Nanoclusters first: a hierarchical phase transformation in a novel Mg alloy

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.

A formation criterion for Order-Disorder (OD) phases of the Long-Period Stacking Order (LPSO)-type in Mg-Al-RE (Rare Earth) Ternary Systems

“…Since the OD phase is characterized by a very high degree of in-plane ordering of dense Al 6 RE 8 atomic clusters with a periodic arrangement on lattice points of a two-dimensional primitive hexagonal lattice, the formability of the OD phase is considered to be closely related to the stability of Al 6 RE 8 atomic clusters in the OD phase as well as the inter-cluster interaction among them 27–30 . Because both the LPSO and OD structures can be described approximately as the structures containing dispersed Al 6 RE 8 atomic clusters embedded in the Mg matrix 27,29 , the size and number density of the Al 6 RE 8 atomic clusters are important factors controlling the stability of the OD phase.…”

“…S1(a) of Supplementary) 15 . In addition, the formation of L1 2 -type atomic clusters is common to both the OD and LPSO phases in the Mg-Al-Gd and Mg-Zn-RE ternary systems 15,16,25,26 and has been considered to play a key role in the formation of the OD and LPSO phases 17,27–30 . Furthermore, recent first-principles calculations have predicted that the OD phases in the Mg-Al-RE and Mg-TM-RE ternary systems are stabilized by the inclusion of an additional Mg atom in the centre of each L1 2 -type atomic cluster 31–36 .…”

Study on the planar segregation of solute atoms in Mg-Al-Gd alloy

“…The 3DAP data was taken by CAMECA LEAP 4000HR at 35 K. A pulse fraction of 20% and a pulse frequency of 200 kHz were applied. Figure 1(a-c) shows some typical atomic-resolution HAADF-STEM images along [11][12][13][14][15][16][17][18][19][20]  and [01-10]  zone axes of hcp (α) matrix in as-casted Mg-3Al-5Gd alloy.…”

Precipitation of Long‐Period Stacking Ordered Structure in Mg–Gd–Zn–Mn Alloy

“…The presence of TM 6 RE 8 clusters in LPSO phases has also been reported in several Mg–TM–RE systems (e.g., Mg–Al–Gd, Mg–Zn–Y,) although a slight difference (e.g., local structures) was observed. The evolution of the solute clusters and the LPSO structure has also been reported in an Mg–Y–Zn alloy, by using in situ synchrotron radiation small angle X‐ray scattering …”