Segregation of solute elements at grain boundaries in an ultrafine grained Al–Zn–Mg–Cu alloy

• Published in: Ultramicroscopy 2011-05, Vol.111 (6), p.500-505
• Main Authors: Sha, Gang, Yao, Lan, Liao, Xiaozhou, Ringer, Simon P., Chao Duan, Zhi, Langdon, Terence G.
• Format: Article
• Language: English
• Subjects: Alloying elements; Aluminium alloy; ALUMINUM ALLOYS (50 TO 99 AL); Aluminum base alloys; Atom probe tomography; BOUNDARIES; Equal channel angular pressing; GRAIN BOUNDARIES; Grain size; GRAIN SIZE AND SHAPE; PRECIPITATION; SEGREGATION; Segregations; Solute segregation; SOLUTES
• ISSN: 0304-3991; 1879-2723
• DOI: 10.1016/j.ultramic.2010.11.013

The solute segregation at grain boundaries (GBs) of an ultrafine grained (UFG) Al–Zn–Mg–Cu alloy processed by equal-channel angular pressing (ECAP) at 200 °C was characterised using three-dimensional atom probe. Mg and Cu segregate strongly to the grain boundaries. In contrast, Zn does not always show clear segregation and may even show depletion near the grain boundaries. Trace element Si selectively segregates at some GBs. An increase in the number of ECAP passes leads to a decrease in the grain size but an increase in solute segregation at the boundaries. The significant segregation of alloying elements at the boundaries of ultrafine-grained alloys implies that less solutes will be available in the matrix for precipitation with a decrease in the average grain size. ► Atom probe tomography has been employed successfully to reveal unique segregation of solutes at ultrafine grained material. ► Mg and Cu elements segregated strongly at the grain boundary of an ultrafine grained Al–Zn–Mg–Cu alloy processed by 4-pass and 8-pass ECAP at 200 °C. Zn frequently depleted at GBs with a Zn depletion region of 7–15 nm in width on one or both sides of the GBs. Only a small fraction (3/13) of GBs were observed with a low level of Zn segregation where the combined Mg and Cu excess is over 3.1 atom/nm 2. Si appeared selectively segregated at some of the GBs. ► The increase in number of ECAP passes from 4 to 8 correlated with the increase in mean level segregation of Mg and Cu for both solute excess and peak concentration. ► The change of plane normal of a grain boundary within 30° only leads to a slight change in the solute segregation level.