Thermodynamic consideration of grain refinement of aluminum alloys by titanium and carbon
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boundaries, stacking faults, and antiphase boundaries. [26] Presumably, segregation of B to these interfaces reduces the strain energy associated with matrix ordering and, in turn, the kinetics of discontinuous grain boundary ordering, as observed for the hypostoichiometric alloy. Further support for this assumption was provided by the observation that the beneficial effect of B on ductility diminishes gradually with extended exposure at 700 ~ as demonstrated in Figure 7. However, after 1000 hours of exposure, about 15 pct elongation was still maintained. One possible explanation for the observed behavior of the stoichiometric alloy is the larger matrix strain energy. In comparison with the hypostoichiometric alloy, a given B content may become less effective in suppressing the grain boundary reaction.
The support of the Research Institute, King Fahd University of Petroleum and Minerals, and its permission to publish this work are greatly appreciated. REFERENCES 1. N.S. Stoloff: Int. Mater. Rev., 1984, vol. 29 (3), pp. 123-35. 2. N.S. Stoloff: Int. Met. Rev., 1989, vol. 34 (4), pp. 153-89. 3. D.P. Pope and C.T. Liu: in Superalloys, Supercomposites and Superceramics, J.K. Tien and T. Caulfield, eds., Academic Press, New York, NY, 1989, pp. 583-624. 4. C.T. Liu, C.L. White, and J.A. Horton: Acta Metall., 1985, vol. 33, lap. 213-29. 5. C.T. Liu, C.L. White, and E.H. Lee: Scripta Metall., 1985, vol. 19, pp. 1247-50. 6. C.T. Liu and C.L. White: High Temperature Ordered Intermetallic Alloys, MRS Symp. Proc., C.C. Koch, C.T. Liu, and N.S. Stoloff, eds., MRS, Pittsburgh, PA, 1985, vol. 39, pp. 365-86. 7. K. Aoki and O. Izumi:Jpn. lnst. Met., 1979, vol. 43, pp. 1190-96. 8. A.I. Taub and C.L. Briant: Metall. Trans. A, 1989, vol. 20A, pp. 2025-32. 9. C.R. Brooks and Y.M. Wang: Mater. Characterization, 1990, vol. 25, pp. 185-97. METALLURGICAL TRANSACTIONS A
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