Twin relationship in between the variant-pair of η precipitates in the Al-Zn-Mg-Cu aluminium alloy

For Al-Zn-Mg-Cu aluminium alloys, 15 types of η precipitates would possess the symmetrically variants distributed on the closed planes of the Al matrix, parallel to the (0001)η, (21�1�0)η, or (101�0)η interfaces of precipitates. The η2 precipitates, possessing the crystallographic orientation of (11�1�)Al // (0001)η2 and [110]Al // [101�0]η2, would exhibit four equivalent variants, i.e., η2(1) to η2(4), on the (11�1� )Al habit planes. In the present work, along the zone axis of [110]Al // [101�0]η, the edge-on configurations showing the twin-like atomic arrays would occur as the growth/coalescence of two η2 variants grown on (11�1�)Al and (11�1)Al planes, respectively. The twin relationship can be revealed in term of the crystallographic relationship at 70.5 ̊ with respect to their habit planes. Alternatively, on the (1�1�0)Al image, it also indicates the nearly-twinning configuration in between the variant-

It has been reported that the distributions in between the orientational variants of precipitates would be easily misunderstood from the 2-dimensional projections of the 3-dimensional phenomena [11,12]. For example, in the Al-Cu-(Li) aluminium alloys [11], the impingements are inevitably caused by the overlap of two edge-on variants of θ'-Al2Cu precipitates, in which their habit planes are parallel to the (100) Al and (010) Al planes of the Al matrix, respectively. On the other hand, in the recent work employed by HAADF-STEM [12], the joint of two edgeon variants of θ' incisively revealed the distributed arrangements, implying that the possibility of the interaction between the orientational variants of precipitates. However, so far there is no direct experimental to support the crystallographic relationship such as twinning relationship in between two variants of θ'. Tacitly, in the Al-Zn-Mg-Cu aluminium alloys, whether the crystallographic distribution of the 15 typed η precipitates (i.e., η1-η14, and η 4 ') and their corresponding variants [1,2] would also exist the crystallographic relationship in between themselves has yet to be elucidated.
In the present work, employed by Cs-corrected HAADF STEM, we especially focused on the crystallographic relationships of

Materials and methods
The AA7050 aluminium alloy (Al-6.25Zn-2.14Mg-2.23Cu, wt.%) was used in this study. After solution treatment (at 475 °C for 1 h with water quenching), the samples were treated with a three-step ageing treatment: (1) ageing at 120 °C for 8 h, (2) ageing at 165 °C for 1 h, and subsequently (3) ageing at 174 °C for 8 h with an applied constant tensile stress of 162.5 MPa at the same time (named as creep-age forming samples, CAF samples). The TEM specimens were prepared by cutting discs from the selected samples for observing the pancaked structures and thinning the discs to the thickness of 65 -70 μm before they were twin-jet electropolished in a mixture of 0.33 HNO3 and 0.67 CH 3 OH at -20 °C with a working voltage of 10-11 V, as shown in Fig. 2. The oxidation layers of specimens was removed and the corresponding thickness was reduced with an M1040 NanoMill system (E.A. Fischione Instrument), as shown in Fig. 3a and then the hydrocarbon contamination was cleaned by an M1070 Nanoclean plasma (E.A. Fischione Instrument), as shown in Fig. 3b. Afterwards, the TEM specimens were observed with an FEI Titan Chemi-STEM equipped with a spherical aberration corrector (Cscorrector), under the 135 nm camera length and with a collecting angle range of ~8.24 mrad (inner angle) to ~143.6 mrad (outer angle) of a HAADF detector.     5a shows that two edge-on configurations of two variants of η 2 (i.e., η 2 (2) and η 2 (4) ) exhibiting the sandwiched stacking structure on the (0001)η 2 // (1 � 11 � ) Al and (0001)η 4 // (1 � 11) Al habit planes, respectively. Their different size can be ascribed to the difference in the growth rate. Particularly, the joint area, i.e., the interweaved interface, between η 2 (2) and η 2 (4) possesses a twinning relationship with an angle of 109.5° between the (002 � ) Al plane, as marked by the red lines in Fig. 5a. Here, it can be ascribed to two phenomena. First, the collision of two variants of η 2 occurred as they grow, the so-called hard impingement [12,14]. Secondly, these two η 2 would develop at the same interface of the nucleation stage. we temporarily call the intrinsic boundary.  [13].

Results & Discussion
For the detailed investigation on the atomic arrangement and microstructural evolution in between two variants of η 2 (i.e., η 2 (2) and η 2 (4) ), the area with the nearly darkness Z-contrast (as marked by the red arrow in Fig 5b) can be recognized as segregation of vacancies or Mg solute atoms. This given area was supposedly ascribed to the early stage of nucleation. As the growth of these two variants of η 2 , vacancies and Mg atoms are gradually annihilated by the substitutions of Zn and Cu solute atoms. Finally, the mismatched interface would turn into the well-matched interface (as marked by the solid dot of the red arrow in Fig. 5b)  Another case crystallographic relationship of different variants of η 2 is shown in Fig. 6a. Specially, the joint area (Fig. 6) is totally different from the end of two variants, as shown in Fig. 5. Along the zone axis of [110] Al , the edge-on configurations of η 2 (2) and η 2 (4) , grown on the (0001)η 2 // (1 � 11 � ) Al and (0001)η 4 // (1 � 11) Al habit planes, respectively, reveal the twinning relationship with an angle of 109.5° or 70.5° with respect to two different twinning planes in between these two variants. Similarly, the detailed study on the atomic configurations in between η 2 (2) and η 2 (4) are illustrated in Fig. 6b. It has been found that the twin relationships of two angles of 109.5° and 70.5° between the (11 � 3) Al and (3 � 31) Al planes of the Al matrix, respectively. The coherent twin boundaries would be caused by the nucleation and growth, instead of the effect of the hard impingement. It could be suggested that the nucleation site would be the partial segregation of Zn/Cu nearby the interfaces between the η 2 precipitate and the Al matrix with the abnormally strong bright Z-contrast, reported in the previous work [2]. In the present case, the nuclei would be located on the interfaces of η 2 (2) and would follow the twinning relationship as the growth of precipitates. Fig. 6 (b) A enlarged image of (a) revealing the twin relationships of two angles of 109.5° and 70.5° between the (11 � 3) Al and (3 � 31) Al planes with respect to η 2 (2) and η 2 (4) precipitates.
Furthermore, in Fig. 7, the edge-on configurations of η 2 (2) and η 2 (4) are specifically observed along the zone axis of [1 � 1 � 0] Al with respect to the resulted HAADF STEM image of Fig. 6. At the joint or intersected position, the crystallographic relationship in twinning would exist. However, one coherent twinning boundary, as indicated by the red arrow in Fig. 7, can be found. It can be suggested that the segregations of Zn/Cu to be the nucleation site of η 2 (4) would locate on the part of the η 2 (2) /Al interfaces.
Conclusively, according to the orientation relationships of 15 typed η precipitates (η 1 -η 14 , and η 4 ') and the corresponding variants, their crystallographic relationships are related to be twinning. In the present work, along the zone axes of [110] Al and [1 � 1 � 0] Al , it can be fount the twinning boundaries in between the edge-on configurations of η 2 (2) and η 2 (4) .