Ultrahigh strength Mg-Y-Ni alloys obtained by regulating second phases

Mg-Y-Ni alloys with different second phases were designed by changing Y/Ni atomic ratio from 1.5 to 0.5. The microstructure and mechanical properties of as-cast and as-extruded alloys were investigated. The as-cast Mg-Y-Ni alloy with Y/Ni ratio of 1.5 is composed of alpha-Mg and long period stacking ordered (LPSO) phase. When Y/Ni ratio is equal to 1, nanoscale lamellar gamma' phase and eutectic Mg2Ni phase are formed in addition to LPSO phase. As Y/Ni ratio decreases further, the amount of eutectic Mg2Ni phase increases, while the amount of LPSO phase decreases. After extrusion, the LPSO and gamma' phases are distributed along the extrusion direction, while eutectic Mg2Ni phase is broken and dispersed in the as-extruded alloys. LPSO phase and Mg2Ni phase in the alloys promote dynamic recrystallization (DRX) during extrusion, while gamma' phase inhibits DRX. Consequently, the Mg96Y2Ni2 (at.%) alloy with LPSO phase and gamma' phase as the main second phases shows the strongest basal texture after extrusion. The tensile yield strength of the as-extruded Mg-Y-Ni alloys increases first and then decreases with decreasing Y/Ni ratio. The as-extruded Mg96Y2Ni2 (at.%) alloy with Y/Ni = 1 exhibits excellent mechanical properties with tensile yield strength of 465 MPa, ultimate tensile strength of 510 MPa and elongation to failure of 7.2%, which is attributed to the synergistic effect of bulk LPSO phase and nanoscale gamma' phase. (C) 2020 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.