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Acta Metall Sin  2018, Vol. 54 Issue (9): 1236-1244    DOI: 10.11900/0412.1961.2017.00563
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Study on the Precipitation of γ' in a Fe-Ni Base Alloy During Ageing by APT
Yuanyuan SONG, Mingjiu ZHAO, Lijian RONG()
CAS Key Laboratory of Nuclear Materials and Safety Assessment, Institue of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
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High strength Fe-Ni base austenitic alloys, such as A286 and JBK-75, are widely used in gas turbine jet engines and hydrogen service and so on because of their excellent corrosion resistance and low hydrogen embrittlement sensitivity. The ordered coherent γ' [Ni3(Al,Ti)], precipitated during ageing, is thought to have the main contribution on the strength. Thus, it is very important to understand the characterization of the precipitation. However, few previous studies are focused on atomic scale evolution of the precipitated phase. Atom probe tomography (APT) is a unique microscopy technique that provides 3D analytical mapping of materials at near atomic resolution and a high detection sensitivity for all elements. The present research is focused on the microstructure evolution at ageing temperature at different time scales using APT. A Fe-Ni base austenite alloy were aged at 620 ℃ for different time after solution treated at 980 ℃ for 2 h. Hardness testing indicates that a sharp increase is observed when the ageing time is less than 6 h. The hardness is up to 205 HV from the initial 145 HV at the ageing time 6 h. After that the hardness increases slowly. The hardness is 251 HV at 120 h. APT results reveal that Ti-rich nanoclusters precipitate obviously at the initial stage of ageing, which contain Fe, Cr, Ni, Mo and Al elements. As the ageing time increases, more Ni and Al atoms are segregated in the Ti-rich nanoclusters while the Fe, Cr and Mo are ejected from the nanoclusters. When the ageing time is up to 120 h, the Ni/(Ti+Al) ration is approximately close to 3. The precipitates can be identified as γ' phase. The results reveal that the formation of γ' involves nucleation and growth. Effect of the number density and the size of the γ' precipitates on the hardening of the alloy has been estimated.

Key words:  Fe-Ni base alloy      γ' phase;      ageing      nanocluster      APT     
Received:  29 December 2017     
ZTFLH:  TG142  
Fund: Supported by National Natural Science Foundation of China and China Academy of Engineering Physics (No.U1730140)

Cite this article: 

Yuanyuan SONG, Mingjiu ZHAO, Lijian RONG. Study on the Precipitation of γ' in a Fe-Ni Base Alloy During Ageing by APT. Acta Metall Sin, 2018, 54(9): 1236-1244.

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Fig.1  Variation of Vickers hardness in the J75 steel solution treated at 980 ℃ for 1 h with ageing time at 620 ℃
Fig.2  Atom distribution maps of Fe, Cr, Mo, Ni, Ti and Al in J75 alloy solution treated at 980 ℃ for 1 h (a) and the nearest-neighbor count distributions of Al and Ti (b) (d-pair—distance between two atoms)
Fig.3  Atom distribution maps of Fe, Cr, Mo, Ni, Ti and Al in J75 alloy aged at 620 ℃ for 1 h (a) and the nearest-neighbour count distributions of Al and Ti (b), the clusters distribution (c) and distribution of Fe, Ni, Cr, Ti and Al atoms in analyzed volume of 20 nm×20 nm×2 nm (d) (The red cycles indicate the nanoclusters)
Fig.4  Atom distribution maps of Fe, Cr, Mo, Ni, Ti and Al in J75 alloy aged at 620 ℃ for 6 h (a) and the nearest-neighbor count distributions of Al and Ti (b), distribution of Fe, Ni, Cr, Ti , Al and Mo atoms in analyzed volume of 20 nm×20 nm×2 nm (Red cycles indicate the nanoclusters) (c) and one dimensional concentration profile along the arrow marked in Fig.4c (d)
Fig.5  Atom distribution maps of Fe, Cr, Mo, Ni, Ti and Al in J75 alloy aged at 620 ℃ for 16 h (a), the distribution of Fe, Ni, Cr, Ti, Mo and Al atoms in analyzed volume of 20 nm×20 nm×2 nm (Red cycles indicate the nanoclusters) (b) and one dimensional concentration profile along the arrow marked in Fig.5b (c)
Fig.6  Atom distribution maps of Fe, Cr, Mo, Ni, Ti and Al atoms in J75 alloy aged at 620 ℃ for 120 h
Fig.7  Isoconcentration surface of 11%(Al+Ti) (atomic fraction) in J75 alloy aged at 620 ℃ for 120 h (a) and one dimensional concentration profile (b), and the distributions of Fe, Ni, Cr, Ti, Mo and Al in analyzed volume of 10 nm× 10 nm× 2 nm (c)
Fig.8  Number density and volume fraction of nanoclusters or precipitates in J75 alloy aged at different time
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