The constitution and phase stability of overlapping melt trails in Ag- Cu alloys produced by continuous laser melt quenc
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I.
INTRODUCTION
S O L I D I F I C A T I O N of undercooled melts is a well established method for synthesizing metastable phases. Two approaches, generally, have been employed to produce undercooled melts, rapid quenching, and nucleant isolation. In rapid quenching, large undercoolings can be attained in the melt prior to nucleation of the solid phase. Also, kinetic undercooling at the interface may occur due to the high solidification rates. In nucleant isolation, high undercooling results from dispersing the sample into fine droplets within an inert carrier fluid, thus isolating the active nucleation catalysts. Previous investigations of the solidification of undercooled Ag-Cu alloys have employed the rapid quenching approach. Duwez et al 1 and others 2-6 employed "splat cooling," a technique in which a small volume of melt is quenched by flattening it against a massive heat sink. Elliot et al 7 employed a pulsed Nd-glass laser to melt small volumes at the surfaces of their samples. In this case, the sample serves as: a heat sink; a seed for crystallization of the equilibrium phases; and, a possible catalyst for nucleation of metastable phases. In both the splat cooling and pulsed laser quenching experiments, kinetic undercooling probably occurs due to the high but variable solidification rates. In our investigation of the solidification of undercooled melts of Ag-Cu alloys, a small volume of material at the surface of the specimen is continuously melted and resolidified by moving the specimen at a constant speed under
the beam of a CW CO2 laser. The sample serves the same purposes as in the pulsed laser experiments; however, in contrast to the pulsed laser and splat cooling experiments, solidification occurs at a constant rate. The solidification rate, amount of kinetic undercooling, and the melt and solid compositions at a specific position on the melt-solid interface are determined by the speed at which the specimen is moved under the beam, the laser power, and the substrate temperature. In most conventional casting processes, kinetic undercooling is only a few hundredths of a degree. If, however, the solidification rate is sufficiently high, the amount of kinetic undercooling may be large. Consider, for example, the coupled eutectic growth of a Cu 50 at. pct Ag alloy. According to Tiller, the amount of undercooling required for a specific solidification rate is plotted in Figure 1 (see Appendix VI-A for details of the calculation). 8 Although the plot should be regarded as approximate, it indicates undercooling increases markedly with increasing solidification 1060 1040 1020 1000 980 T(OK) 960 940 920
DAVID G. BECK is Research Assistant, Department of Materials Science, STEPHEN M. COPLEY is Professor, Departments of Materials Science and Mechanical Engineering, and MICHAEL BASS is Professor, Department of Electrical Engineering and Director, Center for Laser Studies. All are with the University of Southern California, University Park, Los Angeles, CA 90007. Manuscript submitted November 25, 1981.
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