Atomic-scale mechanism of the θ″ → θ′ phase transformation in Al-Cu alloys

doi:10.1016/j.jmst.2016.08.031

Abstract

Abstract:

The phase transformation of θ″ → θ′ in an Al-5.7Cu alloy was investigated by aberration-corrected scanning transmission electron microscopy, and the tranformation mode of θ″ → θ′ during aging treatment was clarified. In the presence of the θ″ phases, θ′ was found to be formed by in-situ transformation from θ″ with the same plate shape, size and broad faces. The transformation starts from multiple sites within the θ″ precipitate and the whole θ′ phase finally forms as the preferential θ′ sections grow and connect with each other. Antiphase domain boundaries are also found in some θ′ precipitates when the disregistry exists between different θ′ sections.

Key words: Al-Cu alloy, θ′phase, Phase transformation, Antiphase domains, High-angle annular dark field (HAADF)

Shen Zhenju, Ding Qingqing, Liu Chunhui, Wang Jiangwei, Tian He, Li Jixue, Zhang Ze. Atomic-scale mechanism of the θ″ → θ′ phase transformation in Al-Cu alloys[J]. J. Mater. Sci. Technol., 2017, 33(10): 1159-1164.

Figures/Tables 6

Table 1 Heat treatment processes of the samples.

	Heat treatment process
Process I	540 °C/1 h WQ + 190 °C/480 min AC
Process II	540 °C/1 h WQ + 190 °C/480 min AC + 190 °C/3 min AC
Process III	540 °C/1 h WQ + 190 °C/480 min AC + 190 °C/154 min AC

Fig. 1. Precipitates in the Al-Cu alloy treated by process I treatment. (a) The low-magnified HAADF image along <100>α-Al; (b) the atomic-resolution HAADF image of the large precipitates in the yellow rectangle in Fig. 1(a); (c) the zoomed-in image showing the small precipitates; (d) the atomic-resolution HAADF image of the small precipitates; (e) the SAED patterns of the small precipitates and α-Al matrix.

Fig. 2. Structure of the precipitates in the Al-Cu alloy treated by process II treatment. (a) The low magnified HAADF image; (b) the SAED patterns; (c) full view of a precipitate; (d) the atomic-resolution HAADF image of the region indicated by the yellow frame in (c), the right inset is the simulation HAADF image of θ″ superposing θ′.

Fig. 3. (a) HAADF image of a precipitate after process II treatment, n1-n4 sections have transformed to θ′. (b and c) Enlarged HAADF images (filtered) of the regions between n1 and n2 (in the red frame), and between n2 and n3 (in the yellow frame) respectively, schematic diagram of extending the sections n2 and n3 is overlaid on (c), orange circles represent Cu atoms in the n2 section and blue the n3 section, Al atoms are omitted. (d) Unit cell of θ′. (e and f) The atomic arrangements and simulated HAADF image for APDB of the type a/2<110> lying on {110}θ′ in θ′.

Fig. 4. Observation of precipitates after process III treatment. (a) A low magnifed HAADF image; (b) SAED patterns of the precipitates embedding in α-Al matrix.

Fig. 5. HAADF-STEM images of the same area in an Al-Cu alloy viewed along <100>α after (a) process I treatment and (b) process III treatment. (c) Filtered atomic-resolution HAADF images of the yellow framed area marked in (a); (d) Filtered atomic-resolution HAADF images of the yellow framed area marked in (b). The insets in (c) and (d) are the HAADF simulation images of θ″ and θ′ phase respectively, and the models used in the two simulated images both have a thickness of 6.0 nm embedded in 14.3 nm thick α-Al matrix.

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