Effect of magnetic field on dislocation morphology and precipitation behaviour in ultrafine-grained 7075 aluminium alloy

doi:10.1016/j.jmst.2021.03.016

Abstract

Abstract:

The influence of magnetic field (1 T) on dislocation morphology and precipitation behaviour of ultrafine-grained (UFG) Al 7075 alloy was investigated after ageing from 90 to 200 °C via wide angle X-ray scattering (WAXS), small angle X-ray scattering (SAXS), and transmission electron microscopy (TEM). Experimental results reveal that the improved precipitation kinetics of alloys in the thickness plane (denoted as sample II) as compared to those in the rolling plane (denoted as sample I), which arises due to a higher dislocation density (morphology of dislocation cells) of the thickness plane than that of the rolling plane (morphology of dislocations and dislocation tangles). Specifically, because of different dislocation morphologies, the magnetic field positively and negatively affects the dislocation activity in samples I and II, leading to enhanced and suppressed precipitation behaviors, respectively. Interestingly, nucleation of the η′ phase is facilitated in the UFG alloy at the critical temperature (140 °C) because it affords a faster rate of atom diffusion and a higher dislocation density as compared to those exhibited by its coarse-grained alloy. This systematic and comprehensive study provides new insights into dislocation morphology and precipitation behaviour of the UFG 7075 Al alloy, while enabling the optimization of precipitation kinetics.

Key words: Ultrafine-grained Al alloy, Precipitation behaviors, Magnetic field, Dislocation morphology, Critical temperature

Jun Luo, Hongyun Luo, Tianshu Zhao, Runze Wang. Effect of magnetic field on dislocation morphology and precipitation behaviour in ultrafine-grained 7075 aluminium alloy[J]. J. Mater. Sci. Technol., 2021, 93: 128-146.

Figures/Tables 18

Table 1. Chemical composition of the AA 7075 alloy (wt.%).

Zn	Mg	Cu	Fe	Cr	Si	Mn	Ti	Al
5.35	2.3	1.41	0.27	0.19	0.08	0.07	0.03	balance

Fig. 1. Schematic representation of rolling samples at liquid nitrogen temperature, sample names and their corresponding TEM microstructures, and the experimental process.

Fig. 2. Schematic of (a) WAXS and (b) SAXS experiments. The insets in (a) reveal the distribution of η′/η phases diffraction peaks derived from the integrated two-dimensional (2D) WAXS pattern. The inset in (b) indicates one-dimensional (1D) SAXS profiles derived from the integrated 2D SAXS pattern.

Fig. 3. Microstructures of samples with different states. (a) Bright-field TEM image of sample I, (b) the corresponding dark-field TEM image of (a), (c) TEM image of sample II, (d) statistical distribution of the width of laminated structures of sample II. The TEM images of samples (e) NH-I-200 °C, (f) MH-I-200 °C, (g) NH-II-200 °C and (h) MH-II-200 °C. The yellow arrows in (e) and (f) denote the dislocation walls, and the red dotted lines in (c), (g) and (h) indicate the laminated structures.

Fig. 4. Typical 2D WAXS patterns of the samples NH-I, MH-I, NH-II and MH-II with different integrated regions under different ageing temperatures (90, 140 and 200 °C). The regions i to iii in the figure illustrate the integrated range. The Debye rings show the {111} and {200} diffraction peaks of Al alloy and the dotted black lines represent the η′/η phases. “BS” represents the area covered by a beam stop. The red triangles, black squares, blue circles and brown stars denote four texture components.

Fig. 5. Intensity distributions of the {111} and {200} diffraction peaks of samples for different integrated regions and ageing temperatures. (a) NH-I, (b) MH-I, (c) NH-II and (d) MH-II.

Fig. 6. Intensity distributions of diffraction peaks associated with the η′/η phases at the ageing temperatures of 90, 140 and 200 °C. Regions i to iii for the samples (a)-(c) NH-I, (d)-(f) MH-I, (g)-(i) NH-II and (j)-(l) MH-II. The arrows in (d)-(e) and (g)-(h) reveal the discernible patterns of the η′/η phases.

Fig. 7. Integral area distributions (area 1, area 2 and area 3) of samples NH-I, MH-I, NH-Ⅱ and MH-Ⅱ in 2D SAXS patterns at 90, 140 and 200 °C. (a) Typical 2D SAXS scattering images of these samples at different ageing temperatures that represent a variety of precipitation states. The evolution of areas 1, 2 and 3 in samples (b-d) NH-I and MH-I, (e-g) NH-II and MH-II.

Fig. 8. SAXS signal evolution associated with peak shifting of samples at different ageing temperatures. Samples of (a) NH-I, (b) MH-I, (c) NH-II, (d) MH-II. The red arrows shown in (a-d) indicate the direction in which the peak moves.

Fig. 9. SAXS signal evolution of samples NH-I, MH-I, NH-II and MH-II at different ageing temperatures. (a) 90 °C, (b) 120 °C, (c) 140 °C, (d) 160 °C, (e) 180 °C, (f) 200 °C. The evolution of (g) Guinier radius, Rg, and (h) volume fraction, fv of these samples from 140 to 200 °C. The qmax in (c) illustrates the scattering vector at which the maximum value of the plot is reached, qmax is easily discernible at 140 °C, which may represent η′/η phases, thereby indicating a critical temperature for the nucleation of η′/η phases. The green regions in (a-c) reveal the signals of clusters (GP zones).

Fig. 10. HRTEM images (taken with the [110]) of the four samples at 90 and 200 °C. Samples of (a) NH-I-90 °C, (b) MH-I-90 °C, (c) NH-II-90 °C, (d) MH-II-90 °C, (e) NH-I-200 °C, (f) MH-I-200 °C, (g) NH-II-200 °C and (h) MH-II-200 °C. The red and white dotted lines in the figures illustrate the GP zones and η′/η phases, respectively. The corresponding FFT patterns of the GP zones and η′/η phases that are generated from the regions of the red and yellow dotted lines in Fig. 10. The red and white arrows in the insets indicate the diffraction patterns of the GP zones and η′/η phases.

Fig. 11. TEM images of the four samples and the selected area diffraction (SAD) patterns at 200 °C. Samples of (a) NH-I-200 °C, (b) MH-I-200 °C, (c) NH-II-200 °C and (d) MH-II-200 °C. The SAD patterns of samples of (a1) NH-I-200 °C, (b1) MH-I-200 °C, (c1) NH-II-200 °C and (d1) MH-II-200 °C. The yellow arrows in (a-d) and white arrows in (a1-d1) represent the η′/η phases and the corresponding diffraction spots, respectively.

Fig. 12. Schematic illustration of the electronically excited state, atomic magnetic moment, and dislocation line motion in the magnetic field. Diagrams corresponding to the (a) electronically excited state, (b) extranuclear electron excitation, (c) atomic magnetic moment and (d) dislocation line motion. σp in (d) represents the critical shear stress of dislocation, which could be reduced by the magnetic field, leading to an acceleration in the dislocation motion.

Fig. 13. Initial microstructure of the samples and evolution of the dislocation activity. (a) The microstructure of the samples after cryo-rolling. The TEM images of sample I (rolling direction, RD-TD plane) and II (thickness direction, RD-ND plane) in (a) show the dislocation tangles and dislocation cells, respectively. (b) The dislocation density evolution of these samples from 90 to 200 °C. The insets in (b) reveal the dislocation motion and annihilation during the ageing process. (c) TEM results of dislocation activity with and without the magnetic field.

Fig. 14. Schematic diagrams of solute atom diffusion and Gibbs free energy evolution at the critical temperature (140 °C) in the UFG alloy. (a) TEM image of the initial state of the CG alloy and schematic of the GP zone coarsening. (b) TEM image of the initial state of the UFG alloy and the schematic showing η′ phase nucleation. (c) The evolution of Gibbs free energy of the UFG alloy at the critical temperature. (d) SAXS signal evolution associated with peak shifting of the CG and UFG alloys at 90 and 140 °C.

Fig. 15. Spatial coordinate distribution associated with a cylindrical precipitate. (a) Coordinate evolution of the precipitate between from a direct space to a reciprocal space. (b) The relationship between the shapes of the η′ phases and the corresponding 2D SAXS image.

Fig. 16. Electron cloud density distribution of the Al matrix and precipitates in sample NH-II at 90, 140 and 200 °C. (a) 90 °C. (b) 140 °C. (c) 200 °C. (d)-(f) shows the corresponding 2D SAXS and HRTEM images of (a)-(c), respectively. (g) The percentage of different areas, (h) Iq2 vs. q plot, and (i) schematic representing the relationship between electron cloud density, 2D SAXS patterns, and precipitation behaviour.

Fig. 17. Evolution of precipitation kinetics during ageing. The SAXS signal evolution associated with peak shifting of samples at 90, 140 and 200 °C: (a) NH-I, (b)MH-I, (c)NH-II and (d) MH-II. (e) Dislocation morphology, dislocation density and precipitation kinetics of these samples.

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