Electron microscopy
Streaking of Diffraction Spots due to Thin Structures in TEM Specimens
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Orientated, very thin structures in TEM specimens can cause streaking of electron diffraction spots. For instance, for the T1 (Al2CuLi) alloys, the orientation relationship between the T1 and the Al matrix is (0001)T1||(111)Al and [10-10]T1||[-110]Al [1]. The T1 precipitates form as extremely thin (< ~2 nm thick) hexagonal-shaped plates which induce alterations in the shape of reciprocal lattice points, resulting in streaking of diffraction spots in the selected-area electron diffraction (SAED) patterns [2].

Figure 1604 shows HRTEM and HAADF images taken from an Mg97Zn1Y2 crystalline alloy. The extra diffraction spots along the c*-direction with strong streaks indicate a correlation length of 6 x d(002)Mg as marked by the arrow in the diffraction pattern. The HAADF-STEM image shows that the heavy Zn and/or Y atoms, indicated by the bright lines with an interval of ~1.56 nm (= 6 x d(002)Mg), are located at the specific positions of the lamellar structure due to chemical ordering.

(a) HRTEM and (b) HAADF-STEM images of nanocrystalline Mg97Zn1Y2 alloy

Figure 1604. (a) HRTEM and (b) HAADF-STEM images of nanocrystalline Mg97Zn1Y2 alloy. The inset in (a) is the corresponding electron diffraction pattern of the HRTEM image. [3]







[1] H.K. Hardy and J.M. Silcock: J. Inst. Met., 1955-56, vol. 84, pp. 423-28.
[2] B. Noble and G. E. Thompson: Met. Sci. Jour., 1972, vol. 6, pp. 167-74.
[3] Eiji Abe, Atomic-Scale Characterization of Nanostructured Metallic Materials by HAADF/Z-contrast STEM, Materials Transactions, 44(10), 2035-2041, (2003).