Influence of electric field on the quenched-in vacancy and solute clustering during early stage ageing of Al-Cu alloy

doi:10.1016/j.jmst.2017.07.020

Abstract

Abstract:

The effects of electric field on the evolution of excess quenched-in vacancy as well as solute clustering in Al-4wt%Cu alloy, and on the vacancy migration and formation enthalpy of pure aluminum were investigated, using positron annihilation lifetime spectroscopy, high-angle annular dark-field scanning transmission electron microscopy, transmission electron microscopy, hardness measurement and four-probe electrical resistivity measurement. The results showed that the electric field improved age hardening response obviously and postponed the decay of excess vacancies for 30 min during the early stage ageing of Al-4wt%Cu alloy. A large number of 2-4 nm GP zones with dense distribution were observed after 1 min ageing with an electric field applied. The electric field-assisted-aged sample owned a lower coarsening rate of GP zone, which was about three fifths of that in the aged sample without an electric field, from 1 min to 120 min ageing. The electric field contributed 8% increase of the vacancy migration enthalpy (0.663 ± 0.021 eV) of pure Al, comparing with that (0.611 ± 0.023 eV) of pure Al without an electric field. The increase of vacancy migration enthalpy, induced by the electric field, was responsible for the difference on evolution of quenched-in vacancy, rapid solute clustering and age hardening improvement during the early stage ageing of Al-4wt%Cu alloy.

Key words: Al-4wt%Cu alloy, Electric field, Quenched-in vacancy, Vacancy migration enthalpy, Solute clustering

Shang Fu, Ying Zhang, Huiqun Liu, Danqing Yi, Bin Wang, Yong Jiang, Zhiquan Chen, Ning Qi. Influence of electric field on the quenched-in vacancy and solute clustering during early stage ageing of Al-Cu alloy[J]. J. Mater. Sci. Technol., 2018, 34(2): 335-343.

Figures/Tables 17

Fig. 1. Schematic of external electric-assisted heat treatment instrument.

Fig. 2. Experimental routes for determination of the formation enthalpy of vacancy without and with an electric field applied in pure aluminum: (a) E = 0 kV/cm; (b) E = +5 kV/cm.

Fig. 3. Experimental routes for determination of the migration enthalpy of vacancy without and with an electric field applied in pure aluminum: (a) E = 0 kV/cm; (b) E = +5 kV/cm.

Fig. 4(a) exhibits the hardness vs ageing time curves of Al-4wt%Cu alloy aged at 453 K without and with an electric field applied. The tendency of the age-hardening curves was consistent in both samples in the early stage of ageing (1-120 min ageing). Hardness increased obviously in Al-4wt%Cu alloy aged under an electric field. The electric field-assisted aged sample showed a relative higher maximum hardness value ～78 HV after 120 min ageing, comparing to maximum hardness value ～72 HV in the artificial aged sample.

Fig. 4. The variation of (a) the Vicker’s hardness and (b) the average positron lifetime of Al-4wt%Cu alloy aged at 453 K for various ageing times without and with an electric field applied.

Fig. 5. HAADF-STEM images of Al-4wt%Cu alloy aged for 1 min without and with an electric field applied: (a) E = 0 kV/cm and (b) E = +5 kV/cm.

Fig. 6. HAADF-STEM images of the Al-4wt%Cu alloy aged for 30 min without and with an electric field applied: (a) E = 0 kV/cm and (b) E = +5 kV/cm.

Fig. 7. TEM images and corresponding diffraction patterns along the [001]Al zone for Al-4wt%Cu alloy aged for 60 min without and with an electric field applied: (a) E = 0 kV/cm and (b) E = +5 kV/cm.

Fig. 8. TEM images and corresponding diffraction patterns along the [001]Al zone for Al-4wt%Cu alloy aged for 120 min without and with an electric field applied: (a) E = 0 kV/cm and (b) E = +5 kV/cm.

Table 1 Quantitative analysis of GP zones by HAADF and TEM: the samples aged at 453 K for various times without and with an electric field applied.

Sample	Ageing time (min)	Average length (nm)	Precipitation fraction (%)
Electric field-assisted-aged sample	1	3.2 ± 0.4	3.3 ± 0.1
	30	5.7 ± 0.8	3.9 ± 0.2
	60	6.8 ± 0.5	4.6 ± 0.3
	120	9.0 ± 1.1	5.6 ± 0.5
Artificial aged sample	1	9.8 ± 0.6	2.8 ± 0.3
	30	11.6 ± 0.7	3.1 ± 0.2
	60	12.4 ± 0.8	3.4 ± 0.4
	120	14.5 ± 1.5	3.8 ± 0.5

Fig. 9. Variation of the square of average length of GP zone as a function of ageing time without and with an electric field applied.

Table 2 Electrical resistivity of annealed pure aluminum samples for determining the vacancy formation enthalpy without and with the effect of an electric field.

Sample	Electric field strength (kV/cm)	Electrical resistivity (μΩ cm)
1-0	0	2.8 ± 0.4
1-1	0	4.3 ± 0.3
1-2	0	14.5 ± 1.3
1-3	0	35.5 ± 1.6
2-0	0	2.8 ± 0.5
2-1	+5	7.2 ± 0.4
2-2	+5	33.5 ± 1.7
2-3	+5	84.1 ± 1.5

Table 3 Electrical resistivity of quenched and annealed pure aluminum samples for determining the vacancy migration enthalpy without and with the effect of an electric field.

Sample	Electric field strength (kV/cm)	Electrical resistivity (μΩ cm)
3-0	0	11495.9 ± 2.1
3-1	0	8181.7 ± 1.6
3-2	0	836.8 ± 1.9
3-3	0	31.2 ± 1.1
4-0	0	11495.9 ± 1.2
4-1	+5	10531.5 ± 1.1
4-2	+5	5707.1 ± 1.5
4-3	+5	1465.8 ± 1.8

Fig. 10. Relationship between ln(ρ1-ρ0ρv) and 1/T for the samples without and with an electric field applied.

Fig. 11. Relationship between ln(ln(ρq-ρ0ρv)-ln(ρa-ρ0ρv)) and 1/T for the samples without and with an electric field applied.

Fig. 12. Variation of excess vacancy concentration as a function of annealing time in pure aluminum at 453 K without and with an electric field applied.

Fig. 13. Schematic mechanism for the effect of vacancy migration ability on solute clustering without and with an electric field applied.

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