No abstract

We investigated the influence of a change in the stacking sequence of the close-packed plane in a Mg 12 ZnY long-period stacking ordered (LPSO) phase on its mechanical properties. A 14H-typed LPSO-phase crystal was fabricated by annealing a directionally solidified (DS) crystal with a 18R-typed LPSO-structure at 525 C for 3 days, and the temperature dependence and orientation dependence of the yield stress were examined via compression tests. (0001)h11 2 20i basal slip was identified as a dominant deformation mode, and deformation kink bands were formed under compression in the case of suppression of basal slip motion. The deformation mechanism of the 14H-typed LPSO-phase is almost similar to that of the 18R-typed LPSO-phase, even though a slight difference was observed at temperatures above 300 C.

Non-basal slip systems in the Mg 12 ZnY long-period stacking ordered (LPSO) phase, the operational frequency of which is increased at high-temperatures and affects the mechanical properties, were clarified. The f1 100gh11 20i prism slip was identified in both 18R and 14H LPSO phases, even though they have the different lattice systems. This behavior is different from that observed in a Ni-based LPSO phase. The peculiar chemical modulation in the Mg 12 ZnY LPSO phase may affect the selection of operative slip systems.

The deformation microstructure and deformation process of the Mg-based long period stacking ordered (LPSO) phase accompanied by the formation of deformation kink bands were examined through dynamical observations during compression tests, and the features were compared to those in Zn single crystals. In both crystals, the formation of deformation kink bands was confirmed in specimens compressed along the direction parallel to the basal plane. The deformation kink bands formed in the directionally solidified (DS) LPSO phase crystals and the Zn single crystals had similar morphologies. Their formation induced plastic strain almost along the c-axis. However, their formation behaviors showed some different features. In the LPSO phase DS crystal, two different migration behaviors of the deformation kink band boundaries existed. The slower migration process was comparable to that observed in Zn single crystals. However, a migration process more than 10 5 times faster than the slower process was also monitored. The results imply that two different formation mechanisms of the deformation kink band might exist in the LPSO phase crystal.

Microstructural factors that govern the plastic deformation of the long-period stacking ordered (LPSO) phase were clarified. The decrease in length of the long-axis for the plate-like shape of LPSO-phase grains increases the yield stress of the alloy in which basal slip is predominant in deformation. On the other hand, the yield stress tended to increase as the thickness of the plate-like shapes of the grains decreased for the alloy in which the formation of deformation kinks carried the strain.