improved magneostriction and mechanical properties in dual-phase FeGa single crystal

ABSTRACT Single-phase Fe81Ga19 and dual-phase (Fe0.81Ga0.19)99.95Tb0.05 single crystals with perfect orientation were prepared by directional solidification technology. The performance of the dual-phase single crystal (SC) can achieve 399 ppm in magnetostriction and 10.2% in tensile fracture strain, which are, respectively, 28% and 5 times larger than those in single-phase Fe81Ga19 SC. The slight solid solution of Tb and dispersive distributed Tb-rich particles in dual-phase SC, respectively, lead to the improvement in magnetostriction and ductility. This dual-phase SC can become the candidate for new-generation magnetostrictive materials combining significant advantages in both structural and functional properties. Impact statement This dual-phase (Fe0.81Ga0.19)99.95Tb0.05 single crystal combining large magnetostriction and excellent mechanical properties can be new-generation magnetostrictive materials which can satisfy the application requirement in structural and functional properties. GRAPHICAL ABSTRACT