Martensitic transformation and mechanical properties of NiMnGaV high-temperature shape memory alloys
Introduction
In the last decade, the Heusler NiMnGa alloy has attracted much attention due to its large magnetic field induced strain and high response frequency [1], [2], [3]. Meanwhile, some Ni-rich Ni2MnGa alloys were developed as promising high-temperature shape memory alloys (HTSMAs) with well-defined shape memory effects, good superelasticity, high thermo-cycling stability and relatively low cost [4], [5], [6], [7], [8], [9]. These excellent properties made NiMnGa alloys be more promising than other HTSMAs, such as TiNi-Hf, CuAlNi and TiNiPd alloys.
It is known that the polycrystalline NiMnGa shape memory alloys (SMAs) are extremely brittle although their single crystals exhibit very high mechanical strain, about 20% [6], [10]. Grain refinement is an effective method to improve the mechanical and shape memory characteristics of the polycrystalline Ni54Mn25Ga21 SMAs [10]. In addition, in order to improve the ductility of NiMnGa alloy, there has also been growing interest in adding the fourth element, such as Fe, Co, Cr, Al, Ti, Cu, Nb, B or rare elements [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]. Adding appropriate amount of Fe, Co, Cr or Cu into NiMnGa alloy can improve the ductility and hot-workability by introducing ductile secondary phase, while the shape memory effect decrease with increasing volume fraction of secondary phase [11], [12], [13], [14]. On the other hand, Nb-doping is not effective in improving the ductility of NiMnGa alloy [15]. Cu or Al addition in NiMnGa alloys can raise the strength and plasticity due to the structural transition of the martensitic phase or solid-solution strengthening without secondary phase precipitation [16], [17]. The rare-earth Gd or Y substitution for Ga in NiMnGa alloys can refine the grains significantly and cause the Gd-rich or Y-rich phase formation, which can enhance the compressive strength and the ductility [18], [19]. A new martensitic transformation sequence of T-7M is found during the deformation process in Ni54Mn25Ga20.9Gd0.1 alloy [20]. Boron addition can also refine the grain and enhance the mechanical properties [21]. The hard-brittle second phase forms when the Gd or B content is very low, which can remarkably decease the compressive ductility and shape memory properties of NiMnGa alloys [21], [22]. Besides the microstructure and mechanical properties, the fourth element also has a great influence on martensitic transformation behavior. Adding Fe, Co, Cr, Al, B, Si and In decrease the martensite transformation temperatures of NiMnGa alloys [11], [12], [13], [21], [23], [24], [25], whereas Cu, Gd, Y and Ti additions can increase the MT temperatures [14], [16], [18], [19], [26]. Cr and Cu addition can expand the transformation hysteresis of the NiMnGa alloys [13], [16].
Therefore, the fourth element has a strong influence on the microstructure and properties of NiMnGa SMAs. However, little information about the addition of V into NiMnGa SMAs has been reported up to now. To our knowledge, only Min et al. and Endo et al. have studied the influence of substituting V for Mn in NiMnGa alloys on the phase transitions and the magnetic characteristics [27], [28]. They found that the Curie temperature and the transition temperatures was decreased with increasing V content [27], [28]. The aim of this work is to investigate the microstructure, transformation behavior, mechanical properties and shape memory effect of the Ni56Mn25Ga19-xVx (x = 0, 1, 2, 4, 6) high-temperature shape memory alloys to show the effect of V addition on the microstructure and properties of NiMnGa SMAs.
Section snippets
Material and methods
The polycrystalline button ingots with the nominal composition of Ni56Mn25Ga19-xVx (x = 0, 1, 2, 4, 6) alloys were prepared with high purity elements Ni (99.9%), Mn (99.7%), Ga (99.99%) and V (99.9%) by arc melting four times under argon atmosphere. The button ingots sealed in vacuum quartz tubes were then homogenized at 1000 °C for 24 h, and then quenched into water. Some slices with 1 mm thickness were cut down from the as-quenched alloys by an electro-discharge machine for microstructure
Microstructures
The XRD patterns of the as-quenched Ni56Mn25Ga19-xVx (x = 0, 1, 2, 4, 6) alloys at room temperature were shown in Fig. 1a. When x = 0 and x = 1, all patterns can be indexed with the non-modulated tetragonal martensite phase (denoted as M). When x ≥ 2, in addition to the seven diffraction peaks from martensite phase, three additional peaks appear, as indicated in the Fig. 1a. The new phase can be confirmed as a face-centered cubic (fcc) γ phase (denoted as γ) with the main diffraction peaks of
Conclusions
The effect of V substitution on microstructure, martensitic transformation behavior, mechanical and shape memory properties of the Ni56Mn25Ga19-xVx (x = 0, 1, 2, 4, 6) alloys was investigated.
- 1)
Single phase of non-modulated martensite with tetragonal structure is present for x = 0 and x = 1, and dual phases with tetragonal martensite and face-centered cubic γ phase are observed for x ≥ 2. The volume fraction of the γ phase increase with the increase of V content up to 41 vol%.
- 2)
The transformation
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (No. 51101057) and the Fundamental Research Funds for the Central Universities of China (No. 2015MS45).
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