A review of Mg alloys containing long-period stacking ordered (LPSO) structures with insight into the application of friction stir processing

Abstract

Owing to their excellent mechanical/functional properties, the lightweight magnesium alloys containing long-period stacking ordered (LPSO) structures have recently received a great deal of attention. In the present overview article, the atomic structures, formation mechanisms, transformations of 18R and 14H phases, and morphology of LPSO phases in various Mg-based systems containing special rare earth elements (such as Y and Gd) and transition metal elements (such as Zn, Ni, and Cu) are reviewed. Then, the effects of thermomechanical processing and severe plastic deformation (SPD) techniques on the grain refinement of the α-Mg matrix by dynamic recrystallization (DRX), kinking as the main deformation mechanism of the LPSO phase, as well as the fragmentation and dispersion of LPSO phase are summarized. Afterward, the reported works on the application of friction stir processing (FSP) and friction stir welding (FSW) for the processing of LPSO-containing Mg alloys regarding the microstructural evolution, mechanical properties, and strengthening mechanisms are critically discussed. Finally, distinct suggestions for future works are proposed, including the investigation of the kinetics of the LPSO phase formation, fabricating by the emerging processing technologies such as additive manufacturing (AM), evaluating the FSP processing parameters (especially multi-pass FSP and applying the Zener-Hollomon parameter), characterizing the DRX mechanisms (such as discontinuous, continuous, and twinning-induced DRX, as well as the particle stimulated nucleation for texture weakening), and studying the superplasticity and superplastic forming of LPSO containing Mg alloys during hot deformation.