Refining 18R-LPSO phase into sub-micron range by pre-kinking design and its prominent strengthening effect on Mg97Y2Zn1 alloy

In this study, a composite deformation strategy of pre-kinking (equal channel angular pressing (ECAP)) followed by large-ratio hot extrusion (HE) was designed to refine the 18R long period stacking ordered (LPSO) phase into sub-micron range in a Mg97Y2Zn1 (at.%) alloy. After the composite processing, the mechanical properties of the alloy are significantly enhanced, superior to the majority of reported Mg97Y2Zn1 and other LPSO-containing Mg alloys. Among the composite deformed alloys, the 16P-HE alloy exhibits the best mechanical properties with tensile yield strength (TYS) of 475 MPa, ultimate tensile strength (UTS) of 526 MPa, and fracture elongation (FE) of 14.5%. Quantitative analysis of 18R phase indicates that increasing ECAP pass from 1 to 16 gradually decreases the average size of 18R phase from 5.1 µm to 2.3 µm. After HE, the 18R phase is further refined with a corresponding decrease in the average size in the descending order of 1P-HE (4.3 µm), 4P-HE (3.2 µm), and 16P-HE (1.4 µm) alloys. Calculation of the strengthening contributions confirms that the superior mechanical properties of 16P-HE alloy are mainly due to its strongest interface strengthening (145 MPa) and grain boundary strengthening (189 MPa) from the sub-micron 18R phase and α-Mg grains. Moreover, the strengthening effect of 18R phase decreases gradually with their morphology changing from particles to fibers, and to blocks. The obtained results further deepen and broaden the strengthening-toughening theory of 18R phase.