Tilt boundaries and associated solute segregation in a Mg-Gd alloy

Microstructures of a Mg-Gd solid solution single phase alloy that has been compressed at room temperature and subsequently annealed have been examined by bright-field and high-angle annular dark-field scanning transmission electron microscopy. It is found that the deformed microstructure contains many nano-sized grains. These nanograins exhibit strong texture: most of them have their 〈12¯10〉 parallel to each other. A range of tilt boundaries are detected between these nanograins, and they can be produced by basal-plane tilt or prismatic-plane tilt about the 〈12¯10〉 axis. Crystallographic analysis indicates that some of these tilt boundaries can be regarded as being generated by impingements of 〈12¯10〉 co-zone variants of {101¯1},{101¯2} and {101¯3} primary twins or secondary or tertiary twins. Segregation of Gd atoms occurs in the tilt boundaries after annealing of the cold deformed sample. The larger size Gd atoms segregate preferentially to dilated atomic sites within, or adjacent to, each tilt boundary to reduce local elastic strain. Consequently, the segregated Gd atoms form a range of unique, chemically ordered patterns specific to tilt boundaries.

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