A lateral remelting phenomenon of the primary phase below the temperature of peritectic reaction in directionally solidi
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ng peritectic solidification, besides the longitudinal remelting of the primary phase at the temperature of peritectic reaction TpK , a lateral remelting phenomenon of the primary phase below TpK is observed under high velocity in directionally solidified Cu–Ge alloys. The lateral remelting occurs continuously along a liquid channel as temperature decreases, and the lateral remelting velocity is larger than that of peritectic transformation. The lateral remelting leads to the morphological change of the primary dendrites, even the fragmentation of dendrite arms. The phenomenon also means that the classical theory calculating the volume fraction of the primary phase during peritectic transformation can need to be modified under some conditions. However, under low velocity, the phenomenon is not so significant. The phenomenon is explained by means of solidification and remelting theory.
I. INTRODUCTION
The subject of peritectic reaction and solidification in recent years has attracted many investigators to research the physical process, which frequently occurs in many important alloys, such as structural alloys (Fe–C, Cu–Sn, Ti–Al, and Al–Fe–Si), YBa2Cu3Oy high temperature superconductors, and Nd–Fe–B rare-earth permanent magnetic materials.1–6 The morphologies and volume fractions of the primary and peritectic phases after peritectic solidification will influence the mechanical and physical properties of the materials directly. Pioneering contributions on the peritectic reaction process were made earlier by Hillert,2 who predicted the morphology of the triple phase region. Boussinot and Stjohn et al. systematically studied the mechanism of peritectic reaction and transformation.6–9 Kerr and Kurz1 reviewed various peritectic systems comprehensively. Sha et al.10 proposed a more detailed theory on the peritectic reaction process in Al–Si–Fe alloy. During peritectic solidification, two stages have been identified in terms of growth of the peritectic phase1: the first stage is the “peritectic reaction,” which occurs at the temperature of peritectic reaction
TpK
and is controlled by solute
diffusion in the liquid phase at the three phase junction; the second stage is the “peritectic transformation,” which occurs below TpK and is controlled by solute diffusion in a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.321 J. Mater. Res., Vol. 28, No. 23, Dec 14, 2013
the secondary phase. It is generally believed that, during peritectic transformation, the primary phase is completely enveloped by the peritectic phase. Thus, the remelting of the primary phase will only occur at TpK during peritectic reaction. 8,10–13 However, in this paper, different from traditional understanding, a lateral remelting phenomenon of the primary phase below TpK is observed in directionally solidified Cu–Ge alloys. In the peritectic system, lateral remelting is a novel nature phenomenon and is found for the first time, as far as we know. The lateral remelting phenomenon accompanies new features and has new fo
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