On the S-phase precipitates in 2024 aluminum alloy: An atomic-scale investigation using high-angle annular dark-field sc
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On the S-phase precipitates in 2024 aluminum alloy: An atomic-scale investigation using high-angle annular darkfield scanning transmission electron microscopy Chenyang Zhu1
, Kexin Lv1, Bin Chen1,a)
1
School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China Address all correspondence to this author. e-mail: [email protected]
a)
Received: 17 November 2019; accepted: 12 March 2020
The present article reports on a comprehensive atomic-scale investigation on S-series precipitates in 2024 aluminum alloy. Cs-corrected high-angle annular dark-field scanning transmission electron microscopy is applied to reveal the fine atomic-scale structure of precipitates at early ageing state. Geometrics phase analysis is used for elucidating the induced strain field from precipitates. The precipitate sequence of S-series precipitates in 2024 Al alloy is identified as follows: super saturated solid solutions (s.s.s.s.) ! clusters (GPB zone) ! S9 phase ! S phase. The interfaces between precipitates acting as precursor of S9 phase are well characterized. One typical characteristic of S-series phase precipitates is the coexistence of clusters and subsequent metastable phases. Transformation of metastable phases is characterized. Corresponding hardness structure relationship is revealed, and S9 phase is considered as the key strengthening structure in S-series precipitates in 2024 Al alloys.
Introduction Aluminum alloys are promising engineering materials, with low density, good plasticity, and formability, which are widely applied in aerospace and transportation field. 2xxx is one series of the age-hardenable Al alloys, which meets the requirement of the combination of specific strength and creep damage tolerance [1, 2, 3, 4]. Copper and magnesium are widely used as alloying elements. Proper heat treatments have been investigated so as to obtain better mechanical properties. One of the major principles used is precipitation hardening (ageing strengthening) [5, 6, 7]. When fine precipitates are at a proper size are used, the chemical compositions in the matrix will greatly increase the strength of the sample. Various influential factors on precipitation hardening have been comprehensively investigated, such as heat treatments and chemical composition [8, 9, 10, 11]. As for 2024 alloy, the dominant precipitate is S-series phases, which is also the significant strengthening phase during ageing treatment. The equilibrium S phase (Al2CuMg) was well studied, the structure of which was first determined by single crystal Brunauer-Emmett-Teller (BET) and modified by experimental and simulation methods [5, 12, 13, 14]. During the ageing process, S phase grows on (210)Al habit planes along h001iAl directions, with an orientation relationship of [100]Al//[100]S,
ª Materials Research Society 2020
½02 1Al //[010]S, [012]Al//[001]S [5]. A second oriented S phase was proposed from styles and coworkers as [100]Al//[100]S, ½02 1Al //[014]S [15]. The strengthening of S phase is also comprehensively understood [2
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