(a)

high creep resistance observed in this alloy. It is also be- lieved that structural stability at large strains, far beyond the minimum creep-rate strain, is largely dependent upon the thermodynamic stability of those carbides pinning the sub- grain boundaries.

A successful attempt to quantify the rate deformation processes in these structure should include the effect of various microstructural and substructural factors in the fol- lowing general form:

= K ~ (A ,d) Pm (try, str) exp - [ ( ac - o'~V*)/RT]

where ~ (A ,d) is a multifunction of both the second-phase particle size d and the subgrain size A; Pm is the mobile dislocation density, which is a function of both the applied stress tra and the initial substructure str; and Qc, V*, and O'e are the classical apparent activation energy, apparent activa- tion volume, and the effective stress of the deformation process, respectively. Quantitative analysis of the above equation is presently under way.

REFERENCES 1. K.E. Amin: Mater. Sci. andEng., 1981, vol. 51, no. 1, p. 123. 2. H.J. McQueen: Metall. Trans. A, 1977, vol. 8A, p. 807. 3. A.W. Thompson: Metall, Trans. A, 1977, vol. 8A, p. 833. 4. O. D. Sherby, R. H. Klundt, and A. K. Miller: Metall. Trans. A, 1977,

vol. 8A, p. 843. 5. R. J. McElroy and Z. C. Szkopiak: Int. Met. Rev., 1972, vol. 17, p. 175. 6. G. C. N. Ahlquist and W.D. Nix: Scripta Met., 1969, vol. 3, p. 679.

(b) A Note on the Microstructural Dependence of Creep , Strength in Incone1700

A.K. KOUL and W. WALLACE

(c) Fig. 5--Transmission electron micrographs of martensite tempered for 24 h at 650 ~ before and after creep deformation under an applied stress of 160 MPa at 650 ~ Top: specimen head, middle: specimen after 1.3 pet creep strain, and bottom: specimen after 22 pet creep strain.

In conclusion, it appears that the formation of fine sub- grain structures, the high dislocation density exhibited at subgrain boundaries of various orientations, and the locking of these boundaries by second-phase particles as a result of thermomechanical treatments are major contributors to the

METALLURGICAL TRANSACTIONS A

Creep strength parameters are often defined in terms of observed minimum creep rate (k) and rupture life (tr) at a given stress and temperature.~ When reference is made to an increase in creep strength, it is invariably accompanied by an increase in rupture life and a decrease in creep rate. 2

Nickel-based superalloys are strengthened through car- bide precipitation at the grain boundaries and 7' precip- itation within the grains. 3,4 Discrete grain boundary carbides are generally considered beneficial since they reduce the extent of grain boundary sliding and therefore the onset of creep cavitation and rupture.3'5 Some workers 6'7 have tried to correlate the creep rate variations with carbide particle size in complex superalloys, e.g., Nimonic 105. The matrix 3" dispersion is established to hinder the dislocation movement within the grain interiors, and the minimum creep rate has been shown to vary as the cube root of the 3" volume fraction. 8 It is, however, unclear as to which microstructural features predominantly influence the individual creep

A.K. KOUL, Research Officer, and W. WALLACE, Head, are both with Structures and Materials Laboratory, National Aeronautical Establish- ment, National Research Council of Canada, Ottawa, Canada.

Manuscript submitted May 6, 1981.

ISSN 0360-2133/82/0311-0673500.75/0 �9 AMERICAN SOCIETY FOR METALS AND VOLUME I3A, APRIL 1982--673

THE METALLURGICAL SOCIETY OF AIME

250 strength parameters of rupture life (tr) and minimum creep rate (k) in a given microstructure.

One of the earlier but simpler International Nickel alloys, i .e. , Inconel 700, was chosen for the present study, and the chemical composition of this alloy in wt pct is given below:

C Mn Si Cr Co Mo Fe Ti A1 Ni

0.12 0.10 0.30 15.0 28.5 3.7 0.7 2.2 3.0 Bal.

The alloy contains three main second phase constituents, MC and M23C6 carbides and the principal strengthening agent 7' Ni3 (AITi), 9

Short term creep-rupture tests were carried out on differ- ent microstructures of Inconel 700 at 345 MN/m 2 and 790 ~ The microstructural features considered in detail were the grain size, grain boundary precipitate morphology, and the matrix 7 ' particle size and its volume fraction. Variations in these microstructures were obtained by re- solution treating and re-aging the fully heat-treated original material at different temperatures and times, as indicated in Table I. Carbon extraction replicas were used to extract both grain boundary and matrix precipitates, and different phases were identified using electron diffraction techniques. To extract carbides, a solution of 10 pct bromine in methyl alcohol was used as etchant, while a 10 pct phosphoric acid water solution was used for 3" extractions.

One of the noticeable features of this investigation was that the original material in the heat-treated condition ex- hibited lower creep rates and longer rupture lives than any of the retreated creep specimens. Two important correlations that explain these results are presented in Fig- ures 1 and 2.

Figure 1 shows a decrease in rupture life with increasing grain size in all retreated specimens. It could be suggested that a coarser grain size increased the stress concentration per unit grain boundary area and this led to early creep rupture. However, if grain size were the only parameter influencing rupture life, then specimens retreated at 1105 ~ and aged at 870 ~ should have shown similar rupture life to that of the original heat-treated material, since their grain sizes were very similar (Table I). However, this was not observed, as shown in Figure 1. When a lower solution temperature of 1105 ~ was used (which is below the MC solvus temperature of 1180 ~ MC carbides precipitated as grain boundary platelets (Fig- ure 3), as opposed to a discrete M23C6 precipitate morphology in the original heat-treated material (Figure 4). The platelet MC carbides are believed to have facilitated crack propaga-

Table I. Heat -Treatment Condit ions Used to

2OO

r

J la.l

~I O0

50

X S.T. 1105"C 0 S.T. 1180"C 1', S.T. 1205"C | HEAT TREATED

VIRGIN MATERIAL

I i t I I

O. 2 0.4 GRAIN SIZE mm

0.6

Fig. 1 - - Microstructural dependence of rupture life in Inconel 700 tested at 790 ~ at 345 M N / m 2.

tion and early rupture, and evidence to this effect is shown in Figure 5, where fracture is predominantly intergranular in nature. Therefore, the rupture life variations in Inconel 700 specimens are primarily governed by their grain size and grain boundary precipitate morphology.

Figure 2 demonstrates that in specimens solution treated at, or above, the MC solvus temperature ( -1180 ~ a definite correlation is observed between 1" particle size and minimum creep rate. The observed minimum creep rate increases with increasing 3/' particle size, and these data points in Figure 2 fit the polynomial between creep rate (k) and 7 ' particle size (d) as follows:

logk = -8 .8 8 + 17.81d - 40 .55d 2 [1]

where e is in s- ' , and d is in/.tm. In contrast, specimens solution treated at 1105 ~ i .e. ,

below the MC solvus temperature, exhibit a higher creep rate for a similar 7 ' particle size (Figure 2). This is attribut- ed to a lower solution treatment temperature of 1105 ~ where some portion of the titanium will be tied up as second- ary TiC (Figure 4). Accordingly, the net amount of titanium available for 7 ' precipitation is reduced, thus forming a lower 3~' volume fraction within the matrix (Table I), which leads to an increased creep rate.8 Therefore, it is concluded

Obtain Microstructural Variat ions in Inconel 700*

Original Material ST 1200 ~ and Aged at 870 ~ and Then Retreated:

Heat Original Material Treatments ST 1200 ~ ST 1105 ~ ST 1180 ~ ST 1205 ~

Used + Age at 870 ~ for + Age at 870 ~ for + Age at 870 ~ for + Age at 870 ~ for

Grain size in mm 24 hours

0.19

- - - - 0 hours 24 hours 24 hours 24 hours

100 hours 100 hours 100 hours

0.21 0.3 0.48

Matrix y' vol pct 19.7 15.3 19.9 20.2

*MC solvus temperature ~ 1180 ~

674--VOLUME 13A, APRIL 1982 METALLURGICAL TRANSACTIONS A

X S.T. 11050C O S.T. 118OOC A S.T. 1205"C | HEAT TREATED

VIRGIN MATERIAL

CD

LU k-

n la_l W r r

I( 0.00

ir

I I 8 , I I I 0.04 0.0 0.12 0.16 0.20 0.24.

! 7' PARTICLE SIZE Fm

Pig. 2 - - 7'particle size dependence of creep rate in Inconel 700 tested at 790 ~ at 345 M N / m 2.

Fig. 4 - -Di sc re t e MzaC6 precipitate morphology in the original heat- treated material.

Fig. 5 Predominantly intergranular failure of the stress-rupture specimen re-solution treated at 1105 ~ -r aged 870 ~ Notice the sharp intergranular facets.

Fig. 3 - - Platelet MC carbide morphology precipitated on re-solution treat- ing at 1105 ~ -- aged 870 ~

that in Inconel 700, creep strength in terms of minimum creep rate is predominantly governed by the 7' particle size and its volume fraction within the matrix.

Finally, both these correlations between the creep strength parameters and microstructural features are consid- ered to provide important guidelines for the development of rejuvenation technology.

REFERENCES

1. N.J . Grant: Deformation and Fracture at Elevated Temperatures. M. I. T. Press. Cambridge, MA, 1965, pp. 91-105, 165.

2. A .K. Koul and F. B. Picketing: presented at TMS-AIME fall meeting m Louisville, KY, October 11-15. 1981.

3. W.C. Hagel and C. T. Sims: Superalloys, 1st edition, John Wiley and Sons Inc.. New York, NY. 1972, vol. 1. p. 114.

4. W. Bettetidge and J. Heslop: The Nimonie Alloys, 2nd edition, Edward Arnold. London. 1974. p. 63.

5. A.J. Perry: J. of Mat. Sci., 1974, vol. 9. pp. 1016-39. 6. P.W. Davies, J. P. Dennison, and D. Sidey: J. Inst. Metals, 1973. vol.

101, pp. 153-61. 7. G.R. Leverant and B. H. Kear: Metall. Trans.. 1970. vol. 1. p. 491. 8. B.E. Hopkins and T. B. Gibbons: Creep Strength in Steels and High

Temperature Alloys, organized by The Iron and Steel Institute, London, September 20-22, 1972. p. 165.

9. D.M. Moon and R. Stickier: "Phase Stability in Super Alloys," pro- ceedings of a symposium at Baden. March 1973, Elsevier Scientific Publishing Company, Amsterdam.

METALLURGICAL TRANSACTIONS A VOLUME 13A, APRIL 1982 675