Homogeneous nucleation kinetics of Al 3 Sc in a dilute Al-Sc alloy
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I.
INTRODUCTION
M O N O L I T H I C A1 alloys tend to achieve commercially useful mechanical property levels mainly through the agency of precipitation reactions, a number of which result in the formation of ordered intermetallic or intermediate phases. The most beneficial combinations of strength and toughness often are derived from a microstructure in which the product phase is uniformly distributed in small, coherent particles that are resistant to dislocation shearing, i . e . , a r e elastically hard. One of the most effective means by which such a microstructure may be generated is through homogeneous nucleation and growth. This process also represents the simplest type of nucleation event to treat theoretically, because an adequate description of the boundary orientation-dependent interactions between the nucleus and the various defects that occur in the parent phase is not yet available for estimating the work of formation of a heterogeneously formed critical nucleus. A review of the literature on comparisons of homogeneous nucleation theory and experiment in solids has been given by Aaronson and LeGoues LIj in these conference proceedings. They report that very little information has as yet been published for homogeneous nucleation kinetics in face-centered cubic (fcc) aluminum solid solutions and that many of the studies performed in other systems suffered from the influence of the overlap of diffusion fields associated with adjacent precipitates and of coarsening, thereby making interpretation of the data problematic. Particularly, in view of the commercial importance of nucleation in aluminum alloys, a study of the nucleation kinetics of A13Sc in a dilute A1-Sc alloy was undertaken in part because A1-Sc alloys have become technologically interesting, but also because this can be a good model system for the study of diffusional nucleation of an ordered precipitate phase
R.W. HYLAND, Jr., Senior Scientist, is with the Alloy Technology Division, ALCOA Technical Center, ALCOA Center, PA 15069-0001. This paper is based on a presentation made in the "G. Marshall Pound Memorial Symposium on the Kinetics of Phase Transformations" presented as part of the 1990 fall meeting of TMS, October 8-12, 1990, in Detroit, MI, under the auspices of the ASM/MSD Phase Transformations Committee. METALLURGICAL TRANSACTIONS A
within a disordered solid solution. The Al-rich portion of the A1-Sc phase diagram is shown in Figure 1. The present article reports the experimental data obtained on nucleation kinetics and on the average values of the nucleus:matrix interfacial back-calculated from these data through classical homogeneous nucleation theory. An independent calculation of the boundary orientation dependence t261of the energy of coherent boundaries between A13Sc and its fcc A1-Sc matrix (and thus of the critical nucleus shape) is presently in progress using the embedded atom method of Daw and Baskes. ~2]
II.
EXPERIMENTAL PROCEDURE
Ingots measuring 5 • 76 • 76 mm of three alloys containing 0.05, 0.11, and 0.16 at. pct
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