Type of presentation: Invited

MS-9-IN-6084 STEM-Based Characterization of Defects and Precipitates in Structural Materials

Smith T. L.¹, Bowers M. L.¹, McAllister D. A.¹, De Graef M.², Mills M. J.¹

¹Department of Materials Science and Engineering, Center for Electron Microscopy and Analysis (CEMAS), The Ohio State University, Columbus OH 43210, USA, ²Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh PA 15213. USA

mills.108@osu.edu

Advancements in our ability to characterize deformation mechanisms and precipitate structures finer length-scales are providing new insights into the governing deformation mechanisms and precipitation processes in several important commercial alloy systems. These advancements are based upon developments using STEM-based approaches to imaging and chemical analysis using EDS.

Diffraction contrast STEM imaging holds significant advantages over conventional TEM (CTEM) imaging of defects [1-3]. The advantages include the suppression of auxiliary contrast effects (bend contours, etc.) and the ability to image in thicker specimens than is practical using conventional TEM at standard operating voltages. Additionally, CTEM visibility rules such as those for stacking faults and “g•b analysis” for dislocations also remain in STEM mode provided the convergence condition and detector geometry are configured appropriately, as in Figure 1 showing a dislocation analysis for a Ni-base superalloy. High contrast, bright-field images along low index zone axes can also be formed – a mode which is not feasible with CTEM. These conditions will be described and image simulations that support these conclusions using the CTEM Soft program will also be presented.

High resolution high angle annular dark field (HAADF) imaging and ChemiSTEM compositional analysis has provided new insights into microstructure development and deformation processes in the commercial superalloy 718. Atomic-scale imaging using HAADF (Figure 2a) is extremely effective for revealing the morphology and size of these precipitates, where the Nb-rich γʹʹ (DO22 structure) phase appears with enhanced intensity relative to the FCC matrix. The highly planar {010} faces bounding the γʹʹ phase are interfaces with the γʹ phase. While Z contrast from the L12 ordering in the γʹ phase is indistinguishable from the matrix, this conclusion is unambiguously supported by ChemiSTEM EDS mapping, as shown in Figure 2b. Deep insight into the mechanisms of plastic deformation have also been gained through HAADF imaging, which in this alloy has advantages relative to diffraction contrast imaging due to the large strain fields of the precipitates that tend to obscure the dislocation structures.

References
[1]P.J. Phillips, M.J. Mills, and M. De Graef, Philosophical Magazine 91 (2011) 2081-2101.
[2] P.J. Phillips, M.C. Brandes, M.J. Mills, and M. De Graef Ultramicroscopy 111 (2011) 1483-1487.
[3] P.J. Phillips, M. De Graef, L. Kovarik, A. Agrawal, W. Windl, M.J. Mills, Ultramicroscopy, 116 (2012) 47–55.

Support from the following program is gratefully acknowledged: the National Science Foundation, Division of Materials Research, under the GOALI program DMR-0907561 (for MLB and MJM), the GE University Strategic Alliance Program (for TLS) and the Metals Affordability Initiative program (for DAM).

Fig. 1: Example of a dislocation analysis in a Ni-base superalloy. Shown are zone axis [001] bright field and two dark field images using g020 and g200. Inset region shows two different families of dislocations.

Fig. 2: (a) HAADF STEM image on <110> zone showing a composite γʹʹ/γʹ particle. (b) ChemiSTEM™ EDS map showing net intensity of Nb and Al peaks which enables discrimination of γʹ and γʹʹ particles.