Effects of annealing on twin boundary segregation and strengthening in Mg–Gd alloys

Twin boundaries decorated with solute segregation or precipitation, either induced by pre-deformation or developed during subsequent annealing, have been recognized as effective in enhancing the strength of Mg alloys. However, the interplay among solute type, concentration, and heat treatment in governing the evolution of twin boundary structures remains insufficiently understood. Here, the role of annealing in regulating segregation at twin boundaries and the corresponding mechanical response was systematically explored in pre-compressed Mg–xGd alloys (x = 1.29 and 6.2 wt%), with Mg–0.45Mn alloy employed as a reference due to the absence of significant segregation or precipitation. Microstructural and mechanical characterizations were carried out using compressive tests together with electron backscatter diffraction and high-resolution transmission electron microscopy. For Mg–0.45Mn alloy, annealing primarily reduced dislocation density without promoting Mn enrichment at twin boundaries, leading to a drop in yield strength. In contrast, Mg–1.29Gd alloy exhibited a balance between dislocation annihilation and strengthening from Gd segregation, giving rise to a slight increase in yield strength. In Mg–6.2Gd alloy, pronounced segregation and precipitation at twin boundaries dominated, and their strengthening effect surpassed the softening associated with dislocation annihilation, producing a notable enhancement in yield strength. These findings indicate that controlling twin boundary structures via alloying and annealing provides an effective route for improving the mechanical performance of Mg alloys.