On Order-Disorder (L21 ? B2?) Phase Transition in Ni2+xMn1?xGa Heusler Alloys

Abstract

Subject classification: 64.60.Cn; S1.2The ferromagnetic Ni 2þx Mn 1Àx Ga Heusler alloys are of considerable interest due to their potential applicability as magnetically driven shape memory materials [1]. For the stoichiometric composition of Ni 2 MnGa the melting temperature is 1382 K [2]. At cooling from the liquid phase these triple alloys usually solidify in the disordered A2 phase characterized by an arbitrary occupation of every site in the crystal lattice. In principle, the chemical ordering in the solid state of Heusler alloys is possible either through an intermediate partially ordered B2 0 phase or directly to the completely ordered body-centered cubic L2 1 phase [3]. In the B2 0 phase Ni atoms order while Mn and Ga atoms occupy their sites in the crystal lattice randomly. However, the neutron diffraction measurements as well as the differential thermal analysis [2] of the Ni-Mn-Ga system presented no clear evidence for the presence of the A2-B2 0 phase transition, meaning presumably that these alloys solidify in the partially ordered B2 0 phase. For the stoichiometric composition of Ni 2 MnGa the L2 1 ! B2 0 phase transition temperature is 1071 K.At further cooling, the Ni 2þx Mn 1Àx Ga Heusler alloys undergo a structural phase transition from the body-centered cubic phase to the body-centered tetragonal (c=a ¼ 0:94) martensitic phase, characterized by pronounced effects of shape memory and superelasticity [4][5][6]. For the stoichiometric Ni 2 MnGa composition the martensitic transition temperature is equal to 202 K. This transition is preceded by a premartensitic transition at 260 K, which is the formation of static displacement waves in the lattice with the wavevector [1/3, 1/3, 0] (Refs. [7,8]). The deviation from stoichiometry in Ni 2þx Mn 1Àx Ga alloys results in merging of the premartensitic and martensitic transitions, so that the tetragonal phase appears to be modulated by static displacement waves.While being in cubic L2 1 phase, Ni 2 MnGa exhibits a ferromagnetic phase transition with the Curie temperature T C ¼ 376 K. The change of composition in the Ni 2þx Mn 1Àx Ga system results in a decrease of Curie temperature and an increase of the martensitic transition temperature until they merge at x ¼ 0:18-0.20 [9]. While the low temperature phase transitions in the Ni 2þx Mn 1Àx Ga system were studied in many aspects, the high temperature L2 1 ! B2 0 phase transition needs further investigation. In the present work we studied this transition in Ni 2þx Mn 1Àx Ga (x ¼ 0:16-0.20) by means of differential scanning calorimetry (DSC) measurements.The Ni 2þx Mn 1Àx Ga polycrystalline ingots were prepared by a conventional arc-melting method under argon atmosphere. The ingots were annealed at 1100 K for nine days in quartz ampoules and quenched in ice water. Samples for the measurements were cut from the middle part of the ingots. The measurements were done by a NETZSCH-404 high temperature differential scanning calorimeter in the temperature range from 750 to 1200 K.The results of DSC measurements for...

Determining the liquidus and ordering temperatures of the ternary NiMn-Ga and quaternary Ni-Mn-Ga-Fe/Cu alloys

“…Alloy compositions of the studied alloys, their phase transformation temperatures, the Curie points, valence electron concentrations both for the alloy in question and for its ternary Ni-Mn-Ga part. The ordering temperature for the L2 1 -B2' transformation [23] was established by observing the minor peaks or slope changes in the DSC curves of the first two measurements both during heating and cooling as well as during heating in the last measurement (insert of Figure 1). The obtained values are shown in Table 2.…”

“…These values are used for comparison of the materials since the original composition is expected to preserve as shown in [23,24]. After melting a clear shift of the transformation temperatures is observed during cooling (Table 2) due to the compositional change [23].…”

“…Furthermore, for the alloy preparation also the melting and solidification temperatures are needed. According to [21,22,23], the critical transformations for the near-stoichiometric Ni-Mn-Ga alloys are the solidificati ordered, cubic L2 1 structure. The former one can be distinguished in the differential calorimeter measurement with a clear transformation peak, while the second one causes a small change of the slope or a minor peak, which can be detected only when the material is in a well-ordered state.…”

“…The former one can be distinguished in the differential calorimeter measurement with a clear transformation peak, while the second one causes a small change of the slope or a minor peak, which can be detected only when the material is in a well-ordered state. So far, the high temperature behavior has been studied merely with the ternary alloys [21,22,23,24,25,26], and the detailed information concerning the quaternary alloys has not been available. Here, it is reported for alloys with a Fe-or Cu-addition.…”

Влияние температуры деформации осадкой на формирование мелкозернистой структуры литого сплава системы Ni-Mn-Ga

“…Это превращение протекает в сплаве вплоть до температуры 1070 K. Согласно литературным данным, в сплавах исследуемой системы в этой области темпе-ратур наблюдается фазовое превращение (при нагреве образца) упорядоченной фазы L2 1 в разупорядоченную фазу B2 [32][33][34]. Второй эндотермический пик на кривой ДСК при температуре 1426 K показывает достаточно большое поглощение теплоты.…”