DISLOCATIONS OBSERVED BY TRANSMISSION ELECTRON-MICROSCOPY IN THE QUASI-CRYSTALLINE PHASE T2 OF AL-LI-CU AFTER DEFORMATION

In the quasicrystalline phase T2 of an Al-Li-Cu alloy, dislocations have been observed after deformation by transmission electron microscopy using diffraction contrast experiments. These dislocations obey different contrast extinction rules according to their Burgers vectors. One of the families obeys the so-called strong extinction condition (SEC) which means that the invisibility criterion g parallel-to .b parallel-to = 0, g perpendicular-to .b perpendicular-to = 0, is satisfied in both subspaces. The general case of contrast extinction in quasicrystals is the weak extinction condition (WEC) where g parallel-to .b parallel-to = -g perpendicular-to .b perpendicular-to not-equal 0. The SEC holds for three independent diffraction vectors which allows the determination of the direction of the Burgers vector components both in physical (b parallel-to) and in perpendicular (b perpendicular-to) spaces. The component b parallel-to was found to be parallel to the twofold axis <0/0, 0/0, 0/2> and the corresponding component b perpendicular-to in perpendicular space is parallel to the axis <0/0, 0/0, 2/0>. The other families of dislocations obey the WEC because their contrast disappears only for some of the excited diffraction vectors in a systematic row. In this case the direction of the Burgers vector in physical space cannot be determined.