Model of banding in diffusive and convective regimes during directional solidification of peritectic systems
- PDF / 1,857,576 Bytes
- 14 Pages / 612 x 792 pts (letter) Page_size
- 92 Downloads / 249 Views
I.
INTRODUCTION
MANY technologically important materials, including magnetic and high-temperature superconducting alloys, are produced by solidification in peritectic systems. Paradoxically, a comprehensive and reliable understanding of the rich class of microstructures that form in these systems has remained largely lacking. Several studies to date[1–6] have reported the observation of banded structures in various peritectic systems, including Sn-Cd,[1,4] Sn-Sb,[2] Zn-Cu,[2] Ag-Zn,[5] and Pb-Bi.[3,6] These structures typically consist of alternate bands of the primary a phase and the secondary b phase. Boettinger[1] first applied the constitutional supercooling criterion of Tiller et al.[7] to this system, which predicts that both a and b phases should grow with a planar solid-liquid interface when the ratio of thermal gradient and pulling velocity, G/vp, exceeds a critical value, in rough quantitative agreement with what is observed experimentally. For smaller G/vp, cellular/dendritic structures of one of the two phases,[1] or more complex banded structures composed of both, alternate phases and alternate planar or cellular/dendritic growth morphologies, have been observed.[2] Interestingly, the formation of banded structures is not a peculiarity of peritectic systems. Similar structures are also known to form during rapid solidification of dendritic and eutectic alloys, and banding appears to be a rather generic phenomenon in soA. KARMA, Professor, and W.-J. RAPPEL, Postdoctoral Research Associate, are with the Department of Physics and Center for Interdisciplinary Research on Complex Systems, Northeastern University, Boston, MA 02115. B.C. FUH, Ph.D. Student, and R. TRIVEDI, Professor, are with Ames Laboratory, United States Department of Energy, and the Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011. Manuscript submitted September 11, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
lidification systems that exhibit time-periodic transitions between two competing steady-state growth morphologies.[8] Recently, Trivedi[9] proposed a simple model of banding in peritectic systems that pertains to the high G/vp regime, where both phases grow with a planar interface. The conceptual banding cycle that forms the basis of this model is shown schematically in Figure 1 for a situation where the growth is purely diffusion controlled. As the parent a phase forms and grows toward its steady state below the peritectic temperature Tp, which is fixed by the alloy composition C0, the heterogeneous nucleation of the second b phase at the solid-liquid boundary of the parent phase becomes possible. The system, instead of reaching this steady state, then transforms into the second phase. This new phase, in turn, grows toward its steady state above Tp. Before it reaches this steady state, the parent phase nucleates heterogeneously on the solid-liquid boundary of the b phase, and the cycle of alternate band formation is then repeated. This model predicts that under purely diffusion-controlle
Data Loading...
