Morphological stability of Sm123 superconductor during peritectic solidification from Sm211+L mixture
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I. INTRODUCTION
WORLDWIDE research efforts are being conducted to improve the bulk processing of the high-Tc-oxide superconductor RE1Ba2Cu3O72d (123, hereafter RE; Sm, Y. Currently, improvement of the critical current density (Jc) is one of the largest problems to be solved. As the high-and low-angle grain boundaries cause weak linkage of the superconductive current flow, a single crystalline 123 phase should have better current-carrying capability, and this theory has been supported by many experimental results. In particular, establishment of the single-crystal fabrication process via melting and solidification is a main subject of many current studies. From the equilibrium-phase relationships of the RE-Ba-CuO system, it is generally recognized that the 123 phase forms via a peritectic reaction between the RE2Ba1Cu1O5 (211) and BaO-CuO liquid (L) phases, with a small amount of dissolved RE. When the peritectic solidification of the 123 phase from the 211 1 L mixture is carried out, the growth is limited by the solute transport through the diffusion in the liquid phase ahead of the 123 growth front.[1–5] The growth morphologies of the solidified 123 phase vary from faceted planar, to cellular, and to equiaxed blocky, due to the increase of the growth rate (R) or the decrease of the temperature gradient at the solidification interface (G). Figure 1 shows the morphological transition of the unidirectionally solidified Sm123 phase from the Sm211 1 L mixture.[6] In these pictures, the faceted Sm123 phase grows from bottom to top. The quenched and faceted planar growth was observed when R was small (Figure 1(a)) and the 123 growth morphology transformed to the cellular morphology (Figure 1(b)) and further to the equiaxed blocky morphology (Figure MASAKI SUMIDA, formerly Research Fellow, Japan Society for the Promotion of Science, Department of Metallurgy, Graduate School of Engineering, The University of Tokyo, is Proposal-Based Researcher, New Energy and Industrial Technology Development Organization, Tokyo. YUH SHIOHARA, Manager, is with the Superconductivity Research Laboratory, Tokyo 135-0062, Japan. TAKATERU UMEDA, Professor, is with the Department of Metallurgy, Graduate School of Engineering, The University of Tokyo, Tokyo 113-8656, Japan. Manuscript submitted October 15, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS B
1(c)) as R increased. Figure 2 shows a typical photomicrograph around the quenched growth interface of the equiaxed blocky morphology. The growing 123 phase contains many equiaxed grains in the solidified portion. Gray and blocky 123 grains are seen in the semisolid region (the white 211 and the dark liquid phase) ahead of the 123 growth front (note the arrows in Figure 2). It appears that the equiaxed morphology is formed by multiple nucleation and growth of the 123 phase in the semisolid.[7,8] It is also shown that the L/211 interfaces are the preferential heterogeneous nucleation sites of new 123 grains.[7] The growth-interface stability has to be considered carefully to achieve the bulk sin
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