Effect of grain refinement and crystallographic texture produced by friction stir processing on the biodegradation behavior of a Mg-Nd-Zn alloy

doi:10.1016/j.jmst.2018.11.025

Abstract

Abstract:

The application of a single pass of friction stir processing (FSP) to Mg-Nd-Zn alloy resulted in grain refinement, texture evolution and redistribution of second phases, which improved corrosion resistance. In this work, an as-rolled Mg-Nd-Zn alloy was subjected to FSP. The microstructure in the processed zone of the FS-400 rpm alloy exhibited refined grains, a more homogenous grain size distribution, less second phases, and stronger basal plane texture. The corrosion behavior assessed using immersion tests and electrochemical tests in Hank’s solution indicated that the FS-400 rpm alloy had a lower corrosion rate, which was attributed to the increase of basal plane intensity and grain refinement. The hardness was lowered slightly and the elongation was increased, which might be attributed to the redistribution of the crushed second phases.

Key words: Mg alloy, Friction stir processing, Grain refinement, Texture evolution, Biodegradation

Wen Zhang, Lili Tan, Dingrui Ni, Junxiu Chen, Ying-Chao Zhao, Long Liu, Cijun Shuai, Ke Yang, Andrej Atrens, Ming-Chun Zhao. Effect of grain refinement and crystallographic texture produced by friction stir processing on the biodegradation behavior of a Mg-Nd-Zn alloy[J]. J. Mater. Sci. Technol., 2019, 35(5): 777-783.

Figures/Tables 14

Fig. 1 Schematic of friction stir processing.

Table 1 Chemical composition of Hank’s solution (g/L).

NaCl	KCl	KH₂PO₄	MgSO₄	NaHCO₃	CaCl₂	Na₂HPO₄	Glucose
8.00	0.40	0.06	0.20	0.35	0.14	0.12	1.00

Fig. 2 Optical micrographs showing the microstructures of NZ20: (a) rolled sheet; (c) FS-400?rpm; (e) FS-600?rpm, and granular discrete second phases in SEM micrographs of NZ20: (b) rolled sheet; (d) FS-400?rpm; (f) FS-600?rpm.

Table 2 Grain size and volume fraction of second phases of NZ20 alloy.

Alloy state	Grain size	Volume fraction of second phase
Rolled sheet	~7μm	4.2?±?0.2%
FS-400rpm	~2μm	0.2?±?0.1%
FS-600rpm	~8μm	1.6?±?0.1%

Fig. 3 (0001) pole figures of NZ20: (a) rolled sheet; (b) FS-400?rpm; (c) FS-600?rpm.

Table 3 Mechanical properties of NZ20 alloy.

Alloy state	UTS (MPa)	YS (MPa)	Elongation (%)
Rolled sheetFS-400?rpmFS-600?rpm	258?±?12150?±?583?±?2	220?±?8 62?±?433?±?3	10.6?±?1.327.3?±?2.423.5?±?1.2

Fig. 4 Hardness (a) and UTS (b) and elongation of NZ20 before and after FSP.

Fig. 5 Polarization curves in Hank’s solution of NZ20 before and after FSP.

Table 4 Electrochemical data for NZ20 in Hank’s solution derived from the polarization curves.

Alloy state	E_corr (V)	i_corr (×10^-6 A cm^-2)	CR (mm year^-1)
Rolled sheet	-1.49?±?0.12	14.0?±?0.2	24.8?±?0.2
FS-400?rpm	-1.54?±?0.04	2.6?±?0.06	4.6?±?0.09
FS-600?rpm	-1.63?±?0.03	31.6?±?0.4	56?±?3

Fig. 6 EIS spectra of NZ20 before and after FSP in Hank’s solution: (a) Nyquist plot; (b) Bode plots of |Z| with frequency; (c) Bode plots of phase angle with frequency; (d) equivalent circuit of EIS spectra.

Table 5 Fitting electrochemical parameters of EIS.

Alloy state	R_s (Ω cm²)	R_ct (Ω cm²)	CPE_ct		R_f (kΩ cm²)	CPE_f
Alloy state	R_s (Ω cm²)	R_ct (Ω cm²)	Q_ct (μF sⁿ?cm^-2)	n	R_f (kΩ cm²)	Q_f (μF sⁿ?cm^-2)	n
RollingFS-400rpmFS-600rpm	31.9130.834.04	71.7166.543.88	7.2710.779.18	0.760.700.74	1.6713.751.33	1.3413.009.31	0.860.820.88

Fig. 7 pH evolution (a) and corrosion rate (b) of NZ20 before and after FSP immersed in Hank’s solution for 14 days.

Fig. 8 SEM images of the corrosion surfaces of NZ20 after removing corrosion products: (a) rolled sheet; (b) FS-400?rpm; (c) FS-600?rpm, and macro corrosion morphologies of NZ20 before and after FSP, (d) rolled sheet; (e) FS-400?rpm; (f) FS-600?rpm.

Fig. 9 Topographic map of NZ20 alloy before and after FSP: (a) rolled sheet; (b) FS-400?rpm; (c) FS-600?rpm.

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