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Journal of Nuclear Materials 479 (2016) 232e239

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Journal of Nuclear Materials

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The isothermal section of the ZreCreCu ternary system at 580 �C

Junkai Tang a, Yuqin Liu a, *, Jianyun Shen b

a School of Materials Science and Technology, China University of Geosciences, Beijing, 100083, Chinab General Research Institute for Nonferrous Metals, Beijing, 100088, China

h i g h l i g h t s

* Corresponding author.E-mail address: liuyuqin@cugb.edu.cn (Y. Liu).

http://dx.doi.org/10.1016/j.jnucmat.2016.07.0040022-3115/© 2016 Elsevier B.V. All rights reserved.

g r a p h i c a l a b s t r a c t

� The isothermal section ofthe ZreCreCu system at 580 �C wasbuilt.

� No ternary compound was found toexist at 580 �C in this system.

� The dissolving of Cr in the CuZr phaseincreases its thermodynamicstability.

� Cu prefers to occupy the position ofCr in ZrCr2 Laves_C15 phase.

a r t i c l e i n f o

Article history:Received 24 January 2016Received in revised form4 July 2016Accepted 4 July 2016Available online 7 July 2016

Keywords:Zr-Cr-Cu ternary systemIsothermal sectionPhase diagram

a b s t r a c t

The 580 �C isothermal section of the ZreCreCu ternary system was determined by means of X-raydiffraction, scanning electron microscopy and electron probe microanalysis. This isothermal sectioncontained 10 single-phase regions, 18 two-phase regions and 9 three-phase regions. No ternary com-pound was found at 580 �C in the system. The solubility of Cu in the ZrCr2 cubic Laves C15 phase and thesolubility of Cr and Cu in the terminal a-Zr solid solution phase were determined. The site occupation ofelement Cu in the ZrCr2 cubic Laves C15 phase was determined by Rietveld refinement. The Cu prefers tooccupy the position of Cr. The CuZr phase, which is not stable at 580 �C in the binary CueZr system, wasconfirmed to exist at this temperature in the ZreCreCu ternary system. This is probably due to that thedissolving of Cr in the CuZr phase increases its thermodynamic stability.

© 2016 Elsevier B.V. All rights reserved.

1. Introduction

Zirconium alloys, known for small of neutron absorption crosssection and other properties such as high strength and hardness,outstanding high-temperature mechanical properties, good corro-sion resistance and radiation stability, specific modulus, and etc.,are being used as cladding and structural materials in light andheavy water nuclear reactors [1e4]. With the development of thenuclear power industry, more research interests are focusing onhow to improve the property of zirconium alloys and how to

increase the service life of the cladding. Recently, it was found thatthe additions of chromium and copper could increase the corrosionresistance property effectively in the Zr-4 alloy [5,6]. The phasediagram of ZreCreCu ternary system is of great importance for thedevelopment of high performance alloys and processing optimi-zation. However, the accurate phase relationship and solubility wasnot reported around the industrially hot-treating temperature.

Many experimental and theoretical works have been carried outto determine the phase diagrams of the binary ZreCr, ZreCu andCreCu systems. Only one compound ZrCr2 which exists in threepolytypes, namely Laves C15 (cubic), Laves C14 (hexagonal) andLaves C36 (hexagonal), was reported in the ZreCr system [7e9].The ZreCu phase diagram contains eight intermetallic compounds,namely Cu9Zr2, Cu51Zr14, Cu8Zr3, Cu2Zr, Cu10Zr7, CuZr, Cu5Zr8 and

Table 1Crystal structure data in the ZreCreCu system.

Phase Pearson symbol Space group Prototype Lattice parameters (Å) Refs.

aa

bb

cg

Cu cF4 Fm-3m Cu 3.61390

3.61390

3.61390

[32]

Cr cI2 Im-3m W 2.88490

2.88490

2.88490

[32]

(a-Zr) hP2 P63/mmc Mg 3.23290

3.23290

5.147120

[33]

(b-Zr) cI2 Im-3m W 3.56890

3.56890

3.56890

[12]

Cu9Zr2 tP24 P4/m Derived AuBe5 6.85690

6.85690

6.88290

[20]

Cu51Zr14 hP65 P6/m Ag51Gd14 11.234890

11.234890

8.2708120

[12]

Cu8Zr3 oP44 Pnma Cu8Hf3 7.868690

8.146790

9.97790

[12]

Cu2Zr oS12 Amm2 Au2V 4.68690

8.49890

4.68690

[32]

Cu10Zr7 oC68 C2ca Ni10Zr7 12.6790

9.3190

9.3490

[11]

CuZr cP2 Pm-3m CsCl 3.26390

3.26390

3.26390

[10]

Cu5Zr8 oP26 Pbam Al2Bi6Ca5 19.73890

7.68890

3.18490

[34]

CuZr2 tI6 I4/mmm CuZr2 3.220490

3.220490

11.183290

[13]

C15/ZrCr2 cF24 Fd-3m MgCu2 7.20590

7.20590

7.20590

[32]

C14/ZrCr2 hP12 P63/mmc MgZn2 5.11390

5.11390

8.309120

[32]

C36/ZrCr2 hP24 P63/mmc MgNi2 5.1190

5.1190

16.56120

[32]

Table 2Summary of the identified phase and their composition for the ZreCreCu alloys at 580 �C.

Sample No. Nominal composition (at. %) Phase present Lattice parameter(Å)

Phase composition (at. %)

Zr Cr Cu a b c Zr Cr Cu

1 75.90 9.90 14.20 a-Zr 3.231 3.231 5.112 99.61 0.16 0.23CuZr2 3.217 3.217 11.263 65.99 0.22 33.79C15 7.199 7.199 7.199 38.57 60.92 0.51

2 64.70 3.80 31.50 CuZr 3.262 3.262 3.262 51.82 1.90 46.28CuZr2 3.229 3.229 11.209 64.87 1.13 34.00C15 7.200 7.200 7.200 36.20 58.16 5.64

3 53.80 25.10 21.10 CuZr2 3.227 3.227 11.231 63.29 1.08 35.64C15 7.218 7.218 7.218 38.60 58.02 3.38

5 33.33 60.30 6.37 C15 7.207 7.207 7.207 32.15 67.54 0.318 26.70 64.20 9.10 Cr 2.884 2.884 2.884 0.72 98.40 0.88

C15 7.225 7.225 7.225 33.86 63.48 2.65Cu54Zr14 11.273 11.273 8.279 22.87 2.49 74.65

9 37.80 4.60 57.60 Cu8Zr3 7.812 8.189 9.939 26.29 0.21 73.50Cu10Zr7 12.703 9.238 9.334 40.10 0.52 59.37C15 7.227 7.227 7.227 33.75 54.43 11.82

11 45.60 4.30 50.10 CuZr 3.262 3.262 3.262 48.00 2.40 49.60Cu10Zr7 12.63 9.325 9.368 40.92 0.50 58.58C15 7.219 7.219 7.219 33.97 55.25 10.77

12 49.70 0.88 49.42 CuZr2 3.221 3.221 11.183 65.54 1.1 33.36Cu10Zr7 9.328 9.242 12.770 41.22 0.3 58.48CuZr 3.256 3.256 3.256 49.97 1.45 48.58

13 6.00 11.22 82.78 Cu9Zr2 6.886 6.886 6.886 17.73 2.51 79.76Cr 2.884 2.884 2.884 0 97.48 2.52Cu 3.615 3.615 3.615 0 0.23 99.77

J. Tang et al. / Journal of Nuclear Materials 479 (2016) 232e239 233

CuZr2 [10e14]. With regard to Cu9Zr2 phase, the composition‘Cu9Zr2’ were suggested by Vitek [15] and Kuznetsov et al. [16],while Phillips [17], Carvalho et al. [18] and Lou et al. [19] reportedthe composition ‘Cu5Zr’. However, the stoichiometry of this com-pound was established conclusively as ‘Cu9Zr2’ by Glimois and

Forey [20,21], who also determined that the room temperaturecrystal structure of Cu9Zr2 is a tetragonal long-period superlatticederived from the AuBe5 type structure. No binary compound wasobserved in the CreCu system [22,23].

The phase equilibrium in the ZreCreCu ternary system is still

Fig. 1. The isothermal section of the ZreCreCu system at 580 �C.

Fig. 3. X-ray diffraction patterns of the samples No. 1 and No. 3 at 580 �C.

J. Tang et al. / Journal of Nuclear Materials 479 (2016) 232e239234

limited andmost of workwas focused on the Cu-rich corner. Glasovet al. [24] investigated the partial isothermal section at 1000 �C forthe composition range of 98e100 at.% Cu by thermal and metal-lographic analysis. Zakharov et al. [25] studied the Cu-rich cornerphase diagrams of this system and seven isothermal sections wereconstructed within the temperature ranges from 600 �C to 1040 �C.Six vertical sections across Cue (Cu-4.6%Cr-2.3%Zr), (Cu-0.61%Cr)e(Cu-0.62%Cr-2.14%Zr), (Cu-2.43%Cr) e (Cu-2.46%Cr-2.45%Zr), (Cu-8.42%Cr) e (Cu-4.98%Zr), (Cu-4.3%Cr) e (Cu-0.3%Zr), (Cu-4.3%Cr-1.4%Zr) e (Cu-1.4%Zr) were constructed. Two invariant eutecticequilibria at 980 �C and 935 �C and a quasibinary CueZrCr2 sectionwere assumed to exist.

Kawakatu et al. [26] determined the solubility of Cr and Zr in theCu solid solutions at 900 �C and 750 �C using metallographic,thermal and dilatometric analyses. Using X-ray diffraction, Fedorovet al. [27] found that the ZrCr2 compound does not exist in thecomposition field investigated (up to 20 at.% Zr and 5 at.% Cr). Zenget al. [28] investigated the phase relations of seven Cu-rich alloysusing microstructure analysis and EDS analyses. Three phases, bcc-Cr, ZrCu5 (based on the older phase name) and fcc-Cu, were re-ported to exist in the seven investigated alloys heat treated at940 �C for 4 h. The results did not support the existence of the

Fig. 2. BSE images of the samples (a

quasibinary CueZrCr2 system, which is in good agreement withKawakatu et al. [26], Fedorov et al. [27] and Kuznetsov et al. [16].

Tregubov et al. [29] established the partial isothermal sections at750 �C and 900 �C for the Zr-rich corner ZreZrCr2eCuZr2, whilethree partial isothermal sections at 875 �C, 800 �C and 700 �C wereconstructed by Malakhova et al. [30]. Zhang et al. [31] determinedthe isothermal section at 700 �C using the XRD and EPMA. Table 1summarizes the crystal structure data and lattice parameters of thesolid phases presented in the ZreCreCu system.

Considering the hot rolling temperature of zirconium alloy is600 �Ce750 �C and the subsequent heat treatment to eliminatestress is normally carried out at 580 �C, this work focuses oninvestigating the phase relations of the ZreCreCu ternary system at580 �C by combining techniques of the XRD, SEM and EPMA.

2. Experimental details

Based on the phase diagrams and thermodynamic calculationsof the three binary systems, ten alloys were designed and theirnominal compositions were shown in Table 2. The samples wereprepared byweighing appropriate amounts of the pure elements Zr99.99 wt %, Cr 99.96 wt % and Cu 99.999 wt %. 10 alloy buttons witharound 18 g each were produced by arc melting in a water-cooled

) No. 1 and (b) No. 3 at 580 �C.

Fig. 4. BSE images and XRD patterns of samples No. 2 (a and b), No. 11 (c and d), No. 12 (e and f) at 580 �C.

J. Tang et al. / Journal of Nuclear Materials 479 (2016) 232e239 235

cast with a non-consumable tungsten electrode under pure argonatmosphere. A pure titanium button was used as an oxygen getterduring themelting process. To beginwith, the samples weremeltedthree times, putting the chromium below zirconium and copper

considering that chromium is relatively volatile. After melting, theingot was taken from the melting furnace, broken into small pieces,and melted again to ensure homogeneity. Each button was cut intosix small parts and two of them were used for further heat

Fig. 5. Crystal structures of ZrCr2 cubic Laves C15 phase. The view (a) perpendicular to the [010] direction, and (b) along the [111] direction.

Fig. 6. Powder XRD pattern of sample No.5 with its corresponding Rietveld refinement(solid line) and residuals (middle).

Table 3Main parameters of processing and refinement of Zr(Cr0.9Cu0.1)2.

Alloy Zr(Cr0.9Cu0.1)2Space Group Fd-3ma, Å 7.206 (94)a, � 90V, Å3 374.32 (79)2q-interval, � 20-135�

Rwp, % 8.171Rexp, % 4.288Rp, % 5.881c2 3.633

J. Tang et al. / Journal of Nuclear Materials 479 (2016) 232e239236

treatment. The 10 alloy samples were sealed in evacuated quartztubes for homogenization heat treatment at 850 �C for 10 days andthen cooled down slowly to 580 �C and kept for 18 days, and finallyquenched in cold water quickly.

One part of the samples was for XRD analysis, while the otherpart was kept for the scanning electron microscopy and electron

probe microanalysis. Because it is difficult to make the alloy sam-ples into powder, the bulk samples were examined by the means ofXRD technique on a Rigaku D/max-rA 12 KW diffractometer withCuKa (l ¼ 0.154056 nm) radiation and graphite monochromatoroperated at 40 kV, 100 mA. In order to understand the site occu-pation of alloying elements in the Laves C15 phase, the powdersample was made for the alloy which composition located in theLaves C15 single phase region to obtain the high quality XRDspectrum for the Rietveld refinement. The phase identification wasperformed by using the software Jade 6.0 applied to PowerDiffraction File (PDF release 2004). The morphology and phasecomposition analysis were carried out by ZRISS scanning electronmicroscope (SEM) with energy dispersive spectrometry (EDS) andJXA-8230 electron probe microanalysis (EPMA) with wave disper-sive spectrometry (WDS).

3. Results and discussion

3.1. The isothermal section at 580 �C

Phase identification was based on the XRD analysis and thephase composition measured by EMPA. The equilibrium phasecomposition of the ZreCreCu ternary system at 580 �C obtainedfrom EPMAwas summarized in Table 2. Based on the experimentaldata obtained in this work, the isothermal section of the ZreCreCuternary system built and shown in Fig. 1. The XRD analysis andEMPA analysis confirmed the existence of seven binary compounds,namely Cu9Zr2, Cu51Zr14, Cu8Zr3, Cu10Zr7, CuZr, CuZr2 and cubicZrCr2 Laves C15 at 580 �C. In this section, nine three-phase regionsof a-Zrþ CuZr2 þ C15, CuZr þ CuZr2 þ C15, CuZrþ CuZr2 þ Cu10Zr7,CuZr þ Cu10Zr7 þ C15, Cu8Zr3 þ Cu10Zr7 þ C15,Cu8Zr3 þ Cu51Zr14 þ C15, Cr þ Cu51Zr14 þ C15,Cu9Zr2 þ Cr þ Cu51Zr14, Cu9Zr2 þ Cr þ Cu appear.

3.2. The Zr-rich corner

Fig. 2 shows the backscattered-electron (BSE) image of alloysNo.1 and No.3, and the corresponding XRD patterns are presentedin Fig. 3. Combining EPMA analysis and XRD analysis results, thealloy No. 1 contains three phases, i.e., the white a-Zr solid solution,the light gray CuZr2 and the black ZrCr2 Laves C15 phase. The sol-ubility of Cr in CuZr2 is 0.22 at. % while the solubility of Cu in ZrCr2 is

Fig. 7. BSE images and the XRD patterns of samples No.8 (a and b), No.9 (c and d), No.13 (e and f) at 580 �C.

J. Tang et al. / Journal of Nuclear Materials 479 (2016) 232e239 237

0.51 at.%. The solubility of Cr and Cu in the a-Zr solid solution is0.16 at. % and 0.23 at. %, respectively. As presented in Fig. 2b, thealloy No. 3 contains two coexisting phases, the light gray CuZr2 andthe black ZrCr2 laves C15 phase. The Cr solubility in CuZr2 is 1.08 at.%, while the Cu solubility in ZrCr2 is 3.38 at. %.

3.3. The phase regions related to the CuZr phase

CuZr phase has a B2 structure (CsCl type, bcc based) and

decomposes eutectoidly into Cu10Zr7 and CuZr2 at 712 �C in thebinary ZreCu system [35]. Additions of the third alloying elementchange the thermodynamic stability of the B2 CuZr phase. Forinstance, the eutectoid decomposition temperature of CuZr phaseincreases with the increases in the amount of Ti addition, implyingthe decrease of the thermodynamic stability of the CuZr phase [36].On the contrary, the Zn addition can effectively lower the eutectoiddecomposition temperature of CuZr phase [37], thus increasing itsthermodynamic stability.

J. Tang et al. / Journal of Nuclear Materials 479 (2016) 232e239238

Fig. 4 presents the BSE images and corresponding XRD patternsof the samples No. 2, No. 11 and No. 12 which contain the CuZrphase. The phases presented in the sample No. 2 are the light grayCuZr phase, the dark gray CuZr2 phase and the black ZrCr2 LavesC15 phase. The solubility of Cu in the ZrCr2 Laves C15 phase is5.64 at.%, while the solubility of Cr in the CuZr phase is 1.90 at. %.The phases presented in the sample No.11 are the black ZrCr2 LavesC15 phase, the white Cu10Zr7 phase and the matrix CuZr. The sol-ubility of Cr in the CuZr phase is 2.40 at.% while the solubility of Cuin the ZrCr2 C15 phase is 10.77 at. %. The sample No. 12 consist ofthree phases CuZr, CuZr2 and ZrCr2 Laves C15. The solubility of Cr inthe CuZr phase is 1.45 at.%. This results indicates that the addition ofCu increases the thermodynamic stability of the CuZr phase.

3.4. The ZrCr2 Laves C15 phase region and Cu site occupation

The AB2 Laves phase has three polymorphs, cubic C15 with thespace group Fd-3m (227), hexagonal C14 with the space group P63/mmc (194) and di-hexagonal C36 with the space group P63/mmc(194). At 580 �C, the ZrCr2 Laves phase exists in cubic C15 form. Asshown in Fig. 1, the isothermal section of the ZreCreCu ternarysystem determined from this investigation indicates that theaddition of Cu can greatly broaden the ZrCr2 cubic Laves C15 phaseregion.

Determining the site occupation of the third alloying elementsCu in the ZrCr2 cubic Laves C15 phase is essential to understand theproperties of this phase and model its thermodynamic propertiestheoretically. Laves C15 phase is an FCC-based structure containingsix atoms in the primitive unit cell. The crystal structure of the ZrCr2cubic Laves C15 was shown in Fig. 5. It could be seen that the Zratom occupies the 8 (a) Wyckoff position while the Cr atom oc-cupies the 16 (d)Wyckoff position. The alloy No. 5, which located inthe single ZrCr2 cubic Laves C15 phase region, was used to deter-mine the Cu site occupation. The Rietveld refinement ofZr(Cr0.9Cu0.1)2 was analyzed via the TOPAS 3 program (Bruker AXS)as shown in Fig. 6. The Rietveld refinement results demonstratethat the Cu atoms prefer to occupy the Cr positions. For the crystalof ZrCr2 host, the lattice parameters were fitted to bea7.12 Å, cellvolume (V)374.32 (79)Å3 and the weighted profile R-factor (Rwp),the expected R factor (Rp) are 8.171% and 5.881%, respectively, assummarized in Table 3.

3.5. Other phase regions

Fig. 7a is the BSE image of the sample No. 8. The XRD pattern ofthis sample is shown in Fig. 7b. Three phases coexist in the sample,namely, the black Cr solid solution, the light gray Cu51Zr14 and thegray ZrCr2 Laves C15. The solubility of Cu in the ZrCr2 Laves C15phase is 2.65 at.%

Fig. 7c is the BSE image of the sample No. 9 and its XRD patternis shown in Fig. 7d. The coexistence of the Cu10Zr7, Cu8Zr3 and ZrCr2phases is found in this alloys. The solubility of Cu in the ZrCr2 C15phase is 11.82 at.%.

Fig. 7e is the BSE image of the sample No. 13 and its corre-spondent XRD pattern is shown in Fig. 7f. The phases presented inthe sample No. 13 are back Cr solid solution, the light gray Cu9Zr2phase and the gray Cu solid solution.

4. Conclusions

The isothermal section of the ZreCreCu ternary system at580 �C was built based on the results of XRD and EMPA analysis.There are nine three-phase regions and eighteen two-phase re-gions and ten single-phase regions. No ternary compound wasfound in this system at 580 �C.The solubility of Cr in the

intermetallic compounds CuZr2, Cu10Zr7 and Cu8Zr3 are smallerthan that in the compounds Cu51Zr14 and Cu9Zr2. The phaseboundary of the ZrCr2 cubic Laves C15 phase was determined andthe maximum solubility of Cu in C15 is around 11.82 at%. Based onthe Rietveld refinement, the site occupation of element Cu in theZrCr2 cubic Laves C15 phase was determined and found that the Cuprefers to occupy the position of Cr. The B2 CuZr phase, which is notstable at 580 �C in the binary CueZr system, was confirmed to existat this temperature in the ZreCreCu ternary system. This is prob-ably due to that the dissolving of Cr in the CuZr phase increases itsstability.

Acknowledgments

This work was supported by the National Natural ScienceFoundation of China (No. 51071142).

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