Abstract
The Mg-Y-Zn ternary alloy system contains a series of novel structures known as long-period stacking ordered (LPSO) structures. The formation process and its key concept from a viewpoint of phase transition are not yet clear. The current study reveals that the phase transformation process is not a traditional spinodal decomposition or structural transformation but, rather a novel hierarchical phase transformation. In this transformation, clustering occurs first, and the spatial rearrangement of the clusters induce a secondary phase transformation that eventually lead to two-dimensional ordering of the clusters. The formation process was examined using in situ synchrotron radiation small-angle X-ray scattering (SAXS). Rapid quenching from liquid alloy into thin ribbons yielded strongly supersaturated amorphous samples. The samples were heated at a constant rate of 10 K/min. and the scattering patterns were acquired. The SAXS analysis indicated that small clusters grew to sizes of 0.2 nm after they crystallized. The clusters distributed randomly in space grew and eventually transformed into a microstructure with two well-defined cluster-cluster distances, one for the segregation periodicity of LPSO and the other for the in-plane ordering in segregated layer. This transformation into the LPSO structure concomitantly introduces the periodical stacking fault required for the 18R structures.
Highlights
The LPSO phase was first observed in MgZnY alloys as a periodic concentration modulation of Zn and Y along the c axis[13,14] in an hcp Mg matrix, and a more refined structure was determined by electron microscopic works focusing on determining equilibrium structures of LPSO phases[17,18]
Upon examining the temporal structural change using synchrotron radiation small-angle scattering (SR-small-angle X-ray scattering (SAXS)), we observed that the distance between the ordered clusters changed continuously with time[19,20] within the segregation plane
To examine the initial stage of the formation process, small-angle X-ray scattering (SAXS) patterns were measured in situ while heating rapidly quenched Mg85Y9Zn6 ribbons
Summary
Hiroshi Okuda[1], Michiaki Yamasaki[2], Yoshihito Kawamura[2], Masao Tabuchi3 & Hajime Kimizuka[4]. Metallic alloys undergo various types of phase transformations These processes, including spinodal decomposition[1], clustering[2], order-disorder transition accompanying spinodal decomposition[3], and displacive transformation[4], have been the subjects of intense interest from a statistical thermodynamics viewpoint[5,6] and from that of strategies for developing innovative materials. The in-plane structure shown in Fig. 1 appears to undergo a two-dimensional order-disorder transition of clusters[21] Such superstructures by clusters in the segregation layer might be examined in terms of two-dimensional phase transformations of particles[22] similar to colloidal crystals. Neither of the mechanisms appears to be compatible with the two-dimensional ordering kinetics of the present alloys described above
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