Abstract: The interaction between dislocations andΩphases in Al-Cu-Mg-Ag alloy during deformation has been studied by traditional transmission electron microscopy and high resolution transmission electron microscopy.It has been shown that dislocations cut through instead of loop around theΩphases,which have semi-coherent relationship with the AI matrix.The color variation and steps due to the dislocation cutting can be observed by TEM dark field and HRTEM.Steps were made by a series of dislocations with high strain energy.This high strain energy inhibits dislocations cutting at the same position or around the step continuously,thus limits the height of the steps.The atom periodical distribution has been destroyed and an edge dislocation forms around the step caused by dislocation cutting theΩphase.The slip systems (slip direction and slip magnitude) are different forΩphase and AI matrix due to the variation in crystal structure. The slip directions of dislocation are along the 011 and 001 in the matrix andΩphase,respectively, with about 30ångle.