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Acta Metall Sin  2016, Vol. 52 Issue (9): 1053-1062    DOI: 10.11900/0412.1961.2016.00164
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CHARACTERIZATION OF NANOSIZED PRECIPITATES IN 9Cr-ODS STEELS BY SAXS AND TEM
Rui XIE1,2,Zheng LU1(),Chenyang LU1,Zhengyuan LI1,Xueyong DING2,Chunming LIU1
1) Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China
2) School of Metallurgy, Northeastern University, Shenyang 110819, China;
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Abstract  

Oxide dispersion strengthened (ODS) steels are the leading candidate structural materials for fast reactor and fusion reactor application due to excellent radiation tolerance and high temperature creep strength. High number density nanoscale oxides play a key role in controlling microstructure and properties. Atomized alloy powders with different ball-milling times were employed to produce 9Cr-ODS steels by hot isostatic pressing (HIP). Nanosized precipitates in 9Cr-ODS steels with different ball-milling times were characterized by synchrotron small angle X-ray scattering (SAXS) together with high resolution transmission electron microscopy (HRTEM). Grain morphology and size were observed by electron backscatter diffraction (EBSD). The effects of nanosized precipitates on grain size and mechanical properties were analyzed. SAXS and TEM results indicated that the size of Y-Ti-O-rich nano-clusters in 9Cr-ODS steels decreases with the increasing milling time, while the distribution density increases. The maximum value of distribution density is about 2.93×1023 m-3 in 9Cr-ODS steel ball milled for 20 h. The maximum value of distribution density of pyrochlore structure Y2Ti2O7 is the highest (1.03×1022 m-3) in 9Cr-ODS steel ball milled for 8 h. Some large-scale Ti-Al-O-rich precipitates are observed and show core/shell structure. Their distribution density increases with ball milling time. With increasing ball milling time, the grain size decreases and the yield strength increases. The contribution of Y-Ti-O-rich nanosized precipitates to yield strength is dominated.

Key words:  9Cr-ODS steel      metal gas atomization      SAXS      Y-Ti-O-rich nano-cluster      Y2Ti2O7      yield strength     
Received:  29 April 2016     
Fund: Supported by National Natural Science Foundation of China (No.51471049) and Specialized Research Fund for the Doctoral Program of Higher Education (No.20130042110014)

Cite this article: 

Rui XIE,Zheng LU,Chenyang LU,Zhengyuan LI,Xueyong DING,Chunming LIU. CHARACTERIZATION OF NANOSIZED PRECIPITATES IN 9Cr-ODS STEELS BY SAXS AND TEM. Acta Metall Sin, 2016, 52(9): 1053-1062.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2016.00164     OR     https://www.ams.org.cn/EN/Y2016/V52/I9/1053

Fig.1  Small angle X-ray scattering (SAXS) curves of 9Cr-ODS steels with different ball milling times (q—scattering vector, I(q)—scattering intensity)
Fig.2  Size distributions of nanosized precipitates obtained from the regions I (a) and II (b) of SAXS curves in Fig.1
Fig.3  High angle annular dark field (HAADF) images of the nanosized precipitates in 9Cr-ODS steels with ball milling times of 0 h (a), 8 h (b) and 20 h (c)
Fig.4  Comparisons of size distribution of nanosized precipitates obtained from SAXS and HAADF data
(a) milling 8 h (b) milling 20 h
Fig.5  HRTEM images and FFT images (insets) of nanosized precipitates in 9Cr-ODS steels
(a) milling 8 h (b) milling 20 h
Precipitate Zone (h1 k1 l1) Interplanar spacing (h2 k2 l2) Interplanar spacing (h3 k3 l3) Interplanar spacing
axis nm nm nm
Exp. Cal. Exp. Cal. Exp. Cal.
A [135] (42) 0.21 0.2061 (4) 0.21 0.2061 (26) 0.13 0.1348
B [129] (33) 0.23 0.2317 (20) 0.22 0.2256 (5) 0.19 0.1943
Table 1  Interplanar spacing of nanosized precipitates in Fig.5
Fig.6  TEM images of large precipitates in 9Cr-ODS steels with different ball milling times
(a) 0 h (b) 8 h (c) 20 h
Fig.7  EDS mapping of large precipitates in 9Cr-ODS steels with different ball milling times
(a) 0 h (b) 8 h (c) 20 h
图8  Size distributions of large precipitates in 9Cr-ODS steels with different ball milling times
Fig.9  EBSD images of 9Cr-ODS steels with ball milling times of 0 h (a), 8 h (b) and 20 h (c), and grain size distribution (d)
Fig.10  Yield strengths of 9Cr-ODS steels with different ball milling times
Fig.11  Room temperature stress-strain curves of 9Cr-ODS steels produced by atomized alloys powders and conventional production technology
Fig.12  Comparison of the measured yield strength at room temperature with the calculated values (σc, σd, σk—contributions of nano-clusters, Y2Ti2O7 and grain boundary to yield strength, respectively; σm—matrix yield strength)
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