The Effect of Rapid Solidification Velocity on the Microstructure of Ag-Cu Alloys
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INTRODUCTION
RAPIDLY solidified crystalline alloys are well known to exhibit refined scales of microsegregation and interdendritic second phase particles. In some cases microsegregation and interdendritic phases can be completely eliminated by rapid solidification. These microsegregation-free structures can occur in alloys with compositions beyond the equilibrium solubility limit of the primary phase and provide an ideal starting microstructure for subsequent thermomechanical processing. The splat-quenching experiments of Duwez, Klement, and Willens ~ showed by X-ray diffraction that complete solubility of Ag and Cu could be obtained in the solid state at all compositions despite the presence of a solid miscibility gap and eutectic reaction in the equilibrium phase diagram. Subsequent TEM examination2'3 showed that splat quenched Ag-Cu solidified from the melt as a single solid phase free of microsegregation. However, for concentrated alloys spinodal decomposition of this structure was difficult to avoid in rapid solidification experiments. Other reports of solubility extension are common, especially in aluminum based alloys. 4 Frequently these alloys exhibit a cellular microsegregation pattern5 but are occasionally found also to be free of microsegregation. 6 The present paper addresses the solidification requirements to produce microsegregationfree crystalline structures. Experiments which pinpoint the rapid solidification conditions required to produce changes in the microstructure of alloys are difficult to perform because of the unknown aspects of the nucleation temperature in many rapid solidification techniques. Surface melting and resolidification experiments using lasers or electron beams permit better control over solidification velocity than other rapid solidification techniques. Surface melting experiments involve no bulk undercooling due to the presence of the alloy W.J. BOETTINGER, R.J. SCHAEFER, and E S. BIANCANIELLO are in the Metallurgy Division, Center for Materials Science, National Bureau of Standards, Washington, DC 20234. D. SHECHTMAN is in the Department of Materials Engineering, Technion, Haifa, Israel, on leave at the Center for Materials Research, The Johns Hopkins University, Baltimore, MD 21218. Manuscript submitted June 24, 1983.
METALLURGICAL TRANSACTIONS A
substrate. Also, because the liquid is contained in its own solid, a knowledge of heat transfer coefficients is not required. This fact greatly increases the accuracy of heat flow models which can be used to determine the imposed solidification velocities] Surface melting experiments on alloys have provided information on the refinement of the dendritic structure and changes in the microsegregation pattern. 8'9 Few have dealt with the formation of microsegregation-free structures. Elliott, Gagliano, and Krauss, ~~using a pulsed laser to melt a spot, showed that supersaturated solid solutions of Ag-Cu could be formed. The solutions, however, contained a rather unusual structure consisting of bands formed parallel to the local
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