Gravity-induced sedimentation during melting and liquid phase sintering of Pb–Sn alloys

1359-6454/03/$30.00 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. doi:10.1016/S1359-6454(02)00428-7
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Y. Du et al. / Acta Materialia 51 (2003) 445–456
Corresponding author. Tel.: +1-906-487-2630; fax: +1906-487-2934. E-mail address: szlu@ (S.Z. Lu).
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resulting stress indicate that creep rates are many orders of magnitude below those required to account for observed sedimentation rates [2]. A second mechanism—termed particle extrication— considers sedimentation a consequence of particle coarsening [3]. It is suggested that during coarsening a particle occasionallys work itself free from the solid skeleton whereupon it falls (if its density exceeds that of the liquid) a distance comparable to the interparticle spacing before resintering to the skeleton. Repetition of the process leads to sedimentation of the solid-plus-liquid region (the mushy zone). Particle extrication has been observed at mushy zone-liquid interfaces and extrication rates measured for this situation [3]. While it is clear that extrication can take place at such interfaces, extrication in the mushy zone interior
Abstract Gravity-induced phase separation during melting of Pb–Sn alloys results in a macroscopic concentration gradient that must be eliminated before phase equilibrium is established. When Pb–Sn alloys are heat treated in solid-plus-liquid regions (akin to liquid phase sintering), recession of the mushy (solid-plus-liquid) region accompanies elimination of the concentration gradient. Sedimentቤተ መጻሕፍቲ ባይዱtion apparently ceases as phase equilibrium is approached. Some solid-plus-liquid alloys do not sediment. These have a solid volume fraction sufficient to sterically hinder the gravity-induced phase separation. This critical volume fraction is about 0.50 for hypoeutectic (Pb-rich) alloys and about 0.75 for hypereutectic (Sn-rich) alloys. 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.
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Department of Materials Science and Engineering, Michigan Technological University, Houghton, MI 49931, USA Received 23 July 2002; received in revised form 13 September 2002; accepted 13 September 2002
Keywords: Semi-solid processing; Percolation; Liquid phase sintering
1. Introduction Solid phase sedimentation often takes place in interconnected solid-liquid mixtures. Since necks exist between the solid particles in such structures, their sedimentation is somewhat perplexing. Two mechanisms have been previously suggested to explain solid sedimentation in interconnected structures. One invokes deformation/creep of the solid skeleton as a consequence of the self-weight of the skeleton plus the stress associated with any liquid head above it [1]. However, estimates of the
is likely to occur at a lesser rate (the solid phase contiguity in the interior is greater—by a factor of about two—than at an interface). Thus, it is not clear that particle extrication can occur at rates sufficient to account for the extent of the observed mushy zone sedimentation. Recent experiments of ours, augmented by a simple model, suggest that another mechanism is primarily responsible for much sedimentation observed in interconnected structures. The ideas were first applied to a binary isomorphous system [4] in which component A melts at a lower temperature than component B, and solid B is denser than liquid A. Provided the amount of B is less than a (system specific) critical value, during melting the B particles sink until their volume fraction becomes sufficient to establish an interconnected structure. During the sinking—which occurs rapidly—little B dissolves in liquid A. Thus, the mushy zone following this initial sinking consists of solid B particles immersed in a liquid of nearly pure A. The composition of the liquid head above this mushy zone is also nearly pure A. The B subsequently dissolves in the mushy zone liquid until the equilibrium liquidus composition is reached there. The dissolution results in some mushy zone recession (sedimentation) since the stipulation of a critical solid volume fraction to maintain an interconnected structure remains. Thus, following this stage, the mushy zone liquid has the equilibrium liquidus composition; however, the liquid head remains nearly pure A. As a consequence, B is transported into the head from the mushy zone. To maintain the equilibrium liquidus composition in the mushy zone requires further dissolution of B there and this is accompanied by additional sedimentation. Additional details are provided in [4]. Experimental studies on sedimentation in a Ni–W binary alloy corroborate, in the main, the above scenario [5]. That is, macrosegregation in the liquid head is observed and sedimentation is linked to elimination of this segregation. Moreover, sedimentation apparently ceases when the liquid head composition reaches the equilibrium liquidus. Similar results are found when a hypoeutectic (Pb-rich) Pb–Sn alloy is heat-treated in a twophase solid-plus-liquid region [6]. Sedimentation in this alloy apparently ceases when the equilib-