Microstructure evolution and mechanical properties of Mg-LPSO two phase Mg96Y2Ni2 (at. %) alloy processed by hot extrusion and decreasing-temperature ECAP

The Mg96Y2Ni2 (at. %) alloy containing ∼50% volume fraction of long period stacking ordered (LPSO) phase was processed by extrusion and subsequent equal channel angular pressing (ECAP). A bimodal α-Mg matrix with different non-dynamic recrystallized (non-DRXed) proportions and a fully dynamic recrystallized (DRXed) matrix with different grain sizes were obtained by regulating extrusion and ECAP processing parameters, and the relationship between tensile properties and microstructure was investigated. The alloy extruded at 390 °C has high area ratio of non-DRXed regions with ∼35% and low area ratio of DRXed regions with ∼15%, as well as fine DRXed grains of ∼0.6 μm, exhibiting ultrahigh tensile yield strength of 505 MPa and low elongation to failure of 3.5%. The alloy extruded at 450 °C exhibited bimodal microstructures containing non-DRXed grains with ∼13% and DRXed grains with ∼37%, and DRXed grains with the size of 2.1 μm, exhibiting tensile yield strength of 400 MPa and elongation to failure of 10%. Fully recrystallized ultra-fine α-Mg grains with the size of ∼0.55 μm was obtained after extrusion alloy was subjected to ECAP for 8 passes at 400 °C and subsequent 4 passes at 300 °C. The ultrafine-grained ECAPed alloy obtains tensile yield strength of 450 MPa and elongation to failure of 8%, exhibiting excellent strength-ductility balance. The lower TYS of the ECAPed alloy compared with the alloy extruded at 390 °C may be ascribed to the break of the LPSO phase into particles, which do not transfer load as effectively as long strips, as well as the fully DRXed grains with weak texture, which has less strengthening effect than that of the non-DRXed grains with strong basal texture. While the fully recrystallized ultra-fine Mg grains contribute to the increase of elongation to failure. This work provides theoretical guidance for develop of Mg-LPSO two-phase alloys with excellent strength-ductility synergy.