Transmission electron microscope observations of rectangular dislocation networks in an Al 70 Co 15 Ni 15 decagonal quas
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The electron diffraction contrast of two types of rectangular dislocation networks in an Al7oCoi5Ni15 decagonal quasicrystal has been analyzed. One type of dislocation network consists of two dislocation sets whose Burgers vectors are parallel to the tenfold axis AlO and a twofold axis A2D. The other type of dislocation network consists of two dislocation sets whose Burgers vectors are parallel to the AlO and the other twofold axis of A2P. The characteristics of the diffraction contrast of the dislocation networks in the Al—Co—Ni decagonal phase are similar to those in conventional crystals.
I. INTRODUCTION The observation of lattice defects is of fundamental importance for the understanding of the structure of quasicrystals (QC's) and their mechanical properties as well. Since the discovery of the quasicrystalline icosahedral phase (/-phase) in rapidly quenched Al-Mn alloys,1 studies of defects in quasicrystals have drawn extensive attention. Dislocations have been observed by electron microscopy in the Al65Cu2oCo15 (Ref. 2) decagonal quasicrystal (DQC) and the icosahedral quasicrystalline phase of Al65Cu2oFei5.3~5 Recently, small dislocation loops6 and stacking faults7 have been observed in the A l - S i - M n /-phase and A l - C o - N i decagonal phase, respectively. Discommensurations and domains have been studied in the M n - S i - A l octagonal phase.8 Smallangle grain boundaries have also been observed in the A l - C o - N i decagonal phase.9 The contrast of dislocations in icosahedral Al62Cu25.5Fe12.5 was investigated in the transmission electron microscope by Wollgarten et al.w In the present paper we report on a study of rectangular dislocation networks in an Ai7oCo15Nii5 decagonal quasicrystal by electron diffraction contrast analysis. II. EXPERIMENTAL The Al7oCoi5Nii5 alloy was prepared by melting the pure metals, using an induction furnace, under an Ar atmosphere. After cooling to room temperature, the ingot was cut into slices. Part of the slices was annealed at 900 K for 24 h under an Ar atmosphere, and then foil specimens for transmission electron microscopy were prepared from the as-cast and annealed slices by mechanical thinning and subsequent ion-milling. The diffraction contrast analysis was carried out in a Phillips CM12 transmission electron microscope at an accelerating voltage 120 kV. 286 http://journals.cambridge.org
J. Mater. Res., Vol. 8, No. 2, Feb 1993 Downloaded: 14 Mar 2015
Wang and Cheng11 extended the dynamical theory of electron diffraction from the case of crystals to the case of QC's and pointed out that one needs only to replace the terms g • R in the case of crystals, where g and R are three-dimensional (3D) reciprocal vectors and displacement vectors, respectively, in the phase factors of the theory, by the corresponding inner products g • R = g • R + g x • R 1 in six-dimensional (6D) space in the case of QC's, with g, R being 6D reciprocal and displacement vectors, g, R their projections in 3D physical space, and g 1 , R 1 in 3D complementary space, respectively. The contr
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