Microstructure Evolution in rapidly solidified immiscible alloys
Final Report Abstract
When a hypermonotectic alloy is cooled from the single-phase liquid state through the miscibility gap, liquid-liquid decomposition begins with the nucleation of the liquid minority phase in the form of droplets, which grow by diffusion, can settle due to gravity or migrate due to a temperature gradient, collide and coagulate thereby. On reaching the non-variant monotectic reaction temperature the matrix liquid decomposes into a solid and a second phase liquid being not distinguishable from the liquid minority phase precipitates produced in the miscibility gap. The size spectra of the droplets of both processes merge the larger the cooling rate. Within a suitable interval of cooling rates they become, however, discriminable and then the spectra stemming from the liquid-liquid decomposition gives unique access to the nucleation process inside the miscibility gap. The project investigated with AlPb and AIBi alloys of different hypermonotectic compositions the demixing process inside a miscibility gap. Special cooling devices were designed such that alloy samples could be solidified with cooling rates ranging from 10K/S to 1000 K/s. The microstructure of the alloys was analyzed in great detail using SEM pictures allowing to statistically analyze per sample around 20000 to 50000 droplets and there from to discern between Pb and Bi drops originating from passing the miscibility gap and those stemming from the monotectic reaction. The experimental results for AlPb alloys suggest that nucleation and diffusional growth are the essential processes occurring. Therefore the average droplet size varies as the inverse square root of the cooling rate. The size distribution of the droplets stemming form the monotectic reaction can be described as a truncated power law of the precipitate diameter. The drops orginating from the liquid-liquid demixing can be described by a log-normal distribution. Adding both with suitable weighting factors allows to describe the observed size spectra in the sections. AIBi alloys behave principally similar to AlPb alloys (the volume fraction of liquid precipitates is much larger than in AlPb). The procedures for de-convolution of the size distribution could be applied and lead to the same functional description of the size spectra for both processes. The difference, however, is that no clear relation exists between cooling rate and average droplet size. X-ray tomography on two samples processed with different cooling rates show unusual features appearing in the AIBi alloy samples (large ball shaped volume elements free of Bi and Bi drops in the shape of a contact lense). Their physical origin could not be clarified till now. Although the project is finished, the research work will continue to clarify the obvious difference between AlPb and AIBi and the strange microstructural features. Our Chinese co-operation partner performed extensive numerical modlling on the pahse separation. Based on a jointly developed physical model that included homogeneous nucleation of the liquid precipitates, diffusional growth, coarsening (curvature dependent solubility), Stokjes and Marangoni motion, coagulation of precipitates due to both drop motiosn and Brownian motion, predictions were made for the size distribution and the average drop radius as varying with cooling rate. For the AlPb alloys a good agreement between theoretical modeling and experimental results could be achieved, The AIBi alloys are still under evaluation theoretically. The numerical model was also applied to alloys exhibiting a metastable miscibility gap like CuCo and CuFe with good success.
Publications
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J. Z. Zhao, J. He, Z. Q. Hu, LRatke Microstructure evolution in immiscible alloys during rapidly directionally solidification Z.Metallkde. 95 (2004) 363-368
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J. Z. Zhao, L L Gao, Jie He, L Ratke Effect of Brownian Coagulation on the Liquid-liquid Decomposition in Gas-atomized Alloy Drops J. Mater. Sei. Technol. 22 (2006) 321 -323
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J. Z. Zhao, M. Kolbe, L L Gao, JR Gao, L Ratke Formation of the Microstructure in a Rapid Solidified Cu-Co Alloy Metallurgical and Materials Transactions A, 38A, (2007) 1162 - 1168
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J. Z. Zhao, Z. Q. Hu, LRatke Modeling of the liquid-liquid phase transformation of immiscible alloys Acta Metall. Sinica 40 (2004) 27-30
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J. Z. Zhao, Z. Q. Hu, LRatke Modeling of the Solidification of Immiscible Alloys, in: Solidification and Crystallisation, D. Herlach (Ed.), Wiley-VCH, Weinheim 2004, 44 - 51
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J. Zhao, LRatke Modeling of the Microstructure Evolution in Continuously Cast Al-Pb Alloys J.Mat.Proc.Tech. 18(2002)306-310
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J. Zhao, LRatke. J. Jia, Q. Li Modeling and Simulation of the Microstructure Evolution during Cooling of Immiscible Alloys in the Miscibility Gap J.Mat.Sci.Tech. 18 (2002) 197 - 205
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J.Z. Zhao, L. Ratke A model describing the microstructure evolution during a cooling of immiscible alloys in the miscibility gap Scripta Mat. 50 (2004) 543 - 546
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Jie He, Jiu Zhou Zhao, L Ratke Solidification microstructure and dynamics of metastable phase transformation in undercooled liquid Cu-Fe alloys, Acta Materialia 54 (2006) 1749-1757
