Surface Energy Reduction In Fibrous Monotectic Structures
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I.
INTRODUCTION
G R O W T H of fibrous structures has been the subject of considerable interest for the past three decades. [1-4] Usually, fibrous structures can be produced in nonfaceted/ nonfaceted (nf/nf) eutectics, but they can also be produced in monotectic systems under the proper compositional and growth conditions. [5-9] Both types of systems are distinguished by an isothermal decomposition of a liquid phase into two different phases. In the eutectic reaction, the resulting phases are both solid, whereas in the monotectic reaction, one of the phases is a liquid. An approximate analytical description of fibrous, rodlike eutectic growth was developed by Jackson and Hunt in 1966. [1] They showed that the interrod spacing could be described by A2V = "constant" under extremum conditions of minimum undercooling and maximum velocity. This relationship was later confirmed by Nash [1~ using a different theoretical approach. An analysis by Derby and Favier t9] showed that monotectic solidification also obeyed the A2V = constant relationship developed by Jackson and Hunt [l[ for n f / n f eutectic growth. In all of these analyses, [1-1~ however, only the growth of the regular, periodic fibrous array has been investigated. No discussion has been made concerning the grain boundary structure in eutectic or monotectic systems. The grain boundaries are an important part of the microstructure, and We report here a morphology in which it appears that the grain boundary length is not minimized, as is the normal case, but is elongated by the development of a highly curved, sinuous morphology. These curved boundaries have been observed experimentally in several eutectic and monotectic systems, such as the C u - C u 2 0 , [111 C u - C u 2 S , [12] U O 2 - W , [131 and Z r O 2 - W [131 e u tectic systems and the Al-In [8A4-161 and C u A 1 - P b [17] monotectic systems. In each case, the grain boundaries were decorated with lenses of the minority phase (actually rods with lens-shaped cross sections) and surrounded by a denuded zone depleted of rods, as shown in Figure 1. These highly curved boundaries seem to contradict the assumption that most systems tend to minimize the inter-
facial energy by a decreasing interfacial area. Therefore, in order to determine whether these curved boundaries actually reduced the overall energy of the system, an analysis of the CuA1-Pb grain boundary morphology was undertaken. II.
A. Alloy Preparation The pure components necessary to produce samples 7.5 cm in length and 0.6 cm in diameter were placed in closed-end fused silica ampoules, which were evacuated, backfilled with 2 x 104 Pa of argon, and sealed. The alloy-filled ampoules were heated to the single liquid phase in an induction furnace (approximately 1350 ~ and held for 5 minutes to homogenize the alloy. The samples were then inverted and the process repeated to ensure complete alloying.
B. Directional Solidification of Experimental Alloys Samples were placed in fused silica crucibles under an argon atmosphere and positioned so that the
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