Structural evolution of oxide dispersion strengthened austenitic powders during mechanical alloying and subsequent consolidation

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Structural evolution of oxide dispersion strengthened austenitic powdersduring mechanical alloying and subsequent consolidation

Man Wang, Hongying Sun, Lei Zou, Guangming Zhang, Shaofu Li,Zhangjian Zhou

PII: S0032-5910(14)00980-2DOI: doi: 10.1016/j.powtec.2014.12.008Reference: PTEC 10669

To appear in: Powder Technology

Received date: 24 June 2014Revised date: 5 November 2014Accepted date: 5 December 2014

Please cite this article as: Man Wang, Hongying Sun, Lei Zou, Guangming Zhang, ShaofuLi, Zhangjian Zhou, Structural evolution of oxide dispersion strengthened austeniticpowders during mechanical alloying and subsequent consolidation, Powder Technology(2014), doi: 10.1016/j.powtec.2014.12.008

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http://dx.doi.org/10.1016/j.powtec.2014.12.008

http://dx.doi.org/10.1016/j.powtec.2014.12.008

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Structural evolution of oxide dispersion strengthened austenitic powders during

mechanical alloying and subsequent consolidation

Man Wang1, Hongying Sun

2, Lei Zou

1, Guangming Zhang

1, Shaofu Li

1, Zhangjian Zhou

1*

1. School of Materials Science and Engineering, University of Science and Technology Beijing,

Beijing 10083, China

2. School of Mechanical Engineering, Anyang Institute of Technology, West of Huanghe Road,

Wenfeng District, Anyang, Henan 455002, China

*Corresponding author

Name: Zhangjian Zhou

E-mail address: [email protected]

Postal address: Laboratory of Special Ceramics and Powder Metallurgy, School of Material

Science and Engineering, University of Science & Technology Beijing,

Beijing 100083, P.R.China

Telephone number: +86-10-62334951

Fax number: +86-10-62334951

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Abstract: Different austenitic steel powders with additions of Y2O3 and Ti were fabricated by

mechanical alloying (MA). The structural evolutions during the process of ball milling and

subsequent annealing were studied by XRD, SEM and TEM. Nano crystalline austenitic powders

were obtained by MA. Different ODS austenitic powders presented different phase transition

during the processed of MA and annealing, which were resulted from different contents of Ni and

Cr. Both ODS-316 and ODS-310 showed a weak diffraction peak of α after annealing and

consolidated by hot isostatic pressing (HIP) due to the addition of Ti. According to the TEM

results, the grain size of all three ODS austenitic steels was around several hundred nanometers.

Key words: Mechanical alloying; ODS austenitic steels; Phase transition

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1. Introduction

Mechanical alloying (MA) was originally developed to produce oxide dispersion

strengthened (ODS) nickel-base superalloys for application in the aerospace industry. Nowadays

MA has shown great potential in fabricating a wide variety of equilibrium and non-equilibrium

alloy phases through high-energy ball milling of blended elemental or pre-alloyed powders [1].

Also, MA has been used to fabricate particle reinforced composite, which can produce uniform

distribution of reinforcement particles in the matrix [2-4]. Recently ODS steels have been

investigated widely, since ODS steels are promising candidate materials for generation-Ⅳ

advanced reactors [5-9]. The extremely thermally stable oxides can improve the high-temperature

properties of materials. Moreover the interface between oxide particles and matrix is effective sink

for point defects and helium atoms, resulting in a high irradiation resistance. Recently ODS

austenitic steels arouse research interests due to the combination of excellent corrosion resistance

and potential reduced irradiation-induced swelling [10-14].

During the process of MA, great energy and heavy deformation are introduced into the

powder particles. At the same time, a high density of crystal defects such as dislocations and grain

boundaries are created in the materials. This defective structure can enhance the diffusion of solute

elements into matrix, which leads to atomic level alloying and extended solid solution. During the

subsequent consolidation, they re-precipitate in the form of complex oxide particle of Y-Ti-O,

which plays an important role in improving the high temperature properties of materials [15-17]. It

is worth noting that the element of Ti is necessary to the formation of nano scale oxide dispersions

[18, 19]. Moreover this process is often accompanied with transformation to metastable phases

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with nano-structure. It was found that MA led to the formation of nanostructured BCC and/or FCC

phases in Fe-Ni and Fe-Cr-Ni alloys [20-24].

It is worth investigating whether the additions of Y2O3 and Ti influence the phase transitions

of ODS austenitic steels or not. In this work, different types of ODS austenitic steels were

fabricated by MA and HIP. The phase transitions and microstructures of ODS austenitic powders

and steels were investigated by XRD and TEM.

2. Experimental

Pre-alloyed 304, 316 and 310 austenitic steel powders were used as the matrix respectively.

The designed chemical compositions are shown in Table 1. Ti and Y2O3 were added to form

nano-sized oxide dispersion particles. Fig. 1 shows the SEM morphologies of the original

materials. The pre-alloyed austenitic steel powders were fabricated by atomization comminuting

process. Therefore different austenitic steel powders had a similar morphology, which was round,

as shown in Fig. 1(a). The size and purity of the powders are shown in Table 2. Normally the

contents of Y2O3 and Ti for ODS steels are 0.35 wt. % and 0.5 wt. % respectively. To investigate

the process of MA, 3 wt. % for both Ti and Y2O3 were used.

The starting materials were mechanical alloyed by a planetary high-energy ball mill equipped

with stainless jars and milling balls. The milling media were consisted of stainless balls with

different sizes and quantities. There were five balls with size of φ20 mm, 400 with size of φ10 mm,

and 2000 with size of φ6 mm, which had a total mass of 3608g. Mechanical alloying was

conducted at 300 rpm with a ball-to-powder mass ratio of 5:1 under nitrogen atmosphere. Samples

were taken at different milling time intervals. Isothermal annealing was carried out at 700, 900 and

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1200 ℃ for 1h in a muffle furnace. The mechanical alloyed powders were consolidated by hot

isostatic pressing (HIP) under a pressure of 100 MPa. The process of HIP consisted of two stages:

first at 1100 ℃ for 2 h and then at 1150 ℃ for 1 h.

The morphology of powders before and after mechanical alloying was investigated by

Scanning Electron Microscope (SEM, LEO-1450). The structural changes were characterized by

X-Ray diffraction using filtered Cu Kα radiation (λ=0.15406 nm). The crystalline size of phases

was obtained by using Hall-Williamson method. A standard sample of silicon was used for

correcting the instrumental broadening. The phase fractions of austenite and martensite were

estimated according to their diffraction intensity. The microstructure of the block samples was

investigated by Transmission Electron Microscopy (TEM, JEM 2010). Foil samples for TEM were

prepared through jet-polishing at -30 ℃ by using 15% perchloric acid and 85% methanol as the

electrolyte. The Vickers hardness of powder particles was determined by a micro-hardness tester

(MH-6) at a load of 100 g and dwell time of 15 s.

3. Results and discussion

3.1 Structural evolution of ODS powders during ball milling

Fig. 2 shows the SEM images of ODS-310 powders at different time intervals. As shown in

Fig. 2(a), the powders became flat after milling of 5 h due to the collisions between balls, powders

and jar. The average diameter of the powders was about 100 μm. After milling of 30 h, the

disk-type particles became round and the size increased to 200 μm resulted from cold welding, as

shown in Fig. 2(b). The particles became fragmented after milling of 50 h and most of them

decreased to 20 μm (Fig. 2(c)). During the process of MA, accumulated strain and work hardening

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led to severe brittleness and therefore the size became smaller. It is worth noting that ODS-304

powders showed similar changes in both morphology and size during the process of MA.

Fig. 3 and Fig. 4 are the XRD patterns of ODS-304 and ODS-310 powders respectively

during the process of MA. We can see that the diffraction peaks of Y2O3 and Ti vanished for both

of them after milling of 5 h. Also the main diffraction peaks displaced towards the smaller angel,

which indicated that the additions of Y2O3 and Ti were dissolved into the matrix. Moreover, all the

diffraction peaks became broader with increasing milling time due to grain refinement and

accumulated strain.

Fig. 5 shows the changes of grain size during the process of milling. After milling of 5 h, the

grain size of both ODS-304 and ODS-310 powders decreased to 18.9 nm, although their phase

component was different. The grain size decreased further to 12.8 nm and 9.4 nm after milling of

50 h respectively, which indicated that both of them achieved nano-crystalline structure after ball

milling. From Fig. 6 we can see that both of them showed an increasing trend in micro-hardness

with increasing milling time. For ODS-304 powders, the micro-hardness increased from 391 Hv to

587 Hv, while the micro-hardness of ODS-310 powders increased from 378 Hv to 556 Hv.

ODS-304 and ODS-310 powders showed similar changing trends in grain size and

micro-hardness, as well as mechanical alloying. However, their phase transitions were different

during the process of ball milling. As shown in Fig. 3, the starting powders of ODS-304 had a

dual-phase structure consisting of γ and α, in which the fraction of γ was higher than α according

to the diffraction intensity. However, the γ phase completely transformed into α after milling of 5

h. On the contrary, ODS-310 powders stayed γ even after milling of 50 h (Fig. 4). This indicated

that ODS-310 powders had greater austenite stability due to relatively higher content of Ni, since

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Ni is a notable austenite forming element.

3.2 Structural evolution of ODS powder during annealing

Fig. 7 shows the XRD patterns of ODS-304 powders after annealing at different temperatures.

We can see that the single-phase milled powders transformed into two phases of γ and α. With

higher annealing temperature, the fraction of γ increased and the fraction of α decreased, as shown

in Fig. 8. The strain was eliminated during annealing, therefore α transformed back to γ. Also the

grain size of both γ and α increased after annealing, and the phase of γ had a relatively larger

average grain size, which was 54.7 nm, as shown in Fig. 9. After annealing at 700 ℃ for 1 h, the

micro-hardness of the as-milled ODS-304 powders decreased from 587 Hv to 394 Hv. As shown

in Fig. 10, there were no obvious differences among different annealing temperatures. Compared

with the as-milled powders, the as-annealed powders showed larger standard deviation in

micro-hardness (Fig. 6 and Fig. 10), which was related to the dual-phase structure.

Fig. 11 shows the XRD patterns of ODS-310 powders after annealing at different

temperatures. It can be seen that γ was the predominant phase in all cases. However, there are

some precipitates of Cr0.99Fe1.01 at 700 and 900 ℃. After annealing at 1200 ℃ for 1 h, these

precipitates disappeared and the phase of α occurred, but its diffraction peaks were relatively

weak.

During the process of MA, a high density of defects are created, which makes material

transfer by diffusion much easier and achieve atomic level alloying [25]. In this case, 3 wt. % of

Y2O3 and Ti were added. The added powders of Y2O3 and Ti dissolved into the matrix in the form

of atoms during the process of MA. They re-precipitated during annealing. ODS-310 powders

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stayed austenite even after milling of 50 h, which was resulted from higher content of Ni.

However, diffraction peaks of α occurred after annealing at 1200 ℃. This may be influenced by

the element of Ti. Ti existed in the form of atom after MA, and during annealing it influenced the

phase transition since it is a notable ferrite forming element.

Fig. 12 shows the micro-hardness of ODS-310 powders during annealing. The

micro-hardness of the as-milled ODS-310 powders decreased from 556 Hv to 391 Hv after

annealing at 700 ℃. There was no obvious difference between 900 and 1200 ℃, both of which

were around 260 Hv. The grain size of ODS-310 powders showed an increasing trend with higher

annealing temperatures, as shown in Fig. 13. After annealing at 1200 ℃, the grain size increased

to 54.7 nm.

3.3 Phase transitions of ODS austenitic powders

The phase transitions of as-milled ODS austenitic powders and as-Hipped ODS austenitic

steels are shown in Fig. 14, Fig. 15 and Fig. 16. It is worth noting that normal contents of Y2O3

(0.35 wt. %) and Ti (0.5 wt. %) were used for these as-milled powders and as-Hipped blocks. It is

very interesting that the phase transitions were different depending on different austenitic matrixes,

namely different contents of Ni and Cr. Ni is a notable austenite forming element and Cr is a

ferrite forming element, therefore they decide the phase transition together. The as-milled

ODS-304 powders were single phase of α, while the as-milled ODS-310 powders were single

phase of γ. Both of them were the same with cases where 3 wt.% of Y2O3 and Ti were used (Fig. 3

and Fig. 4). In contrast to ODS-304 and ODS-310 powders, the as-milled ODS-316 had a dual

phase structure consisting of α and γ. This is because higher content of Ni has greater austenite

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stability and greater resistance of strain-induced transformation. These results were the same with

the phase transitions of traditional Fe-Cr-Ni alloys during ball milling [20-24].

After consolidated by HIP, different ODS austenitic steels showed different structures.

ODS-304 had a dual phase structure, of which γ was the predominant phase. ODS-316 and

ODS-310 were single phase of γ. Moreover, both of them showed a very weak diffraction peak of

α, as shown in Fig. 15 and Fig. 16. As discussed in Part 3.2, ODS-310 powders with higher

content of Ti (3 wt.%) had a relatively obvious diffraction peak of α after annealing at 1200 ℃.

This is different from the phase transitions of traditional Fe-Cr-Ni during annealing [20-24]. For

the traditional Fe-Cr-Ni, if the matrix stayed austenite during ball milling, there would be no phase

transition during annealing. Therefore the addition of Ti may influence the phase transition of γ to

α during annealing or consolidation, and the higher content of Ti makes the transition easier. Since

Ti is necessary for the formation of nano scale oxide dispersions[18, 19], special attention should

be paid to the content of Ti when ODS austenitic steels are fabricated.

3.4 Microstructure of as-hipped ODS austenitic steels

The mechanically alloyed powders were consolidated by HIP first at 1100 ℃ and then at

1500 ℃ totally for 3 h. The microstructures of as-hipped ODS austenitic steels are shown in Fig.

17. From Fig. 17 (a)-(c), we can see that the grain size of these three ODS steels was around a few

hundred nanometers. The corresponding images at a larger amplification were shown in Fig.17

(d)-(f), which showed that nano-sized particles were distributed inside grains. We have

investigated these nano particles in other works [13, 14], and they were characterized to be

enriched in Y-Ti-O, which played an important role in ODS alloys. Among these three ODS steels,

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ODS-316 exhibited a relatively larger grain size, which was around 300 nm, while the grain size

of ODS-310 was around 150 nm. It should be noted that the distribution of grains was not very

uniform, but most of them are around several hundred nanometers. From Fig. 17(d) to (f), we can

see that the distributions of nano-sized particles in ODS-304 and ODS-310 were more uniform

and dense that those in ODS-316. This can be contributed to the different grain sizes of ODS

steels.

4. Conclusions

In this paper, we investigated the structural evolutions of ODS austenitic powders during the

process of ball milling and annealing. Also the phase transition and microstructures of as-Hipped

ODS austenitic steels were studied. The following results can be obtained according to our

research.

(1) Different ODS austenitic powders showed different austenite stability which was related

to the contents of Ni and Cr. ODS-304 powders completely transformed into α after ball milling;

ODS-316 powders had a dual phase of γ and α, while ODS-310 powders stayed austenite even

after milling of 50 h.

(2) A weak diffraction peak of α was found in both the as-Hipped ODS-316 and ODS-310,

which indicates that the addition of Ti may influence the phase transition of ODS austenitic steels.

This should be considered carefully when ODS austenitic steel are fabricated.

(3) According to the XRD results, both the mechanically alloyed powders and the annealed

powders had nano-scaled grain size. This was confirmed by the TEM observation of as-hipped

ODS samples. The grain sizes of all three ODS austenitic steels were around a few hundred

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nanometers.

Acknowledgements

The authors would like to express their thanks for the financial support of National Basic

Research Program of China under Grant No. 2007CB209801, and also thanks for the support of

IAEA Coordinated Research Project (CRP code:T11006) under research contract No. 16763.

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Table Captions

Table 1 The designed chemical compositions of austenitic steel powders (wt. %)

Table 2 Particle size and purity of the starting powders

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Figure Captions

Fig. 1 SEM morphologies of the original materials

Fig. 2 SEM images of ODS-310 powders (a) 5 h (b) 30 h (c) 50 h

Fig. 3 XRD patterns of ODS-304 powders at different milling intervals


Fig. 5 Grain size of ODS austenitic powders at different milling intervals

Fig. 6 Micro-hardness of ODS austenitic powders at different milling intervals

Fig. 7 XRD patterns of ODS-304 powders after annealing at different temperatures

Fig. 8 Phase fractions of ODS-304 powders after annealing at different temperatures

Fig. 9 Grain size of ODS-304 powders as a function of annealing temperature

Fig. 10 Micro-hardness of ODS-304 powders after annealing at different temperatures

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Fig. 12 XRD patterns of ODS-304 powders and block





Fig. 17 TEM images of as-hipped ODS austenitic steels

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Fig. 1 (a)


(a) pre-alloyed 316 powders (b) Ti powder (c) Y2O3 powder

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Fig. 1(b)



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Fig. 1 (c)



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Fig. 2(a)

Fig. 2 SEM images of ODS-310 powders

(a) 5 h (b) 30 h (c) 50 h

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Fig. 2(b)


(a) 5 h (b) 30 h (c) 50 h

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Fig. 2(c)


(a) 5 h (b) 30 h (c) 50 h

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Fig. 5 Grain size of ODS austenitic powders at different milling intervals

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Fig. 6 Micro-hardness of ODS austenitic powders at different milling intervals

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Fig. 8 Phase fractions of ODS-304 powders after annealing at different temperatures

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Fig. 9 Grain size of ODS-304 powders as a function of annealing temperature

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Fig. 13 Grain size of ODS-310 powders after annealing at different temperatures

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Fig. 17(a)


(a), (d) ODS-304; (b), (e) ODS-316; (c), (f) ODS-310

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Fig. 17(b)



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Fig. 17(c)



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Fig. 17(d)



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Fig. 17(e)



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Fig. 17(f)



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Table 1 The designed chemical compositions of austenitic steel powders (wt. %)

kinds of powder Fe Cr Ni Mo

Pre-alloyed 304 Bal. 18 8 1

Pre-alloyed 316 Bal. 16 12 2

Pre-alloyed 310 Bal. 25 20 -

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Table 2 Particle size and purity of the starting powders

kinds of powder purity size

Pre-alloyed 304 99.9% 150 μm



Ti 99.7% 48 μm

Y2O3 99.99% 30 nm

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Graphical abstract

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Highlights

Different ODS austenitic powders showed different phase transitions during MA.

Nano-structural ODS austenitc powders were obtained by MA. The average grain size of

as-hipped samples was around several hundred nanometers.

ODS-304 and ODS-316 austenitic powders completely transformed into α after MA, while

ODS-310 stayed γ.

The element of Ti favored the transformation of γ to α in ODS austenitic powders during

annealing and consolidation

Documents

Structural evolution of oxide dispersion strengthened austenitic powders during mechanical alloying and subsequent consolidation