Effect of yttrium on nanocrystallization of magnesium alloys during cold rotary swaging

Abstract

In this study, binary Mg-2Y alloy, pure Mg, and AZ31 alloy were prepared via cold rotary swaging. Microstructure evolution during the grain refinement process was explored using transmission electron microscopy. The results showed that it was impossible to achieve nanocrystallization in pure Mg via cold rotary swaging, which was attributed to the difficulty of forming dislocation arrays. In contrast, after swaging, nanograins were obtained in an alloy formed by adding 2 wt% Y to Mg. In the early stage of swaging, the initial coarse grains of the Mg-2Y alloy were subdivided by the formation of twin lamellae, dislocation arrays (in the matrix) and deformation bands. With increasing strain, more dislocation arrays were formed within the twin lamellae and deformation bands, which resulted in further refinement of the twin lamellae and deformation bands and promoted the formation of nanoscale subgrains. As the swaging process continued, these nanoscale subgrains transformed into nanograins with high-angle grain boundaries. Nanograins were also developed in the swaged AZ31 alloy. The formation rate of nanograins during swaging was slower, and the grain refinement effect after swaging was weaker, in the Mg-2Y alloy than the AZ31 alloy. • It cannot form nanograins in pure Mg due to the absence of dislocation arrays. • Deformation bands formed in Mg-2Y alloy but not in pure Mg and AZ31 alloy. • Accelerated nanocrystallization in AZ31 alloy due to enhanced twinning activity. • A weaker grain refinement effect obtained in Mg-2Y alloy than in AZ31 alloy.