Microstructures and mechanical properties of a hot-extruded Mg−8Ho−0.6Zn−0.5Zr alloy

The microstructures and mechanical properties of a Mg−8Ho−0.6Zn−0.5Zr alloy were thoroughly investigated. The results reveal five kinds of intermetallic phases, namely the 14H long-period stacking ordered (LPSO) phase, the 18R LPSO phase, Mg24Ho5, Mg0.5Ho1.5, and Mg3Ho. Ordinarily, the 18R LPSO phase locates at a lamellae grain boundary with the misorientation angle of 136.9° and is simultaneously coherent with two α-Mg grains. The corresponding orientation relationships are as (001)18R//(0001)α1 and [010]18R//〈112¯0〉α1, and (001)18R about 43.1° from (0001)α2 and [010]18R//〈112¯0〉α2. After hot-extrusion, the disintegrated intermetallic phases composed of Mg24Ho5 and Mg3Ho mainly distribute at extrusion stringers while the LPSO phases mainly locate in un-recrystallized regions. In addition, amounts of fine spherical intermetallic particles were formed when many parallelly distributed LPSO plates precipitated in a part of the recrystallized grains. The as-extruded Mg−8Ho−0.6Zn−0.5Zr alloy exhibits excellent combination of high strength and high ductility. The underlying strengthening mechanisms were revealed as fine-grain strengthening and dispersion strengthening. •Microstructures and mechanical properties of a Mg−Ho−Zn−Zr alloy were investigated.•Five intermetallic phases including both 14H and 18R LPSO phases were observed.•18R LPSO phase at a new lamellae GB is simultaneously coherent with two α-Mg grains.•Fine H1 phase which are coherent with Mg matrix was formed after hot-extrusion.•The as-extruded alloy exhibits excellent combination of high strength and ductility.