Phase selection during directional solidification of peritectic alloys
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I. INTRODUCTION
DESPITE the widespread commercial use of peritectic alloys (e.g., steels, brass, bronze, intermetallic compounds, cobalt-based superalloys, and A3B-type superconductors), the characterization of the microstructural development during directional solidification of peritectics has historically lagged behind similar efforts directed toward other types of binary invariant reactions such as eutectic or monotectic. A wide variety of possible microstructures has been shown to form in peritectic systems[1–16] depending upon the imposed temperature gradient, G, in the liquid at the interface, the growth velocity, V, the initial composition of the alloy, C0, as well as the presence or absence of convection in the melt. This could have important technological implications since many advanced materials exhibit peritectics and processing methods such as casting and welding often involve widely changing conditions. Microstructural development in peritectic systems has been studied experimentally, focusing on the interface morphology of the primary phase.[1,2,3] The second phase, which we shall call the peritectic phase, i.e., phase in Figure 1, has generally been assumed to form behind the leading interface through the peritectic reaction. However, the peritectic reaction requires diffusion in the solid, a relatively slow process, so that at faster growth rates, the peritectic phase forms directly from the liquid and not through the peritectic reaction. Under these conditions, when the T.A. LOGRASSO, Senior Metallurgist, and B.C. FUH, Graduate Student, are with the Materials and Engineering Physics Program, Ames Laboratory, Ames, IA 50014. Contact e-mail: [email protected] R. TRIVEDI, Senior Metallurgist, Ames Laboratory, United States Department of Energy, is Anson Marston Distinguished Professor, Materials Science and Engineering, Iowa State University, Ames, IA 50011. Manuscript submitted December 20, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
primary phase forms cells or dendrites, the peritectic phase that forms in the interdendritic or intercellular region can exhibit planar, cellular, or dendritic morphology. Experimental studies by Fuh[4] have shown that for small G/V values, both primary and peritectic phases grow dendritically, side by side, with the peritectic solid dendrites lagging behind the primary dendrites or both dendrites can grow at the same isotherm. As V decreased, or G/V increased, the peritectic solid in between the primary dendrites undergoes a transition from dendritic to cellular growth. With further decrease in velocity, the primary solid also undergoes a dendritic to cellular transition. Within the composition range Cp and Cp, the relative positions of the primary and peritectic phase fronts can be described by the distance, , which will decrease as the velocity is decreased. At certain critical velocity, goes to 0, so that the phase will become aligned with the phase. Below this critical velocity, either the phase may prevail or a banded structure may
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