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Microstructure Characterization of Ion-irradiated Ferritic/Martensitic HT9 Steel
Djamel Kaoumi1, Ce Zheng1
1 Department of Nuclear Engineering, North Carolina State University, Raleigh, 27607, NC, USA
HT9 is a 12Cr Ferritic/Martensitic (F/M) steel considered as a promising candidate for structural
and cladding applications in Generation IV reactors [1]. The chemical composition of the alloy is given
in Table 1. The harsh service conditions in Gen IV reactors require that the microstructural response to
irradiation of the candidate structural alloys be investigated and understood to qualify them. For that
matter, a series of ion irradiations were done. Bulk HT9 specimens were irradiated using 5 MeV Fe++
ions to 20 displacements per atom (dpa) at 600 nm depth with a dose rate approximately to 5×10-4 dpa/s,
at irradiation temperatures of 420, 440 and 470°C (with a variation of ± 5°C). The temperature was
monitored using an infrared camera and four attached Type J thermocouples. For post-irradiation
characterization, TEM specimens were firstly prepared by the FIB lift-out method using a FEI Quanta
focused ion beam (FIB) instrument. ChemiSTEM characterizations were then conducted on FIB laminas
using a FEI Titan 80-300 probe aberration corrected microscope. ChemiSTEM characterization was also
conducted on as-received HT9 prior to ion irradiation. Only pre-existed M23C6 type carbides and V-rich
nitride precipitates were observed in the as-received condition. In contrast, Ni-Si-Mn rich precipitates
(also known as G phase) were found in HT9 irradiated to 20 dpa at 420, 440 and 470°C, as shown in
Figure 1. Radiation-induced Ni segregation was also observed at grain boundaries, which is highlighted
by white arrows in Figure 2. In addition, the G phase precipitates were found to nucleate
heterogeneously along lath grain boundaries, as indexed by red arrows in Figure 2. The observed results
indicated that, under self-ion irradiation, alloying elements such as Ni, Si and Mn segregate at defect
sinks, which become thus favourable nucleation sites and promote the radiation-induced G phase
precipitation. While radiation induced precipitation and segregation in neutron irradiated F/M HT9 have
been widely reported in the literature, similar investigations under ion irradiation have been more scarce
[2,3,4]. In fact, this study serves to generate baseline data on ion irradiation effects on F/M HT9 in an
effort to learn how to more accurately choose ion-irradiation experimental conditions to emulate the
irradiated microstructures and effects observed under neutron irradiation.
Ion irradiations were also carried out in the same alloy at similar temperatures in-situ in a TEM
using 1MeV Kr++ ions so that the microstructure characterized in-situ in the TEM can be compared
with the microstructure achieved on the same alloys using self-ion irradiation on bulk samples. The
focus of the comparison is put on the size and density of dislocation loops induced by irradiation, as well
as dislocation loop burgers vector determination. The in-situ experiments provide data on the kinetics of
irradiation induced defect formation and evolution, and on the damage spatial correlation with the pre-
existing microstructure, and thus can help understand how the microstructures observed ex-situ in the
bulk samples have developed for, in these latter cases, only snapshots are available at the limited doses.
By comparing the ex-situ and in-situ irradiation it is also possible to substantiate the free surface effect
on the radiation induced microstructure. The presentation will also report such comparison.
References:
[1] R. L. Klueh, A. T. Nelson, Journal of Nuclear Materials, 371 (2007) p. 37
[2] J. J. Kai, G. L. Kulcinshi, Journal of Nuclear Materials, 175 (1990) p. 227
[3] E. Getto et al, Journal of Nuclear Materials, 480 (2016) p. 159
[4] E. Getto et al, Journal of Nuclear Materials, 484 (2017) p.193
2214
doi:10.1017/S1431927617011734 Microsc. Microanal. 23 (Suppl 1), 2017
© Microscopy Society of America 2017
https://www.cambridge.org/core/terms. https://doi.org/10.1017/S1431927617011734
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Fe
Cr
Mo
Ni
Mn
W
V
Si
C
N
P
S
84.89
11.8
1.03
0.51
0.5
0.5
0.33
0.21
0.21
0.01
0.008
0.003
Table 1. As-received HT9 chemical composition (wt.%)
Figure 1. ChemiSTEM maps showed Ni, Si and Mn enrichment (associated with Fe depletion) in HT9
self-ion irradiated to 20dpa at 420, 440 and 470°C. It indicates the formation of Ni, Si, Mn-rich
precipitates in HT9 under irradiation.
Figure 2. STEM mode bright field micrograph (a) and corresponding Ni Kα map (b) of HT9 self-ion
irradiated to 20 dpa at 440°C; Ni segregated to grain boundaries (indexed by white arrows) and some of
Ni, Si, Mn-rich precipitates nucleated heterogeneously along lath grain boundaries (indexed by red
arrows).
2215Microsc. Microanal. 23 (Suppl 1), 2017
https://www.cambridge.org/core/terms. https://doi.org/10.1017/S1431927617011734
Downloaded from https://www.cambridge.org/core. IP address: 83.136.104.163, on 23 Aug 2017 at 17:50:23, subject to the Cambridge Core terms of use, available at
