Analysis of thermoelectric power measurements in the study of precipitation kinetics in 3003 Al alloy
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EXPERIMENTAL study of the non-heat-treatable 3003 aluminum alloy has been done by a number of researchers using different techniques. Some have used indirect methods,[2,3] and others have used direct ones.[5,6] In each case, they characterized the effect of the main impurities, Mn, Fe, and Si, on the microstructure and their role on the recrystallization[3,7] and precipitation processes[1,2,8] that occur when the alloy is subjected to different thermomechanical treatments. In a recent work,[1] we have studied the isothermal kinetics in the 3003 Al alloy by thermoelectric power technique. In that work, independent of the thermal treatment used, there is evidence of the existence of an equilibrium phase at temperatures about 540 7C, which, when compared against the findings of other authors, could be identified as (MnFe)Al6. At temperatures in the neighborhood of 400 7C, depending on the thermal treatment, the a-(MnFe)3SiAl12 phase could be identified. Also, for strained samples, there seems to exist at low temperatures a metastable phase of lower activation energy than that of the stable phases previously mentioned. Although the results in Reference 1 are not theoretically analyzed, the activation energy for each phase is obtained. Theoretical analysis of the kinetics thus obtained have had little attention in the specialized literature, and this analysis was always made under the premise that only a unique transformation process occurred. That problem has been studied in the reaction rate theory using the Johnson– Mehl–Avrami (JMA) functions.[4,8,9] However, we know that this way of tackling the problem can lead to erroneous results, mainly when different phases coexist simultaneously. NEY JOSE´ LUIGGI, Professor, is with the Department of Physics, Universidad de Oriente, Cumana´, 6161 Venezuela. Manuscript submitted September 11, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS B
Luiggi and Betancourt,[10,11] in their works on reactions rate kinetics, developed equations to follow the decomposition of a solid solution to reach the equilibrium state, by following different paths of atomic exchange between the possible existing phases. This allows them to propose a general equation where a kernel of precipitation is assigned to each existing phase. We define precipitation kernels as the mathematical functions that solve the differential equation for the isothermal evolution of the precipitated fraction Y in a monophase system. Each kernel is identified with those obtained parametrically in the rate reaction theory. In order to evaluate the basic characteristics of the precipitated phases for the 3003 Al alloy after it has been heat treated, this work combines the results of our previous one,[1,10,11] taking into account our models of kernel precipitation for both the JMA and Fujita–Damask (FD) types. The organization of this article is as follows: Section II presents a view of the experimental aspects. Section III states the theoretical aspects heaving a bearing upon the deconvolution of total kinetics. Section IV
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