Effect of Fe addition on transformation temperatures and hardness of NiMnGa magnetic shape memory alloys

In recent years, ferromagnetic shape memory alloys (FSMAs) have been widely investigated in many countries [1–4]. Of the developed FSMAs, NiMnGa alloys have been found to exhibit nearly 10% magnetic-fieldinduced strain which is the maximum obtained MFIS in these FSMAs [5]. Based on NiMnGa alloys, researchers have developed new ferromagnetic shape memory alloys, i.e., NiMnFeGa alloys [6, 7]. The structure, shape memory effect, and magnetic properties of Fe-doped NiMnGa alloys have been studied. It is assumed that the addition of Fe element enhances the alloy plasticity without sacrificing its magnetic and thermoelastic properties [6]. However, little work has been devoted to studying transformation behavior and the mechanical properties of polycrystalline Fe-doped NiMnGa alloys. The aim of the present letter is to study the effect of additions of Fe on transformation behavior and the hardness of NiMnGa alloys. A range of alloys with the nominal composition of Ni48.7Mn30.1−x Fex Ga21.2 (at%) (x = 0, 2, 5, 8, 11), each weighing about 80 g, were prepared. The alloys were fabricated by arc melting under an argon atmosphere using appropriate quantities of nickel (99.99% purity), manganese (99.9% purity), iron (99.9% purity) and gallium (99.99% purity). The ingots obtained were sealed in a quartz tube under a vacuum, followed by annealing at 1123 K for 24 hrs and quenched into ice water for homogeneity. Rectangular samples 4×4×6 mm were cut from the post-treated ingots for hardness measurements. Some 10 mg samples were taken from the ingots for DSC measurements. The transformation temperatures were determined by differential scanning calorimetry using a TA-2920. The heating and cooling rates were 10 K/min. The crystal structure at room temperature was confirmed by powder X-ray diffraction using D/MAX-RA X-ray diffractometer with Cu Kα radiation. Micro Vickers hardness (HV) of eight points was measured using a Akashi HM-102 Vickers Hardness Tester. The applied load and time were 200 g and 15 s, respectively. The HV values were averaged excluding the highest and lowest values [8]. Table I summarizes the transformation temperatures measured by DSC analysis in Ni48.7Mn30.1−x Fex Ga21.2 alloys (x = 0, 2, 5, 8, 11). It can be seen, at room temperature, that the parent phase and martensite coexist in the Ni48.7Mn30.1Ga21.2 alloy and the Ni48.7Mn28.1Fe2Ga21.2 alloys, whereas in the other three alloys only the parent phase exists. The results are in agreement with X-ray diffraction results in Fig. 1. X-ray patterns depict that the cubic parent phase and tetragonal martensite coexist when x = 0, 2, while only the cubic parent phase exists when x = 5, 8, 11. As shown in Table I, the thermal hysteresis of these five alloys are −1.38, −3.30, 0.46, 2.60 and 2.20 ◦C, respectively. For the Ni48.7Mn30.1Ga21.2 alloy and the Ni48.7Mn28.1Fe2Ga21.2 alloys, the thermal hysteresis is negative. Hence Fe-doped NiMnGa alloys exhibit very narrow thermal hysteresis which means that the chemical energy of the phase transformation is very small. The narrow thermal hysteresis also shows that the driving force for inducing the martensitic transformation in Fe-doped NiMnGa alloy is very small. Wayman [9] divided thermal transformations into two types: the first is Af > As > Ms > Mf while the second is Af > Ms > As > Mf. From Table I it can be seen that the first two alloys belong to the second type of thermal transformation, while the other three alloys belong to the first type of thermal transformation. In addition, only single forward and reverse transformation are determined by DSC curves. Fig. 2 depicts the phase transformation temperatures vs. Fe content for Ni48.7Mn30.1−x Fex Ga21.2 alloys (x = 0, 2, 5, 8, 11). It can be seen that with increase of Fe content all the transformation temperatures, Ms (martensitic transformation start temperature), Mf (martensitic transformation finish temperature), As (reverse martensitic transformation start temperature), and A f (reverse martensitic transformation finish temperature), decrease. The relationship between Ms temperature and Fe content is nearly linear, varing from 26.40 ◦C for Ni48.7Mn30.1Ga21.2 without Fe to −55.00 ◦C for Ni48.7Mn19.1Fe11Ga21.2 which has the maximum Fe content. Fig. 3 exhibits the relationship between transformation temperatures and valence electron numbers per atom of Ni48.7Mn30.1−x Fex Ga21.2 alloys (x = 0, 2, 5, 8, 11) and the shape of the curves is similar to Fig. 2. The tendency is clear that all transformation temperatures decrease with increase of electronic concentration in these five alloys, and the increasing rate is also basically consistent with the change of the electronic concentration. Here, it is assumed that the number of valence electrons per atom for Ni, Mn, Fe, and Ga atoms is 10(3d84s2),7(3d54s2),8(3d64s2) and 3(3s24p1), respectively. It is believed that two factors, the conduction electronic concentrations [10] and the size factor [11],