Dynamic dissolution and transformation of LPSO phase during thermomechanical processing of a GWZ magnesium alloy

An extruded Mg-8.2Gd-3.6Y–1.6Zn–0.5Zr alloy containing blocky long-period stacking-ordered (LPSO) phase was subjected to a set of single and multi-pass friction stir processing trials. The evolutions of the microstructure and the secondary phases were investigated in details, and the correlations between the microstructure, the macro-texture and the room-temperature mechanical properties were explained. An outstanding grain refinement was achieved after applying just one friction stir processing pass, while the coarse blocky long-period stacking ordered phase was significantly broken into the fine particles. The occurrence of particle-stimulated nucleation process was identified as the main recrystallization mechanism, and this in turn could lead to appreciable texture weakening. In addition, some of the secondary phases were dissolved in the matrix and successively transformed into cubic RE-rich particles dynamically. The capability of phase transformation and dynamic precipitation were intensified through higher passes and the volume fraction of dispersed RE-rich particles located at the grain boundaries was increased. An ultrafine and relatively homogenous microstructure was also developed at the highest level of imposed equivalent strain. In the current work, the room-temperature mechanical properties of the thermomechanically processed materials were also explored. In spite of previous reports on the severely deformed magnesium alloys, the strength and ductility were significantly improved after friction stir processing. These were explained considering the grain refinement, fragmentation, transformation, suitable distribution of the second phases, and texture weakening which was resulted from the activation of non-basal slip systems.