Achieving ultra-high strength using densely ultra-fine LPSO phase

Long period stacking ordered (LPSO) structures are an effective strengthening phase in Mg alloys. However, the coarse LPSO phases in as-cast alloys are very difficult to refine, and they dramatically reduce the strengthening effect. In the present study, friction stir processing (FSP) was employed to refine the structure of an Mg-12.8Y-4.7 Zn (wt%) alloy with a very high content of coarse LPSO phases. An optimized FSP regime refined the coarse LPSO phases into densely ultrafine blocks with an average width of ∼200 nm and an average length of ∼1 μm. An ultrahigh yield strength of ∼800  MPa was achieved under compression of the FSP region. Theoretical calculations indicated that the strengthening by the densely ultrafine LPSO phases was up to approximately 630 MPa.