Microstructures of Zr-Added Co-Cr-Mo Alloy Compacts

Fabricated with a Metal Injection Molding Process

and Their Metal Release in 1mass% Lactic Acid

Madoka Murakami1, Naoyuki Nomura1;*, Hisashi Doi1, Yusuke Tsutsumi1,Hidefumi Nakamura2, Akihiko Chiba3 and Takao Hanawa1;4

1Institute of Biomaterials and Bioengineering, Tokyo Medical and Dental University, Tokyo 101-0062, Japan2EPSON ATMIX Co., Hachinohe 039-1161, Japan3Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan4Graduate School of Engineering, The University of Tokyo, Tokyo 113-8650, Japan

The microstructures of Zr-added Co-29Cr-6Mo alloy compacts fabricated with a metal injection molding (MIM) process and their metalrelease from the compacts immersed in 1% lactic acid were investigated for medical applications. The relationship between the microstructureand amount of Co released from the compacts is discussed phenomenologically. The relative density of the Co-29Cr-6Mo compacts increasedwhen Zr was added to the powder with amounts of 0.03 and 0.1mass% and sintered in Ar or N2. The amounts of Co released from the compactscontaining 0.03 and 0.1mass% Zr sintered in Ar or N2 were smaller than those from the other compacts. Therefore, the addition of Zr to the Co-29Cr-6Mo powders enhanced the sintering of the compacts and decreased the porosity in the resultant products, leading to the suppression of theCo release from the compacts. When the Zr-added Co-29Cr-6Mo alloy powders were sintered in N2, the relative density of the compacts wassmaller than that of those sintered in Ar. The powders were nitrided during sintering in N2, and the nitrides disturbed the densification duringsintering. In addition, a lamellar structure was formed in the Co-29Cr-6Mo and Co-29Cr-6Mo-0.5Zr compacts. The amount of Co released fromthese compacts was larger than that released from the other compacts because local corrosion occurred at the interface between the differentphases in these compacts during immersion in 1% lactic acid. In the MIM process, a small addition of Zr (less than 0.1mass%) to the Co-29Cr-6Mo alloy is effective for densification during sintering and suppression of the Co release from the compacts.[doi:10.2320/matertrans.M2010040]

(Received February 1, 2010; Accepted April 15, 2010; Published June 2, 2010)

Keywords: cobalt-chromium-molybdenum alloy, metal injection molding (MIM), relative density, microstructure, metal release

1. Introduction

Cobalt-chromium-molybdenum (Co-Cr-Mo) alloys havebeen widely used as orthopedic and dental materials, such asartificial hip- and knee-joints and removable denture bases,because of their high strength, excellent wear, and corrosionresistance. These alloys contain a large amount of nickel(Ni), which accounts for their workability. However, Ni isknown as a high-risk element for metal allergy. When Ni isremoved from the alloys, forging at high temperatures isnecessary, and the shaping becomes hard because of the lowductility and high hardness of the Ni-free alloy. Therefore,investment casting is still an important manufacturingtechnique for such alloys. Thus, Ni-free Co-Cr-Mo alloysare required, but the processing of such alloys should beimproved at the same time.

Recently, the metal injection molding (MIM) process hasattracted much attention for the accurate near-net shaping ofmass products. This process contributes to the decrease ofmanufacturing costs owing to minimization of the machiningprocess and waste. It is possible to fabricate Ti-6Al-4V alloysby MIM with high tensile strength comparable to that of thewrought alloy.1) It has also been reported that the pores in theMIM products did not affect the fatigue strength of Ti-6Al-4V.2) However, pores inevitably occur in the MIM productsdepending on the sintering condition. In the case of MIMproducts for biomedical applications, the metal release from

the products should be considered because the increase of thesurface area due to the existence of pores may affect thecorrosion resistance.

Kurosu et al.3) reported on the effect of alloying elementsto a Co-Cr-Mo-Ni alloy on metal release in 1mass% lacticacid. The amounts of Ni, Co, and Mo released from the alloydecreased with the addition of zirconium (Zr). In addition,the effects of Zr addition to Co-Cr-Mo alloys on themechanical properties have been reported.4,5) The yieldstress, tensile strength, and elongation improved with theaddition of Zr up to 0.5mass%.5) Thus, the addition of Zr tothe Co-Cr-Mo alloy is considered to be a promising solutionto improve not only the corrosion resistance but also themechanical properties.

The as-cast Ni-free Co-Cr-Mo alloys showed a low yieldstrength, a high work-hardening rate, and a lack of elongationat room temperature because of the existence of largeamounts of "martensite (hcp) owing to the low stacking faultenergy of the � phase (fcc). To improve the strength andelongation of the alloys, the addition of nitrogen (N) waseffective because N stabilizes the � phase and decreases theathermal " phase in the alloy.6) For the fabrication of N-addedCo-Cr-Mo alloys, Co-Cr-Mo was melted with the Cr2Npowders.6) However, the yield ratio of N was low owing tothe vaporization of N2. Sato et al.7) controlled the N contentin Co-29Cr-6Mo compacts by changing the mixture ratioof Ar and N2 gases. Therefore, the introduction of N throughN2 gas during sintering is available for the fabrication ofCo-Cr-Mo alloy compacts using the MIM process.*Corresponding author, E-mail: nnomura.met@tmd.ac.jp

Materials Transactions, Vol. 51, No. 7 (2010) pp. 1281 to 1287#2010 The Japan Institute of Metals

The purpose of this study was to evaluate the micro-structures of Zr-added Co-Cr-Mo alloy compacts fabricatedusing the MIM and sintering process and evaluate the metalrelease from the compacts in 1mass% lactic acid. N was alsointroduced to the compacts via N2 gas during sintering. Inthis study, the effect of Zr and N addition to Co-Cr-Mo alloycompacts on the microstructure and metal release from thecompacts is discussed. The mechanical properties of thecompacts have been reported elsewhere.8)

2. Experimental Procedure

2.1 Specimen preparationThree types of Co-29Cr-6Mo alloy powders with various

Zr contents were prepared by a water atomization process.The chemical compositions of the alloys are listed inTable 1. The average particle diameters of these powderswere controlled to about 10 mm by sieving. The Co-29Cr-6Mo and Co-29Cr-6Mo-0.03Zr powders are abbreviatedas 0Zr and 0.03Zr, respectively. Co-29Cr-6Mo-containing0.1mass%Zr and 0.5mass%Zr powders were prepared bymixing Co-29Cr-6Mo-0.03Zr with Co-29Cr-6Mo-10Zr pow-ders. These mixtures are abbreviated as 0.1Zr and 0.5Zr,respectively. 0Zr, 0.03Zr, 0.1Zr, and 0.5Zr were mixed with abinder in a mass ratio of 59 : 41 and then kneaded in thepressurized kneader for 3.6 ks. The compounds were crashedinto grains of 3–4mm and injection-molded to a dimension of10mm� 10mm� 1mm. The green parts were heated in N2

at 743K for 3.6 ks for thermal debinding and then sinteredat 1553K for 10.8 ks in an Ar or N2 atmosphere.

2.2 Characterization of Zr-added Co-29Cr-6Mo alloycompacts

The microstructures of Zr-added Co-Cr-Mo alloy com-pacts were observed through an optical microscope (OM)and a field-emission scanning electron microscope (FE-SEM)equipped with an energy-dispersive X-ray spectrometer(EDS). Phase identification was performed using an X-raydiffractometer (XRD) with Cu K� under 45 kV and 40mA.The specimens for OM, SEM, and XRD were polished withwaterproof emery paper up to 600 grit, a 9 mm diamondsuspension, and 0.04 mm colloidal silica suspension and thenelectropolished with a solution of 10% H2SO4 and 90%CH3OH at 16V and 273K. The densities of the alloys weremeasured with the Archimedes method. The densities of themaster alloys for the atomization (0Zr, 0.03Zr, and 10Zr)were also measured with the Archimedes method forcalculating the relative density of the compacts and thevalues were 8.32Mg�m�3 for 0Zr, 8.32Mg�m�3 for 0.03Zr,and 8.07Mg�m�3 for 10Zr. The densities of the master alloysof 0.1Zr and 0.5Zr were estimated to 8.32 and 8.31Mg�m�3,respectively, with the density of 0.03Zr and 10Zr and their

mixture ratio. The densities of the master alloys were used asthe theoretical density for calculating the relative density ofthe compacts.

2.3 Immersion testEach compact (n ¼ 3) was immersed in 1mass% lactic

acid at 310K for 604.8 ks. The compacts were polished withwaterproof emery paper up to #1000 grid with running waterusing a rotating polisher and then ultrasonically cleaned withacetone. These compacts were placed in polypropylenebottles, and 20ml of 1mass% lactic acid was then pouredinto each bottle. The concentrations of Co, Cr, Mo, and Zrreleased into the solution were determined by inductivelycoupled plasma-mass spectroscopy (ICP-MS). An immersiontest of a Co-29Cr-Mo as-cast alloy with the same specimensize was performed for comparison (n ¼ 3).

3. Results

3.1 Relative density of Zr-added Co-29Cr-6Mo alloycompacts

Figure 1 shows the relative density of Zr-added Co-Cr-Moalloy compacts sintered in (a) Ar and (b) N2. The relativedensities of 0.03Zr and 0.1Zr were larger than those of 0Zrand 0.5Zr, irrespectively of the different sintering atmo-sphere. It was clear that a small addition of Zr increased therelative density of the compacts. The relative density of thecompacts sintered in Ar was higher than that in N2.

3.2 Microstructures and constituent phases of Zr-addedCo-29Cr-6Mo alloy compacts sintered in Ar

Figure 2 shows OM images of Zr-added Co-29Cr-6Moalloy compacts sintered in Ar: (a) 0Zr, (b) 0.03Zr, (c) 0.1Zr,and (d) 0.5Zr. Equiaxed grains with the size of 40–50 mm andspherical pores were observed in each compact. The average

Table 1 Chemical compositions of water-atomized Zr-added Co-29Cr-6Mo alloy powders (mass%).

Alloy powder Co Cr Mo Zr�1 Ni Fe Si�2 Mn�2 C O N

Co-29Cr-6Mo Bal. 28.43 5.76 N.A. 0.01 0.06 0.79 0.42 0.01 0.34 0.014

Co-29Cr-6Mo-0.03Zr Bal. 28.74 5.80 0.03 0.01 0.05 0.85 0.45 0.01 0.27 0.012

Co-29Cr-6Mo-10Zr Bal. 29.06 6.04 9.41 N.A. N.A. N.A. N.A. 0.01 0.28 0.047

�1Zr was not added in Co-29Cr-6Mo. �2Si and Mn may be contained in Co-29Cr-6Mo-10Zr.

Rel

ativ

e de

nsity

, D (

%)

(a) (b)

Ar N2

Fig. 1 Relative densities of Zr-added Co-Cr-Mo alloy compacts sintered in

(a) Ar and (b) N2.

1282 M. Murakami et al.

diameters of these alloy powders were about 10 mm. Thus,grain growth occurred, and the pore was isolated between thegrains during sintering. The volume fraction of the sphericalpore in 0Zr seemed to be larger than that in the othercompacts. Small particles were also observed at the grainboundary for each compact.

Table 2 shows the C, O, and N contents in the Zr-addedCo-29Cr-6Mo alloy compacts sintered in Ar. The oxygencontent in the compacts was as high as that in the powders(Table 1) as a result of the introduction of oxygen to the alloypowders during the water atomization process.

Figure 3 shows the XRD profiles of Zr-added Co-29Cr-6Mo alloy compacts sintered in Ar: (a) 0Zr, (b) 0.03Zr, (c)0.1Zr, and (d) 0.5Zr. Peaks from the " phase (hcp) weredetected with small peaks from the � phase in 0Zr and

0.03Zr. However, peaks from the � phase (fcc) appearedin 0.1Zr and 0.5Zr. The peak intensity from the " phasedecreased with increasing Zr content. The grain size alsoinfluences the phase stability of Co-Cr-Mo alloys.9) However,the grain size of the compacts was similar (Fig. 2). Zr acts asa �-phase stabilizer in the Co-Cr-Mo alloy system.

Figure 4 shows the SEM image and EDS maps of0Zr sintered in Ar: (a) SE image, (b) Cr map, (c) Si map,

100 µm

(d)(c)

(a) (b)

100 µm 100 µm

100 µm

Fig. 2 OM images of Zr-added Co-29Cr-6Mo alloy compacts sintered in

Ar: (a) 0Zr, (b) 0.03Zr, (c) 0.1Zr, and (d) 0.5Zr.

Table 2 C, O, and N contents of the Zr-added Co-29Cr-6Mo alloy

compacts sintered in Ar (mass%).

Compacts C O N

0Zr 0.002 0.32 0.019

0.03Zr 0.002 0.25 0.018

0.1Zr 0.015 0.26 0.053

0.5Zr 0.017 0.26 0.049

35° 55° 65° 75°45° 85°In

tens

ity (

arb.

uni

t)

(a)

(b)

(c)

(d) fcc(

111)

fcc(

200)

hcp(

002)

fcc(

220)

hcp(

100)

hcp(

101)

hcp(

102)

hcp(

103)

2θ

Fig. 3 XRD profiles of Zr-added Co-29Cr-6Mo alloy compacts sintered in

Ar: (a) 0Zr, (b) 0.03Zr, (c) 0.1Zr, and (d) 0.5Zr.

(d) Co

(c) Si(a) SE (b) Cr

(e) Mo (f) O

5 µm

5 µm

5 µm

5 µm

5 µm

5 µm

Fig. 4 SEM image and EDS maps of 0Zr sintered in Ar: (a) SE image, (b) Cr map, (c) Si map, (d) Co map, (e) Mo map, and (f) O map.

Microstructures of Zr-Added Co-Cr-Mo Alloy Compacts Fabricated with a Metal Injection Molding Process and Their Metal Release 1283

(d) Co map, (e) Mo map, and (f) O map. Si and O wereenriched at the particles in the pore (right arrow). Theseparticles correspond to SiO2. On the other hand, particleswith slight concentration differences were observed (upperand lower arrows). These particles may consist of the �phase (CoCr).

Figure 5 shows elemental mapping images of 0.5Zrsintered in Ar: (a) SE image, (b) Cr map, (c) Si map, (d)Co map, (e) Zr map, and (f) O map. Cr, Zr, Si, and O werelocalized at the particles (upper arrow). The solubility limitof Zr in the Co-29Cr-6Mo alloy was 0.12mass%,5) and thus,the excess Zr in the � phase may react with SiO2 to formcomplex oxides containing Cr, Zr, and Si. On the otherhand, particles with slight concentration differences werealso observed (lower arrow). These particles may consist ofthe � phase (CoCr).

3.3 Microstructures and constituent phases of Zr-addedCo-29Cr-6Mo alloy compacts sintered in N2

Figure 6 shows OM images of Zr-added Co-29Cr-6Moalloy compacts sintered in N2: (a) 0Zr, (b) 0.03Zr, (c) 0.1Zr,and (d) 0.5Zr. Spherical pores, observed in Fig. 2, were alsofound in each compact. A lamellar structure was observed in0Zr and 0.5Zr. On the other hand, a grain boundary with azigzag manner was observed in 0.03Zr and 0.1Zr. Smallprecipitates were also observed at the grain boundaries in0.03Zr and 0.1Zr.

Table 3 shows the C, O, and N contents in the Zr-addedCo-29Cr-6Mo alloy compacts sintered in N2. The oxygencontent in the compacts remained at the same level as that inthe alloy powders (Table 1). Nitrogen was introduced in eachcompact during sintering in N2 independently of the Zrcontent.

Figure 7 shows the XRD profiles of Zr-added Co-29Cr-6Mo alloy compacts sintered in N2: (a) 0Zr, (b) 0.03Zr, (c)0.1Zr, and (d) 0.5Zr. Peaks from the � phase and Cr2N wereobserved in each compact. Peaks from the " phase, whichwere dominantly observed in 0Zr and 0.03Zr sintered in Ar

(d) Co

(c) Si(a) SE (b) Cr

(e) Zr (f) O

5 µm 5 µm 5 µm

5 µm 5 µm 5 µm

Fig. 5 Elemental mapping images of 0.5Zr sintered in Ar: (a) SE image, (b) Cr map, (c) Si map, (d) Co map, (e) Zr map, and (f) O map.

(d)(c)

(a) (b)

100 µm

100 µm 100 µm

100 µm

Fig. 6 OM images of Zr-added Co-29Cr-6Mo alloy compacts sintered in

N2: (a) 0Zr, (b) 0.03Zr, (c) 0.1Zr, and (d) 0.5Zr.

Table 3 C, O, and N contents of the Zr-added Co-29Cr-6Mo alloy

compacts sintered in N2 (mass%).

Compacts C O N

0Zr 0.007 0.32 0.42

0.03Zr 0.008 0.24 0.44

0.1Zr 0.016 0.24 0.42

0.5Zr 0.018 0.25 0.43

1284 M. Murakami et al.

(Fig. 7(a) and (b)), were not found in the compacts sintered inN2. N also acted as the �-phase stabilizer for the Co-Cr-Moalloy system.

Figure 8 shows elemental mapping images of 0Zr sinteredin N2: (a) SE image, (b) Cr map, (c) Si map, (d) Co map, (e) Nmap, and (f) O map. Cr and N were enriched, but Co wasdiluted in the lamellar structure. From the phase constitutionof 0Zr detected by XRD (Fig. 7), the lamellar structure mayconsist of Cr2N.

Figure 9 shows elemental mapping images of 0.5Zrsintered in N2: (a) SE image, (b) Co map, (c) Cr map, and(d) Zr map. Cr and Zr were enriched in the Co-diluted area,although the Cr-enriched area was different from the Zr-enriched area. Cr2N may be formed because Cr and N wereenriched in the same area. On the other hand, Zr and Si werelocalized in the same area. Thus, the excess Zr may react withSiO2 to form complex oxides.

3.4 Metal release from Zr-added Co-29Cr-6Mo alloycompacts during immersion test

Figures 10 and 11 show the amounts of Co, Cr, Mo, and Zrreleased from the compacts sintered in Ar and N2, respec-tively. The amounts of Co released from each compact weremuch higher than those of Cr, Mo, and Zr independently ofthe sintering atmosphere. The amount of Co decreased withincreasing Zr content, showed a minimum value at 0.1Zr, andagain increased at 0.5Zr. A small addition of Zr was foundto contribute to the suppression of the Co release from thecompacts. Comparing the Ar and N2 atmospheres, theamounts of each metal released from the compacts sinteredin Ar were lower than those in N2. However, these valuesreleased from the compacts were much higher than thosefrom the as-cast Co-29Cr-6Mo alloy.

4. Discussion

4.1 Effect of Zr addition on the relative density of Co-29Cr-6Mo alloy compacts

A small addition of Zr to Co-29Cr-6Mo contributed to theincrease in the relative density of the compacts (Fig. 1).Nakamura et al.10) reported the effect of Zr addition on thesintering behavior of SUS316L stainless steel powders.The densification of SUS316L containing 0.05mass%Zrpowders started at 1173K, a lower temperature than that forSUS316L powders. These researchers explained that Zr andSi oxides were formed at the surface of powders and thisoxide reduced Fe or Cr oxides. Therefore, neck formationwas faster, and sintering proceeded at a lower temperatureowing to the decrease in disturbance by the oxide layer.10)

This explanation may be applicable to the increase inrelative density in 0.03Zr and 0.1Zr because Zr is the most

(d) Co

(c) Si(a) SE (b) Cr

(e) N (f) O

5 µm 5 µm 5 µm

5 µm 5 µm 5 µm

Fig. 8 Elemental mapping images of 0Zr sintered in N2: (a) SE image, (b) Cr map, (c) Si map, (d) Co map, (e) N map, and (f) O map.

Inte

nsity

(ar

b. u

nit)

(a)

(b)

(c)

(d)

35° 55° 65° 75°45° 85°

fcc(

111)

σ (22

0)

fcc(

200)

Cr 2

N(1

13)

fcc(

220)

Cr 2

N(1

12)

Cr 2

N(1

11)

Cr 2

N(0

02)

σ(4

11)

2θ

Fig. 7 XRD profiles of Zr-added Co-29Cr-6Mo alloy compacts sintered in

N2: (a) 0Zr, (b) 0.03Zr, (c) 0.1Zr, and (d) 0.5Zr.

Microstructures of Zr-Added Co-Cr-Mo Alloy Compacts Fabricated with a Metal Injection Molding Process and Their Metal Release 1285

reactive element in both alloys. However, when the Zrcontent was 0.5mass%, the relative density showed a lowervalue. Excess Zr may react with oxygen to form ZrO2 at thesurface of powders. When the amount of ZrO2 increases atthe surface of powders, densification is delayed because thebinding area between the metal powders decreases and thediffusion at the surface is disturbed. As shown in Fig. 2,all the compacts were in the final stage of sintering becausethe size of pores was large and the shape was spherical.In this stage, densification in the compacts proceeds bythe grain boundary and volume diffusion. Small addition ofZr does not seem to influence the diffusivity of the Co-Cr-Mo alloy. Therefore, the difference of the relative densitymay be reflected in the early stage of sintering, as statedabove. However, further investigation is still required forclarification.

4.2 Effect of the sintering atmosphere on the relativedensity and microstructures of Zr-added Co-29Cr-6Mo alloy compacts

As shown in Fig. 1, sintering in N2 disturbed thedensification of Co-29Cr-6Mo alloy compacts irrespectivelyof the Zr content. Sato11) examined the N content in Co-29Cr-6Mo alloy powers when the powders were heat-treated in aN2 atmosphere in the temperature range from 873 to 1073K.The N content of the powders suddenly increased at 873K.Nitridation of the powders started at around that temperature,although it seems difficult to diffuse Co, Cr, and Mo in thepowders owing to their high melting points (1768K, 2148K,and 2883K, respectively). Accordingly, the nitride wasformed and disturbed the densification of the compacts in N2,leading to lower relative densities than those of the compactssintered in Ar.

(d) Cr

(c) Si(a) SE (b) Zr

(e) N (f) O

5 µm 5 µm 5 µm

5 µm 5 µm 5 µm

Fig. 9 Elemental mapping images of 0.5Zr sintered in N2: (a) SE image, (b) Co map, (c) Cr map, and (d) Zr map.

Am

ount

of R

elea

sed

Met

al ,

w /

µg·c

m2

Fig. 10 Amounts of Co, Cr, Mo, and Zr released from the compacts

sintered in Ar.

Am

ount

of R

elea

sed

Met

al ,

w /

µg·c

m2

Fig. 11 Amounts of Co, Cr, Mo, and Zr released from the compacts

sintered in N2.

1286 M. Murakami et al.

The microstructures of the compacts sintered in N2 wereclearly different from those of the compacts sintered in Ar.An examination of the microstructures of 0Zr showed alamellar structure in 0Zr sintered in N2 (Fig. 6(a)). Thiscompact contained 0.4mass% of N (Table 3). Sato et al.7)

reported that Cr2N particles precipitated in the � phase whenthe N content was more than 0.2mass% in the Co-29Cr-6Moalloy compacts. However, the fraction and morphology ofCr2N in our study were different from those in their results.Taylor et al.12) reported on the aging behavior of a Co-28.3Cr-5.4Mo-0.26C alloy. They showed that a lamellarstructure consisting of carbides appeared in the � phaseduring aging in the range from 973 to 1273K depending onthe aging time. Therefore, the lamellar structure in 0Zrsintered in N2 may be formed during cooling. On the otherhand, the lamellar structure was not found in 0.03Zr and0.1Zr, although the N content of the compacts was almost thesame. It is speculated that the lamellar structure formationwas delayed by the addition of Zr. However, furtherinvestigation is needed for the formation of a complexmicrostructure in 0.5Zr sintered in N2.

4.3 Relationship between metal release from Zr-addedCo-29Cr-6Mo alloy compacts and their microstruc-tures

The surface oxide film on the Co-Cr-Mo alloy plays animportant role in corrosion resistance. The surface oxide filmof the Co-Cr alloy has been reported to consist of oxides ofCo and Cr.13) The corrosion of Co-Cr alloys in neutral or acidsolutions was found to proceed by selective dissolution ofCo.14) When a Co-29Cr-6Mo alloy was immersed in theHank’s solution, Co dissolved from the film, and the filmcomposition changed into Cr oxide containing a smallamount of Mo oxide.15) Therefore, these experimental resultsshowing higher Co release than the other elements in eachcompact are reasonable and in good agreement with thosefrom previous reports.3,16)

As shown in Figs. 10 and 11, a small addition of Zr to theCo-29Cr-6Mo contributed to the decrease of the amount ofCo released from the compacts sintered in Ar or N2. Therelative densities of 0.03Zr and 0.1Zr were higher than thoseof 0Zr and 0.5Zr irrespectively of the sintering atmospheres(Fig. 1). Thus, the surface area of 0Zr and 0.5Zr was higherthan that of 0.3Zr and 0.1Zr owing to their higher porosities.Therefore, the amount of Co release among the compactssintered in Ar or N2 differed depending on the porosity.When the Ar and N2 atmospheres were compared, higherrelative densities were achieved in Ar, and, thus, the amountsof each metal released from the compacts sintered in Ar werelower than those in N2.

The amounts of released Co from 0Zr and 0.5Zr sintered inN2 were found to be higher than those from 0.03Zr and 0.1Zrsintered in N2. The microstructures of both compacts arecharacterized by their lamellar structure. Local corrosionmay occur at the interface between the different phases inthese compacts with the lamellar structure during immersion.The microstructures of the compacts sintered in Ar weresimilar (Fig. 2). Accordingly, the difference in the amount of

released Co among the compacts sintered in N2 was higherthan that in Ar.

5. Conclusion

The Zr addition to the Co-29Cr-6Mo powders enhancedthe sintering of the compacts and suppressed the Co releasefrom the compacts. When the Zr-added Co-29Cr-6Mo alloypowders were sintered in N2, the relative densities of thecompacts were lower than those of powders sintered in Ar.The nitrides disturbed the densification between the powders.In addition, a lamellar structure was formed at the Co-29Cr-6Mo and Co-29Cr-6Mo-0.5Zr compacts. Local corrosionseems to occur at the interface between the different phasesin these compacts during immersion in 1% lactic acid.Accordingly, the amount of Co released from these compactsshowed a higher value than that from other compacts. In theMIM process, a small addition of Zr (less than 0.5mass%) tothe Co-29Cr-6Mo alloy is effective for densification duringsintering and suppression of the Co release from thecompacts.

Acknowledgements

The authors would like to thank Dr. S. Ichinose for theSEM observations. This work was supported by a grantfor Cooperation of Innovative Technology and AdvancedResearch in the Evolutional Area from the Ministry ofEducation, Culture, Sports, Science and Technology ofJapan.

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