Eutectic spacing selection in lead-based alloy systems
- PDF / 1,126,533 Bytes
- 11 Pages / 630 x 792 pts Page_size
- 65 Downloads / 225 Views
I.
INTRODUCTION
T H E development of controlled eutectic structures by the directional solidification technique is important from the technological viewpoint, since eutectic structures generally give rise to a fine microstructural scale and thus exhibit superior mechanical properties. Besides the nature of the phases formed, the mechanical properties of a given eutectic alloy largely depend upon the eutectic spacing and the volume fraction of the phases. Thus, it is important to establish a proper relationship which connects these two variables with the critical processing parameter, i.e., the growth rate of the alloy. Several theoretical models have been proposed in the literature, ll-~2l the most detailed being that by Jackson and Hunt (JH). 151These theoretical models, based on the solute transport in the liquid and capillarity effects, give a relationship between the interface undercooling and the eutectic spacing for a given growth rate, as shown schematically in Figure 1. Experimental studies on regular eutectic growth, on the other hand, show that of the large range of possible spacings, only a narrow band is actually selected by the system at a given growth rate. [13-161 The existence of a finite band of spacings was first rationalized on the basis that of all the possible eutectic spacings, only a small range of spacing values would give stable steady-state growth. In earlier models, [2,3,41it was assumed that this stable spacing corresponded to the spacing, A,,, that gave minimum interface undercooling. Jackson and Hunt, I5[ following Cahn's suggestion, qualitatively showed that spacings less than A,, would be unstable. They also discussed that spacings larger than some
R. TRIVEDI and J.D. VERHOEVEN, Senior Scientists and Professors, Materials Science and Engineering, and J.T. MASON, Associate Metallurgist, are with Ames Laboratory, United States Department of Energy, Ames, IA 50011. W. KURZ, Professor and Head, Department of Materials Engineering, is with the Swiss Federal Institute of Technology, CH-1007 Lausanne, Switzerland. Manuscript submitted December 27, 1990. METALLURGICAL TRANSACTIONS A
critical value, AM, would also be unstable, and they proposed that this maximum stable spacing would correspond to the value where the eutectic interface for the large volume fraction phase acquires an infinite slope. These regimes of stable and unstable eutectic spacings are shown in Figure 1. More detailed stability analyses of the eutectic interface [9-121 have confirmed that spacings A < Am will be unstable. Critical experimental studies by Jordan and Hunt [151in a metallic system and by Seetharaman and Trivedi 116[ in a transparent organic system have clearly shown that the eutectic spacing selection, even in regular (nonfaceted/ nonfaceted) systems, is not very sharp, and the latter authors have determined distributions in spacings at different growth rates. This distribution in spacing is indeed consistent with the JH analysis, since the observed spacings fall in the stable regime shown in Figure 1. Ho
Data Loading...
