Microstructure evolution and mechanical property of Cu-15Ni-8Sn-0.2Nb alloy during aging treatment

doi:10.1016/j.jmst.2021.01.034

Abstract

Abstract:

Aging treatment of Cu-based alloys is essential to enhance their strength that is desirable for their extensive engineering applications in electrical industry, whereas the underlying mechanism of strengthening is essential for massive manufacturing of these alloys. Here, the microstructure evolution of a supersaturated solid solution Cu-15Ni-8Sn-0.2Nb alloy aged at 400 °C for different time was characterized at atomic scale using state-of-the-art transmission electron microscopy (TEM) and the corresponding mechanical property was also measured. The results reveal that the modulated structure, DO₂₂/L1₂ ordering, and discontinuous precipitation (DP) appeared in the advances of aging time. At the early stage of aging treatment, component modulation waves and satellite spots appeared from spinodal decomposition and the modulation wavelength was identified in the range of 1-7 nm. Subsequently the modulated structures formed -poor-rich solute regions, of which DO₂₂ ordering was present in the Ni-poor region while L1₂ ordering appeared in the Ni-rich region. The sequence of ordering precipitates was further verified by density functional theory (DFT) simulations. Furthermore, orientation relationships and interfacial structures between DO₂₂, L1₂ phases and the parent matrix were determined. The measured hardness of alloy reached a maximum value of 335 HV after aging for 120 min due to the coherence between the two ordering phases and matrix. These results illustrated the importance of aging on structural evolution and mechanical property of Cu-15Ni-8Sn alloy at various heat treatment stages, which could potentially help in manufacturing promising alloys for their extensive engineering applications.

Key words: Cu-alloy, Aging, Precipitates, Transmission electron microscopy, Density functional theory

Nan Wang, Yidi Shen, Qi An, Kolan Madhav Reddy, Mingjiang Jin, Rajamallu Karre, Xiaodong Wang. Microstructure evolution and mechanical property of Cu-15Ni-8Sn-0.2Nb alloy during aging treatment[J]. J. Mater. Sci. Technol., 2021, 86: 227-236.

Figures/Tables 14

Table 1 Chemical composition in wt.% of Cu-15Ni-8Sn-0.2Nb alloy.

Cu	Ni	Sn	Nb
Balance	15.10	8.04	0.21

Fig. 1. The generated atomistic structures of L12 (a) and DO22 (b) phases at 18.75 at.% Ni and L12 (c) and DO22 (d) phases at 9.375 at.% Ni. The blue, grey and light purple balls represent the Cu, Sn, and Ni atoms, respectively.

Fig. 2. Hardness of Cu-15Ni-8Sn-0.2Nb alloy samples aged at 400 °C for 0 to 360 min.

Fig. 3. XRD patterns of Cu-15Ni-8Sn-0.2Nb alloy samples aged at 400 °C for 2.5 to 360 min.

Fig. 4. TEM and STEM images of sample A aged at 400 °C for 2.5 min: (a, b) TEM BF and DF images of the matrix, respectively; (c) SADP along [001] direction; (d, e, f) HRTEM image, enlarged view and corresponding FFT diffraction pattern, respectively; (g, h, i) HAADF-STEM image in atomic scale, enlarged view and corresponding FFT diffraction pattern, respectively.

Fig. 5. HAADF-STEM image (a) and corresponding element mapping (b, c, d) of sample A aged at 400 °C for 2.5 min.

Fig. 6. TEM images of sample B aged at 400 °C for 10 min: (a) BF image of the twins; (b) SADP of the twin boundary along [110] direction; (c, d) central-DF images using the diffraction reflection of ((111¯)), marked with yellow circle and red square, of twin and matrix, respectively.

Fig. 7. The TEM and STEM images of sample C aged at 400 °C for 90 min: (a, b) TEM BF and DF images, respectively; (c) SADP image along [111] direction; (d, d') the simulated SADP images of DO22 and L12 ordering along [111] direction, respectively; (e, f) HAADF-STEM image and corresponding element mapping of DP, respectively.

Table 2 STEM-EDS result of the precipitate in sample C aged at 400 °C for 90 min.

Phase	Composition (wt. %)
	Cu	Ni	Sn	Nb
Point M in Fig. 7(e)	75.71	18.13	6.16	-

Fig. 8. TEM and HAADF-STEM images of Cu-15Ni-8Sn-0.2Nb alloy sample D aged at 400 °C for 150 min: (a) BF image of DP; (b) morphology of the matrix; (c) SADP along [001] direction; (d, e) HAADF-STEM image at atomic scale along [001] direction and corresponding filtered IFFT image, respectively; (f, g, h) FFT patterns of DO22 ordering, L12 ordering and FCC matrix shown in Fig. 8(d), respectively.

Fig. 9. HAADF-STEM image of DP (a) and corresponding element mapping (b) in sample D aged at 400 °C for 150 min.

Table 3 STEM-EDS result of DP in sample D aged at 400 °C for 150 min.

Phase	Composition (wt. %)
	Cu	Ni	Sn	Nb
Point N in Fig. 9(b)	55.37	26.40	18.22	-

Fig. 10. TEM images of sample E aged at 400 °C for 360 min: (a) BF image of DP; (b, c) HAADF-STEM image and corresponding element mapping of DP, respectively.

Table 4 STEM-EDS result of DP in sample E aged at 400 °C for 360 min.

Phase	Composition (wt. %)
	Cu	Ni	Sn	Nb
Point P in Fig. 10(b)	39.79	35.31	24.82	0.08

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