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International Journal of Hydrogen Energy 28 (2003) 389–394

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In$uence of annealing treatment on Laves phase compoundcontaining a V-based BCC solid solution phase—Part I:

Crystal structures

Yunfeng Zhu, Hongge Pan ∗, Mingxia Gao, Yongfeng Liu, QidongWangDepartment of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China

Abstract

Two multi-component Laves phase hydrogen storage alloys containing a body centered cubic (BCC) solid solution phasewere prepared and the e8ects of annealing treatment on their crystal structures have been studied in this part. It is found byX-ray powder di8raction and energy dispersive X-ray spectrometer analysis that the as-cast alloys mainly consist of two phases:the C14 Laves phase matrix with hexagonal structure and the dendritic V-based solid solution phase with BCC structure. Inaddition, a small amount of TiNi-based third phase is also found precipitated within both the C14 Laves phase and the V-basedsolid solution phase. However, the content of the TiNi-based phase is decreased greatly by an appropriate annealing treatmentowing to the compositional homogenization. Furthermore, the lattice parameters and unit cell volumes of both the C14 Lavesphase and the V-based solid solution phase have all increased after the annealing treatment.? 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.

Keywords: Hydrogen storage alloy; C14 Laves phase; V-based solid solution; Annealing treatment; Microstructure

1. Introduction

Laves phase hydrogen storage alloys with MgZn2 orMgCu2 structures used for the negative electrode materialsin nickel–metal hydride (Ni-MH) secondary batteries havebeen extensively studied due to their larger hydrogen capac-ities in comparison with the conventional rare earth-basedAB5-type alloys [1–3]. They showed that these alloys wereof either single or combined C14 and C15 Laves phase,and the overall hydrogen storage properties were improvedgreatly by multi-component alloying with A-side elementsbeing Ti and Zr and B-side elements being V, Mn, Cr, andNi and others.It is believed that the body centered cubic (BCC) alloys

generally have a large hydrogen capacity and the solid so-lution phase of a V-based alloy with BCC structure is a rep-resentative one [4]. However, Tsukahara et al. [5,6] pointedthat the V-based solid solution phase alone had very little

∗ Corresponding author. Tel.: +86-571-8795-2576; fax: +86-571-8795-1152.

E-mail address: honggepan@zjuem.zju.edu.cn (H. Pan).

discharge capacity in the alkaline electrolyte due to the lackof electro-catalytic activity, yet it could be activated to ab-sorb and desorb a large amount of hydrogen with the pres-ence of a secondary phase, such as the C14 Laves phaseor the TiNi phase, which was considered to act both asa micro-current collector and as an electro-catalyst. Akibaet al. [7] proposed a new concept of hydrogen absorbing al-loy, namely ‘Laves phase related BCC solid solution’. Theyfound the so-called ‘Laves phase related BCC solid solution’alloys to have large hydrogen capacities (above 2 mass%)and fast hydrogen absorption and desorption kinetics at theambient temperature and pressure.From the above results, we believe that the multi-

component, multi-phase alloys have provided us a newopportunity for the design of high-performance hydrogenstorage alloys as have been proved by Notten et al. [8].They formulated a new class of highly electro-catalyticmaterials with two phases: one being the LaNi5 based bulkphase responsible for hydrogen storage, and the other beingthe precipitated phase, such as MoCo3, responsible for theelectro-catalytic activation of the electrochemical hydrogenreaction of the bulk phase.

0360-3199/02/$ 30.00 ? 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.PII: S 0360 -3199(02)00078 -2

390 Y. Zhu et al. / International Journal of Hydrogen Energy 28 (2003) 389–394

In this paper, we also formulated two multi-component,multiphase Ti-based hydrogen storage alloys. Besides,some scientists have reported that the annealing treatmentcould e8ectively improve the performance of the hydrogenstorage alloys owing to the compositional homogeniza-tion [9–11]. For improving the performance of the alloysinitiated by us, we studied the e8ects of annealing treat-ment on their properties, including the crystal structuresand the electrochemical properties. In this part, we reportonly the e8ects on the crystal structures. The e8ects on theelectrochemical properties will be reported in the secondpart.

2. Experimental

The alloys Ti0:8Zr0:2V3:2Mn0:64Cr0:96Ni1:2 and Ti0:8Zr0:2V3:733Mn0:747Cr1:12Ni1:4 were prepared by induction levita-tion melting of the constituent metals on a water-cooledcopper crucible under argon atmosphere. The ingots wereturned over and remelted twice to ensure a high homogene-ity. Part of the alloys was subject to an annealing treatment.They were put in a small quartz boat and then put in a largequartz tube, which was evacuated Nrst to a high vacuumbelow 10−5 Torr and subsequently purged for several timeswith Ar to get rid of the H2O and O2 remaining in the quartztube. Finally, the alloy samples were annealed for 5–11 hat 1273 K in high purity argon atmosphere (¿ 99:9999%).Each alloy sample was quenched in water with the quartzboat immediately after the assigned duration of annealingwas reached. Before measurements, the surface layer of thealloy was Nled o8.For X-ray powder di8raction (XRD) measurements, part

of the alloys were mechanically crushed and ground topowder of 6 300 mesh size. The experiments were per-formed on a Philips di8ractometer with Cu K� radiation. Formetallographic studies, samples were polished in steps toobtain a mirror-like surface Nrst and then etched with the so-lution of 10%HF, 10%HCl and 80%C2H5OH (by volume).The micrographs were examined with a scanning electronmicroscope (SEM), and the chemical compositions deter-mined with an energy dispersive X-ray spectrometer (EDS)in addition.

3. Results and discussion

Fig. 1 shows the XRD patterns of the as-cast and annealedTi0:8Zr0:2V3:2Mn0:64Cr0:96Ni1:2 alloy samples. The alloy sam-ples were annealed at 1273 K for 5–11 h. From the patterns,two distinct crystallographic phases, namely the C14 Lavesphase withMgZn2-type hexagonal structure and the V-basedsolid solution phase with BCC structure are found to coexistin all the alloy samples, which were conNrmed by the SEM(see Fig. 3) and EDS analysis (Table 3). In addition, it canbe seen that the peak intensity of the C14 Laves phase de-

30 40 50 60 70 80

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OthersV-based solid solutionC14 Laves

Fig. 1. XRD patterns of the Ti0:8Zr0:2V3:2Mn0:64Cr0:96Ni1:2 alloysamples: (a) as-cast; (b) 1273 K× 5 h; (c) 1273 K× 8 h; and (d)1273 K × 11 h.

Table 1The lattice parameters and unit cell volumes of the C14 Laves phaseand the V-based solid solution phase in the as-cast and annealedTi0:8Zr0:2V3:2Mn0:64Cr0:96Ni1:2 alloy samples

Samples Phase Lattice parameters Cell volumes( SA) ( SA3)

As-cast C14 a = 4:897 c = 7:983 165.8BCC a = 2:962 25.99

1273 K × 5 h C14 a = 4:900 c = 7:993 166.2BCC a = 2:963 26.01

1273 K × 8 h C14 a = 4:904 c = 8:000 166.6BCC a = 2:964 26.04

1273 K × 11 h C14 a = 4:910 c = 8:015 167.3BCC a = 2:965 26.07

creases after annealing treatment, which makes us believethat the content of the C14 Laves phase decreases while thecontent of the V-based solid solution phase increases in thealloy during the annealing process. The lattice parametersand unit cell volumes of both phases are calculated and pre-sented in Table 1. It can be found that the annealing treat-ment leads to an increase in lattice parameters and thus theexpansion in crystal lattice of both the C14 Laves phase andthe V-based solid solution phase. Furthermore, with the pro-longation of the holding time from 5 to 11 h at 1273 K, thelattice parameters and unit-cell volumes of both phases arefound to increase proportionately.Fig. 2 shows the XRD patterns of the as-cast and an-

nealed Ti0:8Zr0:2V3:733Mn0:747Cr1:12Ni1:4 alloy samples. Thealloy samples were also annealed at 1273 K for 5–11 h.

Y. Zhu et al. / International Journal of Hydrogen Energy 28 (2003) 389–394 391

30 40 50 60 70 80

d

b

c

a

:*

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V-based solid solutionC14 LavesOthers :

Fig. 2. XRD patterns of the Ti0:8Zr0:2V3:733Mn0:747Cr1:12Ni1:4 alloysamples: (a) as-cast; (b) 1273 K× 5 h; (c) 1273 K× 8 h; and (d)1273 K × 11 h.

Table 2The lattice parameters and unit-cell volumes of the C14 Laves phaseand the V-based solid solution phase in the as-cast and annealedTi0:8Zr0:2V3:733Mn0:747Cr1:12Ni1:4 alloy samples

Samples Phase Lattice parameters Cell volumes( SA) ( SA3)

As-cast C14 a = 4:888 c = 7:979 165.1BCC a = 2:960 25.93

1273 K × 5 h C14 a = 4:896 c = 7:984 165.7BCC a = 2:961 25.96

1273 K × 8 h C14 a = 4:899 c = 7:986 166.0BCC a = 2:962 25.99

1273 K × 11 h C14 a = 4:901 c = 7:988 166.2BCC a = 2:963 26.01

It is found that all the alloy samples are mainly com-posed of two crystallographic phases, namely the C14Laves phase with MgZn2-type hexagonal structure and theV-based solid solution phase with BCC structure similarto the ones of Fig. 1, which were conNrmed by the SEM(see Fig. 4) and EDS analysis (Table 4). Similarly, thepeak intensity of the C14 Laves phase is also found todecrease after annealing treatment, which means that thecontent of the C14 Laves phase decreases while the con-tent of the V-based solid solution phase increases in thealloy. Table 2 shows the lattice parameters and unit-cellvolumes of the two phases. It can be seen that the lat-tice parameters and unit cell volumes of both phases haveincreased after annealing treatment and also increasing

proportionately with the prolongation of the holding timeat 1273 K.Fig. 3 shows the SEM micrographs of the as-cast and

annealed Ti0:8Zr0:2V3:2Mn0:64Cr0:96Ni1:2 alloy samples. Itcan be seen that all the alloy samples are mainly com-posed of two distinct crystallographic phases: one isthe C14 Laves phase matrix in light grey (identiNed asA), and the other is the V-based solid solution phase indark grey (identiNed as B) as conNrmed by EDS anal-ysis, which is in agreement with the XRD result. TheV-based solid solution phase is in the form of den-dritic structures. Besides, a small amount of third phasein black (identiNed as C) is also found precipitated inboth the C14 Laves phase and the V-based solid solu-tion phase. It is determined by EDS analysis that thisis a phase rich in Ti and Ni. However, we cannot de-termine exactly the chemical composition of this phasedue to the great variation of its composition in di8er-ent regions. Moreover, because of the small amount ofthe TiNi-based phase in the alloy samples, it is diUcultto Nnd and identify this phase in the XRD patterns inFig. 1.It is accepted that the segregation in a multi-component

alloy casting is generally inevitable. So the appearance ofthe TiNi-based phase here is just the compositional seg-regation of the alloy during solidiNcation. After annealingtreatment, the content of the precipitated phase decreased,especially when the alloy was annealed at 1273 K for 8 h,which is probably due to the compositional homogeniza-tion by the dissolution and distribution of the TiNi-basedphase into the C14 Laves phase and the V-based solidsolution phase. Zhang et al. [12] reported that three phases,namely the C15 and C14 Laves phases and Zr7Ni10 phasewere found to coexist in the as-cast Zr-based alloys, whilethe Zr7Ni10 phase was decomposed and distributed into theother two phases after an appropriate annealing treatment.In the current study, we also found that the content of theTiNi-based precipitated phase was reduced by an appropri-ate annealing treatment, which is responsible for the changein electrochemical properties as will be discussed in thesecond part.Fig. 4 shows the SEM micrographs of the as-cast and

annealed Ti0:8Zr0:2V3:733Mn0:747Cr1:12Ni1:4 alloy samples.In Fig. 4 (a), it can be seen that three phases, namelythe C14 Laves phase matrix in light grey (identiNed asA), the dendritic V-based solid solution phase in darkgrey (identiNed as B) and a small amount of TiNi-basedblack phase precipitated (identiNed as C), coexist in theas-cast alloy, as conNrmed by EDS analysis. The C14Laves phase and the V-based solid solution phase arealso indicated in Fig. 2, while it is diUcult to Nnd theTiNi-based phase from the XRD patterns owing to itslow abundance. After the alloy was annealed at 1273 K,it is found that the alloy samples still consist of the C14Laves phase matrix and the dendritic V-based solid so-lution phase in large quantities, except that the content

392 Y. Zhu et al. / International Journal of Hydrogen Energy 28 (2003) 389–394

Fig. 3. Scanning electron micrographs of the Ti0:8Zr0:2V3:2Mn0:64Cr0:96Ni1:2 alloy samples: (a) as-cast; (b) 1273 K× 5 h; (c) 1273 K× 8 h;and (d) 1273 K × 11 h.

of the TiNi-based third phase is highly reduced, es-pecially for the alloy sample annealed for 8 h, whichshows no TiNi-based precipitated phase, indicating thatthe TiNi-based phase has completely decomposed anddistributed into the other two phases by homogenizedannealing treatment.The chemical compositions of the C14 Laves phase

and the V-based solid solution phase in both the Ti0:8Zr0:2V3:2Mn0:64Cr0:96Ni1:2 and Ti0:8Zr0:2V3:733Mn0:747Cr1:12Ni1:4alloy samples are determined by EDS analysis and listedin Tables 3 and 4. For both alloys, it can be found that

with annealing and prolongation of the holding time from5 to 11 h at a deNnite temperature (1273 K), the Ti and Nicontents increase in both phases and the V and Cr contentsincrease in the V-based solid solution phase and decreasein the C14 Laves phase, while the Mn content increases inthe C14 Laves phase and decreases in the V-based solid so-lution. In addition, Zr has not been detected in the V-basedsolid solution phase regardless of the annealing conditionsand its content increases slightly in the C14 Laves phase.Hence, it can be concluded that the Zr stays almost entirelyin the C14 Laves phase. The increase in Ti and Ni contents

Y. Zhu et al. / International Journal of Hydrogen Energy 28 (2003) 389–394 393

Fig. 4. Scanning electron micrographs of the Ti0:8Zr0:2V3:733Mn0:747Cr1:12Ni1:4 alloy samples: (a) as-cast; (b) 1273 K×5 h; (c) 1273 K×8 h;and (d) 1273 K × 11 h.

in both the C14 Laves phase and the V-based solid solu-tion phase may result from the dissolution and distributionof the TiNi-based precipitated phase into them during theannealing treatment.

4. Conclusions

Two multi-component Laves phase hydrogen storagealloys containing a BCC solid solution phase were pre-pared and annealed at 1273 K for 5–11 h, and the e8ectsof annealing treatment on their crystal structures were

investigated. Some conclusions can be summarized asfollows:

1. The as-cast alloys are mainly composed of the C14 Lavesphase matrix and the dendritic V-based solid solutionphase with a small amount of TiNi-based third phase pre-cipitated in both phases due to the compositional segre-gation during solidiNcation.

2. The content of the TiNi-based precipitated phaseis highly reduced in both alloys by an appropriateannealing treatment owing to the compositional homo-genization.

394 Y. Zhu et al. / International Journal of Hydrogen Energy 28 (2003) 389–394

Table 3The chemical compositions of the C14 Laves phase and the V-based solid solution phase in the as-cast and annealedTi0:8Zr0:2V3:2Mn0:64Cr0:96Ni1:2 alloy samples

Samples Phase Composition (at%)

Ti Zr V Mn Cr Ni

As-cast C14 21.36 10.14 17.74 8.95 5.01 36.80BCC 4.71 0.00 61.98 8.64 20.39 4.28

1273 K × 5 h C14 21.41 10.16 17.58 8.98 4.88 36.99BCC 4.82 0.00 62.31 7.82 20.51 4.54

1273 K × 8 h C14 21.44 10.17 17.50 9.00 4.80 37.09BCC 4.87 0.00 62.49 7.40 20.57 4.67

1273 K × 11 h C14 21.46 10.18 17.42 9.02 4.74 37.18BCC 4.92 0.00 62.66 7.00 20.62 4.80

Table 4The chemical compositions of the C14 Laves phase and the V-based solid solution phase in the as-cast and annealedTi0:8Zr0:2V3:733Mn0:747Cr1:12Ni1:4 alloy samples

Samples Phase Composition (at%)

Ti Zr V Mn Cr Ni

As-cast C14 20.35 9.75 17.43 8.99 4.98 38.50BCC 3.91 0.00 62.18 8.74 20.50 4.67

1273 K × 5 h C14 20.40 9.78 17.26 9.03 4.83 38.70BCC 4.03 0.00 62.44 7.97 20.64 4.92

1273 K × 8 h C14 20.42 9.80 17.17 9.05 4.75 38.81BCC 4.09 0.00 62.57 7.58 20.71 5.05

1273 K × 11 h C14 20.45 9.81 17.09 9.06 4.68 38.91BCC 4.14 0.00 62.70 7.22 20.77 5.17

3. The annealing treatment leads to the increase of the lat-tice parameters and unit cell volumes of the C14 Lavesphase and the V-based solid solution phase in bothalloys.

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