First-principle investigation on the interfacial structure evolution of the δ'/θ'/δ' composite precipitates in Al-Cu-Li alloys

doi:10.1016/j.jmst.2020.03.065

Abstract

Abstract:

The precipitation sequence in Al-Cu-Li alloys is sensitively dependent on the Cu/Li ratio. In the low ratio Cu/Li alloys of 1-2.5, the δ' phases usually nucleate and grow from the θ' precipitates, forming δ'/θ'/δ' composite precipitates. In this work, we present a first-principle study on atomic structures and their relative stabilities of the growing δ'/θ'/δ' composite precipitates in Al-Cu-Li alloys. Based on the analysis of the interface formation energy, constituted interface and coherent strains energies, an “anti-phase 1/2[110]” relationship for the opposite δ' has been proposed when the inward θ' has an odd number of Cu-layers. It may be achieved by translating one side of the δ' by$\sqrt{2}/2a$ along the [110] slip direction, which is an energetically most favorable path. By analyzing the bonding characteristics, both the “zigzag Al-Li combined with Cu” and the “zigzag Al-Al” interfacial terminals are found to control the interface structure of the growing δ'/θ'/δ'. According to the calculated ideal tensile strength, the “anti-phase 1/2 [110]” structure is most stable to some extent. When Li atoms at the interface enter decohesion mode along the applied strain, the stable δ'/θ'/δ' is prone to failure because of relatively weak Li-Al covalent bonds. Therefore, the really thin δ' in δ'/θ'/δ' composite precipitates may be explained by the continuous disassociation of Li atoms from the interface. In addition, a very weak Cu-Li covalent bond was suggested in the δ'/θ'/δ' composite precipitates. This is in sharp contrast to previous reports.

Key words: Al-Li alloys, Precipitates, Electronic properties, First principles

Shuo Wang, Chi Zhang, Xin Li, Houbing Huang, Junsheng Wang. First-principle investigation on the interfacial structure evolution of the δ'/θ'/δ' composite precipitates in Al-Cu-Li alloys[J]. J. Mater. Sci. Technol., 2020, 58: 205-214.

Figures/Tables 11

Fig. 1. Schematic diagram of the charge density for A and B parts and the entire configuration, where the δ” is an anti-phase response to the δ'. Note, in the following, this designation is used, unless otherwise specified.

Fig. 2. (a) Simplified diagram of the HAADF-STEM image viewed along with the [001]Al direction. (b) According to the side view (a), all possible interface structures: (b1) the hollow-coordinated 1, (b2) the top-coordinated 1, (b3) the top-coordinated 2, (b4) the hollow-coordinated 2. Note the two nearest-neighboring atomic layers around the interfaces are highlighted as black-dashed lines. For the side and top views of each model, the solid balls indicate the atoms of the upper-layer right above the interface (annotated as 1), and the open balls are the opposite (annotated as 0.5).

Fig. 3. The interface formation energy per atom (ΔG) calculated for the top1 (XI) and the hollow2 (XII) interface models as a function of the number of Li-layers (M(Li-layers)) for L12-δ' phase at one side of the θ': (a) two Cu layers (N(Cu-layers) = 2) inside the θ' phase; (b) three Cu layers (N(Cu-layers) = 3) inside the θ' phase.

Fig. 4. Schematic diagram of the transition process from “in-phase” to “anti-phase” in model XII, the transferred δ' as indicated by the dotted line. (a1) and (b1) represent the δ'/θ'/δ' with an in-phase relationship. (a2) and (a3) are the related “anti-phase 1/2[010]” responding to (a1). (b3) is the related “anti-phase 1/2[110]” responding to (b1). (a4) and (b4) show top-views of the interfacial portion (corresponds to a row of atoms in the solid line frame in (a3) and (b3), respectively) during the transition. Here the red dashed line corresponds to the “in-phase”, purple and blue dashed lines correspond to the “anti-phase 1/2[010]”, and the “anti-phase 1/2[110]”, respectively.

Fig. 5. The interface formation energy per atom (ΔG) calculated for the δ'/θ'/δ' with various relationships as a function of the number of Li-layers (M(Li-layers)) for L12-δ' phase at one side of the θ': (a) two Cu layers (N(Cu-layers) = 2) inside the θ' phase; (b) three Cu layers (N(Cu-layers) = 3) inside the θ' phase. Here, ΔE represents the energy barrier from “anti-phase 1/2[010]” to “in-phase”. ΔE' represents the energy difference from “anti-phase 1/2[110]” to “in-phase”.

Fig. 6. The interface formation energy per atom as a function of the normalized interface area per atom for in-phase (blue), out-phase 1/2[010] (red), and out-phase 1/2[110] (purple). The calculated values of interfacial energy γ (eV/?2) and coherent strain energy ΔGcs (eV/atom) have been calculated.

Fig. 7. The contour plots of the charge density difference of the δ'/θ'/δ' with 2 Cu-layers on the (010) plane: (a) the entire configuration with highlighted (010) and 1/2(010) planes. (b1), (c1) the contour plots of the charge density difference of the in-phase interface at 1/2(010) plane and 1(010) planes, respectively; (b2), (c2) the contour plots of the charge density difference of the anti-phase interface of the 1/2(010) plane and 1(010) planes, respectively; (b3), (c3) the contour plots of the charge density difference of the anti-phase interface of the 1/2(010) plane and (010) planes, respectively. The upper and lower-case Roman numbers (I, II, III, i, and ii) inside the graph correspond to Al-Li bond and Al-Al bond.

Fig. 8. The contour plots of the charge density difference of the δ'/θ'/δ' with 3 Cu-layers on the (010) plane. (a) The entire configuration with highlighted (010) and 1/2(010) planes. (b1, c1) The contour plots of the charge density difference of the in-phase interface at 1/2(010) plane and 1(010) planes, respectively. (b2, c2) The contour plots of the charge density difference of the anti-phase interface of the 1/2(010) plane and 1(010) planes, respectively. (b3, c3) The contour plots of the charge density difference of the anti-phase interface of the 1/2(010) plane and (010) planes, respectively. The upper and lower-case Roman numbers (I, II, III, i, and ii) inside the graph correspond to Al-Li bonds and Al-Al bonds.

Fig. 9. The calculated stress-strain relations of δ'/θ'/δ' with 2 Cu-layers in various tension deformation directions and the ideal strengths of the θ' and δ' phases in the [001] direction.

Fig. 10. The calculated Li-Cu and Li-interface distances (where the δ'/θ'/δ' has 2 Cu-layers with an “anti-phase 1/2[110]” relationship) as a function of the applied strain under the [001] tensile direction. (a) The entire configuration with highlighted Li-Cu and Li-interface distances. (c) The side view of the δ'/θ'/δ' in [100] direction at three strain stages corresponding to (b).

Fig. 11. The entire configuration (a) and crystal orbital Hamilton population (COHP) analysis of (b1) Li-Al3, (b2) Al1-Al2, and (b3) Li-Cu bonding interactions in the δ'/θ'/δ' with the “anti-phase 1/2[110]” relationship. The ICOHP values (in eV/bond) are listed here to show the corresponding interactions (black) and main orbital pair contributions to these (colored).

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