Influence of Mg Contents on Aging Precipitation Behavior of Al–3.5Cu–xMg Alloy

doi:10.1007/s40195-014-0033-7

Abstract

Abstract:

The influence of Mg content on the microstructures and mechanical properties at room temperatures of Al–3.5Cu–(0.71–1.81)Mg alloys was studied. Precipitation phases in the alloys were identified by TEM and HRTEM. The results show that when Mg contents increase from 0.71 to 1.81 wt%, the precipitates are transformed from S, S″, Ω, and θ′ phases to S and S″ phases, and Ω phase is first observed in Al–3.48Cu–0.71 Mg alloy with Cu/Mg mass ratio of 5 during the conventional aging heat treatment (190 °C/12 h). Regard to aging hardness effect of the tested alloys, the hardness of the alloys improves with the increase of Mg content, but the increases become slow when Mg content is greater than 1.35 wt%.

Key words: Al–Cu–Mg alloy, Mechanical property, Microstructure, Phase transition

Siyu Li, Jin Zhang, Jinlong Yang, Yunlai Deng, Xinming Zhang. Influence of Mg Contents on Aging Precipitation Behavior of Al–3.5Cu–xMg Alloy[J]. Acta Metallurgica Sinica (English Letters), 2014, 27(1): 107-114.

Figures/Tables 15

Table 1 Chemical composition (wt%) of the experimental Al–Cu–Mg alloys

Alloy	Cu	Mn	Mg	Zn	Ti	Zr	Fe	Si	Al
No. 1	3.48	0.51	0.71	0.22	0.07	0.08	0.08	0.06	Bal.
No. 2	3.51	0.52	1.35	0.21	0.07	0.09	0.08	0.06	Bal.
No. 3	3.46	0.51	1.81	0.21	0.05	0.10	0.08	0.06	Bal.

Fig. 1 Curves of hardness versus aging time for alloy Nos. 1, 2, 3 aging at 190 °C a, 200 °C b

Fig. 2 Mechanical properties of the alloys before aging

Fig. 3 Mechanical properties of the alloys aged at 190 °C for 12 h

Fig. 4 Optical micrographs of casting microstructure of alloy No. 1 a, No. 2 b, No. 3 c

Fig. 5 Backscattered electron images of alloy No. 1 a, No. 2 b, No. 3 c after solid solution treatment

Table 2 Crystal structures and precipitation features of S″, S, θ′, and Ω phases

Phase	Crystal structure	Morphology	Orientation
S″ (Al₁₀Cu_3+xMg_3-x, 0 ≤ x ≤ 1)	Orthorhombic	Needle-like	\( (100)//(100)_{\text{Al}} \)
S″ (Al₁₀Cu_3+xMg_3-x, 0 ≤ x ≤ 1)	Orthorhombic	Needle-like	\( [010]//[010]_{\text{Al}} \)
S (Al₂CuMg)	Orthorhombic	Lath-shaped(front view); needle-like(side view)	\( (010)//(021)_{\text{Al}} \)
S (Al₂CuMg)	Orthorhombic	Lath-shaped(front view); needle-like(side view)	\( [100]//[100]_{\text{Al}} \)
θ′ (Al₂Cu)	Tetragonal	Platelet(front view); needle-like(side view)	\( [001]//[100]_{\text{Al}} \)
Ω (Al₂Cu)	FCC	Platelet	\( (001)//(011)_{\text{Al}} \)
Ω (Al₂Cu)	FCC	Platelet	\( [010]//[10\bar{1}]_{\text{Al}} \)

Fig. 6 Diffraction patterns from precipitation phases: a one variant of S″/S, b two variants of S″/S, c another two variants of S″/S, d two variants of θ′, e one variant of Ω, fS in [100]Al

Fig. 7 TEM micrographs of alloy No. 1 a, No. 2 b, No. 3 c after aging at 190 °C for 12 h and corresponding SAED patterns of No. 1 d, No. 2 e, No. 3 f (the electron beam is parallel to 〈100〉α)

Fig. 8 a 〈100〉Al HRTEM image corresponding the needle-like phases in Fig. 4a, b FFT obtained from the region A of a, c FFT obtained from the region B of a, d FFT obtained from the region C of a

Fig. 9 a 〈100〉Al HRTEM image corresponding the lath-shaped phases and the platelet phase in Fig. 4a, b FFT obtained from the region A of a, c FFT obtained from the region B of a, d FFT obtained from the region C of a

Fig. 10 a 〈100〉Al HRTEM image corresponding needle-like phases in Fig. 4b, b FFT obtained from a, c 〈100〉Al HRTEM image corresponding to lath-shaped phases in Fig. 4b, d FFT obtained from c

Fig. 11 〈100〉Al HRTEM images corresponding the needle-like phases a and the lath-shaped phases c in Fig. 4c; b FFT obtained from a; d FFT obtained from c

Table 3 Cu/Mg ratio (wt%) presented in Al–3.5Cu–xMg alloy with observable phases

Alloy	Cu/Mg ratio	Needle-like phase	Lath-shaped phase
No. 1	4.9	S″ + θ′	S + Ω
No. 2	2.6	S″	S
No. 3	1.9	S″	S

Table 4 Cu/Mg ratio (wt%) presented in Al–Cu–Mg alloy with observable Ω phase

Alloy composition (wt%)	Cu/Mg ratio	Heat-treatment	Reference
Al–2.81Cu–1.05 Mg	2.7	T351+artificial aging	[17]
Al–4.4Cu–1.5 Mg	2.9	Solution+artificial aging, then air cooled	[16]
Al–3.48Cu–0.71 Mg	4.9	Solution+artificial aging	This work
Al–4Cu–0.5 Mg	8	Solution+artificial aging	[18]
Al–5Cu–0.54 M	9.3	Solution+artificial aging	[19]
Al–5.98Cu–0.51 Mg	11.7	Solution+artificial aging	[20]

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