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Acta Metall Sin  2016, Vol. 52 Issue (5): 513-518    DOI: 10.11900/0412.1961.2015.00343
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EFFECTS OF Mn ON MULTI-PRECIPITATES EVOLUTION OF Cu-RICH AND NiAl PHASE IN STEELS
Qin SHEN1,Xiaojiao WANG1,Anyu ZHAO2,Yifeng HE1,Xulei FANG1,Jiarong MA1,Wenqing LIU1()
1 Key Laboratory for Microstructures, Shanghai University, Shanghai 200444, China
2 Changzhou Hanggang Zhuoxin Mechanical Equipment Co., Ltd., Changzhou 213000, China
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Abstract  

Precipitation hardened steels are widely used in various engineering fields due to their high strength, high fracture toughness, good ductility and ease of machinability. As two kinds of common precipitates, Cu-rich and NiAl phases play an important role on the mechanical properties of steels. The obvious effects of Mn on the precipitate evolution of Cu-rich phase and NiAl phase in steel have been observed respectively. However, the effect of Mn is complex, when Cu-rich phase and NiAl phase exist at the same time. This work aims to reveal the effects of Mn on the co-precipitation of Cu-rich phase and NiAl phase in steel. Fe-Cu-Ni-Al and Fe-Cu-Ni-Al-Mn steels were aged at 500 ℃ for different times after solution treatment at 900 ℃ for 2 h. Hardness testing indicates that by adding 2.17%Mn, Fe-Cu-Ni-Al-Mn steel shows a peak hardness of 420 HV which is 80 HV higher than Fe-Cu-Ni-Al steel (about 340 HV). And Fe-Cu-Ni-Al-Mn steel reaches the peak hardness at 1 h which is 1 h earlier as compared with Fe-Cu-Ni-Al steel at 2 h. Moreover, the peak hardness plateau of Fe-Cu-Ni-Al-Mn steel only lasts for 7 h which is far less than that of Fe-Cu-Ni-Al steel. All in all, the addition of Mn enhances the effect of precipitation hardening at early aging, and accelerates the whole process of precipitation hardening. Atom probe tomography (APT) results reveal that Mn increases the nucleation rate of precipitates at early ageing, accelerates the growing and coarsening of precipitates and then accelerates the separation of the Cu-rich phase and NiAl phase. This is due to Mn can reduce the energy for nucleation and accelerate the diffusion rate of elements in the matrix, while the partial substitution of Mn for Al in the NiAl phase can form point defects which can accelerate the diffusion rate of Cu in NiAl phase.

Key words:  precipitation hardening      Cu-rich phase      NiAl phase      atom probe tomography     
Received:  30 June 2015     
Fund: Supported by Steel Joint Funds of the National Natural Science Foundation of China (No.U1460103) and State Key Lab of Advanced Metals and Materials (No.2014-Z08)

Cite this article: 

Qin SHEN,Xiaojiao WANG,Anyu ZHAO,Yifeng HE,Xulei FANG,Jiarong MA,Wenqing LIU. EFFECTS OF Mn ON MULTI-PRECIPITATES EVOLUTION OF Cu-RICH AND NiAl PHASE IN STEELS. Acta Metall Sin, 2016, 52(5): 513-518.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2015.00343     OR     https://www.ams.org.cn/EN/Y2016/V52/I5/513

Steel Cu Ni Al Mn Fe
Fe-Cu-Ni-Al 1.63 3.24 1.09 - Bal.
Fe-Cu-Ni-Al-Mn 1.62 3.26 1.10 2.17 Bal.
Table 1  Chemical compositions of the experimental steels (mass fraction / %)
Fig.1  Vickers microhardness of Fe-Cu-Ni-Al and Fe-Cu-Ni-Al-Mn steels aged at 500 ℃ for different times after solution at 900 ℃ for 2 h (AQ—air quenching)
Fig.2  Three-dimensional atom maps of Fe-Cu-Ni-Al (a, c, e) and Fe-Cu-Ni-Al-Mn (b, d, f) steels after ageing at 500 ℃ for 0.5 h (a, b), 4 h (c, d) and 128 h (e, f)
Ageing time h Rp / nm Nv / 1023 m-3
Fe-Cu-Ni-Al Fe-Cu-Ni-Al-Mn Fe-Cu-Ni-Al Fe-Cu-Ni-Al-Mn
0.5 1.5±0.5 1.4±0.4 12.5 24.7
4 1.8±0.5 2.3±0.9 11.8 6.66
128 3.5±1.4 4.1±2.3 1.1 0.92
Table 2  Average equivalent radius Rp and number density Nv of the precipitates in Fe-Cu-Ni-Al and Fe-Cu-Ni-Al-Mn steels
Steel
Ageing time / h Cu Ni Al Mn Fe
Cu-rich NiAl-rich Cu-rich NiAl-rich Cu-rich NiAl-rich Cu-rich NiAl-rich Cu-rich NiAl-rich
Fe-Cu-Ni-Al 4 47.4±1.8 39.9±4.2 11.3±1.1 12.3±2.8 17.4±1.4 20.3±3.4 - - 23.8±1.5 27.5±3.8
128 76.2±4.6 36.5±4.7 11.9±3.5 28.8±4.4 7.1±2.8 29.8±4.5 - - 6.7±0.7 4.8±2.1
Fe-Cu-Ni-Al-Mn 4 68.0±2.9 20.2±0.5 8.8±1.8 33.1±0.6 8.6±1.8 23.4±0.5 6.5±1.5 12.2±0.4 8.1±1.7 11.0±0.4
128 88.8±1.8 13.7±0.6 2.7±0.9 43.1±0.9 1.4±0.7 26.4±0.8 5.8±1.4 13.9±0.6 1.0±0.6 2.8±0.3
Table 3  Compositions of precipitate cores of Fe-Cu-Ni-Al and Fe-Cu-Ni-Al-Mn steels (atomic fraction / %)
Fig.3  Cu, Ni, Al, Mn atoms distributions of a precipitate in Fe-Cu-Ni-Al (a) and Fe-Cu-Ni-Al-Mn (b) steels after ageing 128 h at 500 ℃
Fig.4  Profiles of Cu, Ni, Al, Mn atoms along the arrows marked in Fig.3a (a) and Fig.3b (b)
Fig.5  Cu, Ni, Al, Mn atoms distributions of a precipitate in Fe-Cu-Ni-Al-Mn steel after ageing 16 h at 500 ℃ (a) and profile along the arrow marked in Fig.5a (b)
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