lable at ScienceDirect

Intermetallics 49 (2014) 121e131

Contents lists avai

Intermetallics

journal homepage: www.elsevier .com/locate/ intermet

Experimental investigation of phase equilibria and microstructurein the CoeTieV ternary system

J.J. Ruan, C.P. Wang, C.C. Zhao, S.Y. Yang, T. Yang, X.J. Liu*

Department of Materials Science and Engineering, College of Materials, and Research Center of Materials Design and Applications, Xiamen University,Xiamen 361005, PR China

a r t i c l e i n f o

Article history:Received 11 July 2013Received in revised form30 December 2013Accepted 9 January 2014Available online 22 February 2014

Keywords:A. Ternary alloy systemsB. Phase diagramsD. Microstructure

* Corresponding author. Tel.: þ86 592 2187888; faxE-mail address: lxj@xmu.edu.cn (X.J. Liu).

0966-9795/$ e see front matter � 2014 Elsevier Ltd.http://dx.doi.org/10.1016/j.intermet.2014.01.011

a b s t r a c t

The phase equilibria in the CoeTieV ternary system have been investigated by means of optical mi-croscopy (OM), electron probe microanalyzer (EPMA), differential scanning calorimetry (DSC), fieldemission scanning electron microscope (SEM) and X-ray diffraction (XRD). The mechanical propertieswere measured by compressive tests. Four isothermal sections of the CoeTieV ternary system at 800 �C,1000 �C, 1100 �C and 1200 �C were experimentally established. The results show that: (1) there is noternary compound in this system; (2) the CoTi2 phase and Co3Ti phase stabilized by the V addition; (3) alarge solubility of Ti in the s-Co2V3 phase was observed at all isothermal sections of 800 �C, 1000 �C,1100 �C and 1200 �C; (4) The alloy with the distribution of fine cuboidal Co3Ti (L12) in (aCo) phase wasobserved. (5) The compressive strength of Co77.29Ti5.83V16.88 (at.%) alloy at room temperature wasmeasured to be about 1985 MPa. The newly determined phase equilibria in this system will provideuseful information for the development of Co-based and Ti-based materials.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Co-based and Ti-based alloys have been widely used in the fieldof high-temperature materials [1e3], magnetic materials [4,5] andhydrogen storagematerials [6e9]. Furthermore, it has been alreadyreported that addition of alloying elements is a method to improvethe performance of high-temperature materials [10e12] andmagnetic materials [13e15]. For instance, alloying with vanadiumenhances the ductility, strength, and corrosion resistance of ma-terials [16]. Therefore, in order to develop high-temperature ma-terials, magnetic recording materials and structural materials, aswell as to understand the relationship between microstructure andproperties, knowledge of the phase equilibria in the CoeTieVternary system is desirable.

Three binary systems CoeV [17], VeTi [18], CoeTi [19],constituting the CoeTieV ternary system, are shown in Fig. 1. Thephase diagram of the CoeV binary system was studied by manyresearchers [20e26]. The CoeV binary system calculated byBratberg and Sundman [17] is consistent with existing experi-mental data. The CoeV binary system has three intermediate

: þ86 592 2187966.

All rights reserved.

phases, Co3V, s-Co2V3 and CoV3. The s-Co2V3 and CoV3 phasesform through peritectic or peritectoid reactions: L þ (V) 4 s-Co2V3 at 1422 �C, s-Co2V3 þ (V) 4 CoV3 at 1025 �C. The VeTibinary system [18] shows that the bcc solid-solution (bTi, V) phaseundergoes a allotropic transformation to (aTi) phase at 882 �C. TheCoeTi binary system [19] has five intermediate phases, CoTi2, CoTi,Co2Ti (c), Co2Ti (h) and Co3Ti. The CoTi phase forms through acongruent reaction, L 4 CoTi at 1500 �C. The Co2Ti (c) phase formsthrough a peritectic reaction, L þ CoTi 4 Co2Ti (c) at 1236 �C. TheCo2Ti (h) phase forms through a peritectic reaction, L þ Co2Ti(c) 4 Co2Ti (h) at 1202 �C. The information of the stable solidphases in three binary systems mentioned above is summarized inTable 1.

The objective of the present work is to experimentallydetermine the phase equilibria of the CoeTieV system at 800 �C,1000 �C, 1100 �C and 1200 �C using OM, EPMA, DSC, SEM andXRD, which is expected to provide a better microstructural un-derstanding in the CoeTieV alloys for practical applications.And the alloy with Co matrix strengthened by Co3Ti which issimilar to traditional Ni þ Ni3Al superalloy system is desired.According to the phase diagram and DSC data, the alloyCo77.29Ti5.83V16.88 (at.%) annealed at 800 �C for 135 days with thedistribution of fine cuboidal Co3Ti (L12) in (aCo) phase was ob-tained. And the compression strength of Co77.29Ti5.83V16.88 (at.%)

Fig. 1. Binary phase diagrams constituting the CoeTieV ternary system [17e19].

J.J. Ruan et al. / Intermetallics 49 (2014) 121e131122

alloy annealed at 800 �C for 135 days at room temperature wasmeasured.

2. Experimental procedure

Cobalt (99.9 wt.%), Vanadium (99.7 wt.%) and Titanium(99.9 wt.%) were used as starting materials. Bulks buttons wereprepared from pure elements by arc melting under high purityargon atmosphere. The ingots were melted at least 8 times in orderto achieve their homogeneity. The weight of each sample wasaround 20 g. The ingots were cut into small pieces by wire-cuttingmachine for heat treatment and further microstructural observa-tion, as well as the analysis of phase structure.

Plate-shaped specimens were put into quartz capsules andbackfilled with argon gas to certain pressure. The specimens were

Table 1The stable solid phases in the three binary systems.

System Phase Pearsonsymbol

Spacegroup

Prototype Strukturbericht References

CoeV (aCo) cF4 Fm-3m Cu A1 [17]( 3Co) hP2 P63/mmc Mg A3 [17]Co3V hP24 P-6m2 Co3V . [17]s-Co2V3 tP30 P42/mnm sCrFe D8b [17]CoV3 cP8 Pm-3n Cr3Si A15 [17](V) cI2 Im-3m W A2 [17]

VeTi (bTi, V) cI2 Im-3m W A2 [18](aTi) hP2 P63/mmc Mg A3 [18]

CoeTi (aTi) hP2 P63/mmc Mg A3 [19](bTi) cI2 Im-3m W A2 [19]CoTi2 cF96 Fd-3m Fe3W3C E93 [19]CoTi cP2 Pm-3m CsCl B2 [19]Co2Ti (c) cF24 Fd-3m Cu2Mg C15 [19]Co2Ti (h) hP24 P63/mmc MgNi2 C36 [19]Co3Ti cP4 Pm-3m Au3Cu L12 [19]( 3Co) hP2 P63/mmc Mg A3 [19](aCo) cF4 Fm-3m Cu A1 [19]

annealed at 800 �C, 1000 �C, 1100 �C, and 1200 �C, respectively.According to the temperature and composition of the specimen,time of the heat treatment varied from 6 h to 135 days. There area few steps for choosing the heating temperature, composition ofalloys and heating time: (1) According to the phase stability ofCo3Ti (L12) and the working temperature of the Co-based high-temperature alloy, the heating temperature of 800 �C, 1000 �C,1100 �C, and 1200 �C were chosen; (2) The choosing of compo-sition of alloys was accorded to binary phase diagrams firstly.Then according to the partial ternary phase diagram that wedetermined, the other composition points were chosen to deter-mine the relationship of phases; (3) According to the ability ofatomic diffusion and heating temperature, the heating time wastaken from 20 days to 135 days, it worth noting that the liquidphase shorten the heating time due to the ability of acceleratingthe atomic diffusion, and the heating time of alloys with liquidphase was taken from 6 h to 12 h. At the end of the heat treat-ment, the specimens were quenched into ice water. Compressiontest specimens with 3 � 3 � 6 mm3 in dimension were cut fromthe alloy Co77.29Ti5.83V16.88 (at.%) ingot annealed at 800 �C for 135days. After the metallographic preparation, microstructuralobservation of specimens was carried out by optical microcopy.The equilibrium composition of each phase in specimens wasmeasured by electron probe microanalyzer (EPMA, JXA-8100,JEOL, Japan), the accelerating voltage and probe current being20 kV and 1.0 � 10�8 A, respectively, and was determined bymean value over five data calibrated by the ZAF (Z: atomicnumber effect, A: absorption effect, F: fluorescence effect)correction, where pure elements were used as standard samples.The XRD was used to identify the crystal structure. The XRDmeasurement was carried out on a Philips Panalytical X-pertdiffractometer using Cu Ka radiation at 40 kV and 30 mA. Thedata were collected in the range of 2q from 30� to 120� at a stepwidth of 0.0167�. The phase transition temperature was deter-mined by DSC at heating and cooling rates of 30 �C/min. Themicrostructure was observed using a field emission scanningelectron microscope (SEM, LEO-1530). An WDW-IOOE III machinewas used for the compression test. And the compression test wascarried out at room temperature with an initial strain rate of1.0 � 10�4 s�1.

3. Results and discussion

3.1. Microstructural morphologies and phase equilibria

Back-scattered electron (BSE) images of the typical ternaryCoeTieV alloys are presented in Figs. 2.1(a)e(f), 2.2(a)e(f),2.3(a)e(f), 2.4(a)e(f), and the XRD results of the typical CoeTieV ternary alloys are presented in Fig. 3(a)e(c). The two-phaseequilibrium of the s-Co2V3 phase and Co2Ti (h) phase wasidentified in the Co58.97Ti31.55V9.48 (at.%) alloy annealed at 800 �Cfor 135 days, as shown in Fig. 2.1(a), where the s-Co2V3 phase isgray and the Co2Ti (h) phase is white, respectively. It is seen thatthe Co2Ti (h) phase distributes in the matrix of the s-Co2V3

phase. The two-phase equilibrium of the Co3Ti phase and Co2Ti(h) phase was identified in the Co70.03Ti24.87V5.10 (at.%) alloyannealed at 800 �C for 135 days, as shown in Fig. 2.1(b), wherethe Co3Ti phase is gray and the Co2Ti (h) phase is light gray,respectively. The two-phase equilibrium of the s-Co2V3 þ Co3Vwas identified in the Co67.12Ti10.86V22.02 (at.%) alloy annealed at800 �C for 135 days, its microstructure is shown in Fig. 2.1(c). Thes-Co2V3 phase is gray, the Co3V phase is light gray. In theCo31.34Ti63.44V5.22 (at.%) alloy that annealed at 800 �C for 135days, the two-phase equilibrium of the CoTi2 phase and CoTiphase was determined, as shown in Fig. 2.1(d). It is seen that the

J.J. Ruan et al. / Intermetallics 49 (2014) 121e131 123

CoTi phase distributes in the matrix of the CoTi2 phase. In theCo18.07Ti5.26V76.67 (at.%) alloy annealed at 800 �C for 135 days, thetwo-phase equilibrium of the (bTi, V) phase and CoV3 phase wasdetermined, as shown in Fig. 2.1(e). The structures of two phaseswere identified by the XRD, as shown in Fig. 3(a). It can be seenthat the characteristic peaks of the (bTi, V) phase and CoV3 phaseare well distinguished by different symbols. The two-phaseequilibrium of the (bTi, V) phase and CoTi2 phase was identi-fied in the Co14.91Ti73.67V11.42 (at.%) alloy annealed at 800 �C for135 days, as shown in Fig. 2.1(f), where the (bTi, V) phase is grayand the CoTi2 phase is white, respectively. It is seen that the CoTi2phase distributes in the matrix of the (bTi, V) phase. The two-phase equilibrium of the Co3Ti þ Co2Ti (h) was identified in theCo71.45Ti19.00V9.55 (at.%) alloy annealed at 1000 �C for 75 days, as

Fig. 2. 2.1. BSE images of typical CoeTieV ternary alloys: (a) Co58.97Ti31.55V9.48 (at.%) alloy an135 days; (c) Co67.12Ti10.86V22.02 (at.%) alloy annealed at 800 �C for 135 days; (d) Co31.34Ti63annealed at 800 �C for 135 days; (f) Co14.91Ti73.67V11.42 (at.%) alloy annealed at 800 �C for 135annealed at 1000 �C for 75 days; (b) Co77.16Ti5.87V16.97 (at.%) alloy annealed at 1000 �CCo57.74Ti36.52V5.74 (at.%) alloy annealed at 1000 �C for 75 days; (e) Co31.10Ti50.59V18.31 (at.%) allofor 75 days. 2.3. BSE images of typical CoeTieV ternary alloys: (a) Co30.87Ti50.75V18.38 (at.%1100 �C for 6 h; (c) Co60.80Ti5.37V33.83 (at.%) alloy annealed at 1100 �C for 40 days; (d) Co32annealed at 1100 �C for 40 days; (f) Co17.48Ti66.80V15.72 (at.%) alloy annealed at 1100 �C for 6annealed at 1200 �C for 12 h; (b) Co62.80Ti3.41V33.79 (at.%) alloy annealed at 1200 �C for 12 h; (calloy annealed at 1200 �C for 20 days; (e) Co17.86Ti51.03V31.11 (at.%) alloy annealed at 1200 �

shown in Fig. 2.2(a), where the light gray phase is Co2Ti (h) andthe gray phase is Co3Ti, respectively.

The three-phase equilibrium of the (aCo) þ Co3V þ Co3Ti wasidentified in the Co77.16Ti5.87V16.97 (at.%) alloy annealed at 1000 �Cfor 75 days, as shown in Fig. 2.2(b), where the light gray phase is(aCo), the white phase is Co3V and the gray phase is Co3Ti,respectively. The morphologies of Co3V phase (needles and strips)suggested that the Co3V phase grew like a lamellar structure inthe sample. The two-phase equilibrium of the s-Co2V3 þ Co3Tiwas identified in the Co65.73Ti17.79V16.48 (at.%) alloy annealed at1000 �C for 75 days, as indicated in Fig. 2.2(c). The s-Co2V3 phaseis light gray and the Co3Ti phase is white. The structures of twophases were identified by the XRD, as shown in Fig. 3(b). In theCo57.74Ti36.52V5.74 (at.%) alloy annealed at 1000 �C for 75 days, the

nealed at 800 �C for 135 days; (b) Co70.03Ti24.87V5.10 (at.%) alloy annealed at 800 �C for.44V5.22 (at.%) alloy annealed at 800 �C for 135 days; (e) Co18.07Ti5.26V76.67 (at.%) alloydays. 2.2. BSE images of typical CoeTieV ternary alloys: (a) Co71.45Ti19.00V9.55 (at.%) alloyfor 75 days; (c) Co65.73Ti17.79V16.48 (at.%) alloy annealed at 1000 �C for 75 days; (d)y annealed at 1000 �C for 75 days; (f) Co17.43Ti72.26V10.31 (at.%) alloy annealed at 1000 �C) alloy annealed at 1100 �C for 40 days; (b) Co17.83Ti62.03V20.14 (at.%) alloy annealed at.52Ti62.93V4.55 (at.%) alloy annealed at 1100 �C for 6 h; (e) Co22.43Ti5.83V71.74 (at.%) alloyh. 2.4. BSE images of typical CoeTieV ternary alloys: (a) Co73.79Ti14.81V11.40 (at.%) alloy) Co31.05Ti50.72V18.23 (at.%) alloy annealed at 1200 �C for 6 h; (d) Co24.48Ti15.39V60.13 (at.%)C for 6 h; (f) Co25.89Ti43.77V30.34 (at.%) alloy annealed at 1200 �C for 6 h.

Fig. 2. (continued).

J.J. Ruan et al. / Intermetallics 49 (2014) 121e131124

two-phase equilibrium of the CoTi þ Co2Ti (h) was determined, asshown in Fig. 2.2(d), where the CoTi phase and Co2Ti (h) phase aregray and white, respectively. In the Co31.10Ti50.59V18.31 (at.%) alloythat annealed at 1000 �C for 75 days, the three-phase equilibriumof the (bTi, V) phase, CoTi2 phase and CoTi phase was determined,as shown in Fig. 2.2(e). The two-phase equilibrium of the (bTi,V) þ CoTi2 was identified in the Co17.43Ti72.26V10.31 (at.%) alloyannealed at 1000 �C for 75 days, as indicated in Fig. 2.2(f). The(bTi, V) phase is gray and the CoTi2 phase is white. The three-phase equilibrium of the (bTi, V) þ CoTi2 þ CoTi was identifiedin the Co30.87Ti50.75V18.38 (at.%) alloy annealed at 1100 �C for 40days, as observed in Fig. 2.3(a). The structures of three phaseswere identified by the XRD, as shown in Fig. 3(c) where thecharacteristic peaks of the (bTi, V) phase, CoTi2 phase and CoTiphase are well distinguished by different symbols. The three-phase equilibrium of the L þ (bTi, V) þ CoTi2 was identified inthe Co17.83Ti62.03V20.14 (at.%) alloy annealed at 1100 �C for 6 h, asshown in Fig. 2.3(b). The CoTi2 phase is white and the (bTi, V)phase is gray. In the Co60.80Ti5.37V33.83 (at.%) alloy that annealed at1100 �C for 40 days, the two-phase equilibrium of the s-Co2V3phase and (aCo) phase was determined, as shown in Fig. 2.3(c).The s-Co2V3 phase is light gray and the (aCo) phase is gray. It is

seen that the (aCo) phase distributes in the matrix of the s-Co2V3phase. The three-phase equilibrium of the L þ CoTi þ CoTi2 wasidentified in the Co32.52Ti62.93V4.55 (at.%) alloy annealed at 1100 �Cfor 6 h, as shown in Fig. 2.3(d), where the CoTi phase and CoTi2phase are white and gray, respectively. The two-phase equilib-rium of the s-Co2V3 þ (bTi, V) was found in the Co22.43Ti5.83V71.74(at.%) alloy annealed at 1100 �C for 40 days, as indicated inFig. 2.3(e). In the Co17.48Ti66.80V15.72 (at.%) alloy that annealed at1100 �C for 6 h, the two-phase equilibrium of the liquid phase and(bTi, V) phase was determined, as shown in Fig. 2.3(f). The two-phase equilibrium of the L þ (aCo) was identified in theCo73.79Ti14.81V11.40 (at.%) alloy annealed at 1200 �C for 12 h, asshown in Fig. 2.4(a). The eutectic morphology suggested that theeutectic react of liquid phase has happened during the coolingprocess of the liquid phase. The three-phase equilibrium of theL þ (aCo) þ s-Co2V3 was identified in the Co62.80Ti3.41V33.79 (at.%)alloy annealed at 1200 �C for 12 h, as indicated in Fig. 2.4(b). Thes-Co2V3 phase is light gray and the (aCo) phase is gray. In theCo31.05Ti50.72V18.23 (at.%) alloy that annealed at 1200 �C for 6 h, thetwo-phase equilibrium of the liquid phase and CoTi phase wasdetermined, as shown in Fig. 2.4(c). The two-phase equilibrium ofthe s-Co2V3 þ (bTi, V) was identified in the Co24.48Ti15.39V60.13

Fig. 2. (continued).

J.J. Ruan et al. / Intermetallics 49 (2014) 121e131 125

(at.%) alloy annealed at 1200 �C for 20 days, as indicated inFig. 2.4(d). The s-Co2V3 phase is light gray and the (bTi, V) phaseis gray. In the Co17.86Ti51.03V31.11 (at.%) alloy that annealed at1200 �C for 6 h, the two-phase equilibrium of the liquid phaseand (bTi, V) phase was determined, as shown in Fig. 2.4(e). Thethree-phase equilibrium of the L þ (bTi, V) þ CoTi was identifiedin the Co25.89Ti43.77V30.34 (at.%) alloy annealed at 1200 �C for 6 h,as indicated in Fig. 2.4(f). The CoTi phase is light gray and the (bTi,V) phase is gray.

3.2. Isothermal sections

The equilibrium compositions of the CoeTieV ternary system at800 �C, 1000 �C, 1100 �C and 1200 �C determined by EPMA aresummarized in Table 2. Based on the experimental data thatmentioned above, the isothermal sections at 800 �C, 1000 �C,1100 �C and 1200 �C were constructed in Fig. 4(a)e(d). Undeter-mined three-phase equilibria are shown in Fig. 4(a)e(d) in dashedlines.

In the isothermal section at 800 �C (Fig. 4(a)), three three-phase regions of the s-Co2V3 þ Co3Ti þ Co2Ti (h),CoTi þ CoTi2 þ (bTi, V) and s-Co2V3 þ CoV3 þ (bTi, V) were

experimentally determined in this work. It is found that thetwo-phase region of Co3V þ Co3Ti is narrow. The Co3Ti phasereach to the CoeV side, but did not form the continuous solidsolution due to the different crystal structure between Co3Vphase and Co3Ti phase. The (bTi, V) phase forms a largecontinuous solid solution from CoeV side to CoeTi side. Thesolubility of V in Co2Ti (h) phase is large than that in Co2Ti (c)phase. The solubility of Ti in CoV3 phase is measured to be about3.86 at.%. The solubility of V in CoTi2 phase is measured to beabout 10.80 at.%. Fig. 4(b) shows the isothermal section at1000 �C, where the three-phase equilibrium of the(aCo) þ Co3V þ Co3Ti and CoTi þ CoTi2 þ (bTi, V) were experi-mentally determined. The results show that the three-phaseregion of (aCo) þ Co3V þ Co3Ti is narrow at 1000 �C. The (bTi,V) phase forms a large continuous solid solution from CoeV sideto CoeTi side which is similar to that in 800 �C. The single phaseregion of (aCo) is large than that in 800 �C. The solubility of V inCoTi2 phase is measured to be about 9.79 at.%. In the isothermalsection at 1100 �C, shown in Fig. 4(c), four three-phase regionsof the s-Co2V3 þ Co3Ti þ Co2Ti (h), CoTi þ CoTi2 þ (bTi, V),L þ CoTi2 þ CoTi and L þ CoTi2 þ (bTi, V) were experimentallydetermined. The single-phase region of CoTi2 phase, rather than

Fig. 2. (continued).

J.J. Ruan et al. / Intermetallics 49 (2014) 121e131126

ternary compound was identified by the present experiment.However, the CoTi2 phase was reported to be stable only below1060 �C in the CoeTi binary system [19]. Therefore, the presentresults suggest that the addition of V stabilized the CoTi2 phaseagainst higher temperatures. The solubility of V in Co3Ti phase islarge than that in 800 �C and 1000 �C. Fig. 4(d) shows theisothermal section at 1200 �C, where the three-phase equilib-rium of the s-Co2V3 þ L þ (aCo) and L þ CoTi þ (bTi, V) wereexperimentally determined. The solubility of V in liquid phase atthe Co-rich corner is measured to be about 32.70 at.%. The sol-ubility of V in liquid phase at the Ti-rich corner is measured tobe about 25.40 at.%. The s-Co2V3 phase with large solubility of Tiwas found in all sections, the solubility of V in the Co2Ti (h)phase is larger than the solubility of V in Co2Ti (c) phase ineach section. In addition, the solubility of V in the Co3Tiphase increases with an increase in temperature region from1000 �C to 1100 �C. And the large solubility of V in the Co3Tiphase also agreed well with the conclusion that Liu Y. et al.made [27].

In present work, the thermal stability of the CoTi2 phase wasinvestigated by DSC. Fig. 5 shows the DSC curve on heating and

cooling of Co30.87Ti50.75V18.38 (at.%) alloy, where the transitiontemperature of CoTi2 phase was determined from the endo-thermic and exothermic peaks. According to the results of thecooling curve and the typical BSE image of Co31.05Ti50.72V18.23(at.%) alloy annealed at 1200 �C for 6 h (Fig. 5(a)), it can beconfirmed that the CoTi phase precipitated from the liquid phaseat w1222.5 �C. It is worthy noting that the composition of theliquid phase (Co22.76Ti54.44V22.80 (at.%)) in Co31.05Ti50.72V18.23(at.%) alloy annealed at 1200 �C for 6 h is in the two-phase regionof CoTi2 þ (bTi, V) at the temperature of 1100 �C (Fig. 4(c)), thephase transformation from Liquid phase to CoTi2 þ (bTi, V) atw1167.7 �C was suggested, and the results are in accord with thetypical BSE image of Co30.87Ti50.75V18.38 (at.%) alloy annealed at1100 �C for 40 days (Fig. 5(b)). The cooling curve in Fig. 5 alsoindicated that there is no peaks between 1100 �C and 1000 �Cwhich is consistent with the results that the Co31.10Ti50.59V18.31

(at.%) alloy annealed at 1000 �C for 75 days (Fig. 5(c)) wasobserved to be three-phase equilibrium (CoTi þ CoTi2 þ (bTi, V)).From the heating curve in Fig. 5, it can be seen that the transitiontemperature of CoTi2 phase in the CoeTieV ternary systemis w1140.5 �C.

Fig. 4. Experimentally determined isothermal sections of the CoeTieV system: (a)800 �C; (b) 1000 �C; (c) 1100 �C; (d) 1200 �C.

Fig. 3. X-ray diffraction patterns obtained from: (a) Co18.07Ti5.26V76.67 (at.%) alloyannealed at 800 �C for 135 days; (b) Co65.73Ti17.79V16.48 (at.%) alloy annealed at 1000 �Cfor 75 days; (c) Co30.87Ti50.75V18.38 (at.%) alloy annealed at 1100 �C for 40 days.

J.J. Ruan et al. / Intermetallics 49 (2014) 121e131 127

3.3. The two phases region of g ((aCo)) þ g0 (Co3Ti) at 800 �C

Co-based L12 (ordered fcc) compounds such as Co3Ti has beenreported [28,29]. The experiment results obtained by Thorntonand Davies shown that the Co3Ti behaved like Ni3Al inmechanically and structurally, thus it has the possibility to pro-duce similar strengthening effects in two phases alloys(fcc þ order-fcc) [30]. The mechanically properties of positivetemperature dependence of strength also be confirmed by WeeDM et al. [31] and this behavior is related to the micro-cross slipsfrom the {111} to {100} plane [32]. And the mechanical proper-ties of polycrystalline Co3Ti were also investigated by Takasugiet al. [33], it suggested that the high ductility of polycrystallineCo3Ti is caused by the high grain-boundary strength. The Taka-sugi et al. also investigated the mechanical properties of singlecrystal of Co3Ti [34]. In addition, the micro-structure such asgrain size and texture which is important for practical applica-tion has also been investigated [35,36], the high critical tem-perature for recrystallization indicated the possibilities of thisphase be used for heat-resistent materials. The mechanicallyproperties of Co3Ti with alloying elements were investigated bysome researchers [37,38], and the yield stress of Co3Ti with asmall addition of Vanadium in tensile test is a little higher thanthat of unalloyed.

Table 2Equilibrium composition of the CoeTieV ternary system determined in the present work. (L indicates the Liquid phase).

T(�C) Alloys (at.%) Annealed time(days or hours)

Equilibria Composition (at.%)

Phase 1/Phase 2/Phase 3 Phase 1 Phase 2 Phase 3

Ti V Ti V Ti V

800 Co80.16Ti14.68V5.16 135 days (aCo)/Co3Ti 5.45 2.66 17.18 6.03Co80.24Ti8.89V10.37 135 days (aCo)/Co3Ti 4.08 6.08 12.17 9.66Co79.72Ti3.81V16.47 135 days (aCo)/Co3Ti 2.88 11.28 6.93 15.36Co66.48Ti22.36V11.16 135 days Co3Ti/Co2Ti (h) 17.65 11.26 27.57 7.04Co58.97Ti31.55V9.48 135 days Co2Ti (h)/s-Co2V3 30.02 7.44 31.74 12.39Co17.07Ti63.00V19.93 135 days CoTi2/(bTi, V) 63.38 5.78 58.60 31.29Co21.69Ti8.97V69.34 135 days (bTi, V)/CoV3/s-Co2V3 5.87 78.29 3.86 73.11 25.56 43.36Co70.03Ti24.87V5.10 135 days Co3Ti/Co2Ti (h) 16.08 9.83 29.99 2.75Co67.12Ti10.86V22.02 135 days Co3V/s-Co2V3 6.47 21.54 19.63 22.49Co61.72Ti23.97V14.31 135 days Co3Ti/Co2Ti (h)/s-Co2V3 11.90 15.35 27.98 8.63 25.88 16.14Co64.30Ti3.54V32.16 135 days Co3V/s-Co2V3 5.09 24.36 3.29 39.77Co30.95Ti50.70V18.35 135 days CoTi/CoTi2/(bTi, V) 46.74 7.82 57.80 10.80 29.78 58.24Co31.34Ti63.44V5.22 135 days CoTi/CoTi2 49.31 3.01 61.95 5.83Co18.46Ti61.54V20.00 135 days CoTi2/(bTi, V) 62.23 6.00 55.88 32.37Co18.07Ti5.26V76.67 135 days CoV3/(bTi, V) 3.80 72.73 4.39 82.71Co14.91Ti73.67V11.42 135 days CoTi2/(bTi, V) 64.54 3.03 75.90 13.87

1000 Co80.52Ti14.61V4.87 75 days (aCo)/Co3Ti 9.85 4.14 15.68 4.73Co80.54Ti9.33V10.13 75 days (aCo)/Co3Ti 6.67 8.79 10.96 9.88Co70.10Ti20.42V9.48 75 days Co2Ti (h)/Co3Ti 31.37 4.46 18.08 10.63Co59.30Ti31.19V9.51 75 days Co2Ti (h)/s-Co2V3 31.89 6.10 30.87 13.02Co21.73Ti9.02V69.25 75 days s-Co2V3/(bTi, V) 28.49 32.82 5.56 77.40Co71.45Ti19.00V9.55 75 days Co2Ti (h)/Co3Ti 29.59 2.97 18.35 8.48Co77.16Ti5.87V16.97 75 days (aCo)/Co3Ti/Co3V 3.83 15.60 7.02 17.16 7.04 17.96Co69.12Ti25.76V5.12 75 days Co3Ti/Co2Ti (h) 19.77 8.44 32.29 2.75Co66.04Ti25.79V8.17 75 days Co3Ti/Co2Ti (h) 17.51 12.04 31.11 5.96Co65.73Ti17.79V16.48 75 days Co3Ti/s-Co2V3 13.05 15.26 22.54 17.29Co64.23Ti3.47V32.30 75 days Co3V/s-Co2V3 3.61 27.15 2.59 38.28Co57.74Ti36.52V5.74 75 days Co2Ti (h)/CoTi 32.36 3.33 37.39 6.64Co84.43Ti10.17V5.40 75 days (aCo)/Co3Ti 9.91 5.81 16.45 5.95Co31.10Ti50.59V18.31 75 days CoTi/CoTi2/(bTi, V) 49.82 7.94 61.08 9.79 46.59 39.36Co17.43Ti72.26V10.31 75 days CoTi2/(bTi, V) 65.45 3.85 71.38 13.95Co17.37Ti67.52V15.11 75 days CoTi2/(bTi, V) 64.70 5.24 65.57 19.40Co17.69Ti61.58V20.73 75 days CoTi2/(bTi, V) 63.36 6.67 59.75 25.35Co13.84Ti74.74V11.42 75 days CoTi2/(bTi, V) 65.85 3.24 75.70 11.77Co56.56Ti39.39V4.05 75 days Co2Ti (h)/CoTi 32.08 2.58 38.98 5.18Co62.20Ti12.41V25.39 75 days Co3V/s-Co2V3 6.59 22.82 10.94 28.68

1100 Co79.83Ti15.21V4.96 40 days (aCo)/Co3Ti 10.94 4.81 15.39 5.12Co80.10Ti9.57V10.33 40 days (aCo)/Co3Ti 8.01 10.32 12.03 10.31Co66.33Ti22.60V11.07 40 days Co3Ti/Co2Ti (h) 16.25 10.74 28.43 5.55Co30.87Ti50.75V18.38 40 days CoTi/CoTi2/(bTi, V) 49.48 7.79 61.26 7.91 48.53 34.50Co17.83Ti62.03V20.14 6 h L/CoTi2/(bTi, V) 65.10 12.33 62.42 7.25 58.11 26.84Co70.09Ti23.72V6.19 40 days Co3Ti/Co2Ti (h) 18.05 8.59 29.39 3.42Co66.50Ti11.62V21.88 40 days Co3Ti/s-Co2V3 8.54 19.37 12.57 23.44Co63.51Ti31.00V5.49 40 days Co2Ti (h)/s-Co2V3 31.03 4.43 31.01 9.52Co66.50Ti22.68V10.82 40 days Co3Ti/Co2Ti (h) 13.76 13.75 27.09 7.62Co65.79Ti18.57V15.64 40 days Co3Ti/Co2Ti (h)/s-Co2V3 12.22 15.59 26.75 8.68 19.86 17.17Co60.80Ti5.37V33.83 40 days (aCo)/s-Co2V3 2.57 30.51 5.61 33.33Co32.52Ti62.93V4.55 6 h L/CoTi/CoTi2 69.79 8.18 49.98 4.80 61.55 7.57Co24.01Ti16.81V59.18 40 days s-Co2V3/(bTi, V) 36.55 21.89 7.90 74.22Co22.43Ti5.83V71.74 40 days s-Co2V3/(bTi, V) 3.84 68.39 3.18 77.60Co16.80Ti73.31V9.89 6 h L/(bTi, V) 70.43 7.01 73.29 15.21Co17.48Ti66.80V15.72 6 h L/(bTi, V) 67.03 10.52 65.50 22.86Co21.05Ti45.65V33.30 40 days CoTi/(bTi, V) 48.05 10.89 42.77 41.49Co14.33Ti76.30V9.37 6 h L/(bTi, V) 72.05 5.90 76.19 12.36Co55.80Ti40.74V3.46 40 days Co2Ti (h)/CoTi 32.12 2.56 37.43 5.35Co60.49Ti11.30V28.21 40 days Co3Ti/s-Co2V3 6.36 23.09 9.14 27.73Co66.85Ti11.87V21.28 40 days Co3Ti/s-Co2V3 8.10 20.40 12.24 24.66

1200 Co73.79Ti14.81V11.40 12 h (aCo)/L 7.23 13.53 15.82 11.43Co59.62Ti18.79V21.59 12 h L/s-Co2V3 14.74 23.76 25.63 15.97Co62.80Ti3.41V33.79 12 h (aCo)/L/s-Co2V3 2.25 31.55 6.01 32.70 4.79 35.03Co74.87Ti19.33V5.80 12 h (aCo)/L 12.75 7.19 19.43 5.71Co69.81Ti23.99V6.20 12 h L/Co2Ti (h) 21.89 9.41 29.36 3.36Co70.75Ti8.09V21.16 12 h (aCo)/L 5.18 22.31 13.88 20.39Co61.20Ti27.46V11.34 12 h L/Co2Ti (h) 22.77 12.80 31.08 7.02Co30.78Ti63.84V5.38 6 h L/CoTi 68.87 7.55 53.85 1.64Co34.36Ti58.56V7.08 6 h L/CoTi 66.65 11.26 50.78 3.36Co28.92Ti63.16V7.92 6 h L/CoTi 60.17 17.48 49.71 5.78Co31.05Ti50.72V18.23 6 h L/CoTi 54.44 22.80 47.98 9.07Co24.48Ti15.39V60.13 20 days s-Co2V3/(bTi, V) 26.65 34.41 8.98 69.01Co22.48Ti5.04V72.48 20 days s-Co2V3/(bTi, V) 3.77 66.65 3.83 73.57Co13.10Ti74.62V12.28 6 h L/(bTi, V) 70.53 11.82 74.87 15.25Co17.30Ti62.12V20.58 6 h L/(bTi, V) 61.92 20.06 60.33 31.33Co17.86Ti51.03V31.11 6 h L/(bTi, V) 57.27 20.16 43.88 45.34Co25.89Ti43.77V30.34 6 h L/CoTi/(bTi, V) 50.58 25.40 43.65 16.97 29.78 53.27Co61.66Ti10.89V27.45 12 h L/s-Co2V3 11.87 25.73 11.01 28.51

Fig. 5. DSC curve obtained from Co30.87Ti50.75V18.38 (at.%) alloy with the typical BSEimages obtained from: (a) Typical BSE image of Co31.05Ti50.72V18.23 (at.%) alloy annealedat 1200 �C for 6 h; (b) Typical BSE image of Co30.87Ti50.75V18.38 (at.%) alloy annealed at1100 �C for 40 days; (c) Typical BSE image of Co31.10Ti50.59V18.31 (at.%) alloy annealed at1000 �C for 75 days.

Fig. 7. DSC heating curves obtained from (A) Co80.24Ti8.89V10.37 (at.%) alloy and (B)Co77.29Ti5.83V16.88 (at.%) with the typical BSE image obtained from: (a) Typical BSEimage of Co80.54Ti9.33V10.13 (at.%) alloy annealed at 1000 �C for 40 days; (b) Typical BSEimage of Co80.10Ti9.57V10.33 (at.%) alloy annealed at 1100 �C for 75 days; (c) Typical BSEimage of Co77.16Ti5.87V16.97(at.%) alloy annealed at 1000 �C for 40 days; (d) Typical BSEimage of Co77.20Ti5.86V16.94 (at.%) alloy annealed at 1100 �C for 75 days.

J.J. Ruan et al. / Intermetallics 49 (2014) 121e131 129

The Co3Ti has not been widely used for commercial Co-basedhigh-temperature alloys mainly due to its low phase stability(i.e., low solvus temperature or low melting point) [19], and thesensitivity of the mechanical behavior of Co3Ti to environmentcondition [39]. Fig. 6(a), (b) and (d) shows the BSE images ob-tained from alloys with the composition of Co80.16Ti14.68V5.16(at.%), Co80.24Ti8.89V10.37 and Co79.72Ti3.81V16.47 (at.%) annealed at

Fig. 6. Electron micrographs: (a) Typical BSE image of Co80.16Ti14.68V5.16 (at.%) alloy annealed at 800 �C for 135 days; (b) Typical BSE image of Co80.24Ti8.89V10.37 (at.%) alloy annealedat 800 �C for 135 days; (c) SEM of Co80.24Ti8.89V10.37 (at.%) annealed at 800 �C for 135 days; (d) Typical BSE image of Co79.72Ti3.81V16.47 (at.%) alloy annealed at 800 �C for 135 days; (e)Typical BSE image of Co77.29Ti5.83V16.88 (at.%) alloy annealed at 800 �C for 135 days; (f) SEM of Co77.29Ti5.83V16.88 (at.%) (at.%) annealed at 800 �C for 135 days.

Fig. 8. Stress strain curve of Co77.29Ti5.83V16.88 (at.%) alloy annealed at 800 �C for 135days obtained at room temperature.

J.J. Ruan et al. / Intermetallics 49 (2014) 121e131130

800 �C for 135 days, and the big irregular white phase and thesmall white phase can be seen in each sample. In order toobserve the microstructure of small white phase, the alloy withthe composition of Co80.24Ti8.89V10.37 (at.%) annealed at 800 �C for135 days was mechanically polished and etched in a solution ofHCl:HNO3 ¼ 1:1. Fig. 6(b) shows the SEM of Co80.24Ti8.89V10.37(at.%). The DSC heating curve of Co80.24Ti8.89V10.37 (at.%) alloy, asindicated in Fig. 7(A) shows that the 800 �C is between Tcs

(indicated as the start transition temperature from magnetic toparamagnetic) and Tcf (indicated as the finish transition tem-perature from magnetic to paramagnetic), consequently, the ex-istence of (afCo) and (apCo) can be suggested at 800 �C in Co richregion which is in accord with microstructure morphologyobserved in this alloy, as shown in Fig. 6(b). Avoiding theappearance of (afCo) is important to achieve the microstructuremorphology which is like the Ni-based super alloys (homoge-neous distribution of fine cuboidal precipitates), thus, an alloywith the composition of Co77.29Ti5.83V16.88 (at.%) was melted inorder to decrease the Tc. Heating treatment of Co77.29Ti5.83V16.88(at.%) alloy was carried out at 800 �C for 135 days after solutiontreatment at 1200 �C for 2 h. The big irregular white phase dis-appeared, as shown in Fig. 6(e), and the fine cuboidal precipitateswere observed in this alloy after etched, as indicated in Fig. 6(f).The DSC heating curve of Co77.29Ti5.83V16.88 (at.%) alloy, as indi-cated in Fig. 7(B) shows that the 800 �C is higher than the Tcf

which suggested that there is no (afCo) in this alloy, this is inagreement with the microstructure morphology observed inCo77.29Ti5.83V16.88 (at.%) alloy annealed 800 �C for 135 days, asindicated in Fig. 6(e). And the compressive strength ofCo77.29Ti5.83V16.88 (at.%) alloy annealed at 800 �C for 135 days atroom temperature was measured to be about 1985 MPa, shownas Fig. 8. The results indicated that it is important to carry out theheat treatment at the temperature which is higher than Tcf forachieving the microstructure morphology like Ni-based super-alloys. In addition, Fig. 7(A) and (B) shows that the Liquidustemperature of Co3Ti (ordered fcc) phase in Co80.24Ti8.89V10.37(at.%) alloy and Co77.29Ti5.83V16.88 (at.%) alloy annealed at 800 �Cfor 135 days is about 1190.3 �C and 1198.7 �C, respectively.However, the Co3Ti phase was reported to be stable only below1181 �C in the CoeTi binary system [19]. Therefore, the additionof V stabilized the Co3Ti phase against higher temperatures wassuggested.

4. Conclusions

Four isothermal sections of the CoeTieV ternary system at800 �C, 1000 �C, 1100 �C and 1200 �C were experimentallydetermined. The experimental results indicated that: (1) there isno ternary compound in this system; (2) the CoTi2 phase andCo3Ti phase were stabilized by the addition of Vanadium againsthigher temperatures; (3) the solubility of Ti in the s-Co2V3 phaseis large at all isothermal sections of 800 �C, 1000 �C, 1100 �C and1200 �C; (4) the solubility of V in the Co3Ti phase increases withan increase in temperature region from 1000 �C to 1100 �C; (5)the solubility of V in the Co2Ti (h) phase is larger than the sol-ubility of V in Co2Ti (c) phase in each section; (6) The alloy withthe homogeneous distribution of fine cuboidal Co3Ti (L12) in(aCo) phase which microstructure morphology likes the Ni-basedsuper alloys was observed; (7) The compressive strength ofCo77.29Ti5.83V16.88 (at.%) alloy at room temperature was measuredto be about 1985 MPa.

The newly determined phase equilibria of the CoeTieV ternarysystem will provide additional support for the thermodynamicassessment of this system and development of Co-based andTi-based materials.

Acknowledgments

This work was supported by the National Natural ScienceFoundation of China (Grant Nos. 51031003 and 51171159), theMinistry of Science and Technology of China (Grant Nos.2009DFA52170), the Ministry of Education of China (Grant Nos.20120121130004) and the National Key Basic Research Program ofChina (973 Program) (No. 2012CB825700). The supports from theChina Aviation Industry Group are also acknowledged.

References

[1] Sullivan CP, Donachie MJ, Morral FR. Centre d’Information du Cobalt; 1970.Brussels.

[2] Sato J, Omori T, Oikawa K, Ohnuma I, Kainuma R, Ishida K. Science 2006;312:90e1.

[3] Cheng J, Yang J, Zhang XH, Zhong H, Ma JQ, Li F, et al. Intermetallics 2012;31:120e6.

[4] Liu ZH, Yu SY, Yang H, Wu GH, Liu YN. Intermetallics 2008;16:447e52.[5] Iwasaki S, Quchi K. IEEE Trans Magn 1978;14:849.[6] Zhang Y, Tsushio Y, Enoki H, Akiba E. J Alloys Compd 2005;393:147e53.[7] Kabutomori T, Takeda H, Wakisaka Y, Ohnishi K. J Alloys Compd 1995;231:

528e32.[8] Cho SW, Han CS, Park CN, Akiba E. J Alloys Compd 1999;288:294e8.[9] Nomura K, Akiba E. J Alloys Compd 1995;231:513e7.

[10] Shinagawa K, Omori T, Oikawa K, Kainuma R, Ishida K. Scr Mater 2009;61:612e5.

[11] Bauer A, Neumeier S, Pyczak F, Göken M. Scr Mater 2010;63:1197e200.[12] Yokokawa T, Osawa M, Nishida K, Kobayashi T, Koizumi Y, Harada H. Scr

Mater 2003;49:1041e6.[13] Qin GW, Oikawa K, Sato M. IEEE Trans Magn 2005;41:918.[14] Loo V, Vrolijk A, Bastin FJ. J Less-Common Met 1981;77:121.[15] Wu ZG, Liu ZH, Yang H, Liu YN, Wu GH. Intermetallics 2011;19:1839e48.[16] Kostov A, Zivkovic D. J Alloys Compd 2008;460:164e71.[17] Bratberg J, Sundman B. J Phase Equilib 2003;24:495e503.[18] Okamoto H. J Phase Equilib 1993;14:266e7.[19] Davydov AV, Kattner UR, Josell D, Blendell JE, Waterstrat RM, Shapiro AJ, et al.

Metall Mater Trans 2001;32A:2175e86.[20] Köster W, Wagner E. Z Met 1937;29:230.[21] Rostoker W, Yamamoto A. Trans Am Soc Met 1954;46:1136.[22] Pietrokowsky P, Duwez P. Trans AIME 1950;188:1283.[23] Pearson WB, Christian JW, W HR. Nature 1951;167:110.[24] Sully AH. J Inst Met 1951;80:9.[25] Pearson WB, Christian JW. Acta Cryst 1952;5:157.[26] Greenfield P, Beck PA. Trans AIME 1954;200:253.[27] Liu Y, Takasugi T, Izumi O. Metall Trans A 1986;17 A:1433e9.[28] Blaise JM, Viatour P, Drapier JM. Cobalt 1970;49:192.[29] Viatour P, Drapier JM, Coutsouradis D. Cobalt 1973;3:67.[30] Thornton PH, Davies RG. Metall Trans 1970;1:549e50.[31] Wee DM, Noguchi O, Oya Y, Suzuki T. Trans JIM 1980;21:237e47.[32] Liu Y, Takasugi T, Izumi O, Takahashi T. Philos Mag A 1989;59:437e54.

J.J. Ruan et al. / Intermetallics 49 (2014) 121e131 131

[33] Takasugi T, Izumi O. Acta Metall 1985;33:39e48.[34] Takasugi T, Hirakawa S, Izumi O, Ono S, Watanabe S. Acta Metall 1987;35:

2015e26.[35] Kaneno Y, Nakaaki I, Takasugi T. J Mater Res 2002;17:2567e77.

[36] Takasugi T, Izumi O. Acta Metall 1985;33:49e58.[37] Liu Y, Takasugi T, Izumi O, Suenaga H. J Mater Sci 1989;24:4458e66.[38] Takasugi T, Takazawa M, Izumi O. J Mater Sci 1990;25:4231e8.[39] Liu Y, Takasugi T, Izumi O, Yamada T. Acta Metall 1989;37:507e17.