Solidification of undercooled molten spinodal
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Solidification of undercooled molten spinodal C. W. Yuen and H. W. Kui Department of Physics, The Chinese University of Hong Kong, Shatin, N. T., Hong Kong (Received 16 July 1996; accepted 25 February 1998)
When molten Pd40.5 Ni40.5 P19 is undercooled way below its liquidus Tl , liquid state spinodal decomposition is observed. Crystallization of this system is particularly interesting because it has plenty of interfaces. The microstructure of an as-crystallized specimen can be divided into four regions, namely, A: random spinodal; B: aligned and elongated spinodal; C: coarsened spinodal and island structure; and D: rod structure and ternary eutectics. The origin of these different microstructures is discussed.
I. INTRODUCTION
Recently, it was demonstrated that molten Pd80 Si20 1 and Pd40.5 Ni40.5 P19 2,3 undergo liquid phase separation (LPS) in the undercooling regime defined as DT Tl 2 T, where Tl is the liquidus of the alloys. There are two modes of LPS which are liquid state nucleation and growth (LNG), and liquid state spinodal decomposition (LSD). Since LNG and LSD result in metastable undercooled liquids, they will crystallize subsequently. In LNG, the crystallization mechanism is fairly simple. The original homogeneous liquid, upon phase separation, is replaced by liquid droplets, denoted by l2 , of composition Pd47 Ni38 P15 , which disperse themselves in a liquid background, denoted by l1 of composition Pd37 Ni42 P21 . Crystallization first appears in l1 producing a dendritic network. As the dendritic network grows into the undercooled liquid of composition l1 , some of its dendrite tips may come across the liquid droplets of composition l2 . The former acts as seeds, triggering the crystallization in the latter. The detailed crystallization behavior in LNG is well characterized in Refs. 2 and 3. In the LSD regime, the characteristic wavelength at the initial site of crystallization of an undercooled specimen with DT 150 K2,3 is of the order of ,0.1 mm. This novel microstructure provides a unique opportunity to study the crystallization behavior where there are plentiful interfaces. Furthermore, since the transformation is from liquid to solid, three effects can bring about the modification of the final morphology of an undercooled specimen. They are the heat of crystallization (termed thermodynamic effect), the stress results from volume contraction on solidification (termed hydrodynamic effect), and gravity segregation. The thermodynamic factor accelerates the coarsening process. It can therefore modify the final morphology of an as-crystallized LSD specimen. Indeed, Elder et al.4 proposed that recalescence may destroy a fine scale phase-separated structure to become dendrites. However, so far very limited work has been done. On the other hand, the effect of stress on spinodal evolution has been J. Mater. Res., Vol. 13, No. 11, Nov 1998
extensively studied both theoretically and experimentally on polymer systems.5–9 It was found that domains of spinodal
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