J. Plasma Fusion Res. SERIES, Vol. 11 (2015)

Effect of Long-term Thermal Aging on Microstructure and Mechanical
Property Changes of Fe-15Cr Ferritic Alloys
Dongsheng CHEN1, Akihiko KIMURA2, Wentuo HAN2, Wenyue TANG3
1

Graduate School of Energy Science, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan
2

Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan3

Beijing Huairou Vocational School, Beijing 100047, China(Received: 13 September 2014 / Accepted: 19 January 2015)

The effects of long-term thermal aging on the mechanical and microstructural behaviors of Fe-15Cr ferritic
alloys were investigated and the mechanism of strengthening was proposed with relevant to the size of Cr-rich α′precipitates. Arc melting was used to produce Fe-15Cr alloys named Fe-Cr, Fe-Cr-C and Fe-Cr-Xs, where Xs
referred to Si, Mn and Ni to investigate the effect of G-phase on age-hardening. Detailed microstructuralcharacterization before and after aging at 475°C up to 10,000 h was conducted to understand the impact of alloying
elements on aging embrittlement; microstructural information before and after tensile deformation, including grainmorphology, dislocation and precipitates, was obtained using SEM, TEM. Ductility loss of Fe-Cr-Xs alloy wasfound different with other alloys; EBSD was used to elucidate the difference in the microstructure. Thermal aging
results in two stages of changes in the strength: age hardening with increasing aging period then followed by overaging. The critical size of α′ precipitates for the appearance of over-aging is around 8 nm in diameter.
Keywords: high chromium ferrttic alloys, phase decomposition, chromium rich precipitates, mechanical properties,
age hardening, over aging.

According to Mueller [14], who studied the effect ofvarying carbon contents on the impact energy of
Fe-17wt. % Cr alloys, there is a small increase in DBTT
as the carbon content increases. Courtnall et al. [15]found that embrittlement was enhanced by the addition ofsubstitutional elements, such as Ti and Nb. As for theeffect of alloy elements in the ferritic steels, it wasreported that the G-phases were formed after long termaging over 8,000 h, and age hardening was enhanced [16].
However, the influence of alloying elements and α/α′phase decomposition on aging hardening behaviors of
Fe-Cr alloys is still not thoroughly understood.
In this work, long-term thermal aging effect on themechanical and microstructural behaviors Fe-15Cr alloys
was investigated up to the aging period of 10,000 h, usingtransmission electron microscopy (TEM), scanningelectron microscopy (SEM) and tensile tests. Effect ofalloying elements as well as the size of α′ precipitates on
age-hardening was focused.

1. Introduction

High-Cr ferritic steels are of technology interest foradvanced fusion blankets as well as nuclear power plants[1-3]. There is, however, a trade-off issue of corrosion
resistant and aging embrittlement, which is attributed tothe phase decomposition to Fe rich α phase and Cr rich α′phase. Binary Fe-Cr alloys exhibit α/α′ phasedecomposition if the Cr content exceeds ~12 wt. %, in theregion of temperatures potentially important fortechnological applications (300~550 °C) [4-7].
Until now, there are several reports on the impact ofα/α′ phase decomposition on mechanical propertychanges in Fe-Cr alloys [8-13]. Cr-rich phases are a bit
oversized coherent body-centered cubic (bcc) particles,with higher shear modulus than Fe-rich matrix [8].
Precipitates formed by α/α′ phase decomposition harden
the material because they obstruct the motion ofdislocations. A few experiments done for neutron
irradiated Fe-Cr alloys suggest that the substantial
contribution to the irradiation hardening comes from thedifficulty for edge dislocations shearing Cr-richprecipitates [12]. The Cr rich α′ precipitation leads to aprogressive hardening and deterioration of fracture
toughness.
Effect of alloying elements on aging embrittlementof high Cr ferritic alloys has been studied [14-16].

2. Experimental

The materials used in this study were Fe-15Cr alloys,
named Fe-Cr, Fe-Cr-C and Fe-Cr-Xs, with keeping Cr
content around 15 wt. %. The chemical compositions areshown in Table 1. Fe-Cr-C was added with 0.02 wt. %carbon. Fe-Cr-Xs was added with Mn, Si and Ni toinvestigate the effect of G-phase formation on

Author’s e-mail: ds-chen@iae.kyoto-u.ac.jp

57©2015 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchD. Chen et al., Effect of Long-Term Thermal Aging on Microstructure and Mechanical Property Changes of Fe-15Cr Ferritic Alloys

is Fe-Cr-Xs, which is represented in a gray area in Fig. 2.
It is considered that the ductility loss of Fe-Cr-Xs needs alarger age-hardening than that of Fe-Cr and Fe-Cr-C.

age-hardening.
Table 1 Chemical compositions of the alloys (wt. %).
Materials CrCMn SiNiFeFe-Cr
14.55 0.001 Bal.
Fe-Cr-C
14.83 0.022 Bal.
Fe-Cr-Xs 14.96 0.002 0.83 0.51 0.49 Bal.
The model alloys were fabricated by arc melting
method in argon atmosphere. The alloy button ingots(φ30mmh5mm thickness) were first homogenized at1100 °C for 72 h followed by furnace cooling, and thencold rolled by 80% at room temperature. All thespecimens were punched out from the alloy sheet with
0.3 mm thick, and finally annealed at 900 °C for 2hfollowed by iced water quenching. Some of specimens
were sealed in quartz tube in high vacuum conditions(10-4 Pa) and isothermally aged at 475 °C for up to10,000 h.
Tensile tests were carried out at room temperature byusing an INTESCO tensile testing machine with acrosshead velocity of 0.2 mm/min. Tensile test specimens,
which had a dimension of 4mm width16mm length
0.25mm thickness were fabricated parallel to the rolling
direction. At least three measurements were made andaveraged. The grain morphology and ruptured surfaces oftensile specimens were examined using a Zeiss Ultra 55
FESEM, equipped with an electron backscatter
diffraction (EBSD) system. Disk-type specimens of 3mm
diameter were punched out from the sheets andmechanically thinned to ~100 Pm. Specimens for TEM
observation were prepared using a twin jet
electro-polishing method with a solution composed of 10
vol. % perchloric acid and 90 vol. % acetic acid at room
temperature. TEM observation has been performed usinga JEOL 2100 operating at 200 kV. The thickness of thinfoils was measured using convergent beam electron
diffraction (CBED) technique.

Fig. 1 Evolution of yield stress, ultimate tensile stress andelongation during aging at 475°C.

Fig. 2 Linear relations between the change in yield stress, Δσy,and the change in elongation, –Δε, of each alloy.

3. Results and discussion
3.1 Tensile properties
Thermal aging at 475°C causes a significant increasein strength accompanied with corresponding decrease intotal elongation. Fig. 1 shows the change in yield stress(YS), ultimate tensile stress (UTS) and total elongation as
a function of aging time. As aging time increases to 5,000h, yield stresses increase after an incubation period for all
the materials. While after aging for 10,000 h, the tensilestresses decrease slightly as compared to the peak values.
Fig. 2 provides the relations between the change inyield stress, Δσy, and the change in total elongation, Δε,of each alloy during aging for different time. There is alinearity between Δσy and –Δε for all the alloys. The
slopes of the plots, Δσy–Δε of each alloy are classified
into two groups, one is Fe-Cr and Fe-Cr-C and the otherFig. 3 Ruptured surfaces of the model alloys: a) Fe-Cr, unaged;b) Fe-Cr, aged for 10,000 h; c) Fe-Cr-C, unaged; d) Fe-Cr-C,aged for 10,000 h; e) Fe-Cr-Xs, unaged; f) Fe-Cr-Xs, aged for
10,000 h.

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D. Chen et al., Effect of Long-Term Thermal Aging on Microstructure and Mechanical Property Changes of Fe-15Cr Ferritic Alloys

3.2 Ruptured surfaces observation
Fig. 3 shows the ruptured surfaces of the model
alloys deformed at room temperature before and afteraging at 475 °C for 10,000 h. Before and after aging, all
the alloys rupture in a ductile manner, indicating that theDBTTs of all the specimens are below room temperatureeven after aging up to 10,000 h.

3.4 Precipitates
The addition of carbon leads to the precipitation ofCr rich M23C6 and Fe2C carbides on or nearby grainboundaries, as presented in Fig. 6(a). In Fe-Cr-Xs alloy,
there are also some Cr rich M 23C6 carbides precipitations.
TEM EDS X-ray analysis shows that the Cr content inmartensite is lower than 12 wt. %. This result can explain
why there is no phase decomposition structure observedin martensite, as shown in Fig. 6(b).

3.3 Microstructures
TEM bright field images of the model alloys areshown in Fig. 4. Before aging, as presented in Fig. 4(a),almost no precipitates exist in Fe-Cr alloy as well as otheralloys except for a small amount of grain boundary
precipitates. Some lath shaped structures are observed in
the ferrite matrix of Fe-Cr-Xs alloy, as shown in Fig. 4(b).
A large number of dislocations can be observed in thesestructures or along their interface with ferrite.

Fig. 6 TEM bright field images of a) Fe-Cr-C alloy, aged for
500 h, b) Fe-Cr-Xs alloy, aged for 5000 h.

The loss of elongation during aging is mainly due toα/α′ phase decomposition. Thus, the reason whyFe-Cr-Xs alloy shows smaller ductility loss than otheralloys can be attributed to the formation of martensitic
phase, in which phase decomposition is not easy to occur.
However, along the interface of ferrite and martensitic
phase, there are some amount of carbides and few
Si-Cr-Mn enriched G phases, which can increase theembrittlement of the material. It is considered that thetransgranular frature characterized by EBSD is due to theformation of Cr rich α′ precipitates as well as carbides
and G phases in Fe-Cr-Xs alloy.

Fig. 4 TEM bright field images of a) Fe-Cr alloy, unaged, b)
Fe-Cr-Xs alloy, unaged.

After tensile testing, the grain morphology andruptured region of Fe-Cr-Xs alloy aged for 5000 h wereexamined by EBSD, as presented in Fig. 5. The EBSD
maps were acquired from the side surface paralleling toRD&TD plane by removing 0.1mm thickness. The
orientation of tensile specimens is defined as: RD (rollingdirection), TD (transverse direction) and ND (normaldirection). A transgranular fracture was found in
Fe-Cr-Xs alloy, as shown in the marked area in Fig. 5(a).
Some phases in ferrite matrix are characterized as
tempered martensite, as presented in Fig. 5(b).

3.5 Critical size of α′ precipitates for over-aging
Fig. 7 shows α/α′ phase decomposition of Fe-Cr
alloy observed by TEM. Cr rich α′ precipitates shows
coherent contrast because of the similar lattice parameterof Fe and Cr. Our previous research [17] showed that themain reason of age-hardening is due to the formation ofα′ precipitates.

Fig. 5 Fe-Cr-Xs alloy after aged for 5000 h: (a) grainmorphology of the ruptured region; (b) phase characteri- zation,ferrite (red color) and martensite (yellow).

Fig. 7 TEM bright field images of a) Fe-Cr, aged for 5000 h, b)
Fe-Cr, aged for 10,000 h.

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D. Chen et al., Effect of Long-Term Thermal Aging on Microstructure and Mechanical Property Changes of Fe-15Cr Ferritic Alloys

4. Conclusions

With increasing aging time, the average size of α′
precipitates increase while the number density decreases.
As aging time increases from 500 h to 5000 h, α′precipitates become larger, with averaged diameterincreasing form 2 nm to about 8 nm. When aging timereaches up to 10,000 h, averaged diameter of α′
precipitates increases to about 11 nm with number density
decreasing from 2.11022 m-3 (5000 h) to 1.21022 m-3.
The present results indicate that the strength decreases
slightly, although α′ precipitates continue coarsening.

The effects of long-term isothermal aging (up to10,000 h, 475°C) on microstructure and mechanicalproperty changes in Fe-15Cr alloys have beeninvestigated. The main results are summarized as follows:(1) Thermal aging causes a remarkable increase instrength with corresponding decrease in elongation.
The lower ductility loss in Fe-Cr-Xs alloy is related
to the formation of few tempered martensitic phases,in which phase decomposition is not observed.
(2) Even after 10,000 h aging, all the materials rupture
in a ductile manner.
(3) The addition of alloying elements leads to theformations of carbides and G phases. Cr-rich α′precipitates as well as carbides and G phase areconsidered to be the source of appearance of age
hardening.
(4) Age-hardening shows two stages, showing a peak inthe strength with increasing aging period, which isassociated with the size of Cr-rich α′ precipitates.
The critical size for α′ precipitates is around 8 nm.
(5) Further aging is necessary to confirm the effect ofalloying additions.

Fig. 8 Age-hardening curve and sketch diagram for precipitation
hardening mechanism.

Acknowledgments

The authors would like to thank Dr. Chonghong
Zhang and Dr. Sosuke Kondo for support with the TEM
observation, and Mr. Yasunori Hayashi for support withEDS X-ray analysis. Financial support from China
Scholarship Council and Institute of Advanced Energy,
Kyoto University, is gratefully acknowledged.

When aging time reaches at 10,000 h, thermal aging
undergoes an over-aging, which leads to a decreasing
hardness. Fig. 8 shows a sketch map of precipitation
strengthening curve of Fe-15Cr alloys. We may define thecritical particle size as dc, then:(1) when d < dc, coherency strengthening increases withincreasing particle size;(2) when d > dc, coherency strengthening decreases withincreasing particle size.
Such a relationship between strengthening effect andparticle size gives a maximum strengthening at a criticalparticle size dc. According to current data, the critical dc
for Fe-15Cr alloy is about 8 nm.
It is considered that dislocation looping can takeplace whenever the precipitates is too big/strong for
dislocation to cut through, regardless of the coherency
nature of the interface between the precipitate and matrix.
Another issue beyond 10,000 h is the continuous
effect of G-phase precipitation. Since Fig. 1 indicates thatthe reduction of strength after 5,000 h is a little bit
smaller in Fe-Cr-Xs alloy than the others, it can be
expected that G-phase precipitation starts to contribute tothe hardening. The alloys without Ni, Mn and Si
additions only show so-called Ostwald ripening of Cr richphases. Further longer aging period is necessary toconfirm the effect of G-phase precipitation on the agehardening.

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