Phase equilibria in the Fe-Nb-Zr ternary system at 1,200℃

Rare Met.(2013)32(2)：201-207DoI 10.10O7，s12598-013-0039-8RARE METALS、Irww.editorialmanager.com/rmetPhase equilibria in the Fe-Nb-Zr ternary system at 1，200 oCYi-Yuan Tang，Chang-Zhong Liao，Jie-Li M eng，Jin-Min Zhu，Qi-Ming Zhu，Liang-Qin Nong，Jian-Lie LiangReceived：18 July 2012/Revised：15 October 2O12/Accepted：20 October 2012/Published online：24 April 2013### The Nonferrous M etals Society of China and Springer-Verlag Berlin Heidelberg 2013Abstract Phase equilibria in the Fe-Nb-Zr system at1。200。C were determined by X-ray diffraction(XRD andscanning electron microscope(SEM)coupled with energv-dispersive X-ray spectroscopy(EDS)techniques.ExtensiveNbF domain was proposed in the current work.Thiscompound existed in the composition range from 35 at% to73 at% Fe.12 at% tO 32 at% Nb.and 0 to 32 at% Zr.In thepresent work，four three-phase regions(1)p-(Nb，Zr)NbFe十NbFe2，(2) p-(Nb，Zr)NbFe2Liquid，(3)NbFe2LiquidZrFe2，and(4)ZrFe2FeNbFe2，were establj shed。

Keywords Phase diagram；Fe-Nb-Zr；X-ray diffraction；Scanning electron microscopy1 IntroductionZirconium-based aloys are widely used as claddingmaterials in nuclear reactor for their excellent mechanicproperties，irradiation stability and corosion resistance.Tomeet the requirement of high bumup of the nuclear fuel，new eneration Nb.containing zirconium-based alloys。

such as Zirlo圆[1-3，M5###，E1 10###，and E635###[3，4]，weredeveloped.M5 and E 1 1 0 were the ZNb-O-based alloys。

while Zirloand E635 contained additional Nb.Fe.andY.-Y.Tang，C.-Z.Liao，J.-L.Meng，J.-M.Zhu，Q.-M.Zhu：L.-Q.Nong.J.-L.LiangInstitute of Materials Science，Colege of Science，GuangxiUniversity for Nationalities，Narming 530006，Chinae-mail：Liangjl197 1###126.comJ.-L.LiangGuangxi Key Laboratory of Hybrid Computational and ICDesign Analysis，Nanning 530006，ChinaSn.CALPHAD is a power method for the materials design5.Recently，a thermodynamic database for zirconiumalloys including related binary systems has been estab-lished by using the CALPHAD approach f6].The databasewas utilized to predict the a/B phase transition temperaturefor the Zircaloy-4.W ell agreement in the calculated andthe experimental results was obtained 61.Though，newexperimental results related to the zirconium being avail-able f7-13]，the information of the phase equilibria in thezirconium aloys are still limited and scared.For thedevelopment and modeling of the zirconium alloys，theexperimenta1 investigation on phase diagrams related tozirconium aloys is quite essentia1。

The temary Fe-Nb-Zr system was investigated early byKorotkova and Alekseeva et a1.9，10]，spanning the wholecomposition range by using X-ray diffraction(XRD)，dif-ferential thermal analysis(DTA)，and microscopy as wellas X.ray microanalytical technique.Seven isothernlalsections at 1200.945.900，800.700，650，and 500。C，respectively，were reported.Later，tentative phase diagramsof this system at 900 and 800。C were presented by Gra-novsky et a1.[11.Recently，by investigating numerousindustrial Zr-Sn -Fe alloys，Barberis et a1.14]pro-posed a phase diagram of the Fe-Nb-Zr system at 580。Cin the Zr-rich comer.The phase diagrams reported byGranovsky et a1.1land Barberis et a1.[14]were diferentto those by Korotkova and Alekseeva et a1.[9，10].Twoternary compounds，(Zr，Nb)2Fe and Zr(Nb，Fe)2，wereidentified in the works in Refs.[1l，14]，while in Refs.[9，101，only(Zr，Nb)2Fe was reported。

(Zr，Nb)，Fe was observed by Korotkova and Alekseevawith composition Fe38NbloZr52 and unknown crystalstructure f91.Granovsky et a1.11recently reported thatthis compound had cubic TiN prototype. Cubic(Zr，Nb)2Fe and hexagonal Zr(Nb，Fe)2 have been observed垒SpringerY.-Y.Tang et alin several Zr-Nb-Fe-related alloys[15-18].Zr(Nb，Fe)2was observed by Woo18and Nikulina[19with latticeparameters a 0.54 and c 0.88 nm and a 0.51 andc 0.83 nm.respectively.This compound was proposedto crystallize in MgZn2 structure type[11。

The Nb-Zr and Fe-Nb binary systems were accessed by0kamoto20，21，No intermediate phase exists in the Nb-Zr system.C ontinuous solid solution of the B-(Nb，Zr)spans the whole composition in the temperature range from977 up to 1，743。C in this system.Two compounds，i.e.，NbFe and NbFe，.exist in Fe-Nb system。

Early works on the FeZr system revealed that therewere four binary intermetalics，namely，Zr3Fe，ZrFe，，Zr2Fe，and Zr6Fez3[221.Recently，Stein et a1.[8rein-vestigated this system and concluded that Zr6Fe，3 was anoxygen-stabilized compound。

As discussed above，knowledge of the phase equilibria inthe Fe-Nb-Zr system was incomplete and contradictory。

Moreover.the information on the Fe-Nb-Zr system at hightemperature was also limited.For the commercial Zr.basedalloys，the temperature of the B solution was usually selectedfrom 1，000t0 1.200。C.Inthiswork，weaim todeterminethephase diagram of the Fe-Nb-Zr system at 1.200。C。

2 ExperimentalSponge Zr(99.8%，mass fraction)，pure Fe(99.8%，massfraction).and Nb(99.9%、mass fraction)were used asstarting materials.Twenty-six alloy ingots(4 g)were arc-melted under the atmosphere of pure argon(99.99%1.Toavoid the contamination of oxygen，titanium getter wasmelted prior to the preparation of the aloys.To insure thehomogeneity，the alloys were turned up side down after themelting and remelted for at least three times.The weightvariation of each alloy was found to be<0.5 wt%.Thealloy buttons were wrapped with tantalum foils and sealedin the clear vacuum silica tubes.Samples were divided intotwo groups.The first group was annealed at 1.200。C for10 h.To determine whether such a short time was suffi-cient for the equilibrium，120 h were used for the othergroup.The experimental results showed that 10 h wereenough to reach the equilibrium for the alloys.During theannealing.the silica tube wal1 was found to become soft。

ening and being compressed to contact with the innertantalum foil.Nevertheless，the tube remained uncrushed。

The buttons were rapidly quenched into cold water bybreaking the tubes.After annealing，a slight thin oxide filmwas found to form on the surface of the specimen.How-ever，the contribution of the oxide film on the weight gainof the sam ples was found negligible。

After removing the surface oxide films，the alloys weresubjected to X-ray diffraction.The XRD was carried out鱼Springerwith the Rikagu-2500 diffractometer with CuK radiationand graphite monochrome.Tube voltage.current.and scanrate were selected as 40 kV，200 mA，and 10r。1.min in20，respectively.The lattice parameter was calculated byusing the 1east-square method.The specimens for theoptical microscopy were prepared in standard metalogra-phy procedure.Dilute solution(HF：HNO3：H20 1：29：70，volume fraction)was used to reveal the microstructure forthe specimens.Microstructure observation and composi-tional analysis were carried out on scanning electronmicroscope(SEM，Hitachi$3400N)coupled with energy-dispersive spectrometer(EDS，EDAX).Chemical compo-sitional measurement for each phase was conducted at leastthree times.Conventional ZAF corection was applied tothe conversion of the measured intensities to weightpercents。

3 Results and discussionTable 1 presents the experimental results of the investi-gated alloys by using XRD an d SEM/EDS techniques.Dueto the slight contamination of oxygen，the oxygen-stabi-lized phases Zr6Fe23 and TiNi2-type Zr2Fe were observedin several alloys and were presented as additional phases。

The phase equilibria of the Fe-Nb-Zr system at 1.200。Cwere determ ined，as shown in Fig.1.Zr6F 3 was excludedin the present phase diagram according to the results ofStein et a1.[81.In this work，the form er known ternarycompound ZrrNbFe)，was found to be the solution ofbinary NbFe，.Evidence to support this conclusion wouldbe presented in the following text.It is worth to noting that，for the specimens in Table 1.aloys 1-14 and 25-26 wereannealed at 1，200。C for 10 h，while alloys 15-24 wereheld at 1.200。C for 120 h。

Temary compound Zr(Nb，Fe)2 was proposed to beMgZn2 structure type11，23].In the binary Fe-Nb side，NbFe，has the same structure type.As reported by Ka。

nematsu24]，in the pseudobinary system (ZrlxNbx)Fe2，the crystal structure of(Zrl- N )Fez phase is MgCu2 typefor x 0.3，MgZn， type for > 0.5.and six layersstructure for 0.35

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Phase equilibria in the Fe-Nb-ZrAcknowledlments This study was financially supposed by theNational Natural Science Foundation of China(No.5 1001033) theNatural SciencFoundation of Guangxi(No.201 1GXNSFA018O3O)，the Program for Excellent Talents in Guan gxi Higher EducationInstitutions，Science Research Program for Education Department ofGuangxi(No.201 1LX140)，and the Science Foundation of GuangxiUniversity for Nationalities fNos.2008ZD010，2010ZD012，201 1QD019，and 201 1MDONO46)。

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