Band Structure Formation in Peritectic Fe-Co and Fe-Ni Alloys

Abstract

In this paper, band structure formation in peritectic Fe-18.0 at%Co and Fe-4.4 at%Ni alloys is reported. Bridgman type directional solidification experiments were performed, and an island band structure was observed in both of these alloys under low velocity solidification conditions. Solid composition profiles along the growth direction in this band region were determined with electron probe measurements. The solute concentration increased during growth, and after an abrupt increase for phase change, it decreased during growth. It then decreased abruptly for the phase change to . Repetition of this process formed bands. Composition profiles calculated by the convection model were compared with these experimental results, suggesting that island bands form by a mechanism similar to that assumed in the model for discrete bands. Furthermore, the nucleation condition in this model was extended. Applying the nucleation constitutional undercooling criterion, a new calculation was developed. The mechanism of band structure formation is discussed in the light of these results.

Growth Analysis of Banded Microstructure Formed during Directional Solidification of Peritectic Alloys

“…A banded microstructure is a characteristic of peritectic alloys, which is composed of alternate layers of primary and secondary phases perpendicular to the growth direction. This microstructure has been observed in low velocity solidification experiments in Cd-Sn, [1][2][3] Sn-Sb, 4) ZnCu, 4) Ag-Zn, 5) Pb-Bi, 3,[6][7][8] Fe-Ni 9,10) and Fe-Co 10) alloys. The microstructures take different forms, such as discrete bands, island bands, and oscillated tree-like morphology, which often coexist in one microstructure.…”

“…The material properties used for these calculations are summarized in Table 1. 10,14) The same magnitude of film thickness with that experimentally determined in the Fe-Ni alloy 9,10) was assumed for Fe-Co. Disregarding the scatter of measurements in thickness and phase change compositions, they fit and represent the starting point, layer thickness, and number of layers. The calculated composition at the starting point of growth is 18.50 at%Co, and, from this value, the interface undercooling from T p is estimated as 0.22 K. On the other side, the composition at the starting point of growth is 17.68 at%Co, then the interface superheating above T p is estimated to be 0.04 K. These values were regulated by the nucleation undercoolings and can be compared well with the experimental values in the former section.…”

“…The transition is supposed to occur if and only if nucleation can occur ahead of the growing phase by constitutional undercooling, assuming that the volume ahead of the growing interface is rapidly filled with the nucleating phase with a high nucleation density and growth rate. 10,12,13) The maximum constitutional undercooling should be taken for the nucleation instead of that at the S/L interface in this context.…”

“…The convection model is extended in the part of the nucleation condition. The nucleation and constitutional undercooling criterion 10,12,13) is adopted for creation of a new solid in front of a growing interface. A new calculation is developed, and its results are compared with those of the experiments in Fe-Co alloy as well as the convection and diffusion models.…”

Effect of Cooling Process on Critical Current Density of Bi-2223/Ag Superconductive Tapes Prepared by Dip-Coating

“…It forms from Bi-2212 (Bi 2 Sr 2 Ca 1 Cu 2 O y ), (Ca, Sr) 2 CuO 3 , (Ca, Sr) 2 PbO 4 , (Ca, Sr) 14 Cu 24 O 41 , and liquid phases, and the heat treatment concentrates into this temperature range. Although this phase formation occurs peritectically, current peritectic solidification models for oxides 16,17) and metals 18,19) are not directly applied to it because of its complicated phase relationships and non-equilibrium phase formation behavior. However, this phase formation takes place under the mechanism of the intercalation and the grain growth according to the reaction equation.…”