P. KISHORE KUMAR et al.: EFFECT OF Y2O3 AND ZrO2 ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES ... 493–497 EFFECT OF Y 2 O 3 AND ZrO 2 ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF NANO-ODS 21Cr-9Mn-6Ni STEELS VPLIV Y 2 O 3 IN ZrO 2 NA MIKROSTRUKTURO IN MEHANSKE LASTNOSTI NANO-ODS 21Cr-9Mn-6Ni JEKEL Paleti Kishore Kumar 1 , Nimmagadda Vijaya Sai 2 , Alluru Gopala Krishna 3 1 Veltech Rangarajan Dr Sagunthala R&D Institute of Science and Technology, Department of Mechanical Engineering, Chennai 600 062, India 2 V R Siddhartha Engineering College, Department of Mechanical Engineering, Viayawada 520 007, India 3 University College of Engineering, Department of Mechanical Engineering, Jawaharlal Nehru Technological University, Kakinada 533 003, India kishore.paleti@gmail.com Prejem rokopisa – received: 2017-11-25; sprejem za objavo – accepted for publication: 2018-02-15 doi:10.17222/mit.2017.199 In this article, nano-oxide dispersion-strengthened (ODS) 21Cr-9Mn-6Ni steels were prepared from elemental powders through vacuum hot pressing. Three ODS steels (21Cr-9Mn-6Ni-0.4Ti-0.3Y2O3, 21Cr-9Mn-6Ni-0.4Ti-0.3ZrO2 and 21Cr-9Mn- 6Ni-0.4Ti-0.15Y2O3-0.15ZrO2) were prepared to study the effect of Y2O3 and ZrO2 additions on the microstructure and mechanical properties. The nano-oxide precipitates within the austenite matrix were identified using a transmission-electron microscope equipped with energy-dispersive X-ray spectrometer (TEM-EDX). An addition of Y2O3 with ZrO2 leads to the formation of a bimodal distribution of grains and fine oxide particles of Y-Zr-O and Y-Ti-O in the microstructure of an ODS steel. The results of the tensile test at different temperatures reveal that the strength and elongation of 21Cr-9Mn-6Ni steels are improved with the addition of Y2O3 and ZrO2 due to a stronger interface bonding between large numbers of oxide particles and the austenite matrix. Keywords: ODS steel, nano-oxide precipitates, tensile strength, elongation, fracture surface V ~lanku avtorji opisujejo izdelavo z nano-oksidnimi delci disperzijsko utrjenega (ODS) jekla 21Cr-9Mn-6Ni. ODS jeklo je bilo izdelano z vro~im stiskanjem iz elementarnih prahov. Tri ODS jekla (21Cr-9Mn-6Ni-0,4Ti-0,3Y2O3, 21Cr-9Mn- 6Ni-0,4Ti-0,3ZrO2 in 21Cr-9Mn-6Ni-0,4Ti-0,15Y2O3-0,15ZrO2) so izdelali z namenom, da bi raziskali vpliv dodatkov Y2O3 in ZrO2 na mikrostrukturo in mehanske lastnosti. Nano-oksidne izlo~ke v austenitu so okarakterizirali s presevnim elektronskim mikroskopom, opremljenim z rentgenskim energijskim disperzijskim spektrometrom (TEM-EDX). Ugotovili so, da dodatek Y2O3 s ZrO2 vodi do tvorbe bimodalne porazdelitve kristalnih zrn in drobnih oksidnih delcev Y-Zr-O in Y-Ti-O v mikrostrukturi ODS jekla. Natezni preizkusi pri razli~nih temperaturah so pokazali, da sta se trdnost in raztezek 21Cr-9Mn-6Ni jekla izbolj{ala z dodatkom Y2O3 in ZrO2 zaradi mo~nej{e vezi med velikim {tevilom oksidnih delcev in austenitno matrico. Keywords: ODS jeklo, nano-oksidni izlo~ki, natezna trdnost, raztezek, prelomna povr{ina 1 INTRODUCTION 21Cr-9Mn-6Ni (Nitronic-40) stainless steel is a high-alloyed, high-manganese (Mn) nitrogen (N) strengthened austenitic stainless steel. It has excellent corrosion and oxidation resistance because of a higher amount of chromium (20–30 %) and an increased amount of manganese. Therefore, this steel can be used at elevated temperatures due to its better creep resistance and high-temperature corrosion resistance. 1 The structural components used in the nuclear industry are subjected to a very high temperature and pressure. Stainless steel of different types, including aus- tenitic steel (AS), martensitic-ferritic steel (M-F/S) and super alloys (nickel based) have good high-temperature properties but poor swelling characteristics. 2 Oxide dispersion strengthened (ODS) steels are the most promising structural materials for nuclear reactors due to many reasons. 3 An ODS steel is manufactured using the dispersion-strengthening mechanism, wherein nano- oxide precipitates are uniformly dispersed in the micro- structure. 4 The superior characteristics of ODS steels depend on the preparation, processing methods 5 and composition design. 6 Y 2 O 3 nanoparticles with a cubic structure were used as the oxide particles in the development of ODS steels in recent years. 7 The titanium element was used, along with Y 2 O 3, to optimize the nano-particles in the microstructure of ODS steels. 8 Researchers also focused on modifying the microstructure of ODS steels by adding zirconium. 9 In this paper, the effect of Y 2 O 3 and ZrO 2 on the development of the ODS 21Cr-9Mn-6Ni steel was inves- tigated. Also, the microstructure and tensile properties (yield strength, ultimate tensile strength and ductility) of three different ODS steels were studied and compared. Materiali in tehnologije / Materials and technology 52 (2018) 4, 493–497 493 UDK 67.017:620.3:669.1 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(4)493(2018) 2 EXPERIMENTAL PART Elemental powders of Fe, Cr, Mn, Ni, Si, Mn 3 N 2 ,T i and C with a purity of above 99.7 % and a size of 45 μm, delivered from Innomet Powders Pvt. Ltd, India, were used as the starting materials. Different ODS materials such as Y 2 O 3 and ZrO 2 powders with a purity of above 99.9 % and a size of 500 nm, from Alfa Aesar, UK, were added to the elemental powders to obtain different ODS steels. The powder blends of three different ODS steel compositions and their designations are listed in Table 1. Table 1: Alloy compositions and designations S. No. Composition Alloy designation 1 Fe-21Cr-9Mn-6Ni-Si-0.15N-0.08C- 0.4Ti-0.3Y 2 O 3 21-9-6/Y 2 O 3 2 Fe-21Cr-9Mn-6Ni-Si-0.15N-0.08C- 0.4Ti-0.3ZrO 2 21-9-6/ZrO 2 3 Fe-21Cr-9Mn-6Ni-Si-0.15N-0.08C- 0.4Ti-0.15Y 2O 3-0.15ZrO 2 21-9-6/Y 2O 3-ZrO 2 The three ODS steel compositions were properly mixed in a high-energy planetary ball mill (Fristch GmbH, Germany) for 20 h using mechanical alloying (MA). The ball mill operated at 300 min –1 and was equipped with a stainless steel vial and balls of a 10-mm diameter. The MA process was conducted at room tem- perature under an argon protective atmosphere and a ball-to-powder weight ratio of 10:1 maintained in a toluene medium. The as-milled powders were hot pressed (VB ceramics Pvt. Ltd, India) with a pressure of 50 MPa at 1170 °C for 60 min under a vacuum of 10 –2 torr. The grain distribution and oxide precipitates were investigated with a high-resolution transmission-electron microscope with energy-dispersive x-ray spectroscopy (HRTEM-EDX) (JEM/2100) operating at 200 kV. Thin-foil specimens of the ODS steel were mechanically thinned to 30 μm and punched into 3-mm discs. Dimpl- ing (GATAN 656) followed by ion milling (GATAN 691) were applied before the TEM studies. Uni-axial tensile tests were carried out at different temperatures of (301, 573 and 873) K, with a strain rate of 1 mm/min, using a universal testing machine (Instron 1362). The test specimens were machined according to the ASTM-E8 standard, having a gauge length of 15 mm. The fracture-surface morphology of the tensile specimens was characterised using a high-resolution scanning-electron microscope (FEI Quanta FEG 200 HR-SEM). 3 RESULTS 3.1 Mechanical alloying The XRD examination of the 21-9-6/Y 2 O 3 -ZrO 2 ele- mental composition is represented in Figure 1. The elemental composition at0hofMAsho wedindividual elemental peaks of Fe, Cr, Mn and Ni. As observed, all the elemental peaks were transformed into a dual-phase structure of austenite ( ) and ferrite ( ) phases after 5 h of MA, indicating a dissolution of the elemental powders into the base matrix. After 20 h of milling, the complete structure of the phase was observed. The MA process was optimized based on the transition rate between and . The other two ODS compositions also follow similar changes. 3.2 Micro-structural analysis The density of the three hot-pressed ODS steels was above 95 % of the theoretical density. The TEM micro- structures of the 21-9-6/Y 2 O 3 and 21-9-6/ZrO 2 steels are shown in Figures 2a and 2b. According to the EDX data, the grains from the figures exhibit an elemental compo- sition (w/%) of Fe, Cr, Mn and Ni. Two different sizes of the grains, namely, a large grain (below 1 μm) and a small grain (below 200 nm) were observed in the bimodal distribution of the grains in the microstructure of the 21-9-6/Y 2 O 3 -ZrO 2 steel (Figure 2c). According to EDX data (Figure 2d), the location (+) of smaller grains contains the main elements of Zr, Y, Ti, Si and O. P. KISHORE KUMAR et al.: EFFECT OF Y2O3 AND ZrO2 ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES ... 494 Materiali in tehnologije / Materials and technology 52 (2018) 4, 493–497 Figure 2: TEM microstructures of ODS steels Figure 1: XRD pattern of 21-9-6/Y 2 O 3 -ZrO 2 powder In order to further study the nano-oxide precipitates, higher-magnification TEM images of the oxide precipi- tates and size distribution (Figure 3) were obtained. The fine oxide precipitates of the Y-Ti-O rich composition of the microstructure of the 21-9-6/Y 2 O 3 steel vary from 5 to 50 nm according to the analysis of the particle-size frequency distribution (Figures 3a and 3b). On the other hand, the analysis of the ZrO 2 dispersed ODS steel (21-9-6/ZrO 2 ) revealed coarse oxide particles of the ZrO 2 composition (Figure 3c) and fine precipitates of the Ti-O composition; a large variation in the size of the precipi- tates can be observed in Figure 3d. In the 21-9-6/Y 2 O 3 - ZrO 2 steel (Figure 3e), the precipitates mainly have the Y-Ti-O and Y-Zr-O compositions. The size of most of the oxide particles is in a range of 10–20 nm (Figure 3f). The properties of the nano-oxide precipitates ob- tained for the three ODS steels are listed in Table 2. The calculations were performed on a large data array (a mi- nimum of 1000 nano-sized particles per composition). As it is seen, the 21-9-6/Y 2 O 3 -ZrO 2 ODS steel exhibited the minimum size and the maximum density of the precipitates. Table 2: Characteristics of nano-oxide precipitates ODS steel Density (per cc) Size (nm) 21-9-6/Y 2 O 3 3.2×10 15 14 21-9-6/ZrO 2 0.9×10 15 26 21-9-6/Y 2 O 3 -ZrO 2 5.6×10 15 12 3.3 Mechanical properties 3.3.1 Tensile properties Figure 4 shows the stress-strain curves for three ODS steels at different temperatures. Five replicate ten- sile tests were performed for each ODS steel at each temperature. The yield strength (YS), tensile strength (UTS) and percent elongation (PE) of the three ODS steels at (301, 573 and 873) K are summarized in Table 3. At all the testing temperatures, the 21-9-6/Y 2 O 3 -ZrO 2 specimen exhibited higher YS and UTS values compared to the other ODS steels, due to the bimodal distribution of powder grains in the microstructure (Figure 2c) and P. KISHORE KUMAR et al.: EFFECT OF Y2O3 AND ZrO2 ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES ... Materiali in tehnologije / Materials and technology 52 (2018) 4, 493–497 495 Figure 3: Appearance of nano-oxide precipitates Figure 5: Fracture-surface morphology Figure 4: Stress-strain curves for hot-pressed ODS steels also due to the presence of nano-sized Y-Zr-O oxide pre- cipitates (Figure 3e), which pinned the dislocations and retarded the grain growth. 9 However, the PE values for the 21-9-6/Y 2 O 3 -ZrO 2 steel were slightly reduced when compared to the 21-9-6/Y 2 O 3 steel at all the testing temperatures due to the high density of the nano-oxide precipitates (Table 2), making the material harder. 10 3.3.2. Fractographic examination The fracture surface of 21-9-6/ZrO 2 (Figure 5a) con- tained rearranged particles in some dimples. This indi- cated that failure occurred only at the particle interface, indicating a brittle failure. Because of this, the specimen exhibited poor mechanical properties (Table 3). How- ever, the 21-9-6/Y 2 O 3 -ZrO 2 steel (Figure 5b) clearly showed a ductile fracture evidenced by fine dimples over the entire surface. This was also validated by the tensile properties (Figure 4 and Table 3) of the specimen, showing the highest elongation among the ZrO 2 dis- persed ODS steels. 4 DISCUSSION 4.1 Microstructure The microstructure of the 21Cr-9Mn-6Ni ODS steel was studied based on the additions of different ODS ma- terials (Y 2 O 3 and ZrO 2 ). The microstructure of the Y 2 O 3 ODS steel has the appearance of Y-Ti-O nano-oxide precipitates, indicating the presence of Y 2 O 3 dissolved in the austenite matrix (Figure 3a). It is then combined with Ti to form nano-oxide precipitates of Y–Ti–O in the ODS steel. 8 The microstructure of the ZrO 2 ODS steel contains ZrO 2 particles (Figure 3c) indicating that it does not dissolve in an austenite lattice and may not form a solid solution with Ti. However, in the cases of Y 2 O 3 and ZrO 2 added to steel, the ZrO 2 particles are dissolved in the austenite matrix, forming a thermodynamically favour- able oxide precipitate with Y 2 O 3 (Figure 3e). Similar observations 8,11,12 were reported for the behaviour of ZrO 2 during the consolidation of ODS steel. 4.2 Mechanical properties The mechanical properties of the 21Cr-9Mn-6Ni ODS steel were studied as a function of different ODS materials (Y 2 O 3 and ZrO 2 ) added. The Y 2 O 3 ODS steel from the present work exhibited a lower UTS value (Table 4) compared to the other ODS austenitic steels developed previously. This is due to the HIP 4,10,13,14 consolidation process used for developing ODS steels, which is an effective process compared to the HP process used in this study. The Y 2 O 3 and ZrO 2 ODS steels from the present wok can have the tensile-strength values comparable to the other ODS steels developed previously. 10–14 The bimodal distribution of the grains (Figure 2c) and the formation of the Y-Ti-O and Y-Zr-O nano-oxide precipitates (Table 2 and Figure 3e) in the microstructure explain the high tensile-strength values of the ODS 21Cr-9Mn-6Ni steel. 5 CONCLUSIONS In this research, the authors investigated the micro- structure and mechanical properties of Y 2 O 3 and ZrO 2 21Cr-9Mn-6Ni-0.4Ti ODS steels. Three ODS steels (21-9-6/Y 2 O 3 , 21-9-6/ZrO 2 , 21-9-6/Y 2 O 3 -ZrO 2 )w e r e made from elemental powders and consolidated by hot pressing. The results were compared and the conclusions are summarized as follows: According to the TEM results, a bimodal distribution of the grains was found in the microstructure of the P. KISHORE KUMAR et al.: EFFECT OF Y2O3 AND ZrO2 ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES ... 496 Materiali in tehnologije / Materials and technology 52 (2018) 4, 493–497 Table 3: Mechanical properties of ODS steels at different test temperatures Composition 301 K 573 K 873 K YS (MPa) UTS (MPa) PE (%) YS (MPa) UTS (MPa) PE (%) YS (MPa) UTS (MPa) PE (%) 21-9-6/Y 2 O 3 466 734 19 421 667 17 181 291 12 21-9-6/ZrO 2 419 696 9 374 626 6 132 269 3 21-9-6/Y 2 O 3 -ZrO 2 494 820 17 452 756 15 199 321 10 Table 4: Comparison of microstructures and tensile strengths of different ODS austenitic steels Composition Process Nano-oxide precipitate (nm) UTS at 301 K (MPa) UTS at 873 K (MPa) Ref. 18Cr–8Ni–1Mo–0.5Ti–0.35Y 2 O 3 HIP Y-Ti-Si-O (20) 1000 500 4 SS 316 +0.3 Ti + 0.35 Y2O3 HIP YAH (20) 729 459 10 16Cr-13Ni-0.2 Y 2 O 3 -0.45Zr HIP+HR —— 970 370 11 14Cr-Zr HIP Y-Zr-O (8) 994 358 12 SS 304+0.35Y 2 O 3 +0.5Ti HIP Y-Ti-O (13) 940 415 13 SS 304 +0.3 Ti + 0.35Y 2 O 3 HIP Y-Al-O (20) 1000 – 14 21-9-6/Y 2 O 3 HP Y-Ti-O (14) 734 291 Present work 21-9-6/Y 2 O 3 -ZrO 2 HP Y-Ti-O & Y-Zr-O (12) 820 321 Present work 21-9-6/Y 2 O 3 -ZrO 2 ODS steel, which indicates that ZrO 2 was decomposed and dissolved into the austenite matrix only in the presence of Y 2 O 3 . The nano-oxide precipitates in the 21-9-6/Y 2 O 3 -ZrO 2 steel have a smaller size and a higher density compared to the other two ODS steels. This is due to the formation of Y-Zr-O and Y-Ti-O oxide precipitates with the addi- tions of Y 2 O 3 and ZrO 2 particles. The composition of the 21-9-6/Y 2 O 3 -ZrO 2 ODS steel showed higher yield and tensile strengths compared to the other two ODS steels studied. This is due to the homogeneous dispersion of the nano-oxide particles and bimodal distribution of powder grains. The fractography studies of the surface of the 21-9-6/Y 2 O 3 -ZrO 2 ODS steel revealed that the presence of regular-shaped dimples indicates a ductile failure. Based on the above findings, it can be concluded that an addition of Y 2 O 3 and ZrO 2 facilitates attractive mechanical properties of 21-9-6 steels. Acknowledgements The authors would like to express their gratitude to Veltech & JNTUK, India, for giving them an opportunity to publish this paper. 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KISHORE KUMAR et al.: EFFECT OF Y2O3 AND ZrO2 ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES ... Materiali in tehnologije / Materials and technology 52 (2018) 4, 493–497 497