The role of nucleation entropy in the formation of carbide phases in cobalt base superalloy HS 21 is investigated. X-ray and electron diffraction analyses show that M23C6 is the primary carbide, the composition approximating (Cr0·77Co0·15Mo0·08)23C6. The composition of HS 21 was varied by remelting the as-received alloy in a vacuum induction furnace and adding small amounts of Ta and Nb. The addition of ∼1 at.-%Ta or 1·5 at.-%Nb resulted in the coarser M23C6 carbide being entirely replaced by a considerably finer MC carbide (TaC or NbC). It is shown that the carbide refinement is in agreement with the increased nucleation entropy and corresponding nucleation rate of the MC v. M23C6 carbide.
Get full access to this article
View all access options for this article.
References
1.
HEBEISENM. C., WAGNERH. J., NELSONS. G., ALEXANDERC. A., FOSTERE. R., MOAKD. P., BLOCKERJ. M., and PFEIFERW. H.: NASA SP-5095,1971.
2.
YOUDELISw. V.: Met. Sci., 1975, 9, 464.
3.
YOUDELISW. V.: Met. Sci., 1979, 13, 540.
4.
YOUDELISW. V. and YANGc.-s.: Met. Sci., 1982, 16, 275.
5.
CISSEJ., ICERRH. W., and BOLLINGG. F.: Mesa/I. Trans., 1974, 5, 633.
6.
YOUDELISW. V., YANGC.-S. and SRINIVASANM. N.: Aluminium, 1979, 55, 533.
7.
SIMSC. T.: J. Met., Dec. 1969, 27.
8.
LANEJ. R. and GRANTN. J.: Trans. ASM, 1952, 44, 113.
9.
wouLosM. J. and CASST. R.: Cobalt, 1969, 42, 3.
10.
‘Handbook of chemistry and physics, 56th edn, D-62, 63;1975–76, CRC Press.
11.
HULTGRENR., DESAIP. D., HAWKINSD. T., GLEISSERM., and KELLEYK. K.: ‘Selected values of the thermodynamic properties of binary alloys’, 503; 1973, Metals Park, Ohio, American Society for Metals.
12.
KUBASCHEWSKIO., EVANSE. L. L., and ALCOCKC. B.: ‘Metall-urgical thermochemistry’, 4th edn, 212; 1967, London, Pergamon.
13.
KAUFMANL. and NESORH.: Calphad, 1978, 2, 295.
14.
HULTGRENR., DESAIP. D., HAWKINSD. T., GLEISERM., and KELLEYK. K.: ‘Selected values of the thermodynamic properties of binary alloys’, 473; 1973.
15.
GOLDSCHMIDTFl. J.: ‘Interstitial alloys’, 92–94; 1967, New York, Plenum.
16.
SWALINR. A.: ‘Thermodynamics of solids’, 209; 1972, New York, John Wiley.
