Formation of a new intermediate phase and its evolution toward θ' during aging of pre-deformed Al-Cu alloys

doi:10.1016/j.jmst.2018.11.022

Abstract

Abstract:

The frequent occurrence of hitherto unknown phase Pre-θ'-2 and unusual 1.5 c_θ' thick θ' precipitate was observed by atomic-resolution scanning transmission electron microscopy in the well-studied Al-Cu alloys. This phenomenon is associated with heterogeneous precipitate nucleation and growth on pre-existing dislocations introduced by slight deformation prior to aging. In this study, the precise structure details of Pre-θ'-2 was determined by atomic scale imaging, image simulation based on image forming theories and first principle calculations. Pre-θ'-2 has a well-defined ordered structure sandwiched between two 2a_Al (～1.5c_θ') spaced Cu layers on {200}_Al planes. The strong structural similarities between Pre-θ'-2 and 1.5 c_θ' thick θ' in terms of interfacial structure and thickness, coupled with energetic calculations and preliminary in-situ observations, lead us to propose a new precipitation path toward key strengthening phase θ'.

Key words: Aluminum alloys, Precipitation, Deformation, Age-hardening

Peipei Ma, Chunhui Liu, Ziyao Ma, Lihua Zhan, Minghui Huang. Formation of a new intermediate phase and its evolution toward θ' during aging of pre-deformed Al-Cu alloys[J]. J. Mater. Sci. Technol., 2019, 35(5): 885-890.

Figures/Tables 8

Fig. 1 Age hardening curve at 165 ℃ for the Al-Cu alloy with and without pre-deformation.

Fig. 2 HAADF-STEM images showing the typical microstructure in the alloy aged at 165 ℃ for 9?h: (a) without pre-deformation; (b) overall mixed-precipitates morphology in pre-deformed sample; (c-g) high resolution images of the large precipitates marked by the arrows P1-P4, respectively, in (a); (e) a magnified image of the lower tip of P2. The electron beam is parallel to [001]Al direction.

Fig. 3 Atomic-resolution HAADF-STEM images of (a) pre-θ′-2 and (b) 1.5 cθ′ thick θ′. The yellow arrow points to an enlarged part of the precipitate indicated by the solid-line box. The atomic model and the simulated image are overlaid on the experimental image. Inset with dashed-line box shows a HAADF-STEM image reported in ref. [18] of the complex hetero-phase θ′′+ θt′ at the semi-coherent interface (rim region) between Al matrix and θ′ phase. A 1-nm-wide region of θ′′ is present at the rim area of each precipitate. The arrow at the rim interface represents a dislocation with Burgers vector (aAl/2) [100]. The electron beam is parallel to [001]Al direction.

Fig. 4 Calculated total energies of various proposed structures of Pre-θ'-2 with the first one as the most favorable one.

Fig. 5 Formation energy per Cu atom compared to Cu in solid solution of different precipitate structures appearing in Al-Cu alloy with pre-deformation. The energy of a pre-θ'-2 was set as the zero energy.

Fig. 6 TEM images showing the precipitates and dislocations in the pre-deformed Al-Cu alloy after ageing. The large precipitates normally exist near or at irregular dislocation segments marked by the arrows.

Fig. 7 HAADF-STEM images showing the typical microstructure in the pre-deformed high-purity Al-Cu alloy aged at 165 ℃ for 9?h: (a) overall mixed-precipitates morphology; (b-d) high resolution images of the large precipitates marked by the arrows P1-P3, respectively, in (a). The electron beam is parallel to [001]Al direction.

Fig. 8 Structural change from Pre-θ'-2 to θ' induced by e-beam exposure under TEM.

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