Simultaneously enhanced strength and ductility of 6xxx Al alloys via manipulating meso-scale and nano-scale structures guided with phase equilibrium

doi:10.1016/j.jmst.2019.09.029

Abstract

Abstract:

Excellent comprehensive mechanical properties including good formability, high strength and high ductility are prior demands for Al-Mg-Si-Cu alloys. This study utilizes calculation of phase diagram (CALPHAD) to simplify the alloy design and meet these demands. Specifically, CALPHAD was used to finely tune the Mg/Si atomic ratio in solid solution and accurately control the type and content of second phases, especially to avoid the formation of the harmful constituent phase β-AlFeSi. Constituents and dispersoids of only α-AlFeMnSi phase were found in the alloy prepared. An optimized microstructure with fine grains, micro scale constituents, densely distributed submicron scale dispersoids and extremely dense nano precipitates provides effective impediment to dislocation gliding and induces transgranular fracture. Therefore, the designed alloy has better comprehensive mechanical properties than other 6xxx series aluminum alloys, including excellent formability, strength and ductility. The low T4P strength of 149 MPa as well as the high elongation of 26.1% implies the alloy's applicability to automobile body panel forming. The yield strength was rapidly improved from 149 MPa to 277 MPa during the paint bake ageing, because the number density of precipitates is twice as high as that of some other 6xxx alloys. Meanwhile, the elongation was kept at a high level of 20.0%.

Key words: Aluminum alloys, Mg/Si atomic ratio, Constituent, Precipitates, Three-dimensional atom probe (3DAP)

Qiang Lu, Kai Li, Haonan Chen, Mingjun Yang, Xinyue Lan, Tong Yang, Shuhong Liu, Min Song, Lingfei Cao, Yong Du. Simultaneously enhanced strength and ductility of 6xxx Al alloys via manipulating meso-scale and nano-scale structures guided with phase equilibrium[J]. J. Mater. Sci. Technol., 2020, 41: 139-148.

Figures/Tables 11

Fig. 1. CALPHAD results of the Al-Fe-Mg-Si system: (a) quasi-ternary isothermal section with fixed iron content of 0.3 wt% at 550 °C; (b) vertical section of the quaternary phase diagram. The Mg/Si atomic ratio of red and blue dotted line is 2:1 and 1:1, respectively. The red cross is a composition with the same contents of Mg and Si as the finally designed alloy.

Table 1 Effect of Mn on the calculated phase equilibrium of Al-1.05 Mg-0.85Si-0.5Cu-0.3Fe Alloy at 550 °C.

Mn (wt%)	FCC (vol.%)	α-AlFeMnSi (vol.%)	β-AlFeSi (vol.%)	Mg₂Si (vol.%)	Mg:Si (FCC) (at.%)
0	99.07	0	0.84	0.07	1.71
0.1	99.00	0.32	0.60	0.08	1.72
0.2	98.84	0.95	0.14	0.07	1.75
0.3	98.66	1.29	0	0.05	1.82
0.4	98.47	1.51	0	0.02	1.91
0.5	98.26	1.74	0	0	2.01

Table 2 Driving forces of Mg2Si and Q phases in alloys with different Mg/Si atomic ratios.

Composition (wt%)	Mg₂Si (kJ/mol)	Q (kJ/mol)	Mg:Si (at.%)
Al-1.05 Mg-0.85Si-0.5Mn-0.5Cu-0.3Fe	20.34	14.73	2:1
Al-0.84Mg-1.03Si-0.5Mn-0.5Cu-0.3Fe	19.93	14.63	5:4
Al-0.68Mg-1.20Si-0.5Mn-0.5Cu-0.3Fe	19.56	14.55	5:6

Fig. 2. Microstructures of the alloy at micrometer scale: EBSD images of the grain size of the alloy before (a) and after (e) the rolling process; (b, f) morphologies of the constituents in the homogenized and SS + PBA samples, respectively. The constituent compositions determined by EPMA are inserted in (b); (c, g) STEM images of the dispersoids in the homogenized and SS + PBA samples, respectively. The dispersoid compositions determined by EDX in TEM are inserted in (g); (d) HRTEM image of an α-AlFeMnSi constituent along the zone axis of [-1-11]α, with the corresponding selected area electron diffraction pattern inserted; (h) HRTEM image (and the FFT pattern inserted) of a dispersoid along the zone axis of [001]α; (i, j, k) EDX elemental maps of the region 1 in (c); (l, m, n) EDX elemental maps of the region 2 in (g).

Fig. 3. Microhardness (a) and tensile properties (b) of the alloy samples directly aged for different times after solid solution heat treatment.

Fig. 4. Mechanical properties of the alloy under different conditions: (a) stress-strain curves of solid-solutionizied (SS) and paint bake ageing (PBA) samples directly after the solid solution heat treatment; (b) stress-strain curves of similar samples but with nature ageing (NA) for 30 d; (c) paint bake ageing response of samples pre-aged in various temperatures and times and naturally aged for 30 d; (d) stress-strain curves of the samples before and after PBA, with an optimal pre-ageing (PA) treatment at 100 °C for 30 min. The y values are the yield ratios.

Fig. 5. Summary of the tensile properties of 6xxx aluminum alloy body panels based on experimental data. The points 1, 2 and 3 represent the mechanical properties of the alloy studied in the T4P, PB and T6 states, respectively. The specific data can be found in Table S1.

Fig. 6. Secondary electron images of the fracture surface of the tensile tested sample aged at 180 °C for 30 min: (a) primary voids and the secondary voids produced during tensile deformation; (b) high magnification SEM image showing the primary voids; (c, d) the secondary voids at higher magnifications.

Fig. 7. Observation of precipitates in differently aged samples; (a) SAED image with β″ and GP zone patterns of the alloy aged for 30 min; (b, c) dark field and bright field images of the alloy aged for 30 min, respectively. The objective aperture position used for dark field imaging is marked with a yellow circle in (a); (d) HRTEM image of the alloy aged for 30 min; (e, g) bright field images (with corresponding SAED pattern inserted) of the alloy aged for 110 and 360 min, respectively; (f, h) HRTEM images (with corresponding FFT pattern inserted) of the alloy aged for 110 and 360 min, respectively.

Fig. 8. 3DAP result showing the solute aggregates with more than 278 solute atoms detected.

Table 3 Number density and volume fraction at different ageing time.

Parameters	110 min	200 min	360 min
l (nm)	22.9(8)	30.8(2)	43.2(4)
A_CS (nm²)	4.7(5)	6.6(6)	10.0(5)
t (nm)	124.8(4)	129.9(4)	250.8(8)
n (nm^-3)	9.1(10) × 10^-5	7.6(8) × 10^-5	3.8(4) × 10^-5
V_f (%)	0.99(15)	1.55(24)	1.65(26)

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