Trans. Nonferrous Met. Soc. China 24(2014) 126-130

Microstructure and transformation behavior of Ni₅₄Mn₂₅Ga₁₅Al₆ high-temperature shape memory alloy

Yan Xin¹, Yan Li²

1. School of Energy, Power and Mechanical Engineering, North China Electric Power University, Beijing 102206, China;

2. School of Materials Science and Engineering, Beihang University, Beijing 100191, China

Received 11 December 2012; accepted 13 April 2013

Abstract: The microstructure, martensitic transformation behavior, mechanical properties and shape memory effect of Ni₅₄Mn₂₅Ga₁₅Al₆ high-temperature shape memory alloy were investigated. By comparing with the Ni₅₄Mn₂₅Ga₂₁ alloy, the effect of Al addition on the properties of Ni-Mn-Ga alloys was analyzed. The results show that the Ni₅₄Mn₂₅Ga₁₅Al₆ alloy has a single-phase tetragonal non-modulated martensite structure with lamellar twins_. The martensitic transformation start temperature of this alloy is up to 190 °C, displaying the promising application as a high-temperature shape memory alloy. Al addition in Ni-Mn-Ga alloy can decrease the martensitic transformation temperatures due to the effect of size factor and improve the strength and plasticity. However, the shape memory effect is reduced remarkably with the Al addition.

Key words: shape memory alloy; Ni-Mn-Ga; Al addition; martensitic transformation

1 Introduction

In the last decade, Ni₂MnGa ferromagnetic shape memory alloys have been extensively explored as candidates for magnetic actuator materials due to their large magnetic-field-induced strains [1-4]. Meanwhile, these alloys also undergo a thermoelastic martensitic transformation under cooling. It has been demonstrated that the martensitic transformation temperature of the off-stoichiometric Ni₂MnGa alloys is very sensitive to the composition ranging from liquid helium temperature up to over 350 °C [5,6]. Accordingly, some Ni-rich Ni₂MnGa alloys were developed as promising high temperature shape memory alloys with well-defined shape memory effects [7-9], good superelasticity [10] and high thermo-cycling stability [11]. These excellent properties and relatively low cost made Ni-Mn-Ga alloys more promising than other high-temperature shape memory alloys, such as Ti-Ni-Hf, Cu-Al-Ni and Ti-Ni-Pd alloys.

It is known that the polycrystalline Ni-Mn-Ga alloys are extremely brittle although their single crystals exhibit very high mechanical strain of about 20% [7]. The rod sample prepared by LI et al [8] showed that grain refinement is an effective method to improve the mechanical and shape memory characteristics of Ni₅₄Mn₂₅Ga₂₁ alloys. The compressive strength and compressive strain of the button sample are 440 MPa and 10%, respectively, and the corresponding values of the rod sample are 970 MPa and 16%, respectively. On the other hand, many efforts have been made to improve the plasticity of Ni-Mn-Ga alloy by adding the fourth element, such as Fe, Ti, Co, Cu, Cr, Gd or rare elements [12-20]. The ductility has been improved by introducing sufficient amount of ductile γ phase in the Ni-Mn-Ga alloys with Fe, Co, Cu or Cr additions [12,14,16-18]. However, little information about the addition of Al into Ni-Mn-Ga alloys has been reported up to now. The purpose of this work is to investigate the microstructure, transformation behavior, mechanical properties and shape memory effect of Ni₅₄Mn₂₅Ga₁₅Al₆ alloy to show the effect of Al addition on the properties of Ni-Mn-Ga alloys.

2 Experimental

The polycrystalline button ingots with the nominal composition of Ni₅₄Mn₂₅Ga₁₅Al₆ and Ni₅₄Mn₂₅Ga₂₁ alloy were prepared with high-purity Ni (99.9%), Mn (99.9%), Ga (99.99%) and Al (99.99%) by melting 4 times in an arc-melting furnace under argon atmosphere. Ni₅₄Mn₂₅Ga₂₁ alloy was prepared for comparison. Some button ingots of these alloys were re-melted into rods of approximately 6.8 mm in diameter using a cylindrical copper mold which was set at the bottom of the furnace. The rod ingots sealed in vacuum quartz ampoules were annealed at 1000 °C for 24 h followed by water quenching. Some slices with 1 mm thickness were cut down from the as-quenched rod alloys by an electro-discharge machine for microstructure observation and phase structure analysis. The samples for phase transformation measurement were cut down from the slices by a slow-speed diamond saw.

The microstructures were observed by optical microscopy (OM) and transmission electron microscopy (TEM, Hitachi H-800 operating at 200 kV). Samples for OM observation were mechanically polished and chemically etched in a solution of 4 g CuSO₄ + 20 mL hydrochloric acid (HCl) + 20 mL H₂O. The phase structure was identified at room temperature by X-ray diffraction (XRD, Rigaku D/Max 2200PC diffractometer with Cu K_α radiation). The phase transformation temperatures of the alloy were measured by differential scanning calorimetry (DSC, Perkin-Elmer DSC-7) with a cooling/heating rate of 10 °C/min.

From the middle part of the rod alloy, some samples with dimensions of d3 mm×5 mm were also cut for mechanical property testing. Compressive stress-strain curves were tested by a MTS-800 master material tester at room temperature. The shape memory effect of the samples was measured after unloading. The height of the samples was measured before loading (h₀), after loading (h₁) and after heating to 400 °C for 15 min (h₂) by a micrometer with an accuracy of 0.01 mm. The shape memory strains were obtained as: (h₂-h₁)/h₀ ×100%. The recovery ratio (R) was calculated as: (h₂-h₁)/(h₀-h₁)× 100%.

3 Results and discussion

3.1 Microstructure

The powder XRD patterns of the as-quenched Ni₅₄Mn₂₅Ga₁₅Al₆ and Ni₅₄Mn₂₅Ga₂₁ alloys are shown in Fig. 1. The reflection peaks in the XRD pattern of Ni₅₄Mn₂₅Ga₁₅Al₆ alloy can be indexed with the unique tetragonal non-modulated martensite phase, identical to that of the single-phase Ni₅₄Mn₂₅Ga₂₁ alloy. Therefore, substituting Al for Ga in Ni-Mn-Ga alloy doesn’t introduce any new phase, which is different from the Ni-Mn-Ga alloys with Fe, Co, Cu, Cr, Gd additions [12,14,16-19]. By calculating from the XRD results, the crystal lattice parameters of martensite phase in Ni₅₄Mn₂₅Ga₁₅Al₆ alloy are a=b=0.7664 nm, c=0.6631 nm, c/a=0.8652 and the corresponding unit-cell volume is 0.3895 nm³. In comparison, the crystal lattice parameters of martensitic phase in Ni₅₄Mn₂₅Ga₂₁ alloy are a=b=0.7656 nm, c=0.6633 nm, c/a=0.8664 and the corresponding unit-cell volume is 0.3888 nm³. It can be seen that the crystal lattice parameter c almost has no change but the a parameter increases with substituting Al for Ga in the Ni₅₄Mn₂₅Ga₂₁ alloy, which leads to a reduction in tetragonality (c/a). Simultaneously, substituting Al for Ga in Ni-Mn-Ga alloy expands the unit-cell volume.

Fig. 1 Powder XRD patterns of Ni₅₄Mn₂₅Ga₁₅Al₆ and Ni₅₄Mn₂₅Ga₂₁ alloys

The XRD measurement result can also be confirmed by microstructure observation. Figure 2 shows the optical micrograph, TEM images and selected area electron diffraction (SAED) pattern of the as-quenched Ni₅₄Mn₂₅Ga₁₅Al₆ alloy. It can be obviously observed in Fig. 2(a) that this alloy has a single martensite phase structure with lamellar twins. The TEM bright-field image in Fig. 2(b) shows that the martensite variants are twinned and well self-accommodated. Figure 2(c) is the enlargement of the framed area B in Fig. 2(b), which shows that each variant has high-density inner microtwins. The corresponding SAED pattern of the circle area A in Fig. 2(b) is shown in Fig. 2(d). Twin-related double sets of reflections are observed in this SAED pattern, which can be identified as a tetraganal non-modulated martensite structure, as shown in Fig. 2(d).

3.2 Martensitic transformation behavior

Figure 3 shows the DSC curves of Ni₅₄Mn₂₅Ga₁₅Al₆ alloy. The endothermic peak on the heating curve is associated with the reverse martensitic transformation from tetragonal martensite to cubic austenite, where the reverse martensitic transformation start temperature (A_s), peak temperature (A_p) and finish temperature (A_f) are 202, 210 and 214 °C, respectively. The exothermic peak occurring on the cooling curve indicates the forward martensitic transformation with the start temperature (M_s), peak temperature (M_p) and finish temperature (M_f) of 190, 184 and 179 °C, respectively. The corresponding temperatures of A_s and M_s for Ni₅₄Mn₂₅Ga₂₁ alloy are 273 and 260 °C, respectively. Therefore, Al addition in Ni-Mn-Ga alloy can reduce the martensitic transformation temperatures. The transformation temperature (M_s) of the Ni₅₄Mn₂₅Ga₁₅Al₆ alloy is still as high as about 190 °C, much higher than room temperature, which means this alloy can be used as a high-temperature shape memory alloy. It is acknowledged that the electron concentration of martensite and the size factor (the unit-cell volume and the tetragonality c/a) can affect the transformation temperatures of the Ni-Mn-Ga based alloys [18,21]. Since Al and Ga belong to the same family with the same number of outer electrons, no electron concentration changes during substituting Al for Ga in Ni-Mn-Ga alloy. Thus, the decrease of the transformation temperatures by substituting Al for Ga is attributed to the size factor changes, that is, the reduction of tetragonality (c/a) and expansion of the unit-cell volume, as obtained from the XRD results in section 3.1. A smaller tetragonality (c/a) of martensite indicates less lattice deformation and less energy variation during the martensitic transformation, which leads to lower martensitic transformation temperatures. These results are consistent with the previous works [15,16,18,21].

Fig. 2 Microstructures and SAED pattern of Ni₅₄Mn₂₅Ga₁₅Al₆ alloy

Fig. 3 DSC curves of as-quenched Ni₅₄Mn₂₅Ga₁₅Al₆ alloy

3.3 Mechanical and shape memory properties

The compressive stress-strain curves of the Ni₅₄Mn₂₅Ga₂₁ and Ni₅₄Mn₂₅Ga₁₅Al₆ alloys are shown in Fig. 4, in which Curve a and Curve b correspond to Ni₅₄Mn₂₅Ga₂₁ and Ni₅₄Mn₂₅Ga₁₅Al₆ alloys, respectively. These two curves were obtained by compressing the sample until it cracked. The compressive fracture strength and strain of the Ni₅₄Mn₂₅Ga₂₁ alloy are 873 MPa and 14.3%, respectively, and the corresponding values of the Ni₅₄Mn₂₅Ga₁₅Al₆ alloy are 1027 MPa and 15.8%, respectively. Therefore, Al substitution for Ga can improve the fracture strength and plasticity of Ni-Mn-Ga alloy due to the solid solution of Al atoms, which is similar to the Ti-Ni-Al alloy [22].

Fig. 4 Compressive stress–strain curves of Ni₅₄Mn₂₅Ga₂₁ (a) and Ni₅₄Mn₂₅Ga₁₅Al₆ (b) alloys

In order to investigate the shape memory effect, the Ni₅₄Mn₂₅Ga₁₅Al₆ and Ni₅₄Mn₂₅Ga₂₁ alloys were compressed at room temperature to different total strains (4%-10%). The compressive stress-strain curve at total strain of 8% for Ni₅₄Mn₂₅Ga₁₅Al₆ alloy is shown in Fig. 5 and the arrowed line represents the shape memory strain after heating to 400 °C. The residual strain after unloading is 3.6%. The shape memory strain is 1% and the recovery ratio is 22%. The shape memory strain and recovery ratio of the Ni₅₄Mn₂₅Ga₁₅Al₆ and Ni₅₄Mn₂₅Ga₂₁ alloy at different total strains are shown in Fig. 6. It can be seen that the maximum shape memory strain of Ni₅₄Mn₂₅Ga₁₅Al₆ alloy is 1% at the total strain of 6% and 8% with the recovery ratio of 36% and 22%, respectively.

Fig. 5 Compressive stress-strain curve of Ni₅₄Mn₂₅Ga₁₅Al₆ alloy with 8% total strain

Fig. 6 Shape memory strain (a) and recovery ratio (b) of Ni₅₄Mn₂₅Ga₁₅Al₆ and Ni₅₄Mn₂₅Ga₂₁ alloys

While the maximum shape memory strain of Ni₅₄Mn₂₅Ga₂₁ alloy is 4.2% at the total strain of 10% with the recovery ratio of 57%. Consequently, the shape memory effect of Ni₅₄Mn₂₅Ga₁₅Al₆ alloy is worse than that of Ni₅₄Mn₂₅Ga₂₁ alloy. Similar to adding Cu and Cr [16,18], the shape memory effect is reduced remarkably with the Al addition.

4 Conclusions

1) The Ni₅₄Mn₂₅Ga₁₅Al₆ alloy has a single-phase tetragonal non-modulated martensite structure and the martensite variants are twinned and well self- accommodated.

2) The martensitic transformation start temperature of Ni₅₄Mn₂₅Ga₁₅Al₆ is up to 190 °C, displaying the promising application as a high-temperature shape memory alloy. Al addition in Ni-Mn-Ga alloy can reduce the martensitic transformation temperatures, which is attributed to the size factor changes, that is, the reduction of tetragonality (c/a) and expansion of the unit-cell volume.

3) Al substitution for Ga can improve the mechanical properties of the Ni-Mn-Ga alloy. The compressive fracture strength and strain of the Ni₅₄Mn₂₅Ga₁₅Al₆ alloy are 1027 MPa and 15.8%, respectively. However, the shape memory effect is reduced remarkably with Al addition. The maximum shape memory strain of Ni₅₄Mn₂₅Ga₁₅Al₆ alloy is 1% at the total strain of 6% and 8% with the recovery ratio of 36% and 22%, respectively.

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Ni₅₄Mn₂₅Ga₁₅Al₆高温形状记忆合金的微观组织和相变行为

辛 燕¹，李 岩²

1. 华北电力大学 能源动力与机械工程学院，北京 102206；2. 北京航空航天大学 材料与工程学院，北京 100191

摘  要：研究Ni₅₄Mn₂₅Ga₁₅Al₆高温形状记忆合金的微观组织、马氏体相变特性、力学性能和形状记忆效应。通过与Ni₅₄Mn₂₅Ga₂₁合金对比，分析添加第四组元Al对Ni-Mn-Ga合金性能的影响。结果表明：Ni₅₄Mn₂₅Ga₁₅Al₆合金为单一的四方结构非调制马氏体相并呈片状的马氏体孪晶板条形貌。该合金的马氏体相变开始温度超过190 °C，具有发展成为高温形状记忆合金的潜力。在Ni-Mn-Ga合金中添加Al会降低马氏体相变温度，这主要归因于Al添加引入的晶格尺寸因素的改变。添加Al元素能有效提高合金的强度和塑性，但降低合金的形状记忆性能。

关键词：形状记忆合金；Ni-Mn-Ga；Al添加；马氏体相变

(Edited by Chao WANG)

Foundation item: Project (51101057) supported by the National Natural Science Foundation of China; Project (12MS10) supported by the Fundamental Research Funds for the Central Universities of China

Corresponding author: Yan XIN; Tel: +86-10-61772646; Fax: +86-10-61772383; E-mail: xinyan@ncepu.edu.cn

DOI: 10.1016/S1003-6326(14)63037-5