Microstructural evolution of primary dendrite trunks in directionally solidified, hypoeutectic, aluminum-silicon alloys
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6. F.A. Mohamed and T.G. Langdon: Phil. Mag., 1975, vol. 32, pp. 697-769. 7. P.K. Chaudhury and F.A. Mohamed: Acta Metall., 1988, vol. 36, pp. 1099-110. 8. P.K. Chaudhury, V. Sivaramakrishnan, and F.A. Mohamed: Metall. Trans. A, 1988, vol. 19A, pp. 2741-52. 9. F.A. Mohamed: J. Mater. Sci., 1983, vol. 18, pp. 582-92. 10. P.K. Chaudhury, K.T. Park, and F.A. Mohamed: Metall. Trans. A, in press. 11. K.L. Murty, F.A. Mohamed, and J.E. Dorn: Acta Metall., 1972, vol. 20, pp. 1009-18. 12. S. Yan, J.C. Earthman, and F.A. Mohamed: Phil. Mag., 1994, vol. 69, pp. 1017-38. 13. K.-I. Hirano, R.P. Agarwala, and M. Cohen: Acta Metall., 1962, vol. 10, pp. 856-62.
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Microstructural Evolution of Primary Dendrite Trunks in Directionally Solidified, Hypoeutectic, Aluminum-Silicon Alloys R.N. G R U G E L
'~, MPa Fig. 5 - - T h e dependence of the apparent activation energy for superplastic flow on applied shear stress for grade 0 of Zn-22 pct Al (1460 ppm of Fe) with a grain size of 2.5 p,m. Also, previously reported data I"~j for grades A, B, C, and 3 containing 423, 125, 40, and 1 ppm of Fe (high-purity grade), respectively, are included for the purpose of comparison.
steady-state creep rates of Zn-22 pct A1 in region II (the superplastic region) are insensitive to Fe level and (b) the threshold stress for superplastic flow, To, in the alloy, when doped with Fe, approaches a limiting value when Fe concentration reaches a critical value of about 400 ppm; the limiting value for % may be related to saturation effects that result from strong segregation of Fe at boundaries.
This work was supported by the National Science Foundation under Grant No. DMR 9024255. Thanks are extended to Dr. K.T. Park for his assistance.
REFERENCES 1. A.H. Chokshi, A.K. Mukherjee, and T.G. Langdon: Mater. Sci. Eng., 1993, vol. R10, pp. 237-74. 2. O.D. Sherby and J. Wadsworth: Prog. Mater. Sci., 1989, vol. 33, pp. 169-221. 3. R.C. Gifkins: in Superplastic Forming of Structural Alloys, N.E. Paton and C.H. Hamilton, eds., TMS-AIME, San Diego, CA, 1982, pp. 2-26. 4. F.A. Mohamed and T.G. Langdon: Acta Metall., 1975, vol. 23, pp. 117-24. 5. F.A. Mohamed, S.-A. Shei, and T.G. Langdon: Acta Metall., 1975, vol. 23, pp. 1443-50. 496
VOLUME26A, FEBRUARY 1995
The primary dendrite arms in directionally solidified materials, e.g., turbine blades, serve as continuous fibers that extend the casting length and contribute to strength and ductility. [~j Previously, [2j it has been shown that the primary dendrite trunk diameter (d), like other microstructural constituents, exhibits predictable behavior as a function of the solidification processing parameters of growth velocity (V), temperature gradient (G), and composition (Co). Additionally, it responds more quickly to changes in these parameters than, for example, the primary (A~) arm spacing and is thus a better indicator of the local solidification conditions. 131 The work presented here will follow and discuss evo
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