Fig. 1 Schematic illustration of the pre-treatment process for the Mg-Zr master alloy

Fig. 2 SEM micrographs a-f, proportion of clustered Zr g, and size distribution of particle Zr h of Mg-Zr master alloys. a Master Alloy I, b enlarged views of Region D in a, c Master Alloy II, d enlarged views of Region E in c, e enlarged views of Region F in d, f EDS surface scanning results of e

Fig. 3 a TEM morphology of Master Alloy II, b the enlarged view of Region A in a

Fig. 4 Microstructures of Mg-6Zn alloy and Mg-6Zn-0.6Zr alloy refined by various Mg-Zr master alloys

Fig. 5 Zr content a and grain size b of Mg-6Zn-0.6Zr alloys refined by various Mg-Zr master alloys at different holding time

Fig. 6 Zr nuclei in Mg–6Zn–0.6Zr alloys refined by different Mg–Zr master alloys: a SEM image of alloys refined by Master Alloy I; b enlarged views of the rectangular region in a; c right-field image of Zr nucleus in alloys refined by Master Alloy II; d HRTEM micrograph of the interface between α–Mg matrix and Zr nucleus (B = [2 $\overline{1 }\overline{1 } $ 0]Mg); e, f the corresponding selected area electron diffraction pattern (SADP) from α–Mg and the Zr particle, respectively

Fig. 7 DSC curve of Mg-6Zn-0.6Zr alloy refined by Master Alloy II

Fig. 8 Synergistic strategy of Zr on grain refinement of Mg alloy: a diagram of a typical columnar grain structure of Mg-6Zn alloys without Zr; b diagram of the synergistic grain-refining strategy including heterogeneous nucleation and solute-driven growth restriction

Fig. 9 Effect of melt temperature and Zr particle size on settling speed: a effect of melt temperature, assume the particle diameter is 1 mm; b effect of Zr particle size, assume the melt temperature is 720 ℃

Fig. 10 Comparison of grain refinement efficiency of Mg-Zr master alloys treated by various methods. The holding time of 0 min indicates that the melt is added with Mg-Zr master alloy and poured immediately after stirring. (UHFR-TIGR: tungsten inert gas arc re-melting with ultra-high frequency pulses; FSP: friction stir processing; ECAE: equal channel angular extrusion)