Enhanced cyclic stability of elastocaloric effect in oligocrystalline Cu–Al–Mn microwires via cold-drawingAmélioration de la stabilité cyclique de l’effet élastocalorique dans des microfils oligocristallins Cu-Al-Mn en utilisant l'étirage à froid
Introduction
Solid-state refrigeration based on caloric materials is a promising alternative to conventional vapor compression techniques due to its high energy efficiency, eco-friendliness and cost effectiveness (Aprea et al., 2018). Caloric materials such as magneto-, electro- and mechano-caloric alloys and compounds may be driven by a magnetic, electric and stress field respectively. When the applied uniaxial stress or hydrostatic pressure changes its intensity adiabatically, a temperature variation due to elastocaloric (eCE) or barocaloric (bCE) effect occurs (Moya et al., 2014; Mañosa and Planes, 2017), respectively.
Shape memory alloys (SMAs) are considered as promising working materials for eCE refrigeration systems (S. Qian et al., 2016; Ossmer and Kohl, 2016; Bruederlin et al., 2018). The energy source in the eCE originates from the stress induced martensitic transformation (MT) and inverse MT upon unloading, which can be quantified by the isothermal entropy change (ΔS) or the adiabatic temperature change (ΔT). For example, Ni-Ti alloys exhibited a large ΔT as high as 25 K related to eCE (Cui et al., 2012), which was superior to many kinds of caloric materials (Zhang et al., 2019;; X. Zhang et al., 2018; R. Zhang et al., 2018). Exploring novel and superior elastocaloric materials is a central issue for the development of eCE refrigeration technique. In the last decade, the exploration had expanded from Ni-Ti-based (Bechtold et al., 2012; Ossmer et al., 2015; Tušek et al., 2018; Aaltio et al., 2019) to Cu-based (S. Qian et al., 2016; Wang et al., 2017; Mañosa et al., 2013; Niitsu et al., 2018) and Ni-Mn-based (Tang et al., 2019; Shen et al., 2019; Li et al., 2019) SMAs. In these alloys, Cu-based SMAs showed attractive eCE because of their high thermal conductivity, giant ΔT and low cost (Xu et al., 2016; Yang et al., 2017; Bonnot et al., 2008). However, bulk Cu-SMAs showed high intergranular fracture tendency due to the stress concentration formed at grain boundaries in polycrystalline alloys (Liu et al., 2014), which limited their application as effective working refrigerants.
Single crystalline Cu-SMAs showed enhanced ductility and better eCE properties as compared to polycrystalline alloys. Mañosa et al. (Mañosa et al., 2009; Vives et al., 2011; Gràcia-Condal et al., 2018) reported that large ΔT of ~ 12.3 K over a very broad temperature span of ~ 130 K was achieved in Cu-Zn-Al single crystals. Nevertheless, the fabrication of single crystalline SMAs is difficult because of the composition segregation, low growth rate and high cost (Dutkiewicz, 1994; Dutkiewicz et al., 1995). Recently, a grain-architecture design strategy, i.e. the creation of columnar grains or oligocrystalline grains was proposed to enhance the eCE in Cu-SMAs. For example, Xu et al. (Xu et al., 2016) demonstrated Cu-Al-Mn alloys with columnar grains exhibited a large recoverable superelasticity strain of about 9% and large ΔT of 12–13 K. In addition, Sutou et al. (Sutou et al., 2005) stated that Cu-Al-Mn-based microwires with oligocrystalline bamboo grains showed lower superelastic critical stress and larger transformation strain compare to the alloys with randomly orientated grains. Such oligocrystalline grains have also been reported in Cu-Al-Ni (Chen et al., 2009; Chen and Schuh, 2011) and Cu-Zn-Al (Ueland et al., 2012; Ueland and Schuh, 2012; Ueland and Schuh, 2014) microwires. Furthermore, the orientation of grains significantly affects the transformation strain in SMAs. Studies showed that the transformation strain during superelastic cycling was high between 〈001〉 and 〈101〉 orientation, which was favorable for a high ΔT during eCE (Tuncer and Schuh, 2016; Sutou et al., 2002; Fornell et al., 2017). Although some feasible methods are easy to fabricate textured bulk material, however, there is still a challenge on controlling grain orientation in miniature-sized materials.
Besides the larger ΔT, the cyclic stability is also a bottleneck for the application of elastocaloric materials. The cyclic stability, i.e. fatigue behavior, consists of functional (degradation of ΔT and the reversibility) and structural (fracture of the material) fatigue (Chluba et al., 2015;Hou et al., 2018). Generally, a basic strategy to enhance the eCE stability is grain refinement, thus enhance the yield strength of materials. Yang et al. (Yang et al., 2019) reported that Fe and B co-doped Ni–Mn-In alloys had fine grains and stable ΔT = 5.6 K for up to 2700 cycles. Chen et al. (Chen et al., 2019) showed that nanocrystalline Ni–Ti sheet obtained via cold-rolling exhibited a stable ΔT of 23 K for 100 cycles. Therefore, the grain refinement in Cu-based microwires may be a feasible method to enhance the cyclic stability of eCE cooling.
Currently, the developments in elastocaloric materials especially the micro- or nano- scale SMA films or microwires marked the possibility of new cooling and heat pumping systems for applications at the miniature scale (Bruederlin et al., 2018; Ossmer and Wendler, 2016). The SMAs with thin geometry provided high surface-to-volume ratio for micro-cooling applications such as in medical applications (Petrini and Migliavacca, 2011; Carmo et al., 2011), microelectronic devices (Fujita and Toshiyoshi, 1998), smart sensors and lab-on-a-chip systems (El-Ali et al., 2004), where the integration of various functionalities in small spaces lead to highly localized generation of heat requiring active temperature control (Greco et al., 2019). There are many obstacles need to overcome to achieve these goals, such as design of materials and engineering structure. It is mandatory to fabricate such appropriate materials which has large eCE ability and high fatigue life. On the other hand, the separation of cold and heat region will also a challenge for design and fabrication technologies at small scales (Bruederlin et al., 2017).
In this work, we focused on the issue of high performance elastocaloric materials. The novel Cu-Al-Mn microwires of diameter 130 µm and 80 µm with grain size of ~ 112 µm and 70 µm respectively were prepared via a combined technique of Taylor-Ulitovsky method and multi-step cold-drawing. Preferential 〈101〉-orientated grains along the wire axis was obtained by controlling texture through this combined technique. A large but unstable ΔT = 4.5 K was obtained in the as-drawn microwire with diameter 130 µm. When the wire was further drawn to a diameter 80 µm, a stable ΔT = 3.0 K was achieved for up to 275 cycles. The underlying positive effect of cold-drawn on eCE cyclic stability can be attributed to the enhanced yield strength due to the reduced grain size and cold work hardening via cold-drawing. The present Cu–Al–Mn microwires may have a great potential in micro- or nano-size elastocaloric cooling applications due to the low cost, which is easy to commercialize compare to the Ni–Ti alloy. In addition, this criterion is also applicable for other similar elastocaloric materials, showing good practical application value.
Section snippets
Sample preparation
The parent ingots with nominal composition Cu71Al18Mn11 (atomic percent) were prepared by vacuum induction melting with pure Cu (99.99%), Al (99.99%) and Mn (99.99%) under argon atmosphere and casting into a copper mold. The cast ingots were cut into φ3 × 4 mm3 by electron discharge machining (EDM), which were used to prepare rods with diameter 260 µm and 130 µm by a Taylor-Ulitovsky method (Yuan et al., 2019). The as-prepared rods were placed in a quartz tube, evacuated, back-filled with argon
Grain morphology and orientation of the microwires
The supplementary material Fig. S1 shows the morphology of the grains in the annealed and as-drawn microwires. It is seen that cold-drawing is effective in grain refinement. The 130 µm annealed microwire (Fig. S1a) shows coarse bamboo-grains, with an average grain length ~ 345 µm measured by a linear intercept method (Sutou et al., 2005). By contrast, the 130 µm as-drawn microwire (Fig. S1b) has an average grain size ~ 112 µm. When the wire was further drawn to a diameter 80 µm (Fig. S1c),
Further perspective
Table 2 summarized the key issues for excellent elastocaloric materials and the current situation of Cu-Al-Mn microwires in this paper. As can be seen, the Cu-Al-Mn microwires are basically meet the requirements of the excellent elastocaloric materials, especially showing low driving force, excellent heat transfer performance and giant commercial potential, but the fatigue life needs further improvement. Not only the microwires in this work, the vast majority of elastocaloric materials still
Conclusions
The oligocrystalline Cu70.3Al18.4Mn11.3 microwires were prepared by a combined technique of Taylor–Ulitovsky method and cold-drawing. The textured microwires can be obtained by multi-step cold-drawing and most of grains were gradually turn to 〈101〉 direction along the wire axis. The elastocaloric effect (eCE) of annealed and as-drawn microwires was studied by indirect superelastic tests and direct temperature change measurements. By controlling the grain orientation, the elastocaloric cycling
Declaration of competing interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors greatly acknowledge the financial supports from National Key R&D program of China (Grant Number 2017YFB0703103), National Natural Science Foundation of China (NSFC) (Grant Number 51701052), China Postdoctoral Science Foundation (grant number 2017M620114, 2019T120262), Heilongjiang Provincial Natural Science Foundation of China (grant number QC2018062), Heilongjiang Provincial Postdoctoral Science Foundation of China (grant number LBH-Z17085).
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