Type of presentation: Poster

MS-4-P-1731 Scanning transmission electron microscopy study of Mg-RE solid solutions

Bugnet M.1, Kula A.1,2, Niewczas M.1, Botton G. A.1
1Materials Science and Engineering, McMaster University, Hamilton, ON, Canada, 2AGH-University of Science and Technology, Cracow, Poland
bugnetm@mcmaster.ca

Magnesium-Rare Earth (RE) alloys exhibit favorable combination of high specific strength and good ductility, making them suitable for various applications in aerospace, aircraft, and automotive industry. Understanding the effect of RE elements on alloys’ properties is considered a key aspect in alloy develepoment and processing. RE elements tend to segregate to the grain boudaries affecting kinetics of the grain growth, texture and mechanical properties of Mg-based alloys. The phenomenon generates much interest from the academic and industrial communities and recently has been a subject of intense studies [1].

In the present work, we investigate the distribution of solute elements in the structure of Mg-0.28at.%Gd and Mg-0.36at.%Sm binary alloys by high-angle annular dark field (HAADF) imaging in scanning transmission electron microscopy (STEM) and by electron energy loss spectroscopy (EELS) techniques [2]. The structure of Mg-0.28at.%Gd and Mg-0.36at.%Sm solid solutions is characterized by STEM in Z-contrast, down to the atomic scale. We show that Gd(Sm) are present in two different forms: (i) in solid solution as quasi-random atoms distributed in Mg matrix and (ii) as segregates at high angle grain boundaries forming 1-2 nm Gd(Sm)-rich clusters (Figure 1). The analysis of the structural models for atomically resolved images of the clusters at grain boundaries suggests the stabilization of the face-centered cubic Gd phase (Figure 2). The results validate the already reported predictions about segregation of RE elements at grain boundaries in Mg-RE alloys, and ultimately provide a direct visualization of the distribution of the solute atoms in the structure of Mg-Gd and Mg-Sm alloys. The segregation phenomenon of solute atoms at grain boundaries can be directly correlated to the decrease of the grain size in the Mg-Gd and Mg-Sm alloys as compared to pure Mg. The present study provides new insight towards understanding the effect of RE elements on the texture development during alloy processing and recrystallization, and thereby the mechanical behavior and properties of Mg-RE alloys [3,4].

References:

[1] J.D. Robson, Metall Mater Trans A, (2013). DOI: 101007/s11661-013-1950-1

[2] M. Bugnet, A. Kula, M. Niewczas, G.A. Botton, submitted.

[3] A. Kula, R.K. Mishra, M. Niewczas, in preparation.

The Authors are grateful to NSERC for financial support. The STEM work was carried out at the Canadian Centre for Electron Microscopy, a national facility supported by NSERC and McMaster University.

Fig. 1: HAADF-STEM image of a high angle grain boundary in a Mg-0.28at.%Gd alloy, and EELS line scans illustrating the substantial segregation of Gd.

Fig. 2: (a-c) High resolution HAADF-STEM image of Gd-rich clusters at high angle grain boundaries in a Mg-0.28at.%Gd alloy.