Rapid solidified ductile Cu-Al-Mn ribbon and its elastocaloric potential

Cu-Al-Mn alloys display martensitic transformation over a wide range of temperatures. In addition to low cost, this alloy is known for its low transformation stress with reasonable latent heat favoring elastocaloric applications. However, the ductility of Cu-Al-Mn can be limited owing to ordering and intergranular fracture. Through rapid solidification by melt spinning, we show that Cu-Al-Mn ribbon can be made highly ductile (greater than 8% tensile strain in the as-spun state and 10% tensile strain after heat treatment). The ductility of the melt-spun ribbon is related to the suppression of L21 ordering that is characterized through magnetic property measurement. Heat treatment of the ribbon promotes bamboo grain formation, and the latent heat is increased to 6.4 J g−1. Under tensile conditions, we show that the ribbon exhibited about 4 °C temperature change (4.4 °C on heating and 4.2 °C on cooling from 6.3% strain).


Introduction
Elastocaloric cooling (EC) exploits the latent heat of the reversible martensite transformation of shape memory alloys for refrigeration and space conditioning applications [1,2]. Instead of cycling between the gas and liquid phases of the refrigerant in vapor-compression technology that causes concerns on global warming, EC uses a solid crystalline material and cycles between its two different crystal structures (i.e. austenite and martensite) through mechanical loading and unloading. This gives EC the inherent advantage of being environmentally friendly. EC system has also been identified to be more efficient than the vapor-compression systems [3,4]. It is estimated that the EC system can deliver substantial energy savings to the nation with 790 TWh of energy per year and has been identified as the most promising non-vapor compression-based cooling and refrigeration technology [5].
Since the first discovery of EC effect more than 160 years ago [6], significant progress has been made in this field, including the discovery of many alloys that show prominent EC effects. The effectiveness of the EC materials can be roughly estimated by its latent heat as the associated adiabatic temperature change ∆T ad is proportional to L/C p , where L is the latent heat and C p is the heat capacity. Figure 1 summarizes the latent heat of various EC materials. Heusler alloys have low latent heat values, typically less than 5 J g −1 , except for that of Ni-Mn-Sn-Cu and Ni-Mn-Sb-Co, which have 17.3 J g −1 and 10.2 J g −1 , respectively. Copper-based alloys typically exhibit latent heat values of around 5 J g −1 . Ni-Ti alloys typically show a latent heat of 10-20 J g −1 with the highest value of 35.1 J g −1 seen in a Ni 49.8 Ti 30.2 Hf 20 alloy [7]. VO 2 has been identified as a new candidate [8], and its latent heat can be as high as 51.5 J g −1 induced by hydrostatic stress [9]. Due to the large latent heat from Ni-Ti, a large cooling effect of 17 • C was demonstrated on Ni-Ti via wire tensile experiments [10]. And a recent breakthrough in adiabatic temperature change of as high as 31.5 • C was demonstrated on a Ni-Mn-Ti alloy [11].  [1,[7][8][9][11][12][13][14][15][16][17][18][19][20][21][22]; Ni-Mn-Z are alloys where Z is Ga, In, or Sn (p-block); Ni-Mn-Ti are all-d-metal full Heusler [23] alloys.
In addition to latent heat, other factors such as materials cost and availability, transformation stress, hysteresis, ductility, and transformation temperature window can be equally important. Copper-based alloys (i.e. Cu-Al-Mn, Cu-Al-Ni) stand out due to their low cost, low transformation stress, and wide compositional-dependent transformation temperature window [24,25]. Cu-Al-Mn alloy can also be ductile through composition tuning [26,27] and processing design [28,29]. The high-temperature bcc β phase of the alloy responsible for the martensitic transformation can experience an order-disorder transition: A2 (disordered bcc Cu)-B2 (CuAl)-D0 3 (Cu 3 Al) and embrittle the alloy [26]. Increasing Mn content widens the β phase region and results in Heusler L2 1 (Cu 2 AlMn) magnetic order, while increasing Al content (which strongly affects the martensitic transformation temperature [28]) leads to a higher degree of order [26]. The order-disorder transition may be fully bypassed if the Al content is less than 18 at.% [28]. The alloy's ductility and shape memory properties are also affected by grain size and orientation. Larger β grains are desirable for improving recoverable strain due to the relaxation of grain geometrical constraints [30]. However, β alloy with coarse grains is susceptible to intergranular fracture because of its abnormally high elastic anisotropy [31]. Therefore, the microstructure of the alloy must be appropriately controlled for a balance of ductility and shape memory properties [32]. Recent studies show that columnar or bamboo (or oligocrystalline) grain is preferred for improved shape memory properties [32], cyclic stability [33], and elastocaloric effect [34][35][36] for Cu-Al-Mn alloy. However, such improved grain structure is achieved either by directional solidification [35] or repeated thermomechanical processing [32], which is cost-ineffective and energy-intensive.
Rapid solidification by melt spinning is known to bypass order-disorder transformation [37] and promotes the formation of columnar grains [38]. The process directly produces continuous thin ribbons (thickness ∼20 µm, width ∼1 mm) that may be of interest to miniature EC systems. These miniature EC systems may find various demanding applications, such as microelectronics, smart sensors/actuators, biomedicine (e.g. local cooling of biological tissue and rapid blood cooling in microsurgery), and chemical analytics (e.g. lab-on-chip system) [39,40]. In addition to miniaturization, compared to bulk EC materials, smaller-scale ribbons or foil offers a higher surface-to-volume ratio which may lead to increased power output and frequency capability [41][42][43]. The foil or ribbon may also offer improved fatigue properties as they can be immune to inclusions [44]. These ribbons may also be consolidated or put into assembly [43], enabling their use in larger-scale EC devices. However, studies on melt-spun copper-based ribbons are rare and focused mainly on the transition temperatures [45][46][47][48]. There has been renewed interest in melt-spun ribbons, and a recent work reported the microstructure and thermal properties of copper-based ribbons after heat treatment [49]. This work systematically studied the order-disorder transition, microstructure, tensile property, phase transformation characteristics, and elastocaloric potential for melt-spun Cu-Al-Mn ribbons.

Materials and methods
The Cu 72 Al 17 Mn 11 (nominal composition in at.%) ingot was prepared by arc melting of elemental Cu, Al, and Mn chunks (>99.9%) acquired from the Materials Preparation Center at Ames Laboratory. The alloy ingot was melt spun to ribbons using a custom-built melt spinner with a vacuum chamber partially filled with 1/3 of ultra-high purity helium. The quartz crucible nozzle diameter, melt shot temperature, overheat pressure, and the copper wheel (φ25 cm × 2.5 cm) speed was 0.81 mm, 1150 • C, 120 Torr, and 30 m s −1 , respectively. The annealing was done in a helium-filled quartz ampule at 900 • C for 2 h, followed by quenching in brine ice water. The subsequent 200 • C aging treatment was carried out in the air.
The cross-section microstructures (along the ribbon length direction) of the ribbons were analyzed using scanning electron microscope (Teneo, FEI Inc.) equipped with energy dispersive x-ray spectroscopy (EDS) detector. The ribbons were mounted on their side and polished and etched with 5% Nital prior to imaging. The magnetic moment of the ribbon was measured with vibrating sample magnetometer (VSM) (Versalab, Quantum Design, Inc.). Tensile tests were conducted using a universal testing machine (Zwick/Roell, zwickiLine) equipped with a laser extensometer using a strain rate of 1 × 10 −3 s −1 on a single ribbon. The phase transformation characteristics of the sample were measured by differential scanning calorimetry (DSC) (Netsch DSC 214 Polyma) on a few ribbon pieces from −150 to 150 • C with a heat/cool rate of 10 • C min −1 . Temperature changes under tensile loading/unloading at ambient temperature were measured on a single aged ribbon using a custom-built device [50]. The strain of the ribbon was measured with a displacement sensor at the grip, and the temperature of the ribbon was measured by an infrared camera (FLIR A8303sc).

Results and discussion
Figure 2(a) shows a collection of ribbons after the melt-spinning process. The ribbon has a width of 1 mm and a thickness of 20-30 µm, controlled mainly by nozzle size and wheel speed. The ribbon is continuous (tens of meters long) with excellent surface quality and can be wound onto a cylinder. The ribbon is also highly ductile as it can be bent repeatedly at 180 • with zero bend radius without fracture. The as-spun ribbon is also strong showing a yield strength (YS) of ∼400 MPa, and tensile ductility of at least 8% as in figure 2(b) (likely fails pre-maturely due to defects). The fast cooling rate achievable from the melt spinning process, which is on the order of 10 6 • C s −1 , may fully suppress the ordering transition, as discussed in the magnetic measurement below, and refine the microstructure contributing to a well-balanced strength and ductility property of Cu-Al-Mn far exceeding the properties reported [28] in the literature (typically 100-200 MPa YS and less than 10% tensile strain). The as-spun ribbon shows columnar grain microstructure with the grains aligned parallel to the thickness direction ( figure 2(c)). Such columnar gain microstructure is commonly observed in melt-spun ribbons as it follows the heat extraction from the wheel side to the free side of the ribbon [38].
However, the as-spun ribbon does not show any stress-induced martensitic (SIM) transformation at room temperature, presumably due to the large quench in vacancy density and unstable martensite formation (see discussion below under the DSC section). To facilitate SIM, the ribbon was annealed and aged. The heat treatment resulted in significant grain growth, and the formation of oligocrystalline grains ( figure 2(d)). The height of the grains extends to the ribbon's full thickness, while the width of the grains is 3-4 times the ribbon's thickness, highly resembling the cellular structure seen in bamboo. EDS confirmed the composition of the ribbons, and it matches the nominal composition. Figure 2(b) shows that the SIM has been activated for the ribbon after the heat treatment. The stress for SIM, which depends on austenite finishing (Af) temperature according to the Clausius-Clapeyron relation, for the ribbon, is ∼120 MPa, matching what is typically reported for this alloy composition [28]. Due to the pre-load applied to the ribbon necessary for accurate laser strain measurement, quantifying the recoverable strain is difficult. Still, it is at least 4% out of the 5% total strain. After the stress plateau for the SIM, there is some work hardening before the ribbon fractures. Due to the superelastic strain accommodated by the martensitic reorientation, the aged sample showed much higher tensile strain (∼10%) before failure.
The order-disorder transition between A2-B2-L2 1 in Cu 72 Al 17 Mn 11 is known to significantly affect the ductility of the alloy [26]. The L2 1 (Cu 2 AlMn) is a Heusler compound with unique ferromagnetic properties, while the other two phases are paramagnetic. VSM measurement characterizing the magnetic properties is, therefore, instrumental for evaluating the ordering degree. Figure 3(a) shows that the as-spun ribbon after high-temperature annealing and water quench is paramagnetic, confirming the disordered structure. Aging treatment between 1 min and 30 min decreases the magnetic susceptibility of the paramagnetic alloy. According to Kainuma [26], paramagnetic L2 1 forms before the ferromagnetic L2 1 from the disordered structure [26]. Therefore, paramagnetic L2 1 likely formed in the samples that were aged under 30 min. Apparently, the paramagnetic L2 1 phase has a smaller magnetic susceptibility than its parent disordered  phase. The magnetic susceptibility surpasses the as-quenched ribbon after 3 h of 200 • C aging and the ribbon becomes highly ferromagnetic after aging for 6 h, suggesting strong ferromagnetic L2 1 order. Figure 4(a) shows the transformation characteristics of the ribbon sample after different heat treatments. The as-spun ribbon exhibits no martensitic transformation in the current measurement temperature range. Hsu and Linfah [51] showed that a drop in martensite start (Ms) temperature can be significant when the cooling rate is 2500 • C s −1 . This is because large amounts of clustered quench-in vacancies form during quenching, and they are pinned to partial dislocations hindering the nucleation of martensite. In contrast, the cooling rate of melt spinning is 10 6 • C s −1 , three orders of magnitude higher than 2500 • C s −1 . Therefore, the suppression of Ms can be tremendous in the melt-spun ribbon. Subsequent 200 • C aging treatments without 900 • C annealing for the as-spun ribbon have minimum impact on the martensitic transition.
The effect of annealing and quenching on the ribbon is evident, and it brings the transition above −150 • C as shown on the DSC curve labeled as AS_ANQ in figure 4(a). Aging treatment at 200 • C for 1 min decreases the quench-in vacancy densities and results in some increase in the transition temperature. An increase in the aging time from 1 min to 5 min has resulted in a stabilized martensitic transformation near room temperature. The latent heat of the 200 • C 5 min aged ribbon is 6.4 J g −1 . In comparison, the latent heat was reported to be 4.8 J g −1 on a single crystal Cu 72 Al 17 Mn 11 alloy [13], and 6.4 J g −1 on a directional solidified Cu 71.5 Al 17.5 Mn 11 alloy [35]. It suggests that columnar or bamboo grain may be responsible for higher latent heat. Aging at 200 • C for a longer time tends to decrease the latent heat because of ordered phase formation, as confirmed by the VSM measurement discussed above. Once the ribbon becomes fully ordered with strong ferromagnetism (200 • C, 6 h aging), it does not show any martensite transformation. A second peak was observed above 100 • C for ribbon aged for longer than 10 min, which can be correlated to the Curie point for the ferromagnetic L2 1 phase as reported for similar compositions [26].
The reversibility of the martensitic transformation is confirmed on the 200 • C, 5 min aged ribbon. Figure 4(b) shows that the martensitic transition is mostly stabilized after the first heating and cooling cycle. There is still some shift in the transition temperatures in the subsequent cycles, but its magnitude is decreasing. The thermal hysteresis, austenite finishing temperature (Af) -martensite finishing temperature (Mf), is ∼55 • C for this ribbon under the current heat treatment conditions.
To demonstrate the elastocaloric effect of the ribbon, a single aged ribbon was fitted into a custom-built [50] device for tensile test. The ribbon was sprayed with matte dark paint to enhance thermal emissivity for infrared thermography and pre-loaded into the tensile grip to ensure it was fully stretched before loading ( figure 5(a)). Figures 5(b) and (c) show that the ribbon becomes hotter than the surroundings when loaded and cooler than the surroundings when unloaded, confirming its EC effect. The temperature changes in the loading and unloading states increase with higher strain. So far, the highest temperature change recorded on this ribbon was 4.4 • C on loading and −4.2 • C on unloading from a 6.3% strain before it fractures at 7%. Such temperature change is higher than the value (3.9 • C) reported on a single-crystalline Cu 72 Al 17 Mn 11 alloy under compression [13]. However, it is much lower than the adiabatic temperature change of 12.8 • C reported [35] for a columnar-grained directional solidified Cu 71.5 Al 17.5 Mn 11 alloy which was tensile strained (under a fast strain rate of 1.3 × 10 −1 s −1 ) to 10%.
The adiabatic limit for the temperature change ∆T achievable is estimated by ∆T = L/C p , where L is the latent heat, and C p is the heat capacity. Using L = 6.4 J g −1 from the DSC results and C p = 0.44 J g −1 • C at 20 • C as determined by physical property measurement system (DynaCool, Quantum Design), the ∆T is expected to be 14.5 • C. This theoretical ∆T is significantly higher than other elastocaloric materials except for NiTi. However, our current ribbon tensile demonstration only unleashed 30% of its potential. The reason for the low heat extraction can be two-fold. First, the experimental setup has a poor adiabatic condition. The strain rate (3 × 10 −2 s −1 ) is not optimized, and the EC test is highly strain rate dependent, where a higher ∆T is correlated with a higher strain rate [39]. There may also be significant heat loss to the ambient due to the large surface area of the ribbon sample. Second, the ribbon may have some thickness variation and defects (though it can be optimized through melt spinning process control), which prevents sufficient strain loading for a complete transition. The ribbon's heat treatment is not optimized, and there could be a large energy dissipation ∆W due to the friction at the residual untransformed austenite/martensite interface. Using where the density ρ = 7.4 × 10 3 kg m −3 , and the hysteresis loop area¸σ (ε) dε = 3.5 × 10 6 J m −3 according to figure 2(b), the dissipation heat ∆W is calculated to be 473 J kg −1 . This results in an irreversible temperature change because of heat dissipation ∆T dis = 0.54 • C according to the following equation, where C p = 0.44 J g −1 • C, This heat dissipation can be significant as it increases with larger strains, mainly due to the large hysteresis loop. Optimized heat treatment should minimize the stress-strain hysteresis and reduce the internal friction energy loss [11].

Conclusion
Melt spinning was used to prepare a strong and highly ductile continuous Cu-Al-Mn ribbon. The ribbon has a favorable columnar grain structure in the as-spun state and is transformed into a bamboo grain structure after heat treatment. The rapid solidification inherent to the melt spinning suppressed the deleterious order-disordered transition responsible for the alloy's brittleness. The martensitic transformation is temporally suppressed in the melt-spun ribbon but can be reactivated by annealing and aging. Heat-treated ribbon shows the highest latent heat of 6.4 J g −1 , much higher than what is typically reported (4-5 J g −1 ) for this alloy composition. Such latent heat is expected to deliver 14.5 • C temperature change under adiabatic conditions. The ribbon was demonstrated (though under less ideal adiabatic and non-optimized heat treatment conditions) to display a temperature change of 4.4 • C on loading and −4.2 • C on unloading, showing great potential for low-cost elastocaloric applications.

Data availability statement
The data cannot be made publicly available upon publication because they are not available in a format that is sufficiently accessible or reusable by other researchers. The data that support the findings of this study are available upon reasonable request from the authors.