Investigation of spark plasma sintered oxide-dispersion strengthened steels by means of small-angle neutron scattering

Journal of Alloys and Compounds 685 (2016) 927e935

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Investigation of spark plasma sintered oxide-dispersion strengthened steels by means of small-angle neutron scattering €rber c, B. Kieback b, c, I. Hilger a, b, *, F. Bergner a, A. Ulbricht a, A. Wagner a, T. Weißga C. Heintze a, C.D. Dewhurst d a

Institute of Ion-Beam Physics and Materials Research, Helmholtz-Zentrum Dresden - Rossendorf, Bautzner Landstr. 400, 01328 Dresden, Germany €t Dresden, 01062 Dresden, Germany Institut für Werkstoffwissenschaft, Technische Universita Fraunhofer Institute for Manufacturing Technology and Advanced Materials, Branch Lab Dresden, 01277 Dresden, Germany d Institut Laue-Langevin, 71 Avenue des Martyrs, 38000 Grenoble, France b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 February 2016 Received in revised form 21 June 2016 Accepted 23 June 2016 Available online 24 June 2016

Spark plasma sintering (SPS) is an advanced consolidation technique, which particularly allows coarsening of the microstructure to be limited. The basis for the characteristics of the strengthening nanofeatures is already set in the milling process preceding SPS. The present study is focused on the dependence of the size distribution and nature of the nanofeatures in an ODS Fe-14Cr-1W-0.4Ti alloy as a function of the applied milling parameters and amount of added Y2O3 while keeping the SPS parameters constant. Statistically reliable averages of the particle characteristics representative of a macroscopic sample volume have been obtained by means of small-angle neutron scattering (SANS). The measured magnetic-to-nuclear scattering ratios have been critically compared to values calculated on the basis of structures and compositions reported in the literature. Milling parameters suitable to completely transform the added Y2O3 into Ti-containing nm-sized oxide particles have been identified. Two size ranges of particles have been analyzed separately: 0.5e3 nm and 3e15 nm (radius). The former size range is dominant in all ODS samples, the magnetic-to-nuclear scattering ratio indicates these particles to be predominantly Y2TiO5 and/or Y2Ti2O7. © 2016 Elsevier B.V. All rights reserved.

Keywords: Nuclear reactor materials Nanostructured materials Powder metallurgy Mechanical alloying Sintering Small-angle neutron scattering

1. Introduction Oxide dispersion strengthened (ODS) steels are considered as candidate materials for components of generation IV (Gen IV) fission and fusion nuclear reactors [1,2]. Ferritic Fe-14Cr alloys exhibit good resistance to corrosion and swelling compared to austenitic steels [3]. They are creep resistant up to 550e600
C [4], but in order to apply these materials at higher operation temperatures (e.g. 650
C in Gen IV fission reactors [5]), the creep properties have to be improved. The addition of ODS particles (e.g. Y2O3) was found to be a good option [2,6,7]. Besides refining the grain size of the matrix, the particles themselves have an influence on the mechanical behavior at high temperatures. Moreover, the interface between small particles (few nm) and the matrix is assumed to act

* Corresponding author. Institute of Ion-Beam Physics and Materials Research, Helmholtz-Zentrum Dresden - Rossendorf, Bautzner Landstr. 400, 01328 Dresden, Germany. E-mail address: [email protected] (I. Hilger). http://dx.doi.org/10.1016/j.jallcom.2016.06.238 0925-8388/© 2016 Elsevier B.V. All rights reserved.

as a sink for radiation-induced primary defects, such as vacancies and interstitials, and as a nucleation site for small Heagglomerations which are uncritical as opposed to large pressurized He bubbles [8,9]. Along with the grain size, the oxide particle size distribution is an important factor with regard to the radiation resistance. Here, two factors play a role: the mean free diffusion path towards sinks and the sink capacity. The finer the dispersion of ODS particles (at constant volume fraction) is, the larger is their specific area and, simultaneously, the smaller is the mean free diffusion path towards sinks/nucleation sites. Usually, ODS steels are produced applying a powder-metallurgy route which consists of mechanical alloying (MA) with subsequent consolidation via hot isostatic pressing (HIP) [10e12] or hot extrusion [12e15] and thermal/thermomechanical treatments. Recently, MA followed by spark plasma sintering (SPS) was shown to be suitable to achieve oxide particle sizes in the lower nm range in ODS Fe-Cr alloys [12,16e19]. It has been observed that changes in the structural properties of the Y2O3 take place during the MA process. After milling for several

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hours, the intensities of the Y2O3 peaks in powder X-ray diffraction (XRD) patterns decrease and finally disappear [20e26]. This may be due to either complete dissolution of Y and O [23,24] or, alternatively, to fragmentation of Y2O3 down to the sub-nm scale [21,22]. The details are still under debate. Finally, the milling parameters govern the size distribution, spatial distribution and structure of oxide particles after consolidation. During sintering of the milled powders, Y2O3 reappears [17] or, if the matrix contains Ti, YeTi complex oxides form [27]. The variation of the Y2O3 content allows the Y/Ti ratio to be adjusted. Consolidation by means of SPS is based on heat generation directly at the powder particle contacts. The main advantage of SPS is the shorter holding time and a limitation of grain growth [28] compared to conventional techniques such as HIP. The nature of nanoclusters in ODS steels is still a matter of debate [18,29e31]. Yttrium-containing oxide particles have been extensively studied by means of TEM, atom probe tomography (APT) and XRD [32e35]. Zhang et al. applied TEM and APT to identify the particles in a spark plasma sintered material to be Y2Ti2O7, which is known to form stable particles [36,37]. Nagini et al. investigated the influence of milling time on the microstructure, including the particle size and composition, of a 9Cr ODS steel by means of TEM. However, the volume analyzed using these techniques is very small, which can cause problems in samples with an inhomogeneous spatial distribution of particles. In contrast, small-angle neutron scattering (SANS) provides information about the size distribution averaged over a statistically representative number of oxide particles randomly sampled from a macroscopic volume (~50 mm3 in the present case) [38]. In existing SANS studies, several aspects of the ODS particle distribution and its role in the fabrication process are considered [39,40]. This work is focused on the influence of milling parameters on the size distribution and composition of oxide particles in ODS Fe14Cr alloys consolidated by means of SPS. For this purpose, SANS is applied to ODS and non-ODS samples. The SANS analysis allows bimodal particle size distributions to be explored and both particle populations to be analyzed individually. The main aim is to identify a set of milling parameters suitable to obtain a high number density of nm-size stable strengthening particles in the steel matrix. The suitability of the non-ODS samples as reference samples is examined in order to distinguish the desired strengthening particles from contamination oxides. Moreover, the ratio between the magnetic and nuclear scattering derived from SANS is critically compared to values calculated on the basis of the structure and composition of oxide particles reported in the literature.

2. Experiments 2.1. Materials The investigated samples were fabricated by MA of a gas atomized Fe-14Cr based pre-alloyed powder provided by Nanoval GmbH & Co. KG and Y2O3 powder from PCT Ltd. (particle size z 30 nm) with subsequent consolidation via SPS. The composition of the steel powder is given in Table 1. MA was carried out in a Pulverisette P5 planetary ball mill under purified argon atmosphere. The ball-to-powder weight ratio was 10:1 and the

Table 1 Composition provided by the supplier of the pre-alloyed steel powder. All values are given in wt%. Fe

Cr

W

Ti

Mn

Si

Ni

Bal.

14.10

0.99

0.32

0.34

0.18

0.17

milling tools (bowls and balls) were made of stainless steel. In order to study the influence of the Y2O3 content and milling parameters, the composition and milling intensity was varied as shown in Table 2. The mechanically alloyed powders were then consolidated using SPS at 1050
C applying the same sintering parameters for all samples. All samples reached relative densities higher than 98%. The microstructure of each of the compacts is ferritic with bimodal grain size distributions for the ODS alloys as was found in previous work on the same materials [41]. The overview TEM micrograph shown in Fig. 1 confirms exemplarily the presence of particles in the lower nm-size range. 2.2. SANS measurements The SANS measurements were carried out at the instrument D33 of ILL Grenoble [42]. The twin multi-tube detector system consisting of a 4-panel front detector and a single panel rear detector was used in the monochromatic mode at a neutron wavelength of 0.6 nm. Two setups for the sample-detector distances, 1.2 m and 12 m for the front detector together with 2 m and 12.8 m for the rear detector, were applied to cover a Q-range of 0.15 nm1 < Q < 3 nm1. The collimation lengths of the incident neutron beam were 2.8 m and 12.8 m, respectively. The samples with dimension 10 mm  10 mm x 1 mm were placed in a magnetic field of 1.4 T in order to separate magnetic and nuclear scattering cross-sections. Absolute calibration was done by means of a direct beam measurement [43]. The data reduction was performed using the software package GRASP [44]. The scattering intensity is proportional to the number density and composition of the scatterers in terms of squared scattering contrast, whereas the radius R of scatterers governs the curve shape of the scattering intensity dS/dU (or I) versus scattering vector Q. It is important to recognize that scattering is caused by a hierarchy of microstructural features in a broad range of sizes. In order to focus on the nanoscale oxide particles, the Porod approximation [45] is assumed to be applicable to the entirety of particles with sharp boundaries and sizes larger than about 20 nm. Accordingly, a lower bound Q4-dependence was fitted to the logI-logQ plots and subtracted from the measured scattering curves. This procedure approximately allows the size distribution of the oxide nanoparticles to be extracted. Another option might have been to subtract the scattering curves obtained for the non-ODS reference samples from the respective curves of ODS samples. However, it was found that this option is not applicable in the present case, because the ODS materials are not just non-ODS reference plus a population of oxide nano-particles. Instead, constituent elements in the ODS materials modify the nano-oxides already present in the non-ODS samples. Below, we refer to the difference between measured scattering curves and the lower-bound Q4-fit as ‘difference scattering curves’. For the analysis of the magnetic difference scattering curves, non-magnetic spherical particles, randomly distributed in the steel matrix were assumed with Fe being the only element that contributes to the magnetic scattering length density. The

Table 2 Y2O3 content and milling parameters of the analyzed materials. Sample

Y2O3 content [wt%]

Time [h]

Speed [rpm]

A B C D E F

0.0 0.0 0.3 0.6 0.6 0.6

30 50 20 20 30 50

250 250 150 150 250 250

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Z

Z 

 cR Dh2 dR mag þ1¼ Z  þ1 A¼ Z  cR Dh2 ðdS=dUÞnuc dQ dR ðdS=dUÞmag dQ

(3)

nuc

In some cases including the present one, as will be shown below, there is a pronounced bimodality of the size distribution of scatterers with the first and second component of the size distribution extending from RL1 to RU1 and from RL2 to RU2, respectively. The right-hand-side part of Eq. (3) can then be used in order to ascribe individual average A-ratios to the two components. For known structure and composition of both scatterers and matrix, and assuming that the measured magnetic and nuclear scattering contributions arise from the same family of objects, the A-ratio can be calculated starting from the right-hand-side part of Eq. (3):

A¼

Dh2mag Dh2nuc 0 P

B S ¼ @X2 P

Z Z

transformation from scattering curves into particle-size distributions was performed using a Monte Carlo code [46] based on an algorithm introduced by Martelli et al. [47]. Here, no assumption on the shape of the size distribution is necessary. By randomly varying the starting conditions of the fit procedure, an estimate of the fit stability was derived. These ranges will be indicated in the figures as colored bands. An important quantity related to the composition of the scatterers is the A ratio. The A-ratio was originally defined according to Eq. (1) (left-hand-side part) as the ratio of the scattering cross section perpendicular to the applied saturation magnetic field and the scattering cross section parallel to the applied field [48]. It is a function of the scattering vector calculated as the ratio of magnetic (subscript ‘mag’) and nuclear (subscript ‘nuc’) scattering cross sections after separation of both contributions to the total scattering (right-hand-side part of Eq. (1)).

AðQ Þ ¼

ðdS=dUÞ⊥ ðdS=dUÞmag ¼ þ1 ðdS=dUÞjj ðdS=dUÞnuc

(1)

Upon transforming dS/dU(Q) into size space, the A-ratio transforms into a function of radius, R, according to Eq. (2), where cR is the size distribution of scatterers in terms of volume fraction per size increment and Dh is the contrast (see below) between scatterer and matrix.




 cR Dh2 mag  AðRÞ ¼
þ1 cR Dh2 nuc

(2)

The average A-ratio frequently given in the literature, e.g. Refs. [29], is defined by Eq. (3). Integration extends either over the Q-range or over the R-range covered in the SANS experiment.

þ1¼

cR;nuc dR nSX bSmag;X 

X2 S

Fig. 1. TEM bright field image of an ODS Fe-14Cr compact produced by means of mechanical alloying and spark plasma sintering.

cR;mag dR

nSX bSnuc;X 

P X2M

P

X2M

Dh2mag þ1 Dh2nuc

M nM X bmag;X M nM X bnuc;X

12 C A þ1

(4)

bX is the magnetic or nuclear scattering length of element X (including vacancies with bmag ¼ bnuc ¼ 0), nX is the fraction of element X per unit volume and superscripts S and M refer to cluster and matrix, respectively. Eq. (4) will be used in the discussion to calculate the A-ratio of several candidates of oxides in the steel matrix for the sake of comparison with the experimental results.

3. Results The measured and fitted magnetic difference scattering curves and the reconstructed size distributions of scatterers are shown in Figs. 2 to 4. The fits covered the Q-range from 0.04 nm1 to 2.5 nm1. The fit curves are the Fourier transforms of the given size distributions. The goodness of fit therefore confirms the validity of the reconstructed size distributions. In Table 3, the integral characteristics of the calculated particle-size distributions, i.e. number density N, mean particle radius Rm, particle volume fraction c and A-ratio, are given for all analyzed samples. The errors were obtained by randomly changing the starting conditions and repeating the fit. They do not contain systematic experimental errors. However, the latter were minimized by means of a set of corrections and absolute calibration. Fig. 2(b) indicates that after 30 h and 50 h of milling both nonODS samples, A and B, exhibit a pronounced bimodality of the size distribution of scatterers with the first component ranging from 0.5 nm (lower detection limit of SANS) to 3 nm and the second component ranging from 3 nm to about 15 nm. It is reasonable to assume that both components correspond to different populations of nano-particles. In order to pursue this idea in more detail, all values of c and A-ratio in Table 3 (but not Rm and N) are given individually for the respective size ranges. It is interesting to note that the assumption of two different populations of nano-particles is justified ex post by the systematically different A-ratios found for both populations. Comparison of the non-ODS samples consolidated from powders milled for 30 h versus 50 h (samples A versus B in Fig. 2(b) and Table 3) suggests that increasing milling time at constant milling speed gives rise to:

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Fig. 2. a) Fitted scattering curve and b) reconstructed particle size distribution of non-ODS samples for different milling times. The solid line marks the part of the distribution which is most important for the application, as the very small particles provide most of the particle e matrix interface for defect annihilation and helium entrapment.

Fig. 3. a) Scattering curve and b) particle size distribution of samples with and without Y2O3 added. The solid line marks the part of the distribution which is most important for the application, as the very small particles provide most of the particle e matrix interface for defect annihilation and helium entrapment.

 a pronounced increase of the volume fraction of nano-particles for the first (lower size) population,  a slight increase of the volume fraction for the second population, and  a decrease of size for the second population. The size distributions of the ODS samples, C to F, and their characteristics are summarized in Figs. 3(b) and 4(b) and Table 3. It is observed that the bimodality, if any, is less pronounced than for the non-ODS samples. However, it is still meaningful to maintain the same distinction of size ranges as for the non-ODS samples for the sake of comparison. Indeed, Fig. 3(b) indicates that the volume fraction of the first population of the 0.6 wt%-ODS sample is larger than the volume fraction of the second population, whereas the opposite is true for the non-ODS sample treated with the same milling parameters. Similar observations are applicable to the other ODS samples. Fig. 3(b) and Table 3 also show that the first

population of the ODS sample assumes a much higher volume fraction than the first population of the non-ODS sample. For the second populations, the volume fractions of the ODS and non-ODS samples are of the same order of magnitude. In samples C and D, which were made of powder milled for 20 h at 150 rpm, total particle volume fractions of 0.30% and 0.46%, respectively, were estimated for the total Q-range analyzed. These values are significantly below the theoretical values of 0.52% (0.3 wt % Y2O3) and 1.04% (0.6 wt% Y2O3) derived from the Y2O3 additions. In contrast, the particle volume fractions of ODS samples E (1.18%) and F (1.16%) slightly exceed the theoretical value of 1.04%. The size distributions of scatterers for the ODS compacts with 0.6 wt% Y2O3 added, D to F, are compared in Fig. 4(b) as a function of milling parameters. The fact that sample D exhibits a much smaller volume fraction of scatterers in the size range between 0.5 nm and 3 nm than sample E indicates that the formation of oxide nanoparticles in this size range is not yet complete after

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Fig. 4. a) Scattering curve and b) particle size distribution of 0.6 wt% Y2O3 samples with different milling conditions applied. The solid line marks the part of the distribution which is most important for the application, as the very small particles provide most of the particle e matrix interface for defect annihilation and helium entrapment.

Table 3 Mean radius Rm, number density N, volume fraction c and A-ratio of the scatterers derived from the SANS measurements. Mean radius and number density refer to the entire size distribution. Volume fraction and A-ratio refer to the size ranges indicated. Sample Rm total R range [nm] N total R range [1016 cm3] c 0.5 nm < R < 3 nm [vol%] c 3 nm < R < 15 nm [vol%] A-ratio 0.5 nm < R < 3 nm A-ratio 3 nm < R < 15 nm A B C D E F

1.10 1.43 1.28 1.15 1.57 1.47

± ± ± ± ± ±

0.15 0.09 0.09 0.02 0.04 0.03

8±3 20 ± 4 14 ± 5 25 ± 20 90 ± 9 71 ± 34

0.04 0.12 0.19 0.26 1.07 1.01

± ± ± ± ± ±

0.005 0.005 0.01 0.02 0.03 0.05

milling for 20 h at 150 rpm. With an increase of the milling time from 30 h (sample E) to 50 h (sample F) at 250 rpm there is no significant further change in the particle size distribution and volume fraction. It is interesting to note that bimodal size distributions of oxide particles in ODS samples mechanically alloyed by cryomilling and consolidated by hot isostatic pressing were also observed by means of SANS in Ref. [49]. These authors report a larger mean size of the second component of the size distribution and A-ratios in the range between 1.8 and 2.3 for both populations. In order to derive conclusions about the nature of the scatterers, it is useful to compare measured A-ratios with calculations based on typical structures and compositions of oxide types reported in the literature. Based on Eq. (4), the A-ratio of a number of oxide types being candidates for the observed scatterers was calculated. It was assumed that the matrix is bcc with a composition according to the analysis given in Table 1. The oxides were assumed to be nonmagnetic in agreement with [50] and Fe was considered to be the only element with non-zero magnetic moment in the matrix, bmag,Fe ¼ 6 fm. The remaining task is to calculate the first term of the denominator in parentheses in Eq. (4) with the composition including the vacancy fraction and structure of the oxides given. The results are collected in Table 4. It is important to keep in mind for the discussion that an increase of the Y:Ti ratio with all other parameters kept constant gives rise to an increase of the A-ratio. This is because of the nuclear scattering length of Y (bnuc ¼ 7.75 fm) being higher than that of Ti (bnuc ¼ 3.37 fm).

0.13 0.27 0.11 0.20 0.11 0.15

± ± ± ± ± ±

0.005 0.01 0.01 0.03 0.005 0.03

2.76 2.12 2.48 2.84 2.58 2.50

± ± ± ± ± ±

0.01 0.01 0.01 0.02 0.03 0.05

1.59 1.53 2.07 2.10 2.06 2.00

± ± ± ± ± ±

0.01 0.02 0.02 0.03 0.02 0.04

4. Discussion 4.1. Non-ODS samples Multimodal particle size distributions are usually caused by the presence of different particle types with respect to their chemical composition and/or structure. It is therefore reasonable to interpret the two components of the observed bimodal size distributions of scatterers as manifestation of (at least) two different populations of particles. Bimodal particle size distributions were also reported by Mathon et al., Olier et al. and Sakasegawa et al. [14,50,62]. The shift of the coarser-particle fraction towards smaller size from sample A to B is likely to be caused by a refinement of the oxide particles already existing in sample A. They were formed during gas atomisation and the first 30 h of milling. A similar refinement of oxide particles during milling was shown by Nagini et al. [15] and Laurent-Brocq et al. [63]. For both non-ODS samples, the A-ratio of the coarser-particle fraction with a size of 3 nme15 nm is constant at a value of 1.55 ± 0.05, i.e. independent of the milling time (see Fig. 5). This clearly corresponds to the theoretical value for TiO in the possibly defective NaCl structure. In contrast to that, the A-ratio of particles smaller than 3 nm decreases significantly with increasing milling time from 2.76 to 2.12. This range cannot be assigned to only one of the structures of Y-free compounds included in Table 3. It is rather assumed that a mixture of different types of Ti oxides is present, e.g. corundum-type Ti2O3, rutile-type TiO2 and NaCl-type TiO with the latter contributing more for the longer milling time. Also a

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Table 4 Calculated values of the A-ratio for different kinds of scatterers in a Fe-14Cr matrix. No.

Composition

Structure

A-ratio

1 2 3

Nanovoids CrO Cr2O3

1.36 2.50 4.90

4 5

TiOa TiO2

6

TiO2

7

Ti2O3

8

Y2O3

9

Y2TiO5

10

Y2Ti2O7

11 12

O-vac-Y-Ti Fe3.4-Cr1.1-Y6.9-Ti43.9-O44.7, 10%vac [30,34] 0.1 < Y:Ti < 0.6 [30,34] Cr4.6-Y14.6-Ti37.9-O42.8 [60]

e NaCl, a ¼ 0.465 nm [51] Hexagonal [52] a ¼ 0.4959 nm, c ¼ 1.359 nm 6 formula units per unit cell NaCl, a ¼ 0.418 nm [53] Rutile (tetragonal) [37] a ¼ b ¼ 0.459 nm, c ¼ 0.296 nm 2 formula units per unit cell Anatas (tetragonal) [54] a ¼ b ¼ 0.378 nm, c ¼ 0.951 nm 4 formula units per unit cell Hexagonal [55] a ¼ 0.5155 nm, c ¼ 1.361 nm 6 formula units per unit cell Bixbyite (cubic) [56] a ¼ 1.0604 nm 16 formula units per unit cell Orthorhombic [57] a ¼ 0.1326 nm 32 formula units per unit cell Pyrochlore (fcc) [50] a ¼ 1.00947 nm 8 formula units per unit cell bcc Fe, O at octahedral sites, Y:Ti ratio between 0.2 and 0.4 [58,59] NaCl [55] NaCl [55] NaCl [55]

13

1.53 2.47

2.25

2.20

3.2 [50] 3.25 2.55

2.54 [50] 2.57 1.41e1.71 1.67 1.6e2.2 2.03

a

NaCl-type defective TiO in bcc Fe are highly coherent in the Baker-Nutting orientation relationship, (001)TiO || (001)Fe, [100]TiO || [110]Fe [61]. The mismatch is as small as 3% [53].

Fig. 5. A-ratios of different particle populations in non-ODS samples A and B in relation to A-ratios calculated or reported in the literature (compare Table 4). The volume fraction of each particle population is given in vol% close to the data point, respectively.

contribution of NaCl-type CrO cannot be excluded. As all A-ratios are clearly above 1.34, nanovoids are excluded to be a main scattering feature in the present samples. 4.2. ODS samples The finding that the total volume fraction of oxide particles observed by SANS for samples C and D (lower milling time and speed) is smaller than the volume fraction corresponding to the

added Y2O3 is probable to originate from Y-containing particles of size outside the detection range of SANS. These may be Y-containing particles larger than 15 nm, smaller than 0.5 nm or dissolved Y2O3. Particles significantly larger than 15 nm have been observed by other groups [15,39,62,64]. Particles smaller than 0.5 nm may correspond to the X-ray-amorphous particles suspected in Refs. [22,65]. For samples E and F (higher milling time and speed), the total volume fraction of oxide particles observed by SANS is significantly higher than for samples C and D and slightly exceeds the volume fraction corresponding to the added Y2O3. The increase of the volume fraction with increasing milling time and speed is expected to originate from both, the refinement of particles with sizes beyond the measured Q-range as reported by Nagini et al. [15] and the progressing involvement of Ti, Cr and O from the matrix into the particles. The latter effect is also present in the nonODS samples A and B. It is interesting to note that the sum of measured particle volume fractions of non-ODS samples A and B and the theoretical volume fraction of added Y2O3 corresponds to the measured particle volume fractions of the ODS samples E and F, respectively. Thus, non-ODS samples produced with the same milling and sintering parameters pose proper references concerning the contamination level due to the fabrication process. For the ODS samples D, E and F the bimodality of the particle size distribution is less pronounced than for the non-ODS samples A and B (see Fig. 3). Here, the coarser-particle fraction poses a comparably low volume fraction of 0.15e0.28 vol%, as opposed to 0.26e1.45 vol% for the finer-particle fraction. Bimodal particle size distributions with similar size ranges of the two components were also observed by Mathon et al. [50] and Zhong et al. [66,67]. For each of the ODS samples, the A-ratio of the coarser-particle fraction with a size of 3 nme15 nm is constant at a value of 2.05 ± 0.05, i.e. independent of the milling time, milling speed and content of added Y2O3 (0.3 wt% versus 0.6 wt%) (see Fig. 6). In particular, the agreement of the A-ratios for samples C (0.3 wt% Y2O3) and D (0.6 wt% Y2O3, same milling parameters) indicates that the same

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Fig. 6. A-ratios of different particle populations in ODS samples C - F in relation to Aratio values calculated or reported in the literature (compare Table 4). The numbers 11e13 of the marked theoretical A-ratios or A-ratio ranges refer to the structure number in Table 4. The volume fraction of each particle population is given in vol% close to the data point, respectively.

type of yttrium-containing oxide particles is formed in both cases. The observed A-ratio is higher than the corresponding A-ratio for the non-ODS samples, which can be explained by the presence of yttrium in the clusters. On the one hand, this A-ratio is in agreement with a particle type exhibiting the composition reported by Marquis et al. [60] (No. 13 in Table 4) and consistent with the range calculated on the basis of the compositional range reported by Hirata et al. [30] (No. 12 in Table 4), including Y:Ti ratios between 0.1 and 0.6. On the other hand, the measured A-ratio is inconsistent with the A-ratio calculated on the basis of the structures considered in Refs. [58,59] with Y and Ti at substitutional sites and O at octahedral sites of the bcc Fe lattice (No. 11 in Table 4). Moreover, the complex Y-Ti containing oxide phases fcc Y2Ti2O7 and orthorhombic Y2TiO5 ascribed by London et al. [68] and Hirata et al. [30] to coarser particles can also be excluded as structures of dominant scatterers in the present case. Contrary to the coarser-particle fraction, the A-ratio of the finerparticle fraction does depend on the amount of added Y2O3 and the milling parameters. An increase of the amount of added Y2O3 from 0.3 wt% to 0.6 wt% (at constant milling parameters) leads to a significant increase of the A-ratio from 2.48 to 2.84. This is due to an increase of the Y:Ti ratio in the finer particles. Obviously, the titanium in the matrix is distributed among more of the small Y-oxide particles, leading to either a lower Ti content in the Y-Ti-oxides or a coexistence of such particles (e.g. Y2Ti2O7) with Y2O3 particles in the same size range. A combined increase of milling time and speed (at constant content of added Y2O3) gives rise to a decrease of the Aratio from 2.84 to 2.50. This may be caused by a progressing incorporation of Ti from the matrix into the clusters during milling, which reduces the Y:Ti ratio, or by a reduction of the Y2O3 content in favor of Ti-containing particles. Comparison of the A-ratio measured for the finer-particle fraction, which poses the dominant contribution for the ODS alloys, with the theoretical data in Table 4 allows the composition and structure of the particles to be specified in more detail. Indeed, the measured A-ratios of the finer particles are close to the values of 2.55 and 2.57 calculated for the orthorhombic Y2TiO5 structure and the fcc Y2Ti2O7 structure, respectively. The presence of these structures was reported by other groups [8,18,39,68] for nanoparticles in ODS steels. Only sample D exhibits a noticeably higher A-ratio of the finer particles. The only structure in Table 4 that

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exhibits a higher A-ratio than Y2Ti2O7 is Y2O3 as already considered above. The latter was also detected by Klimiankou et al. [8] in Tibearing ODS steel after MA and consolidation by means of HIP. On the other hand, the non-stoichiometric compounds proposed by Posselt et al. [58,59], Hirata et al. [30] and Marquis [59] are inconsistent with the present A-ratios (Table 4 and Fig. 6). As a limitation of SANS, the measured A-ratio does not allow phases of coincidentally similar theoretical A-ratios to be distinguished. This is the case for Y2TiO5 and Y2Ti2O7. Unpublished APT results [69] obtained for the same set of samples indicate both phases to be present. From the characterization of samples produced with systematically varied milling parameters, we conclude that a milling speed of 250 rpm and milling time of 30 or 50 h is suitable to obtain complete transformation of the added Y2O3 into nm-sized Ti-containing hardening particles after consolidation. In contrast, a milling speed of 150 rpm in combination with a milling time of 20 h is not sufficient. It is important to note that the conclusions drawn about the dominant nature of the observed finer and coarser particles is counterintuitive and in contrast to some evidence in the literature, with the finer particles here found to be stoichiometric stable phases such as Y2TiO5 and Y2Ti2O7 and the coarser particles found to be coherent Y-Ti-O-containing particles ([30,34] and references therein). However, the presence of Y2Ti2O7 particles in the size range from 0.5 to 3 nm (radius) is in remarkable agreement with findings recently reported by Zhang et al. [8,18,39] for Fe-14Cr ODS alloys also consolidated by means of SPS. Their conclusion is based on evidence derived from STEM/EDS and APT measurements. The special feature of SANS is that it reveals the nature of the dominant type of nm-sized scatterers in a macroscopic volume (some 10 mm3) with reliable statistics. Insofar, the occasional presence of nm-sized coherent Y-Ti-O-containing particles and coarser Y2Ti2O7 particles as suspected by Hirata et al. [30,34] is not excluded.

5. Conclusions Oxide dispersion strengthened (ODS) Fe-14Cr alloys were produced using mechanical alloying and spark plasma sintering, varying the Y2O3 content and milling parameters. The compacts were analyzed using small-angle neutron scattering. The scattering curves, particle size distributions and A-ratios were critically compared among the investigated alloys and with results from the literature. The following conclusions were drawn: (1) Non-ODS samples exhibit bimodal particle size distributions with a finer-particle fraction in the size range 0.5 nm < R < 3 nm and a coarser-particle fraction in the size range 3 nm < R < 15 nm, corresponding to different particle types. These are assumed to be different types of Ti oxides and CrO. In ODS samples, a dominant fraction of the small particles was observed instead of a pronounced bimodal particle size distribution. (2) In the ODS samples, a milling speed of 250 rpm and milling time of 30 or 50 h is suitable under the present conditions to obtain complete transformation of the added Y2O3 into nmsized Ti-containing hardening particles after consolidation. (3) At a milling speed of 250 rpm, the non-ODS samples pose a good reference for the ODS samples with respect to contamination from the milling process. Indeed, the particle volume fractions in the ODS samples correspond to the ones in the non-ODS reference plus the theoretical volume fraction of added Y2O3, respectively.

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