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Microstructure and mechanical properties of ultrafine-grained Fe-14Cr and ODS Fe-14Cr model alloys.

01 Oct 2011-Journal of Nuclear Materials (North-Holland)-Vol. 417, Iss: 1, pp 213-216

Abstract: Reduced activation ferritic Fe–14 wt%Cr and Fe–14 wt%Cr–0.3 wt%Y 2 O 3 alloys were produced by mechanical alloying and hot isostatic pressing followed by forging and heat treating. The alloy containing Y 2 O 3 developed a submicron-grained structure with homogeneous dispersion of oxide nanoparticles that enhanced the tensile properties in comparison to the Y 2 O 3 free alloy. Strengthening induced by the Y 2 O 3 dispersion appears to be effective up to 873 K, at least. A uniform distribution of Cr-rich precipitates, stable upon a heat treatment at 1123 K for 2 h, was also found in both alloys.
Topics: Hot isostatic pressing (57%), Heat treating (55%), Alloy (54%), Microstructure (52%)

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Journal of Nuclear Materials 417 (2011) 213–216
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Microstructure and mechanical properties of ultrane-grained Fe14Cr and ODS
Fe14Cr model alloys
M.A. Auger
, T. Leguey
, A. Muñoz
, M.A. Monge
, V. de Castro
, P. Fernández
, G. Garcés
, R. Pareja
Departamento de Física-IAAB, Universidad Carlos III de Madrid, 28911-Leganés, Spain
Department of Materials, University of Oxford, OX1 3PH, United Kingdom
National Fusion Laboratory-CIEMAT, Avda. Complutense 22, 28040 Madrid, Spain
Departamento de Metalurgia Física, CENIM (CSIC), Avda. Gregorio del Amo 8, 28040 Madrid, Spain
Abstract: Reduced activation ferritic Fe14 wt%Cr and Fe14 wt%Cr0.3 wt%Y
alloys were produced by mechan-ical alloying and hot isostatic
pressing followed by forging and heat treating. The alloy containing Y
developed a submicron-grained structure with homogeneous dispersion
of oxide nanoparticles that enhanced the tensile properties in comparison to the Y
free alloy. Strengthening induced by the Y
appears to be effective up to 873 K, at least. A uniform distribution of Cr-rich precipitates, stable upon a heat treatment at 1123 K for 2 h, was also found
in both alloys.
1. Introduction
FeCr binary alloys are nowadays the most promising base to
fabricate reduced-activation ferritic/martensitic (RAFM) and fer-
ritic (RAF) steels for structural applications in future fusion reac-
tors as well as in generation IV ssion reactors [13]. The reasons
are their high resistance to swelling, helium embrittlement and
irradiation creep, in combination with their mechanical prop-erties
at elevated temperatures and corrosion resistance. However,
conventional RAFM steels have limited the upper operating tem-
perature to 823 K because of their low thermal creep rupture
strength above this temperature. A goal of fusion reactor designs is
to enhance efciency and safety by increasing the operating tem-
peratures using fusion devices cooled by helium or liquid metals.
This would require the development of RAFM/RAF steels with
upper operating temperatures of 923 K, at least. Oxide dispersion
strengthening appears to have the capability to improve the ele-
vatedtemperature strength of high-chromium steels, thus
increasing the operating temperatures up to 950 K [47]. Conse-
quently, the development of oxide dispersion strengthened (ODS)
steels forces to investigate the effect of the fabrication and process-
ing techniques on the mechanical behavior of ODS model alloys as
well as to understand their strengthening mechanisms. The aims of
the present work are: (1) to fabricate non-ODS and ODS Fe14 wt%
Cr alloys, via powder metallurgy methods, and (2) to inves-
tigate the effect of the processing parameters on their microstruc-
ture and mechanical behavior, in order to elucidate the oxide
dispersion capacity for strengthening ferritic steels and extending
their operating temperature window.
2. Experimental
Powder blends with target compositions: Fe14%Cr and Fe
(wt%), hereafter designed respectively as refer-
ence Fe14Cr and ODS Fe14Cr, were mechanically alloyed at
300 rpm for 60 h inside a chromium steel vessel (1112% Cr) under
a He atmosphere in a highenergy planetary mill (Fritsch Pulveris-
ette 6). Chromium steel balls (11.7% Cr, 10 mm diam.) with a ball-
topowder mass ratio of 10:1 were used as grinding media. The
alloyed powders were packed in 304 stainless steel cans (42 mm
diam. 57 mm height), degassed at 823 K for 24 h in vacuum
and then the cans were vacuum sealed. The starting powders, as
well as the blended and alloyed powders, were manipulated in
each step of the procedure under Ar atmosphere inside a glove
box. The canned powder was consolidated by hot isostatic pressing
(HIP) for 2 h at 1373 K and a pressure of 200 MPa. The starting
powders were spherical 99.7% pure Fe and 99.8% pure Cr powder,
both with mean particle size <10
m supplied by Alfa Aesar, and
99.5% pure monoclinic Y
with particle sizes 650 nm from Nano-
phase Technologies.
The cans containing the HIP consolidated alloys were forged into
the form of 12 mm 12 mm 170 mm bars. The forging temper-
ature, which was 1323 K for reference Fe14Cr, had to be increased
to 1373 K for ODS Fe14Cr. After forging, the bars were heat trea-
ted at 1123 K for 2 h and air cooled. Finally, the remaining material
coming from the can was removed. The chemical composition was
Corresponding author. Tel.: +34 916249184; fax: +34 916248749.
E-mail addresses: (M.A. Auger), (T.
Leguey), (A. Muñoz), (M.A. Monge), (V. de Castro), (P.
Fernández), (G. Garcés), (R.

determined as follows: O and C contents in TC500 and CS600 ele-
mental analyzers, manufactured by LECO Corporation; Y and Cr
contents by X-ray uorescence analysis. The microstructure was
characterized by X-ray diffraction (XRD), laser diffraction, scanning
and transmission electron microscopy (SEM and TEM), and energy
dispersive spectroscopy (EDS). For microscopy characterization
the following instruments were used: a Philips XL30 scanning elec-
tron microscope equipped with an EDAX (AMETEK, Inc., Materials
Analysis Division) analyzer and TEM images were obtained with
Philips CM20 and JEOL 3000F microscopes operated at 200 and
300 keV, respectively. Tensile tests in the temperature range 300
975 K at a constant crosshead rate of 0.1 mm/min were performed
on at tensile specimens with 20 mm gauge length 3 mm
width 1 mm thickness cut parallel to the bar axis. Above room
temperature, the tests were performed with the specimens under
a ow of pure Ar. In addition, Vickers microhardness was measured
using an applied load of 300 g for 20 s.
3. Results and discussion
3.1. Chemical composition and density
Table 1 shows the chemical composition of the starting elemen-
tal powders and the HIP consolidated alloys after forging and heat
treating. The compositional analyses indicated a signicant Fe
enrichment in the alloys, attributable to the observed Cr sticking
on the grinding media surface. Under the present processing con-
ditions, no signicant intake of O and C appears to occur. The high
oxygen content in the Fe and Cr starting powders is responsible of
the oxygen content measured in the consolidated alloys. This may
induce a detrimental effect on the mechanical properties.
The average particle size of the alloyed powders before canning,
determined from SEM and laser diffraction analyses, is given in Ta-
ble 2. This value was found to be 18% smaller in the ODS alloy than
in the reference Fe14Cr alloy.
The measured densities for the forged and heat treated materi-
als are shown in Table 2. The density for the ODS Fe14Cr alloy re-
sults in a higher value than the one for the reference Fe14Cr alloy.
This result can be related to the ner grain size developed in the
ODS alloy.
3.2. Microstructure
3.2.1. Fe14Cr
TEM observations of the Fe14Cr alloy after forging and heat
treating at 1123 K revealed a duplex microstructure formed by
large ferrite grains with sizes in the range 37
lm, and regions
containing submicron ferrite grains with an average size of
380 nm, as Fig. 1a shows. A dispersion of Cr-rich precipitates with
sizes ranging between 50 and 400 nm were found distributed in-
side the large ferrite grains and aligned along the grain boundaries,
besides small voids which would very likely contain entrapped gas,
as Fig. 1b reveals. EDS and electron diffraction analyses suggested
that the nature of these Cr-rich precipitates may differ in structure
and composition. In addition to Cr and Fe, these particles contained
C, N, O and other impurities. A detailed study of the microstructure
of these alloys and nature of the precipitates are reported in[8].
3.2.2. ODS Fe14Cr
Fig. 2a shows the microstructure of the ODS Fe14Cr alloy after
forging and heat treating. It exhibited a homogeneous submicron
structure of ferritic grains, apparently unrecovered, with an esti-
mated average size of 360 nm. Alike the reference Fe14Cr, a ne
uniform distribution of Cr-rich precipitates is found in the ferritic
matrix. The TEM/EDS analyses did not show traces of Y in these
particles. Also, the TEM analyses revealed the presence of a disper-
sion of YO rich nanoparticles, as shown in Fig. 2b. Voids were also
observed in ODS Fe14Cr, frequently associated to the nanoparti-
cles [8].
3.3. XRD measurements
XRD patterns from both alloys showed a single ferrite phase. The
lattice parameter determined for the ODS Fe14Cr in forged-heat
treated condition was 0.7% larger than the one corresponding to the
reference Fe14Cr. These calculations were made by a least squares
tting of the XRD data using the WINPLOTR software [9]. The
average crystallite size and accumulated strain were estimated
from the analyses of the width and shape of the diffraction peaks,
by applying the De Keijser formula [10] included in WINPLOTR
Table 1
Chemical composition of the starting elemental powders and HIP consolidated alloys,
measured after forging and heat treating at 1123 K (Fe mass balance).
O (wt%) C (wt%) Y (wt%) Cr (wt%)
Fe powder 0.28 0.02
Cr powder 0.67 0.006 99.81
Fe14Cr 0.36 ± 0.01 0.07 ± 0.01 13.1 ± 0.1
ODS Fe14Cr 0.54 ± 0.11 0.05 ± 0.01 0.20 ± 0.01 13.4 ± 0.1
Table 2
Average particle size of the milled powders and characteristic parameters for the HIP
consolidated alloys after forging and heat treating.
Alloy Powder
size (nm)
Fe14Cr 44.69 7.654 22 3.1 2.71 ± 0.04
ODS Fe14Cr 36.68 7.711 16 4.5 4.17 ± 0.05
Fig. 1.TEM images of the reference Fe14Cr alloy forged and heat treated at
1123 K. (a) Duplex microstructure. (b) Dispersion of Cr-rich precipitates and voids.
Fig. 2.
TEM images of the ODS Fe14Cr alloy forged and heat treated at 1123 K. (a)
Submicron-grained structure. (b) Dispersion of YO rich nanoparticles (black
arrows). Void attached to a nanoparticle is marked by a white arrow.

software [9] processing. Table 2 shows these values for both alloys
in the forged-heat treated condition. The XRD results in conjunc-
tion with the TEM observation indicated that the ne YO rich
nanoparticle dispersion had the capability of promoting and
retaining grain renement upon forging and subsequent heat
treatment at 1123 K. Also, the Cr-rich particles should have a role in
constraining recovery and grain growth as the results for refer-ence
Fe14Cr suggested.
3.4. Mechanical properties
The tensile properties for the alloys in the forged-heat treated
condition are summarized in Figs. 3 and 4. The tensile properties
for the ODS Fe14Cr alloy in the range 300973 K were remarkably
enhanced in relation to those for the reference Fe14Cr alloy. The
enhancement factor of the yield strength in the ODS alloy com-
pared to the one in the non-ODS alloy was of 1.7 at room temper-
ature but 2.6 at 973 K. The fact that this enhancement is higher
with increasing temperature is a very interesting result. It demon-
strates the real capability of the nanoparticle dispersion in combi-
nation with the submicron-grained structure, for strengthening a
ferritic matrix up to 873 K, at least. Moreover, Fig. 4 discloses a very
interesting characteristic of the ODS Fe14Cr in relation to the non-
ODS counterpart alloy. The hardening ratio, dened as ten-sile
strength/yield strength, appears to increase steeply in the ODS Fe
14Cr at temperatures above 773 K, while it does not exhibit a
dened trend in the reference Fe14Cr. Another relevant charac-
teristic of the ODS Fe14Cr compared to the non-ODS counterpart
alloy is the shift in the onset of the sharp drop of ow stress, from
673 K to 773 K, coinciding with a sharp increase of the uniform
elongation and hardening ratio, which are not observed in the non-
ODS alloy.
The microhardness results shown in Table 2 also reect the
capability of the nanoparticle dispersion to strength and stabilize
the grain structure of ferritic Fe14Cr alloys. The fact that the
ODS alloy required a higher temperature to be forged compared
to that for the non-ODS alloy, as well as the considerable effect
of forging on the microstructure and microhardness, demonstrate
that the grain growth during the HIP treatment and subsequent
Fig. 3.Tensile properties for the HIP consolidated alloys in the forged-heat treated condition.
Fig. 4.
Hardening ratio as a function of temperature for the HIP consolidated alloys
in the forged-heat treated condition.

thermomechanical treatments is mainly controlled by the presence
of the YO rich nanoparticles in the alloyed powder.
4. Conclusions
The addition of Y
to the Fe14Cr blend appears to promote
particle size renement of the mechanically alloyed powder and
favors its densication. The fabrication and processing route herein
applied produced an ODS ferritic Fe14Cr alloy with a submicron-
grained structure and a dispersion of YO rich nanoparticles and
Cr-rich precipitates.
The YO rich dispersion is responsible for the enhancement of
the tensile properties, grain renement and stability of the induced
structure. This submicron-grained structure was stable upon heat
treatment at 1123 K. The ODS ferritic Fe14Cr alloy fabricated and
processed under the present conditions appears to exhibit superior
tensile properties than ODS Fe17Cr alloys processed by extrusion
followed by rolling and annealing [5] and the ODS (12
14)Cr2WTi ferritic steels fabricated and processed by a similar
route [11].
This investigation was supported by the Spanish Ministry of Sci-
ence and Innovation (project No ENE 2008-06403-C06-04 and Juan
de la Cierva program), the Comunidad de Madrid through the pro-
gram ESTRUMAT-CM (grant S0505/MAT/0077), and the European
Commission through the European Fusion Development Agree-
ment (contract No. 09-240), the IP3 FP6 ESTEEM project (contract
No. 026019) and the Fusion Energy Materials Science (FEMaS)
FP7 coordination action. All the fundings are gratefully
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