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I.

INTRODUCTION

THE microstructural characteristics of undercooled and rapidly solidified Ag-Cu alloys have been extensivel;: investigated in the past. Duwez et al. lll reported a comPIete solubility of Ag and Cu in the solid state at all compositions. These investigators used the gunquenching process for rapid solidification (RS), and the samples were analyzed by an X-ray diffraction technique. Nagakura et alYl have termed this metastable solid solution phase as 1" to distinguish it from the equilibrium a and {3 phases of this alloy. When the splat quenching was less effective, metastable silver-rich a' and copper-rich {3' phases appeared concurrently with the 1" phase. Subsequent transmission electron microscope (TEM) examination of splat-quenched foils by Stoering and Conard[3] and Boswell and Chadwick(4 ) indicated a solid solution phase free of microsegregation in a wide range of composition of this alloy. However, the spinodal decomposition of the metastable phase was difficult to avoid for the concentrated alloys. Stoering and Conard[3] obtained yet another metastable solid solution phase i' which was different from 1" reported earlier by Nagakura et al yl In thicker regions of the foil, only the metastable a' phase was observed without evidence for the existence of y' and i' phases. The copperrich {3' phase was detected only during the decomposition of solid phase produced by splat quenching. It is not clear whether in all of the preceding cases these structures form from one another or whether they are nucleated independently. However, a decrease in the O.P. PANDEY, Lecturer, is with the Department of Materials Science, Thapar Insitute of Engineering and Technology, Patiala14700 I, India. N. S. MISHRA, Research Manager, is with the Research and Development Center, SAIL, Ranchi-834002, India. C. RAMACHANDRA, Reader, and S. LELE and S.N. OJHA, Professors, are with the Center for Advanced Study, Department of Metallurgical Engineering, Banaras Hindu University, Varanasi221005, India. Manuscript submitted August 2, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A

cooling rate during quenching or an increase in annealing temperature in their experiments led to the formation of structures closer to those existing under equilibrium conditions. The formation and stabilization of metastable phases have been understood in the light of undercooling induced in the melt during the rapid quenching process. [S.6] It has been shown that at a large undercooling of the melt, a metastable phase may nucleate and grow, preferentially over a stable phase. Because a quantitative measurement of undercooling is difficult in RS processes, Prasad et al.[7l employed the mushy-state quenching technique to quantify the effect of quenching on microstructural evolution in Ag-Cu alloys. In their experiment, a hypoeutectic Cu-13 pct Ag alloy* was

*All

compositions will be given in atomic percent.

quenched from the two-phase liquid-solid region. The undercooling of the melt was estimated from the phase analysis to be 233 K below the l