Phase selection during directional solidification of peritectic alloys

Abstract

Directional solidification studies have been conducted using Pb-Bi peritectic alloys over a wide range of compositions, temperature gradients, and growth velocities to characterize the primary α-to primary β-phase transitions, which have been observed at both very low and very high velocities. The critical conditions for these transitions correspond to the simultaneous growth of the α and β phases at or close to a single isotherm. The low velocity transition occurs under very specific conditions of composition, temperature gradient, and growth velocity. Since the transition conditions are composition dependent, they change continuously under terrestrial conditions where rejected solute is convectively mixed into the liquid. Detailed experimental studies have been carried out to examine the phase selection in the immediate vicinity of the critical velocity for the α to β transition, and the effect of convection on this transition is examined experimentally in the Pb-Bi system. The dynamic condition, at which both phases are present at the same isotherm, was shown to depend not only on velocities, temperature gradients, and bulk (nominal) alloy compositions, but also on the volume fractions of solid. A quantitative expression for the α-to β-phase transition condition was obtained by using the boundary layer model of fluid flow, which showed good agreement with the experimental results. It is shown that the transition occurs at the volume fraction where the bulk composition reaches the critical composition value predicted by the diffusive model. The modification in the microstructure map for the trailing planar or nonplanar β phase is discussed. Disciplines Metallurgy CommentsCopyright 2005 ASM International. This paper was published in Metallurgical and Materials Transactions A, Vol. 36, Issue 5, pp. 1287-1300, and is made available as an electronic reprint with the permission of ASM International. One print or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplications of any material in this paper for a fee or for commercial purposes, or modification of the content of this paper are prohibited. Directional solidification studies have been conducted using Pb-Bi peritectic alloys over a wide range of compositions, temperature gradients, and growth velocities to characterize the primary ␣-to primary ␤-phase transitions, which have been observed at both very low and very high velocities. The critical conditions for these transitions correspond to the simultaneous growth of the ␣ and ␤ phases at or close to a single isotherm. The low velocity transition occurs under very specific conditions of composition, temperature gradient, and growth velocity. Since the transition conditions are composition dependent, they change continuously under terrestrial conditions where rejected solute is convectively mixed into the liquid. Detailed experimental studies have been carried out to examine the phase selection in ...

Investigation of TiC–C Eutectic and WC–C Peritectic High-Temperature Fixed Points

“…The structure and the plateau behavior observed for the Cr-C system seems to conform to this theory [9,10]. Many directional solidification studies indicate, however, that very often the primary α-phase forms cells or dendrites, while the peritectic β-phase forms in the interdendritic or intercellular region, depending on the freezing condition and on the material condition [14][15][16]. Furthermore, in most peritectic systems, the peritectic β-phase can form directly from the liquid [17].…”

Solidification microstructures and solid-state parallels: Recent developments, future directions

“…Observations of growth in peritectic systems have remained rather sparse until recently [179,180]; rather, mostly other microstructures, such as discrete bands, oscillatory structures and disordered composites, have been found [181][182][183][184][185]. The situation has now been considerably clarified.…”

Primary dendrite arm spacing during unidirectional solidification of Pb–Bi peritectic alloys

“…Investigations on directional solidification of peritectic alloys have been carried out on Zn-Ag [2], Sn-Cd [3][4][5][6], Pb-Bi [7][8][9][10], Zn-Cu [11][12][13], Sn-Sb [14,15], high temperature intermetallics Ti-Al [16,17] and Ni-Al [18], superconducting materials YBCO [19], magnetic materials Nd-Fe-B [20], and structural materials Fe-Ni [21][22][23][24][25]. Generally, peritectic solidification involves three stages: (1) the peritectic reaction in which the direct interaction between the primary phase and liquid occurs;…”