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Investigation On Phase Equilibriums, Phase Transformation And Diffusion At Low Cu Side In Al-Zn-Cu Ternary System

Posted on:2006-04-10Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y P RenFull Text:PDF
GTID:1101360155956104Subject:Materials science
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The Al-Zn-Cu ternary system is one of practically important systems among Al-based light alloys, and there is inconsistent acknowledge on miscibility gap of fcc solid solution in this system. If the miscibility gap of fcc solid solution can be correctly obtained, there is practical importance for understanding precipitation of Al alloys containing Zn and Cu during earlier aging after quenching and controlling microstructure and properties of these alloys. In this paper, miscibility gap of fcc solid solution in the Al-Zn-Cu ternary system has been systematically investigated by combining experimental study with thermodynamical calculation. The effect of Cu addition on interdiffusion behavior in Al-Zn alloy has been analyzed by Matano method on the basis of experimental measurement. Also, the influence of metastable phase transformation on microstructure has been studied in Al-Zn alloy containing low Cu. The boundary trend of α1/(α1+α2) has been successfully determined for miscibility gap of fcc solid solution at low Cu side between 277 and 351℃by means of diffusion couple-EPMA technique. It is determined that the solubility of Zn in both α1 and α2 phase decreases with the increasing of Cu. At the same time, the partitioning ratios-K Cuα2/α1 are remarkably more than 1, i.e. Cu is mainly distributed in the α2 phase. It suggests that miscibility gap of fcc solid solution appears divergent, transforming continuously from Al-Zn side to Al-Cu side. It is also found that miscibility gap of fcc solid solution still exists above 351℃at low Cu side in the Al-Zn-Cu ternary system. The Zn content in the α1 phase decreases and the Zn content in the α2 phase increases with the increasing of Cu contens. At the same time, the partitioning ratios-K Cuα2/α1are still markedly more than 1. It means that the existing temperature of miscibility gap of the Al-Zn system is improved due to the addition of Cu. It is concluded from experimental results that the miscibility gap of fcc solid solution is divergent to form connective tunnel space, which is in agreement with thermodynamic calculation, but not convergent to form bell-shaped space as considered previously. Correct acknowledge of the complete shape of miscibility gap of fcc solid solution has been obtained by thermodynamic calculation in the Al-Zn-Cu ternary system. It is clearly indicated that its shape should be connective tunnel space, transforming continuously from Al-Zn side to Al-Cu side, not bell-shaped space as considered previously. The calculated result has been strongly supported by experimental data. It has been shown through thermodynamic analysis and calculation that the whole shape of miscibility gap is well reproduced for the fcc solid solution in the Al-Zn-Cu system by means of SSOL data, which can not be obtained by thermodynamic parameters of Wisconsin. This is because the ternary interactive parameter is so large in Wisconsin data that there is rose-shaped miscibility gap inconsistent with experimetnal one, such as the trend of α2/(α1+α2) boundary at 300340℃. Miscibility gap island still exists in the broad range of temperatures and compositions in the Al-Zn-Cu ternary system even if the calculation is carried out only by the ternary parameter which is just introduced to simulate phase equilibrium relationship at higher temperatures. It has shown from calculated results obtained by either SSOL data or Wisconsin parameters that there is metastable miscibility gap of fcc solid solution in Al-rich corner of Al-Cu binary system. Its top temperature is about 500℃and the corresponding composition is 13.5at%Cu. Besides, it is found that the Al content in βphase increases due to the addition of Cu. And the partitioning ratios-βα2KC u are clearly more than 1, i.e. Cu is mainly distributed in the βphase, when equilibrium composition between α2(α) and βphase is measured at 260360℃. There is only equilibrium relationship between αand βphase under 277℃, and the Zn content in αphase increases with the increasing of Cu contents. After diffusion couples were annealed at 260360℃and concentration profiles of Zn were measured by EPMA, it has been first quantitatively determined by Matano method that interdiffusion coefficient in fcc solid solution containing high Zn contents...
Keywords/Search Tags:Al-Zn-Cu ternary system, phase diagram, fcc solid solution, miscibility gap, diffusion couple-EPMA technique, phase equilibrium composition, thermodynamic calculation, Matano method, interdiffusion coefficient, metastable phase transformation
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