• PDF / 733,531 Bytes
• 5 Pages / 612 x 792 pts (letter) Page_size
• 23 Downloads / 239 Views

DOWNLOAD

REPORT

---

of approximately 270 kJ/mol. This activation energy is no longer significantly different from the activation energies obtained for alloys A2 and A3 and is of the order of the activation energy of interdiffusion in dilute Ni-Al alloys, 234 to 268 kJ/mol.[6] Assuming, however, that the longrange diffusion of Mo is rate controlling, the same procedure would yield a higher true activation energy of approximately 320 kJ/mol. We note that this value is still larger than the 272 to 284 kJ/mol activation energies reported for interdiffusion in the binary dilute Ni-Mo alloys.[6] Microstructural coarsening data have been obtained for three low-volume fraction, two-phase (g 2 g ') Ni-Al-Mo alloys in the temperature range 775 7C ≤ T ≤ 903 7C. The cube of the average particle radius was found to depend linearly on time for the aging times and temperatures studied. The coarsening rate was observed to decrease monotonously with increasing Mo content of the alloy. The experimentally determined apparent activation energies of coarsening for alloys containing 5 and 8 at. pct Mo were 276 5 46 and 276 5 25 kJ/mol, respectively, whereas for an alloy containing 13 at. pct Mo, a significantly higher apparent activation energy of 360 5 33 kJ/mol was obtained. The increased apparent activation energy of coarsening in the high Mo alloy is considered strong evidence that a change in the rate-controlling mechanism of coarsening from long-range diffusion of Al to long-range diffusion of Mo occurred in this series of alloys. This conclusion is based on a comparison of the activation energies of interdiffusion with those of the respective binary systems. It is further supported by an estimate of the contribution of each alloying element to the overall coarsening resistance of the alloy.

We are indebted to W.H. Hort, J.M. Howe, J.E. Morral, J.A. Smith, and P.W. Voorhees for several discussions of METALLURGICAL AND MATERIALS TRANSACTIONS A

this work and for making unpublished material available to us. This work has been supported by the Division of Materials Science, Department of Energy, through Grant No. DE-FG05-93ER45507 (WCJ), by the National Science Foundation through Grant No. NSF-DMR9258297 (TMP), and by the Alexander von Humboldt Foundation, Germany (MF).

REFERENCES 1. M. Fa¨hrmann, P. Fratzl, O. Paris, E. Fa¨hrmann, and W.C. Johnson: Acta Metall. Mater., 1995, vol. 43, pp. 1007-22. 2. J.A. Smith: preprint of unpublished work, 1992. 3. C.J. Kuehmann and P.W. Voorhees: Metall. Mater. Trans. A, 1996, vol. 27A, pp. 937-44. 4. A. Umantsev and G.B. Olson: Scripta Metall. Mater., 1993, vol. 29, pp. 1135-40. 5. J.E. Morral and G.R. Purdy: Scripta Metall. Mater., 1994, vol. 30, pp. 905-10. 6. G.E. Murch and C.M. Bruff: in Landoldt-Bornstein, Numerical Data and Functional Relationships in Science and Technology, H. Mehrer, ed., Springer, Berlin, 1990, vol. 26, p. 279. 7. D.B. Miracle, V. Srinivasan, and H.A. Lipsitt: Metall. Trans. A, 1984, vol. 15A, pp. 481-86. 8. V. Biss and D.L. Sponseller: Metall. Trans., 1973, vol. 4, pp. 195360. 9. W.T. Lo