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Abstract
Mg is the most important lightweight engineering alloy enabling future weight-reduced and fuel-saving engineering solutions. Yet, Mg is soft. Long-period stacking ordered (LPSO) structures in Mg alloys have unique crystal structures, characterized by both complex chemical and stacking order. They are essential for strengthening of Mg alloys. The formation mechanism of these LPSO structures is still under discussion. Here we report that Y/Zn enriched Guinier-Preston (GP) zones observed in a lean Mg-Y-Zn model alloy are precursors of early stage LPSO structures. We provide evidence of a new type of phase transformation mechanism which comprises the diffusional formation of Y/Zn enriched GP zones and their subsequent shear transformation into LPSO building blocks. The mechanism constitutes a new type of coupled diffusional-displacive phase formation sequence which may also be applicable to other alloy systems.
Highlights
Mg and Mg based alloys are key materials for future lightweight applications due to their low mass density[1]
Our study provides direct evidence of the phase transformation from Y/Zn enriched GP zones to Long-period stacking ordered (LPSO) building blocks indicating that Y/Zn enriched GP zones are a pre-stage of LPSO structures
The plate-type defects on the basal plane shown in Fig. 1a exhibit bright contrast in the High-angle annular dark-field (HAADF) imaging mode, which indicates that the defects are enriched in Y and Zn
Summary
Mg and Mg based alloys are key materials for future lightweight applications due to their low mass density[1]. A recent study on the formation of LPSO structures in a highly alloyed Mg-Y-Zn system using small-angle X-ray scattering suggests a hierarchical phase transformation sequence of atomic clustering, followed by cluster motion and the formation of stacking faults to form LPSO structures[28]. Our study provides direct evidence of the phase transformation from Y/Zn enriched GP zones to LPSO building blocks indicating that Y/Zn enriched GP zones are a pre-stage of LPSO structures. The formation of these LPSO structures represents a new type of coupled diffusional-displacive phase formation sequence which may be applicable to other alloy systems
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