A Note on the Microstructural Dependence of Creep Strength in Inconel 700
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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.
A Note on the Microstructural Dependence of Creep , Strength in Incone1700
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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 subgrain 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 METALLURGICALTRANSACTIONSA
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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 carbide precipitation at the grain boundaries and 7' precipitation within the grains. 3,4Discrete 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 Establishment, National Research Council of Canada, Ottawa, Canada. Manuscript submitted May 6, 1981.
ISSN 0360-2133/82/0311-0673500.75/0 AMERICANSOCIETYFOR METALSAND THE METALLURGICALSOCIETY OF AIME
VOLUME I3A, APRIL 1982--673
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
250 2OO
S.T. 1105"C S.T. 1180"C S.T. 1205"C HEAT TREATED VIRGIN MATERIAL
r
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 car
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