On the Elastic Accommodation Between the Structural Units in the LPSO Structures

Abstract

Long range periodic stacking ordered (LPSO) structures in magnesium alloy consist of regular arrangements of four layer height fcc structural units separated by several Mg layers paralleling to the base plane. The shear transformation of the structural units from hcp structure involves large transformation strain. This strain can be significantly accommodated by the combination of the structure units with different shears. In addition, the possible lowest elastic energy configuration in both R and H type of LPSO structure is obtained. For H type LPSO structure, the opposite shear directions (or Burgers vectors of the partial dislocations) are operated in nearest structural units, while all three types of shear directions are operated successively in the nearest structural units for R type LPSO structure. This study also intends to discuss the inter spacing between the structural units in terms of the elastic interaction. It is found the elastic interaction alone cannot rationalize the spacing between the structural units.

Effects of Pre-Strain and Ageing on the LPSO Structure in Mg₉₇Zn₁Y₂ Alloy

“…Again, mainly 14H LPSO structure is observed, thus 14H type LPSO seems to be stable during the ageing process. This observation is consistent with the consideration in terms of the elastic interaction energy, 14) which shows that H type LPSO is stable than R type. In summary, it shows that 14H is generated without precursor phase such as 18R type LPSO, which is similar to the result in Mg-Zn-Gd alloy, 19) i.e.…”

“…Similar result could be obtained by adopting the same calculation procedure in Ref. 14), and the interaction energy could be lowered when the number structure units in the growth ledge increased as shown in Fig. 9(b) for both 18R and 4H type LPSO.…”

“…In addition, we also show that 14H type LPSO has lower elastic interaction energy than 18R type LPSO. 14) This may explain the observation that 14H LPSO is stable in Mg-Zn-Y alloy 1518) and Mg-Zn-Gd alloy. 19) It would be supposed that 14H LPSO will precipitate from a supersaturated matrix during ageing.…”

“…12) In our recent work, the elastic interaction between neighboring structural units with different variants of shear directions has been studied. 13,14) It shows that the configuration with lowest elastic interaction energy for R or H type LPSO structure is reached when macroscopic net shear is zero. In addition, we also show that 14H type LPSO has lower elastic interaction energy than 18R type LPSO.…”

“…The second way is that the shear combinations for transformation front could be changed but the LPSO structure keeps the same, since there are three different but equivalent shear directions available for each fcc structural unit. 14) For the alloy system with low stacking fault energy such as Mg-Zn-Y alloy, partial dislocations could be generated by dissociation of full dislocations introduced by deformation (pre-strain). Therefore, pre-strain before ageing may be applied to change the structure of LPSO.…”