Phase transformation, thermoelastic and magnetic properties of Ni50Mn30Ga20−xCux ferromagnetic shape memory alloys

Materials Science and Engineering B 178 (2013) 857–862

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Phase transformation, thermoelastic and magnetic properties of Ni50 Mn30 Ga20−x Cux ferromagnetic shape memory alloys G.F. Dong a,c,∗ , L.P. Gai b,∗∗ , X.L. Zhang a a b c

Department of Physics, Dalian University, Dalian 116622, China Department of Physics, Dalian Medical University, Dalian 116044, China Dalian University of Technology, College of Physics and Optoelectronic Engineering, Dalian 116024, China

a r t i c l e

i n f o

Article history: Received 8 November 2012 Received in revised form 5 March 2013 Accepted 16 April 2013 Available online 14 May 2013 Keywords: Intermetallics Martensitic transformations Magnetic properties Thermal properties

a b s t r a c t Effect of addition of Cu on phase transformation temperatures, enthalpy and entropy changes, Curie temperature, magnetization saturation were investigated in Ni–Mn–Ga ferromagnetic shape memory alloy. The results show that the Ni50 Mn30 Ga20−x Cux alloys exhibit thermoelastic martensitic transformation. The martensitic and reverse martensitic transformation temperatures, enthalpy and entropy changes and thermal hysteresis increase with increase of Cu content. Martensite structure changes from 7 M with 0–0.5 at.% Cu to non-modulated T martensite when the content of Cu is more than 0.5 at.%. In addition, the Curie temperature almost remains unchanged at low-Cu content and subsequently decreases obviously. Magnetization saturation of alloys decrease with increasing Cu content since it is sensitive to ordered atomic arrangement. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.

1. Introduction Shape memory alloys (SMAs) are attractive smart materials which have an intrinsic ability to recover their initial configuration upon phase transformation temperature [1]. During the past few decades, great efforts have been find to investigate various aspects of SMAs [2–5], owing to their wide applications in medical, aerospace and marine industries [1]. However, the working frequency is relatively low due to the conventional thermally controlled SMAs. This greatly limits their practical applications in certain fields. The newly developed ferromagnetic shape memory alloys (FSMAs) [6–15] integrate the advantages of both conventional SMAs [1–5] and magnetostrictive materials [16], showing giant magnetic-field-induced-strain (MFIS) and high respond frequency [17,18], which makes them extremely promising for use in sensors, actuators, proportional fluid valves and linear motors. Ni–Mn–Ga alloys are the most intensively studied FSMAs, because they exhibit large magnetic-field-induced strain (MFIS) as a result of magnetic-field-induced twin boundary motion [19–30]. However, practical application of Ni–Mn–Ga alloy is limited due to its extreme brittleness and low strength. Improving the mechanical properties has become a priority in the development of Ni–Mn–Ga

∗ Corresponding author at: Department of Physics, Dalian University, Dalian 116622, China. Tel.: +86 411 87402712; fax: +86 411 87403963. ∗∗ Corresponding author. Tel.: +86 411 87402712; fax: +86 411 87403963. E-mail addresses: [email protected] (G.F. Dong), gai [email protected] (L.P. Gai).

alloys. Recently, several rare elements and Fe, Co have been added to ternary Ni–Mn–Ga alloys to improve the mechanical properties by the introduction of a ductile phase. As is well known, Copper is an elements with excellent ductility, consequently, we try to substitute Cu for Ga in Ni–Mn–Ga alloy to improve the mechanical properties. Very recently, we reported that the highest compressive strength of 3178.9 MPa is obtained in Ni50 Mn30 Ga16 Cu4 alloy, which is about 2900 MPa higher than that of the Ni50 Mn30 Ga20 alloy [31]. Up to now, this is the highest compressive strength reported in the Ni–Mn–Ga alloy system. Therefore, Cu doping can significantly enhance the compressive strength and improve the ductility of Ni–Mn–Ga alloy. So far, however, there have been no results published concerning the effect of Ni–Mn–Ga doping with different Cu content on the magnetic properties. In this work, we significant investigate the effect of Cu addition on the microstructure, phase transformation behavior, thermoelastic and magnetic properties of polycrystalline Ni50 Mn30 Ga20−x Cux alloys, and the main purpose is to identify the martensitic transformation behavior, microstructure changes, thermoelastic and magnetic properties. 2. Experimental The nominal composition of the alloy Ni50 Mn30 Ga20−x Cux (x = 0, 0.2, 0.5, 1, 2, 4) was prepared with high purity elements melted by melting four times in an arc-melting furnace under an argon atmosphere. The melted alloys were casted into a chilled copper mold to obtain a master rod with a dimension of 10 mm in diameter and 70 mm in length. The master rod was sealed in a

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Fig. 1. Optical micrograph of the Ni50 Mn30 Ga20−x Cux (x = 0, 0.2, 0.5, 1, 2, 4) alloys. (a) x = 0; (b) x = 0.1; (c) x = 0.5; (d) x = 1; (e) x = 2; (f) x = 4.

quartz tube under a vacuum, then annealed at 850 ◦ C for 24 h and quenched into ice water for homogeneity. The phase transformation temperatures are determined by Perkin-Elmer diamond differential scanning calorimetry (DSC), the rate of heating and cooling is 10 ◦ C/min. X-ray diffraction (XRD) measurements were performed as a means of crystal structure determination using a Rigaku D/max-rB with Cu K␣ radiation. The microstructures morphologies were observed by optical microscopy. Samples for optical observation were mechanically polished and etched in a solution consisting of 20% nitric acid and 80% methanol. The Curie temperatures of the alloys were measured by AC susceptibility. Saturation magnetization measurements were taken using the physical property measurement system (Quantum Design) in an applied field up to 5 T.

3. Results and discussion The optical images in Fig. 1 show representative micrographs of Ni50 Mn30 Ga20−x Cux (x = 0, 0.2, 0.5, 1, 2, 4) alloys at room temperature. As can be seen from Fig. 1, all samples have typical lath-martensite morphology, yet in this alloys precipitates are not visible. It can be concluded that the Ni50 Mn30 Ga20−x Cux alloys exhibit monophase structure. In addition, it is found that the size of the grain decreases with the increasing Cu contents. The martensitic variants exhibit configurations with self-accommodation arrangement, while the Cu content increased to 4 at.%, although there is no obvious change in the martensitic variants. The XRD patterns for all samples were collected at RT. Fig. 2a shows the RT XRD patterns of Ni50 Mn30 Ga20 and Ni50 Mn30 Ga16 Cu4 samples. The main matrix phases in the both samples are martensites. The martensitic phases of these alloys changes from orthogonal structure 7 M to tetragonal non-modulated T martensite at room temperature according to the patterns. In order to confirm the structure of both alloys, transmission electronic microscopy (TEM) is also carried out, and the results are shown in Fig. 2b–d. It can be seen from the SADPs that the martensitic structure of both alloys can be indexed within orthogonal structure 7 M and tetragonal non-modulated T martensite, respectively. It is consistent with the XRD and OM results above. The lattice parameters of Ni50 Mn30 Ga20−x Cux (x = 0, 0.2, 0.5, 1, 2, 4) alloys gradually increase with an increase of Cu content, as shown in Fig. 2f. Here, the partial replacement of Ga by Cu results in a non-modulated martensite. This is attributed to the smaller Cu atom substituting

for the larger Ga atom, as the same martensitic structure induced by Ni or Mn substituting for Ga in Ni–Mn–Ga SMAs [32]. Fig. 3a summarizes the effect of Cu content on transformation temperatures and thermal hysteresis in Ni50 Mn30 Ga20−x Cux (x = 0, 0.2, 0.5, 1, 2, 4) alloys. The martensitic transformation start temperatures (Ms ), martensitic transformation finish temperature (Mf ), reversible martensitic transformation start temperature (As ) and reversible martensitic transformation finish temperature (Af ) of Ni50 Mn30 Ga20−x Cux (x = 0, 0.2, 0.5, 1, 2, 4) are summarized in Fig. 3a. It can be seen that martensite transformation temperatures (Ms , Mf , As , Af ) gradually increase with the increasing Cu content. It is seen that the thermal hysteresis increased with increasing the content of Cu in Fig. 3. As the Cu content increases from 0 to 1 at.%, the thermal hysteresis approximately increase linearly. When the content of Cu more than 1 at.%, the thermal hysteresis are increase slowly and still has a tendency to increase. Wang and Jiang [32] reported that the thermal hysteresis in the Ni–Mn–Ga ternary shape memory alloys originates from the friction of phase boundary motion. Thus, it is speculated that the Cu-doping increase the friction losses of phase boundary motion in the Ni50 Mn30 Ga20−x Cux alloys. The typical DSC curve of Ni50 Mn30 Ga16 Cu4 alloy exhibits an exothermic peak on cooling and an endothermic peak during heating, corresponding to the martensitic and reverse transformations, respectively, as shown in Fig. 3b. This implies that the ratio of Mn and Ga increases when the content of Ni appears to be approximately unchanged, which can be accounted for the increase of martensitic transformation temperatures. This is consistent with the results obtained by Wang [33]. The enthalpy changes H and entropy changes S of Ni50 Mn30 Ga20−x Cux alloys achieved from the DSC curves are shown in Fig. 3c. It can be seen that the enthalpy changes H and entropy changes S of Ni50 Mn30 Ga20−x Cux alloys firstly rapidly increase then slowly increase with the increase in Cu content. The phase transformation temperatures and H are obtained by marking the DSC curves with tangent method using the software of DSC [34] equipment. S is calculated by H/Tm , where Tm = (Af + Ms )/2. Enthalpy and entropy changes increase with the increase the content of Cu. With the increase in Cu content, the degree of ordered alloy decrease, leading to an increase in both the internal energy and the latent heat being released during phase transformation. The Curie temperatures firstly almost unchanged and then rapidly decrease, and finally slowly decrease with the increase of Cu content, as shown in Fig. 4a. It can be seen from Fig. 4a

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Fig. 2. (a) XRD patterns of the Ni50 Mn30 Ga20 and Ni50 Mn30 Ga16 Cu4 alloys; (b) bright field image and the corresponding SADPs of (c) from area A of the Ni50 Mn30 Ga20 alloy; (d) bright field image and the corresponding SADPs of (e) from area B of the Ni50 Mn30 Ga16 Cu4 alloy; (f) effect of Cu content on the lattice parameters of Ni50 Mn30 Ga20−x Cux (x = 0, 0.2, 0.5, 1, 2, 4) alloys.

that increasing Cu content has an effect on Curie temperature. In addition, there is only one abrupt change of the curves of AC susceptibility during the heating process, as shown in Fig. 4b–g. According to the DSC results, the martensitic transformation of these alloys takes place more than 350 K. Therefore, it is confirmed that the abrupt change in Fig. 4b–g corresponds to the Curie temperature of the martensitic phase. As is well known, the chemical

composition of material significantly affects the Curie temperatures of Heusler-type Ni–Mn–Ga ferromagnetic shape memory alloy. Thus it is reasonable to believe that the content of matrix is changed by partial substitution of Cu for Ga. However, our experimental data is not sufficient to make an unambiguous conclusion about the mechanism responsible for the change of Tc with increase content Cu, and further investigation is still needed.

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Fig. 3. (a) Effect of Cu content on transformation temperatures and thermal hysteresis of Ni50 Mn30 Ga20−x Cux (x = 0, 0.2, 0.5, 1, 2, 4) alloys. (b) DSC curve of Ni50 Mn30 Ga16 Cu4 alloy. (c) Effect of Cu content on enthalpy changes H and entropy changes S of Ni50 Mn30 Ga20−x Cux (x = 0, 0.2, 0.5, 1, 2, 4) alloys. Insert is the.

The magnetic behavior in M–H curves of Ni50 Mn30 Ga20−x Cux alloys was measured to evaluate the saturation magnetization, since its large values is recognized as favorable features to the MFIS [35]. Fig. 4 shows the representative M-H curves from 0 to 30 kOe for all Ni50 Mn30 Ga20−x Cux samples. As for samples annealed at 1073 K/24 h, the saturation magnetizations with 30 kOe are 60.2, 52.3, 50.6, 46.2, 41.8 and 38.1 emu/g for Ni50 Mn30 Ga20−x Cux (x = 0, 0.2, 0.5, 1, 2, 4), respectively. For all the alloys, the magnetization curves quickly increased until saturation for an applied magnetic field close to 5 kOe, exhibiting the typical characteristics of ferromagnetic materials. It is seen that the saturated magnetization decreased abruptly from 60.2 to 38.1 emu/g, which might be related with a different occupancy of Cu atoms. In the present

work, the Ga was chemically substituted by Cu. The atomic sites and possible alteration of the ferromagnetic coupling are still unknown, which requires further study. Additionally, according to the M–H curves, it is worth noting that with increasing Cu, samples show harder magnetization. The fact of a stronger antiferromagnetic coupling between two Mn atoms sited in two sublattices is accepted as an explanation for the magnetic deterioration of Ni–Mn–Ga alloys with increase Cu content. Comparing all samples with different Cu content, part of the reason of magnetization deterioration and magnetic hardening could be explored from the decrease itinerant electron of Ga and weakening the exchange effect of Mn–Mn atoms, resulting in the decrease in the saturation magnetization (Fig. 5).

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Fig. 4. (a) Effect of Cu content on the Tc of the Ni50 Mn30 Ga20−x Cux (x = 0, 0.5, 1, 2, 4) alloys; (b–g) the ac susceptibility curve of Ni50 Mn30 Ga20−x Cux (x = 0, 0.5, 1, 2, 4) alloys during the heating process.

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References

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4. Conclusions In conclusion, the microstructure, martensitic transformation and magnetic properties of Ni50 Mn30 Ga20−x Cux alloys were investigated. The following results were obtained. (1) The martensitic transformation temperatures, enthalpy changes, entropy changes and thermal hysteresis of the Ni50 Mn30 Ga20−x Cux sample increase with increase of Cu content. (2) The main matrix phases in the all samples are martensites and martensitic phases of these alloys changes from orthogonal structure 7 M to tetragonal non-modulated T martensite at room temperature according to the patterns. The lattice parameters of Ni50 Mn30 Ga20−x Cux alloys gradually increase with an increase of Cu content. (3) While the Curie temperature almost remains unchanged at low-Cu content and subsequently decreases obviously. Magnetization saturation of martensitic phase decreases with increasing Cu content. Acknowledgements This study is supported by Postdoctoral Science Foundation of China (Grant No. 20100481218), Natural Science Foundation of China (Grant No. 21173028) and Doctor Startup Foundation of Dalian University.

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