behavior ds mm247lc(bae2001)

8/4/2019 Behavior DS MM247LC(Bae2001)

1/6

Formation of MC-c=cH eutectic bers and their eecton stress rupture behavior in D/S Mar-M247LC

superalloyJ.S. Baea, J.H. Leea*, S.S. Kimb, and C.Y. Joc

a

Department of Metallurgy and Materials Science, Changwon National University, Sarim-dong, Changwon 641-773,South Korea

bDivision of Materials Science and Engineering, Advanced Materials Research Center, Gyeongsang National University,

Chinju 660-701, South KoreacHigh Temperature Materials Laboratory, KIMM, Changwon 641-010, South Korea

Received 21 November 2000; received in revised form 2 April 2001; accepted 13 April 2001

Keywords: Solidication; Nickel; Liquids; Interface; Fibers

Introduction

Nickel-based superalloys are extensively used in turbine blades and other engine

parts on account of their creep strength and resistance to oxidation at elevated tem-

peratures [1]. Mar-M247LC is a modied version of superalloy Mar-M247 with en-

hanced carbide stability, castability and ductility, achieved by lowering the carbon

content. The carbides precipitated at grain boundaries play an important role in the

strengthening of grain boundaries at elevated temperatures. Uniformly distributed ne

carbides inhibit grain boundary sliding and enhance elevated temperature strength;

however, coarse or continuous script-like carbides are detrimental because they serve as

crack initiation sites and crack propagation paths [28]. Much attention has been givento the production of aligned brous dispersions of carbides. If the rod phase is strong

enough and the matrix phase is suciently ductile, a composite material with high

strength and ductility could be obtained [9]. There has been considerable interest in

developing in situ composite superalloys, such as TaC or NbC eutectic composites in c

matrix, for use at high temperatures [10].

At a relatively low solidication rate of 0.5 lm/s, MC-c=cH eutectic composed ofcontinuous MC bers in the c=cH matrix was found to form in the directional solidi-cation of the Mar-M247LC. The MC-c eutectic could be grown such that it comprised

Scripta Materialia 45 (2001) 503508www.elsevier.com/locate/scriptamat

* Corresponding author.

E-mail address: [email protected] (J.H. Lee).

1359-6462/01/$ - see front matter 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

PII: S1359-6462(01)01049-1


2/6

more than 60% of the steady state structure of the directionally solidied rod between

solid fractions of 0.2 and 0.8. The creep-rupture time of these composite structures

proved to be as long as that found for the Mar-M247LC single crystal superalloy in a

196 MPa/982C creep-rupture test.

Experimental

A master ingot of Mar-M247LC superalloy was obtained from CannonMuskegon

Co. (MI, USA); its nominal composition is shown in Table 1. Mar-M247LC superalloy

rods were directionally solidied at a low withdrawal rate of 0:5 lm/s in a modiedBridgeman type furnace at 1630C. Eutectic solidication with a planar solid/liquid

interface could be obtained at this solidication rate. An 8 mm OD 5 mm ID alu-mina tube was used and the growing solid/liquid interface was quenched during

directional solidication. Longitudinal and transverse sections of the directionally so-

lidied specimen were polished and etched using Kalling's reagent. The microstructure

of the specimen was observed by OM and SEM (JEOL JSM5800). In addition,

a 17 mm OD 12 mm ID tube was used to obtain a directionally grown MC-c=cH

eutectic sample for high temperature creep-rupture test. In order to evaluate the high

temperature creep properties of the MC-c=cH composite, a directionally solidied rodwith a diameter of 12 mm was prepared to sucient length such that the composite

microstructure was at least 50 mm. The directionally solidied composite was aged at

871C, and the creep-rupture test was carried out according to ASTM E139 (gauge

length 23 mm, gauge diameter 4.5 mm) at 196 MPa/982C. The microstructure after

creep failure was observed by OM and SEM.

Results and discussion

MC-c=cH rod eutectic growth

At the low solidication rate of 0.5 lm/s, a planar interface was observed, but the

directionally solidied microstructure changed along the rod, where the sample lengthfor the directional solidication was 80 mm. As the directional solidication progressed,

the composition of liquid increased due to the segregation of elements (except c phase

forming elements). When the solidication fraction reached a certain value, the com-

position of solid/liquid interface front came to a eutectic composition. The experimental

results showed that the c planar interface was formed at low solidication fractions

Table 1

Chemical composition of Mar-M247LC (wt.%)

Element C Co Cr Al Ti Ta Mo W Zr B Hf Ni

Composition 0.07 9.5 8 5.5 0.7 3.3 0.5 9.4 0.02 0.016 1.4 Balance

504 J.S. Bae et al./Scripta Materialia 45 (2001) 503508


3/6

fs 0:2, where the MC-c eutectic composite was formed at higher solidicationfractions ranging from 0.4 to 0.8. The c=cH eutectic was observed near the end of the rodfs 0:95, as shown in Fig. 1.

Table 2 shows the relationship between solidication fraction and solid/liquid in-

terface structure. These results indicate that solidication starts with a c planar struc-

ture. As solidication proceeds, the structure changes to MC-c eutectic planar before

becoming c=cH eutectic. The sequence of solidication in the sample at fs 0:95 isshown schematically in Fig. 2. This result clearly demonstrates that the nal freezing

microstructure in the interdendritic region during Ni-base superalloy solidication is

c=cH eutectic. The c phase with a planar solid/liquid interface was formed as an initial

transient of the directionally solidied rod, where the c=cH eutectic was formed as a naltransient.

Fig. 1. Quenched solid/liquid interface morphologies with solidication fractions at 0:5 lm/s. (a) c planar at fs 0:2,

(b) MC-c=cH eutectic at fs 0:4, (c) c=cH eutectic at fs 0:95.

Table 2

Microstructure of the solid/liquid interface at a solidication rate of 0 :5 lm/s

Solidication fraction fs

in 80 mm D/S rod

Interface structure Carbide shape

0.2 c planar Octahedral blocky

0.4 MC/c eutectic Continuous rod

0.8 MC/c eutectic Continuous rod0.95 c=cH eutectic Elongated blocky

Fig. 2. Schematic of the directionally solidied rod quenched at fs 0:95.

J.S. Bae et al./Scripta Materialia 45 (2001) 503508 505


4/6

The MC-c eutectic could form more than 60% of the steady state structure of the

directionally solidied rod. Fig. 3 presents a transverse section of the eutectic composite

microstructure, where the MC rods are uniformly distributed in the c matrix. It appears

that carbides of 23 lm diameter are dispersed at the grain boundaries and throughout

the matrix. To observe the accurate morphology of the MC ber, the sample was deeply

etched with modied Kalling's reagent.

Creep-rupture properties

To understand the mechanical properties of the MC-c composite in the directionally

solidied Mar-M247LC superalloy, specimens were prepared from a directionally so-lidied 12 mm diameter rod for creep-rupture testing. The creep-rupture specimens

were aged at 872C for 20 h for the cH precipitation in the c phase, which resulted in the

MC-c=cH composite.The creep-rupture time for equiaxed, single crystal and MC-c=cH composite structure

Mar-M247LC, respectively, is presented in Table 3. The average creep-rupture time for

Fig. 3. Micrographs of the MC-c=cH eutectic at fs 0:4 of Fig. 1(b). (a, b) Transverse section, (c, d) longitudinal section.

Table 3

Comparison of creep-rupture times between equiaxed, MC-c=cH composite and single crystal Mar-M247LC alloys

Equiaxed [11] MC/c eutectic composite Single crystal [11]

Rupture time (h) 74.4 116 119.4



5/6

MC-c=cH composite was 116 h at 196 MPa/982C. This value, which was the averageof two test results, was comparable to that of the single crystal Mar-M247LC alloy.

Longitudinal views after the creep-rupture test are shown in Fig. 4.

The creep deformation in the MC-c=cH composite structured Mar-M247LCwas found to be very uniform throughout the matrix, as demonstrated in Fig. 4(a). In

addition, the grain boundary area of the MC-c=cH composite was largely intact. Suchbehavior in the MC-c=cH composite structured Mar-M247LC does not resemble that ofthe typical directionally solidied microstructure, which cracked along the grain

boundary ledges that were perpendicular to the direction of load, or the single-crys-talline microstructure, which underwent interdendritic fracture during creep defor-

mation. Interestingly, Fig. 4(b) and (c) suggests that the MC carbide bers would

eectively block crack growth, as well as acting as crack-nucleating sites. As shown in

Fig. 4(c), the coalescence of cracks, as initiated at the interface between ber and

matrix, was often stopped by the presence of another ber. Although the rupture life of

the composite structure of the alloy was either comparable to, or slightly lower than,

that of the single crystal in the present study, the high-temperature mechanical prop-

erties of the MC-c=cH composite including creep strain, tensile ductility and fracture

toughness may overall exceed those of the single-crystalline counterpart. Moreover,high-magnication SEM observation demonstrated that the carbides in the present

alloy were free from any reactions with the matrix even after the creep test, as shown in

Fig. 4. Micrographs after creep failure. (a, b) OM, (c, d) SEM micrographs. The loading direction was bottom to top.

(a) Shows homogeneous crack initiation, (b, c) show that the MC carbides initiate and block cracks, and (d) shows MC

carbides free from reaction with the c=cH

matrix.

J.S. Bae et al./Scripta Materialia 45 (2001) 503508 507


6/6

Fig. 4(d). In conclusion, the presence of ne, continuous and in situ carbide bers in the

c matrix during solidication could be benecial for the Mar-M247LC alloy by ho-

mogenizing the deformation, and blocking crack growth during creep deformation.

Further studies are, however, required to quantify the eect of bers on the mechanical

properties of Mar-M247LC alloys.

Conclusions

A MC-c composite structure in Mar-M247LC was obtained by directional solidi-

cation at a relatively low growth rate of 0:5 lm/s. As solidication proceeded, thestructure changed from c planar to the MC-c eutectic planar before ending up with the

c=cH eutectic structure. The MC-c eutectic could be induced to form more than 60% ofthe directionally solidied rod. The bers of MC carbides were continuous and ne,with diameters of 23 lm. The carbide, as composed of refractory elements in Mar-

M247LC displayed an excellent stability at high temperatures. In the present study, the

rupture life of the composite structure of the alloy was observed to be slightly lower

than that of single crystal. The creep deformation was, however, extremely uniform in

the composite structure. It is therefore possible that the high-temperature mechani-

cal properties (including creep strain, tensile ductility and fracture toughness) of Mar-

M247LC could be improved by use of the MC-c=cH composite structure that provideshomogenization of creep deformation and blocking of crack growth during creep de-

formation.

References

[1] Weetham, G. W. (1986). High Temp Alloys for Gas Turbines and Other Applications (Eds.). Betz, et al. Reidel:

Dordrecht.

[2] Gell, M., & Leverant, G. R. (1968). Trans AIME 242, 1869.

[3] Kotval, P. S., Venables, J. D., & Calder, R. W. (1972). Met Trans 3, 453.

[4] Bhambri, A. K., Kattamis, T. Z., & Morral, J. E. (1975). Met Trans B 6, 523.

[5] Fernandez, R., Lecomte, J. C., & Kattamis, T. E. (1978). Met Trans A 9, 1381.

[6] Codkcroft, S. L., Mitchell, A., & Schmalz, A. J. (1996). J High Temp Mater Proc 15, 173.

[7] Liu, L., Sommer, F., & Fu, H. Z. (1994). Scripta Metall Mater 30, 587.[8] Chen, J., Lee, J. H., & Jo, C. Y. (1998). Mater Sci Eng A 247, 113.

[9] Kraft, R. W. (1966). J Metals 18, 192.

[10] Buchanan, E. R., & Rarshis, L. A. (1973). Metall Trans A 4, 1895.

[11] Jo, C. Y. (1996). Ph.D. Thesis, Pusan National University, Korea.


behavior ds mm247lc(bae2001)

Documents