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Microstructural studies of oxide dispersion strengthened austenitic steels
Materials and Design 110 (2016) 519–525
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Microstructural studies of oxide dispersion strengthened austenitic steels Koppoju Suresh ⁎, M. Nagini, R. Vijay, M. Ramakrishna, Ravi.C. Gundakaram, A.V. Reddy, G. Sundararajan International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), Balapur PO, Hyderabad 500 005, Telengana, India
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• Designed a new ODS austenitic alloy composition to suppress the detrimental (delta and sigma) phases • Synthesized the alloy through powder metallurgical process and has the hardness of 340 HV. • Ultra-fine grain structure and even distribution of dispersoids (9 nm) in the alloy the observed to enhance hardness.
A new oxide dispersion strengthened austenitic steel Fe-16.8Cr-22Ni-2W-2.4Mo-1.5Mn-0.62Si-0.15La0.2Ti + 0.35Y2O3, wt%.
a r t i c l e
i n f o
Article history: Received 28 April 2016 Received in revised form 3 August 2016 Accepted 5 August 2016 Available online 06 August 2016 Keywords: Austenite Dispersoids size Microstructure Hardness
a b s t r a c t Oxide dispersion strengthened (ODS) austenitic steel (Fe-16.8Cr-22Ni-2W-2.4Mo-1.5Mn-0.62Si-0.15La0.2Ti + 0.35Y2O3, wt%) was prepared by mechanical alloying and consolidated by hot extrusion followed by solutionization at 1150 °C for 1 h. The composition of this steel is designed to suppress the formation of deleterious delta (δ-) ferrite and sigma (σ-) FeCr phases, which reduces the life the materials. Various microscopic, spectroscopic and diffraction techniques were employed to study the hierarchical microstructural features of this alloy. X-ray diffraction studies confirm the austenitic phase and no traces of δ-ferrite phase were observed in as-extruded and solutionized conditions. The solutionized austenitic ODS steel exhibited ultra-fine grain structure having mean sizes of 280 ± 5 and 440 ± 9 nm in transverse and longitudinal sections of the bar, respectively. Evenly dispersed high density of Y-Ti-O nano-oxides with a mean size of 9 nm was achieved along with low fractions of TiC and La rich Y2Si2O7. The crystal structure of Y-Ti-O nano-oxides was determined by combining HRTEM and FFT techniques and was found to be pyrochlore Y2Ti2O7. These dispersoids and ultra-fine grain structure of austenitic steel contribute to an enhanced hardness of 346 ± 25 HV. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Austenitic stainless steels possess good corrosion and oxidation resistance at elevated temperatures and are, therefore, used for many high temperature applications upto about 600 °C. The major limitation of Fe-based austenitic steels is their low yield strength and poor creep ⁎ Corresponding author. E-mail address: [email protected] (K. Suresh).
http://dx.doi.org/10.1016/j.matdes.2016.08.020 0264-1275/© 2016 Elsevier Ltd. All rights reserved.
rupture strength as compared to Ni-based superalloys above 600 °C [1]. Although, high strength materials like precipitation hardened stainless steels are developed essentially for use in land/natural gas turbines, their use is limited to about 600 °C because of the growth of precipitates at temperatures beyond 600 °C [2]. Oxide dispersion strengthened (ODS) austenitic steels, in view of the stability of fine oxide dispersoids, should be lucrative alternatives to nickel base superalloys for use in hot sections of various turbines like casings, last stages of compressor blades, and various fasteners. The relatively better resistance of ODS
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austenitic steels at high temperatures even in marine environments can be gainfully used in marine gas turbines. Processing of ODS steel involves mechanical alloying of elemental/ pre-alloy powder of the desired steel composition with nano-Y2O3 particles, followed by consolidation of the powder either by hot extrusion or hot isostatic pressing. Since sufficient progress and understanding have already been achieved in fabrication of ferritic/martensitic and ferritic ODS steels with a controlled microstructure, a similar strategy can be applied to synthesize austenitic ODS steels. Recently, Kim et al. reported synthesis of austenitic ODS steel with composition close to AISI 316L through powder metallurgical process and obtained an ultimate tensile strength of 660 MPa at room temperature [3]. However, dispersoids of Y-Ti-O based complex oxides were not found. Instead, Y2Si2O7 and TiO2 particles with a size of few hundreds of nanometers were reported to be present in the steel matrix. Zhou et al. also reported the presence of Y-Ti-Si-O complex oxides of 20 nm with an ultimate tensile strength of 1000 MPa in ODS austenite steels [4]. It is also reported that addition of Zr and Hf to PNC316 austenitic ODS steel further reduces the average size of the complex oxides down to 6 nm from 14 nm on annealing at 1200 °C [5]. These complex oxides exhibit faceted type morphology and an anion-deficient fluorite Y2Hf2O7 type crystal structure [6]. Zhou et al. reported improved room temperature tensile strength (940 MPa) in HIPed austenite ODS steel having Y-Ti-Si oxide dispersoids of 17 nm when compared to base AISI304 austenite steel (300 MPa) [7, 8]. Incorporation of a high number density of nano-oxides in austenitic ODS steels enhances yield and ultimate tensile strength by almost 3 times even at temperatures beyond 700 °C [4,7]. However, austenitic ODS steels exhibit inferior ductility in a temperature range of 600– 900 °C, which may be due to the formation of σ-phase (FeCr) that causes the brittleness [8]. During prolonged exposure to high temperatures, austenitic steels suffer from embrittlement due to nucleation of Crrich (α′), Fe-rich (α), ordered FeCr and Laves phases [9,10,11]. The presence of a very low fraction of the δ-ferrite, which co-exists also causes the embrittlement [12]. These detrimental phases can be suppressed in the austenite steel by choosing the right combination of chromium equivalent (Creq) and nickel equivalent (Nieq) for δ-ferrite phase [13, 14,15] and electron vacancy number ( Nv ) for the σ-phase [16,17]. Woodyatt et al. developed an empirical relation to estimate the Nv from the composition of the alloy [16]. If the Nv is higher than 2.52,
the σ-phase precipitates in the alloy, with the propensity of precipitation increasing with increase of Nv . Table 1 summarizes the reported austenitic ODS steel compositions along with calculated Creq, Nieq and Nv values in addition to results from mechanical properties. The Nv value of each of the alloys studied is much higher than 2.52, which indicates that the appearance of σ-phase is inevitable in these austenitic ODS steels [4–8,18,19]. Moreover, the Creq and Nieq values indicate that the reported alloys are not completely in the austenite phase field [3–8,18]. In this study, the alloy composition was carefully designed such that the alloy is completely in austenitic phase field without the formation of δ-ferrite, while suppressing the tendency to form σ-phase under prolonged exposures at high temperatures. Therefore, the nominal chemical composition of the austenitic steel selected for the present study is Fe-16.8Cr-22Ni-2W-2.4Mo-1.5Mn-0.62Si-0.15La-0.2Ti (wt%), where Ni and Mn stabilize the austenitic phase and Cr improves resistance to corrosion and oxidation. Addition of La further enhances high temperature oxidation resistance by forming a thin layer of La rich chromium oxide below the chromia scale (surface Cr2O3 layer), thereby preventing further sublimation of the Cr [20]. W and Mo contribute to solid solution strengthening [21]. In the presence of Ni and Cr, Mo improves resistance to pitting corrosion [22,23]. A small fraction of Ti in this alloy greatly helps to reduce the size of the complex and stable YTi-O based oxides [24]. The alloy was designed such that primary strengthening is due to oxide dispersoids (5–10 nm) and not with the carbides and the nitrides of MX type. The present paper discusses the development of microstructure of the matrix and the structure of dispersoids in ODS austenitic steel. 2. Experimental The ODS austenitic steel with composition of Fe-16.8Cr-22Ni-2W2.4Mo-1.5Mn-0.62Si-0.15La-0.2Ti + 0.35Y2O3, wt% was prepared by mechanical alloying, followed by hot extrusion. Austenitic SS316L alloy powder was prepared by water atomization technique. To attain the proposed composition of the alloy, Ni, Cr, W, Ti, NiLa powder were added to the SS316L powder and the mixture was blended along with Y2O3 powder and mechanically milled in Zoz Simoloyer CM-08 mill at 700 rpm for 4 h with a ball to powder ratio of 10:1 using SS vial and
Table 1 Summary of the compositions, mechanical properties and other structural data in comparison with the reference data. Creq/Nieq
Presence of δ, σ phase
Composition (wt%)
Nv
Fe-16.56Cr-11.23Ni-2.19Mo-0.12Mn-0.81Si-0.029C-0.30Ti + 0.35Y2O3
2.76 21.86/12.16 δ, σ
Fe-18Cr-8Ni-1Mo + 0.5Ti + 0.35Y2O3
2.89 20.25/8.0
δ, σ
Processing stages
Precipitates/oxide dispersoids (size, nm)
Mechanical Ref. property
HIPed @ 1150 °C
TiO2 (~300) Y2Si2O7 (~400)
UTS 670
[3]
MPa UTS 1000
[4]
HIPed @ 1100 °C
Y-Ti-Si-O (~20) TiN (N100)
MPa YS 960 MPa
Fe-16.16Cr-13.66Ni-2.33Mo-1.82Mn-0.18Ti-0.75Si-0.08Nb-0.05C-0.5Zr 2.68 21.97/16.07 δ, σ + 0.35Y2O3 Fe-16.16Cr-13.66Ni-2.33Mo-1.82Mn-0.18Ti-0.75Si-0.08Nb-0.05C + 0.6Hf + 0.35Y2O3 Fe-18Cr-8Ni-2W-1 Ti-0.35Y2O3 Fe-18Cr-8Ni-1Mo + 0.5Ti + 0.35Y2O3
2.67 22.29/16.07 δ, σ 2.86 21.0/8.0 2.89 20.25/8.0
δ, σ δ, σ δ, σ
Fe-20Ni-14Cr-2.5Mo-2.5 Al-2Mn + 0.5 & 5% Y2O3
2.64 31.5/21.0
Fe-24.5Cr-19.2Ni-0.46Ti-0.44N-0.019C-0.22Y-0.2O (ex.)
2.92 25.19/30.77 σ
Fe-16.8Cr-22Ni-2W-2.4Mo-1.5Mn-0.62Si-0.15La-0.022C-0.2Ti + 0.35Y2O3
2.54 23.44/23.41 –
Powder annealed @ 400–1200 °C
Y2Hf2O7 (~5.9) Y2Zr2O7 (~6.0)
[5]
Hot extrusion @ 1100
Y2Hf2O7 (~10)
[6]
°C HIPed @ 1150 °C
Y-Ti-O (10–80)
HIPed @ 1100 °C
TiN (large) Y-Ti-O (fine ~17)
UTS 775
[7]
MPa UTS 940
[8]
MPa
Powder annealed and YAlO3, Y2O3, hot pressing @ 1150 °C Al2O3, Y2Al5O12 HIPed @ 1100 °C Y-Ti-O (10−20)
UTS 904
Hot extrusion @ 1150
MPa VH 346 ±
°C
Y2Si2O7 (~100) TiC (~40) Y2Ti2O7 (~9)
[18]
20 HV
[19] Present work
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hardened steel balls of 5 mm diameter. Ethylene glycol (0.1 wt%) was added as process controlling agent. Both milling and powder handling were carried out under Ar atmosphere. The milled powder was filled in a mild steel container (ϕ:50 mm, L:75 mm), followed by degassing at 450 °C under vacuum of 5.3 × 10−3 Pa and sealed. The sealed can was upset at 1050 °C and 510 MPa and hot extruded at 1150 °C and 1070 MPa with an extrusion ratio of 9 to obtain a rod of 16 mm diameter. An extrusion speed of 8 mm/s was maintained for all the extrusions. The density of the as-extruded sample was 9.8 g/cm3. Finally, the extruded bar was solutionized at 1150 °C for 1 h followed by quenching in water. The chemical composition was measured using ICP-AES (Jobin-Yvon France, Model ultima-2CHR), carbon analysis was carried out using LECO carbon analyzer (Model CS444) and oxygen and nitrogen analysis were carried out using LECO oxygen and nitrogen determinator (Model TC-600). It was found that the concentration of trace elements C, N, O (excess), P and S in the alloy are 0.022, 0.014, 0.126, 0.021 and 0.01 wt%, respectively. Samples for structural and mechanical characterization were chosen at various stages of processing such as asmilled powder, as-extruded and solutionized. The as-milled powder particles were mounted and polished as per standard metallographic procedures. The hardness of the powder was evaluated using a Vickers micro hardness tester (VMHT-10) with a load of 50 g and dwell time of 10 s. An average value from 10 indentations made on the powder samples is reported. Bulk hardness was measured on consolidated (asextruded and solutionized) samples with a load of 5 kg using Vickers macro hardness tester (LECO, Model: LV-700AT). The structural and microstructural studies have been carried out using powder X-ray diffraction (Bruker D8 Advance with Cukα radiation), optical microscopy (OM Olympus Model: GX51), scanning electron microscopy (Hitachi Model S-4300SE/N), electron back scatter diffraction (EBSD, EDAX-TSL) and transmission electron microscopy (TEM, FEI-G2, 200 kV) with energy dispersive spectroscopy (EDS) and Gatan energy filtered (EF-TEM) techniques.
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Fig. 1 shows the Vickers hardness at different processing stages of the austenitic ODS steel. Hardness of the as-milled powder, as-extruded and solutionized specimens was found to be 425 ± 30, 330 ± 29 and 346 ± 25 HV respectively. It is clear that the as-milled powder exhibits higher hardness than the as-extruded and solutionized samples, since it undergoes severe cold working during the mechanical milling, resulting in changes in dislocation density and formation of sub-structure [25]. The bulk hardness of the as-extruded and solutionized samples suggests that the hardness in solutionized condition is slightly higher than the corresponding extruded sample because of the dissolution of carbides
and enhancement of the solid solution strengthening effect. The measured hardness is found to be higher than reported earlier and the vales are summarized in Table 1. The secondary electron micrographs of the milled powder are shown in Fig. 2. It can be seen from the figure that the as milled powder shows flake type morphology with a bi-model distribution in size. The size of the finest powder particles is around 2 μm whereas the largest particles are around 60 μm in size. The secondary electron image of the as-milled powder (inset to Fig. 2) exhibits a lamellar type flow line structure. This kind of structure evolves during the milling, which is due to repeated fracture and welding of the powder. In the early stages of mechanical milling, the ductile component, that is austenite steel powder, flattens to platelet shape by repeated micro-forging processes. In the next stage, these flattened particles are cold welded together and form a composite lamellar structure [26,27]. With increasing milling time, the composite powder particles get work hardened resulting in increase in hardness and consequently fragment to finer particles with more equiaxed dimensions. On further continuation of milling, dissolution of Yttria, which is trapped between the lamella, begins to occur at this stage. The hardness, and particle size and substructure tend to reach a saturation value at this stage. The crystal structure and phases present in the samples with different processing conditions were studied by X-ray diffraction (XRD). The XRD patterns (Fig. 3) of the blended powder shows the presence of austenitic (fcc) and ferrite (bcc) phase. The bcc phase found in the blended powder matches with ferrite (Pearson crystal data 558498), which is considered to have been inherited from 316L powder. The milled powder also exhibits two peaks corresponding to fcc and bcc phases but the intensity of bcc phases reduced significantly. This reduction is considered to be due to non-equilibrium compositional changes (probably enhancement of Ni content) in the ferrite phase, thus destabilizing the ferrite phase [28]. XRD patterns of as-extruded and solutionized samples revealed the presence only of a single fcc (austenite) phase with a lattice constant of 3.602 ± 0.004 Å. The observed lattice parameter of the as-extruded and solutionized samples is close to the previously reported lattice parameter of ASI316 austenite steel [29]. In order to examine the detailed grain structure from the relatively large area of solutionized austenitic ODS steel, electron back scattered diffracted (EBSD) high angle (15–180° miss-orientation) grain boundary maps have been obtained from the transverse (perpendicular to the extrusion direction) and longitudinal (parallel to the extrusion direction) sections of the steel bar is shown in Fig. 4. It can be observed from the Fig. 4(a) that equiaxed grains are present, whereas Fig. 4(b) shows elongated grains with bimodal distribution. The grain size (diameter) calculated from the EBSD data by measuring the area of each grain, converting the area to that of a circle and taking the diameter of the
Fig. 1. Vickers hardness of the as-milled, as-extruded and solutionized austenitic ODS steel.
Fig. 2. Secondary electron (SE) image of the as-milled austenite ODS powder after milling for 4 h and (inset) higher magnification SE images showing the internal structure of the as-milled powder.
3. Results
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Fig. 3. XRD patterns of blended, as-milled, extruded and solutionized austenitic ODS steels.
circle as grain size. The area fraction of the grains in the EBSD maps was plotted as a function of grain diameter and is shown in Fig. 4(c & d). The estimated finer grains exhibit log-normal size distribution with mean grain size observed from transverse and longitudinal sectioned steel bar being 280 ± 5 and 440 ± 9 nm, respectively. The observed minor fraction of larger grains in the longitudinal section is in the range of 1 to 5 μm and the similar microstructural observations are already reported by Mao et al. [30]. Transmission electron microscopy studies have been carried out on the as-milled and solutionized austenite ODS steel specimens. The BF images from the different regions of the as-milled powder are shown in Fig. 5(a & b) and the corresponding selected area electron diffraction (SAED) patterns are shown as insets in the BF images. The microstructure observed in one of the regions (Fig. 5a) showed strained patterns, and the corresponding electron diffraction pattern (inset to Fig. 5a) revealed that this region is crystallized with bcc structure. Microstructure observed in the other region confirmed strain-free, fine and elongated grains. The electron diffraction pattern (inset to Fig. 5b) obtained from this region revealed the structure of these grains to be fcc. TEM study of the as-milled powder confirms the presence of two phases as seen in the SAED patterns and the phases identified are ferrite (bcc) and austenite (fcc) (Fig. 5a & b). XRD studies also confirm the presence of bcc
Fig. 5. Transmission electron bright field (BF) micrographs of as-milled powder showing two phase structure (a) ferritic and (b) austenitic phases, and corresponding SAD patterns [inset to BF images]. Dark field (DF) images (c) and (d) obtained from the SAD pattern [inset to (b)] of austenitic region marked as 1 and 2, respectively.
Fig. 4. Electron backscattered diffraction (EBSD) high angle (15–180° miss-orientation) grain boundary maps of (a) transverse, and (b) longitudinal section of solutionized austenite ODS steel bar, (c) and (d) are the plot of the area fraction with grain size, estimated from the (a) and (b), respectively.
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structure along with fcc structure in the as-milled powder. Therefore, TEM and XRD studies are consistent with each other. In addition to the diffracted rings (inset to Fig. 5b), a set of diffraction spots can be seen, which are inside the first diffraction ring. From these diffracted spots, dark field (DF) images of these regions were obtained (Fig. 5c & d), which show fine and coarse particles. Low magnification TEM bright field (BF) micrograph of the solutionized sample in the transverse section of the bar is shown in Fig. 6 from which it can be seen that the as-solutionized sample possesses ultra-fine equiaxed grain structure. It is also observed that a few larger grains are present, which is consistent with EBSD data inferred from the grain maps (Fig. 4). A similar microstructure has been reported in mechanically alloyed, hot isostatic pressed (HIPed) ODS austenitic steel [7,8,19,21]. SAED patterns from the solutionized sample could be indexed with the fcc austenite phase, while the bcc ferritic phase was not observed. In the microstructure, various other features were observed such as twins, coarse and fine particles. Three types of particles were observed within the size range of 45–200 nm (Fig. 7). Energy dispersive spectroscopy (EDS) studies confirmed that some of these particles are silicon oxides. The other oxide particles are Lanthanum-rich Yttrium-Silicon based oxides having the chemical composition 41.6, 4.2 and 54.2 at.% of Y, La and Si, respectively and during the quantification, O is not considered, as the quantifying the same yields error. The crystal structure of the these particles was identified using the SAED pattern obtained from one of those particles with different orientations of [100] and [110], which confirms that these oxide particles are (Y,La)2Si2O7 with monoclinic crystal structure (Fig. 7b). However, occurrence of these particles is rare (b 1% of the observed particles). Kim et al. have also reported sub-micron sized Y2Si2O7 type complex and TiO2 based oxides in 304L grade austenitic ODS steel after milling, HIPing and hot rolling at 1150 °C [4]. The other type of coarse particles with size around 40–50 nm are found be Ti rich carbides (Fig. 8). Crystal structure of this carbide was identified using 〈100〉 and 〈110〉 zone axes of SAED patterns which confirm the face centered cubic structure and a Fm-3m symmetry and a lattice parameter of 4.26 ± 0.03 Å. In order to further study the nano-dispersoids, high magnification bright field TEM images were recorded. From Fig. 9a, finely dispersed nano-oxides can be observed with a size b10 nm. A few oxide particles with a size of about 25 nm can also be seen. For evaluation of the size distribution of nano-oxides, a total 2257 particles were counted from N40 micrographs and the distribution is shown in Fig. 9b. The number fraction with size fitted with log-normal size distribution yields a mean size of 9.1 nm and variance of 0.45. The obtained small oxide dispersoids size may be related to the homogeneous microstructure (uniform yttria segregation) of the as-milled powder by adoption of very high energy Simoloyer Zoz mill which imparts almost 9 times higher
energy to the powder than the other regularly used planetary ball mills [31,32,33]. These nano-oxides were found to be dispersed evenly in the matrix, which are responsible for dispersion strengthening of the austenitic steel. EDS results reveal that the fine particles with size b20 nm consist of Y, Ti and O, whereas the large oxide particles with size larger than 100 nm (shown in Fig. 7) consist of Si along with Y, La and O, and do not contain Ti. Thus, the presence of Ti in ODS steel refines the oxide particle size [34]. Estimated chemical composition of the fine oxides is 53.7 and 46.3 at.% for Y and Ti, respectively.
Fig. 6. TEM BF image of solutionized austenite ODS steel in transverse section of the bar, and (inset) SAD pattern obtained from the same region.
Fig. 8. BF micrograph showing particles with different sizes of austenite ODS steel solutionized at 1150 °C/1 h. Inset shows SAD patterns from the precipitate with [100] and [110] zone axes.
Fig. 7. (a) TEM BF image showing different sizes of oxide particles. Inset shows the SAED pattern obtained from the particle indicated as “c”, and (b) superimposed EDS profiles obtained from locations as-indicated “a”: matrix, “b”: small oxide particle (Y-Ti-O) and “c” big oxide (La rich Y2Si2O7) particle of austenite ODS steel solutionized at 1150 °C/1 h.
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Fig. 9. (a) Higher magnification BF micrograph of solutionized austenitic ODS steel showing fine dispersoids and (b) size distribution plot of the dispersoids.
In order to study the structure of the nano Y-Ti-O complex oxides with a mean size of 10 nm, high resolution TEM images were recorded, while the matrix was oriented to 〈011〉 zone axis as shown in Fig. 10. The high resolution image reveals distinguishable lattice planes of the oxide particles. Further, Fast Fourier transformation (FFT) and inverse FFT techniques were applied to the high resolution lattice image to extract the lattice symmetry and inter-planar distance. The FFT pattern shows a [004] diffraction pattern related to diamond cubic (Fd-3m) pyrochlore Y2Ti2O7 type structure. Inverse FFT obtained from [004] reflections shows an inter-planar distance of 2.52 Å, therefore, the lattice parameter is 10.08 Å, which is close to the lattice parameter of Y2Ti2O7 type pyrochlore structure [35,36]. 4. Discussion The microstructure of the solutionized sample shows a bimodal grain structure (Figs. 4 & 6). A high density of the nano-oxides was observed in the finer grains, while large grains have relatively fewer nanooxides [8]. The presence of nano-oxides inhibited the grain growth which led to the retention of finer grains [37], whereas the absence of oxide particles due to inhomogeneous distribution during milling led to grain growth with a larger grain size of 1–5 μm [5]. The area fraction of large grains is considerably lower than that of fine grains. Large grains should be suppressed in order to obtain a homogeneous microstructure, thereby leading to enhanced mechanical properties. In the present study, microstructural inhomogeneities were present which probably could be eliminated by prolonged milling time. However, the milling duration could not be increased due to processing difficulties due to the sticky nature of powder particles to the milling media, which would also increase contamination levels.
Fig. 10. (a) High resolution image of the Y-Ti-O oxide dispersoids in the austenitic (fcc) matrix along 〈011〉 orientation. Inset is the Fast Fourier transform (FFT) of the whole area exhibiting diffraction spots from dispersoids (400) and matrix. (b) Inverse FFT created using (400) spot of dispersoid. The top inset shows the magnified view of the dispersoid. The intensity profile obtained from across the lattice planes in the bottom inset. The measured inter-planar distance from the profile is 2.56 Å.
In contrast to ferritic/martensitic ODS steels, various precipitates, oxides and dispersoids in austenite ODS steels were found. In the solutionized sample, a small number fraction of coarse La rich (La,Y)2Si2O7 particles, and a large number fraction of very fine Y2Ti2O7 dispersoids were observed. In the present study, the base powder consists of 0.62 wt% Si and during consolidation, some Si might have precipitated with Y, La and O forming monoclinic type (Y,La)2Si2O7 oxide particles. Gibbs free energy (ΔG) calculations suggest that the formation of (Y,La)2Si2O7 phase is favorable due to the lower ΔG of Y2Si2O7 when compared to Y2Ti2O7 and TiO2 [3]. Bulk hardness of the solutionized austenitic ODS steel in excess of 340 HV has been achieved, which is higher than the published data of ODS austenite steels [3,4,21] and 316 austenitic steel (160 HV) [23,29] and comparable to the hardness of the Inconel718 super alloy of 330 ± 10 HV [38]. The obtained higher hardness is attributed to a combination of dispersoids, grain size and solid solution strengthening. Further enhancement of hardness could be achieved with a better control
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of microstructure by the complete suppression of undesirable oxides such as La rich Y2Si2O7. 5. Conclusions Oxide dispersion strengthened (ODS) austenitic steel was developed by designing the composition such that the formation of delta (δ-) ferrite and sigma (σ-) FeCr phases, which are deleterious for prolonged high temperature exposure are suppressed. This steel is made by mechanical alloying followed by hot extrusion and solutionization. X-ray and electron diffraction studies of solutionized ODS austenitic steel confirm the presence of only austenitic phase (fcc), and no other phases (esp. δ-ferrite) were detected. The microstructure of the solutionized sample comprises fine equiaxed grains in transverse section and elongated grains in longitudinal section of the steel bar with mean size of 280 ± 5 and 440 ± 9 nm, respectively. The dispersoids in this steel are mainly of the pyrochlore type Y2Ti2O7 with low fraction of TiC, and La rich Y2Si2O7. The size of the Ti-containing yttrium oxide particles is much finer with a mean size of 9 nm than the Si containing Y-Si-O oxide particles. Bulk hardness of 346 ± 25 HV was achieved in this alloy. Acknowledgements The authors would like to thank Dr. K. Satya Prasad, Mr. B. Manjunath, and Mr. G. V. R. Reddy for their kind support during TEM, SEM and metallographic studies. References [1] P. Marshall, Austenitic Stainless Steels, Elsevier, London–New York, 1984. [2] T. Sourmail, Precipitation in creep resistant austenitic stainless steels, Mater. Sci. Technol. 17 (2001) 1–14. [3] T.K. Kim, C.S. Bae, D.H. Kim, J. Jang, S.H. Kim, C.B. Lee, D. Hahn, Microstructural observation and tensile isotropy of an austenitic ODS steel, Nucl. Eng. Technol. 40 (2008) 305–310. [4] Z. Zhou, S. Yang, W. Chen, L. Liao, Y. Xu, Processing and characterization of a hipped oxide dispersion strengthened austenitic steel, J. Nucl. Mater. 428 (2012) 31–34. [5] H. Oka, M. Watanabe, S. Ohnuki, N. Hashimoto, S. Yamashita, S. Ohtsuka, Effects of milling process and alloying additions on oxide particle dispersion in austenitic stainless steel, J. Nucl. Mater. 447 (2014) 248–253. [6] H. Oka, M. Watanabe, N. Hashimoto, S. Ohnuki, S. Yamashita, S. Ohtsuka, Morphology of oxide particles in ODS austenitic stainless steel, J. Nucl. Mater. 442 (2013) 164–168. [7] Y. Xu, Z. Zhou, M. Li, P. He, Fabrication and characterization of ODS austenitic steels, J. Nucl. Mater. 417 (2011) 283–285. [8] M. Wang, Z. Zhou, H. Sun, H. Hu, S. Li, Microstructural observation and tensile properties of ODS-304 austenitic steel, Mater. Sci. Eng. A 559 (2013) 287–292. [9] P.J. Mazias, Developing an austenitic stainless steel for improved performance in advanced fossil power facilities, JOM J. Miner. Met. Mater. Soc. 41 (1989) 14–20. [10] Y. Yamamoto, M.P. Brady, Z.P. Lu, P.J. Maziasz, C.T. Liu, B.A. Pint, K.L. More, H.M. Meyer, E.A. Payzant, Creep-resistant, Al2O3-forming austenitic stainless steels, Science 5823 (2007) 433–436. [11] B. Weiss, R. Stickler, Phase instabilities during high temperature exposure of 316 austenitic stainless steel, Metall. Trans. 3 (1972) 851–866. [12] D. Peckner, M. Burnstein, Handbook of Stainless Steels, McGraw-Hill, New York, 1977 (ISBN 007049147X).
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Microstructural studies of oxide dispersion strengthened austenitic steels Koppoju Suresh ⁎, M. Nagini, R. Vijay, M. Ramakrishna, Ravi.C. Gundakaram, A.V. Reddy, G. Sundararajan International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), Balapur PO, Hyderabad 500 005, Telengana, India
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• Designed a new ODS austenitic alloy composition to suppress the detrimental (delta and sigma) phases • Synthesized the alloy through powder metallurgical process and has the hardness of 340 HV. • Ultra-fine grain structure and even distribution of dispersoids (9 nm) in the alloy the observed to enhance hardness.
A new oxide dispersion strengthened austenitic steel Fe-16.8Cr-22Ni-2W-2.4Mo-1.5Mn-0.62Si-0.15La0.2Ti + 0.35Y2O3, wt%.
a r t i c l e
i n f o
Article history: Received 28 April 2016 Received in revised form 3 August 2016 Accepted 5 August 2016 Available online 06 August 2016 Keywords: Austenite Dispersoids size Microstructure Hardness
a b s t r a c t Oxide dispersion strengthened (ODS) austenitic steel (Fe-16.8Cr-22Ni-2W-2.4Mo-1.5Mn-0.62Si-0.15La0.2Ti + 0.35Y2O3, wt%) was prepared by mechanical alloying and consolidated by hot extrusion followed by solutionization at 1150 °C for 1 h. The composition of this steel is designed to suppress the formation of deleterious delta (δ-) ferrite and sigma (σ-) FeCr phases, which reduces the life the materials. Various microscopic, spectroscopic and diffraction techniques were employed to study the hierarchical microstructural features of this alloy. X-ray diffraction studies confirm the austenitic phase and no traces of δ-ferrite phase were observed in as-extruded and solutionized conditions. The solutionized austenitic ODS steel exhibited ultra-fine grain structure having mean sizes of 280 ± 5 and 440 ± 9 nm in transverse and longitudinal sections of the bar, respectively. Evenly dispersed high density of Y-Ti-O nano-oxides with a mean size of 9 nm was achieved along with low fractions of TiC and La rich Y2Si2O7. The crystal structure of Y-Ti-O nano-oxides was determined by combining HRTEM and FFT techniques and was found to be pyrochlore Y2Ti2O7. These dispersoids and ultra-fine grain structure of austenitic steel contribute to an enhanced hardness of 346 ± 25 HV. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Austenitic stainless steels possess good corrosion and oxidation resistance at elevated temperatures and are, therefore, used for many high temperature applications upto about 600 °C. The major limitation of Fe-based austenitic steels is their low yield strength and poor creep ⁎ Corresponding author. E-mail address: [email protected] (K. Suresh).
http://dx.doi.org/10.1016/j.matdes.2016.08.020 0264-1275/© 2016 Elsevier Ltd. All rights reserved.
rupture strength as compared to Ni-based superalloys above 600 °C [1]. Although, high strength materials like precipitation hardened stainless steels are developed essentially for use in land/natural gas turbines, their use is limited to about 600 °C because of the growth of precipitates at temperatures beyond 600 °C [2]. Oxide dispersion strengthened (ODS) austenitic steels, in view of the stability of fine oxide dispersoids, should be lucrative alternatives to nickel base superalloys for use in hot sections of various turbines like casings, last stages of compressor blades, and various fasteners. The relatively better resistance of ODS
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austenitic steels at high temperatures even in marine environments can be gainfully used in marine gas turbines. Processing of ODS steel involves mechanical alloying of elemental/ pre-alloy powder of the desired steel composition with nano-Y2O3 particles, followed by consolidation of the powder either by hot extrusion or hot isostatic pressing. Since sufficient progress and understanding have already been achieved in fabrication of ferritic/martensitic and ferritic ODS steels with a controlled microstructure, a similar strategy can be applied to synthesize austenitic ODS steels. Recently, Kim et al. reported synthesis of austenitic ODS steel with composition close to AISI 316L through powder metallurgical process and obtained an ultimate tensile strength of 660 MPa at room temperature [3]. However, dispersoids of Y-Ti-O based complex oxides were not found. Instead, Y2Si2O7 and TiO2 particles with a size of few hundreds of nanometers were reported to be present in the steel matrix. Zhou et al. also reported the presence of Y-Ti-Si-O complex oxides of 20 nm with an ultimate tensile strength of 1000 MPa in ODS austenite steels [4]. It is also reported that addition of Zr and Hf to PNC316 austenitic ODS steel further reduces the average size of the complex oxides down to 6 nm from 14 nm on annealing at 1200 °C [5]. These complex oxides exhibit faceted type morphology and an anion-deficient fluorite Y2Hf2O7 type crystal structure [6]. Zhou et al. reported improved room temperature tensile strength (940 MPa) in HIPed austenite ODS steel having Y-Ti-Si oxide dispersoids of 17 nm when compared to base AISI304 austenite steel (300 MPa) [7, 8]. Incorporation of a high number density of nano-oxides in austenitic ODS steels enhances yield and ultimate tensile strength by almost 3 times even at temperatures beyond 700 °C [4,7]. However, austenitic ODS steels exhibit inferior ductility in a temperature range of 600– 900 °C, which may be due to the formation of σ-phase (FeCr) that causes the brittleness [8]. During prolonged exposure to high temperatures, austenitic steels suffer from embrittlement due to nucleation of Crrich (α′), Fe-rich (α), ordered FeCr and Laves phases [9,10,11]. The presence of a very low fraction of the δ-ferrite, which co-exists also causes the embrittlement [12]. These detrimental phases can be suppressed in the austenite steel by choosing the right combination of chromium equivalent (Creq) and nickel equivalent (Nieq) for δ-ferrite phase [13, 14,15] and electron vacancy number ( Nv ) for the σ-phase [16,17]. Woodyatt et al. developed an empirical relation to estimate the Nv from the composition of the alloy [16]. If the Nv is higher than 2.52,
the σ-phase precipitates in the alloy, with the propensity of precipitation increasing with increase of Nv . Table 1 summarizes the reported austenitic ODS steel compositions along with calculated Creq, Nieq and Nv values in addition to results from mechanical properties. The Nv value of each of the alloys studied is much higher than 2.52, which indicates that the appearance of σ-phase is inevitable in these austenitic ODS steels [4–8,18,19]. Moreover, the Creq and Nieq values indicate that the reported alloys are not completely in the austenite phase field [3–8,18]. In this study, the alloy composition was carefully designed such that the alloy is completely in austenitic phase field without the formation of δ-ferrite, while suppressing the tendency to form σ-phase under prolonged exposures at high temperatures. Therefore, the nominal chemical composition of the austenitic steel selected for the present study is Fe-16.8Cr-22Ni-2W-2.4Mo-1.5Mn-0.62Si-0.15La-0.2Ti (wt%), where Ni and Mn stabilize the austenitic phase and Cr improves resistance to corrosion and oxidation. Addition of La further enhances high temperature oxidation resistance by forming a thin layer of La rich chromium oxide below the chromia scale (surface Cr2O3 layer), thereby preventing further sublimation of the Cr [20]. W and Mo contribute to solid solution strengthening [21]. In the presence of Ni and Cr, Mo improves resistance to pitting corrosion [22,23]. A small fraction of Ti in this alloy greatly helps to reduce the size of the complex and stable YTi-O based oxides [24]. The alloy was designed such that primary strengthening is due to oxide dispersoids (5–10 nm) and not with the carbides and the nitrides of MX type. The present paper discusses the development of microstructure of the matrix and the structure of dispersoids in ODS austenitic steel. 2. Experimental The ODS austenitic steel with composition of Fe-16.8Cr-22Ni-2W2.4Mo-1.5Mn-0.62Si-0.15La-0.2Ti + 0.35Y2O3, wt% was prepared by mechanical alloying, followed by hot extrusion. Austenitic SS316L alloy powder was prepared by water atomization technique. To attain the proposed composition of the alloy, Ni, Cr, W, Ti, NiLa powder were added to the SS316L powder and the mixture was blended along with Y2O3 powder and mechanically milled in Zoz Simoloyer CM-08 mill at 700 rpm for 4 h with a ball to powder ratio of 10:1 using SS vial and
Table 1 Summary of the compositions, mechanical properties and other structural data in comparison with the reference data. Creq/Nieq
Presence of δ, σ phase
Composition (wt%)
Nv
Fe-16.56Cr-11.23Ni-2.19Mo-0.12Mn-0.81Si-0.029C-0.30Ti + 0.35Y2O3
2.76 21.86/12.16 δ, σ
Fe-18Cr-8Ni-1Mo + 0.5Ti + 0.35Y2O3
2.89 20.25/8.0
δ, σ
Processing stages
Precipitates/oxide dispersoids (size, nm)
Mechanical Ref. property
HIPed @ 1150 °C
TiO2 (~300) Y2Si2O7 (~400)
UTS 670
[3]
MPa UTS 1000
[4]
HIPed @ 1100 °C
Y-Ti-Si-O (~20) TiN (N100)
MPa YS 960 MPa
Fe-16.16Cr-13.66Ni-2.33Mo-1.82Mn-0.18Ti-0.75Si-0.08Nb-0.05C-0.5Zr 2.68 21.97/16.07 δ, σ + 0.35Y2O3 Fe-16.16Cr-13.66Ni-2.33Mo-1.82Mn-0.18Ti-0.75Si-0.08Nb-0.05C + 0.6Hf + 0.35Y2O3 Fe-18Cr-8Ni-2W-1 Ti-0.35Y2O3 Fe-18Cr-8Ni-1Mo + 0.5Ti + 0.35Y2O3
2.67 22.29/16.07 δ, σ 2.86 21.0/8.0 2.89 20.25/8.0
δ, σ δ, σ δ, σ
Fe-20Ni-14Cr-2.5Mo-2.5 Al-2Mn + 0.5 & 5% Y2O3
2.64 31.5/21.0
Fe-24.5Cr-19.2Ni-0.46Ti-0.44N-0.019C-0.22Y-0.2O (ex.)
2.92 25.19/30.77 σ
Fe-16.8Cr-22Ni-2W-2.4Mo-1.5Mn-0.62Si-0.15La-0.022C-0.2Ti + 0.35Y2O3
2.54 23.44/23.41 –
Powder annealed @ 400–1200 °C
Y2Hf2O7 (~5.9) Y2Zr2O7 (~6.0)
[5]
Hot extrusion @ 1100
Y2Hf2O7 (~10)
[6]
°C HIPed @ 1150 °C
Y-Ti-O (10–80)
HIPed @ 1100 °C
TiN (large) Y-Ti-O (fine ~17)
UTS 775
[7]
MPa UTS 940
[8]
MPa
Powder annealed and YAlO3, Y2O3, hot pressing @ 1150 °C Al2O3, Y2Al5O12 HIPed @ 1100 °C Y-Ti-O (10−20)
UTS 904
Hot extrusion @ 1150
MPa VH 346 ±
°C
Y2Si2O7 (~100) TiC (~40) Y2Ti2O7 (~9)
[18]
20 HV
[19] Present work
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hardened steel balls of 5 mm diameter. Ethylene glycol (0.1 wt%) was added as process controlling agent. Both milling and powder handling were carried out under Ar atmosphere. The milled powder was filled in a mild steel container (ϕ:50 mm, L:75 mm), followed by degassing at 450 °C under vacuum of 5.3 × 10−3 Pa and sealed. The sealed can was upset at 1050 °C and 510 MPa and hot extruded at 1150 °C and 1070 MPa with an extrusion ratio of 9 to obtain a rod of 16 mm diameter. An extrusion speed of 8 mm/s was maintained for all the extrusions. The density of the as-extruded sample was 9.8 g/cm3. Finally, the extruded bar was solutionized at 1150 °C for 1 h followed by quenching in water. The chemical composition was measured using ICP-AES (Jobin-Yvon France, Model ultima-2CHR), carbon analysis was carried out using LECO carbon analyzer (Model CS444) and oxygen and nitrogen analysis were carried out using LECO oxygen and nitrogen determinator (Model TC-600). It was found that the concentration of trace elements C, N, O (excess), P and S in the alloy are 0.022, 0.014, 0.126, 0.021 and 0.01 wt%, respectively. Samples for structural and mechanical characterization were chosen at various stages of processing such as asmilled powder, as-extruded and solutionized. The as-milled powder particles were mounted and polished as per standard metallographic procedures. The hardness of the powder was evaluated using a Vickers micro hardness tester (VMHT-10) with a load of 50 g and dwell time of 10 s. An average value from 10 indentations made on the powder samples is reported. Bulk hardness was measured on consolidated (asextruded and solutionized) samples with a load of 5 kg using Vickers macro hardness tester (LECO, Model: LV-700AT). The structural and microstructural studies have been carried out using powder X-ray diffraction (Bruker D8 Advance with Cukα radiation), optical microscopy (OM Olympus Model: GX51), scanning electron microscopy (Hitachi Model S-4300SE/N), electron back scatter diffraction (EBSD, EDAX-TSL) and transmission electron microscopy (TEM, FEI-G2, 200 kV) with energy dispersive spectroscopy (EDS) and Gatan energy filtered (EF-TEM) techniques.
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Fig. 1 shows the Vickers hardness at different processing stages of the austenitic ODS steel. Hardness of the as-milled powder, as-extruded and solutionized specimens was found to be 425 ± 30, 330 ± 29 and 346 ± 25 HV respectively. It is clear that the as-milled powder exhibits higher hardness than the as-extruded and solutionized samples, since it undergoes severe cold working during the mechanical milling, resulting in changes in dislocation density and formation of sub-structure [25]. The bulk hardness of the as-extruded and solutionized samples suggests that the hardness in solutionized condition is slightly higher than the corresponding extruded sample because of the dissolution of carbides
and enhancement of the solid solution strengthening effect. The measured hardness is found to be higher than reported earlier and the vales are summarized in Table 1. The secondary electron micrographs of the milled powder are shown in Fig. 2. It can be seen from the figure that the as milled powder shows flake type morphology with a bi-model distribution in size. The size of the finest powder particles is around 2 μm whereas the largest particles are around 60 μm in size. The secondary electron image of the as-milled powder (inset to Fig. 2) exhibits a lamellar type flow line structure. This kind of structure evolves during the milling, which is due to repeated fracture and welding of the powder. In the early stages of mechanical milling, the ductile component, that is austenite steel powder, flattens to platelet shape by repeated micro-forging processes. In the next stage, these flattened particles are cold welded together and form a composite lamellar structure [26,27]. With increasing milling time, the composite powder particles get work hardened resulting in increase in hardness and consequently fragment to finer particles with more equiaxed dimensions. On further continuation of milling, dissolution of Yttria, which is trapped between the lamella, begins to occur at this stage. The hardness, and particle size and substructure tend to reach a saturation value at this stage. The crystal structure and phases present in the samples with different processing conditions were studied by X-ray diffraction (XRD). The XRD patterns (Fig. 3) of the blended powder shows the presence of austenitic (fcc) and ferrite (bcc) phase. The bcc phase found in the blended powder matches with ferrite (Pearson crystal data 558498), which is considered to have been inherited from 316L powder. The milled powder also exhibits two peaks corresponding to fcc and bcc phases but the intensity of bcc phases reduced significantly. This reduction is considered to be due to non-equilibrium compositional changes (probably enhancement of Ni content) in the ferrite phase, thus destabilizing the ferrite phase [28]. XRD patterns of as-extruded and solutionized samples revealed the presence only of a single fcc (austenite) phase with a lattice constant of 3.602 ± 0.004 Å. The observed lattice parameter of the as-extruded and solutionized samples is close to the previously reported lattice parameter of ASI316 austenite steel [29]. In order to examine the detailed grain structure from the relatively large area of solutionized austenitic ODS steel, electron back scattered diffracted (EBSD) high angle (15–180° miss-orientation) grain boundary maps have been obtained from the transverse (perpendicular to the extrusion direction) and longitudinal (parallel to the extrusion direction) sections of the steel bar is shown in Fig. 4. It can be observed from the Fig. 4(a) that equiaxed grains are present, whereas Fig. 4(b) shows elongated grains with bimodal distribution. The grain size (diameter) calculated from the EBSD data by measuring the area of each grain, converting the area to that of a circle and taking the diameter of the
Fig. 1. Vickers hardness of the as-milled, as-extruded and solutionized austenitic ODS steel.
Fig. 2. Secondary electron (SE) image of the as-milled austenite ODS powder after milling for 4 h and (inset) higher magnification SE images showing the internal structure of the as-milled powder.
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Fig. 3. XRD patterns of blended, as-milled, extruded and solutionized austenitic ODS steels.
circle as grain size. The area fraction of the grains in the EBSD maps was plotted as a function of grain diameter and is shown in Fig. 4(c & d). The estimated finer grains exhibit log-normal size distribution with mean grain size observed from transverse and longitudinal sectioned steel bar being 280 ± 5 and 440 ± 9 nm, respectively. The observed minor fraction of larger grains in the longitudinal section is in the range of 1 to 5 μm and the similar microstructural observations are already reported by Mao et al. [30]. Transmission electron microscopy studies have been carried out on the as-milled and solutionized austenite ODS steel specimens. The BF images from the different regions of the as-milled powder are shown in Fig. 5(a & b) and the corresponding selected area electron diffraction (SAED) patterns are shown as insets in the BF images. The microstructure observed in one of the regions (Fig. 5a) showed strained patterns, and the corresponding electron diffraction pattern (inset to Fig. 5a) revealed that this region is crystallized with bcc structure. Microstructure observed in the other region confirmed strain-free, fine and elongated grains. The electron diffraction pattern (inset to Fig. 5b) obtained from this region revealed the structure of these grains to be fcc. TEM study of the as-milled powder confirms the presence of two phases as seen in the SAED patterns and the phases identified are ferrite (bcc) and austenite (fcc) (Fig. 5a & b). XRD studies also confirm the presence of bcc
Fig. 5. Transmission electron bright field (BF) micrographs of as-milled powder showing two phase structure (a) ferritic and (b) austenitic phases, and corresponding SAD patterns [inset to BF images]. Dark field (DF) images (c) and (d) obtained from the SAD pattern [inset to (b)] of austenitic region marked as 1 and 2, respectively.
Fig. 4. Electron backscattered diffraction (EBSD) high angle (15–180° miss-orientation) grain boundary maps of (a) transverse, and (b) longitudinal section of solutionized austenite ODS steel bar, (c) and (d) are the plot of the area fraction with grain size, estimated from the (a) and (b), respectively.
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structure along with fcc structure in the as-milled powder. Therefore, TEM and XRD studies are consistent with each other. In addition to the diffracted rings (inset to Fig. 5b), a set of diffraction spots can be seen, which are inside the first diffraction ring. From these diffracted spots, dark field (DF) images of these regions were obtained (Fig. 5c & d), which show fine and coarse particles. Low magnification TEM bright field (BF) micrograph of the solutionized sample in the transverse section of the bar is shown in Fig. 6 from which it can be seen that the as-solutionized sample possesses ultra-fine equiaxed grain structure. It is also observed that a few larger grains are present, which is consistent with EBSD data inferred from the grain maps (Fig. 4). A similar microstructure has been reported in mechanically alloyed, hot isostatic pressed (HIPed) ODS austenitic steel [7,8,19,21]. SAED patterns from the solutionized sample could be indexed with the fcc austenite phase, while the bcc ferritic phase was not observed. In the microstructure, various other features were observed such as twins, coarse and fine particles. Three types of particles were observed within the size range of 45–200 nm (Fig. 7). Energy dispersive spectroscopy (EDS) studies confirmed that some of these particles are silicon oxides. The other oxide particles are Lanthanum-rich Yttrium-Silicon based oxides having the chemical composition 41.6, 4.2 and 54.2 at.% of Y, La and Si, respectively and during the quantification, O is not considered, as the quantifying the same yields error. The crystal structure of the these particles was identified using the SAED pattern obtained from one of those particles with different orientations of [100] and [110], which confirms that these oxide particles are (Y,La)2Si2O7 with monoclinic crystal structure (Fig. 7b). However, occurrence of these particles is rare (b 1% of the observed particles). Kim et al. have also reported sub-micron sized Y2Si2O7 type complex and TiO2 based oxides in 304L grade austenitic ODS steel after milling, HIPing and hot rolling at 1150 °C [4]. The other type of coarse particles with size around 40–50 nm are found be Ti rich carbides (Fig. 8). Crystal structure of this carbide was identified using 〈100〉 and 〈110〉 zone axes of SAED patterns which confirm the face centered cubic structure and a Fm-3m symmetry and a lattice parameter of 4.26 ± 0.03 Å. In order to further study the nano-dispersoids, high magnification bright field TEM images were recorded. From Fig. 9a, finely dispersed nano-oxides can be observed with a size b10 nm. A few oxide particles with a size of about 25 nm can also be seen. For evaluation of the size distribution of nano-oxides, a total 2257 particles were counted from N40 micrographs and the distribution is shown in Fig. 9b. The number fraction with size fitted with log-normal size distribution yields a mean size of 9.1 nm and variance of 0.45. The obtained small oxide dispersoids size may be related to the homogeneous microstructure (uniform yttria segregation) of the as-milled powder by adoption of very high energy Simoloyer Zoz mill which imparts almost 9 times higher
energy to the powder than the other regularly used planetary ball mills [31,32,33]. These nano-oxides were found to be dispersed evenly in the matrix, which are responsible for dispersion strengthening of the austenitic steel. EDS results reveal that the fine particles with size b20 nm consist of Y, Ti and O, whereas the large oxide particles with size larger than 100 nm (shown in Fig. 7) consist of Si along with Y, La and O, and do not contain Ti. Thus, the presence of Ti in ODS steel refines the oxide particle size [34]. Estimated chemical composition of the fine oxides is 53.7 and 46.3 at.% for Y and Ti, respectively.
Fig. 6. TEM BF image of solutionized austenite ODS steel in transverse section of the bar, and (inset) SAD pattern obtained from the same region.
Fig. 8. BF micrograph showing particles with different sizes of austenite ODS steel solutionized at 1150 °C/1 h. Inset shows SAD patterns from the precipitate with [100] and [110] zone axes.
Fig. 7. (a) TEM BF image showing different sizes of oxide particles. Inset shows the SAED pattern obtained from the particle indicated as “c”, and (b) superimposed EDS profiles obtained from locations as-indicated “a”: matrix, “b”: small oxide particle (Y-Ti-O) and “c” big oxide (La rich Y2Si2O7) particle of austenite ODS steel solutionized at 1150 °C/1 h.
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Fig. 9. (a) Higher magnification BF micrograph of solutionized austenitic ODS steel showing fine dispersoids and (b) size distribution plot of the dispersoids.
In order to study the structure of the nano Y-Ti-O complex oxides with a mean size of 10 nm, high resolution TEM images were recorded, while the matrix was oriented to 〈011〉 zone axis as shown in Fig. 10. The high resolution image reveals distinguishable lattice planes of the oxide particles. Further, Fast Fourier transformation (FFT) and inverse FFT techniques were applied to the high resolution lattice image to extract the lattice symmetry and inter-planar distance. The FFT pattern shows a [004] diffraction pattern related to diamond cubic (Fd-3m) pyrochlore Y2Ti2O7 type structure. Inverse FFT obtained from [004] reflections shows an inter-planar distance of 2.52 Å, therefore, the lattice parameter is 10.08 Å, which is close to the lattice parameter of Y2Ti2O7 type pyrochlore structure [35,36]. 4. Discussion The microstructure of the solutionized sample shows a bimodal grain structure (Figs. 4 & 6). A high density of the nano-oxides was observed in the finer grains, while large grains have relatively fewer nanooxides [8]. The presence of nano-oxides inhibited the grain growth which led to the retention of finer grains [37], whereas the absence of oxide particles due to inhomogeneous distribution during milling led to grain growth with a larger grain size of 1–5 μm [5]. The area fraction of large grains is considerably lower than that of fine grains. Large grains should be suppressed in order to obtain a homogeneous microstructure, thereby leading to enhanced mechanical properties. In the present study, microstructural inhomogeneities were present which probably could be eliminated by prolonged milling time. However, the milling duration could not be increased due to processing difficulties due to the sticky nature of powder particles to the milling media, which would also increase contamination levels.
Fig. 10. (a) High resolution image of the Y-Ti-O oxide dispersoids in the austenitic (fcc) matrix along 〈011〉 orientation. Inset is the Fast Fourier transform (FFT) of the whole area exhibiting diffraction spots from dispersoids (400) and matrix. (b) Inverse FFT created using (400) spot of dispersoid. The top inset shows the magnified view of the dispersoid. The intensity profile obtained from across the lattice planes in the bottom inset. The measured inter-planar distance from the profile is 2.56 Å.
In contrast to ferritic/martensitic ODS steels, various precipitates, oxides and dispersoids in austenite ODS steels were found. In the solutionized sample, a small number fraction of coarse La rich (La,Y)2Si2O7 particles, and a large number fraction of very fine Y2Ti2O7 dispersoids were observed. In the present study, the base powder consists of 0.62 wt% Si and during consolidation, some Si might have precipitated with Y, La and O forming monoclinic type (Y,La)2Si2O7 oxide particles. Gibbs free energy (ΔG) calculations suggest that the formation of (Y,La)2Si2O7 phase is favorable due to the lower ΔG of Y2Si2O7 when compared to Y2Ti2O7 and TiO2 [3]. Bulk hardness of the solutionized austenitic ODS steel in excess of 340 HV has been achieved, which is higher than the published data of ODS austenite steels [3,4,21] and 316 austenitic steel (160 HV) [23,29] and comparable to the hardness of the Inconel718 super alloy of 330 ± 10 HV [38]. The obtained higher hardness is attributed to a combination of dispersoids, grain size and solid solution strengthening. Further enhancement of hardness could be achieved with a better control
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of microstructure by the complete suppression of undesirable oxides such as La rich Y2Si2O7. 5. Conclusions Oxide dispersion strengthened (ODS) austenitic steel was developed by designing the composition such that the formation of delta (δ-) ferrite and sigma (σ-) FeCr phases, which are deleterious for prolonged high temperature exposure are suppressed. This steel is made by mechanical alloying followed by hot extrusion and solutionization. X-ray and electron diffraction studies of solutionized ODS austenitic steel confirm the presence of only austenitic phase (fcc), and no other phases (esp. δ-ferrite) were detected. The microstructure of the solutionized sample comprises fine equiaxed grains in transverse section and elongated grains in longitudinal section of the steel bar with mean size of 280 ± 5 and 440 ± 9 nm, respectively. The dispersoids in this steel are mainly of the pyrochlore type Y2Ti2O7 with low fraction of TiC, and La rich Y2Si2O7. The size of the Ti-containing yttrium oxide particles is much finer with a mean size of 9 nm than the Si containing Y-Si-O oxide particles. Bulk hardness of 346 ± 25 HV was achieved in this alloy. Acknowledgements The authors would like to thank Dr. K. Satya Prasad, Mr. B. Manjunath, and Mr. G. V. R. Reddy for their kind support during TEM, SEM and metallographic studies. References [1] P. Marshall, Austenitic Stainless Steels, Elsevier, London–New York, 1984. [2] T. Sourmail, Precipitation in creep resistant austenitic stainless steels, Mater. Sci. Technol. 17 (2001) 1–14. [3] T.K. Kim, C.S. Bae, D.H. Kim, J. Jang, S.H. Kim, C.B. Lee, D. Hahn, Microstructural observation and tensile isotropy of an austenitic ODS steel, Nucl. Eng. Technol. 40 (2008) 305–310. [4] Z. Zhou, S. Yang, W. Chen, L. Liao, Y. Xu, Processing and characterization of a hipped oxide dispersion strengthened austenitic steel, J. Nucl. Mater. 428 (2012) 31–34. [5] H. Oka, M. Watanabe, S. Ohnuki, N. Hashimoto, S. Yamashita, S. Ohtsuka, Effects of milling process and alloying additions on oxide particle dispersion in austenitic stainless steel, J. Nucl. Mater. 447 (2014) 248–253. [6] H. Oka, M. Watanabe, N. Hashimoto, S. Ohnuki, S. Yamashita, S. Ohtsuka, Morphology of oxide particles in ODS austenitic stainless steel, J. Nucl. Mater. 442 (2013) 164–168. [7] Y. Xu, Z. Zhou, M. Li, P. He, Fabrication and characterization of ODS austenitic steels, J. Nucl. Mater. 417 (2011) 283–285. [8] M. Wang, Z. Zhou, H. Sun, H. Hu, S. Li, Microstructural observation and tensile properties of ODS-304 austenitic steel, Mater. Sci. Eng. A 559 (2013) 287–292. [9] P.J. Mazias, Developing an austenitic stainless steel for improved performance in advanced fossil power facilities, JOM J. Miner. Met. Mater. Soc. 41 (1989) 14–20. [10] Y. Yamamoto, M.P. Brady, Z.P. Lu, P.J. Maziasz, C.T. Liu, B.A. Pint, K.L. More, H.M. Meyer, E.A. Payzant, Creep-resistant, Al2O3-forming austenitic stainless steels, Science 5823 (2007) 433–436. [11] B. Weiss, R. Stickler, Phase instabilities during high temperature exposure of 316 austenitic stainless steel, Metall. Trans. 3 (1972) 851–866. [12] D. Peckner, M. Burnstein, Handbook of Stainless Steels, McGraw-Hill, New York, 1977 (ISBN 007049147X).
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