Model of banding in diffusive and convective regimes during directional solidification of peritectic systems

Journal Article · Fri May 01 04:00:00 EDT 1998 · Metallurgical and Materials Transactions. A, Physical Metallurgy and Materials Science

DOI:https://doi.org/10.1007/s11661-998-0361-1· OSTI ID:616268

Karma, A; Rappel, W J ^[1]; Fuh, B C; Trivedi, R ^[2]

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1. Northeastern Univ., Boston, MA (United States)
2. Ames Lab., IA (United States)

The formation of banded microstructure in peritectic systems is examined theoretically in both diffusive and convective regimes. A rigorous model is developed in the diffusive regime that describes the non-steady-state growth of alternate solid {alpha} and {beta} phase bands with a planar solid-liquid interface. The model is extended to incorporate the effect of convection by assuming that solute diffusion takes place within a boundary layer of constant thickness, with a uniform composition in the mixed liquid zone outside this layer. The model predicts that convection effects in a semi-infinite sample narrow the composition range over which extended banding can occur, and the spacing of bands is reduced compared to the diffusive growth model. In a finite length sample, convection is shown to lead only to the transient formation of bands. In this transient banding regime, only a few bands with a variable width are formed, and this transient banding process can occur over a wide range of compositions inside the two-phase peritectic region. Directional solidification studies in the Pb-Bi system show transient bands and agree qualitatively with these predictions. However, the basic mechanism of band formation observed in this system is found to be significantly different from the one assumed in the model. A new mechanism of banding is proposed in which continuous growth of both phases is present instead of nucleation at the boundary of the pre-existing phase. This mechanism yields an oscillatory structure with a shorter spatial periodicity than the band spacing predicted by the purely diffusive or boundary layer convective models.

• Sponsoring Organization: National Aeronautics and Space Administration, Washington, DC (United States); USDOE, Washington, DC (United States)
• DOE Contract Number: W-7405-ENG-82
• OSTI ID: 616268
• Journal Information: Metallurgical and Materials Transactions. A, Physical Metallurgy and Materials Science, Journal Name: Metallurgical and Materials Transactions. A, Physical Metallurgy and Materials Science Journal Issue: 5 Vol. 29; ISSN MMTAEB; ISSN 1073-5623
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
BISMUTH ALLOYS
CONVECTION
DIFFUSION
LEAD ALLOYS
MATHEMATICAL MODELS
MICROSTRUCTURE
SOLIDIFICATION