Strengthening mechanisms acting in extruded Mg-based long-period stacking ordered (LPSO)-phase alloys

The unusual increase in the strength by extrusion is a unique feature of recently developed Mg alloys containing the LPSO phase. In this study, we first elucidated the detailed mechanisms that induce this drastic strengthening. The dependencies of the deformation behavior of a Mg88Zn4Y7 extruded alloy, which contains ∼86-vol% LPSO phase, on the temperature, loading orientation, and extrusion ratio were examined. It was found that the yield stress of the alloy is drastically increased by extrusion, but the magnitude of the increase in the yield stress is significantly different depending on the loading orientation. That is, the strengthening of the LPSO phase by extrusion shows a strong anisotropy. By the detailed analyses, this was clarified to be derived from the variation in the deformation mechanisms depending on the loading orientation and extrusion ratio. Basal slip was found to govern the deformation behavior when the loading axis was inclined at a 45° to the extrusion direction, while the predominant deformation mechanism varies from basal slip to the formation of deformation kink bands as the extrusion ratio increased when the loading axis was parallel to the extrusion direction. Moreover, it was clarified in this study that the introduction of a deformation-kink-band boundary during extrusion effectively acts as a strong obstacle to basal slip. That is, "the kink band strengthening" was first quantitatively elucidated, which contributes to the drastic increase in the yield stress of the extruded LPSO-phase alloys in the wide temperature range below 400 °C.