Tailoring precipitation/properties and related mechanisms for a high-strength aluminum alloy plate via low-temperature retrogression and re-aging processes

doi:10.1016/j.jmst.2021.10.057

Abstract

Abstract:

The retrogression and re-aging (RRA) processes, aimed mainly at tailoring intergranular precipitates, could significantly improve the corrosion resistance (i.e., stress corrosion cracking resistance) without considerably decreasing the strength, which signifies that an efficient control of the size, distribution and evolution of intergranular and intragranular precipitates becomes critical for the integrated properties of the (mid-)thick high-strength Al alloy plates. Compared to RRA process with retrogression at 200 °C (T77), this study investigated the impact of a modified RRA process (MT77) with lower retrogression temperatures (155-175 °C) and first-stage under-aging on the properties of a high-strength AA7050 Al alloy, in combination with detailed precipitate characterization. The study showed that the strength/microhardness of the RRA-treated alloys decreased with raising retrogression temperature and/or prolonging retrogression time, along with the increased electrical conductivity. The rapid responsiveness of microstructure/property typical of retrogression at 200 °C was obviously postponed or decreased by using MT77 process with longer retrogression time that was more suitable for treating the (mid-)thick plates. On the other hand, higher retrogression temperature facilitated more intragranular η precipitates, coarse intergranular precipitates and wide precipitate free zones, which prominently increased the electrical conductivity alongside a considerable strength loss as compared to the MT77-treated alloys. With the preferred MT77 process, the high strength approaching T6 level as well as good corrosion resistance was achieved. However, though a relatively homogeneous through-thickness strength was obtained, some small discrepancies of properties between the central and surface areas of an 86-mm thick 7050 Al alloy plate were observed, possibly related to the quenching sensitivity. The precipitate evolution and mechanistic connection to the properties were discussed and reviewed for high-strength Al alloys along with suggestions for further RRA optimization.

Key words: Aluminum alloy, Heat treatment, Precipitation, Strength, Grain boundary

L.G. Hou, H. Yu, Y.W. Wang, L. You, Z.B. He, C.M. Wu, D.G. Eskin, L. Katgerman, L.Z. Zhuang, J.S. Zhang. Tailoring precipitation/properties and related mechanisms for a high-strength aluminum alloy plate via low-temperature retrogression and re-aging processes[J]. J. Mater. Sci. Technol., 2022, 120: 15-35.

Figures/Tables 17

Fig. 1. Strength (a), microhardness (b) and electrical conductivity (c) of AA7050 Al alloys after T6 (time=0 min), T77 and MT77 treatments.

Fig. 2. Intragranular precipitates (a, c, e, g), related selected area electron diffraction (SAED) patterns (b, d, f) and HREM images (h-j) of the T6- (a, b) and T77-treated alloys: (c, d) T77 (200 °C/5 min), (e-g, h-j) T77 (200 °C/40 min). Fast Fourier Transform (FFT) images of regions 1-6 in (h-j) correspond to 1-FFT to 6-FFT, respectively. The IFFT images of regions 1, 2 and 3 are also given. (i1) after adjusting brightness and gamma values of IFFT image of region 3 that can display bright contrast. The diffraction spots of Al3Zr phase with L12 superlattice structure also appear in (b, d, f).

Fig. 3. Intragranular precipitates in the MT77 (155 °C/300 min) alloy: (a, b) TEM image, (c, d) SADE pattern, (e-g) HREM images. (e1) FFT images of the white region in (e), (e2) masked FFT image of GP II zone in (e), (e3) IFFT images of GP I zone in (e), (e4) IFFT images of (e3), (f1) FFT images of white region in (f). (g1) FFT image of η’ phase in (g), (g2) and (g3) are IFFT images along g(11$\bar{1}$)Al and g($\bar{2}$$\bar{2}$0)Al of (g1), (g4) IFFT image of (g1). The FFT/IFFT images of white regions 1-4 in (g) are shown with corresponding numbers. The yellow-marked precipitates also display similar morphology with white region 3 in (g).

Fig. 4. Intragranular precipitates in the MT77 (165 °C/300 min) alloy: (a, b) TEM image, (c-e) SADE patterns, (f-h) HREM images of the precipitates. FFT images of (f, g) are inserted. (g1) IFFT images of (g) with removing Al matrix (FFT image as inserted), (g2) after adjusting image parameters (Brightness=0.43, Contrast=0.67, Gamma=0.71) of (g1) that can display bright contrast (marked FFT image as inserted). (g3) and (g5) are the IFFT images of region 4 and 5 in (g), respectively. (g4) and (g6): after adjusting brightness and gamma values of (g3) and (g5), respectively. (h1) IFFT image based on marked FFT image (as inserted) of region 6, (h2) IFFT image of region 6, (h3) after adjusting brightness and gamma values of (h2).

Fig. 5. TEM (a-c), SADE patterns (d-f) and HREM (g-i) images of the MT77 (175 °C/300 min) alloy. (g1) IFFT image of (g, FFT image as inserted) after adjusting image parameters (Brightness=0.47, Contrast=0.67, Gamma=0.69), (h1) IFFT image of (h, FFT image as inserted) after adjusting image parameters (Brightness=0.43, Contrast=0.65, Gamma=0.69), (i1) IFFT images of (i) with removing Al matrix (FFT image as inserted), (i2) marked FFT image of FFT image in (i4) from the white region in (i), (i3) IFFT image from (i2), (i5) IFFT image from the marked FFT image (as inserted) along [$\bar{2}$40]Al direction.

Fig. 6. TEM (a-c, a: bright field, b: dark field) and HREM (g-j) images of the MT77 (160 °C/600 min) alloy. (d-f) SADE patterns, (g1) IFFT images of (g, FFT image as inserted) with removing Al matrix, (g2, g5, h1, h4) FFT images of regions 1-4 in (g, h), (g3, g6, h2, h5) IFFT images of regions 1-4 in (g, h) (marked FFT images as inserted), (g4, g7, h3, h6) IFFT image of regions 1-4 in (g, h) after adjusting image parameters (Brightness=0.47, Contrast=0.66, Gamma=0.68). (i1) IFFT images of the precipitate in (i, FFT image as inserted), (i2) IFFT image after adjusting image parameters of (i1), (j1) and (j2) are the FFT images of regions 5 and 6 in (j), respectively.

Fig. 7. TEM (a-d, b is dark-field image) and HREM (g, j) images of the MT77 (165 °C/600 min) alloy. (d, e, f) SADE patterns. The FFT and IFFT images of upper and bottom white regions in (g) are shown at right; (i) FFT image of (h); the FFT and IFFT images of regions 1 and 2 are shown at right.

Fig. 8. TEM (a, b), SADE (c-e) patterns and HREM (f-j) images of the MT77 (175 °C/600 min) alloy. (f1) IFFT images of (f) with removing Al matrix (FFT image in (f2)), (f3) and (f4) are the FFT image of regions 2 and 1 in (f), respectively. (f5) IFFT image of (f4) (Brightness=0.49, Contrast=0.67, Gamma=0.64). (g1) IFFT images of (g) with removing Al matrix (FFT image as inserted in (g)), (g2) IFFT image of region 3 in (g1) (FFT image as inserted), (g3) IFFT image of region 3 in (g1) after adjusting image parameters (Brightness=0.42, Contrast=0.56, Gamma=0.68). (h1) IFFT image of (h, FFT image as inserted) after adjusting image parameters (marked FFT image as inserted in (h1). (i1) and (j1) are FFT images of (i) and (j), respectively. (i2) FFT image of region 4 in (i).

Fig. 9. TEM (a, b) images and SADE patterns along [001]Al (c) and [112]Al (d) zone axis of the MT77 (175 °C/900 min) alloy.

Fig. 10. GPBs and PFZs near GBs in the T6-, T77- and MT77-treated alloys. A high magnification GB image of MT77 (200 °C/40 min) alloy and GBPs possibly along high angle grain boundaries (HAGBs) of MT77 (165 °C/600 min) alloy are shown.

Fig. 11. The properties along the thickness of the MT77 (165 °C/600 min) treated 7050 Al alloy plate.

Fig. 12. Intragranular and intergranular precipitates (a, b, f, g) and SAED patterns (c-e, h-j) near the plate surface (a-e) and center (f-m) of the MT77-treated 7050 Al alloy plate. (k, m) HREM images, (l) FFT image of coarse precipitate in (k).

Fig. 13. Temperature-time transformation of precipitates in Al-Zn-Mg-(Cu) 7XXX-series alloys (single-step aging data are collected from published data) (the inset shows the possible transformation sequence with changing temperatures and times according to ref. [67]).

Fig. 14. TEM (a), SADE (b-d) patterns and HREM (e-g) images of the 120 °C/300 min aged alloy. The FFT or IFFT images of region 1-3 are marked with corresponding numbers.

Fig. 15. Schematic connection between composition, process, microstructure and properties for high-strength Al alloys.

Fig. 16. Schematical evolution of strength during retrogression and T6 re-aging treatments (a), and aging temperature and time ranges of typical two-step aging and RRA processes: (b) first and/or third steps (the dark area represents the first and/or third step aging parameters), (c) second or retrogression step of different RRA processes.

Fig. 17. TEM (a), SADE patterns (b-d) and HREM (e-g) images of the 165 °C/600 min retrogressed alloy with the first-step 120 °C/300 min aging. (g1) and (g2) are the FFT and IFFT images of the white-box area in (g), respectively. FFT images of other white-box areas are shown with corresponding numbers.

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