On the origin of the high coarsening resistance of Ω plates in Al–Cu–Mg–Ag Alloys

Effect of pre-straining on the aging behavior and mechanical properties of an Al–Cu–Mg–Ag alloy

“…The angle between the normal of the boundary plane and the normal of the Ω-phase platelets ranges from 0 to 451 to fit the orientation of this phase to a {111} α habit plane. Only one broad interface of a thick Ω-phase retains fully coherent configuration and specific OR [1,[3][4][5]8,12,14], while the opposite broad interface has a fully or partially semi-coherent structure, and OR at this interface deviate from the (001) Ω //(111) α on the value of misorientation (Z41) of a deformation-induced boundary (Fig. 16).…”

“…16). This deviation is attributed to the high density of the thickening ledge [4,8,17,25] on the semi-coherent board interfaces. Therefore, the deformation banding leads to the appearance of relatively coarse platelets of the Ω-phase along the {111} α plane with a semi-coherent interface due to heterogeneous nucleation and accelerated growth at dislocation boundaries or boundaries of shear bands.…”

“…Age-hardenable aluminum alloys belonging to the Al-Cu-MgAg system are used in the aerospace industry due to their attractive combination of high specific strength, good fracture toughness and enhanced creep resistance attributed to the highly efficient strengthening by the Ω-phase [1][2][3][4][5][6][7][8][9][10]. This phase is a coherent modification of the equilibrium θ-phase (Al 2 Cu) that forms as a uniform dispersion of thin, hexagonal plates with large aspect ratios (diameter/thickness) on the {111} α habit planes [2][3][4][5][11][12][13][14][15].…”

“…This phase is a coherent modification of the equilibrium θ-phase (Al 2 Cu) that forms as a uniform dispersion of thin, hexagonal plates with large aspect ratios (diameter/thickness) on the {111} α habit planes [2][3][4][5][11][12][13][14][15]. The replacement of the Al atoms in the entire first and second layers on the broad faces of the Ω-phase plate in the direction normal to the Ω/Al interface with either Ag or Mg, respectively, lowers the interfacial energy of the (001) Ω //(111) α interface and aids in accommodation of the misfit and the volumetric strain that exist between the Ω-phase and the Al lattice [4,5]. Thus, Ag and Mg segregation at the interfaces provides a high degree of coherence on the broad faces.…”

“…Thus, Ag and Mg segregation at the interfaces provides a high degree of coherence on the broad faces. The edges of the plates can lose coherency through Ω plate thickening [4]. The precipitation sequence of Al-Cu-Mg-Ag can be summarized as follows [4,6,7,11]: SSSS-Guinier-Preston GP ð Þzones-Ω À phase-θ À phase…”