Studies of the effect of melt spinning on the electrochemical properties of the AB 2 Laves phase alloys

A comparative study of the effect of melt spinning on the electrochemical properties of the C14 and C15 AB2 alloys has been performed. The wheel speeds of 630, 2100, and 4100 cm/s were applied during the rapid solidiﬁcation of both alloys. The structural analysis of the formed phases was performed by X-ray powder diffraction (XRD), while their microstructural morphology was studied by scanning electron microscopy (SEM). In both alloys a tremendous grain reﬁnement due to the melt spinning process was observed: In addition, melt spinning also signiﬁcantly contributed to the morphological variation of the microstructural changes in C14 alloys which showed changes from the equiaxed grain at lower speed to the small dendrites at higher speed. In contrast to the C14 alloys, the morphological variation was not observed for the C15 alloys. Furthermore, for both C14 and C15 alloys melt-spun at 2100 cm/s the maximum capacities of 435 and 414 mAh/g were achieved, respectively. As both alloys revealed the signiﬁcant grain reﬁnement due to the melt spinning, an increase in electrochemical capacity was achieved. However, the melt spinning parameters need to be further optimized to improve poor activation behavior of the rapidly solidiﬁed alloys.


Introduction
Laves phases can adopt hexagonal MgZn 2 -(C14), hexagonal MgNi 2 -(C36) and cubic MgCu 2 -(C15) types of structures [1]. C14 and C15 type intermetallic are often utilized as negative electrodes in Nickel-Metal Hydride (Ni-MH) batteries [2]. While the C14-type alloys usually achieve higher storage capacity and excellent cycle stabilities, the C15 phases reveal the tendency to reach a better high-rate performance and are easier activated [3,4]. However, an optimization of the performance of both alloys is required to mitigate their shortcomings Zr-based AB2 type Laves phase alloys have attracted great attention due to their promising application as anode materials in Nickel-Metal Hydride (Ni-MH) batteries [5,6]. However, a poor activation behavior and insufficient cycle stability of the C14-and C15-type intermetallics, respectively, need to be overcome to allow them reaching an improved performance.
Rapid solidification is one of the techniques used to improve the properties of the alloys by modifying their microstructures, extending solid solubility limits and forming metastable constituent phases, while in turn, enhancing their hydrogen sorption properties [7]. A number of rapid solidification parameters need to be adjusted in order to reach improved performance of the rapidly solidified material. Melt spinning is one of the rapid solidification methods, where a molten metal, pushed from the crucible and ejected through the spinning drum, is rapidly cooled to form a ribbon with a specific atomic structure, which may positively affect the formed material properties [8].
In this study, a comparative overview of the melt spinning effects on C14-and C15-type alloys is presented [9,10]. The comparison included the analysis of the samples phase compositions, microstructural properties and their electrochemical performances The present study provides a broad overview of the studied melt spun inter-metallics based on which a general relationship between the applied synthesis technique and the properties of the investigated materials, is established.

Experimental method
The prepared Zr-based AB2-type intermetallics got a nominal composition close to Ti 0.15 Zr 0.85 La 0.03 Ni 1.2 Mn 0.7 V 0.12 Fe 0.12 and crystallised with the C14-and C15-type structures.
Further details concerning the chemical compositions of the studied samples can be found elsewhere [9,10]. The alloys were prepared by a rapid solidification technique with an Edmund Buehler melt spinner (Institut de Chimie de la Matière Condensée de Bordeaux (ICMCB, France)) with the wheel rotation speeds of 630, 2100, and 4100 cm/s.
The melt-spun samples were manually ground and sieved to the powders with the size of 40 -60 µm which were used for the electrochemical testing. The sieved powders of smaller than 40 µm were used for the XRD characterization with a D8 Focus Bruker diffractometer (a step size of 0.00157 • in the range of 10 − 90 • 2θ). GSAS software was used for the Rietveld refinement analysis [11]. The material microstructure was characterized by a Zeiss Supra-55 VP Field Emission SEM.
The electrochemical characterization was carried out with a three-electrode-based CT 2001 Land Battery Tester,. The test details can be found elsewhere [9][10][11][12][13]. Figure 1. shows the phase compositions of the ascast and melt-spun samples with the C14-and C15-type alloys as obtained from the Rietveld refinements [9,10]. For the C14-type alloys, the higher speed of the melt spinner resulted in a larger abundance of the C14 phase which is probably caused by the tendency of the C14 Laves phase to preferably form at higher rotation wheel speeds and higher solidification rates [12]. Such effect is even more pronounces at higher speed of the melt spinner for the C15 phases leading even higher increased abundance.

SEM characterization
The SEM micrographs of the C14 and C15 alloys can be seen in Figure 2. The results obtained for the as-cast materials confirm formation of two-phase alloys. In the C14-type alloys, the C14 phase becomes the main phase, while C15 phase becomes the main phase in the C15-type alloys. For the initial alloys the grain size in the as-cast alloys was significantly larger for the C14 type intermetallic as compared to the C15 one (15 and 3.5, respectively). This indicates that C14 phase in likely to crystallise at lower temperatures as compared to the C15 one. Further to this, a significant grain refinement was observed for both types of intermetallics in the analysed microstructures of all melt-spun samples. The grain size reduction effect is proportional to the increase of the wheel rotation speed. Figure 1. Sample phase compositions based on Rietveld refinement results. The label C14 means that C14 phase as the major phase. Similar, the label C15 means that C15 phase is the major phase. For C14 melt-spun alloys, when the low speed was applied, the equiaxed grains have been formed. On the other hand, increasing of the speeds, results in forming the dendrites. These dendrites became smaller when the speed increased up to 33 Hz. For C15 melt-spun alloys, there was no change in the microstructural morphology, except of the grain refinement.
Indeed, in the melt-spun C14-type alloys, the grain size refinement resulted in reaching the grain size of 2 mm. This value went down even to the submicron size (0.25 mm) for the C15-type alloys (see Table 1). In addition, melt spinning significantly contributes to the morphological variations among studied samples in C14 alloys, in which the equiaxed grains at lower speed transform to the small dendrite-like regions formed at higher speed. On the contrary, the morphology of the C15 samples remains unchanged for all investigated samples.

Electrochemical measurements
The results of electrochemical measurements are listed in Table 2. From the data it is clear that the activation performance of the melt-spun materials appears to be worse than that of the as-cast alloys. Indeed, all samples from the C14 alloys series require more then 10 cycles to get activated, with the material formed at the wheel speed of 4100 cm/s performing the worst. This is in accordance with the previous observations [14], where the increased speed of a melt spinner resulted in a greater abundance of the C14-type, which led to the poorer activation performance of the melt-spun alloys. On the other hand, at the same speed, melt-spun C15 alloys perform better and are able to get activated after 10 cycles. In this case, the reason of the difficult sample activations is likely related to the formation of the thick oxide layer(s) on the alloy surface.
When it comes to the best performance, the maximum energy storage capacity of 435 and 414 mA.h/g for the C14 and C15 alloys, respectively, are reached at the same wheel speed of 2100 cm/s. These values are higher by 8.8 and 31.8% as compared to the numbers obtained for the as-cast C14 and C15 alloy series, respectively.
The A and B melt-spun alloys achieve their best rate performance at the same speed of 2100 cm/s (see Table  2). The smaller decrease in the discharge capacity at the increasing current density is observed for the samples of C15 melt-spun alloy at 2100 cm/s (Figure 3(a)). On the other hand, a significant difference in the discharge capacity of approximately 76% took place for the C14 melt-spun 2100 cm/s alloy (see Figure 3(a)). At the lower current density, the C14 alloys reach higher discharge capacities than the C15 alloys. In contrast to the higher current density, a higher energy storage capacity was obtained by the C14 alloy samples. Indeed, this phenomenon confirms that the C15 alloys achieve a better rate performance compared to the C14 alloys. In addition, the melt spinning process works more effectively in improving the properties of the alloys by achieving the refined phase structures and microstructures in their optimized formation. Therefore, the C15 alloy sample obtained at 2100 cm/s reaches a better rate performance than the corresponding as-cast material, and presents the best rate performance among all investigated alloys (see Figure 3(a)) [9,10].
When it comes to the comparison of the ate performance of both alloys in terms of potential vs. discharge capacity, the C14 alloys sample processed at a wheel speed of 2100 cm/s shows a dramatic decrease in the discharge capacity, which is pronounced to a much higher extent at the increasing current densities (see Figure 3(c)). As shown in Figure 3(c), longer plateau, wider range, and higher potential vs. discharge capacity values are obtained for the C15 alloy sample melt-spun at 2100 cm/s. At the current density of 300 mA/g the sample energy storage capacity is still at the level of 300 mA.h/g, which indicates that the this material is superior in achieving advanced rate performance.

Conclusions
Melt spinning has been performed on the C14 and C15 alloys with the nominal compositions: Ti 0.15 Zr 0.85 La 0.03 Ni 1.2 Mn 0.7 V 0.12 Fe 0.12 at a wheel speeds of 630, 2100, 4100 cm/s causing a significant change in the material electrochemical performance as compared to the as-cast samples. Those samples processing resulted in the remarkable grain size refinement optimized phase composition, and modified microstructural state. An excellent value of the maximum discharge capacity above 400 mA.h/g was achieved for each melt-spun alloy series at the same spinning wheel speed of 2100 cm/s. The melt spinning process works effectively in improving the rate performance of the compositions based on the C15-type Laves phases. However, the poor activation performance of the melt-spun alloys requires future optimization of the spinning parameters.