The formation of grain boundary void in cast nickel base alloys after cold working and annealing

Scripta METALLURGICA

Vol. 16, pp. 209-212, 1982 Printed in the U.S.A.

Pergamon Press Ltd. All rights reserved

THE FORMATION OF GRAIN BOUNDARY VOIDS IN CAST NICKEL BASE ALLOYS AFTER COLD WORKING AND ANNEALING

M.Y. Nazmy and T.W. Duerig Brown Boveri Research Center, CH-5405 Baden Switzerland

{ R e c e i v e d December 1, 1981)

Introduction The service life of some nickel base superalloys at elevated temperatures can be notabaly shortened by prior deformation at room temperature (I). Dyson and Henn (2), in their work on the wrought superalloy Nimonic 80 A, showed that numerous submicron size voids were developed on the grain boundaries after deforming a few percent and subsequently annealing at 750°C. Hence the period of time usually required for the nucleation of cavities in a component under stress at high temperature is eliminated by the prestrain, and thereby failure is hastened. Dyson et al. (3) conducted a systematic study of prestain-induced cavitation in Nimonic 80 A as a function of the state of stress in the material. They reported that cavitation is most likely to occur on those boundaries which lie roughly parallel to the direction of maximum principal stress. Dyson et al. (3) also postulated that cavitation is initiated by the large stresses associated with the intersection of slip band with a grain boundary or grain boundary particle. When the material is then heated, the voids grow to their equilibrium size (about 0.3 ~m in diameter) by stress directed diffusion under the effect of residual tensile stresses. They attributed the presence of the residual stresses to the grain-to-grain variation deformability associated with differences in the Schmid factor. Recently, Kikuchi and Weertman (4) and Saegusa et al. (5) reported similar cavitation behaviour in wrought nickel base Astroloy. In a detailed investigation on the void nucleation in the same alloy, Kikuchi et al. (6) showed that voids are nucleated at the interface between matrix and grain boundary M2aC 6 carbides, and they grow during annealing by vacancy diffusion driven by residual tensile stresses. They attributed the origin of these residual stresses to the difference in deformability between the hard carbides and the matrix. The present investigation has been carried out to study the process of void nucleation and growth after room temperature deformation and annealing in nickel base alloys with cast structures and thus with relatively large grain size. The alloys chosen for the study were cast IN 738, IN 939 and IN 100". Experimental

Procedure

Strips of 16 x 8 x 4 mm were milled from fully heat treated cast to size blocks of the three alloys and then surface ground. The grain size for the three alloys was about 2 mm. The room temperature deformation of 4 ~ was carried out by rolling. After cold deformation, the strips were heated at i120°C for 2 hours.

*Trade names of the International

Nickel Company

209 0036-9748/82/020209-04503.00/0 C o p y r i g h t (c) 1982 Pergamon P r e s s Ltd.

210

GRAIN BOUNDARY VOIDS

Vol.

16, No.

2

For the alloy IN 738, cold rolling was carried out on fully heat treated strips as well as on ones that were only solution treated at I120°C for 2 hr. Different types of deformation modes like bending, torsion and tension were utilized for the same alloy. The metallographic sections were prepared by mechanical grinding and final polishing was done using 1/4 ~m diamond. Results

and Discussion

In all of the three alloys, grain boundary voids of about 2 ~m in diameter were observed after room temperature deformation and subsequent heating at I120°C for 2 hr. Figure 1 a, b and c illustrate typical grain boundary pore chains in the three alloys. As shown in the same figure, the grain boundary voids were nucleated at MC carbide/matrix interfaces as well as at carbide free sites along the grain boundary. For these three alloys in the fully heat treated condition, the grain boundary structure consists mainly of M23C 6 carbides, blocky MC carbides and ¥' particles. The prior solution treatment that was carried out on some of the strips of IN 738, rendered the grain boundaries free of M2sC 6 carbide. For these strips, grain boundary voids also were developed, as shown in figure 2. This result is different than that reported by Saegusa et al. (5) on Astroloy, in which they related the grain boundary voids only to the decohesion of M23C6/matrix interface. Dyson et al. (3) postulated that void nucleation can take place where slip bands intersect an obstacle like a grain boundary or precipitate at a grain boundary in the manner suggested by Stroh (7) or Smith and Barnby (8). Hondros and McLean (9), in their work on cohesion in copper alloys, found that during room temperature deformation cracks can be nucleated where a slip band impinges on a grain boundary. They also showed that small amount of segregates at grain boundaries have a great effect on cohesion. They also proposed that cracking is easier to nucleate where two slip bands on approximately mirror image planes meet a grain boundary. It was shown by Kikuchi and Weertman (4) that grain boundary voids due to decohesion of M23C6/matrix interface were associated with slip bands. A strip of IN 738 was solution treated, cold rolled 4 ~, and again solution treated at i120°C for 2 hr. Grain boundary voids were observed. The same specimen was then bent at room temperature and the surface was examined again. As shown in figure 3 grain boundary voids are generally associated with slip bands and the slip band spacing is of the same order as the void spacing. Hence, it is suggested that in these cast alloys void nucleation takes place by grain boundary particles (M23C6, and MC) decohesion as well as by grain boundary cracking, and both processes are induced by stress concentrations generated by dislocations piling up at the tips of lattice slip bands. After nucleation, the grain boundary voids grow to their stable size by stress directed diffusion (3). As reported by Dyson et al. (3), and Saegusa et al. (5), the voids tend to develop more frequently on the grain boundaries parallel to the maximum principal stress. In this investigation on the three cast nickel base superalloys, these observations by Dyson et al. (3) and Saegusa et al. (5) were not confirmed. Figure 4 a, b shows voids on grain boundaries inclined to, as well as perpendicular to, the rolling direction. For the alloy IN 738, room temperature plastic deformation was performed on the fully heat treated specimens by torsion, tension, bending as well as by r o l l i n g Grain boundary voids were developed in all cases after solution treatment. However, it was observed that void formation was less extensive in torsion and tension specimens than in rolled ones for the same amount of deformation. This result agrees with the findings of Dyson and McLean (i0) on Nimonic 80 A. In the strips of IN 738 that were deformed by bending at room temperature and given the same solution treatment, voids were observed only near the concave surface, of the strips and only on grain boundaries normal to it. This implies that macro-residual tensile stresses play an important role, in the stress directed diffusion process needed for the growth of these voids. Conclusions i.

Grain boundary voids can develop in cast nickel base superalloys IN 738, IN I00, and IN 939 due to room temperature plastic deformation followed by solution treatment at I120°C for 2 hr.

Vol.

16, No.

2

GRAIN BOUNDARY VOIDS

211

2.

Grain boundary voids were found associated with grain boundary carbides M2sC~) as well as on carbide free grain boundary sites.

3.

Voids were developed inhomogenously on grain boundaries parallel to as well as prependicular to the principal stress axis, i.e. rolling direction in this case.

(MC,

References i. 2. 3. 4 5 6 7 8 9 i0

B.F. Dyson and M.J. Rodgers, Metal Science 8, 261 (1974). B.F. Dyson and D.E. Henn, J. Microsc. 97, 165 (1973). B.F. Dyson, M.S. Loveday, and M.J. Rodgers, Proc. R. Soc. London A, 349, 245 (1976). M. Kikuchi, and J.R. Weertman, Scripta Met. i_44, 797 (1980). T. Saegusa, M. Uemura, and J.R. Weertman, Met. Trans. IIA, 1453 (1980). M. Kikuchi, K. Shiozawa, and J.R. Weertman, Acta Met. 2_99, 1747 (1981). A.N. Stroh, Proc. R. Soc. London, A223, 404 (1954). E. smith and J.T. Barnby, Met. Sci. J., l, 1 (1967). E.D. Hondros, and D. McLean, Phil. Mag. XXIX, No. 4, 771 (1974). B.F. Dyson and D. McLean, Metal Science l_!l, 37 (1977). Acknowledgement

The authors acknowledge and Dr. O. Szabo.

(a)

400

x

the helpful comments

(b)

270 x

and discussion

of Dr. G.H. Gessinger

(c)

200 X

FIG. 1 Grain boundary voids in cast nickel base alloys IN 738 (a), IN 939 (b), and IN i00 (c) developed due to cold rolling und subsequent solution treatment.

212

GRAIN BOUNDARY VOIDS

•

FIG. 2 Grain boundary voids developed after solution treatment in a strip of IN 738 that was prior solution treated then cold rolled. 560 x

(a)

150 x

Vol.

16, No. 2

¸4¸•¸

FIG. 3 Grain boundary voids, associated with slip lines in the specimen of Figure 2 after bending.

(b)

150 x

FIG. 4 Grain boundary voids in IN 939 cold rolled strip and then solution treated, rolling direction is in the vertical direction.

the