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Influence of heat treatment on the microstructure and hydrogen storage properties of Ti10V77Cr6Fe6Zr alloy
Journal of Alloys and Compounds 529 (2012) 128–133
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Influence of heat treatment on the microstructure and hydrogen storage properties of Ti10 V77 Cr6 Fe6 Zr alloy Zhouming Hang a,b , Xuezhang Xiao a , Shouquan Li a , Hongwei Ge a , Changpin Chen a , Lixin Chen a,∗ a b
Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China Zhejiang Test Academy of Quality and Technical Supervision, Hangzhou 310023, China
a r t i c l e
i n f o
Article history: Received 12 November 2011 Received in revised form 20 February 2012 Accepted 10 March 2012 Available online 17 March 2012 Keywords: V-based solid solution alloy Heat treatment Microstructure Hydrogen storage properties
a b s t r a c t The as-cast Ti10 V77 Cr6 Fe6 Zr alloy was heat-treated at 1373 K for 8 h or 1523 K for 5 min and then quenched in water. The influence of heat treatment on the microstructure and hydrogen storage properties of Ti10 V77 Cr6 Fe6 Zr alloy was investigated systematically. The results show that all of the as-cast and heattreated alloys consist of BCC main phase and C14 Laves secondary phase. After heat treatment, the phase abundance of BCC enhances and the plateau region of P–C–T curve is flattened, but the hydrogen absorption capacity is decreased. However, the alloy heat-treated at 1523 K for 5 min achieves a enhanced hydrogen desorption capacity of 1.82 wt.% at 333 K against 0.1 MPa, which is higher than 1.44 wt.% hydrogen desorption capacity of the as-cast alloy. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Vanadium or vanadium-based solid solutions, as the third generation of hydrogen storage alloys, have been widely studied and several series of V-based multi-component alloys with good hydrogen storage properties have been developed. Among which Ti–Cr–V alloys attracted much more attention for its high capacity of absorbing hydrogen and excellent kinetics for hydrogen absorption and desorption at moderate conditions [1–4]. However, these alloys exists some shortcomings, such as difficulty in activation, poor P–C–T plateau characteristics and low hydrogen desorption capacity for the much lower dehydrogenation pressure plateau region of mono-hydride (e.g. plateau pressure of about 1 Pa at room temperature for the hydrogen desorption of VH) [3–8]. It is well known that the partial substitution of V with other transition elements, such as Fe, Zr and Mn, is a very effective way to improve the overall hydrogen storage properties and lowering their cost of the Ti–Cr–V alloys [9,10,6,11–23]. On the other hand, it was reported that the heat treatment could also impact the microstructure and hydrogen storage properties of the Ti–Cr–V alloys [3,24,25]. Akiba and Iba [3] reported that the hydrogen storage properties of Ti–Cr–V alloys in as-cast state were sensitive to heating, therefore it was reasonable to believe that the heat treatment might affect the hydrogen storage properties of these alloys. Okada et al. [24] investigated the effect of heat treatment on the
∗ Corresponding author. Tel.: +86 571 8795 1152; fax: +86 571 8795 1152. E-mail address: [email protected] (L. Chen). 0925-8388/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2012.03.044
hydrogen storage properties of the Ti–Cr–V alloys, and reported that moderate heat treatment could enhance the hydrogen storage capacity and flatten the hydrogen desorption pressure plateau, e.g. the Ti25 Cr40 V35 alloy annealed at 1573 K for 1 min and then quenched in water achieved a hydrogen desorption capacity of 2.4 mass% at 313 K against 0.01 MPa. Cho et al. [25] also reported that Ti32 Cr43 V25 alloy annealed at 1653 K for 1 min achieved a hydrogen desorption capacity of 2.3 mass% at 303 K against 0.001 MPa. However, the influence of the heat treatments with the relatively lower temperature and longer time (such as 1523 K for 5 min or 1373 K for 8 h) on the microstructure and hydrogen storage properties was not reported. In this paper, the corresponding heat treatment conditions (annealed at 1523 K for 5 min and 1373 K for 8 h) were employed for Ti10 V77 Cr6 Fe6 Zr alloy which was explored and optimized in our previous work [22,23], and the influence of heat treatment on the microstructure and hydrogen storage properties of this alloy was investigated in detail. 2. Experimental The as-cast Ti10 V77 Cr6 Fe6 Zr alloy was prepared by levitation induction melting under argon atmosphere, and the ingot was turned over and remelted four times to ensure its homogeneity. Then two methods of heat treatment were adopted: (1) annealing at 1373 K for 8 h and then quenching in water; (2) annealing at 1523 K for 5 min and then quenching in water. In order to investigate the phase structure, the morphology and the phase composition of the alloy, X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS) were performed, respectively. For activation procedure, the sample of 4 g was placed in the reactor of Sieverts type apparatus and evacuated for 15 min under room temperature firstly, and then
Z. Hang et al. / Journal of Alloys and Compounds 529 (2012) 128–133
129
Fig. 1. XRD patterns of the as-cast and heat-treated Ti10 V77 Cr6 Fe6 Zr alloys: (a) ascast; (b) heat-treated at 1373 K for 8 h and then quenched in cold water; and (c) heat treated at 1523 K for 5 min and then quenched in cold water.
hydrogen was introduced gradually into the reactor up to a pressure of 4 MPa for the absorption process. Also the hydriding testing was performed under room temperature with a starting pressure of 4 MPa. The dehydriding kinetics curves were obtained against 0.1 MPa at 333 K. After dehydriding, the reactor was evacuated for 30 min at 673 K to extract the residual hydrogen for the next hydriding process. After three hydriding-dehydriding cycling, the dehydrogenation P–C–T measurement were carried out at 333 K. The change in pressure with time was recorded. In this study, the effective hydrogen desorption capacity is defined as the amount of hydrogen desorbed when the hydrogen pressure is decreased from 3 MPa to 0.1 MPa. Differential scanning calorimetry (Netzsch STA 449F3) measurements of the samples were performed under the pressure of 0.1 MPa with a constant heating rate of 10 K/min. The powder size used for the XRD and DSC measurements is under 30 m.
3. Results and discussion Fig. 1 shows the XRD patterns of the studied alloys. It can be seen from Fig. 1 that the as-cast alloy consists of BCC main phase and C14 Laves secondary phase. After heat treatment, the intensity of diffraction peak of the BCC phase enhances, while that of C14 Laves phase becomes weak, which implies that these heat treatments promote the growing of the BCC phase. The diffraction peak shifts to higher angle and lower angle for samples b and c, respectively. The lattice parameters of the BCC main phase for the studied alloys were determined by the Retvield refinement
Fig. 3. SEM micrographs of the as-cast and heat-treated Ti10 V77 Cr6 Fe6 Zr alloys: (a) as-cast; (b) heat-treated at 1373 K for 8 h; and (c) heat-treated at 1523 K for 5 min.
Fig. 2. The relationship between heat treatment conditions and lattice parameters and hydrogen absorption capacities: (a) as-cast; (b) heat-treated at 1373 K for 8 h; and (c) heat-treated at 1523 K for 5 min.
analyses and calculated as 0.30363 nm, 0.30191 nm and 0.30283 nm, which are shown in Fig. 2. The reason why the lattice parameter decreased by the heat treatment might be attributed to the change in the element contents of the BCC main phase after different heat treatments. Fig. 3 shows the SEM micrographs of the as-cast and heattreated Ti10 V77 Cr6 Fe6 Zr alloys. It can be observed that all the alloys consist of two phases, which is in good agreement with the results
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Z. Hang et al. / Journal of Alloys and Compounds 529 (2012) 128–133
Table 1 The results of EDS analyses for the as-cast and heat-treated Ti10 V77 Cr6 Fe6 Zr samples. Samples
Phase
As-cast
BCC C14 BCC C14 BCC C14
Heat-treated at 1373 K for 8 h Heat-treated at 1523 K for 5 min
Composition (at.%) Ti
V
Cr
Fe
Zr
8.00 19.98 9.39 9.77 10.02 14.38
80.81 66.88 79.14 79.39 77.93 64.17
6.10 4.78 5.29 5.41 5.54 11.99
4.90 7.66 5.86 5.10 6.11 8.48
0.18 0.71 0.31 0.33 0.40 0.98
of XRD analysis. For the as-cast alloy, the secondary phase precipitated along the grain boundary of main phase and formed a three-dimension network. After heat treatment at 1373 K for 8 h, the amount of secondary phase decreases and segregates as small point shape. After heat treatment at 1523 K for 5 min, the secondary phase appears as both networking and small point shape. The chemical composition of the sample is semi-quantitatively determined by EDS analysis and the results are listed in Table 1. It can be seen that the change in composition is noticeable owing to the differences in annealing temperature and keeping time. During heat treatment of 1373 K for 8 h, the V element diffuses from BCC phase to C14 Laves, resulting in the increase of V content in C14 phase. As a whole, the content of each element in the two phases becomes closely, which is due to the long temperature-keeping process. During heat treatment of 1523 K for 5 min, the Cr element diffuses from BCC phase to C14 Laves phase, while the elements of Ti, Fe and Zr from C14 phase to BCC phase, however, no obvious change in V content is observed. Fig. 4 shows the hydrogen absorption curves of the as-cast and heat-treated Ti10 V77 Cr6 Fe6 Zr alloys at 298 K. It can be seen that all the samples can be activated easily without previous thermal treatment, thereinto, the as-cast alloy has the best activation behavior. After activation, above 90% of maximum hydrogen absorption capacities are achieved in 5 min for all the samples. The samples a, b and c have maximum hydrogen absorption capacity of 3.11 wt.%, 2.272 wt.% and 2.671 wt.%, respectively. Fig. 2 shows the relationship between the maximum hydrogen absorption capacities and lattice parameters of the samples. It can be seen that the maximum hydrogen absorption capacities show a rigorously direct proportion with the lattice parameters of the BCC main phase. Obviously, the phase abundance is an overwhelming part for the consideration of the hydrogen absorption capacity for the multiphase alloys [22,23,26], which is not changed distinctly in the studied samples. In the previous report [27], the maximum hydrogen absorption capacities is related to the electron concentration e/a, while the hydrogen absorption capacities are decreased after heat treatment without the change in the value of e/a. This implies that the dependence of hydrogen absorption capacities on the value of e/a should be restricted to the as-cast alloy. Generally, the heat treatment could homogenize the composition and reduce the lattice strain. Therefore, it can be concluded that the factors related to the maximum hydrogen absorption capacities involves four aspects: lattice parameters, phase abundances, the values of e/a and internal lattice strain. In the present study, the hydrogen absorption capacities are the result of compensative effect of the four aspects. Fig. 5 shows the dehydriding curves of the as-cast and heattreated Ti10 V77 Cr6 Fe6 Zr alloys at 333 K against 0.1 MPa. Obviously, the excellent kinetics can be seen for all samples, and the maximum hydrogen desorption capacities are basically obtained within 5 min. From sample (a) to (c), the hydrogen desorption capacity first decreases and then increases, which is paralleled with the change in the hydrogen absorption capacity of the three samples. After heat treatment at 1523 K for 5 min, the hydrogen desorption capacity of 1.82 wt.% is achieved.
Fig. 4. The hydrogen absorption curves of the as-cast and heat-treated Ti10 V77 Cr6 Fe6 Zr alloys: (a) as-cast; (b) heat-treated at 1373 K for 8 h; and (c) heattreated at 1523 K for 5 min.
Z. Hang et al. / Journal of Alloys and Compounds 529 (2012) 128–133
Fig. 5. The hydrogen desorption curves of the as-cast and heat-treated Ti10 V77 Cr6 Fe6 Zr alloys at 333 K: (a) as-cast; (b) heat-treated at 1373 K for 8 h; and (c) heat-treated at 1523 K for 5 min.
Fig. 6 shows the P–C–T hydrogen desorption curves of the as-cast and heat-treated Ti10 V77 Cr6 Fe6 Zr samples. The plateau hydrogen pressure (Peq ) and the slope factor (Sf ) are defined as follows: Peq =
Sf =
p1 + p2 , 2
d(ln P) d(H/M)
where the P1 and P2 are the tuning pressures indicated by the arrows in Fig. 6. The calculated results are listed in Table 2. Compared with the as-cast alloy, after heat treatment at 1373 K for 8 h, the hydrogen desorption plateau pressure increases and the plateau width (effective hydrogen desorption capacity) decreases; however, after heat treatment at 1523 K for 5 min, the hydrogen desorption plateau pressure decreases and the plateau width (effective hydrogen desorption capacity) increases. For the XRD data obtained from the samples with the same powder size distribution, the full width half maximum (FWHM) of the X-ray diffraction can be used to qualitatively evaluate the lattice strain in the V-based solid solution alloys [28], which may also contribute to determining the plateau slope behavior in addition to the chemical energy effect [29].
Fig. 6. The dehydriding P-C-T curves of the as-cast and heat-treated Ti10 V77 Cr6 Fe6 Zr alloys: (a) as-cast; (b) heat-treated at 1373 K for 8 h; (c) heat-treated at 1523 K for 5 min.
131
Fig. 7. The relationship between FWHM and desorption plateau slope of the ascast and heat-treated samples: (a) as-cast; (b) heat-treated at 1373 K for 8 h; (c) heat-treated at 1523 K for 5 min.
Fig. 7 shows the variation in the FWHM of [1 1 0] peaks (BCC main phase) and desorption plateau slope for the samples with different heat treatment condition. It can be seen that the value of FWHM for the sample noticeably decreases after heat treatment, which implies that the lattice strain is reduced effectively. This phenomenon can be interpreted as one of the effects of the heat treatment. Meanwhile, the change of hydrogen desorption plateau slope exhibits the same trend as that of the value of FWHM. This indicates that the plateau slope is strongly influenced by the lattice strain. Thus, the heat treatment plays an important role in flattening the plateau through decreasing the lattice strain. Among the studied alloys, the sample heat treated at 1523 K for 5 min has high hydrogen absorption capacity, moderate plateau pressure, wide and flattened plateau region, and hydrogen desorption capacity of 1.82 wt.%. To investigate the residual hydrogen in the samples after P–C–T measurement at 333 K, the XRD analysis was employed (shown in Fig. 8). It is found that the main phase of samples has transformed from fully hydrided VH2 -based phase with FCC structure to the V2 H0.85 -based phase with BCT structure, which could be implied by the dehydriding P–C–T measurement. Fig. 9 shows the relationships between lattice parameters and plateau pressure and hydrogen desorption capacities of the as-cast and heat-treated Ti10 V77 Cr6 Fe6 Zr alloys. Clearly, the plateau pressure and the hydrogen desorption capacity show an inverse and
Fig. 8. XRD patterns of the as-cast and heat-treated Ti10 V77 Cr6 Fe6 Zr alloys after P–C–T measurement: (a) as-cast; (b) heat-treated at 1373 K for 8 h; and (c) heattreated at 1523 K for 5 min.
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Table 2 The data for hydrogen storage of the as-cast and heat-treated Ti10 V77 Cr6 Fe6 Zr alloys: (a) as-cast; (b) heat-treated at 1373 K for 8 h; and (c) heat-treated at 1523 K for 5 min. Sample
Electron-to-atom ratio (e/a)
Maximum hydrogen absorption capacity at 298 K (wt.%)
Effective hydrogen desorption capacity at 333 K (wt.%)
Hydrogen desorption plateau pressure, Peq at 333 K (MPa)
Slope factor (Sf )
a b c
5.13
3.11 2.272 2.671
1.44 1.03 1.82
1.11 1.17 0.75
0.64 0.31 0.10
temperatures is around 660 K for all the samples. By integrating the function in the endothermic region, the decomposition enthalpies of 153.8 J/g, 191 J/g and 192.7 J/g for V2 H0.85 phase are calculated for the samples a, b and c, respectively, which implies that the heat treatments are harmful for the complete release of residual hydrogen for the samples after P–C–T measurements at 333 K. 4. Conclusions
Fig. 9. The relationships between dehydriding plateau pressure and hydrogen desorption capacities and lattice parameter of the as-cast and heat-treated Ti10 V77 Cr6 Fe6 Zr alloys: (a) as-cast; (b) heat-treated at 1373 K for 8 h; and (c) heattreated at 1523 K for 5 min.
direct proportion with the lattice parameters, respectively. The effective hydrogen desorption capacity is enslaved to the hydrogen absorption capacity, the pressure range for testing, the plateau pressure, the width and slope of the hydrogen desorption plateau, and so on. Then, the dependence of hydrogen desorption capacities on lattice parameters should be comprehended as the synergetic effects of the above factors. The DSC curves of the samples after dehydriding P–C–T measurement at 333 K are obtained to examine the hydrogen desorption of the samples at high temperature, i.e. the hydrogen release of V2 H0.85 phase indicated by Fig. 8. It can be seen from Fig. 10 that the temperatures for hydrogen release locate in the range between 600 K and 700 K, among which the peak
Fig. 10. DSC curves of the as-cast and heat-treated Ti10 V77 Cr6 Fe6 Zr alloys: (a) ascast; (b) heat-treated at 1373 K for 8 h; and (c) heat-treated at 1523 K for 5 min.
The influence of heat treatment on the microstructure and hydrogen storage properties of Ti10 V77 Cr6 Fe6 Zr alloy has been investigated systematically. It is found that all alloys consist of BCC main phase and C14 Laves secondary phase. After heat treatment, the content of C14 Laves phase decreases and the hydrogen absorption capacity is lowered, while the hydrogen desorption plateau is flattened distinctly. The sample heat-treated at 1523 K for 5 min has the best overall hydrogen storage properties, with hydrogen desorption capacity of 1.82 wt.%, dehydriding plateau pressure of 0.75 MPa and sloping factor of the plateau of 0.1. Acknowledgments This work is jointly supported by the National High Technology Research and Development Program of China (2012AA051503 and 2006AA05Z144). References [1] G. Sandrock, J. Alloys Compd. 293–295 (1999) 877. [2] T. Kabutomori, H. Takeda, Y. Wakisaka, K. Ohnishi, J. Alloys Compd. 231 (1995) 528. [3] E. Akiba, H. Iba, Intermetallics 6 (1998) 461. [4] S. Cho, C. Han, C. Park, E. Akiba, J. Alloys Compd. 288 (1999) 294. [5] T. Kuriiwa, T. Tamura, T. Amemiya, T. Fuda, A. Kamegawa, H. Takamura, M. Okada, J. Alloys Compd. 293–295 (1999) 433. [6] S. Cho, H. Enoki, E. Akiba, J. Alloys Compd. 307 (2000) 304. [7] C. Seo, J. Kim, P. Lee, J. Lee, J. Alloys Compd. 348 (2003) 252. [8] M. Okada, T. Kuriiwa, A. Kamegawa, H. Takamura, Mater. Sci. Eng. A 329–331 (2002) 305. [9] T. Tamura, Y. Tominaga, K. Matsumoto, T. Fuda, T. Kuriiwa, A. Kamegawa, H. Takamura, M. Okada, J. Alloys Compd. 330–332 (2002) 522. [10] S. Cho, C. Han, C. Park, E. Akiba, J. Alloys Compd. 289 (2002) 244. [11] T.Z. Huang, Z. Wu, J.Z. Chen, X.B. Yu, B.J. Xia, N.X. Xu, Mater. Sci. Eng. A 385 (2004) 17. [12] X.B. Yu, Z. Wu, B.J. Xia, N.X. Xu, J. Alloys Compd. 372 (2004) 272. [13] T.Z. Huang, Z. Wu, B.J. Xia, N.X. Xu, Intermetallics 13 (2005) 1075. [14] Y.G Yan, Y.G. Chen, H. Liang, C.L. Wu, M.D. Tao, M.J. Tu, J. Alloys Compd. 426 (2006) 253. [15] X.B. Yu, Z.X. Yang, S.L. Feng, Z. Wu, N.X. Xu, Int. J. Hydrogen Energy 31 (2006) 1176. [16] S. Basak, K. Shashikala, P. Sengupta, S.K. Kulshreshtha, Int. J. Hydrogen Energy 32 (2007) 4973. [17] S. Challet, M. Latroche, F. Heurtaux, J. Alloys Compd. 439 (2007) 294. [18] H. Liang, Y.G. Chen, Y.G. Yan, C.L. Wu, M.D. Tao, Mater. Sci. Eng. A 459 (2007) 204. [19] Y.G. Yan, Y.G. Chen, H. Liang, X.X. Zhou, C.L. Wu, M.D. Tao, L.J. Pang, J. Alloys Compd. 454 (2008) 427. [20] Y.G. Yan, Y.G. Chen, X.X. Zhou, H. Liang, C.L. Wu, M.D. Tao, J. Alloys Compd. 453 (2008) 428. [21] X.P. Liu, F. Cuevas, L.J. Jiang, M. Latroche, Z.N. Li, S.M. Wang, J Alloys Compd. 476 (2009) 403.
Z. Hang et al. / Journal of Alloys and Compounds 529 (2012) 128–133 [22] Z.M. Hang, X.Z. Xiao, D.Z. Tan, Z.H. He, W.P. Li, S.Q. Li, C.P. Chen, L.X. Chen, Int. J. Hydrogen Energy 35 (2010) 3080. [23] Z.M. Hang, L.X. Chen, X.Z. Xiao, S.Q. Li, C.P. Chen, Y.Q. Lei, Q.D. Wang, J. Alloy Compd. 493 (2010) 396. [24] M. Okada, T. Kuriiwa, T. Tamura, H. Takamura, A. Kamegawa, J. Alloys Compd. 330–322 (2002) 511. [25] S. Cho, G. Shim, G. Cho, C. Park, J. Yoo, J. Choi, J. Alloys Compd. 430 (2007) 136.
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[26] Z.M. Hang, X.Z. Xiao, K.R. Yu, S.Q. Li, C.P. Chen, L.X. Chen, Int. J. Hydrogen Energy 35 (2010) 8143. [27] J.F. Lynch, A.J. Maeland, G.G. Libowitz, Z. Phys. Chem. 145 (1985) 51. [28] X.B. Yu, J.Z. Chen, Z. Wu, B.J. Xia, N.X. Xu, Int. J. Hydrogen Energy 29 (2004) 1377. [29] X.B. Yu, S.L. Feng, Z. Wu, B.J. Xia, N.X. Xu, J. Alloys Compd. 393 (2005) 129.
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Influence of heat treatment on the microstructure and hydrogen storage properties of Ti10 V77 Cr6 Fe6 Zr alloy Zhouming Hang a,b , Xuezhang Xiao a , Shouquan Li a , Hongwei Ge a , Changpin Chen a , Lixin Chen a,∗ a b
Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China Zhejiang Test Academy of Quality and Technical Supervision, Hangzhou 310023, China
a r t i c l e
i n f o
Article history: Received 12 November 2011 Received in revised form 20 February 2012 Accepted 10 March 2012 Available online 17 March 2012 Keywords: V-based solid solution alloy Heat treatment Microstructure Hydrogen storage properties
a b s t r a c t The as-cast Ti10 V77 Cr6 Fe6 Zr alloy was heat-treated at 1373 K for 8 h or 1523 K for 5 min and then quenched in water. The influence of heat treatment on the microstructure and hydrogen storage properties of Ti10 V77 Cr6 Fe6 Zr alloy was investigated systematically. The results show that all of the as-cast and heattreated alloys consist of BCC main phase and C14 Laves secondary phase. After heat treatment, the phase abundance of BCC enhances and the plateau region of P–C–T curve is flattened, but the hydrogen absorption capacity is decreased. However, the alloy heat-treated at 1523 K for 5 min achieves a enhanced hydrogen desorption capacity of 1.82 wt.% at 333 K against 0.1 MPa, which is higher than 1.44 wt.% hydrogen desorption capacity of the as-cast alloy. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Vanadium or vanadium-based solid solutions, as the third generation of hydrogen storage alloys, have been widely studied and several series of V-based multi-component alloys with good hydrogen storage properties have been developed. Among which Ti–Cr–V alloys attracted much more attention for its high capacity of absorbing hydrogen and excellent kinetics for hydrogen absorption and desorption at moderate conditions [1–4]. However, these alloys exists some shortcomings, such as difficulty in activation, poor P–C–T plateau characteristics and low hydrogen desorption capacity for the much lower dehydrogenation pressure plateau region of mono-hydride (e.g. plateau pressure of about 1 Pa at room temperature for the hydrogen desorption of VH) [3–8]. It is well known that the partial substitution of V with other transition elements, such as Fe, Zr and Mn, is a very effective way to improve the overall hydrogen storage properties and lowering their cost of the Ti–Cr–V alloys [9,10,6,11–23]. On the other hand, it was reported that the heat treatment could also impact the microstructure and hydrogen storage properties of the Ti–Cr–V alloys [3,24,25]. Akiba and Iba [3] reported that the hydrogen storage properties of Ti–Cr–V alloys in as-cast state were sensitive to heating, therefore it was reasonable to believe that the heat treatment might affect the hydrogen storage properties of these alloys. Okada et al. [24] investigated the effect of heat treatment on the
∗ Corresponding author. Tel.: +86 571 8795 1152; fax: +86 571 8795 1152. E-mail address: [email protected] (L. Chen). 0925-8388/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2012.03.044
hydrogen storage properties of the Ti–Cr–V alloys, and reported that moderate heat treatment could enhance the hydrogen storage capacity and flatten the hydrogen desorption pressure plateau, e.g. the Ti25 Cr40 V35 alloy annealed at 1573 K for 1 min and then quenched in water achieved a hydrogen desorption capacity of 2.4 mass% at 313 K against 0.01 MPa. Cho et al. [25] also reported that Ti32 Cr43 V25 alloy annealed at 1653 K for 1 min achieved a hydrogen desorption capacity of 2.3 mass% at 303 K against 0.001 MPa. However, the influence of the heat treatments with the relatively lower temperature and longer time (such as 1523 K for 5 min or 1373 K for 8 h) on the microstructure and hydrogen storage properties was not reported. In this paper, the corresponding heat treatment conditions (annealed at 1523 K for 5 min and 1373 K for 8 h) were employed for Ti10 V77 Cr6 Fe6 Zr alloy which was explored and optimized in our previous work [22,23], and the influence of heat treatment on the microstructure and hydrogen storage properties of this alloy was investigated in detail. 2. Experimental The as-cast Ti10 V77 Cr6 Fe6 Zr alloy was prepared by levitation induction melting under argon atmosphere, and the ingot was turned over and remelted four times to ensure its homogeneity. Then two methods of heat treatment were adopted: (1) annealing at 1373 K for 8 h and then quenching in water; (2) annealing at 1523 K for 5 min and then quenching in water. In order to investigate the phase structure, the morphology and the phase composition of the alloy, X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS) were performed, respectively. For activation procedure, the sample of 4 g was placed in the reactor of Sieverts type apparatus and evacuated for 15 min under room temperature firstly, and then
Z. Hang et al. / Journal of Alloys and Compounds 529 (2012) 128–133
129
Fig. 1. XRD patterns of the as-cast and heat-treated Ti10 V77 Cr6 Fe6 Zr alloys: (a) ascast; (b) heat-treated at 1373 K for 8 h and then quenched in cold water; and (c) heat treated at 1523 K for 5 min and then quenched in cold water.
hydrogen was introduced gradually into the reactor up to a pressure of 4 MPa for the absorption process. Also the hydriding testing was performed under room temperature with a starting pressure of 4 MPa. The dehydriding kinetics curves were obtained against 0.1 MPa at 333 K. After dehydriding, the reactor was evacuated for 30 min at 673 K to extract the residual hydrogen for the next hydriding process. After three hydriding-dehydriding cycling, the dehydrogenation P–C–T measurement were carried out at 333 K. The change in pressure with time was recorded. In this study, the effective hydrogen desorption capacity is defined as the amount of hydrogen desorbed when the hydrogen pressure is decreased from 3 MPa to 0.1 MPa. Differential scanning calorimetry (Netzsch STA 449F3) measurements of the samples were performed under the pressure of 0.1 MPa with a constant heating rate of 10 K/min. The powder size used for the XRD and DSC measurements is under 30 m.
3. Results and discussion Fig. 1 shows the XRD patterns of the studied alloys. It can be seen from Fig. 1 that the as-cast alloy consists of BCC main phase and C14 Laves secondary phase. After heat treatment, the intensity of diffraction peak of the BCC phase enhances, while that of C14 Laves phase becomes weak, which implies that these heat treatments promote the growing of the BCC phase. The diffraction peak shifts to higher angle and lower angle for samples b and c, respectively. The lattice parameters of the BCC main phase for the studied alloys were determined by the Retvield refinement
Fig. 3. SEM micrographs of the as-cast and heat-treated Ti10 V77 Cr6 Fe6 Zr alloys: (a) as-cast; (b) heat-treated at 1373 K for 8 h; and (c) heat-treated at 1523 K for 5 min.
Fig. 2. The relationship between heat treatment conditions and lattice parameters and hydrogen absorption capacities: (a) as-cast; (b) heat-treated at 1373 K for 8 h; and (c) heat-treated at 1523 K for 5 min.
analyses and calculated as 0.30363 nm, 0.30191 nm and 0.30283 nm, which are shown in Fig. 2. The reason why the lattice parameter decreased by the heat treatment might be attributed to the change in the element contents of the BCC main phase after different heat treatments. Fig. 3 shows the SEM micrographs of the as-cast and heattreated Ti10 V77 Cr6 Fe6 Zr alloys. It can be observed that all the alloys consist of two phases, which is in good agreement with the results
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Z. Hang et al. / Journal of Alloys and Compounds 529 (2012) 128–133
Table 1 The results of EDS analyses for the as-cast and heat-treated Ti10 V77 Cr6 Fe6 Zr samples. Samples
Phase
As-cast
BCC C14 BCC C14 BCC C14
Heat-treated at 1373 K for 8 h Heat-treated at 1523 K for 5 min
Composition (at.%) Ti
V
Cr
Fe
Zr
8.00 19.98 9.39 9.77 10.02 14.38
80.81 66.88 79.14 79.39 77.93 64.17
6.10 4.78 5.29 5.41 5.54 11.99
4.90 7.66 5.86 5.10 6.11 8.48
0.18 0.71 0.31 0.33 0.40 0.98
of XRD analysis. For the as-cast alloy, the secondary phase precipitated along the grain boundary of main phase and formed a three-dimension network. After heat treatment at 1373 K for 8 h, the amount of secondary phase decreases and segregates as small point shape. After heat treatment at 1523 K for 5 min, the secondary phase appears as both networking and small point shape. The chemical composition of the sample is semi-quantitatively determined by EDS analysis and the results are listed in Table 1. It can be seen that the change in composition is noticeable owing to the differences in annealing temperature and keeping time. During heat treatment of 1373 K for 8 h, the V element diffuses from BCC phase to C14 Laves, resulting in the increase of V content in C14 phase. As a whole, the content of each element in the two phases becomes closely, which is due to the long temperature-keeping process. During heat treatment of 1523 K for 5 min, the Cr element diffuses from BCC phase to C14 Laves phase, while the elements of Ti, Fe and Zr from C14 phase to BCC phase, however, no obvious change in V content is observed. Fig. 4 shows the hydrogen absorption curves of the as-cast and heat-treated Ti10 V77 Cr6 Fe6 Zr alloys at 298 K. It can be seen that all the samples can be activated easily without previous thermal treatment, thereinto, the as-cast alloy has the best activation behavior. After activation, above 90% of maximum hydrogen absorption capacities are achieved in 5 min for all the samples. The samples a, b and c have maximum hydrogen absorption capacity of 3.11 wt.%, 2.272 wt.% and 2.671 wt.%, respectively. Fig. 2 shows the relationship between the maximum hydrogen absorption capacities and lattice parameters of the samples. It can be seen that the maximum hydrogen absorption capacities show a rigorously direct proportion with the lattice parameters of the BCC main phase. Obviously, the phase abundance is an overwhelming part for the consideration of the hydrogen absorption capacity for the multiphase alloys [22,23,26], which is not changed distinctly in the studied samples. In the previous report [27], the maximum hydrogen absorption capacities is related to the electron concentration e/a, while the hydrogen absorption capacities are decreased after heat treatment without the change in the value of e/a. This implies that the dependence of hydrogen absorption capacities on the value of e/a should be restricted to the as-cast alloy. Generally, the heat treatment could homogenize the composition and reduce the lattice strain. Therefore, it can be concluded that the factors related to the maximum hydrogen absorption capacities involves four aspects: lattice parameters, phase abundances, the values of e/a and internal lattice strain. In the present study, the hydrogen absorption capacities are the result of compensative effect of the four aspects. Fig. 5 shows the dehydriding curves of the as-cast and heattreated Ti10 V77 Cr6 Fe6 Zr alloys at 333 K against 0.1 MPa. Obviously, the excellent kinetics can be seen for all samples, and the maximum hydrogen desorption capacities are basically obtained within 5 min. From sample (a) to (c), the hydrogen desorption capacity first decreases and then increases, which is paralleled with the change in the hydrogen absorption capacity of the three samples. After heat treatment at 1523 K for 5 min, the hydrogen desorption capacity of 1.82 wt.% is achieved.
Fig. 4. The hydrogen absorption curves of the as-cast and heat-treated Ti10 V77 Cr6 Fe6 Zr alloys: (a) as-cast; (b) heat-treated at 1373 K for 8 h; and (c) heattreated at 1523 K for 5 min.
Z. Hang et al. / Journal of Alloys and Compounds 529 (2012) 128–133
Fig. 5. The hydrogen desorption curves of the as-cast and heat-treated Ti10 V77 Cr6 Fe6 Zr alloys at 333 K: (a) as-cast; (b) heat-treated at 1373 K for 8 h; and (c) heat-treated at 1523 K for 5 min.
Fig. 6 shows the P–C–T hydrogen desorption curves of the as-cast and heat-treated Ti10 V77 Cr6 Fe6 Zr samples. The plateau hydrogen pressure (Peq ) and the slope factor (Sf ) are defined as follows: Peq =
Sf =
p1 + p2 , 2
d(ln P) d(H/M)
where the P1 and P2 are the tuning pressures indicated by the arrows in Fig. 6. The calculated results are listed in Table 2. Compared with the as-cast alloy, after heat treatment at 1373 K for 8 h, the hydrogen desorption plateau pressure increases and the plateau width (effective hydrogen desorption capacity) decreases; however, after heat treatment at 1523 K for 5 min, the hydrogen desorption plateau pressure decreases and the plateau width (effective hydrogen desorption capacity) increases. For the XRD data obtained from the samples with the same powder size distribution, the full width half maximum (FWHM) of the X-ray diffraction can be used to qualitatively evaluate the lattice strain in the V-based solid solution alloys [28], which may also contribute to determining the plateau slope behavior in addition to the chemical energy effect [29].
Fig. 6. The dehydriding P-C-T curves of the as-cast and heat-treated Ti10 V77 Cr6 Fe6 Zr alloys: (a) as-cast; (b) heat-treated at 1373 K for 8 h; (c) heat-treated at 1523 K for 5 min.
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Fig. 7. The relationship between FWHM and desorption plateau slope of the ascast and heat-treated samples: (a) as-cast; (b) heat-treated at 1373 K for 8 h; (c) heat-treated at 1523 K for 5 min.
Fig. 7 shows the variation in the FWHM of [1 1 0] peaks (BCC main phase) and desorption plateau slope for the samples with different heat treatment condition. It can be seen that the value of FWHM for the sample noticeably decreases after heat treatment, which implies that the lattice strain is reduced effectively. This phenomenon can be interpreted as one of the effects of the heat treatment. Meanwhile, the change of hydrogen desorption plateau slope exhibits the same trend as that of the value of FWHM. This indicates that the plateau slope is strongly influenced by the lattice strain. Thus, the heat treatment plays an important role in flattening the plateau through decreasing the lattice strain. Among the studied alloys, the sample heat treated at 1523 K for 5 min has high hydrogen absorption capacity, moderate plateau pressure, wide and flattened plateau region, and hydrogen desorption capacity of 1.82 wt.%. To investigate the residual hydrogen in the samples after P–C–T measurement at 333 K, the XRD analysis was employed (shown in Fig. 8). It is found that the main phase of samples has transformed from fully hydrided VH2 -based phase with FCC structure to the V2 H0.85 -based phase with BCT structure, which could be implied by the dehydriding P–C–T measurement. Fig. 9 shows the relationships between lattice parameters and plateau pressure and hydrogen desorption capacities of the as-cast and heat-treated Ti10 V77 Cr6 Fe6 Zr alloys. Clearly, the plateau pressure and the hydrogen desorption capacity show an inverse and
Fig. 8. XRD patterns of the as-cast and heat-treated Ti10 V77 Cr6 Fe6 Zr alloys after P–C–T measurement: (a) as-cast; (b) heat-treated at 1373 K for 8 h; and (c) heattreated at 1523 K for 5 min.
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Z. Hang et al. / Journal of Alloys and Compounds 529 (2012) 128–133
Table 2 The data for hydrogen storage of the as-cast and heat-treated Ti10 V77 Cr6 Fe6 Zr alloys: (a) as-cast; (b) heat-treated at 1373 K for 8 h; and (c) heat-treated at 1523 K for 5 min. Sample
Electron-to-atom ratio (e/a)
Maximum hydrogen absorption capacity at 298 K (wt.%)
Effective hydrogen desorption capacity at 333 K (wt.%)
Hydrogen desorption plateau pressure, Peq at 333 K (MPa)
Slope factor (Sf )
a b c
5.13
3.11 2.272 2.671
1.44 1.03 1.82
1.11 1.17 0.75
0.64 0.31 0.10
temperatures is around 660 K for all the samples. By integrating the function in the endothermic region, the decomposition enthalpies of 153.8 J/g, 191 J/g and 192.7 J/g for V2 H0.85 phase are calculated for the samples a, b and c, respectively, which implies that the heat treatments are harmful for the complete release of residual hydrogen for the samples after P–C–T measurements at 333 K. 4. Conclusions
Fig. 9. The relationships between dehydriding plateau pressure and hydrogen desorption capacities and lattice parameter of the as-cast and heat-treated Ti10 V77 Cr6 Fe6 Zr alloys: (a) as-cast; (b) heat-treated at 1373 K for 8 h; and (c) heattreated at 1523 K for 5 min.
direct proportion with the lattice parameters, respectively. The effective hydrogen desorption capacity is enslaved to the hydrogen absorption capacity, the pressure range for testing, the plateau pressure, the width and slope of the hydrogen desorption plateau, and so on. Then, the dependence of hydrogen desorption capacities on lattice parameters should be comprehended as the synergetic effects of the above factors. The DSC curves of the samples after dehydriding P–C–T measurement at 333 K are obtained to examine the hydrogen desorption of the samples at high temperature, i.e. the hydrogen release of V2 H0.85 phase indicated by Fig. 8. It can be seen from Fig. 10 that the temperatures for hydrogen release locate in the range between 600 K and 700 K, among which the peak
Fig. 10. DSC curves of the as-cast and heat-treated Ti10 V77 Cr6 Fe6 Zr alloys: (a) ascast; (b) heat-treated at 1373 K for 8 h; and (c) heat-treated at 1523 K for 5 min.
The influence of heat treatment on the microstructure and hydrogen storage properties of Ti10 V77 Cr6 Fe6 Zr alloy has been investigated systematically. It is found that all alloys consist of BCC main phase and C14 Laves secondary phase. After heat treatment, the content of C14 Laves phase decreases and the hydrogen absorption capacity is lowered, while the hydrogen desorption plateau is flattened distinctly. The sample heat-treated at 1523 K for 5 min has the best overall hydrogen storage properties, with hydrogen desorption capacity of 1.82 wt.%, dehydriding plateau pressure of 0.75 MPa and sloping factor of the plateau of 0.1. Acknowledgments This work is jointly supported by the National High Technology Research and Development Program of China (2012AA051503 and 2006AA05Z144). References [1] G. Sandrock, J. Alloys Compd. 293–295 (1999) 877. [2] T. Kabutomori, H. Takeda, Y. Wakisaka, K. Ohnishi, J. Alloys Compd. 231 (1995) 528. [3] E. Akiba, H. Iba, Intermetallics 6 (1998) 461. [4] S. Cho, C. Han, C. Park, E. Akiba, J. Alloys Compd. 288 (1999) 294. [5] T. Kuriiwa, T. Tamura, T. Amemiya, T. Fuda, A. Kamegawa, H. Takamura, M. Okada, J. Alloys Compd. 293–295 (1999) 433. [6] S. Cho, H. Enoki, E. Akiba, J. Alloys Compd. 307 (2000) 304. [7] C. Seo, J. Kim, P. Lee, J. Lee, J. Alloys Compd. 348 (2003) 252. [8] M. Okada, T. Kuriiwa, A. Kamegawa, H. Takamura, Mater. Sci. Eng. A 329–331 (2002) 305. [9] T. Tamura, Y. Tominaga, K. Matsumoto, T. Fuda, T. Kuriiwa, A. Kamegawa, H. Takamura, M. Okada, J. Alloys Compd. 330–332 (2002) 522. [10] S. Cho, C. Han, C. Park, E. Akiba, J. Alloys Compd. 289 (2002) 244. [11] T.Z. Huang, Z. Wu, J.Z. Chen, X.B. Yu, B.J. Xia, N.X. Xu, Mater. Sci. Eng. A 385 (2004) 17. [12] X.B. Yu, Z. Wu, B.J. Xia, N.X. Xu, J. Alloys Compd. 372 (2004) 272. [13] T.Z. Huang, Z. Wu, B.J. Xia, N.X. Xu, Intermetallics 13 (2005) 1075. [14] Y.G Yan, Y.G. Chen, H. Liang, C.L. Wu, M.D. Tao, M.J. Tu, J. Alloys Compd. 426 (2006) 253. [15] X.B. Yu, Z.X. Yang, S.L. Feng, Z. Wu, N.X. Xu, Int. J. Hydrogen Energy 31 (2006) 1176. [16] S. Basak, K. Shashikala, P. Sengupta, S.K. Kulshreshtha, Int. J. Hydrogen Energy 32 (2007) 4973. [17] S. Challet, M. Latroche, F. Heurtaux, J. Alloys Compd. 439 (2007) 294. [18] H. Liang, Y.G. Chen, Y.G. Yan, C.L. Wu, M.D. Tao, Mater. Sci. Eng. A 459 (2007) 204. [19] Y.G. Yan, Y.G. Chen, H. Liang, X.X. Zhou, C.L. Wu, M.D. Tao, L.J. Pang, J. Alloys Compd. 454 (2008) 427. [20] Y.G. Yan, Y.G. Chen, X.X. Zhou, H. Liang, C.L. Wu, M.D. Tao, J. Alloys Compd. 453 (2008) 428. [21] X.P. Liu, F. Cuevas, L.J. Jiang, M. Latroche, Z.N. Li, S.M. Wang, J Alloys Compd. 476 (2009) 403.
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