Effects of Ag Addition on Precipitation and Fatigue Crack Propagation Behavior of a Medium-Strength Al-Zn-Mg Alloy

doi:10.1016/j.jmst.2016.11.008

Abstract

Abstract:

Effects of trace addition of Ag on the fatigue crack propagation behavior and microstructure of a medium-strength aged Al-Zn-Mg alloy were investigated in the present work. The results show that a combination of enhanced tensile strength and improved fatigue crack propagation resistance in Al-Zn-Mg alloys is achieved with small addition of Ag. The enhanced strength is attributed to the high density of η′ precipitates within the grains and narrow precipitate free zones in the vicinity of grain boundaries. The main contribution to the improvement of fatigue crack propagation resistance comes from the coarser precipitates within the grains. When subjected to two-step aging, Ag-added alloy shows larger semi-coherent matrix precipitates. These relatively coarser precipitates increase the homogeneity of deformation and therefore improve the fatigue crack propagation resistance. In addition, microstructure analysis indicates that the size and distribution of inclusions as well as the grain structures of Al-Zn-Mg alloys are independent of Ag addition.

Key words: Aluminum alloy, Fatigue crack propagation, Precipitate free zones, Silver

Lianghua Lin, Zhiyi Liu, Wenjuan Liu, Yaru Zhou, Tiantian Huang. Effects of Ag Addition on Precipitation and Fatigue Crack Propagation Behavior of a Medium-Strength Al-Zn-Mg Alloy[J]. J. Mater. Sci. Technol., 2018, 34(3): 534-540.

Figures/Tables 9

Table 1 Chemical composition (in wt%) of the alloys

Alloy	Zn	Mg	Mn	Cr	Zr	Ti	Ag	Al
Al-Zn-Mg	4.6	1.2	0.4	0.2	0.15	0.02	/	Bal.
Al-Zn-Mg-Ag	4.6	1.2	0.4	0.2	0.15	0.02	0.2	Bal.

Fig. 1. Hardness-time curves illustrating the artificial aging response of the studied alloys aged at 120 °C.

Table 2 Influences of Ag addition and aging treatments on the tensile properties of Al-Zn-Mg alloys

Alloy	Aging treatment	Tensile strength	Yield strength	Elongation
Alloy	Aging treatment	σ (MPa)	σ_0.2 (MPa)	δ (%)
Al-Zn-Mg	One-step peak aging	391	354	9.9
Al-Zn-Mg	Two-step aging	424	389	9.5
Al-Zn-Mg-Ag	One-step peak aging	442	392	10.3
Al-Zn-Mg-Ag	Two-step aging	458	411	9.6

Fig. 2. Representative bright field TEM micrographs showing the microstructures in (a) one-step aged Al-Zn-Mg alloy, (b, c) two step-aged Al-Zn-Mg alloy, (d) one-step aged Al-Zn-Mg-Ag alloy and (e, f) two-step aged Al-Zn-Mg-Ag alloy. (a, b) <100>Al zone axis, (c, d) <110>Al zone axis.

Fig. 3. HRTEM micrographs showing the presence of η′ phase in (a) Al-Zn-Mg alloy and (b) Al-Zn-Mg-Ag alloy after two-step aging. The marked sections of the region in (a) and (b) with high magnification are shown in (c) and (d) respectively. Images were taken along the <110>Al zone axis. The corresponding FFT spectrums are inserted.

Fig. 4. Size distribution of η′ phase in (a) Ag-free and (b) Ag-added alloys after two-step aging.

Fig. 5. Effect of Ag addition on the fatigue crack propagation rate (da/dN) in two-step aged Al-Zn-Mg alloys.

Fig. 6. SEM fractographs of fatigue fracture surface for (a-c) Al-Zn-Mg and (d-f) Al-Zn-Mg-Ag alloys. (a, d) near threshold regime, (b, e) Paris regime with ΔK ≈ 25 MPa m0.5 and (c, f) final fatigue fracture surface. The crack propagation directions were all from top to bottom.

Fig. 7. SEM micrograph showing the inclusions (marked with arrow) in (a) Al-Zn-Mg and (b) Al-Zn-Mg-Ag alloys. The typical grain structures are inserted.

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