A new insight into LPSO phase transformation and mechanical properties uniformity of large-scale Mg-Gd-Y-Zn-Zr alloy prepared by multi-pass friction stir processing

Abstract

A large-scale fine-grained Mg-Gd-Y-Zn-Zr alloy plate with high strength and ductility was successfully prepared by multi-pass friction stir processing (MFSP) technology in this work. The structure of grains and long period stacking ordered (LPSO) phase were characterized, and the mechanical properties uniformity was investigated. Moreover, a quantitative relationship between the microstructure and tensile yield strength was established. The results showed that the grains in the processed zone (PZ) and interfacial zone (IZ) were refined from 50 µm to 3 µm and 4 µm, respectively, and numerous original LPSO phases were broken. In IZ, some block-shaped 18R LPSO phases were transformed into needle-like 14H LPSO phases due to stacking faults and the short-range diffusion of solute atoms. The severe shear deformation in the form of kinetic energy caused profuse stacking fault to be generated and move rapidly, greatly increasing the transformation rate of LPSO phase. After MFSP, the ultimate tensile strength, yield strength and elongation to failure of the large-scale plate were 367 MPa, 305 MPa and 18.0% respectively. Grain refinement and LPSO phase strengthening were the major strengthening mechanisms for the MFSP sample. In particularly, the strength of IZ was comparable to that of PZ because the strength contribution of the 14H LPSO phase offsets the lack of grain refinement strengthening in IZ. This result opposes the widely accepted notion that IZ is a weak region in MFSP-prepared large-scale fine-grained plate.

Keywords

• Friction stir processing
• Multi-pass
• Mg-Gd-Y-Zn-Zr alloy
• LPSO phase transformation
• Mechanical properties