Diffusional-displacive transformation enables formation of long-period stacking order in magnesium
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.
Fig.: Sequential diffusional-displacive transformation mechanism. Y/Zn atomic clusters are expected to form first in the super-saturated Mg matrix (step 1) followed by the formation of Y/Zn enriched GP zones on the basal planes of the Mg matrix (step 2). Above a critical amount of Y and Zn atoms in the GP zones, LPSO building blocks are formed from the GP zones by the generation and propagation of Shockley partial dislocations (step 3). LPSO structures can be formed during annealing (step 4).