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ODS alloys at high temperatures
Intermetallics 13 (2005) 387–392 www.elsevier.com/locate/intermet
Stability of ferritic MA/ODS alloys at high temperatures M.K. Miller*, D.T. Hoelzer, E.A. Kenik, K.F. Russell Microscopy, Microanalysis, Microstructures Group, Metals and Ceramics Division, Oak Ridge National Laboratory, Building 4500S, MS 6136, P.O. Box 2008, Oak Ridge, TN 37831-6136, USA Available online 2 September 2004
Abstract The ultrafine scale microstructure of three MA/ODS ferritic alloys (12YWT, 14YWT and MA957) have been characterized by atom probe tomography in the as-received condition and after isothermal heat treatments at temperatures of up to 24 h at 1300 8C (w85% Tm). Atom probe tomography revealed the presence of a high number density of 2–4 nm diameter titanium-, yttrium- and oxygen-enriched particles. These ultrafine particles were found to be extremely resistant to coarsening during isothermal aging at 1300 8C. Solute segregation of the alloying elements to dislocations and grain boundaries was also observed. q 2004 Elsevier Ltd. All rights reserved. Keywords: B. Precipitates; C. Mechanical alloying and milling; D. Microstructure
1. Introduction Some mechanically alloyed (MA), oxide dispersion strengthened (ODS) ferritic alloys fabricated from powders of a prealloyed mixture of iron, chromium, yttrium, titanium, and tungsten and yttria have exhibited dramatically improved high temperature creep properties at temperatures of up to 850 8C [1–12]. This temperature is significantly higher than the traditionally accepted operating range of ferritic alloys. Therefore, there appears to be considerable potential for developing ferritic and possibly other alloys with significantly improved high temperature properties for a number of different applications. Atom probe tomography characterizations of a MA/ODS 12YWT alloy with a nominal composition of Fe–12.3 wt% Cr–3% W–0.39% Ti–0.25% Y2O3 [Fe–13.3 at.% Cr, 0.92% W, 0.46% Ti, 0.13% Y and 0.19% O] revealed the presence of high number densities of 2–4 nm diameter titanium-, yttrium- and oxygen-enriched particles in both the asprocessed and crept conditions [7–12]. These ultrafine particles were found to be present in the ferrite after high temperature creep for up to 14,500 h at 800 8C (O50% Tm). In this study, the microstructures of three MA/ODS ferritic * Corresponding author. Tel.: C1-865-574-4719; fax: C1-865-2413650. E-mail address: [email protected] (M.K. Miller). 0966-9795/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2004.07.036
alloys (designated 12YWT and 14YWT and a commercial MA957 alloy) have been characterized in the as-received condition and after isothermal heat treatments at temperatures up to 1300 8C (w85% Tm). The aim of this study was to investigate the stability of these titanium-, yttrium- and oxygen-enriched particles during high temperature annealing. Previous studies indicated that tungsten had an important role in the improved mechanical properties [6–12]. The tungsten-free commercial MA957 alloy was included in this study to ascertain the importance of tungsten or whether other alloying elements such as molybdenum had similar beneficial effects. Atom probe tomography was used to characterize the size, composition and number density of the titanium-, yttrium- and oxygen-enriched particles and the matrix composition [13]. Transmission and scanning electron microscopy were also used to study the dislocation structure and to determine whether recrystallization had occurred during the high temperature annealing treatment.
2. Experimental Three MA/ODS alloys, 12YWT, 14YWT and a commercial MA957 alloy, were characterized in this study. The 12YWT alloy had a nominal composition of Fe–12.3 wt% Cr–3% W–0.39% Ti–0.25% Y 2O 3
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[Fe–13.3 at.% Cr, 0.92% W, 0.46% Ti, 0.13% Y and 0.19% O]. The 14YWT alloy that was developed at Oak Ridge National Laboratory (ORNL) had a composition of Fe–14.2 wt% Cr–1.95% W–0.22% Ti–0.25% Y 2O 3 [Fe–15.0 at.% Cr, 0.6% W, 0.26% Ti, 0.13% Y and 0.19% O]. The commercial MA957 alloy had a nominal composition of Fe–14 wt% Cr, 0.9% Ti, 0.3% Mo, 0.1% Al and 0.25% Y2O3 [Fe–14.8 at. % Cr, 0.17% Mo, 1.0% Ti, 0.2% Al, 0.13% Y and 0.19% O] and contained trace levels of Mn, Si, B and C. The 12YWT alloy was reportedly prepared by milling 70-mm-diameter pre-alloyed metal and 20-nm-diameter Y2O3 powders in a high-energy attritor mill for 48 h under an argon atmosphere. The mechanically alloyed flakes were then degassed for 2 h at 400 8C in vacuum at a pressure of !2!10K2 Pa, canned in mild steel and then directly consolidated into bar by hot extrusion at 1150 8C. The alloys were hot-rolled at 1150 8C into 7 mm sheet, warm-rolled at 600 8C to 2.7 mm sheet and then annealed for 1 h at 1050 8C in vacuum. The 14YWT was prepared by milling 50–150-mm-diameter pre-alloyed metal and 17–31-nm-diameter Y2O3 powders in a high-energy attritor mill for 80 h under an argon atmosphere. The resulting mechanically alloyed flakes were canned in mild steel and extruded into a 25-mm-diameter rod at 850 8C. The commercial MA957 MA/ODS alloy was fabricated into an extruded tube by a proprietary process. The 12YWT and MA957 alloys were characterized in the as-received
state and after annealing for times up to 24 h at 1300 8C. The 14YWT alloy was characterized only in the asextruded state. Atom probe tomography characterizations were performed in the ORNL energy-compensated optical positionsensitive atom probe (ECOPOSAP) and the ORNL local electrode atom probe (LEAP). The experiments were performed with a specimen temperature of 50–60 K, pulse repetition rates of 1.5 kHz in the ECOPOSAP and 100 kHz in the LEAP, and a pulse fraction of 20% of the standing voltage. All compositions quoted in this paper are expressed in atomic percent. The Guinier radius, rG, and the composition of each particle were determined from the positions of the solute atoms associated with the particles with the use of the maximum separation method [13]. Particles containing less than 10 atoms were not included in the analyses.
3. Results and discussion The general microstructures of the 12YWT and MA957 alloys in the as-received condition and after isothermal aging at 1300 8C are shown in the optical micrographs in Fig. 1. These micrographs revealed a heavily deformed microstructure and some coarse particles in the 12YWT material. Transmission electron microscopy of this material revealed a high dislocation density. Some grain
Fig. 1. Optical micrographs showing the general microstructures of the 12YWT alloy in (a) the as-received condition and (b) after annealing for 1 h at 1300 8C and the MA957 alloy in (c) the as-received condition and after annealing for (d) 1 h and (e) 24 h at 1300 8C.
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Fig. 2. TEM micrograph and energy-dispersive spectroscopy spectra of Ti-, Y- and O-enriched particles along a grain boundary in the as-received MA957 alloy.
growth had occurred during annealing for 1 h at 1300 8C. The MA957 alloy exhibited a uniform microstructure of elongated grains, some cavities and a low number density of micron-sized particles. Transmission electron microscopy (TEM) revealed that these particles were the Al2O3 phase. Formation and coarsening of the cavities occurred during the annealing at 1300 8C. No significant grain growth was observed in the MA957 alloy. TEM also revealed Ti-, Y- and O- enriched particles along the grain boundary, as shown in Fig. 2. Some smaller (!0.4 mm) TiO particles with little or no yttrium were also observed in the interior of the grains by electron microscopy. TEM revealed partial recovery of the dislocation structure but no recrystallization had occurred during the high temperature anneal. The solute distribution in the 14YWT alloy immediately after the extrusion at 850 8C is shown in the atom maps in Figs. 3 and 4. Chromium, tungsten, carbon, and oxygen segregation to a grain boundary are evident in the atom maps shown in Fig. 3. Enrichments of up to 20 at.% Cr and up to 2.7 at.% W were measured at this grain boundary. A high number density of ultrafine Ti-, Y- and O-enriched particles is also evident in the atom maps shown in Fig. 4. This observation suggests that these particles formed during the extrusion process. Although preliminary experiments have been unable to detect these particles in the milled powders, the possibility that these particles formed during the milling process cannot be excluded. The distributions of tungsten appeared to be randomly distributed in the interior of the grains. Some other atom maps of the intragranular regions showed no evidence of the ultrafine particles. This behavior may be due to sampling of the fine- and coarse-grained regions evident in the optical micrographs. The solute distribution in the 12YWT alloy after isothermally annealing for 10 h at 1300 8C is shown in the atom maps in Fig. 5. A high number density (w2!1024 mK3) of ultrafine Ti-, Y- and O-enriched
particles is evident. The compositions of the individual particles were determined by the envelope method [2] with a maximum separation distance of 0.7 nm for yttrium, titanium and oxygen and a grid spacing of 0.1 nm. The average compositions and the solute partitioning factors of the solute between the particles and the matrix are given in Table 1. The results indicate that the particles are enriched in yttrium, titanium and oxygen and depleted in tungsten and chromium and have a stoichiometry close to (TiCY):O. Low levels of iron and chromium were also detected in the particles. The oxygen content in the matrix was estimated to be !0.13, !0.15 and !0.13 at.% O, respectively for the as-received, 1 and 10 h at 1300 8C annealed conditions, respectively. The solute distribution in the MA957 alloy is shown in the atom maps in Fig. 6 for the as-received condition and after isothermally annealing for 24 h at 1300 8C. A high number density (w2!1024 mK3) of ultrafine Ti-, Y- and O-enriched particles is evident in the as-received condition. The yttrium atoms were predominantly found in the central region of the particle and were surrounded with a Ti- and O-enriched shell. The distribution of molybdenum appeared to be randomly distributed in all conditions. The number density of the particles decreased by an order of magnitude to w2!1023 mK3 after annealing for 1 h at 1300 8C and further decreased to w8!1022 mK3 after annealing for 24 h at 1300 8C. The average Guinier radius of the particles, rG, was determined from the positions of the yttrium, titanium and oxygen atoms within the particle with the use of the maximum separation method [2] to be 1.2G0.4, 1.7G 0.7 nm, and 4.6G1.1 nm for the as-received, 1 and 24 h at 1300 8C conditions, respectively. In all conditions, the rG(Y) was approximately 90% of the overall rG value. These results indicate that some coarsening of the particles had occurred during the annealing treatments at 1300 8C. The compositions of the individual particles were determined by the envelope method [2] with a grid spacing of 0.1 nm. The average compositions and the solute partitioning factors are
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Fig. 3. Atom maps showing chromium, tungsten, carbon and oxygen segregation to a grain boundary in the 14YWT alloy after extrusion at 850 8C. The atom map has been oriented so that the plane of the grain boundary is into the page.
given in Table 2. The results indicate that the particles are enriched in yttrium, titanium and oxygen and depleted in molybdenum and chromium. The oxygen content in the matrix was estimated to be !0.14, and !0.19 at.% O, respectively, for the as-received and 1 h at 1300 8C annealed conditions. These estimates are upper bounds due to
Fig. 4. Atom maps showing titanium-, oxygen- and yttrium-enriched particles in the 14YWT alloy after extrusion at 850 8C. The lines are the three-dimensional box bounding the data.
the possible presence of Mo3C and Ti3C ions superimposing with the TiO2C and OC ions, respectively. The milling process introduced strain and a large number of dislocations into the lattice. A high dislocation density was observed in all alloys by TEM even after the high temperature annealing treatment indicating that these alloys are far from equilibrium. Solute segregation to
Fig. 5. Atom maps showing titanium-, oxygen- and yttrium-enriched particles in the 12YWT alloy after 10 h at 1300 8C.
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Table 1 and partitioning factors (PF) of the particles in the 12YWT alloy Compositions ðXÞ at.%
Fe Cr W Ti Y O
As received
1 h 1300 8C
X
PF
X
10 h 1300 8C PF
X
PF
4.1G4.1 0.8G0.8 0.13G0.13 42.1G5.6 8.1G5.2 44.4G8.2
0.05 0.07 0.17 435 77 325
4.2G4.2 1.6G1.6 0.02G0.02 43.1G6.9 5.2G5.2 45.8G6.9
0.05 0.14 0.02 334 38 295
7.6G7.6 1.3G1.3 0.04G0.04 36.8G8.1 8.6G4.4 45.5G10.2
0.09 0.10 0.04 382 75 360
The balance is iron.
the dislocations has been previously observed by atom probe tomography in the 12YWT alloy and may pin the dislocations [11]. The atom probe tomography results indicate the ultrafine Ti-, Y- and O-enriched particles are present in all three alloys at all stages after the initial processing. The titanium and yttrium contents of these particles indicate that they are not remnants of the original yttria powder. These results suggest that the yttria particles are completely broken up during the mechanical alloying process and the yttrium and oxygen are put into solution in the ferrite despite the thermodynamic stability of yttria.
The particles are surprisingly stable and resistant to coarsening after long term high temperature heat treatment. However, a limited amount of coarsening was observed after 24 h at 1300 8C. In order for the particles to coarsen, all the solute elements associated with the particles (i.e. the yttrium, titanium and oxygen) have to dissolve from the surface of the smaller particles, diffuse in the ferrite matrix and to deposit on the surface of the larger particles. Titanium and yttrium atoms are significantly larger than iron atoms and therefore, may diffuse slowly in an iron lattice. However, this large misfit may be alleviated by solute-vacancy interactions and it is possible that a titanium-vacancy and a yttrium-vacancy may diffuse significantly more rapidly than isolated solute atoms. The oxygen content of the ferrite was found to be significantly higher than the parts per million expected from previous estimates of the solubility [14]. In all three alloys, the majority of the titanium and oxygen both in the particles and the matrix were detected as TiO2C ions. In addition, some CrO, WO and YO ions were also detected. The presence of these molecular ions indicates a strong affinity of solute atoms for oxygen. This affinity is expected due to the large heats of formation of all the oxide phases. This strong solute-oxygen affinity may retard or even inhibit the diffusion process because, in order for the oxygen to diffuse the solute-oxygen bond has either to be broken, or the soluteoxygen complex has to diffuse as an entity. Both these processes are not energetically favorable. In addition, the strong solute-oxygen affinity may be the reason for the high oxygen content in the ferrite. The solute distribution in the intragranular regions and the formation of the ultrafine particles in the three alloys are Table 2 and partitioning factors (PF) of the particles in the Compositions ðXÞ as-received and annealed conditions of the commercial MA957 alloy at.%
Cr Mo Ti Y O Fig. 6. Atom maps showing titanium-, oxygen- and yttrium-enriched particles in the MA957 alloy after (a) 1 and (b) 24 h at 1300 8C.
1 h at 1300 8C
As received X
PF
X
PF
1.7G1.7 0.02G0.02 32.9G5.3 15.4G7.3 39.9G6.9
0.1 0.1 41 91 285
4.2G4.2 0.03G0.03 21.4G7.9 8.9G5.8 47.8G23.3
0.3 0.2 26 52 294
The balance is iron.
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similar indicating that the molybdenum or tungsten additions do not significantly change the intragranular precipitation in these MA/ODS ferritic alloys. However, the particles had coarsened slightly more in the MA957 alloy as compared to the 12YWT and therefore, molybdenum may not be as effective as tungsten in retarding the coarsening process. This trend is consistent with the free energies of formation of the molybdenum and tungsten oxides which suggests that the tungsten-oxygen interaction is stronger than the molybdenum-oxygen interaction.
4. Conclusions Ultrafine Ti-, Y- and O-enriched particles were found to be extremely resistant to coarsening during isothermal aging at 1300 8C. Solute segregation of the chromium and tungsten to grain boundaries was observed in as-extruded 14YWT. After annealing the 12YWT and MA957 alloys at 1300 8C, partial recovery of the dislocation structure was observed. However, no significant grain growth was evident. The ultrafine particles and the solute segregation to the dislocations appear to act as pinning sites for the dislocations and their stability may explain the improved high temperature mechanical properties.
Acknowledgements Research at the Oak Ridge National Laboratory SHaRE Collaborative Research Center was sponsored by
the Division of Materials Sciences and Engineering and the Office of Nuclear Energy, Science and Technology (I-NERI 2001-007-F), US Department of Energy, under contract DE-AC05-00OR22725 with UT-Battelle, LLC.
References [1] Smith GD, deBarbadillo JJ. In: deBarbadillo JJ et al, editor. Structural Applications of Mechanical Alloying. Materials Park, OH: ASMInternational; 1994. p. 117–23. [2] Ukai S, Harada M, Okada H, Inoue M, Nomura S, Shikakura S, Asabe K, Nishida T, Fujiwara M. J Nucl Mater 1993;204:65. [3] Ukai S, Harada M, Okada H, Inoue M, Nomura S, Shikakura S, Nishida T, Fujiwara M. J Nucl Mater 1993;204:74. [4] Ukai S, Nishida T, Okada H, Okuda T, Fujiwara M, Asabe K. J Nucl Sci Tech 1997;34:256. [5] Ukai S, Yoshitake T, Mizuta S, Matsudaira Y, Hagi S, Kobayashi T. J Nucl Sci Tech 1999;36:710. [6] Kim I-S, Okuda T, Kang C-Y, Sung J-H, Maziasz PJ, Klueh RL, Miyahara K. Met Mater 2000;6:513. [7] Larson DJ, Maziasz PJ, Kim I-S, Miyahara K. Scripta Mater 2001; 44:359. [8] Kim I-S, Hunn JD, Hashimoto N, Larson DJ, Maziasz PJ, Miyahara K, Lee EH. J Nucl Mater 2000;280:264. [9] Kenik EA, Hoelzer DT, Maziasz PJ, Miller MK. Microsc Microanal 2001;7:550. [10] Miller MK, Kenik EA. Microsc Microanal 2002;8(Suppl 2):1126CD. [11] Miller MK, Kenik EA, Russell KF, Heatherly L, Hoelzer DT, Maziasz PJ. Mat Sci Eng A 2003;353:140–5. [12] Miller MK, Hoelzer DT, Kenik EA, Russell KF. Microsc Microanal 2003;9(Suppl 2):44–5. [13] Miller MK. Atom Probe Tomography. Dordrecht/New York: Kluwer Academic/Plenum; 2000. [14] Swisher JH, Turkdogan ET. Trans Met Soc AIME 1967;239:426.
Stability of ferritic MA/ODS alloys at high temperatures M.K. Miller*, D.T. Hoelzer, E.A. Kenik, K.F. Russell Microscopy, Microanalysis, Microstructures Group, Metals and Ceramics Division, Oak Ridge National Laboratory, Building 4500S, MS 6136, P.O. Box 2008, Oak Ridge, TN 37831-6136, USA Available online 2 September 2004
Abstract The ultrafine scale microstructure of three MA/ODS ferritic alloys (12YWT, 14YWT and MA957) have been characterized by atom probe tomography in the as-received condition and after isothermal heat treatments at temperatures of up to 24 h at 1300 8C (w85% Tm). Atom probe tomography revealed the presence of a high number density of 2–4 nm diameter titanium-, yttrium- and oxygen-enriched particles. These ultrafine particles were found to be extremely resistant to coarsening during isothermal aging at 1300 8C. Solute segregation of the alloying elements to dislocations and grain boundaries was also observed. q 2004 Elsevier Ltd. All rights reserved. Keywords: B. Precipitates; C. Mechanical alloying and milling; D. Microstructure
1. Introduction Some mechanically alloyed (MA), oxide dispersion strengthened (ODS) ferritic alloys fabricated from powders of a prealloyed mixture of iron, chromium, yttrium, titanium, and tungsten and yttria have exhibited dramatically improved high temperature creep properties at temperatures of up to 850 8C [1–12]. This temperature is significantly higher than the traditionally accepted operating range of ferritic alloys. Therefore, there appears to be considerable potential for developing ferritic and possibly other alloys with significantly improved high temperature properties for a number of different applications. Atom probe tomography characterizations of a MA/ODS 12YWT alloy with a nominal composition of Fe–12.3 wt% Cr–3% W–0.39% Ti–0.25% Y2O3 [Fe–13.3 at.% Cr, 0.92% W, 0.46% Ti, 0.13% Y and 0.19% O] revealed the presence of high number densities of 2–4 nm diameter titanium-, yttrium- and oxygen-enriched particles in both the asprocessed and crept conditions [7–12]. These ultrafine particles were found to be present in the ferrite after high temperature creep for up to 14,500 h at 800 8C (O50% Tm). In this study, the microstructures of three MA/ODS ferritic * Corresponding author. Tel.: C1-865-574-4719; fax: C1-865-2413650. E-mail address: [email protected] (M.K. Miller). 0966-9795/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2004.07.036
alloys (designated 12YWT and 14YWT and a commercial MA957 alloy) have been characterized in the as-received condition and after isothermal heat treatments at temperatures up to 1300 8C (w85% Tm). The aim of this study was to investigate the stability of these titanium-, yttrium- and oxygen-enriched particles during high temperature annealing. Previous studies indicated that tungsten had an important role in the improved mechanical properties [6–12]. The tungsten-free commercial MA957 alloy was included in this study to ascertain the importance of tungsten or whether other alloying elements such as molybdenum had similar beneficial effects. Atom probe tomography was used to characterize the size, composition and number density of the titanium-, yttrium- and oxygen-enriched particles and the matrix composition [13]. Transmission and scanning electron microscopy were also used to study the dislocation structure and to determine whether recrystallization had occurred during the high temperature annealing treatment.
2. Experimental Three MA/ODS alloys, 12YWT, 14YWT and a commercial MA957 alloy, were characterized in this study. The 12YWT alloy had a nominal composition of Fe–12.3 wt% Cr–3% W–0.39% Ti–0.25% Y 2O 3
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[Fe–13.3 at.% Cr, 0.92% W, 0.46% Ti, 0.13% Y and 0.19% O]. The 14YWT alloy that was developed at Oak Ridge National Laboratory (ORNL) had a composition of Fe–14.2 wt% Cr–1.95% W–0.22% Ti–0.25% Y 2O 3 [Fe–15.0 at.% Cr, 0.6% W, 0.26% Ti, 0.13% Y and 0.19% O]. The commercial MA957 alloy had a nominal composition of Fe–14 wt% Cr, 0.9% Ti, 0.3% Mo, 0.1% Al and 0.25% Y2O3 [Fe–14.8 at. % Cr, 0.17% Mo, 1.0% Ti, 0.2% Al, 0.13% Y and 0.19% O] and contained trace levels of Mn, Si, B and C. The 12YWT alloy was reportedly prepared by milling 70-mm-diameter pre-alloyed metal and 20-nm-diameter Y2O3 powders in a high-energy attritor mill for 48 h under an argon atmosphere. The mechanically alloyed flakes were then degassed for 2 h at 400 8C in vacuum at a pressure of !2!10K2 Pa, canned in mild steel and then directly consolidated into bar by hot extrusion at 1150 8C. The alloys were hot-rolled at 1150 8C into 7 mm sheet, warm-rolled at 600 8C to 2.7 mm sheet and then annealed for 1 h at 1050 8C in vacuum. The 14YWT was prepared by milling 50–150-mm-diameter pre-alloyed metal and 17–31-nm-diameter Y2O3 powders in a high-energy attritor mill for 80 h under an argon atmosphere. The resulting mechanically alloyed flakes were canned in mild steel and extruded into a 25-mm-diameter rod at 850 8C. The commercial MA957 MA/ODS alloy was fabricated into an extruded tube by a proprietary process. The 12YWT and MA957 alloys were characterized in the as-received
state and after annealing for times up to 24 h at 1300 8C. The 14YWT alloy was characterized only in the asextruded state. Atom probe tomography characterizations were performed in the ORNL energy-compensated optical positionsensitive atom probe (ECOPOSAP) and the ORNL local electrode atom probe (LEAP). The experiments were performed with a specimen temperature of 50–60 K, pulse repetition rates of 1.5 kHz in the ECOPOSAP and 100 kHz in the LEAP, and a pulse fraction of 20% of the standing voltage. All compositions quoted in this paper are expressed in atomic percent. The Guinier radius, rG, and the composition of each particle were determined from the positions of the solute atoms associated with the particles with the use of the maximum separation method [13]. Particles containing less than 10 atoms were not included in the analyses.
3. Results and discussion The general microstructures of the 12YWT and MA957 alloys in the as-received condition and after isothermal aging at 1300 8C are shown in the optical micrographs in Fig. 1. These micrographs revealed a heavily deformed microstructure and some coarse particles in the 12YWT material. Transmission electron microscopy of this material revealed a high dislocation density. Some grain
Fig. 1. Optical micrographs showing the general microstructures of the 12YWT alloy in (a) the as-received condition and (b) after annealing for 1 h at 1300 8C and the MA957 alloy in (c) the as-received condition and after annealing for (d) 1 h and (e) 24 h at 1300 8C.
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Fig. 2. TEM micrograph and energy-dispersive spectroscopy spectra of Ti-, Y- and O-enriched particles along a grain boundary in the as-received MA957 alloy.
growth had occurred during annealing for 1 h at 1300 8C. The MA957 alloy exhibited a uniform microstructure of elongated grains, some cavities and a low number density of micron-sized particles. Transmission electron microscopy (TEM) revealed that these particles were the Al2O3 phase. Formation and coarsening of the cavities occurred during the annealing at 1300 8C. No significant grain growth was observed in the MA957 alloy. TEM also revealed Ti-, Y- and O- enriched particles along the grain boundary, as shown in Fig. 2. Some smaller (!0.4 mm) TiO particles with little or no yttrium were also observed in the interior of the grains by electron microscopy. TEM revealed partial recovery of the dislocation structure but no recrystallization had occurred during the high temperature anneal. The solute distribution in the 14YWT alloy immediately after the extrusion at 850 8C is shown in the atom maps in Figs. 3 and 4. Chromium, tungsten, carbon, and oxygen segregation to a grain boundary are evident in the atom maps shown in Fig. 3. Enrichments of up to 20 at.% Cr and up to 2.7 at.% W were measured at this grain boundary. A high number density of ultrafine Ti-, Y- and O-enriched particles is also evident in the atom maps shown in Fig. 4. This observation suggests that these particles formed during the extrusion process. Although preliminary experiments have been unable to detect these particles in the milled powders, the possibility that these particles formed during the milling process cannot be excluded. The distributions of tungsten appeared to be randomly distributed in the interior of the grains. Some other atom maps of the intragranular regions showed no evidence of the ultrafine particles. This behavior may be due to sampling of the fine- and coarse-grained regions evident in the optical micrographs. The solute distribution in the 12YWT alloy after isothermally annealing for 10 h at 1300 8C is shown in the atom maps in Fig. 5. A high number density (w2!1024 mK3) of ultrafine Ti-, Y- and O-enriched
particles is evident. The compositions of the individual particles were determined by the envelope method [2] with a maximum separation distance of 0.7 nm for yttrium, titanium and oxygen and a grid spacing of 0.1 nm. The average compositions and the solute partitioning factors of the solute between the particles and the matrix are given in Table 1. The results indicate that the particles are enriched in yttrium, titanium and oxygen and depleted in tungsten and chromium and have a stoichiometry close to (TiCY):O. Low levels of iron and chromium were also detected in the particles. The oxygen content in the matrix was estimated to be !0.13, !0.15 and !0.13 at.% O, respectively for the as-received, 1 and 10 h at 1300 8C annealed conditions, respectively. The solute distribution in the MA957 alloy is shown in the atom maps in Fig. 6 for the as-received condition and after isothermally annealing for 24 h at 1300 8C. A high number density (w2!1024 mK3) of ultrafine Ti-, Y- and O-enriched particles is evident in the as-received condition. The yttrium atoms were predominantly found in the central region of the particle and were surrounded with a Ti- and O-enriched shell. The distribution of molybdenum appeared to be randomly distributed in all conditions. The number density of the particles decreased by an order of magnitude to w2!1023 mK3 after annealing for 1 h at 1300 8C and further decreased to w8!1022 mK3 after annealing for 24 h at 1300 8C. The average Guinier radius of the particles, rG, was determined from the positions of the yttrium, titanium and oxygen atoms within the particle with the use of the maximum separation method [2] to be 1.2G0.4, 1.7G 0.7 nm, and 4.6G1.1 nm for the as-received, 1 and 24 h at 1300 8C conditions, respectively. In all conditions, the rG(Y) was approximately 90% of the overall rG value. These results indicate that some coarsening of the particles had occurred during the annealing treatments at 1300 8C. The compositions of the individual particles were determined by the envelope method [2] with a grid spacing of 0.1 nm. The average compositions and the solute partitioning factors are
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Fig. 3. Atom maps showing chromium, tungsten, carbon and oxygen segregation to a grain boundary in the 14YWT alloy after extrusion at 850 8C. The atom map has been oriented so that the plane of the grain boundary is into the page.
given in Table 2. The results indicate that the particles are enriched in yttrium, titanium and oxygen and depleted in molybdenum and chromium. The oxygen content in the matrix was estimated to be !0.14, and !0.19 at.% O, respectively, for the as-received and 1 h at 1300 8C annealed conditions. These estimates are upper bounds due to
Fig. 4. Atom maps showing titanium-, oxygen- and yttrium-enriched particles in the 14YWT alloy after extrusion at 850 8C. The lines are the three-dimensional box bounding the data.
the possible presence of Mo3C and Ti3C ions superimposing with the TiO2C and OC ions, respectively. The milling process introduced strain and a large number of dislocations into the lattice. A high dislocation density was observed in all alloys by TEM even after the high temperature annealing treatment indicating that these alloys are far from equilibrium. Solute segregation to
Fig. 5. Atom maps showing titanium-, oxygen- and yttrium-enriched particles in the 12YWT alloy after 10 h at 1300 8C.
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Table 1 and partitioning factors (PF) of the particles in the 12YWT alloy Compositions ðXÞ at.%
Fe Cr W Ti Y O
As received
1 h 1300 8C
X
PF
X
10 h 1300 8C PF
X
PF
4.1G4.1 0.8G0.8 0.13G0.13 42.1G5.6 8.1G5.2 44.4G8.2
0.05 0.07 0.17 435 77 325
4.2G4.2 1.6G1.6 0.02G0.02 43.1G6.9 5.2G5.2 45.8G6.9
0.05 0.14 0.02 334 38 295
7.6G7.6 1.3G1.3 0.04G0.04 36.8G8.1 8.6G4.4 45.5G10.2
0.09 0.10 0.04 382 75 360
The balance is iron.
the dislocations has been previously observed by atom probe tomography in the 12YWT alloy and may pin the dislocations [11]. The atom probe tomography results indicate the ultrafine Ti-, Y- and O-enriched particles are present in all three alloys at all stages after the initial processing. The titanium and yttrium contents of these particles indicate that they are not remnants of the original yttria powder. These results suggest that the yttria particles are completely broken up during the mechanical alloying process and the yttrium and oxygen are put into solution in the ferrite despite the thermodynamic stability of yttria.
The particles are surprisingly stable and resistant to coarsening after long term high temperature heat treatment. However, a limited amount of coarsening was observed after 24 h at 1300 8C. In order for the particles to coarsen, all the solute elements associated with the particles (i.e. the yttrium, titanium and oxygen) have to dissolve from the surface of the smaller particles, diffuse in the ferrite matrix and to deposit on the surface of the larger particles. Titanium and yttrium atoms are significantly larger than iron atoms and therefore, may diffuse slowly in an iron lattice. However, this large misfit may be alleviated by solute-vacancy interactions and it is possible that a titanium-vacancy and a yttrium-vacancy may diffuse significantly more rapidly than isolated solute atoms. The oxygen content of the ferrite was found to be significantly higher than the parts per million expected from previous estimates of the solubility [14]. In all three alloys, the majority of the titanium and oxygen both in the particles and the matrix were detected as TiO2C ions. In addition, some CrO, WO and YO ions were also detected. The presence of these molecular ions indicates a strong affinity of solute atoms for oxygen. This affinity is expected due to the large heats of formation of all the oxide phases. This strong solute-oxygen affinity may retard or even inhibit the diffusion process because, in order for the oxygen to diffuse the solute-oxygen bond has either to be broken, or the soluteoxygen complex has to diffuse as an entity. Both these processes are not energetically favorable. In addition, the strong solute-oxygen affinity may be the reason for the high oxygen content in the ferrite. The solute distribution in the intragranular regions and the formation of the ultrafine particles in the three alloys are Table 2 and partitioning factors (PF) of the particles in the Compositions ðXÞ as-received and annealed conditions of the commercial MA957 alloy at.%
Cr Mo Ti Y O Fig. 6. Atom maps showing titanium-, oxygen- and yttrium-enriched particles in the MA957 alloy after (a) 1 and (b) 24 h at 1300 8C.
1 h at 1300 8C
As received X
PF
X
PF
1.7G1.7 0.02G0.02 32.9G5.3 15.4G7.3 39.9G6.9
0.1 0.1 41 91 285
4.2G4.2 0.03G0.03 21.4G7.9 8.9G5.8 47.8G23.3
0.3 0.2 26 52 294
The balance is iron.
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similar indicating that the molybdenum or tungsten additions do not significantly change the intragranular precipitation in these MA/ODS ferritic alloys. However, the particles had coarsened slightly more in the MA957 alloy as compared to the 12YWT and therefore, molybdenum may not be as effective as tungsten in retarding the coarsening process. This trend is consistent with the free energies of formation of the molybdenum and tungsten oxides which suggests that the tungsten-oxygen interaction is stronger than the molybdenum-oxygen interaction.
4. Conclusions Ultrafine Ti-, Y- and O-enriched particles were found to be extremely resistant to coarsening during isothermal aging at 1300 8C. Solute segregation of the chromium and tungsten to grain boundaries was observed in as-extruded 14YWT. After annealing the 12YWT and MA957 alloys at 1300 8C, partial recovery of the dislocation structure was observed. However, no significant grain growth was evident. The ultrafine particles and the solute segregation to the dislocations appear to act as pinning sites for the dislocations and their stability may explain the improved high temperature mechanical properties.
Acknowledgements Research at the Oak Ridge National Laboratory SHaRE Collaborative Research Center was sponsored by
the Division of Materials Sciences and Engineering and the Office of Nuclear Energy, Science and Technology (I-NERI 2001-007-F), US Department of Energy, under contract DE-AC05-00OR22725 with UT-Battelle, LLC.
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