Cellular-dendritic transition in directionally solidified binary alloys
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The cause of moisture sensitivity of A1203 scales is not known, but has been observed for undoped NiAI. t2 Here a weight loss of 0.44 mg/cm2 per hundred hours was produced by cyclic oxidation at 1100 ~ and exposure to moisture after each cooldown as compared to 0.12 mg/cm 2 for samples not exposed to moisture. The effect was discussed in relation to a moisture-assisted crack growth phenomenon (stress corrosion) reported for bulk A1203. In conclusion, the present study shows that adherent scales can form on undoped NiCrA1 alloys by repeated heating and sanding, presumably due to the successive removal of interfacial segregants such as sulfur. While adherent scales form after 13 purging cycles, spalling can still be caused by water immersion. With further purging this susceptibility is also diminished. Any additional strengthening of the oxide-metal bond for doped NiCrA1 alloys due to segregation of the oxygen-active dopants themselves is not required for scale retention during cycling and consequently appears to be a secondary effect. The results therefore strongly support the model proposed by Smeggil and coworkers that (1) trace levels of sulfur in undoped MCrA1 alloys are sufficient to cause A1203 scale spaUation, (2) removal of sulfur from the alloy is sufficient to allow scale adhesion, and (3) one major effect of oxygen-active dopants on adhesion is to prevent sulfur segregation at the interface.
REFERENCES I, A.W. Funkenbusch, J.G. Smeggil, and N.S. Bornstein: Metall. Trans. A, 1985, vol. 16A, p. 1164. 2. J.G. Smeggil, A.W. Funkenbusch, and N.S. Bornstein: Metall. Trans. A, 1986, vol. 17A, p. 923. 3. K.L. Luthra and C. L. Briant: Electrochem. Soc. Extended Abstracts, 1984 Spring Mtg., Cincinnati, OH, p. 26. 4. R. Browning, C. Park, F.A. Marks, J.L. Smialek, and C. Papageorgopoulas: NASA Ames, Nasa Lewis, unpublished research, 1985. 5. L. A Larson, M. Prutton, H. Poppa, and J. L. Smialek: J. Vac. Sci. Technol., 1982, vol. 20, p. 1403. METALLURGICALTRANSACTIONS A
6. M.M. El Gomati, C.G.H. Walker, D.C. Peacock, M. Prutton, H.E. Bishop, R.M. Hawes, and J.L. Smialek: Surf. Sci., 1985, vol. 152/153, p. 917. 7. M.M. E1 Gomati, C. Walker, D. C. Peacock, and M. Prutton: Corr. Sci., 1985, vol. 25, p. 351. 8. J.L. Smialek and R. Browning: in Symp. Proc. on High Temperature Materials Chemistry 111, Z.A. Munir and D. Cubicciotti, eds., Electrochemical Soc., Pennington, NJ, 1986, p. 258; also NASA TM 87168, 1985. 9. A.B. Anderson, S. P. Mehendru, and J. L. Smialek: J. Electrochem. Soc., 1985, vol. 132, p. 1695. 10. J.L. Smialek and R. Gibala: Metall. Trans. A, 1983, vol. 14A, p. 2143. 11. J.L. Smialek: in Proc. N. L. Peterson Mem. Symp. on "Oxidation of Metals and Associated Mass Transport," AIME, in press. 12. J.L. Smialek: Metall. Trans. A, 1978, vol. 9A, p. 309.
Cellular-Dendritic Transition in Directionally Solidified Binary Alloys S. N. TEWARI and V. L A X M A N A N
It is important to study the microstructural development during solidification because the physical and mechanical properties of castings are determin
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