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Rapid synthesis of oxide dispersion strengthened ferritic alloys through Spark plasma sintering
Materials Today: Proceedings xxx (xxxx) xxx
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
Rapid synthesis of oxide dispersion strengthened ferritic alloys through Spark plasma sintering B.V. Ponraj ⇑, A. Dinesh Kumar, S. Kumaran Department of Metallurgical and Materials Engineering, National Institute of Technology Tiruchirappalli, Tamilnadu 620015, India
a r t i c l e
i n f o
Article history: Received 5 November 2019 Accepted 7 November 2019 Available online xxxx Keywords: Oxide dispersion strengthening Nanostructured ferritic alloy Mechanical alloying Spark plasma sintering Yttria
a b s t r a c t Oxide dispersion strengthened (ODS) ferritic alloys of composition Fe-16Cr-2Al-2W-0.3Ti with varying additions of Y2O3 (0, 0.25, 0.5, 0.75 wt%) were fabricated by mechanical alloying (MA) and Spark plasma sintering (SPS) technique swiftly. The influence of SPS temperature in phase evolution and densification kinetics of ODS ferritic alloy was studied at two different temperatures of 900 °C and 950 °C with 5 min soaking duration at the imposed pressure of 50 MPa. X-ray diffraction (XRD) study was carried out on milled powders and sintered bulk samples. XRD confirmed that alloying has taken place and revealed the nano-crystalline nature. Additionally, crystallite size and micro strain were also evaluated. Uniaxial compression test was performed on sintered bulk samples to understand the mechanical properties of these ODS ferritic alloys. Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International conference on Materials and Manufacturing Methods.
1. Introduction By the year 2030, Generation IV nuclear fission reactors would come into existence. These reactors operate at a temperature upward of 700 °C [1]. Current generation materials could not operate in such harsh conditions. A new breed of cladding materials is required to meet the stringent requirements of the 4th generation nuclear reactors. Development of cladding materials is a vital issue to accomplish higher burn-up operation of Generation IV systems such as lead bismuth-cooled fast reactor (LFR), sodium-cooled fast reactor (SFR), supercritical pressurized water reactor (SCPWR), and so on [2]. The aspiring cladding materials should have an adequate resistance to embrittlement caused by neutron irradiation and void swelling. Additionally, even at elevated temperatures they should possess sufficient mechanical properties [3]. Besides, claddings should exhibit optimum corrosion resistance for pragmatic long term application of state-of-the-art fission nuclear systems [4]. Similar to the Generation IV nuclear systems, exceptional performance is mandatory for fusion blanket materials. Furthermore, fusion systems require stringent materials for their inherent predicaments pertaining to helium/hydrogen transmutation issues and also require low activation grade materials [5]. ⇑ Corresponding author. E-mail address: [email protected] (B.V. Ponraj).
Ferritic/martensitic steels enhanced by oxide dispersion strengthening (ODS) comprising of 9–12% Cr (by weight) are proposed to be the cladding component for uranium fuel rods in sodium environment, thanks to their superficial resistance to neutron irradiation embrittlement and superior creep strength at elevated temperatures [6]. Nevertheless, the aforementioned alloys are unsuitable for supercritical water environment due to inadequate corrosion resistance of these steels. It is well documented that corrosion resistance of ferrous alloys is predominantly determined by Cr and Al content [2]. For every distinct blanket system, it is anticipated to have appropriate synergistic levels of Cr and Al combination, as required by the system. The Cr level has to be determined so as to maintain a balance between the positive trait of offering corrosion resistance and the negative trait of aging embrittlement while upholding strength at sufficiently high temperatures. Ferritic ODS steels with less than 16% or Cr was prone to corrosion under service conditions. Ferritic ODS steels with more than 19% Cr were susceptible to ageing embrittlement [7]. Nano-sized oxide dispersion is the desired technology for enhancing the mechanical properties of Al-added steels.
2. Materials and methods Elemental powders of iron (Fe), chromium (Cr), aluminium (Al), tungsten (W), titanium (Ti) with varying proportions of nano
https://doi.org/10.1016/j.matpr.2019.11.094 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International conference on Materials and Manufacturing Methods.
Please cite this article as: B. V. Ponraj, A. Dinesh Kumar and S. Kumaran, Rapid synthesis of oxide dispersion strengthened ferritic alloys through Spark plasma sintering, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.094
2
B.V. Ponraj et al. / Materials Today: Proceedings xxx (xxxx) xxx
Table 1 Chemical composition (in weight %). Sample ID
Fe
Cr
Al
W
Ti
Y2O3
NFA-0Y NFA-0.25Y NFA-0.50Y NFA-0.75Y
Balance Balance Balance Balance
16 16 16 16
2 2 2 2
2 2 2 2
0.3 0.3 0.3 0.3
Nil 0.25 0.50 0.75
yttria (Y2O3) powder were chosen as starting materials to synthesize Ferritic ODS alloys, also referred to as nanostructured ferritic alloys (NFA). The composition of the NFA systems considered for this study are presented in Table 1. 2.1. Processing Ferritic ODS alloy powders were produced by High energy planetary ball mill (Retsch PM - 400). The following milling parameters were adopted for optimum milling. The ball to powder ratio (BPR) was maintained at 15:1. The milling speed was set at 250 rpm. Tungsten carbide vials with Tungsten carbide balls were used for this milling process. The milling was carried out for 15 h with 20 min idle time for every 10 min of mill time. 0.5 wt% of Stearic Acid was used as the Process Control Agent for this milling process. Spark Plasma Sintering was done in vacuum (10 2 torr) at temperatures of 900 °C, 950 °C with a heating rate of 100 °C per minute at a constant pressure of 50 MPa. The sample was held at the sintering temperature for 5 min. It is pertinent to note that a single sintering cycle time is an astonishing 14 min; hence it is a very rapid synthesis technique.
respectively. There were no significant changes in the crystallite size at 20 h of milling time, as compared to 15 h. Broadening of peaks were observed from 10 h milled sample onwards which is an indicator of particle size refinement and straining of powder particles during high energy ball milling. 3.1.1. Peak shifting The X-Ray diffraction patterns show that the shifting of peaks takes place in the alloyed powder compared to unmilled powder, shown in the Fig. 1(c). So there is a decrease in 2h position which corresponds to an increase in the d-spacing which further serves as a proof for the solid solution taking place in the system. Peak broadening of the alloyed powder shows that there is a corresponding decrease in the crystallite size and the same is presented in Table 2. Crystallite size which was measured using WilliamsonHall method, since it factors in micro strain, was found to be in the order of nanometers, supplementing the claim that milling has resulted in nano powders. Mechanical alloying induces lots of mechanical strain in the lattice. 3.2. Scanning electron microscopy
3. Results and discussion
The process of mechanical alloying fundamentally involves two mechanisms – Fracturing and cold welding. The unmilled powders, shown in Fig. 2(a), due to the impact of the high energy balls get fractured and broken down into smaller fragments, step by step which can be seen in the consecutive Fig. 2(a)–(e). Initially, powder gets flattened due to ball-powder-ball impact that can be seen in 5 h milled powder. When the particle size reaches a lower critical level, further fracture and breakdown becomes difficult. As the balls impinge on the powder particles, these fragments are forced to collide on each other, so that cold welding takes place which in turn leads to alloying. Thus, mechanical alloying facilitates even for the formation of non-equilibrium phases. The start of this process could be noticed in Fig. 2(c) and is well established in Fig. 2(d). As the process of cold-welding progresses, the particle size continues to increase, only to be fractured again by the impinging balls. Both fracturing and cold welding takes place simultaneously to create multiple layered powder particles. This cycle continues until the system reaches a saturation particle size. In our case, the systems tend to saturate after 15 h of ball milling.
3.1. X-ray diffraction
3.3. Spark plasma sintering
The XRD patterns of the mechanically alloyed powders, achieved through ball milling were compared to unmilled powders. The unmilled powder pattern exhibits the presence of elemental powders such as Fe, Cr, W, Al. After 10 h of milling, elements have started forming a solid solution. At 15 h milling time, the XRD patterns showed that aluminium (FCC) and tungsten (BCC) formed a complete solid solution with iron (BCC) [Fig. 1(a)]. Here all the peaks correspond to a-Fe. Initially, the aluminium (FCC) peaks were found at the 2h positions of 38.5° and 78.44° which corresponds to (1 1 1) and (1 1 3) planes respectively. Similarly the tungsten (BCC) peaks were present at the positions of 73.38° and 40.34° that correlates to (1 1 2) and (0 1 1) planes
Consolidation of the mechanically alloyed powders was done by spark plasma sintering (SPS), as shown in Table 3. Since the alloy system does not undergo reactive sintering; a fairly high heating rate of 100 °C/min was justified. Sintering at a temperature of 950 °C produced samples with higher density compared to 900 °C, which can be observed from Table 3. Smaller holding time of 5 min and higher heating rate helps prevent dispersoid coarsening. Samples sintered above 950 °C developed cracks while handling. Densification for all the combinations started at around 450 °C, as can be seen from Fig. 3. Crystallite size increased after consolidation but was still very much lower than the as-mixed powder, hence the nanostructure is still retained.
2.2. Characterisation X-Ray Diffraction profiles of as-mixed powders, also referred to as unmilled powders and mechanically alloyed powders were recorded using ULTIMA-III (Rigaku, Japan). The samples were scanned from 30 °C to 90 °C with a step size of 0.02°. Cu-Ka radiation was used for the diffraction studies. Scanning Electron Microscopy images of powders at different milling stages were taken in FEI ESEM Quanta 200 in SE mode. For compression test, cylindrical specimens were cut from the sintered samples with an aspect ratio of 2:1 (Length: Diameter), as per ASTM E9-09 standards. Sample flatness was ensured. Test was conducted in Tinius Olsen machine at a strain rate of 10 3 s 1. Vickers Micro hardness was carried out using Wilson Micro Hardness Tester (402MVD). Sintered samples were mechanically polished before indentation. 0.5 kgF load was applied with a dwell time of 10 s.
Please cite this article as: B. V. Ponraj, A. Dinesh Kumar and S. Kumaran, Rapid synthesis of oxide dispersion strengthened ferritic alloys through Spark plasma sintering, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.094
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B.V. Ponraj et al. / Materials Today: Proceedings xxx (xxxx) xxx
Fig. 1. XRD patterns of (a) powder samples (b) sintered solid samples (c) Peak shift.
Table 2 Crystallite size and micro strain. Alloy System
Interplanar Spacing ‘‘d” (Å)
Lattice parameter ‘‘a” (Å)
Crystallite Size (Å)
Micro strain (%)
Unmilled NFA-0Y - Powder NFA-0.25Y - Powder NFA-0.50Y - Powder NFA-0.75Y - Powder NFA-0Y - Solid NFA-0.25Y - Solid NFA-0.50Y - Solid NFA-0.75Y - Solid
2.0192 2.0363 2.0356 2.0305 2.0306 2.0398 2.0388 2.0288 2.0377
2.8551 2.8793 2.8783 2.8712 2.8713 2.8843 2.8829 2.8687 2.8813
1753 75 67 67 63 142 135 206 222
0.0334 0.8794 0.6882 0.3125 0.2560 0.4952 0.4606 0.2577 0.2427
3.4. Hardness test
3.5. Compression test
Microhardness of the solid samples were observed using Micro Vickers hardness tester. The average hardness for NFA-0Y was found to be 586 Hv, for NFA-0.25Y it was 572 Hv, whereas for NFA-0.50Y it was found to be 534 Hv and finally NFA-0.75Y was in the range of 542 Hv. It was noted that there was no significant difference in hardness values. Even though a slight decreasing trend could be observed, the maximum difference among various compositions were still less than 10%.
In a previous study, compression behaviour for similar compositions except for aluminium was studied. System Fe-16Cr-2W-0.3Ti with no aluminium (which will be referred to as NFA-0Al-0Y), resulted in a compression strength of 2131 MPa. 0.25% of yttria was added to the above composition (referred to as NFA-0Al-0.25Y), which resulted in a compression strength of 3123 MPa, which is 1.5 times more than NFA-0Al-0Y. This corroborates the fact that Yttria acts as dispersion strengthener. Along
Please cite this article as: B. V. Ponraj, A. Dinesh Kumar and S. Kumaran, Rapid synthesis of oxide dispersion strengthened ferritic alloys through Spark plasma sintering, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.094
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Fig. 2. SEM images of (a) Unmilled powder, (b) 5 h milled, (c) 10 h milled, (d) 15 h milled, (e) 20 h milled powders.
with titanium, yttria gets refined into Y-Ti-O compound which is much smaller in size compared to yttria that contributed to the final strength [8]. In the current study, 2% Al was added to improve corrosion resistance. Addition of aluminium without yttria resulted in a similar compression strength (2090 MPa) to that of System NFA-0Al-0Y. Aluminium along with 0.25% yttria was added to improve both corrosion resistance and sustain strength. But the
resulting material, as shown in Fig. 4, had largely reduced compression strength (1820 MPa), compared to NFA-0Al-0.25Y. The reason for the decrease in compressive strength is due to the formation of Y-Al-O compounds. There is a competition between Ti and Al to form tertiary oxides with yttria. Al having favorable enthalpy of formation [9], forms Y-Al-O readily. This oxide is larger than yttria and thus reduces the effect of dispersion strengthening [10].
Please cite this article as: B. V. Ponraj, A. Dinesh Kumar and S. Kumaran, Rapid synthesis of oxide dispersion strengthened ferritic alloys through Spark plasma sintering, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.094
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B.V. Ponraj et al. / Materials Today: Proceedings xxx (xxxx) xxx Table 3 SPS Densification at 900 °C and 950 °C. S. No
Alloy System
Temperature (°C)
Holding Time (Min)
Pressure (MPa)
Theoretical Density (g/cc)
Sintered density (g/cc)
Relative density (%)
1 2 3 4 5 6 7 8
NFA-0Y NFA-0.25Y NFA-0.50Y NFA-0.75Y NFA-0Y NFA-0.25Y NFA-0.50Y NFA-0.75Y
900 900 900 900 950 950 950 950
5 5 5 5 5 5 5 5
50 50 50 50 50 50 50 50
7.523 7.514 7.504 7.495 7.523 7.514 7.504 7.495
6.469 5.94 6.11 6.13 6.97 6.94 6.881 6.835
86 79.16 81.4 81.78 92.6 92.3 91.7 91.2
Fig. 3. Densification Plots (a) Instantaneous Relative Density (b) Instantaneous Densification Rate.
Fig. 4. Compression studies.
Please cite this article as: B. V. Ponraj, A. Dinesh Kumar and S. Kumaran, Rapid synthesis of oxide dispersion strengthened ferritic alloys through Spark plasma sintering, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.094
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B.V. Ponraj et al. / Materials Today: Proceedings xxx (xxxx) xxx
4. Conclusion ODS ferritic alloys have been rapidly synthesised through mechanical alloying and SPS process. XRD revealed complete solid solution and nanostructure of the alloy powders. Fracturing and cold welding during mechanical alloying was evident from SEM images. Sintering at 900 °C, did not yield adequate densification, whereas sintering done at 950 °C resulted in sufficient density. Compression studies revealed that addition of Al resulted in lowering of mechanical strength due to combined effect of Al and yttria, nevertheless Al is preferred for superior corrosion resistance.
[3]
[4]
[5]
[6]
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[7]
[8]
References [9] [1] N. Muthukumar, J.H. Lee, A. Kimura, SCC behavior of austenitic and martensitic steels in supercritical pressurized water, J. Nucl. Mater. 417 (2011) 1221–1224, https://doi.org/10.1016/j.jnucmat.2011.02.033. [2] A. Kimura, R. Kasada, N. Iwata, H. Kishimoto, C.H. Zhang, J. Isselin, P. Dou, J.H. Lee, N. Muthukumar, T. Okuda, M. Inoue, S. Ukai, S. Ohnuki, T. Fujisawa, T.F.
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Abe, Development of Al added high-Cr ODS steels for fuel cladding of next generation nuclear systems, J. Nucl. Mater. 417 (2011) 176–179, https://doi. org/10.1016/j.jnucmat.2010.12.300. J. Isselin, R. Kasada, A. Kimura, T. Okuda, M. Inoue, S. Ukai, S. Ohnuki, T. Fujisawa, F. Abe, Evaluation of fracture behavior of recrystallized and aged high-Cr ODS ferritic steels, J. Nucl. Mater. 417 (2010) 185–188, https://doi.org/ 10.1016/j.jnucmat.2010.12.061. J. Isselin, R. Kasada, A. Kimura, Corrosion behaviour of 16 % Cr – 4 % Al and 16 % Cr ODS ferritic steels under different metallurgical conditions in a supercritical water environment, Corros. Sci. 52 (2010) 3266–3270, https://doi.org/10.1016/ j.corsci.2010.05.043. J. Chen, M.A. Pouchon, A. Kimura, P. Jung, W. Hoffelner, Irradiation creep and microstructural changes in an advanced ODS ferritic steel during helium implantation under stress, J. Nucl. Mater. 386–388 (2009) 143–146, https:// doi.org/10.1016/j.jnucmat.2008.12.081. C.H. Zhang, Y.T. Yang, Y. Song, J. Chen, L.Q. Zhang, J. Jang, A. Kimura, Irradiation response of ODS ferritic steels to high-energy Ne ions at HIRFL, J. Nucl. Mater. 455 (2014) 61–67, https://doi.org/10.1016/j.jnucmat.2014.04.015. S. Li, Z. Zhou, J. Jang, M. Wang, H. Hu, H. Sun, L. Zou, G. Zhang, L. Zhang, The influence of Cr content on the mechanical properties of ODS ferritic steels, J. Nucl. Mater. 455 (2014) 194–200, https://doi.org/10.1016/j. jnucmat.2014.05.061. I. Hilger, M. Tegel, M.J. Gorley, P.S. Grant, T. Weißgärber, B. Kieback, The structural changes of Y2O3 in ferritic ODS alloys during milling, J. Nucl. Mater. 447 (2014) 242–247, https://doi.org/10.1016/j.jnucmat.2014.01.026. R. Chinnappan, Thermodynamic stability of oxide phases of Fe-Cr based ODS steels via quantum mechanical calculations, Procedia Eng. 86 (2014) 788–798, https://doi.org/10.1016/j.proeng.2014.11.099. L. Dai, Y. Liu, Z. Dong, Size and structure evolution of yttria in ODS ferritic alloy powder during mechanical milling and subsequent annealing, Powder Technol. 217 (2012) 281–287, https://doi.org/10.1016/j.powtec.2011.10.039.
Please cite this article as: B. V. Ponraj, A. Dinesh Kumar and S. Kumaran, Rapid synthesis of oxide dispersion strengthened ferritic alloys through Spark plasma sintering, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.094
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
Rapid synthesis of oxide dispersion strengthened ferritic alloys through Spark plasma sintering B.V. Ponraj ⇑, A. Dinesh Kumar, S. Kumaran Department of Metallurgical and Materials Engineering, National Institute of Technology Tiruchirappalli, Tamilnadu 620015, India
a r t i c l e
i n f o
Article history: Received 5 November 2019 Accepted 7 November 2019 Available online xxxx Keywords: Oxide dispersion strengthening Nanostructured ferritic alloy Mechanical alloying Spark plasma sintering Yttria
a b s t r a c t Oxide dispersion strengthened (ODS) ferritic alloys of composition Fe-16Cr-2Al-2W-0.3Ti with varying additions of Y2O3 (0, 0.25, 0.5, 0.75 wt%) were fabricated by mechanical alloying (MA) and Spark plasma sintering (SPS) technique swiftly. The influence of SPS temperature in phase evolution and densification kinetics of ODS ferritic alloy was studied at two different temperatures of 900 °C and 950 °C with 5 min soaking duration at the imposed pressure of 50 MPa. X-ray diffraction (XRD) study was carried out on milled powders and sintered bulk samples. XRD confirmed that alloying has taken place and revealed the nano-crystalline nature. Additionally, crystallite size and micro strain were also evaluated. Uniaxial compression test was performed on sintered bulk samples to understand the mechanical properties of these ODS ferritic alloys. Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International conference on Materials and Manufacturing Methods.
1. Introduction By the year 2030, Generation IV nuclear fission reactors would come into existence. These reactors operate at a temperature upward of 700 °C [1]. Current generation materials could not operate in such harsh conditions. A new breed of cladding materials is required to meet the stringent requirements of the 4th generation nuclear reactors. Development of cladding materials is a vital issue to accomplish higher burn-up operation of Generation IV systems such as lead bismuth-cooled fast reactor (LFR), sodium-cooled fast reactor (SFR), supercritical pressurized water reactor (SCPWR), and so on [2]. The aspiring cladding materials should have an adequate resistance to embrittlement caused by neutron irradiation and void swelling. Additionally, even at elevated temperatures they should possess sufficient mechanical properties [3]. Besides, claddings should exhibit optimum corrosion resistance for pragmatic long term application of state-of-the-art fission nuclear systems [4]. Similar to the Generation IV nuclear systems, exceptional performance is mandatory for fusion blanket materials. Furthermore, fusion systems require stringent materials for their inherent predicaments pertaining to helium/hydrogen transmutation issues and also require low activation grade materials [5]. ⇑ Corresponding author. E-mail address: [email protected] (B.V. Ponraj).
Ferritic/martensitic steels enhanced by oxide dispersion strengthening (ODS) comprising of 9–12% Cr (by weight) are proposed to be the cladding component for uranium fuel rods in sodium environment, thanks to their superficial resistance to neutron irradiation embrittlement and superior creep strength at elevated temperatures [6]. Nevertheless, the aforementioned alloys are unsuitable for supercritical water environment due to inadequate corrosion resistance of these steels. It is well documented that corrosion resistance of ferrous alloys is predominantly determined by Cr and Al content [2]. For every distinct blanket system, it is anticipated to have appropriate synergistic levels of Cr and Al combination, as required by the system. The Cr level has to be determined so as to maintain a balance between the positive trait of offering corrosion resistance and the negative trait of aging embrittlement while upholding strength at sufficiently high temperatures. Ferritic ODS steels with less than 16% or Cr was prone to corrosion under service conditions. Ferritic ODS steels with more than 19% Cr were susceptible to ageing embrittlement [7]. Nano-sized oxide dispersion is the desired technology for enhancing the mechanical properties of Al-added steels.
2. Materials and methods Elemental powders of iron (Fe), chromium (Cr), aluminium (Al), tungsten (W), titanium (Ti) with varying proportions of nano
https://doi.org/10.1016/j.matpr.2019.11.094 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International conference on Materials and Manufacturing Methods.
Please cite this article as: B. V. Ponraj, A. Dinesh Kumar and S. Kumaran, Rapid synthesis of oxide dispersion strengthened ferritic alloys through Spark plasma sintering, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.094
2
B.V. Ponraj et al. / Materials Today: Proceedings xxx (xxxx) xxx
Table 1 Chemical composition (in weight %). Sample ID
Fe
Cr
Al
W
Ti
Y2O3
NFA-0Y NFA-0.25Y NFA-0.50Y NFA-0.75Y
Balance Balance Balance Balance
16 16 16 16
2 2 2 2
2 2 2 2
0.3 0.3 0.3 0.3
Nil 0.25 0.50 0.75
yttria (Y2O3) powder were chosen as starting materials to synthesize Ferritic ODS alloys, also referred to as nanostructured ferritic alloys (NFA). The composition of the NFA systems considered for this study are presented in Table 1. 2.1. Processing Ferritic ODS alloy powders were produced by High energy planetary ball mill (Retsch PM - 400). The following milling parameters were adopted for optimum milling. The ball to powder ratio (BPR) was maintained at 15:1. The milling speed was set at 250 rpm. Tungsten carbide vials with Tungsten carbide balls were used for this milling process. The milling was carried out for 15 h with 20 min idle time for every 10 min of mill time. 0.5 wt% of Stearic Acid was used as the Process Control Agent for this milling process. Spark Plasma Sintering was done in vacuum (10 2 torr) at temperatures of 900 °C, 950 °C with a heating rate of 100 °C per minute at a constant pressure of 50 MPa. The sample was held at the sintering temperature for 5 min. It is pertinent to note that a single sintering cycle time is an astonishing 14 min; hence it is a very rapid synthesis technique.
respectively. There were no significant changes in the crystallite size at 20 h of milling time, as compared to 15 h. Broadening of peaks were observed from 10 h milled sample onwards which is an indicator of particle size refinement and straining of powder particles during high energy ball milling. 3.1.1. Peak shifting The X-Ray diffraction patterns show that the shifting of peaks takes place in the alloyed powder compared to unmilled powder, shown in the Fig. 1(c). So there is a decrease in 2h position which corresponds to an increase in the d-spacing which further serves as a proof for the solid solution taking place in the system. Peak broadening of the alloyed powder shows that there is a corresponding decrease in the crystallite size and the same is presented in Table 2. Crystallite size which was measured using WilliamsonHall method, since it factors in micro strain, was found to be in the order of nanometers, supplementing the claim that milling has resulted in nano powders. Mechanical alloying induces lots of mechanical strain in the lattice. 3.2. Scanning electron microscopy
3. Results and discussion
The process of mechanical alloying fundamentally involves two mechanisms – Fracturing and cold welding. The unmilled powders, shown in Fig. 2(a), due to the impact of the high energy balls get fractured and broken down into smaller fragments, step by step which can be seen in the consecutive Fig. 2(a)–(e). Initially, powder gets flattened due to ball-powder-ball impact that can be seen in 5 h milled powder. When the particle size reaches a lower critical level, further fracture and breakdown becomes difficult. As the balls impinge on the powder particles, these fragments are forced to collide on each other, so that cold welding takes place which in turn leads to alloying. Thus, mechanical alloying facilitates even for the formation of non-equilibrium phases. The start of this process could be noticed in Fig. 2(c) and is well established in Fig. 2(d). As the process of cold-welding progresses, the particle size continues to increase, only to be fractured again by the impinging balls. Both fracturing and cold welding takes place simultaneously to create multiple layered powder particles. This cycle continues until the system reaches a saturation particle size. In our case, the systems tend to saturate after 15 h of ball milling.
3.1. X-ray diffraction
3.3. Spark plasma sintering
The XRD patterns of the mechanically alloyed powders, achieved through ball milling were compared to unmilled powders. The unmilled powder pattern exhibits the presence of elemental powders such as Fe, Cr, W, Al. After 10 h of milling, elements have started forming a solid solution. At 15 h milling time, the XRD patterns showed that aluminium (FCC) and tungsten (BCC) formed a complete solid solution with iron (BCC) [Fig. 1(a)]. Here all the peaks correspond to a-Fe. Initially, the aluminium (FCC) peaks were found at the 2h positions of 38.5° and 78.44° which corresponds to (1 1 1) and (1 1 3) planes respectively. Similarly the tungsten (BCC) peaks were present at the positions of 73.38° and 40.34° that correlates to (1 1 2) and (0 1 1) planes
Consolidation of the mechanically alloyed powders was done by spark plasma sintering (SPS), as shown in Table 3. Since the alloy system does not undergo reactive sintering; a fairly high heating rate of 100 °C/min was justified. Sintering at a temperature of 950 °C produced samples with higher density compared to 900 °C, which can be observed from Table 3. Smaller holding time of 5 min and higher heating rate helps prevent dispersoid coarsening. Samples sintered above 950 °C developed cracks while handling. Densification for all the combinations started at around 450 °C, as can be seen from Fig. 3. Crystallite size increased after consolidation but was still very much lower than the as-mixed powder, hence the nanostructure is still retained.
2.2. Characterisation X-Ray Diffraction profiles of as-mixed powders, also referred to as unmilled powders and mechanically alloyed powders were recorded using ULTIMA-III (Rigaku, Japan). The samples were scanned from 30 °C to 90 °C with a step size of 0.02°. Cu-Ka radiation was used for the diffraction studies. Scanning Electron Microscopy images of powders at different milling stages were taken in FEI ESEM Quanta 200 in SE mode. For compression test, cylindrical specimens were cut from the sintered samples with an aspect ratio of 2:1 (Length: Diameter), as per ASTM E9-09 standards. Sample flatness was ensured. Test was conducted in Tinius Olsen machine at a strain rate of 10 3 s 1. Vickers Micro hardness was carried out using Wilson Micro Hardness Tester (402MVD). Sintered samples were mechanically polished before indentation. 0.5 kgF load was applied with a dwell time of 10 s.
Please cite this article as: B. V. Ponraj, A. Dinesh Kumar and S. Kumaran, Rapid synthesis of oxide dispersion strengthened ferritic alloys through Spark plasma sintering, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.094
3
B.V. Ponraj et al. / Materials Today: Proceedings xxx (xxxx) xxx
Fig. 1. XRD patterns of (a) powder samples (b) sintered solid samples (c) Peak shift.
Table 2 Crystallite size and micro strain. Alloy System
Interplanar Spacing ‘‘d” (Å)
Lattice parameter ‘‘a” (Å)
Crystallite Size (Å)
Micro strain (%)
Unmilled NFA-0Y - Powder NFA-0.25Y - Powder NFA-0.50Y - Powder NFA-0.75Y - Powder NFA-0Y - Solid NFA-0.25Y - Solid NFA-0.50Y - Solid NFA-0.75Y - Solid
2.0192 2.0363 2.0356 2.0305 2.0306 2.0398 2.0388 2.0288 2.0377
2.8551 2.8793 2.8783 2.8712 2.8713 2.8843 2.8829 2.8687 2.8813
1753 75 67 67 63 142 135 206 222
0.0334 0.8794 0.6882 0.3125 0.2560 0.4952 0.4606 0.2577 0.2427
3.4. Hardness test
3.5. Compression test
Microhardness of the solid samples were observed using Micro Vickers hardness tester. The average hardness for NFA-0Y was found to be 586 Hv, for NFA-0.25Y it was 572 Hv, whereas for NFA-0.50Y it was found to be 534 Hv and finally NFA-0.75Y was in the range of 542 Hv. It was noted that there was no significant difference in hardness values. Even though a slight decreasing trend could be observed, the maximum difference among various compositions were still less than 10%.
In a previous study, compression behaviour for similar compositions except for aluminium was studied. System Fe-16Cr-2W-0.3Ti with no aluminium (which will be referred to as NFA-0Al-0Y), resulted in a compression strength of 2131 MPa. 0.25% of yttria was added to the above composition (referred to as NFA-0Al-0.25Y), which resulted in a compression strength of 3123 MPa, which is 1.5 times more than NFA-0Al-0Y. This corroborates the fact that Yttria acts as dispersion strengthener. Along
Please cite this article as: B. V. Ponraj, A. Dinesh Kumar and S. Kumaran, Rapid synthesis of oxide dispersion strengthened ferritic alloys through Spark plasma sintering, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.094
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B.V. Ponraj et al. / Materials Today: Proceedings xxx (xxxx) xxx
Fig. 2. SEM images of (a) Unmilled powder, (b) 5 h milled, (c) 10 h milled, (d) 15 h milled, (e) 20 h milled powders.
with titanium, yttria gets refined into Y-Ti-O compound which is much smaller in size compared to yttria that contributed to the final strength [8]. In the current study, 2% Al was added to improve corrosion resistance. Addition of aluminium without yttria resulted in a similar compression strength (2090 MPa) to that of System NFA-0Al-0Y. Aluminium along with 0.25% yttria was added to improve both corrosion resistance and sustain strength. But the
resulting material, as shown in Fig. 4, had largely reduced compression strength (1820 MPa), compared to NFA-0Al-0.25Y. The reason for the decrease in compressive strength is due to the formation of Y-Al-O compounds. There is a competition between Ti and Al to form tertiary oxides with yttria. Al having favorable enthalpy of formation [9], forms Y-Al-O readily. This oxide is larger than yttria and thus reduces the effect of dispersion strengthening [10].
Please cite this article as: B. V. Ponraj, A. Dinesh Kumar and S. Kumaran, Rapid synthesis of oxide dispersion strengthened ferritic alloys through Spark plasma sintering, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.094
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B.V. Ponraj et al. / Materials Today: Proceedings xxx (xxxx) xxx Table 3 SPS Densification at 900 °C and 950 °C. S. No
Alloy System
Temperature (°C)
Holding Time (Min)
Pressure (MPa)
Theoretical Density (g/cc)
Sintered density (g/cc)
Relative density (%)
1 2 3 4 5 6 7 8
NFA-0Y NFA-0.25Y NFA-0.50Y NFA-0.75Y NFA-0Y NFA-0.25Y NFA-0.50Y NFA-0.75Y
900 900 900 900 950 950 950 950
5 5 5 5 5 5 5 5
50 50 50 50 50 50 50 50
7.523 7.514 7.504 7.495 7.523 7.514 7.504 7.495
6.469 5.94 6.11 6.13 6.97 6.94 6.881 6.835
86 79.16 81.4 81.78 92.6 92.3 91.7 91.2
Fig. 3. Densification Plots (a) Instantaneous Relative Density (b) Instantaneous Densification Rate.
Fig. 4. Compression studies.
Please cite this article as: B. V. Ponraj, A. Dinesh Kumar and S. Kumaran, Rapid synthesis of oxide dispersion strengthened ferritic alloys through Spark plasma sintering, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.094
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B.V. Ponraj et al. / Materials Today: Proceedings xxx (xxxx) xxx
4. Conclusion ODS ferritic alloys have been rapidly synthesised through mechanical alloying and SPS process. XRD revealed complete solid solution and nanostructure of the alloy powders. Fracturing and cold welding during mechanical alloying was evident from SEM images. Sintering at 900 °C, did not yield adequate densification, whereas sintering done at 950 °C resulted in sufficient density. Compression studies revealed that addition of Al resulted in lowering of mechanical strength due to combined effect of Al and yttria, nevertheless Al is preferred for superior corrosion resistance.
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Please cite this article as: B. V. Ponraj, A. Dinesh Kumar and S. Kumaran, Rapid synthesis of oxide dispersion strengthened ferritic alloys through Spark plasma sintering, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.094