Evolution of Quasicrystals and Long-Period Stacking Ordered Structures During Severe Plastic Deformation and Mixing of Dissimilar Mg Alloys Upon Friction Stir Welding

Microstructural evolution during severe plastic deformation and mixing of Mg(95.8)Zn(3.6)Gd(0.6)and Mg97Cu1Y2(at%) alloys upon friction stir welding was studied. A laminated onion-ring structure composed of alternative distribution of layers with significantly refined microstructures from different alloys was formed in the stirred zone. Coarse quasicrystals were broken up and dispersed with most of them being transformed into cubic W-phase particles, and thick 18R long-period stacking ordered plates were fractured and transformed into fine 14H-LPSO lamellae in the stirred zone (SZ) experiencing complex material flow under high strain rate. Fine W-phase particles and 14H-LPSO lamellae formed during dissimilar friction stir welding (FSW) usually have no specific orientation relationship with surrounding Mg matrix. Chemical measurements demonstrated occurrence of interdiffusion between dissimilar layers in the SZ. Phase transformation was observed for some particles of quasicrystals and long-period stacking ordered (LPSO) in regions slightly outside the SZ. An ultimate tensile strength of similar to 415 MPa and an elongation to failure of similar to 27.8%, both exceeding those of base materials, were obtained in the SZ, due to microstructural refinement and formation of a laminated structure.

Automated summary

Microstructural evolution of Mg-based alloys during severe plastic deformation and mixing via friction stir welding was studied. The formation of a laminated onion-ring structure with significantly refined microstructures was observed in the stirred zone, leading to improved ultimate tensile strength and elongation to failure.