Modification of long period stacking ordered phase and improvement of mechanical properties of Mg–Gd–Zn–Zr alloy by friction stir processing

• The coarse LPSO phase at grain boundary is modified to smaller size less than 5 μm during FSP.
• FSP causes remarkable grain refinement (1.6±0.3 µm).
• The β-phases are formed during friction stir processing (FSP).
• Enhanced mechanical properties, especially yield strength are obtained after FSP.

The effect of friction stir processing (FSP) on the modification of microstructure and mechanical properties of Mg–14Gd–2.3Zn–0.6Zr (wt%) alloy has been investigated. The coarse long period stacking ordered (LPSO) phase at grain boundary in the alloy was successfully broken into fine particles (~1 µm) due to the complicated deformation beneath the shoulder and around the pin. The grain size was also significantly refined from 23±4 μm to 1.6±0.3 µm through dynamic recrystallization (DRX) during FSP. Yield strength of the FSP sample was increased by ~30% probably due to the grain size and LPSO phase refinement as well as fine precipitation around the LPSO phase while the elongation decreased by ~39% maybe because of the heterogeneous microstructure.