Distinct Roles of Individual Mg-0.3 at.% X (X = Gd, Y, Al and Zn) Trace Solutes in Microstructure Evolution and Mechanical Properties of Mg Sheets: A Combined Experimental and First-Principles Study

Understanding the distinct roles of representative alloying elements, including rare-earth (RE) solutes such as Gd and Y and non-rare-earth (non-RE) solutes such as Al and Zn, in regulating microstructural evolution and mechanical response is essential for developing high-strength Mg sheets. In this study, Gd and Y are typical RE solutes, whereas Al and Zn are common non-RE alloying elements, and were individually added to pure Mg at an identical concentration of 0.3 at.% to enable a controlled comparison of their effects on microstructural evolution and mechanical response during hot rolling and subsequent annealing. The effects of alloying elements on hot-rolling behavior were first investigated. Mg-Al and Mg-Zn exhibited improved hot rollability, whereas Mg-Gd and Mg-Y showed higher edge-cracking susceptibility after 90% hot rolling. The hot-rolled (HR) Mg-Gd and Mg-Y sheets achieved yield strength (YS) values of 235.5 and 226.4 MPa, respectively, exceeding those of Mg-Al (170.0 MPa) and Mg-Zn (162.8 MPa). Combined microstructural characterizations and DFT calculations revealed that RE solutes strongly segregated at grain boundaries, inducing solute drag, suppressing dynamic recrystallization (DRX), and promoting dislocation accumulation. During subsequent annealing, the grain size increased by only ~1.5 times in Mg-Gd/Mg-Y, whereas increases of 2.6–2.96 times were observed in pure Mg and Mg-Al/Mg-Zn alloys. The RE-containing alloys retained a UTS of ~190 MPa and elongations above 12% after annealing at 500 °C for 240 min. The identical atomic fraction design (0.3 at.%) enabled a direct comparison of the element-specific effects of different alloying elements, providing insights into alloy design and thermomechanical processing strategies for high-performance Mg sheets.