The microstructure of metastable phases in Ag-Cu alloys generated by continuous laser melt quenching
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BY "splat cooling," Duwez et al were the first to produce metastable extended solid solutions in the Ag-Cu system. ~Subsequently, the Ag-Cu system has often been used in the examination of new techniques for non-equilibrium phase formation. It exhibits a simple eutectic phase diagram (Fig. 1), with the supersaturated solid solutions' lattice parameters showing an almost linear dependence on composition? Recently, Elliot et al produced metastable extended solid solutions in Ag-Cu alloys by self-substrate_ quenching small regions melted with a pulsed Nd:glass laser. 2 We have previously demonstrated the feasibility of forming continuous trails of metastable extended solid solutions melted by scanning a CW CO 2 laser beam along the surface of Ag-Cu alloy specimens? In this paper, we present the results of a detailed investigation of the constitution and microstructure of continuous melt trails in Ag-Cu alloys containing 25, 50, and 75 at. pct Ag. These compositions were chosen so that a direct comparison could be made with the microstructures observed in earlier experiments employing a pulsed Nd: glass laser. 2 SAMPLE P R E P A R A T I O N A N D OPERATING CONDITIONS Starting with 99.995 pct purity silver and 99.9999 pct purity copper, three alloys were formed; 25, 50, and 75 at. pct Ag. The samples were repeatedly mixed with an arc button melter to promote homogenization, turning over the cast buttons before each individual melting process was begun. The buttons were cold rolled and then annealed at 725 ~ for approximately one month. A low speed abrasive saw was employed to slice individual specimens. To enhance absorption, the scanned surfaces were coated with a trichloroethane DAVID G. BECK is Research Assistant, and S. M. COPLEY is Professor, Department of Materials Science, University of Southern California. M. BASS is Director, Center for Laser Studies, University of Southern California, University Park, Los Angeles, CA 90007. Manuscript submitted December 29, 1980.
suspension of graphite. Prior to coating, the top surfaces were polished in order to insure uniform surface roughness. Full details of sample preparation were previously presented. 4 The laser, operating at a wavelength of 10.6/~m in the TEMoo mode, had an output power of 1250 W and an incident power of 1050 W. After coating, the samples absorbed an average of 26 percent as calculated using a melting efficiency approach. Focussing with a 63.5 mm focal length lens, an average power density of 13.4 M W cm-2 was achieved. Sample translation rates ranged from 10 to 115 cm s -I with an inert atmosphere being employed to prevent oxidation reactions from occurring. The laser beam was oriented at normal incidence to the samples. MICROSTRUCTURE Figure 2 (a) through (c) show cross-sectional views of a trail in each composition range. A low speed abrasive saw was used to section the trails which were then mechanically polished down to a finish of 0.3/tm alumina. The etching solution was made up of 7.6 gm CrO 3, 5 ml H2SO4, and 1 liter HE0, which had previously
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