Structures and formation mechanisms of dislocation-induced precipitates in relation to the age-hardening responses of Al-Mg-Si alloys

doi:10.1016/j.jmst.2019.11.001

Abstract

Abstract:

In the slightly deformed Al-Mg-Si alloys, dislocation-induced precipitates are frequently observed, and they usually line up, forming sophisticated precipitation microstructures. Using atomic-resolution electron microscopy in association with hardness measurements, we systematically investigated these precipitates in relation to the age-hardening responses of the alloys. Our study reveals that the majority of dislocation-induced complex precipitates are actually short-range ordered while long-range disordered polycrystalline precipitates and multiphase composite precipitates, including polycrystalline U2 precipitates, B'/U2, B'-2/U2, B'/B'-2/U2 and β'/U2 composite precipitates. It is suggested that the formation of these complex precipitates is mainly owing to a high nucleation rate and rapid growth of different precipitate phases parallel to the associated dislocation lines. Since dislocation-induced precipitates consume more Mg than Si from the matrix and have a high formation kinetics, they will have different impacts on the matrix precipitation in different types of Al-Mg-Si alloys. Our results further demonstrate that for the “normally-β"-hardened” alloy, their formation leads to a coarser precipitate microstructure in the matrix, whereas for the “normally-β'-hardened” alloy, their formation reverses the precipitation pathway in the matrix, resulting in a reduced age-hardening potential of the former alloy and an improved age-hardening potential of the latter alloy.

Key words: Al-Mg-Si alloys, Precipitation, Dislocation, Age-hardening, Electron microscopy

Y.X. Lai, W. Fan, M.J. Yin, C.L. Wu, J.H. Chen. Structures and formation mechanisms of dislocation-induced precipitates in relation to the age-hardening responses of Al-Mg-Si alloys[J]. J. Mater. Sci. Technol., 2020, 41: 127-138.

Figures/Tables 17

Fig. 1. Variations of hardness for the AA-treated samples and for the PDA-treated samples with aging time at 250 °C: (a) Alloy A, (b) Alloy B.

Fig. 2. Overall precipitate microstructures in the AA-treated (a) and the PDA-treated (b) Alloy A after peak-aging at 250 °C. The dislocation-induced precipitates in (b) are marked by dotted-line circles.

Fig. 3. HRTEM images showing the typical precipitates in the AA-treated (a) and the PDA-treated (b-f) Alloy A after peak-aging at 250 °C. The inset in each image is the corresponding FFT pattern.

Fig. 4. Overall precipitate microstructures and the corresponding HRTEM images showing the main matrix precipitates in the AA-treated (a, c) and the PDA-treated (b, d) Alloy B after peak-aging at 250 °C. The dislocation-induced precipitates in (b) are marked by dotted-line circles. The inset in each HRTEM image is the corresponding FFT pattern.

Fig. 5. Schematic illustration of the unit cell of B’ [11] (a), U2 [13] (b), β’ [10] (c) and β’-2 [14] (d) in Al-Mg-Si alloys.

Fig. 6. HAADF-STEM images of dislocation-induced precipitates in the PDA-treated Alloy A after peak-aging at 250 °C, (a) low-magnification image, (b, c) atomic-resolution images of two polycrystalline U2 precipitates marked in (a).

Fig. 7. Atomic-resolution HAADF-STEM images of dislocation-induced’/B’-2/U2 composite precipitate (a, b) and B’-2 phase (c, d) in the PDA-treated Alloy A after peak-aging at 250 °C, enlarged from the precipitates indicated by blue and yellow dotted circles in Fig. 6(a), respectively. Atomic structures of half unit cell of the B'-2 phase are overlaid in (b) and (d).

Fig. 8. Atomic-resolution HAADF-STEM images of dislocation-induced B'-2/U2 composite precipitates (a) and B'-phase (b) in the PDA-treated Alloy A after peak-aging at 250 °C. The insert in (b) is an enlarged image of the area marked by the white dashed box in (b).

Table 1 Experimentally suggested atomic coordinates of the B'-2 phase. The phase has a monoclinic unit cell with a =3.426 nm, b =0.405 nm, c = 1.04 nm and β = 102.5°.

Atom	x/a	y/b	z/c	Occupancy
Si	0	0	0	1
Si	0.05	0	0.45	1
Si	0.07	0	0.78	1
Si	0.11	0.5	0.17	1
Si	0.15	0.5	0.55	1
Si	0.23	0.5	0.30	1
Si	0.31	0.5	0.07	1
Mg	0.11	0	0.36	1
Mg	0.17	0	0.18	1
Mg	0.21	0	0.48	1
Mg	0.03	0.5	0.22	1
Mg	0.07	0.5	0.58	1
Mg	0.02	0.5	0.84	1
Mg	0.13	0.5	0.90	1
Al	0.08	0	0.02	1
Al	0.14	0	0.69	1
Al	0.28	0	0.27	1
Al	0.01	0	0.60	1
Al	0.23	0.5	0.07	1
Al^†	0.17	0.5	0.35	0-1

Fig. 9. Atomic-resolution HAADF-STEM images of dislocation-induced B'/U2 composite precipitates in the PDA-treated Alloy A after peak-aging at 250 °C.

Fig. 10. Atomic-resolution HAADF-STEM images of dislocation-induced β'/β'-2 phase (a) and β'/U2 composite precipitate (c) in the PDA-treated Alloy A after peak-aging at 250 °C. (b) is an enlarged image of the area marked by the white dashed box in (a).

Fig. 11. Atomic-resolution HAADF-STEM images of dislocation-induced β'/U2 composite precipitates (a, b), polycrystalline U2 precipitate (c) and β'-precipitate (d) in the PDA-treated Alloy B after peak-aging at 250 °C.

Fig. 12. Formation frequencies of different dislocation-induced precipitates for the PDA-treated Alloy A and Alloy B after peak-aging at 250 °C.

Fig. 13. 3D illustrations of dislocations introduced by small-deformation (a) and dislocation-induced precipitates (b, c). (b) Nucleation of different precipitates with their easy-growth directions all parallel to the dislocation lines. (c) The polycrystalline precipitates and multiphase composite precipitates formed by the parallel growth of nuclei of either the same phase type or different phase types.

Fig. 14. TEM dark-field images of Alloy A after aging at 250 °C for different time: (a) AA-treated, 10 s, (b) AA-treated, 20 s, (c) PDA-treated, 10 s.

Table 2 Lattice misfit between atomic rows under different ORs.

OR	Parallel directions	Distance between two atomic rows (nm)		Misfit (%)
OR1	[100]_U2 // <310>_Al	D_[100]U2 = 0.675	D_<310>Al = 0.640	5.5
OR2	[100]_U2 // <110>_Al	2D_[100]U2 = 1.350	5D_<110>Al = 1.432	5.7
OR3	<11$\bar{2}$0>_B' // <510>_Al	D_<11-20>B' = 1.040	D_<510>Al = 1.032	0.7
OR4	[100]_U2 // <710>_Al	2D_[100]U2 = 1.350	D_<710>Al = 1.432	5.7
OR5	[100]_U2 // <410>_Al	4D_[100]U2 = 2.700	3D_<410>Al = 2.505	7.8
OR6	<11$\bar{2}$0>_B' // <710>_Al	4D_<11-20>B' = 4.160	3D_<710>Al = 4.296	3.2

Fig. 15. An illustration of the effect of matrix Si-concentration on nucleation energy barriers for β" and β' under high Mg-concentration and low Mg-concentration. $C_{eqβ"}^{Si}$ and $C_{eqβ'}^{Si}$ mean the equilibrium matrix Si-concentrations for β" and β', respectively.

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