Solidification behavior of Sn-15 wt pct Pb alloy under a high shear rate and high intensity of turbulence during semisol
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I. INTRODUCTION
SINCE the discovery of the unique rheological behavior of nondendritic semisolid slurries at Massachusetts Institute of Technology in the early 1970s,[1] efforts have been dedicated to the understanding of microstructural evolution under forced convection, as summarized in Ref. [2,3]. Spencer et al.[1] showed that stirring the previously unstirred partially solidified alloy fragmented the initial dendritic structure, producing nondendritic solid particles suspended in a liquid matrix. Many of the particles contained liquid. Joly and Mehrabian[4] obtained similar microstructures while the alloy was cooled and stirred from a temperature above the liquidus, i.e., with shear being applied during the early stages of nucleation and growth of the solid. Studies by Vogel et al.[5] on Al alloys showed that, when stirring was applied at the earliest stages of freezing, there was no significant increase in the number of nuclei in the first few minutes of freezing. Molenaar et al.[6] investigated the structural evolution of Al-Cu alloys under simple shear flow and found that shearing increases neither the undercooling nor the particle density. They attributed the formation of rosettes to cellular growth under forced convection.[7] Vogel and Cantor[8] analyzed the stability of the solid/liquid interface during shearing using the boundary-layer theory and concluded that shearing destabilizes the solid/liquid interface and favors dendritic growth. This conclusion is in contradiction to the experimental observation that the solid particles are nondendritic in nature. To explain such experimental observations, Vogel et al.[5,9] proposed that viscous forces during stir casting cause bending of dendritic arms, and that the bent arms detach through wetting of high-angle grain boundaries by S. JI, Research Fellow, and Z. FAN, Professor of Materials, are with the Department of Mechanical Engineering, Brunel University, Middlesex UB8 3PH, United Kingdom. Contact e-mail: [email protected] Manuscript submitted July 27, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
the liquid phase. The detached dendritic arms act as nuclei on which further grains grow, accounting for grain multiplication. Doherty[10] further pointed out that when there is a high density of growing nuclei, even with full diffusion control, then a stable nondendritic growth form may be expected, because the overlapping diffusion fields from adjacent growing crystals interact with each other, reducing the concentration gradients that cause instabilities. However, if the dendrite fragmentation mechanism is considered to be reasonable for shearing of the initially dendritic structure, it is clearly due to the lack of unambiguous experimental evidence in the case of continuous shearing during the early stages of nucleation and growth of the solid phase. A further objection to this mechanism is the fact that not all the alloys (Al alloys, for instance) show dynamic recrystallization.[10] In addition, having analyzed the strength of alloys at temperatures c
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