Ni-Mn-Ga high temperature shape memory alloys: Function stability in β and β + γ regions
Graphical abstract
Introduction
Over the past decades, shape memory alloys (SMAs) have received considerable attention as functional materials due to their ability to recover the shape by heating and/or to exhibit superelasticity; both effects are associated with the thermoelastic martensitic transformation (MT) [1], [2]. NiTi- and Cu-based alloys are the most commonly used SMA materials [2]. These materials show MT temperatures limited to about 100 °C, whereby preventing their application at higher temperatures [3], [4], [5]. NiTi-based alloys are widely used in biomedical applications thanks to their good biocompatibility and the fact that in most of these applications the material needs to be deformed only once, as is the case of stents [5], [6], [7].
A growing interest has recently arisen in the development of high temperature SMAs (HTSMAs), working up to 550 °C, as a response to the demand from the different highly technological areas, such as automotive or aerospace industries [3], [4]. Currently, the FeMnSi-, CuAlNi-, NiMn-, NiAl-, Ti(Pt, Pd, Au)- and NiTi-based alloys systems are under intense research as HTSMAs candidates [4], [8], [9], [10], [11], [12]. While many of these systems exhibit a high temperature MT, no real breakthrough has been made yet in their actuation capability at high temperatures, due to issues like martensite stabilization [2], low strain output [7], thermal and thermomechanical instability, etc. [13], [14]. In order to meet the industrial requirements, new low cost alloys with higher operational actuation range, larger work output and stable properties are highly desirable [2], [7], [8], [9]. Among all known today HTSMAs, suitable for applications in the range of 400–500 °C, only Ti-Pd-Ni alloys with an actuation strain of about 4% seem promising, although their high cost constitutes a big disadvantage for practical implementation (see Ref. [4] and references therein).
Ni-Mn-Ga alloys have been widely studied as magnetic and non-magnetic shape memory materials [15], [16], [17], [18]. The composition-sensitive dependence of their properties [17], [19], [20], [21] makes them easily tunable materials, which can exhibit high-temperature, low-hysteresis thermoelastic martensitic transformations [22], [23], with over 9% recoverable strains [24] and good compressive and tensile superelastic behavior [25], [26]. Should these properties being combined in a single alloy, they would make it a real candidate for a highly efficient low-cost HTSMA.
The design of new Ni-Mn-Ga HTSMAs should not only focus on the increase of the MT temperature [23], [27], [28] but also on the improvement of their properties' stability [29], [30]. In the Ni-Mn-Ga phase diagram there are two regions of single phase: β (B2-ordered body-centered cubic (bcc)) and γ (Ni-rich and Ga-depleted disordered face-centered cubic (fcc)), as well as a region of a β + γ dual phase [29]. Mechanical properties [31] and thermal stability [13], [31], [32] of the alloy may strongly depend on its phase content.
Research on Ni-Mn-Ga HTSMAs has been mostly focused on improving ductility by the precipitation of second phases [33], [34], increasing MT [23], [27], [28] and studying aging effects [13], [31], [35], [36]. However, little has been done on understanding the underlying physics behind the martensite stabilization [37] or evaluating the thermal [38], [39] and thermomechanical stability [40], [41] of these alloys. Indeed, experiments on cyclic stability would be essential in order to disclose the mechanisms for aging and stable actuation at high temperature for single and dual phase samples. Furthermore, one should also emphasize that thermal treatments of the samples prior to the thermomechanical cycling, can strongly modify the physical and transformation characteristics of SMAs by changing the stabilization processes [35], [42].
The aim of this work was to elaborate Ni-Mn-Ga HTSMAs with MT higher than 400 °C exhibiting a stabilized actuation response of nearly 4% strain, after a cyclic training. Our approach to reach this goal was to explore different compositions of these alloys that show either a single β phase or a dual β + γ phase, and to study the mechanisms leading to the cycling stability (thermal and thermomechanical) in single crystal and polycrystalline forms. The influence of annealing (prior to a thermomechanical cycling) on the transformation characteristics of the single crystalline samples was also examined.
Section snippets
Experimental
Polycrystalline Ni-Mn-Ga alloys were prepared from high purity elements (Alfa-Aesar Ga 99.999% Ni 99.95+% and Mn 99.95%) by melting in an Induret compact Reitel induction furnace under argon atmosphere. The furnace chamber was evacuated and refilled with argon gas several times before melting to reduce the amount of oxygen present. The molten alloy was cast into a cold copper mold. The final compositions of the alloys were measured by EDX using a Bruker Quantax 70 detector in a Hitachi TM3000
Results and discussion
Two groups of alloys were selected in the ternary phase diagram (Fig. 1) with the aim of finding compositions with the highest possible transformation temperatures. One group of the alloys should exhibit, at room temperature a non-modulated tetragonal martensite (2M) originated from a single β phase. The second group, exhibiting a dual β + γ phase structure, should show at room temperature a mixture of the 2M martensite, resulting from the transformation of the austenite, with the precipitates
Conclusions
We have studied the cyclic behavior of four polycrystalline Ni-Mn-Ga HTSMAs. Then, we have grown two single crystals with composition pertaining to a single (β) and a dual (β + γ) phase regions on the phase diagram, which show thermo-mechanical actuation in the temperature range of 400–500 °C.
Two very distinct behaviors are observed in the thermal and thermomechanical cycling behavior of the samples. Under thermal cycling: (i) the samples from the dual (β + γ) phase regions show a decrease of
Acknowledgements
Authors would like to acknowledge the financial support from Alstom Inc. (project ANS GTA-2014-0410) and from Spanish Ministry of Economy and Competitiveness (project MAT2014-56116-C4-3-4-R). A. Pérez-Checa acknowledges a PhD grant from the Basque Government. Technical and human support provided by SGIker (UPV/EHU, MINECO, GV/EJ, ERDF and ESF), and in particular by A. Larrañaga, is also gratefully acknowledged.
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