Effect of Cr Doping on Microstructure and Hydrogen Storage Properties of Zr_0.8Ti_0.2Cr_xCo_1−x (x = 0, 0.05, 0.1, 0.15) Alloys

Abstract

The ZrCo hydrogen storage alloy is a relatively good hydrogen isotope carrier applied in the National Thermonuclear Fusion Reactor. However, the intrinsic disproportionation characteristics of ZrCo alloy reduces its cyclic service life and limits its further application. To address this issue, Zr_0.8Ti_0.2Co alloy is developed and exhibits good anti-disproportionation performance than pure ZrCo. Nevertheless, Zr_0.8Ti_0.2Co suffers from relatively poor hydrogen absorption kinetics. In this study, the effects of Cr substitution on its microstructure and hydrogen storage performance are investigated. Zr_0.8Ti_0.2Cr_xCo_1−x (x = 0, 0.05, 0.1, 0.15) samples are composed of the ZrCo main phase. After Cr substitution, the second phases of CoZr₂ and TiCr₂ Laves phases appear. With the increase in Cr content, the lattice constant and unit cell volume of the Zr_0.8Ti_0.2Co alloy increase. Meanwhile, the hydrogen absorption incubation time of the Zr_0.8Ti_0.2Co alloy is shortened, and the activation performance is enhanced, which is attributed to the catalytic effect of the Laves second phases. The enthalpy of hydrogen absorption of the Zr_0.8Ti_0.2Co alloy increases, and the stability of the hydride is enhanced with increasing Cr addition. Zr_0.8Ti_0.2Cr_0.05Co_0.95 demonstrates excellent hydrogen desorption kinetics while maintaining robust anti-disproportionation performance. The element substitution and the composition design are effective approaches to improving the comprehensive hydrogen storage performance of ZrCo-based alloys, which provides guidance for its further application.

1. Introduction

With the growing number of the energy demand and the increasingly serious environmental pollution, it is crucial that humans seek an environmentally friendly energy source to replace traditional fossil fuels. The International Thermonuclear Experimental Reactor (ITER) is a renowned project with immense potential to alleviate the energy crisis. It generates energy through the fusion of deuterium and tritium nuclei, resulting in the formation of a helium nucleus and the release of the energy stored within the atomic cores. Tritium, due to its radioactive, poisonous, rare, and expensive nature, requires a safe, environmentally friendly, and effective storage and delivery system (SDS). The choice of hydrogen storage material is crucial for ensuring the integrity and reliability of the SDS. At present, Uranium (U) has been applied for deuterium and tritium during storage and delivery system due to fast de-/hydriding kinetics and low equilibrium pressure at room temperature. However, the presence of defects inhibits its further application of uranium owing to the radioactivity, pyrophoricity, and pulverization. To solve this thorny problem, many researchers try out finding a dependable material to supersede U in SDS. Due to the high creep resistance and high mechanical properties, Zr-based alloy is used in the nuclear energy [1]. ZrCo alloy is a spotlighted alternative material and exhibits splendid performance, such as plateau characteristics, superior high hydrogen capacity, no radiation exposure, and the ability of capture ³He, which displays similar properties for U [2]. Unfortunately, ZrCo is limited in its applications because of the poor kinetic properties and the awful cycling performance due to the stable ZrH₂ and the induced ZrCo₂ phase during the disproportionation reaction [3,4]. The poor cyclic stability of ZrCo would be displayed by the disproportionation reaction of ZrCo alloy: 2ZrCo + H₂ → ZrH₂ + ZrCo₂. The stability of the ZrH₂ and ZrCo₂ phases can be attributed to their high hydrogen desorption temperature, which reaches up to 700 °C, and their high hydrogen absorption pressure, respectively [5,6,7].

Many efforts have been attempted to address the aforementioned issues, such as element substitution, surface modification, and optimization of preparation methods. Among these, element substitution is an important means to improve the hydrogen storage performance of ZrCo-based alloys. Elements such as Ti, Hf, and Nb have been used to substitute Zr, which has a positive effect on improving the anti-disproportionation performance of ZrCo alloys [8,9,10]. Elements such as Mn, Mo, Fe, Ni, and Pd have been used to substitute Co, which has certain effects on improving the hydrogen storage performance of ZrCo alloys [11,12,13,14,15,16]. Among them, Ti has achieved the most significant anti-disproportionation effect, which has been unanimously recognized by researchers [17,18,19]. It has been shown that Ti substitution for Zr exhibits super disproportionation properties when the addition of Ti reached 20 at.% in the as-cast Zr_0.8Ti_0.2Co ingot [8]. Zr_0.8Ti_0.2Co bed shows better hydrogen delivery amount than ZrCo bed due to the retarded disproportionation reaction [9]. Besides disproportionation reaction, the sluggish kinetics also hinders its further practical application of ZrCo [20,21]. Zr_0.8Ti_0.2Co exhibits slower hydrogen absorption and desorption rates compared to ZrCo, despite its good cyclic stability and resistance to disproportionation. The ZrCo alloy reaches hydrogen absorption saturation in approximately 500 s, whereas the Zr_0.8Ti_0.2Co alloy requires about 800 s [8].

Addressing the hydrogen absorption and desorption kinetics of ZrTiCo alloy is a crucial step in promoting its practical application. Single element substitution has certain limitations in improving the hydrogen storage performance of ZrCo hydrogen storage alloys. For example, the hydrogen absorption rate of ZrCo alloy increases after Mn substitution for Co but the disproportionation reaction is exacerbated [11]. In contrast, Mo substitution for Co in ZrCo alloy alleviates the disproportionation reaction, albeit at the cost of reduced hydrogen absorption capacity [12]. Dual substitution is considered to be a comprehensive and effective approach. Wan et al. [22] proposed that the combined substitution of Ti and Ni in ZrCo alloy enhances the resistance to pulverization while maintaining good anti-disproportionation performance, thereby improving the cyclic stability of ZrCo. Xu et al. [23] proposed that the substitution of Fe for Co in the ZrTiCo alloy enhances the hydrogen absorption and desorption kinetic properties. Zhang et al. [24] investigated the effects of co-doping with Nb and Ta on ZrCo alloy using first-principles calculations. They found that the dissociation energy of hydrogen molecules on the alloy surface and the diffusion energy of hydrogen atoms was reduced, which enhanced the hydrogen storage properties. Zhang et al. [25] also revealed the synergistic effect of co-doping with Ti, Nb, and Ni on the hydrogen storage capacity of ZrCo. Therefore, co-substitution is an effective method for addressing the issues in ZrCo hydrogen storage alloys. However, the combinations of different elements, the optimal doping amounts, and the improvement mechanisms still need to be further investigated and elucidated. Therefore, in this paper, we selected the ternary alloy Zr_0.8Ti_0.2Co alloy with good anti-disproportionation performance as a basic research object. And elemental doping was performed to investigate the effect of elemental doping on the hydrogen storage performance of Zr_0.8Ti_0.2Co alloy and to improve the kinetic property. Along with the previous examples, Cr addition in ZrCo alloy seems a promising approach to improve the hydrogen kinetics properties. Luo et al. [26] observed that kinetics can be enhanced by selecting Cr to replace Co, and the activation of pure ZrCo alloy requires about 7700 s, while ZrCr_0.1Co_0.9 alloy only requires 195 s to reach saturation hydrogen absorption. Interestingly, it has been demonstrated that doping Cr not only could form the metal passivation layer to prevent alloy oxidation, but also effectively avoid the surface of the alloy being infected by impurity gases. Considering that Ti substitution can improve the anti-disproportionation performance of ZrCo alloy and Cr substitution can enhance its hydrogen absorption kinetics, this study proposes to adopt a combined Ti/Cr substitution method. It is expected that the synergistic effect of Ti and Cr will improve the comprehensive hydrogen storage performance of ZrCo alloy. This study focuses on the Zr_0.8Ti_0.2Co alloy and investigates the effects of partial substitution of Zr by Cr on the microstructure and hydrogen storage performance of the ZrCo alloy. The Zr_0.8Ti_0.2Cr_xCo_1−x (x = 0, 0.05, 0.1, 0.15) series of alloys were prepared, and their microstructures, hydrogen absorption and desorption kinetics, and anti-disproportionation performance were investigated. The ZrCo alloy with combined Ti/Cr substitution not only maintains good anti-disproportionation performance but also achieves a significant improvement in hydrogen absorption and desorption kinetics. Meanwhile, the intrinsic mechanisms underlying the enhanced hydrogen absorption and desorption performance are further elucidated. This work provides guidance for the compositional design and practical application of ZrCo alloys.

2. Materials and Methods

The quaternary Zr_0.8Ti_0.2Cr_xCo_1−x (x = 0, 0.05, 0.1, 0.15) alloys were prepared by arc melting the metals (Zr, Ti, Cr, and Co, purity > 99.9%, purchased from ZhongNuo Advanced Material (Beijing) Technology Co., Ltd., Beijing, China) in a water-cooled copper crucible under a high-purity argon atmosphere, which were named Cr000, Cr005, Cr010, and Cr015, respectively. To ensure compositional homogeneity, the ingot was remelted at least four times. Then, the ingot alloy was polished by a fine sand wheel to remove the surface oxide layer and cut into pieces by pliers. A portion of the ingot alloy was selected as an inlay sample, while the other portion was crushed into a fine powder with a particle size of 200–300 mesh.

X-ray diffraction (XRD, Rigaku D/max2550 VB, Showima City, Tokyo, Japan) was employed to analyze the phases of the samples before and after hydrogen absorption and desorption, using a Cu target with a scanning angle ranging from 20° to 90°. Scanning electron microscopy (SEM, Quanta 450 FEG, FEI, Hillsboro, OR, USA) was utilized to examine the microstructure of the polished samples. Energy-dispersive spectroscopy (EDS, Oxford Instruments, Oxfordshire, England) mapping was applied to analyze the elemental composition of the materials. The alloy powder was wrapped with nickel mesh and placed in a stainless-steel container (FINESORB-3110, Zhejiang Finetec Instruments Co., Ltd., Hangzhou, China), vacuuming at 473 K for 2 h to take away the gas impurities adsorbed on the surface of the alloy. Then, all alloys were activated at 473 K for 1 h under 20 bar H₂ (Guilin Xinhongyi Ltd., Guilin, China, with purity of 99.999%) to reach hydrogen absorption saturation. Meanwhile, the initial hydrogen absorption kinetics curves were collected. To obtain the thermodynamic parameters of the Cr000, Cr005, Cr010, Cr015 samples, Pressure–Composition–Temperature (PCT) curves were conducted at 623 K, 653 K, and 683 K. The hydrogen desorption kinetics curves of the samples were tested at 713 K, 743 K, and 773 K. To evaluate the anti-disproportionation property, the test of anti-disproportionation property was performed on the same apparatus. The hydriding Zr_0.8Ti_0.2Cr_xCo_1−x (x = 0, 0.05, 0.1, 0.15) alloys were loaded into the reaction vessel, and the entire apparatus was vacuumed below 10 Pa. Subsequently, the sample was heated from room temperature to 773 K for dehydrogenation and the change of hydrogen pressure was recorded for 10 h at a constant temperature until the end of the test.

3. Results and Discussion

3.1. Structural Characterizations

The XRD patterns of the Zr_0.8Ti_0.2Cr_xCo_1−x alloy (x = 0, 0.05, 0.1, 0.15) samples are shown in Figure 1. It is observed that all the samples form the cubic ZrCo main phase with a CsCl-type structure [10]. As the Cr content increases, two types of Laves phase secondary structures gradually emerge, and the intensity of these secondary phases strengthens with increasing Cr content, indicating an increasing amount of secondary phases. Compared with the standard powder diffraction files, the three phases correspond to the ZrCo main phase (JCPDS#04-001-1671), the CoZr₂ secondary phase (JCPDS#04-001-1672), and the TiCr₂ secondary phase (JCPDS#01-071-7628), respectively. The unit cell parameters of the samples were obtained using refinement software (GSASII), as shown in

Table 1. The lattice parameters and cell volume of Zr_0.8Ti_0.2Cr_xCo_1−x (x = 0, 0.05, 0.1, 0.15) alloys.

Sample	Lattice Parameters (Å)	Cell Volume (Å3)
Cr000	3.156 (4)	31.43
Cr005	3.163 (1)	31.85
Cr010	3.168 (5)	32.14
Cr015	3.177 (8)	32.86

. As the Cr content increases, the unit cell parameters gradually increase. This is likely due to the atomic radius of Cr (128 pm) being larger than that of Co (125 pm). The substitution of Co by Cr leads to an increase in lattice constants and expansion of the unit cell volume, which is also confirmed in the early report that the unit cell volume was slightly increased after Cr doped Co in ZrCo system [26].

From the backscattered electron images of the Zr_0.8Ti_0.2Cr_xCo_1−x (x = 0, 0.05, 0.1, 0.15) alloys, as shown in Figure 2a–d, it can be observed that the Cr000 alloy exhibits a light gray single phase, representing a solid solution structure. With the incorporation of Cr, white and dark gray secondary phases gradually emerge. Analysis of the elemental composition reveals that the white phase corresponds to a Zr/Co atomic ratio close to 2:1, which is indicative of the CoZr₂ Laves phase, while the dark gray phase corresponds to a Cr/Ti atomic ratio close to 2:1, representing the TiCr₂ Laves secondary phase. When the Cr content reaches 0.1, the dark gray TiCr₂ Laves phase appears, and the amounts of both Laves phases increase significantly. As the Cr content increases, the proportion of the secondary phases becomes more pronounced. Studies have shown that in the Zr–Cr phase diagram [27], a eutectic reaction occurs in the alloy at 1605 K, leading to the formation of ZrCr₂. The formation of ZrCr₂ consumes Zr from the ZrCo₂ phase, thereby reducing the formation of the ZrCo₂ phase. In the Zr_0.8Ti_0.2Cr_xCo_1−x (x = 0, 0.05, 0.1, 0.15) alloys, the formation of CoZr₂ consumes Co from the ZrCo₂ phase, resulting in negligible formation of ZrCo₂. A similar result was found in the reported reference that Zr₂Co and ZrCr₂ second phase were formed in the Cr-doped ZrCo system [26]. In this study, when Zr is partially substituted by Ti, TiCr₂ is more likely to form preferentially. To further analyze the composition, the Cr010 alloy was characterized using EDS mapping (Figure 2e). The distribution of Zr and Ti elements is relatively uniform. Segregation of Co corresponds to the white CoZr₂ phase, while segregation of Cr corresponds to the dark gray TiCr₂ Laves phase, consistent with the XRD analysis.

3.2. Hydriding Kinetics of Zr_0.8Ti_0.2Cr_xCo_1−x Alloy

The rate of hydrogen absorption directly determines whether hydrogen isotopes can be supplied in time in nuclear fusion reactions. Compared with ZrCo alloy, Zr_0.8Ti_0.2Co alloy has improved anti-disproportionation performance, but its hydrogen absorption kinetics are still not satisfactory. To investigate the effect of Cr substitution for Co on the kinetics of ZrCo-based alloys, hydrogen absorption kinetics tests were conducted. Figure 3 shows the hydrogen absorption kinetics curves of Zr_0.8Ti_0.2Cr_xCo_1−x (x = 0, 0.05, 0.1, 0.15) alloys at 473 K with hydrogen pressure equal to 20 bar. Before Cr doping for Co, the incubation period of Zr_0.8Ti_0.2Co alloy can reach about 1000 s. After Cr doping, the incubation period is significantly shortened to 700 s, 540 s, and 470 s, respectively. The time to reach hydrogen absorption saturation for Zr_0.8Ti_0.2Cr_xCo_1−x (x = 0, 0.05, 0.1, 0.15) alloys is 2600 s, 1800 s, 1220 s, and 990 s, respectively. Cr doping for Co significantly enhances the hydrogen absorption rate, thereby improving the hydrogen absorption kinetics.

The improvement in the hydriding kinetics of Zr_0.8Ti_0.2Cr_xCo_1−x (x = 0, 0.05, 0.1, 0.15) alloys can be attributed to the following three aspects: (1) the substitution of Cr for Co increases the brittleness of the ZrCo-based hydrogen storage alloy. This makes the samples more prone to cracking and spalling after hydrogen absorption. The formation of cracks facilitates the entry of hydrogen atoms into tetrahedral or octahedral interstitial sites. When the alloy expands upon hydrogen absorption, internal stresses are generated between the hydride and the matrix. The increase in internal stress further promotes the rupture of the hydride, forming additional cracks that provide more diffusion pathways for hydrogen atoms, thereby accelerating the hydrogen absorption reaction [28]. (2) The improved hydrogen absorption kinetics are closely related to the presence of Laves phases. The interfaces between the Laves phases and the main phase provide additional diffusion channels for hydrogen atoms. Initial hydrogen absorption can induce microcracks, which increase the contact area between the sample surface and hydrogen atoms. Similar improvements in initial activation behavior have been observed in Ti-V-based hydrogen storage alloys, where the introduction of hexagonal C14 and cubic C15 Laves phases enhanced the alloy’s performance [29,30]. (3) It has been reported that the unique surface structure of the TiCr₂ phase exhibits catalytic activity, promoting the dissociation of H₂ into atomic hydrogen (H) and reducing the activation energy barrier for hydrogen absorption [26]. As the Cr content increases, the amount of the second phase (such as TiCr₂) increases, enhancing the catalytic effect and further improving the kinetics.

In terms of the maximum hydrogen absorption capacity, the hydrogen uptake of Zr_0.8Ti_0.2Cr_xCo_1−x (x = 0, 0.05, 0.1, 0.15) decreases from 2.20 (f.u.) to 2.06 (f.u.), 2.02 (f.u.), and 1.875 (f.u.) with increasing Cr content (corresponding to the decreased Co content). This reduction in hydriding capacity is attributed to the presence of the TiCr₂ Laves phase, which does not nearly react with hydrogen at the testing condition, thereby lowering the effective hydrogen uptake. The hydrogenated samples were subjected to XRD analysis, as shown in Figure 3b. The results indicate that all hydrogenated samples exhibit the orthorhombic ZrCoH₃ phase (PDF card 33-0416). As the Cr content increases, the intensity of the diffraction peaks decreases, and the peaks broaden, indicating a reduction in the crystallinity of the hydride. Additionally, the presence of the TiCr₂ phase becomes more pronounced after hydrogenation, which is attributed to the intensity of the diffraction peaks of the hydride being significantly lower than that of the sample before hydrogen absorption. Since the TiCr₂ phase does not participate in the hydrogen absorption reaction, the sample still retained TiCr₂, which also validated the reason for the decreased hydrogen absorption capacity after Cr substitution for Co. Similarly, in the ZrCo-Mo and ZrCo-Al system, the emergence of the secondary phase (ZrMo₂ and Zr₆CoAl₂, respectively) has improved activation performance and hydrogen absorption rate, but reduced hydrogen capacity due to its much lower hydrogen absorption than the primary phase ZrCo [12,20].

3.3. Thermodynamics Property of the Zr_0.8Ti_0.2Cr_xCo_1−x Alloy

Thermodynamics property is an important indicator of whether the Zr_0.8Ti_0.2Cr_xCo_1−x (x = 0, 0.05, 0.1, 0.15) alloy is suitable for practical uses. To investigate the thermodynamics property of Cr-doped alloys, the pressure-composition- temperature (PCT) curves of hydrogenation for the Zr_0.8Ti_0.2Cr_xCo_1−x (x = 0, 0.05, 0.1, 0.15) alloys were measured at 543 K, 573 K and 603 K, shown in Figure 4. It is evident from Figure 4 that all the samples demonstrate a single plateau at different Cr concentrations with a temperature range of 543 K–603 K, and this can be considered as only one type of hydrogen absorption reaction occurs in the alloys because the alloy samples reacted with hydrogen to generate one kind of hydride corresponding to the orthorhombic ZrCoH₃ phase. With the increased Cr content (corresponding to the decreased Co content), the hydrogen storage capacity and the plateau width decrease. It is noticeable that the hydrogen absorption capacity of pure Cr000 alloy is at 1.85 (f.u.) while it declines to 1.47 (f.u.) for Cr015. The magnitude and width of the plateau pressure for hydrogen absorption PCT curves are related to the stability of the hydrides.

In order to further research the behavior of the alloy’s thermodynamics, thermodynamic calculations were performed on Zr_0.8Ti_0.2Cr_xCo_1−x (x = 0, 0.05, 0.1, 0.15) alloys by combining the plateau pressure data with the Van’t Hoff formula of Figure 4e, and the reaction can be written as follows:

$$\begin{array}{l}\ln {\displaystyle \frac{P_{eq}}{P_0}}={\displaystyle \frac{\Delta H}{RT}}-{\displaystyle \frac{\Delta S}{R}}\end{array}$$

(1)

where ΔH and ΔS are the enthalpy and entropy, R is the gas constant, T is the test temperature, P_eq is the plateau pressure, respectively. The thermodynamic parameters of the hydride are obtained by combining the Van’t Hoff formula with the fitted curve of the Van’t Hoff diagram, as shown in

Table 2. Thermodynamic data for hydrogen absorption of Zr_0.8Ti_0.2Cr_xCo_1−x (x = 0, 0.05, 0.1, 0.15) alloys.

Samples	ΔH (kJ/mol H2)	ΔS (kJ/mol·K H2)
ZrCo-H [10]	89.71	226.16
ZrCo-H [32]	90.12	226.14
Cr000	70.75	214.50
Cr005	72.66	218.66
Cr010	73.99	221.65
Cr015	76.16	226.64

. The obtained parameters are compared with the previously reported thermodynamic parameters of ZrCo [10]. It can be concluded that the thermodynamic parameters take an increasing trend by adding Cr to Zr_0.8Ti_0.2Co alloy. Thermodynamic properties are related to hydride stability while the stability of hydrides is related to crystal structure. As the lattice parameters and cell volume were magnified, which is favorable for the stable occupancy of hydrogen atoms and then the formed hydrides are more stable [31]. In the study of this system, the substitution of Cr for Co promotes the lattice expansion of Zr_0.8Ti_0.2Co alloy, enlarging the space to storage hydrogen and making hydrogen atoms more stable in tetrahedral and octahedral, and, thus, the hydrogenation plateau is reduced during the PCT testing and hydride formation enthalpy is increased.

3.4. Dehydrogenation Properties

Figure 5 shows the hydrogen desorption kinetics curves of Zr_0.8Ti_0.2Cr_xCo_1−x (x = 0, 0.05, 0.1, 0.15) hydrides at 743 K. The samples reached hydrogen desorption saturation within 900 s. The hydrogen desorption process can be divided into three stages: (1) Initial desorption stage, where hydrogen atoms in the metastable α-phase, which are less thermodynamically stable, are preferentially released. The desorption amount increases slowly at this stage; (2) Accelerated desorption stage, where the desorption amount increases significantly due to the release of hydrogen atoms that were more stably positioned in the hydride structure at this stage; (3) Final desorption stage, where the desorption capacity remains essentially constant. With increasing Cr content, the desorption capacity gradually decreases, with values of 1.90 (f.u.), 1.54 (f.u.), 1.49 (f.u.), and 1.28 (f.u.) for Cr000, Cr005, Cr010, and Cr015, respectively. Since the hydrogen absorption capacities differ among the samples, we introduced the desorption ratio, defined as the ratio of desorption capacity to absorption capacity to better evaluate the extent of hydrogen desorption. The desorption ratios for Zr_0.8Ti_0.2Cr_xCo_1−x (x = 0, 0.05, 0.1, 0.15) were 86.49%, 74.62%, 73.51%, and 68.01%, respectively. The hydrogen absorption and desorption properties of the samples are shown in

Table 3. The hydriding/dehydriding data of the samples.

Samples	The Hydriding Incubation Period (s)	The Maximum Hydriding Capacity (f.u.)	The Desorption Capacity (f.u.)	The Desorption Ratio (%)
Cr000	1000	2.20	1.90	86.49
Cr005	700	2.06	1.54	74.62
Cr010	540	2.02	1.49	73.51
Cr015	470	1.87	1.28	68.01

for comparison.

As the Cr content increases (corresponding to the decreased Co content), the desorption ratio decreases progressively. This is consistent with the enthalpy results of the hydride formation, which showes that the stability of the hydride increases with increasing Cr content, thereby making the desorption reaction more difficult. The order of desorption rates is Cr005 > Cr010 > Cr000 > Cr015. The desorption rate initially increases and then decreases with Cr substitution. Specifically, the Cr005 sample (Zr_0.8Ti_0.2Cr_0.05Co_0.95) exhibits the fastest desorption rate. It takes only 748 s for this sample to achieve 90% of the desorption progress, whereas the unmodified sample requires 852 s to reach the same level of progress.

The hydrogen desorption kinetics curves of Zr_0.8Ti_0.2Cr_xCo_1−x (x = 0, 0.05) at 713 K, 743 K, and 773 K are shown in Figure 6a,b. For the same sample, the desorption rate increases, and the desorption amount rises with increasing desorption temperature. Higher temperature facilitates the diffusion of hydrogen atoms and the decomposition of the hydride. The desorption kinetics curves were fitted to the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model, which is described by the equation as follows:

$$\ln \left[-\ln \left(1-\alpha \right)\right]=\eta \ln \left(\mathrm{k}\right)+\eta \ln \left(\mathrm{t}\right)$$

(2)

where α represents the degree of hydrogen desorption reaction, t is the reaction time, k is the reaction rate constant, and η is the Avrami exponent (reflecting nucleation and growth mechanisms) [33,34]. The fitted isothermal desorption curves show R² values exceeding 0.93 (Figure 6a′,b′), indicating that the desorption kinetics of Zr_0.8Ti_0.2Cr_xCo_1−x (x = 0, 0.05) alloys conform to the JMAK model.

To further evaluate the desorption performance of Zr_0.8Ti_0.2Cr_xCo_1−x (x = 0, 0.05) alloys, the apparent activation energy of the desorption reaction was calculated using the combination of the Arrhenius equation and the JMAK equation. The Arrhenius equation is listed as follows:

$$\ln \left(\mathrm{k}\right)=-\mathrm{Ea}/\left(\mathrm{RT}\right)+\ln \left(\mathrm{A}\right)$$

(3)

where k is the reaction rate constant, Ea is the activation energy, R is the gas constant, T is the testing temperature, and A is the pre-exponential factor [35]. The fitting results are shown in Figure 7. For Zr_0.8Ti_0.2Cr_xCo_1−x (x = 0, 0.05) alloys, the desorption activation energy of the ZrCoH₃ phase decreased from 99.78 kJ/mol to 87.48 kJ/mol with Cr doping. This indicates that Cr substitution reduces the apparent activation energy for hydrogen desorption in the alloy hydride, making the desorption reaction easier and, thus, accelerating the desorption rate. Luo et al. also confirmed that the Ea of hydrogen desorption is decreased after Cr doping for Co in the ZrCo system [26]. The enhanced hydrogen desorption rate is associated with the promotion of hydrogen atom combination and diffusion by the Laves second phase.

3.5. Disproportionation Property

The disproportionation effect is a key barrier to the practical application of the ZrCo alloy in the ITER project. Temperature and hydrogen pressure are two critical factors affecting hydrogen-induced disproportionation. Figure 8 shows the hydrogen pressure variation curves of Zr_0.8Ti_0.2Cr_xCo_1−x (x = 0, 0.05, 0.1, 0.15) alloys during desorption mode at 773 K for 10 h. As can be seen from the figures, disproportionation can be divided into two stages: Stage A and Stage B. Stage A represents the hydrogen desorption process of the hydride in Zr_0.8Ti_0.2Cr_xCo_1−x (x = 0, 0.05, 0.1, 0.15) alloys as they are heated from room temperature to 773 K. The reaction equation is 2ZrCoH₃ → 2ZrCo + 3H₂, which corresponds to the phase transformation from ZrCoH₃ to ZrCo.

Stage B is the isothermal process at 773 K for 10 h, aimed at testing the stability of Zr_0.8Ti_0.2Cr_xCo_1−x (x = 0, 0.05, 0.1, 0.15) alloys at high temperatures. The reaction equation is 2ZrCo + H₂ → ZrCo₂ + ZrH₂. The formation of the disproportionation phases ZrCo₂ and ZrH₂ leads to a decrease in hydrogen pressure. As observed in Figure 8, during Stage B, the hydrogen pressure variation in Cr-doped ZrTiCo alloys is not significant and closely follows the trend of the undoped ZrTiCo alloy. In ZrCo system, the hydrogen pressure drops sharply due to the disproportionation product [32]. This indicates that Cr substitution for Co maintains the excellent anti-disproportionation stability of the ZrTiCo alloy, which is far superior to that of pure ZrCo. This enhanced stability is highly favorable for the further implementation of the alloy in the ITER project.

4. Conclusions

In this study, the microstructure, activation performance, hydrogen desorption kinetics, and anti-disproportionation properties of Zr_0.8Ti_0.2Cr_xCo_1−x (x = 0, 0.05, 0.1, 0.15) alloys were systematically investigated. Cr doping is an effective means of improving the performance of ZrTiCo alloys. The specific conclusions are as follows:

(1)

The ZrTiCo-based alloy is composed of the ZrCo main phase. After Cr substitution, two Laves phases appear in Zr_0.8Ti_0.2Cr_xCo_1−x (x = 0.05, 0.1, 0.15), namely CoZr₂ phase and TiCr₂ phase. With increasing Cr content, the lattice constant and unit cell volume increase, and the amount of the secondary phases also increases;

(2)

Cr doping improves the initial activation kinetics of Zr_0.8Ti_0.2Co alloy. The hydriding time is reduced from 2600 s to 990 s for Zr_0.8Ti_0.2Cr_0.15Co_0.85. This improvement is attributed to the catalytic effect of the TiCr₂ Laves phase on the dissociation and diffusion of hydrogen molecules;

(3)

A certain amount of Cr doping enhances the hydrogen desorption kinetics. The time required for Zr_0.8Ti_0.2Cr_0.05Co_0.95 to achieve 90% of the desorption progress is reduced to 748 s compared with Zr_0.8Ti_0.2Co of 852 s. This is due to the decreased desorption activation energy resulting from Cr doping;

(4)

With the addition of Cr, the hydrogen absorption enthalpy of Zr_0.8Ti_0.2Cr_xCo_1−x (x = 0, 0.05, 0.1, 0.15) increases, leading to enhanced stability of the hydride phase. Meanwhile, the alloys maintain good anti-disproportionation performance, which is beneficial for the further application of ZrTiCo-based alloys.