13. A. Manzano, E. Nava, and J. Beech: Trans. 16th Int. Die Casting Congress and Exposition, Detroit, MI, 1991, pp. 153-59. 14. E. Nava, A. Manzano, J. Torres, and M. Mendez: AMT '92 Advanced Materials and Technologies, Proc. Conf., Warsaw, Sept. 23-25, 1992, p. 231.

(a) r-~ Cooauot,ng m

Ph.,.

(b)

Insulating Phase

Fig. 10--Schematic representation of the possible impediment to electron flow by (a) a fine structure and (b) a coarse structure.

Nevertheless, it should be noted in Figures 1 and 7 and Tables I and II that the changes in the electrical conductivity values are not of orders of magnitude, indicating conduction in the metallic regime of a percolation curve. Furthermore, due to the determined chemical composition of second-phase particles, it is believed that the aluminum silicon eutectic phase acts as the best insulating phase. In conclusion, the changes in electrical conductivity in aluminum alloys can be correlated to the characteristics of the metallurgical structure, i.e., modification of the eutecfic phase and refinement of the structure through either the addition of a grain refinement element or high solidification rates. However, since these changes are not of orders of magnitude and knowing that the chemical composition of the conducting phase does not change, the changes in those properties are ascribed to the size and distribution of the second-phase particles. Nevertheless, it has been observed that the changes are enough to indicate different microstructural conditions; therefore, it is believed that the measurement of either electrical conductivity or resistivity can be used as a reliable method to assess metal quality.

The authors wish to acknowledge A.R. Chan for the SEM analysis and Dr. M. Mendez for providing laboratory facilities.

REFERENCES I. J. Charbonnier, J. Morice, and R. Partalier: Int. Cast Met. J., 1979, vol. 4 (3), pp. 39-44. 2. D. Apelian, G.K. Sigworth, and K.R. Whaler: AFS Trans., 1984, vol. 92, pp. 297-307, 3. J. Charbonnier: AFS Trans., 1984, vol. 92, pp. 907-23. 4. Thermal Analysis of Molten Aluminum, Proc. AFS Conf., Rosemont, IL, 1984. 5. International Molten Aluminum Processing, Proc. AFS Conf., City of Industry, CA, 1986. 6. H. Oger, B. Closset, and J.E. Gruzleski: AFS Trans., 1983, vol. 91, pp. 17-20. 7. D. Argo, R.A.L. Drew, and J.E. Gruzleski: AFS Trans., 1987, vol. 95, pp. 455-64. 8. M.H. Mulazimoglu, R.A.L. Drew, and J.E. Gruzleski: Metall. Trans. A, 1987, vol. 18A, pp. 941-47. 9. M.H. Mulazimoglu, R.A.L. Drew, and J.E. Gruzleski: Metall. Trans. A, 1989, vol. 20A, pp. 383-89. 10. M.H. Mulazimoglu and J.E. Gruzleski: Modern Casting, 1990, vol. 80 (1), pp. 29-31. 1 I. T.J. Hurley: AFS Trans., 1986, vol. 94, pp. 159-72. 12. A. Manzano, E. Nava, E. Carrasco, and J. Mendez: CINVESTAVIPN Unidad Saltillo, Saltillo, Coahuila, MExico, unpublished research, 1993. 2358--VOLUME 24A, OCTOBER 1993

Synthesis of TiC Particulates and Their Segregation during Solidification in In Situ Processed AI-TiC Composites M.K. PREMKUMAR and M.G. CHU High modulus aluminum materials are being increasingly sought after by designers for many aerospace and automotive applications, both from the point of view of the ability to save weight by down-gaging, as well as other mechanical design limitations. A class of materials that has the ability to provide the required property improvements are metal matrix composites (MMCs), and consequently, these materials are receiving a lot of attention in the research community. In particular, particulate-reinforced MMCs, also known as discontinuously reinforced aluminum (DRAs), are of interest due to their ease of fabrication, lower costs, and more isotropic properties. Of the several ways of producing these materials, the technologies where the particles are introduced or formed in the molten aluminum prior to its solidification are more attractive, primarily due to their potential of being commercially economic processes on a large scale. However, solidification of melt-containing particulates subsequent to casting, poses an interesting challenge in controlling the final microstructure, as a result of the interaction between the solidifying front and the suspended particulates. Assuming that the initial distribution of particulates in the molten metal is homogeneous, the solid particles can be either trapped by the advancing solidification front, thus globally retaining a homogeneous distribution in the cast product, or rejected by the front, resulting in segregation of the particulates to specific locations, such as the intercellular/interdendritic regions. Subsequent deformation of a cast product with nonuniform particulate distribution would result in a final microstructure consisting of bands of reinforcement particles which would have adverse effects on damage tolerance properties, such as fracture toughness. Thus, it is vital to understand this phenomenon and thereby control it favorably. This is necessary for the commercial success of MMCs, an important class of emerging new materials. This communication introduces a novel technology M.K. PREMKUMAR, Staff Engineer, Fabricating Technology Division, and M.G. CHU, Technical Specialist, Molten Metal Processing Division, are with Alcoa Technical Center, Alcoa Center, PA 15069. Manuscript submitted May 12, 1993. METALLURGICAL TRANSACTIONS A