Grain Boundary Chemical Analysis Using Intense Electron Beams
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* School of Metallurgy and Materials + IRC in Materials for High Performance Applications The University of Birmingham, Birmingham B15 2TT UK -Department of Materials Science and Engineering, The University of Liverpool, Liverpool L69 3BX, UK ABSTRACT Scanning Transmission Electron Microscope (STEM) Energy Dispersive X-Ray analysis (EDX) linescans and mapping have been used to examine the large angle grain boundary chemistry of Ni-rich Ni 3AI both with and without boron. The results show that the Al content is reduced while the Ni content is unchanged at the grain boundaries in all these alloys, with the percentage of reduction of Al at the grain boundaries decreasing as boron concentration increases. This is ascribed to differential surface sputtering of the lighter Al atoms. INTRODUCTION There is still considerable controversy surrounding the chemical composition and state of order of grain boundaries in Ni3AI optionally doped with boron (for review see Horton et al. (1991) (1]). This paper presents STEM/EDX linescans and maps at large angle grain boundaries of Ni-rich Ni3A1 both with and without boron. EXPERIMENTAL Alloys of three compositions (Ni-23.5A1, Ni-23.5A1-0.2B and Ni-23.5-0.5B) were prepared by plasma melting under an argon atmosphere. The alloys were then homogenised at 1050TC for 3 days in a vacuum furnace at 10-6Pa. In order to obtain a sufficient number of grain boundaries, so that there would be a reasonable chance of there being one in the visible area of a TEM specimen, specimens 5x5x8 mm3 were cold deformed to 80% at room temperature and then recrystallized at 900TC for I hour under vacuum. The grain size after such treatment was found to be about 2-3 Aim. TEM specimens were made by electrochemical polishing using a twin jet polisher and a solution of 55% ethanol, 37% butoxyethanol and 8% perchloric acid at about -8°C. Before the STEM observations the specimens were cleaned by ion beam for 0.5 hour. Grain boundary composition analysis was carried out on a VG601UX STEM operated at 100kV. It is a dedicated cold field-emission instrument with a windowless X-ray detector. Point to point image resolution of 0.25 nm is attainable and the analytical probe size is lnm at a probe current of 0.5 nA. Large angle grain boundaries were identified using selected area diffraction on both sides of the boundary. Linescans were performed across the grain boundaries. X-rays corresponding to the Ni(K), Ni(L) and Al(K) emission peaks were collected. The dwell time (time per point) for the linescans was set to 10 seconds. To remove the background, neighbouring windows were set above and below the peaks. The total number of background channels was arranged to equal the number of channels on the peak.
KK10.2.1 Mat. Res. Soc. Symp. Proc. Vol. 552 0 1999 Materials Research Society
To set the electron beam parallel to the grain boundary, the specimen was adjusted repeatedly in order to obtain the narrowest grain boundary contrast. This process is critical for the chemical analysis in the grain boundary as illustrated
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