Temperature-induced successive martensitic and inter-martensitic phase transformations of Ni2.15Mn0.85Ga Heusler alloy

Abstract

1. Entropy change of successive martensitic transformations in Ni-Mn-Ga Heusler alloys can be utilized to realize enhanced magnetocaloric properties. A detailed study of phase transformations of Ni2.15Mn0.85Ga, (ΔQ = 4900 J/kg at 343 K and 140 kOe) is reported here. Upon cooling, paramagnetic austenite (L21) transforms into a ferromagnetic 7M monoclinic martensitic phase. This phase is stable in a narrow temperature range and, upon further cooling, transforms into a non-modulated ferromagnetic tetragonal (L10) phase. The separation between the equilibrium temperatures of the L21 and the L10 phases is only ~50 K. The alloy undergoes reversible temperature-induced martensitic and inter-martensitic phase transformations with thermal hysteresis about 25 K. The conclusions from the detailed study of the phase transformations lead to new possibilities to enhance the magnetocaloric effect by utilizing the entropy associated with multi-structural transformations.2. IntroductionMagnetic cooling based on the giant magnetocaloric effect (GMCE) is a proposed solution to the low-efficiency and eco-adverse refrigerant of current technologies [1]. Developing a material that produces a GMCE near room temperature is key to the more efficient and economical cooling technology. Ni-Mn-X alloys are under the spotlight because they are inexpensive and non-toxic. The isothermal entropy changes exhibited by some Ni-Mn-Ga alloys are in close range or even higher than those reported in Gd and La-based compounds [2]. Several techniques are available to enhance the GMCE demonstrated by Ni-Mn-X alloys. Combining magnetocaloric and elastocaloric effects by applying stress to the refrigerants is one technique [3]. The latest trend is to utilize the two consecutive structural transformations of these alloys [4]. Here we report a study of the inter-martensitic phase transformations of such an alloy. The analysis of the structures, phase transformation temperatures, and the thermal hysteresis will support the search for new avenues to enhance the magnetocaloric effect by utilizing the entropy associated with multi-structural transformations [5].3. ExperimentalA polycrystalline sample was prepared by arc-melting in an argon atmosphere and then annealed at 1073 K for 50 hours [6]. The chemical composition, Ni2.15Mn0.85Ga, was determined by energy dispersive spectroscopy. In-situ synchrotron diffraction data were collected at beamline 11-ID-C of the Advanced Photon Source at Argonne National Laboratory (λ = 0.010804 nm). Diffraction data were analyzed using the General Structure Analysis System EXPGUI [7]. Magnetization measurements were done using a SQUID magnetometer.4. Results and DiscussionThe diffraction data (Fig. 1) at selected temperatures (400-100 K) shows that the alloy undergoes martensitic and inter-martensitic transformations and mixtures of phases in different temperature intervals. The austenitic structure is cubic L21 (Fm-3m space group). Upon cooling, the alloy transforms into a 7M monoclinic (P 1 2/m 1 space group). Upon further cooling, it transforms into a non-modulated tetragonal L10 structure (I 4/m m m space group). The thermograms (Fig. 2) were constructed by the calculated phase fractions obtained from Rietveld refinements. The alloy undergoes martensitic transformation around 310 K from austenite to 7M modulated martensite. Upon further cooling (~ 260 K), the inter-martensitic phase transformation [8] occurs from monoclinic to tetragonal phase. The reverse-phase transformations occur while heating (see fig. 2).The characteristic temperatures of the austenitic (TMH), martensitic (TMC), and inter-martensitic transformations (TIMH and TIMC) were defined at 50% phase fraction of the respective phases upon cooling and then heating, respectively. Thermal hystereses of the transformations, calculated by the differences between TMH & TMC and between TIMH & TIMC, respectively, are very close (~25 K). Because of the similar hysteresis, the separation between the martensitic and inter-martensitic transformation temperatures, while heating and cooling, are the same and is ~50 K. The austenitic phase is paramagnetic, and the martensitic phases are ferromagnetic. The austenitic phase transformation temperature (332 K) and Curie temperature (337 K) are very close. Under a suitable field, these two transformations could coincide to give a much higher MCE.5. Conclusions and Future workThe alloy undergoes two successive phase transformations with narrow separation between them (~50 K). The hystereses are less than 25 K. These conditions suggest that utilizing two successive phase transformations may result in higher GMCE. To optimize the GMCE, extensive knowledge of two transformations is required. The magnetic phase transformation temperature increases with increasing magnetic field [6]. However, the behavior of the crystalline phase transformation temperatures under magnetic fields is unknown and essential to study. Also, isothermal magnetization measurements could reveal the possible magnetic phase transformations. **