Materials Science and Engineering, A 179/A 180 (1994) 163-167 163 Microstructural developments in rapidly solidified monotectic alloys R. Goswami and K. Chattopadhyay Department of Metallurgy, Indian Institute of Science, Bangalore560 012 (India) Abstract The origin of the finely dispersed second-phase morphology observed in rapidly solidified near-monotectic alloys of liquid-immiscible systems has been discussed. It is shown that two competing mechanisms, namely ( 1 ) phase separation in the liquid state followed by trapping of the dispersed liquid by the growing solid and (2) massive solidification followed by solid state decomposition, exist. A combination of thermodynamics and homogeneous nucleation kinetics can predict the possible origin. This is confirmed by analysing the symmetry of the shape of the dispersoids and invoking the principle of the intersection group. It is also shown that in the case of liquid trapping, a simple analysis based on Ulmann et al.'s work can predict the hierarchy of ease of trapping of the dispersed liquid by different matrices. 1. Introduction A reasonable number of studies of microstructure development in miscibility gap alloys around mono- tectic composition under rapid solidification con- ditions exist [1-6]. In general, rapid solidification is reported to promote a random dispersion of the second phase [7]. Recently the possibility of producing nanodispersed materials through this route has been discussed [8]. A careful evaluation of rapidly solidified monotectic microstructures, however, reveals several distinctive features which depend on the system as well as the processing conditions. For example, several alloys exhibit denuded zones near grain boundaries [2, 9[. The fine dispersoids also have distinctive shapes and precise orientation relations [3, 5, 6]. Understand- ing the origin of these features is of vital importance to enable control of the microstructures. The aim of this paper is to analyse the conditions for the development of these monotectic microstructures. 2. Experimental details Alloys of various compositions of the binary systems Zn-Pb, Zn-Bi, AI-Pb, Al-In and Cu-Pb near mono- tectic composition were rapidly solidified using the melt-spinning technique. The rapidly solidified ribbons were characterized using transmission electron micros- copy (Jeol 2000 FX-II). 3. Rapidly solidified monotectic microstructure In this section we present the observed microstruc- tures of few selected monotectic systems. Representa- tive microstructures in two alloys, namely Zn-10wt.%Pb and Al-10wt.%Cd, are shown in Fig. 1. From the micrographs both bimodular and uniform distributions are observed. The scale of the microstruc- tures can be reduced by employing processing condi- tions favourable for a higher cooling rate. The scale also depends on the alloy system. For example, the dispersoids in A1-Cd are always fine, while Zn-Pb yields a relatively coarse morphology. The average sizes of dispersoids obtained in a large number of monotectic alloys are summarized in Table 1. The dispersoids are faceted and exhibit a well-defined polyhedral shape which is unique to a given system. Figure 2 gives a selection of the particles exhibiting this feature. They also have a precise orientation relation with the matrix. Table 2 summarizes the orientation relations and shapes observed in a variety of mono- tectic systems during the course of the present investi- gation. In some of the systems a dispersion-free zone similar to a precipitate-free zone can be clearly seen near grain boundaries. Figure 3 shows a typical example from the Al-In system. Thus the rapidly solidified dispersed second-phase microstructures in near-monotectic alloys exhibiting liquid miscibility contain a variety of features. Understanding of these requires knowledge of the conditions needed for their 0921-5093/94/$7.00 © 1994 - Elsevier Sequoia. All rights reserved SSDI 0921-5093(93)05688-L