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Metallurgical and MaterialsTransactions A ISSN 1073-5623Volume 43Number 6 Metall and Mat Trans A (2012)43:2125-2132DOI 10.1007/s11661-011-1009-0

Precipitates in Biomedical Co-Cr-Mo-C-N-Si-Mn Alloys

Alfirano, Shingo Mineta, ShigenobuNamba, Takashi Yoneda, Kyosuke Ueda& Takayuki Narushima

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Precipitates in Biomedical Co-Cr-Mo-C-N-Si-Mn Alloys

ALFIRANO, SHINGO MINETA, SHIGENOBU NAMBA, TAKASHI YONEDA,KYOSUKE UEDA, and TAKAYUKI NARUSHIMA

The microstructures of biomedical ASTM F 75/F 799 Co-28Cr-6Mo-0.25C-0.175N-(0 to 1)Si-(0to 1)Mo alloys (mass pct) were investigated before and after heat treatment, with specialattention paid to the eect of nitrogen on the phases and the dissolution of precipitates. Theheat treatment temperatures and holding periods employed ranged from 1448 to 1548 K (1175to 1275 C) and 0 to 43.2 ks, respectively. A blocky-dense -phase precipitate and a lamellarcellular colony, which consisted of an M2X type precipitate and a phase, were mainly detectedin the as-cast alloys with and without added Si, respectively. The addition of nitrogen causedcellular precipitation, while the addition of Si suppressed it and enhanced the formation of the phase. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM)analyses suggested that a discontinuous reaction, i.e., 1 2 + M2X, might be a possibleformation mechanism for the lamellar cellular colony. Nitrogen was enriched in the M2X type,-phase, and -phase precipitates, but was excluded from the M23X6 type precipitate. Completeprecipitate dissolution was observed in all of the alloys under varied heat treatment conditionsdepending on the alloy composition. The addition of nitrogen decreased the time required forcomplete precipitate dissolution at low heat-treatment temperatures. At high temperatures, i.e.,1548 K (1275 C), complete precipitate dissolution was delayed by the partial melting thataccompanied the formation of the precipitates such as the phase resulting in the boundarybetween the complete and incomplete precipitate dissolution regions in having a C-curvedshape.

DOI: 10.1007/s11661-011-1009-0 The Minerals, Metals & Materials Society and ASM International 2012

I. INTRODUCTION

THE Co-Cr-Mo alloys were registered in ASTM F 75and F 799 standards as surgical implant materials forcastings and forgings, respectively.[1,2] Precipitates inbiomedical Co-Cr-Mo alloys are known to aect thewear and corrosion-resistant properties of castings[35]

as well as the workability of forgings.[6] As a result,precipitates in biomedical Co-Cr-Mo alloys were studiedby several research groups including ours.[716] Wereported on the phases and dissolution behavior ofprecipitates in Co-Cr-Mo-C[7] and Co-Cr-Mo-C-Si-Mn[8,9] system alloys under as-cast condition and afterheat treatment at 1448 to 1548 K (1175 to 1275 C). Theprecipitate evaluation in this temperature range provesuseful for selecting the hot working conditions andcontrolling the as-cast microstructure of biomedicalCo-Cr-Mo alloys. In our previous studies, two types ofprecipitates, i.e., a phase (M2T3X-type carbide with a-Mn structure, where M and T are metallic elementsand X is carbon)[7] and a phase (intermetallic

compound with an -Mn structure),[9] were detectedfor the rst time in biomedical Co-Cr-Mo alloys.It was reported that nitrogen in Co-Cr-Mo alloys

stabilizes a face-centered-cubic (fcc) Co-based metallicphase ( phase) and improves the hot-workability[4] andmechanical properties[17] of the alloys. In biomedicalCo-Cr-Mo alloys containing nitrogen, the formation ofM23C6 type,

[17,18] -phase (M6C),[17] and Cr2X

[19,20]

precipitates was reported. However, the eect of nitro-gen on the phases and dissolution of precipitates inASTM F 75/F 799 Co-Cr-Mo alloys has not yet beenclaried in detail.In this study, the eect of nitrogen, Si, and Mn on the

phases and dissolution of the precipitates in ASTM F75/F 799 Co-Cr-Mo alloys was investigated based onthe observation of the microstructures of as-cast andheat-treated Co-Cr-Mo-C-N-Si-Mn alloys with a focuson the precipitates.

II. MATERIALS AND METHODS

A. Specimens

Table I gives the chemical composition of the alloysused in this study. The nitrogen content was controlledto the level of 0.175 0.025 mass pct. The carboncontent was approximately 0.25 mass pct, and thecontents of Si and Mn were 0 or 1 mass pct. Thechemical composition of the alloys is denoted by masspercent, although the notation mass pct is omitted.

ALFIRANO and SHINGO MINETA, Graduate Students,KYOSUKE UEDA, Assistant Professor, and TAKAYUKINARUSHIMA, Professor, are with the Department of MaterialsProcessing, Tohoku University, Sendai 980-8579, Japan. Contacte-mail: [email protected] SHIGENOBU NAMBA, SeniorResearchMetallurgist, iswith theMaterialsResearchLaboratory,KobeSteel, Ltd., Kobe 651-2271, Japan. TAKASHI YONEDA, President, iswith Yoneda Advanced Casting Co., Ltd., Takaoka 933-0951, Japan.

Manuscript submitted June 30, 2011.Article published online January 24, 2012

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, JUNE 20122125

Author's personal copy

The alloy is referred to by the notation in the left columnof Table I.Alloy ingots (diameter: 34 mm, height: 100 mm) were

prepared using an induction-melting furnace under anitrogen-containing atmosphere and cast in a coppermold. These ingots were cut into disks with a diameterof 34 mm and a thickness of 5 mm, which were then cutinto four equal parts.

B. Heat Treatment

Heat treatments were performed in a horizontalelectric resistance tube furnace. The temperatures ofthe heat treatments were 1448, 1473, 1498, 1523, and1548 K (1175, 1200, 1225, 1250, and 1275 C) forholding periods of 0, 1.8, 7.2, 21.6, and 43.2 ks. Tosuppress the reduction of carbon and nitrogen from thespecimen during heat treatment, the specimen wasplaced in a sealed silica ampoule. The atmosphere insidethe ampoule was either high vacuum or argon. The silicaampoule with the specimen was then inserted into thehot zone of the tube furnace maintained at the speciedtemperature to initiate the heat treatment. The specimentemperature reached the specied value 0.6 ks after itwas placed in the tube furnace. After the completion ofheat treatment, the specimen was water quenched withbreaking silica ampoule. A holding time of 0 ks meansthat the specimens were water quenched immediatelyafter the temperature of the specimen reached thespecied value.

C. Analysis of Specimens

The microstructures of the as-cast and heat-treatedalloys were observed using an optical microscope(Olympus, Tokyo, BX60M) and a scanning electronmicroscope (PHILIPS,* XL30FEG) after wet polishing

with emery paper (maximum grit size: 1500), bupolishing with 0.1-m diamond paste, and electrolyticetching in a 10 pct H2SO4-methanol solution at 6 V. Aeld emissionelectron-probe microanalyzer (FE-EPMA, JEOL, Tokyo, JXA-8350F) and a transmissionelectron microscope (JEOL, JEM-2100) were used forcompositional and structural analyses of the precipi-tates, respectively. The precipitates in the as-cast andheat-treated alloys were electrolytically extracted atroom temperature in a 10 pct H2SO4 aqueous solutionat 2 V. The phases of the extracted precipitates were

identied using X-ray diraction (XRD, Bruker AXS,Karlsruhe, Germany, D8Advance) with Cu K radia-tion, and the morphologies of the extracted precipitateswere observed using scanning electron microscopy(SEM).

III. RESULTS

A. As-Cast Alloys

Figures 1(a) through (d) show the microstructures ofthe as-cast alloys. The SEM image in the upper rightcorner is a higher magnication of the precipitates shownin each gure. In the alloys containing Si, i.e.,1Si0Mn0.175N and 1Si1Mn0.175N, blocky-dense pre-cipitates were observed in the interdendritic region. Incontrast, in the alloys without added Si, i.e.,0Si1Mn0.175N and 0Si0Mn0.175N, lamellar cellularprecipitation was observed and was primarily locatedon the grain boundaries. The area percents of theprecipitates in the as-cast alloys are shown in Figure 2.The error bars in this gure show the standard deviationof measurements using four optical micrographs with amagnication of 200. All of the areas of the cellularcolonies were counted in the calculation of area percents.The XRD patterns of the precipitates that were

electrolytically extracted from the as-cast alloys areshown in Figure 3, along with JCPDS card nos. 026-0428 ((Cr,Mo)12(Fe,Ni)8xN4z, phase) and 035-0803(Cr2N). The phases of the precipitates in the as-castalloys are summarized in Table II; the Cr2N type phaseis referred to as M2X because Cr and nitrogen werepartially substituted with other elements. phase andM2X type were the main precipitates in the as-cast alloyswith and without added Si, respectively. The appear-ances of the -phase and M2X type precipitates wereblocky-dense and platelike, respectively, as shown inFigure 4. Currently, the reection observed at around45.8 deg in the XRD patterns of 1Si1Mn0.175N,1Si0Mn0.175N, and 0Si1Mn0.175N alloys could notbe assigned. Transmission electron microscopy (TEM)analysis showed that the cellular colony in the as-cast0Si0Mn0.175N alloy consisted of platelike M2X typeprecipitates and the phase (Figure 5). These resultsindicate that the white precipitates in the cellular colonyshown in the SEM images in Figures 1(b) and (d) areM2X type.

B. Heat-Treated Alloys

Figure 6 shows the microstructure evolution in the1Si0Mn0.175N alloy during heat treatment. Complete

Table I. Chemical Composition of the Co-Cr-Mo-C-N-Si-Mn Alloys Used in This Study (Mass Percent)

Abbreviation Cr Mo C N Si Mn P S Co

1Si0Mn0.175N 27.90 6.17 0.25 0.15 1.21 0.001 0.002

precipitate dissolution was achieved at 1498 K (1225 C)with a holding period of 43.2 ks. The phase of theprecipitates in the incomplete dissolution region and thecomplete precipitate dissolution region are summarizedin Figure 7. In this gure, the phases are listed from leftto right in order of decreasing content, which wasevaluated using XRD. At high temperatures such as1548 K (1275 C), the phase was the major precipitate.Meanwhile, at temperatures below 1523 K (1250 C),

Fig. 1Microstructure of the as-cast (a) 1Si0Mn0.175N, (b) 0Si1Mn0.175N, (c) 1Si1Mn0.175N, and (d) 0Si0Mn0.175N alloys.

1Si0Mn

0.175N

0Si1Mn

0.175N

1Si1Mn

0.175N

0Si0Mn

0.175N

Prec

ipita

te c

onte

nt, C

p (%

)

15

12

9

6

3

0

Fig. 2Contents of precipitate in the as-cast alloys.

Inte

nsity

, I (a

.u.)

35 40 5045 55

M2X

0Si0Mn0.175N

1Si1Mn0.175N

0Si1Mn0.175N

1Si0Mn0.175N

Cr2NJCPDS card no. 035-0803

(Cr,Mo)12(Fe,Ni)8-xN4-z, JCPDS card no. 026-0428

Fig. 3XRD patterns of the precipitates electrolytically extractedfrom the as-cast alloys.



-phase and -phase precipitates were primarily observedin the alloys with and without added Si, respectively.Complete precipitate dissolution was achieved in all

of the alloys and the temperature conditions for thecomplete precipitate dissolution depended on the alloycomposition. In the cases of the 1Si0Mn0.175N,0Si1Mn0.175N, and 1Si1Mn0.175N alloys, the holdingtime required for complete precipitatedissolutionat 1548K(1275 C) was longer than that at 1523 K (1250 C), andthe boundary between the complete and incomplete pre-cipitate dissolution regions can be described as C curvedwith a nose at ~1523 K (1250 C).The chemical compositions of the precipitates formed

during heat treatment of the alloys were determined usingFE-EPMA and are listed in Table III. Nitrogen wascontained in the -phase, -phase, and M2X type precip-itates. The nitrogen content of theM23X6 type precipitatewas approximately 25 pct of that in the metallic matrix( phase), which was evaluated from the signal intensitiesof nitrogen in FE-EPMA measurements. This resultsuggests that the nitrogen contents of the M23X6 typeprecipitate were less than that of the metallic matrix( phase) and were too low to determine quantitatively.

IV. DISCUSSION

A. M2X Type Precipitate

Since the M2X type precipitate was not formed in theCo-Cr-Mo-C[7] and Co-Cr-Mo-C-Si-Mn[8,9] alloys,nitrogen appears to be the cause of the formation ofthe lamellar cellular colony with M2X type precipitates.

The formation of M2X type precipitate in ASTM F75/F 799 Co-Cr-Mo alloys containing nitrogen wasreported by Kilner et al.[19] and Kurosu et al.[20]

However, their formation mechanisms of M2X wereinternal nitridation in nitrogen-containing atmospheresat 1473 K (1200 C) with the form of large particles[26]

and the eutectoid transformation of + Cr2N in theaging at 1073 K (800 C).[27] They did not agree with theformation mechanism of M2X type precipitate in thisstudy, where the lamellar cellular colony of +M2Xwasobtained.A lamellar cellular colony with M2X type precipitates

was reported in aged[2125] and as-cast[26] stainless steels

Table II. Phases of Precipitates Formed in the As-CastAlloys

Alloy -Phase M2X Type

1Si0Mn0.175N N.D.0Si1Mn0.175N 1Si1Mn0.175N N.D.0Si0Mn0.175N

M: metallic element, X: carbon and nitrogen, N.D.: not detected,: major precipitate.

Fig. 4SEM images of the (a) phase formed in the as-cast 1Si0Mn0.175N alloy and (b) M2X type precipitate formed in the as-cast0Si0Mn0.175N alloy after electrolytic extraction.

Fig. 5TEM analysis of the cellular colony formed in as-cast0Si0Mn0.175N alloy. (a) Bright-eld image and associated dirac-tion patterns of the (b) M2X type precipitate with the beam directionalong 212

and (c) phase with the beam direction along 112

.

2128VOLUME 43A, JUNE 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A


Fig. 6Microstructure of the 1Si0Mn0.175N alloy after heat treatment at (a) through (c) 1548 K (1275 C), (d) through (f) 1498 K (1225 C),and (g) through (i) 1448 K (1175 C) for (a), (d), and (g) 0 ks; (b), (e), and (h) 7.2 ks; and (c), (f), and (i) 43.2 ks.

Fig. 7Phases of the precipitates formed in the as-cast and heat-treated (a) 1Si0Mn0.175N, (b) 0Si1Mn0.175N, (c) 1Si1Mn0.175N, and(d) 0Si0Mn0.175N alloys and the complete precipitate dissolution regions.



with high nitrogen and low carbon contents. In Co-Cr-Mo alloys, Rothenthal et al.[12] reported the formation ofcoarse lamellar cellular colonies in an as-cast Co-28Cr-6Mo-0.25C alloy; the cellular colonies comprised anM23C6 type precipitate and a phase. The appearance ofthose cellular colonies was very similar to that observedin this study. They stated that discontinuous[27] andeutectoid[11] reactions are possible mechanisms for theformation of the cellular colonies; a discontinuousreaction mechanism was specically proposed for theformation of the coarse cellular colonies. In this study,since the cellular colonies consisted of M2X type precip-itates and the phase, the 1 2 + M2X discontinuousreaction is a potential mechanism for the formation ofcellular precipitation during cooling in casting. The 1and 2 phases describe the fcc Co-based metallic phasewith dierent chemical compositions. Precipitation ofthe cellular colonies on the grain boundary supports thediscontinuous reaction mechanism.In this study, lamellar cellular precipitation of M2X

type precipitates was not observed in the alloys withadded Si. This result suggests that Si suppressed theformation of the M2X type precipitate. Si is known toincrease the thermodynamic activity of carbon insteels.[28] The increase in carbon activity due to theaddition of Si causes the formation of carbon-richcarbonitrides such as the phase (Section IVB).Escobede et al.[17] and Yamashita et al.[18] reported theformation of carbide precipitates in as-cast biomedicalCo-Cr-Mo alloys with added nitrogen. The alloy thatEscobede et al.[17] used contained carbon in 0.41 to0.45 mass pct and nitrogen in 0.035 to 0.15 mass pct.The carbon contents higher than that of ASTMF 75maystabilize carbides and cause the formation of -phase andM23C6 type precipitates. Yamashita et al.

[18] obtainedM23C6 type precipitates in a Co-28Cr-6Mo-0.23C-0.14Nalloy. This could be due to the fact that its nitrogencontent was slightly less than that in the 0Si0Mn0.175Nalloy, which could lead to the formation of M23C6 typeprecipitates. These results including ours suggest that theformation of M2X type precipitates is suppressed by theaddition of Si or carbon that stabilizes -phase andM23X6(or -phase) type precipitates, respectively, though nitro-gen is an essential element for the formation of M2X typeprecipitates.

B. Compositions of Precipitates

The relationship between the nitrogen content in theprecipitates and alloys is illustrated in Figure 8. Thedotted line in the gure represents the conditions under

which the nitrogen contents in the precipitates andalloys are equal. Nitrogen was enriched in the M2X type,-phase, and -phase precipitates, but was excludedfrom the M23X6 type precipitate. These results agreewell with the microstructural observations; the region inwhich the M2X type, -phase, and -phase precipitateswere detected in the incomplete precipitation regions(Figure 7) was signicant, as compared to the alloyswithout added nitrogen.[8] The promotion of the for-mation of the phase by the addition of nitrogen wasreported by Escobedo et al.[17]

As shown in Table III, carbon was also detected in theM2X type precipitate, which suggests that the M2X typeprecipitate is a carbonitride. The molar ratio of nitrogento carbon was calculated to be 1.56 from the data inTable III. The -phase and -phase precipitates werealso recognized as carbonitrides with molar ratios ofnitrogen to carbon of 0.37 and 0.22, respectively. Thenitrogen-to-carbon molar ratio in the phase was nearlyconstant at 1548 K (1275 C) for up to 21.6 ks. Theseresults show that nitrogen is highly distributed in theM2X type precipitate.

C. Dissolution Behavior of Precipitates

The C-curved shape of the boundary between thecomplete and incomplete precipitate dissolution regionswas reported to be caused by partial melting of theinterdendritic parts in the alloys that accompanied-phase formation.[9,29] This mechanism can be applied

Table III. Chemical Compositions of the M23X6 Type, -Phase, -Phase, and M2X Type Precipitates (Mass Percent)

Phase Co Cr Mo C N Si Mn

M23X6 type 16.1 67.4 10.8 5.6 N.D. N.D. 0.1 phase 34.4 38.1 21.7 2.7 0.7 1.3 1.1 phase 32.5 21.6 38.6 2.3 1.0 4.0 N.D.M2X type 4.1 76.6 9.7 3.4 6.2 N.D. N.D.

N.D.: not detected.

Fig. 8Nitrogen content of the precipitates as a function of that inthe alloys.



to 1Si0Mn0.175N, 0Si1Mn0.175N, and 1Si1Mn0.175Nalloys, in which the formation of the precipitates such asthe phase was observed under the as-cast conditionand after heat treatment at 1523 and 1548 K (1250 and1275 C) for short holding periods.A comparison between the complete dissolution

conditions of alloys with and without nitrogen is shownin Figure 9. The enhancement of precipitate formationat higher temperatures such as 1548 K (1275 C) afterthe addition of nitrogen could be the reason for theincrease in the holding time required for completeprecipitate dissolution. In contrast, at lower tempera-tures, the addition of nitrogen decreased the holding

time required for complete precipitate dissolution.Figures 10(a) and (b) show the shape of the precipitatesin the 1Si0Mn0.175N and 0Si0Mn0.175N alloys, respec-tively, after heat treatment at 1473 K (1200 C) for 0 ks.A breakup of the precipitates was observed, which mightincrease the surface area of the precipitates. In addition,the diusion coecients of nitrogen and carbon werereported as 3.3 1010 m2/s[30] and 1.1 1010 m2/s,[31]

respectively, in Co at 1473 K (1200 C). The precipitatedispersion and the higher diusion coecient of nitrogenas compared to that of carbon appear to cause theincreased apparent precipitate dissolution rates anddecreased required holding time for complete precipitate

Fig. 9Heat-treatment conditions for complete precipitate dissolution in the Co-Cr-Mo-C-N-Si-Mn alloys.

Fig. 10SEM images of the precipitates in the (a) 1Si0Mn0.175N and (b) 0Si0Mn0.175N alloys after heat treatment at 1473 K (1200 C) for0 ks.



dissolution in the alloy with added nitrogen at lowtemperatures.In the 0Si1Mn0.175N alloy, the area of complete

precipitate dissolution was wider than that of the alloyswith added Si. Because the addition of Mn was reportedto decrease the carbon activity in Fe-based alloys,[28]

which is opposite to the eect observed upon adding Si,the Mn supplementation could increase the apparentdissolution rate of precipitates, resulting in a shortenedholding time for complete precipitate dissolution.[8]

V. CONCLUSIONS

The microstructures of biomedical ASTM F 75/F 799Co-Cr-Mo-C-N-Si-Mn alloys were investigated beforeand after heat treatment in the temperature range of1448 to 1548 K (1175 to 1275 C) with a focus on theeect of nitrogen on the phases and dissolution of theprecipitates. The following results were obtained.

1. A blocky-dense -phase precipitate and lamellar cellu-lar colony, which consisted of an M2X type precipitateand a phase, were mainly detected in the as-cast al-loys with and without added Si, respectively. Nitrogencaused the formation of the M2X type precipitate, butSi appeared to suppress cellular precipitation, resultingin the formation of the -phase precipitate.

2. The phases of the precipitates observed in the heat-treated alloys were M23X6 type, M2X type, phase, and phase. The addition of nitrogen strongly enhanced theformation of the M2X type and the -phase precipitates athigh and low heat treatment temperatures, respectively.

3. Nitrogen was enriched in the M2X type, -phase,and -phase precipitates, but excluded from the M23X6type precipitate.

4. Complete precipitate dissolution was observed in all ofthe alloys after heat treatment for up to 43.2 ks, andthe temperatures at which complete precipitate dissolu-tion took place depended on alloy composition. Nitro-gen appears to decrease the holding time required forcomplete precipitate dissolution at low heat treatmenttemperatures. It also delays complete precipitate disso-lution at 1548 K (1275 C) due to the enhancement ofthe precipitate formation such as the phase.

ACKNOWLEDGMENTS

The authors thank Yoneda Advanced Casting Co.,Ltd. for supplying the Co-Cr-Mo alloy ingots used inthis study, Mr. K. Suda for his FE-EPMA measure-ments, and Dr. K. Kobayashi for assistance with TEM.

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Precipitates in Biomedical Co-Cr-Mo-C-N-Si-Mn AlloysAbstractIntroductionMaterials and MethodsA. SpecimensB. Heat TreatmentC. Analysis of Specimens

ResultsA. As-Cast AlloysB. Heat-Treated Alloys

DiscussionA. M2X Type PrecipitateB. Compositions of PrecipitatesC. Dissolution Behavior of Precipitates

ConclusionsACKNOWLEDGMENTSREFERENCES

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No137Alfirano3MMTA=