Effect of heat treatment cooling way on the microstructure and mechanical property of Mg-Sc alloys

• The effect of cooling way on the microstructure and mechanical property of Mg-Sc alloy is studied for the first time. • The model for Burgers orientation relationship between α and β phases has been established. • The strengthening mechanism of Mg-Sc alloy is analyzed. • The nanohardness of the β and α phases is analyzed by nanoindentation test for the first time. • The Mg-Sc alloy has a superior balance performance, with ultimate tensile strength exceeding 320 MPa and elongation over 15 %. In this study, the mechanical property and microstructure of Mg-Sc alloys obtained by quenching to room temperature with water quenching (WQ), air cooling (AC) and furnace cooling (FC) were investigated. The results indicate that the β contents increase from 1.3 % in FC alloy to 4.9 % for AC sample, and finally increase to 12.8 % of WQ specimen, as the heat treatment cooling rate increases. Concurrently, the grain sizes of β phase and α phase rapidly increase and gradually decrease, respectively. Moreover, α and β phases follow the the Burgers orientation relationship. The result from the nanoindentation tests indicates that the nanohardness of the β phase exceeds that of the α phase due to solid solution strengthening effect by Sc in the Mg and higher dislocation density of the β phase. Vickers hardness for WQ sample exceeds 95 HV due to an increment in the β phase. As the cooling rate increases, the WQ, AC and FC alloys demonstrate the similar ultimate tensile strength (UTS) while elongations gradually decrease. In comparison to the Mg-Sc, AZ, ZK and Mg-Li series Mg alloys, the present Mg-Sc alloys exhibit better balance performance, with UTS value exceeding 320 MPa and elongation surpassing 15 %.