the wear performance of yttrium-modified stellite 712 at elevated temperatures

Tribology International 40 (2007) 254–265

The wear performance of yttrium-modified Stellite 712at elevated temperatures

Iulian Radu, D.Y. Li

Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alta., Canada T6G 2G6

Received 1 December 2004; accepted 1 September 2005

Available online 31 March 2006

Abstract

Cobalt-based alloys are often used for bearing applications, especially at elevated temperatures. One of the newly developed 700 series

cobalt-based alloys, Stellite 712, has been demonstrated to possess high resistance to wear and corrosion in aggressive environments.

Continuous efforts have been made to further improve this alloy for enhanced resistance to high-temperature wear involving oxidation.

Recent studies showed that the improvement of the oxide scale on Co-base alloys by alloying with yttrium was an effective way to

diminish wear of the alloys at elevated temperatures.

In this work, sliding wear performances of yttrium-free and yttrium-containing Stellite 712 samples at elevated temperatures were

evaluated. The mechanism responsible for changes in its wear performance was investigated by studying the effects of alloying yttrium on

microstructure and mechanical properties of the bulk alloy and its oxide scale, employing various experimental methods including micro-

and nano-mechanical probing, XRD, SEM-EDS, AFM and high-temperature pin-on-disc wear testing. The research demonstrated that

alloying a small amount of yttrium (e.g. less than 1%Y) rendered the oxide scale on Stellite 712 stronger with higher adherence to the

substrate, which was largely beneficial to the wear performance of the alloy at elevated temperatures. Mechanisms involved are discussed

in this article.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Yttrium; Stellite 712; High-temperature wear; Oxide scale

1. Introduction

Stellites represents cobalt-base alloys typically used inapplications that require high resistance to wear inaggressive environments, particularly at elevated tempera-tures [1,2]. One of the newly developed 700 series cobalt-base alloys, Stellite 712, has been demonstrated to possesshigh resistance to wear and corrosion in aggressiveenvironments. This type of alloy is suitable for bearingapplications in nuclear industry, e.g. impeller bearing forslurry pumps operated in nuclear waste tanks [3,4]. Theexcellent resistance of Co-base alloys to mechanical attackand chemical degradation over a wide temperature rangebenefits from the low stacking fault energy of the cobaltmatrix, high corrosion resistance, solid solution hardening,and carbide precipitation-hardening [2].

Pure cobalt exhibits an allotropic phase transformationon cooling at 390 1C (Ms) from a high-temperature stable gphase (FCC) to a low-temperature stable e phase (HCP).The transformation is reversible on heating, occurring at430 1C (As) [2]. When alloying elements are present, thetransformation temperatures may be affected, e.g. As isincreased to 970 1C for a Co–27Cr–5Mo–0.05C alloy [5].However, under most cooling conditions, the transforma-tion tends to be sluggish in a cobalt-based alloy, thereforethe cobalt matrix is a mixture of e and metastable g at roomtemperature [2,5]. The volume fractions of the two cobaltallotropes determine the mechanical behavior of thealloy, e.g. the ductility is increased with increasing gphase [6,7]. As a result, alloying elements are carefullybalanced to avoid lower ductility. The g-e transformationmay also be induced by aging at temperatures near 800 1C[5,8] or induced by plastic deformation (strain inducedtransformation—SIT) [9,10]. The SIT can be linked to theabsorption of energy in various wear situations, which

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Corresponding author. Tel.: +1780 492 5157; fax: +1 780 492 2881.

E-mail address: [email protected] (I. Radu).

explains the excellent galling resistance of cobalt basealloys [11].

The strengthening of Stellite alloys benefits from therefractory elements (tungsten or molybdenum) solid solu-tion hardening and carbide precipitation. Stellite 712(Table 1) contains high amounts of molybdenum for solidsolution hardening. This particular alloy has relatively highcarbon content (1.8%) that is necessary for increasing theamount of carbides for higher hot hardness and enhancedinhibition to grain boundary sliding, dislocation move-ment, and grain growth, compared to conventional Stellitealloys. The remarkable corrosion and oxidation resistanceof this alloy benefits from its high concentrations ofchromium and molybdenum.

Efforts have been made to further improve this alloyparticularly for applications at elevated temperatures.Recent studies [12,13] have demonstrated that the resis-tance of Stellite alloys to wear at elevated temperatures canbe markedly enhanced by improving their oxide scale’smechanical properties and adherence to the substrate. Oneapproach is to use a small amount of oxygen-activeelements, such as yttrium, to positively modify the oxidescale’s properties.

In this work, sliding wear performances of yttrium-free(Y-free) and yttrium-modified Stellite 712 at elevatedtemperatures were evaluated. The mechanism responsiblefor changes in its wear performance was investigated bystudying effects of alloying yttrium on microstructure andmechanical properties of both the bulk alloy and its oxidescale, employing various experimental methods includingoptical microscopy, X-ray diffraction, SEM (scanningelectron microscopy), EDS (energy dispersive spectro-scopy), micro- and nano-mechanical probes, micro-scratchtesting, AFM (atomic force microscopy), and pin-on-discwear testing.

2. Experimental procedure

2.1. Alloy preparation and heat treatment

Stellite 712 alloy provided by Deloro Stellite Inc. was re-melted with added yttrium powder (40 mesh) in an arc-melting furnace (MRF INC. SA338-V&G). The meltingand solidification conditions are given in Table 2. Allsamples were turned over and re-melted consecutively forfive times in order to increase the degree of homogeniza-tion. Several groups of samples were made, whichcontained 0, 0.5, 1, 2, and 5-wt% yttrium, respectively.The cast samples had the following dimensions:

15 10 30mm. The samples were homogenized at815 1C for three hours followed by cooling in air. Afterthe homogenization treatment, samples were cut using anabrasive disc cutter machine.

2.2. Microstructure examination

Specimens for optical metallography were polished withSiC sand papers and finally polished using a 0.05 mmalumina slurry. A solution of HCl and 30% H2O2 (volumefraction 6:1) was used to etch the samples. Microstructuresof the samples were observed using an optical microscope(Olympus PME3-ADL). SEM and EDS techniques werealso employed to investigate microstructure and composi-tion distribution of the specimens. Specimens for X-rayexamination had dimensions of 15 10 1mm. A RikaguX-ray diffractometer with Cu Ka radiation (l ¼ 1:54056 A)was used to determine crystal structures of phasesdeveloped in the samples. Prior to X- ray investigation,specimens were polished using a 0.05 mm alumina slurrythen etched to remove a deformed surface layer caused bypolishing.

2.3. Mechanical property evaluation

Macro-hardness was measured using a conventionalRockwell hardness tester under a load of 1471N. Eachreported HRC value is an average of six measurements.Micro-hardness values of different phases or domainsdeveloped in the samples were determined using a micro-mechanical probe (Fisher Technology Ltd., Winsor, CT,USA) under different maximum loads (10, 50, 100, 200,400, and 600mN). Each reported value is an average offifteen measurements. Z value and indentation depth werealso determined. The Z value is the ratio of elasticdeformation energy (We) to the total deformation energy(Wtot), and represent a measure of the contribution ofelasticity to the deformation. Wtot is determined by the areaenclosed by the loading curve and the maximum depth ofpenetration, while We is represented by the area enclosedby the unloading curve and the maximum depth ofpenetration.

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Table 1

Nominal composition of Stellite 712

Element Co Cr Mo Ni Fe Si Mn C

Percentage composition (wt%) Bal 29 8.5 2.5 2.5 1.5 1 1.8

Table 2

Experimental conditions for re-melting of Stellite 712

Parameters Melting conditions

Vacuum pressure before filling with Ar gas 27–28 psi

Ratio of vacuum/back-fill with Ar gas 3 times

Protection gas Argon

Gas flow during melting and cooling 2 l/min

Pressure during melting and cooling 0 psi

Electrode type Thoriated tungsten

Electrode diameter 4.8mm

Arc current 160/240A

No. of turning over and remelting 5 times

I. Radu, D.Y. Li / Tribology International 40 (2007) 254–265 255

2.4. Evaluation of oxide scale’s mechanical properties

Y (Yttrium)-free and Y-containing Stellite 712 specimenswere heated in a furnace and oxidized in air at 600 1C for20min. The sample dimensions were 15 20 7mm. The

surface of the specimens was polished with a 0.05 mmalumina slurry before oxidation treatment. The scratchresistance of oxide scales on specimens with and withoutyttrium were investigated using a micro-scratch tester(Center for Tribology Inc., Mountain View, CA, USA).The micro-scratch test was performed by scratching thesurface of a specimen using a pyramidal tungsten carbidetip under a linearly increasing load from 0 to 10N. The tipmoved at a horizontal velocity of 0.05mm/s. The electricalcontact resistance (ECR) between the tip and the samplewas monitored in situ. The oxide scale was scratched offwhen the applied load reached a critical value. The failureof the oxide scale resulted in a drop of the electrical contactresistance (ECR). The critical load under which the oxide

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Fig. 1. Optical microstructures of Stellite 712 alloy samples with (a) 0%Y,

(b) 0.5%Y, and (c) 5%Y. The light and dark domains are dendritic and

interdendritic regions, respectively.

Inte

nsity

[ar

bitr

ary

unit]

8070605040302010

2θ angle

Stellite 712 -Y-freeXRD - bulk

12

3

4

5

6

7

8

Inte

nsity

[ar

bitr

ary

unit]

8070605040302010

2θ angle

Stellite 712 -0.5%YXRD - bulk

1 2

3

4

5

6

78

Inte

nsity

[ar

bitr

ary

unit]

8070605040302010

2θ angle

Stellite 712-5%YXRD - bulk

7

86

5

432

0'0 6'8'

Fig. 2. X-ray diffraction patterns of Y-free, 0.5%, and 5% yttrium-

containing samples, respectively (see Table 3 for detailed information on

the peaks marked with numbers).

I. Radu, D.Y. Li / Tribology International 40 (2007) 254–265256

scale was scratched off can be regarded as a measure of theoxide adherence to substrate.

The oxide scale’s hardness was evaluated using a nano-mechanical probe (Hysitron, Minneapolis, USA). Forthis investigation, the oxide was allowed to grow at600 1C for 25 h. The dimensions of the specimens were15 10 6mm. The nano-mechanical probe was a three-sided diamond pyramid (tip angle ¼ 901). The test wasperformed under three maximum loads (20, 40, and 80 mN)and corresponding load–depth curves were recorded.Hardness and elastic behavior of the oxide scales were

determined from the force–displacement curves and eachvalue reported was an average of thirty measurements. Thesmaller the indentation depth under a given maximumload, the harder is the material.X-ray diffraction analysis was performed at a glancing

angle of 21 on specimens oxidized at 600 1C for 25 h inorder to obtain the information about the phase structureof the thin oxide scales. The dimensions of specimens were15 10 1mm.Atomic force microscopy technique was used to study

the morphology of oxide films developed after isothermal

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Table 3

XRD phases corresponding to the peak numbers

Phase Peak number

00 0 1 2 3 4 5 6 60 7 80 8

eCo — — — — (1 0 0) — (0 0 2) (1 0 1) — — — (1 1 0)

a-Co — — — — — — (1 1 1) — — (2 0 0) — (2 2 0)

Ni–Cr–Co–Mo — — — — — — (1 1 1) — — (2 0 0) — (2 2 0)

Co7Mo6 — (1 1 0) — — (1 0 0) — (0 2 1) — — — — —

Co3Mo — — — (2 0 0) — — (0 0 2) (2 0 1) — — (2 2 0) —

Cr23C6 — (4 2 0) — — (4 2 2) — (5 1 1) — (4 4 0) (6 0 0) — (8 2 2)

Fe2Mo3 — — X X — X — X — — — X

C — — — — (1 0 0) (1 0 1) (0 0 4) (1 0 2) — — — (1 1 0)

Co2YSi2 (1 0 3) (1 1 2) — — — — — (2 0 0) — — — —

20 µm

20 µm

20 µm

20 µm

20 µm

(a)

(c)

(e)

(d)

(b)

Fig. 3. SEM images of Stellite 712 alloy samples with (a) 0%Y—untreated; (b) 5%Y—untreated; (c) 0%Y—heat treated; (d) 5%Y—heat treated; (e)

0%Y—heat treated, a closer view.


oxidation at 600 1C for 25 h on Y-free and Y-containingStellite 712 samples. An atomic force microscope (DigitalInstruments, Santa Barbara, CA, USA) was used tomeasure the three-dimensional topography of a samplesurface with a scan size of 10 mm 10 mm and a scanrate of 1Hz. The cantilever probe was a silicon nitride tip.Mean surface roughness (Ra) and 10-point mean roughness(Rz) were determined from the recorded AFM imagesusing the NanoScope SPM software, version 4.42. EachRa and Rz value reported is an average of fifty measure-ments.

2.5. Wear test

Sliding wear tests were performed at room temperatureand 600 1C, respectively, on a pin-on-disc tribometer(CSEM Instruments, Neuchatel, Switzerland). The discwas the sample under study (15 10 6mm) and the pinwas an alumina ball with its diameter equal to 6mm.All tests were performed at a sliding speed of 0.5 cm/salong a circular path of 0.8mm in radius under a normalload of 2N for 10 000 cycles. Prior to each wear test, thesurface of a specimen was polished using a 6 mm-diamondpaste.

The volume loss was determined by measuring theprofile of wear track using a contact profilometer (TencorInstruments, USA). Reflected light microscopy and SEMwere used to observe morphologies of worn surfaces. EDSwas used to determine the local chemical compositions ofthe worn surfaces.

3. Results and discussion

3.1. Microstructure and phase analysis

Microstructures of Y-free and Y-containing Stellite 712specimens after homogenization treatment are shown inFig. 1. All the specimens showed a typical dendriticmicrostructure. No marked difference in microstructurewas observed among the specimens; however, the inter-dendritic regions of the 5%Y-containing specimens showeda slightly different response to etching.Fig. 2 presents X-ray diffraction patterns of Y-free,

0.5% and 5% Y-containing samples, respectively. X-raypatterns of 1 and 2% Y-containing Stellite 712 specimensare similar to that of the specimen containing 0.5%yttrium. Identified phases were (Table 3): Cr23C6 (FCC),an intermetallic compound—Co3Mo (HCP), m phase—Co7Mo6 (rhombohedral), NiCrCoMo (FCC), carbon(HCP), Fe2Mo3, g-Co (FCC), and e-Co (HCP). TheX-ray result was consistent with those reported in literaturefor cobalt-based alloys [2]. As shown, a small amountof yttrium did not result in the formation of any newX-ray peak, probably because the amount of yttriumwas not sufficient for the formation of new phases, orpossibly formed new phases were too small to be detectedby XRD. However, a new phase (Co2YSi2) was observedat higher yttrium contents, e.g. 5%Y. All specimenscontained both g and e cobalt allotropes at roomtemperature. These results are consistent with previousstudies on Stellite 21 [12].

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Fig. 4. EDS compositional dot maps of Stellite 712 alloy containing 5%Y. The bright regions show the presence of that particular element in the image.


SEM images of heat-treated (homogenization) anduntreated Y-free and 5%Y- containing specimens arepresented in Fig. 3. Fig. 4 presents EDS maps ofmolybdenum, chromium, cobalt, nickel and yttrium inStellite 712 sample containing 5-wt% yttrium after heattreatment. It was demonstrated that rich molybdenum andchromium eutectics were formed in the interdendriticregions. The 0.5, 1 and 2%Y-containing samples hadsimilar molybdenum and chromium distributions but as theamount of yttrium increased it seems that the chromium-rich eutectic became slightly coarser due to enhancedatomic diffusion at the high homogenization temperature.No silicon was detected in the chromium-rich phase in allspecimens. Before homogenization treatment, yttrium wasnot homogeneously distributed in the 5%Y-containingsample but appeared to be concentrated in some smallregions in the interdendritic regions. According to binaryphase diagrams (e.g. Co–Y, Ni–Y) [14], yttrium has verylimited solid solubility in the elements present in Stellite 712at room temperature. Therefore, yttrium segregated in theinterdendritic regions. Exposure at 815 1C for 3 h promotedthe diffusion of yttrium to the domains that contained

molybdenum-rich phases. It is known that molybdenumsolutes diffuse preferentially to stacking faults [15], whichmight also provide more space to accommodate yttriumatoms that have a larger radius (0.181nm), compared toother elements in the alloy (e.g. Co, Ni, Fe, Cr, and Mo).This might be a reason why yttrium was more concentratedin the molybdenum-rich regions. However, according to a

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(a)

(b)

2.5

2.0

1.5

1.0

0.5

0.0

Dep

th o

f In

dent

atio

n [µ

m]

6005004003002001000

Load [mN]

2.5

2.0

1.5

1.0

0.5

0.0

Dep

th o

f In

dent

atio

n [µ

m]

6005004003002001000

Load [mN]

Stellite 712

Depth of IndentationInterdendritic region

Y free 0.5%Y

5%Y

Stellite 712

Depth of IndentationInterdendritic region

Y free 0.5%Y

5%Y

35

30

25

20

15

η [%

]

6005004003002001000Load [mN]

35

30

25

20

15

η [

%]

6005004003002001000

Load [mN]

Stellite 712

η–Interdentritic region

Y free 0.5%Y5%Y

Stellite 712

η–Interdentritic region

Y free 0.5%Y5%Y

Fig. 5. Curves of Z value and indentation depth versus indentation load: (a)interdendritic regions; (b) dendritic regions.

40

30

20

10

0

Forc

e [

µN]

1086420Displacement [nm]

Y free0.5%Y

5%Y

Fig. 6. Nano-indentation curves of the oxide scales on yttrium-free, 0.5%

and 5% yttrium-containing samples, respectively.


binary Mo–Y diagram [14], molybdenum and yttrium areinsoluble in each other at room temperature.

3.2. Mechanical properties

Macro-hardness tests were performed on Y-free andY-containing Stellite 712 samples. It was demonstrated

that yttrium did not influence the hardness of the alloy(52 HRC for all samples).Localized mechanical properties of various phases or

domains were evaluated using a micro-mechanical probe.The technique is based on Vickers indentation, whichprovides information on local hardness and elasticbehavior. The elastic behavior was evaluated using theratio (Z) of elastic deformation energy to the totaldeformation energy.Fig. 5 presents plots of Z value and depth versus

indentation load for dendritic and interdendritic regionsas shown in Fig. 1. As illustrated, yttrium did notsignificantly modify mechanical properties of these regions.The interdendritic regions showed slightly higher hardnessand larger Z values than the dendritic regions.

3.3. Mechanical properties of the oxide scale

Effects of alloyed yttrium on the oxide scale on Stellite712 were evaluated by nano-indentation testing. Fig. 6shows force–displacement curves of Y-free, 0.5% and 5%Y-containing samples, respectively, under a maximum loadof 40 mN. Indentation depths and Z value of oxide scales onY-free and Y-containing specimens determined from theindentation tests are given in Table 4. As shown, the oxidescale formed on the 0.5% Y-containing Stellite 712 samplewas the hardest with the lowest indentation depth and thehighest Z value. However, high Y concentrations did notbenefit the oxide scale in terms of hardness and Z value asTable 4 and Fig. 6 illustrate.Fig. 7 illustrates results of micro-scratch test for Y-free

and Y-containing Stellite 712 specimens. The Y-containing

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Table 4

Mechanical properties of the oxide scale obtained from nano-indentation

test (error range depth: 70.4 nm; Z: 73%)

Material Maximum

load (mN)

Maximum

depth of

indentation

(nm)

Z (%)

Yttrium free Stellite

712

20 4.6 33.8

40 8.4 31.0

80 13.4 34.0

Stellite712–0.5%Y 20 4.4 42.3

40 6.8 40.0

80 11.3 51.7

Stellite 712–1%Y 20 5.1 32.2

40 7 34.2

80 12.7 35.2

Stellite 712–2%Y 20 5.6 27.4

40 9.5 22.4

80 13.8 21

Stellite 712–5%Y 20 5.8 37.0

40 9.4 32.0

80 14.7 29.5

-10

-8

-6

-4

-2

0

Loa

d [N

]

605040302010

1000

800

600

400

200

Ele

ctri

cal C

onta

ctR

esis

tanc

e [m

Ω]]

Stellite 712-Y free

Load

3.5N

1000

800

600

400

200

Ele

ctri

cal C

onta

ctR

esis

tanc

e [m

Ω]

605040302010Time [s]

-10

-8

-6

-4

-2

0

Loa

d [N

]

Stellite 712-1%YLoad

4.0N

1000

800

600

400

200

Ele

ctri

cal C

onta

ctR

esis

tanc

e [m

Ω]

605040302010Time [s]Time [s]

-10

-8

-6

-4

-2

0

Loa

d [N

]Stellite 712 -0.5%Y

4.3N

Load

(a)

(b)

(c)

1000

800

600

400

200

Ele

ctri

cal C

onta

ctR

esis

tanc

e [m

Ω]

605040302010Time [s]

-10

-8

-6

-4

-2

0

Loa

d [N

]

Stellite 712 -5%Y

Load

5.4N

(d)

Fig. 7. Micro-scratch test with in situ monitoring changes in electric contact resistance: (a) Yttrium-free; (b) 0.5%Y; (c) 1%Y; (d) 5%Y.


samples showed higher values of critical load than theY-free specimen. The micro-scratch results showed that theoxide scales developed on Y-containing specimens weremore adherent to the substrate, compared to that on theY-free specimen. The improved adherence of the oxide scaleto substrate by yttrium was also demonstrated previously

and possible mechanisms were proposed [16–18], such aspegging of oxide into the substrate [19].AFM three-dimensional topographic pictures (10 10mm)

and roughness profiles of the oxide surface layer devel-oped on Y-free, 0.5%Y, and 5%Y-containing specimensafter oxidation at 600 1C for 25 h are presented in Fig. 8.

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Fig. 8. AFM three-dimensional topographic pictures (10 10 mm) and roughness profiles of the oxide scales developed at 600 1C for 25 h on (a) Yttrium-

free; (b) 0.5%Y; (c) 5%Y. The full vertical scale of the AFM images is 400 nm.

Table 5

Surface roughness of oxide scales developed on yttrium-free and yttrium-containing Stellite 712 samples after oxidation at 600 1C for 25 h

Material Mean roughness (nm), Ra 10 Pt. Mean roughness (nm), Rz

Yttrium-free Stellite 712 40.7 137.5

Stellite 712–0.5%Y 42.7 147.5

Stellite 712–1%Y 49.2 167.5

Stellite 712–2%Y 49 167.1

Stellite 712–5%Y 59 184


The roughness profiles presented in Fig. 8 are typicalresults selected from those determined along two horizon-tal directions. Table 5 presents the corresponding Ra andRz values. The Y-free Stellite 712 specimen showed thelowest roughness and the most homogeneous outer surfaceof the oxide layer. When focused on a smaller scanned area(top view) of 2 2 mm, one may also see that the oxidedeveloped on Y-free specimen was smooth (Fig. 9(a))compared to those on Y-containing specimens (Figs. 9(b)and (c)). Yttrium is an oxygen-active element, whichmay affect the oxide growth in addition to its properties.Therefore, the growth of oxide on a specific area should

be influenced by the local yttrium concentration. Sinceyttrium concentration was not very homogeneous as, Fig. 4illustrates, a relatively rough oxide scale could thusbe generated, compared to that developed on a Y-freespecimen.Fig. 10 presents grazing incidence X-ray diffraction

patterns of oxide scales developed at 600 1C on Y-free,0.5%, and 5%Y-containing specimens, respectively. Theoxide scale developed on Y-free Stellite 712 consisted of thefollowing phases: (Cr,Fe)2O3, Cr2O3, Mo17O47, and CoM-n2O4. The oxide scale grown on the samples containing 0.5and 1% yttrium did not have the cobalt–manganese oxide

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Fig. 9. AFM images of oxide scales developed at 600 1C for 25 h on different samples: (a) Yttrium-free; (b) 0.5%Y; (c) 5%Y. The full vertical scale of the

AFM images is 250 nm.


but contained a new phase: Y2O3. The presence of theyttria phase could be a reason why the oxide scaledeveloped on 0.5 and 1% yttrium containing samplesshowed higher hardness. Previous studies on another Co-base alloy, Stellite 6, demonstrated that Y2O3 strengthenedthe oxide scale on the alloy, leading to a stronger oxidescale [13]. Besides, the absence of the cobalt–manganeseoxide could also be beneficial to the oxide scale. It wasreported that manganese was richer in the outer region of

oxide scales on Cr-containing alloys [16–19]. It appearsthat the diffusion of manganese ions in chromia scales ismuch faster than chromium ions [20]. Consequently, if thediffusion of Mn is reduced possibly by the existence oflarge yttrium atoms, the Cr2O3 phase may thus take morevolume fraction in the oxide scale, resulting in a strongeroxide scale.However, Y2O3 phase as well as Mo17O47 were not

detected in the oxide scales developed on 2 and 5%

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CPS

8070605040302010

2θ Angle

(110

)a, c

,s

(113

) a,

c,s

ss

s

s

s

(222

) y

(400

) y

(104

) a,

c

(024

)c

(116

) a,

c

(012

) c,

(440

) b

(001

) b

Stellite 21 -0.5%YXRD Oxide Film

(012

)a

(440

) y

CPS

8070605040302010

2θ Angle

(110

)a, c

, d,s

(113

)a, c

,ss

ss

s

s

(104

)a, c

(116

) a,

c

(012

) c,

(440

) b

(001

) b

d

Stellite 21 -Y freeXRD Oxide Film

(012

)a

CPS

8070605040302010

Stellite 712 -5%YXRD Oxide Film

s

s(104

) a,

c

(110

) a,

c,d

, s

(012

)a

ss

s

(113

) a,

c,s

(024

) a,

c

(116

)a, c

sd

2θ Angle

Fig. 10. Low angle X-ray diffraction patterns of the oxide scales developed on different samples: (a) yttrium free; (b) 0.5%Y; (c) 5% yttrium containing

samples, respectively, a—(Cr,Fe)2O3; b—Mo17O47; c—Cr2O3; d—CoMn2O4; y—Y2O3; s—substrate (CPS stands for counts per second).


Y-containing specimens. The absence of Mo17O47 may bedue to possible effect of yttrium on diffusion of Mo intothe oxide scale. It is unclear why Y2O3 phase was notdetected in the oxide scales on specimens with 2 and 5%yttrium. It might be related to the enhanced interactionbetween Y and Mo, which might lead to the following twoeffects: (1) the enhanced interaction could also affect themigration of yttrium itself towards the oxide scale whenMo was blocked by yttrium in the surface region, and (2)the formed Y2O3 particles in the oxide scale might be toosmall to be detected using XRD. Further studies arenecessary in order to clarify this issue. The absence of theyttrium and molybdenum oxide phases in the oxide scalesdeveloped on specimens containing 2 and 5% yttriumcould explain their lower hardness (Table 4).

3.4. Wear behavior

Sliding wear tests of Y-free and Y-containing Stellite 712samples were performed at room temperature and at 600 1C,respectively. The wear tests showed that the alloyed yttriumdid not affect the wear behavior at room temperature,mainly because the microstructure and mechanical proper-ties of the alloy were not modified by the yttrium addition.However, the situation changed at elevated temperatures.Fig. 11 illustrates wear volume losses of Y-free and Y-containing specimens at 600 1C. As shown, the presence ofsmall amounts of yttrium, particularly at 0.5% level,considerably enhanced the high-temperature wear perfor-mance of Stellite 712. The improvement in wear perfor-mance could be mainly attributed to improved oxidationbehavior that enhanced oxide scales on Stellite 712. Theoxide scales on 0.5% and 1%Y-containing specimens weremore protective than that developed on Y-free specimen asthe nano-indentation and micro-scratch tests demonstrated.This is consistent with previous studies on Stellite 6 and 21[12,13]. Recent studies on Stellite 21 by performing weartests at elevated temperatures in air and Ar atmospheredemonstrated that the enhanced oxide scale was the mainfactor responsible for improved high-temperature wearresistance of the alloy [20].

However, when a larger amount (e.g. 2 or 5%) ofyttrium was added to the alloy, the wear performance was

negatively influenced. In this case, the hardness of the oxidescale was reduced as the nano-indentation test demon-strated, although its adherence to substrate was improved.As a result of the decrease in hardness, the oxide scale maynot effectively withstand the wearing force, leaving largerbare metal surface area under oxidation attack. Further-more, the oxide scale developed on specimens containingmore yttrium could be less homogeneous and rougher thanoxide scales developed on Y-free specimen and thosecontaining a small amount of yttrium. A rough and lesshomogeneous oxide scale with possibly higher residualstress due to lower homogeneity would have experiencedlarger stress concentration when in contact with a counter-face. As a result, the probability of failure would be higherwhen the surface is subjected to wear attack.It should be indicated that the improved high-tempera-

ture wear resistance by yttrium is unlikely due to improvedhigh-temperature strength of bulk material. As demon-strated, yttrium did not affect mechanical properties of thebulk material at room temperature even with the formationof a new second phase (Co2YSi2) when the yttrium contentwas high enough. If the new second phase can help to holdthe material strength to elevated temperatures, one mayexpect that the sample containing the highest yttriumcontent (5%) could perform the best. However, suchphenomenon was not observed. Therefore, the enhancedoxide scale should play a main role in improving the high-Twear performance of Y-containing Stellite 712.Fig. 12 illustrates optical micrographs of wear tracks on

the Y-free and 0.5% Y-containing samples after slidingwear tests at 600 1C. After sliding at elevated temperatures,no glaze layer was observed on the worn surfaces. As aresult, the improvement in the wear behavior of Stellite 712with a small amount of yttrium could mainly benefit fromthe positive effects of yttrium on the mechanical behaviorof the oxide scale and its adherence to substrate.

4. Conclusions

1. Stellite 712 consisted of Cr23C6, Co3Mo, Co7Mo6,NiCrCoMo, carbon, Fe2Mo3, e, and g cobalt phases.

2. Mechanical properties and microstructure of Stellite 712were not significantly affected by the yttrium addition.

ARTICLE IN PRESS

Stellite712 - Wear Loss at 600°C; Load: 2N(Pin-on-disc)

154.8

84.5

122.3

162.7182

0

50

100

150

200

Y free 0.5%Y 1%Y 2%Y 5%YAmount of Yttrium Added to the Alloy

Wea

r L

oss

[µm

3 /mm

]x10

4

Fig. 11. Wear losses of different samples at 600 1C.


3. Alloying small amounts of yttrium (less than 1%)markedly enhanced the mechanical properties of theoxide scale and its adherence to the substrate, whichbenefited the high-temperature wear performance of

Stellite 712. However, when more than 1% yttrium wasadded, the hardness of oxide scale was lowered whichnegatively affected the resistance to high-temperaturewear.

4. The 0.5%Y—containing Stellite 712 performed the bestat elevated temperatures. Such improvement largelybenefited from the superior mechanical properties of itsoxide scale containing Y2O3 phase.

References

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[14] Spedding FH, Daane AH, editors. The rare earths. New York: Wiley;

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ARTICLE IN PRESS

Fig. 12. Micrographs of wear tracks (wear at 600 1C): (a) yttrium-free;

(b) 0.5%Y; (c) yttrium-free—a closer view. No glaze layer was observed

on both samples.


the wear performance of yttrium-modified stellite 712 at elevated temperatures

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