Orginal Article

# Study on the Precipitation of γ' in a Fe-Ni Base Alloy During Ageing by APT

SONG Yuanyuan, ZHAO Mingjiu, RONG Lijian

CAS Key Laboratory of Nuclear Materials and Safety Assessment, Institue of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China

Abstract

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.

Keywords： Fe-Ni base alloy ; γ' phase; ; ageing ; nanocluster ; APT

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SONG Yuanyuan, ZHAO Mingjiu, RONG Lijian. Study on the Precipitation of γ' in a Fe-Ni Base Alloy During Ageing by APT[J]. Acta Metallurgica Sinica, 2018, 54(9): 1236-1244 https://doi.org/10.11900/0412.1961.2017.00563

γ'-Ni3(Al, Ti)相为fcc结构的Cu3Au型有序相,在时效过程中析出,与基体保持共格关系,是Fe-Ni基合金中的主要强化相[5]。研究[5,6,7,8,9,10,11,12]发现,铁镍基合金的强度和耐氢脆能力与γ'相间具有强烈的依赖关系。首先,合金的高强度与γ'相的尺寸、分布和数量有关,且显著受γ'相中Ti/Al值的影响[5,6,7,8,9]。如Fe-Ni基合金通常都采用较高的Ti/Al比,其目的就在于增加γ'相中的Ti/Al比,使γ'的反向畴界能提高,提高合金强度[5,8]。其次,Fe-Ni基合金的氢损伤研究发现,基于位错切割机制,γ'相一方面可起到强化基体、提高合金强度的作用。 另一方面亦会增加位错平面滑移[10,11,12],在氢的共同作用下(氢降低层错能[2]、加剧位错平面滑移),导致严重局部塑性变形[10,11,12],诱发沿晶界、沿滑移带和沿孪晶界开裂[11,12],造成合金氢损伤。受分析手段的限制,有关铁镍基合金中γ'相形成过程中组成元素的变化还不清楚,因此开展时效过程中γ'相析出行为研究,尤其是从原子尺度上来研究其演化规律,是实现γ'相的性能及数量、尺寸和分布调控,优化Fe-Ni基合金性能的基础。

## 1 实验方法

$Nv=nηNtotΩ$(1)

$f=NpNtot$(2)

## 2 实验结果与讨论

### 2.1 硬度分析

Fig.1   Variation of Vickers hardness in the J75 steel solution treated at 980 ℃ for 1 h with ageing time at 620 ℃

### 2.2 APT分析

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)

### 2.3 析出相数密度与体积分数分析

$Δτ=Cf1/2d1/2$(3)

Fig.8   Number density and volume fraction of nanoclusters or precipitates in J75 alloy aged at different time

## 3 结论

(1) 在时效初始阶段,合金的硬度增加速率较快,随着时效时间的延长,合金的硬度增加速率有所降低, 较为缓慢。时效过程中硬度的变化与γ'析出相的尺寸和数量密度有关。

(2) APT结果表明,在时效最初阶段,Ti发生更为明显的偏聚,形成含有Fe、Ni和Al等元素组成的富Ti纳米团簇;随着时效时间延长,富Ti纳米团簇中的Ni和Al原子的含量逐步增多,而Fe、Cr及Mo等原子的含量不断减少,当时效至120 h时,团簇中Ni与Ti+Al比值近似于3,即已形成γ'相。

(3) 在时效初始阶段,γ'相处于形核阶段,析出的纳米团簇数量密度高。随着时效时间的延长,析出相开始长大,因而导致了数量密度的降低,而析出相的体积分数随着时效时间的延长而增大。

The authors have declared that no competing interests exist.

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